Aggregate Base Course
Aggregate Base Course often referred simply as ABC, has certain desirable properties. Base Course in pavements refers to the sub-layer material of an asphalt roadway and is placed directly on top of the undisturbed soil (Sub-Grade) so as to provide a foundation to support the top layers of the pavement. It is typically made of a recipe of different sizes of aggregate rock inclusive of 1″ to fine dust. Aggregate is made from quarried rock, recycled asphalt, or concrete.
Aggregate Base is used as the base course under asphalt pavement roadways, under concrete slabs and structural foundations, and as backfill material for underground pipelines and other underground utilities within a roadway. It is placed by means of attentive spreading and compacting to a minimum of 95% relative compaction, providing the stable foundation needed to support a pathway, foundation, driveway or roadway.
The Sub-Base is a layer of small chipped aggregate and dust, typically Crushed Fines, which is laid above the ABC on driveways or heavy traffic areas. The thickness of sub-base can range from 1″ to 2″ inches on light weight traffic areas like pathways and paver patios above the sub-grade when a ABC is not required,
ABCM Aggregate Base Coarse is a hard pack sub-base compaction material. Contains a mix of crushed stone, topsoil and dust.
Cross-Section layers that make up a mortar-less or “dry-laid” pavement. A. Sub-Grade or undisturbed soil B. Aggregate Base Course C. Sub-Base Crushed Fines D. Paver Sand base bed E. Pavers F. Polymeric Sand
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Subgrades and Subbases for Concrete Slabs
A well-compacted subgrade keeps construction out of the mud and provides uniform slab support. Lippincott & Jacobs
What lies below your concrete slab is critical to a successful job. This is no different than the foundation for a building. A slab on ground (or slab on grade) by definition is not intended to be self-supporting. The "soil support system" beneath it is there to support the slab.
WHAT IS A SUBBASE/SUBGRADE?
The terminology used for soil support systems, unfortunately, is not completely consistent, so let's follow the American Concrete Institute's definitions, starting from the bottom:
- Subgrade—this is the native soil (or improved soil), usually compacted
- Subbase—this is a layer of gravel on top of the subgrade
- Base (or base course)—this is the layer of material on top of the subbase and directly under the slab
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A compacted subbase keeps workers out of the mud. Energy Efficient Building Network
The only layer that is absolutely required is the subgrade—you have to have ground to place a slab on ground on top of. If the natural soil is relatively clean and compactable, then you can put a slab right on top of it without any extra layers. The problems with that are that the soil may not drain well and it can be muddy during construction if it gets wet, it may not compact well, and it can be difficult to get it flat and to the proper grade. Typically, the top of the subgrade should be graded to within plus or minus 1.5 inches of the specified elevation.
A subbase and base course, or both, provide several good things. The thicker the subbase, the more load the slab can support, so if there are going to be heavy loads on the slab—like trucks or fork lifts—the designer will probably specify a thick subbase. A subbase can also act as a capillary break, preventing water from wicking up from the groundwater table and into the slab. The subbase material is usually a reasonably low cost gravel without a lot of fines.
Recycled crushed concrete is an excellent source for subbase material. The Concrete Producer
A base course on top of the subbase makes it easier to get to the proper grade and to get it flat. If you use some sort of a choker course of finer material on the top of the subbase, it will support your people and equipment during concrete placement. It will also keep your slab thickness uniform, which will save money on concrete—the most expensive part of the system. A flat base course will also allow the slab to slide easily as it shrinks, reducing restraint and the risk of cracks as the concrete contracts after placement (drying shrinkage).
The entire subbase and base system should be at least 4 inches thick—thicker if the engineer feels it is needed for proper support. The base course material, according to ACI 302, "Concrete Floor and Slab Construction," should be "compactible, easy to trim, granular fill that will remain stable and support construction traffic." ACI 302 recommends material with 10 to 30% fines (passing the No. 100 sieve) with no clay, silt, or organic materials. Manufactured aggregate works well—crushed recycled concrete aggregate can also work well. Tolerances on the base course are +0 inches and minus 1 inch for Class 1 through 3 floors (typical low tolerance floors) or +0 inches and minus ¾ inches for higher tolerance floors.
WHAT ABOUT THE SOIL?
A sand base course is easy to compress, but can rut easily during construction. Free Reformed Church of Southern River
The weight of the slab and anything on top of it is going to eventually be supported by the soil. When a building site is excavated, usually the soil gets moved around—high spots are cut and low spots are filled. Everything then should get compacted before you place the concrete, subbase and base.
The type of soil determines what needs to happen before placing a slab. There are three basic types of soil and here's what you should know about each:
- Organic soils , what you might call top soils, are great in your garden, but awful beneath a slab. Organic soils can't be compacted and must be removed and replaced with a compressible fill.
- Granular soils are sand or gravel. You can easily see the individual particles and water drains quite easily from them. Just like at the beach when you make a sand castle, if you take a wet handful of granular soil and make a ball, as soon as it dries it will crumble. Granular soils have the highest bearing strength and compact easily.
- Cohesive soils are clays. If you take a wet handful, you can roll it into a string just like with modeling clay. It has a greasy, smooth feeling between your fingers and individual particles are too small to see. Cohesive soils are often difficult to compact and take on a rock-hard consistency when dry, but they have a lower bearing strength than granular soils. Some clays expand when wet and shrink when dry making them particularly difficult as subgrade materials. The best way to counter this problem is first with good compaction, then to not let them get wet (by providing drainage). But as the ground beneath the slab dries over time, it will shrink and the slab will sink. That's not a big problem as long as the slab is isolated from the footings and columns and from any pipes that penetrate the slab so that it can settle a little and settle evenly. Often, with expansive clays, the best approach is a structural slab that doesn't bear on the soil at all or a post-tensioned slab that floats atop the soil but doesn't rely on it for structural support.
Post tensioning is often the best solution for a slab on poor soil. J.C. Escamilla's Concrete
Most natural soil, of course, is a mixture and so is characterized by the type of material that is predominant. The amount of weight the soil can support before it fails is its bearing capacity, typically given in pounds per square foot. The design, however, is based on the allowable soil pressure, which adds a safety factor to the ultimate bearing capacity.
Let's look at the weight the subgrade soil will typically need to support. A 6-inch-thick slab weighs about 75 pounds per square foot. According to the International Residential Code, the live load (anything that is not part of the building itself), varies from about 20 to about 60 pounds per square foot—50 pounds per square foot in a garage. That gives us 125 pounds per square foot for the soil to support. A clean sandy soil might have an allowable soil pressure as high as 2000 pounds per square foot. Even a poor soil—silt or soft clay—might have an allowable soil pressure of 400 pounds per square foot.
We can see then that the allowable soil pressure for a slab is seldom a problem. However, there is a need for uniform support because if one part of the slab settles more than another, that's when we get bending in the slab—and potentially cracks and differential settlement. Knowing which areas have been cut and which filled is important—make sure the fill areas have been well compacted. In fact, any soil that's been disturbed during excavation must be compacted.
The key for the soil support system is uniform support rather than strong support. Sure, it has to be able to support the slab, and on most ground that's not a big problem, at least across the middle of the slab, since the load is spread across so much area. Good strong support at the edges and at any joints can be a different matter—to prevent cracking and joint spalling we need to support the slab at those locations where it can behave like a cantilever and bend into the subbase. But with a good subbase that's really not a big issue either.
What happens to a concrete slab if support isn't uniform?
Concrete is very strong in compression and not so strong in tension. In a slab, tension is often created by bending. When a piece of concrete bends, it is in compression on one side and tension on the other side. A concrete slab may bend concave up (like a smile) if the subgrade has a soft spot in the middle, putting the bottom in tension. It may bend down (like a frown) at free edges or at joints, putting the top in tension. So if your entire concrete slab isn't being supported from below, by the "soil support system," it's going to bend more easily and is probably going to crack.
Why do the subgrade and subbase allow the concrete to move at all, shouldn't it be completely rigid?
The fact is that any soil or gravel base course is going to compress if the load is high enough, unless the slab is placed on solid rock. And in some ways that's good, because slabs curl and if the base can deflect a little, it can continue to provide support for the slab even when it curls. But if it doesn't provide uniform support, if the slab has to bridge over soft spots, the slab will probably crack. There doesn't even need to be much of a load on the slab--its own weight is usually enough since a slab on grade is not typically designed to even carry the dead load. And when it does crack, that crack is going to go all the way through the slab. If the under-slab support is bad enough, you can then get differential settlement across the crack that leaves a very unfortunate bump and a very unhappy owner.
After compaction the soil density may be tested with nuclear testing equipment. Bechtel
HOW DOES THE SUBGRADE/BASE AFFECT SLAB DESIGN?
We go to all this effort to get the proper soil support system and what we end up with is a single input value for the slab design. The most commonly used value is the modulus of subgrade reaction, k . This value is not directly related to bearing capacity and k does not tell the designer if there is compressible or expansive soil. What it does is indicate how stiff the subbase/subgrade is over small deflections (about 0.05 inches).
Now let's look at why we need to know how flexible the subgrade is. To start with it's important to understand that a slab on ground is designed as "plain" concrete. That means that we do not count on the reinforcing steel to carry any of the load. But wait, you say, there's steel in the slab—mesh and rebars. Yes, but that steel is only there for crack control—to hold any cracks tightly together. It normally does not extend through the joints—at joints we only want to transfer shear forces, not bending moments and certainly not lateral restraint. That's what the joint is there for in the first place, to allow lateral shrinkage in the slab.
If the subgrade settles under the middle of the slab or at the edges, the unsupported portion can lead to cracks or slab failure.
So if we aren't counting on the steel to carry any load, then the concrete has to be strong enough to carry the bending. And the support it is getting from below determines how much it will bend. As we've already discussed, concrete isn't that strong in tension, and since half of bending is tension, it's not that strong in bending. What makes it stronger in bending, though, is a thicker slab.
A poorly compacted subgrade or more load than the slab was designed to carry can result in cracking at joints. Bill Palmer
The weaker the subgrade, or the heavier the loads, then, the thicker the slab needs to be. Concrete strength also comes into play, but most slab concrete is around 3000 to 4000 psi, so it's not a major factor. The tensile strength of concrete is typically taken as 10 to 15% of the compressive strength, so only about 400 or 500 psi. Compare that to the tensile strength of Grade 60 rebar, which is 60,000 psi.
The thing to remember here is that a concrete slab is intended to be rigid, but we don't expect the base to be infinitely stiff. A slab will settle a little and that's OK from a design standpoint—again, as long as the settlement is uniform. The danger, though, is at the edges of the slab or at joints that are wide enough to let the slab on either side settle independently. At those free edges, the weight the slab can carry depends on the stiffness of the base and the flexural strength of the slab, which is mostly a function of slab thickness.
Read Preventing Concrete Cracks for more information.
HOW CAN WE IMPROVE THE SUBGRADE?
Most subgrade improvement is accomplished by compacting the soil. In extreme situations, when the soil is particularly bad or the loads high, soil stabilization can be used. In this process, portland cement, calcium chloride, or lime are mixed into the soil then it is compacted. The subgrade soil can also be excavated and mixed with gravel then compacted.
For some difficult soils, the subbase may be placed atop a layer of geogrid.
Soil compaction is the act of squeezing out as much air and moisture as possible to push the solid soil particles together—this makes the soil more dense and typically the higher the soil's density, the higher its bearing capacity. Well-compacted soils also do not allow moisture to move in and out as easily.
So, compaction accomplishes the following:
- Reduces the amount that the soil will compress (settle) when the slab is on it
- Increases the amount of weight we can put on it (bearing capacity)
- Prevents frost damage (heave) if the soil under the slab freezes
- Reduces swelling and contraction
How much a soil can be compacted is measured by a geotechnical (or soils) engineer by placing the soil in a cylinder and beating on it—seriously. The standard or modified Proctor tests (each uses different weights to compress the soil) determine the relationship between soil density and moisture and tell us the maximum reasonable soil density that can be achieved in the field.
What we are trying to determine with the Proctor test is the moisture content in the soil that will make it easiest to compact and result in the highest density—remember that density is directly related to compaction. Too little moisture and the soil is dry and doesn't compress easily; too much moisture and you can't easily squeeze the water out. To get the best compaction, the optimal moisture content will typically be in the range of 10% to 20%. So when you hear that according to the specification the soil needs to be at 95% of the maximum modified Proctor density, you will know that you need the moisture content to be about right in order to get to that level of compaction.
A soil density-moisture curve defines the optimum moisture content and the maximum density achievable in the field.
If you aren't going to get Proctor tests done, there are some simple field tests for getting a rough idea of bearing capacity and moisture content:
- For moisture content use the hand test. Squeeze a ball of soil in your hand. If it's powdery and won't hold a shape, it's too dry; if it molds into a ball then breaks into a couple of pieces when dropped, it's about right; if it leaves moisture on your hand and doesn't break when dropped, it's too wet.
- Clay that you can push your thumb a few inches into with moderate effort has a bearing strength in the range of 1000 to 2500 psf
- Loose sand that you can just barely push a #4 rebar into by hand has a bearing capacity of 1000 to 3000 psf
- Sand that you can drive a #4 rebar into about 1 foot with a 5-pound hammer has a bearing capacity over 2000 psf
Also, remember that it's not just the soil (the subgrade) that needs to be compacted. Any subbases or base courses, which will typically be granular materials, also need to be well compacted in the proper lift thicknesses.
See more on building high-quality slabs on grade .
Plate Compactor Video Time: 02:18 Proper function and use of the vibratory plate compactor tool for preparing the concrete subgrade before placing the concrete
There are two ways to compact the soil or subgrade—static force or vibratory force. Static force is simply the weight of the machine. Vibratory force uses some sort of mechanism to vibrate the soil, which reduces the friction between the soil particles, allowing them to squeeze together more easily.
The type of soil (or subgrade material) determines the type of equipment needed for compaction:
- Cohesive soils need to be sheared to get compaction, so you need a machine that has high impact force. A rammer is the best choice, or for bigger jobs, a pad-foot roller (similar to a sheepsfoot roller). Lifts for compaction of cohesive soils should be no thicker than 6 inches.
- Granular soils only need the particles to be vibrated to move them closer together. Vibrating plates or rollers are the best choice. Lifts for gravel can be as thick as 12 inches; 10 inches for sand.
For big jobs, such as highways or large slabs, big ride-on vibratory rollers, either with smooth rollers or sheepsfoot rollers, are used for compaction. Walk-behind rollers, either with padded rollers that knead the soil, or with smooth vibrating rollers, are good for medium-sized jobs. For smaller jobs, the two most common types of compaction equipment are vibratory plate compactors (either one-way or reversible) and rammers .
Here are some details on each of the types of equipment:
- Rammers , sometimes called jumping jacks, vary in weight from about 130 pounds to 185 pounds. These tools are great for compacting the soil in a footing trench or for cohesive clays in smaller areas since they deliver a high impact force (high amplitude, lower frequency). They are not good for compacting granular materials-such as base courses.
- Vibratory plates are ideal for compacting granular soils and subbases. Available in weights of 100 to 250 pounds with plate size of 1 to 1.5 feet by 2 feet. The vibration is at a lower amplitude but higher frequency than with a rammer and is balanced to cause the machine to move forward.
- Reversible vibratory plates work well on granular soils or with granular-cohesive mixes. With two eccentric weights, the vibration can be reversed to move the machine forward or backward or to stop to compress a single soft spot. For the money, these are good machines due to their versatility.
Read more on compaction requirements for concrete pavers .
PLACING THE CONCRETE
So we've finally got the subgrade compacted and the subbase and base course placed and compacted. But what happens if there is a delay at this point before the concrete is placed? If the subbase gets rained on or frozen prior to concrete placement, it can go from being ready to being too soft.
For most interior slabs, the vapor barrier should be placed on top of the subbase before placing the concrete.
The best way to know if the subbase is properly compacted and ready for the slab is by proof-rolling, which is running a heavily loaded truck (such as a fully loaded concrete truck) across the subbase immediately before placing the concrete to see if any areas sink more than others. This should be done on some sort of grid pattern and the tires should not sink into the surface more than ½ inch. If there is any rutting or pumping of water in any part of the subbase or subgrade, then that area needs more compaction or addition of granular materials—or simply to be allowed to dry out. In the worst cases, trenches or sumps can be cut and the water pumped out.
Just prior to placing the concrete, you may also want to place a moisture barrier. For interior floors, the best location is usually between the base course and the concrete. For more on this see Vapor Barriers for Concrete Slabs .
Learn more about proper subgrade preparation for industrial floors and driveways .
Last updated: July 31, 2018
Practices for Unbound Aggregate Pavement Layers (2013)
Chapter: chapter three - granular base and subbase construction practices.
Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
40 chapter three GRANULAR BASE AND SUBBASE CONSTRUCTION PRACTICES INTRODUCTION Aggregate storage, transportation, and construction practices are critical to ensuring adequate performance of constructed UAB and subbase layers under loading. Improper material handling and construction procedures often lead to aggre- gate segregation and/or degradation, ultimately resulting in a poorly compacted aggregate layer. Because unbound aggre- gate layers function primarily through interparticle load trans- mission at aggregate contact points, such poorly compacted layers may undergo excessive shear deformation, leading to pavement failure. This chapter comprises an overview of common construc- tion and material handling practices adopted by transportation agencies as far as UAB and subbase layers are concerned. Extensive review of published literature was conducted to identify different methods identified by researchers in the past as being adequate or inadequate for aggregate base and sub- base construction. A survey of U.S. state and Canadian pro- vincial agencies was conducted to gather information on the state of the practice on this topic, and an analysis of the find- ings presented to highlight areas where significant improve- ments are still needed. Moreover, applications of nonstandard or unconventional pavement types using unbound aggregate layers and related construction practices, such as the inverted pavement concept of a granular layer over a stiff layer at depth, are described in this chapter. The overall objective is to identify gaps in knowledge concerning the âeffective prac- ticesâ for UAB and subbase layer construction, along with research needs to address these gaps. IMPORTANCE OF STANDARDIZED CONSTRUCTION SPECIFICATIONS A âpocket-sizedâ handbook published by the NSSGA (1989) contains important guidelines for UAB construction. Simi- larly, different transportation agencies have adopted different guidelines to help the construction of âgood qualityâ base and subbase layers. Apart from providing the contractors with a definite set of guidelines to be followed during construction, these guidelines help the field engineers with QA of con- structed pavement layers. However, the survey of state and Canadian provincial transportation agencies conducted under the scope of this synthesis study indicated that only 37% of the responding agencies (17 of 46) currently have specific guidelines regarding the transportation and storage (stock- piling) of aggregate materials for base and subbase construc- tion. Approximately 25 agencies reported not having any such guidelines, whereas the remaining four agencies indicated the presence of generic guidelines without specific instructions. AGGREGATE STORAGE AND CONSTRUCTION PRACTICES AFFECTING CONSTRUCTED LAYER PERFORMANCE To fulfill the overall objectives of this synthesis study, it is important to first present a summary of material handling and construction practices that have been identified as adequate or inadequate as far as ensuring the construction of good qual- ity unbound aggregate pavement layers. Inadequate material handling and construction practices may lead to aggregate seg- regation and/or degradation affecting the gradation or particle size distribution of the constructed aggregate layer. The fol- lowing sections discuss different material storage and con- struction practices that may lead to the problems of aggregate segregation and degradation. Aggregate Stockpiling as a Source of Segregation The Aggregate Handbook defines aggregate segregation as the separation of one size of particles from a mass of par- ticles of different sizes, caused by the methods used to mix, transport, handle or store the aggregate in the plant under conditions favoring nonrandom distribution of the aggregate sizes (Barksdale 1991). Certain practices magnify the segre- gation problem and thus are best restricted by transportation agencies. One possible source of segregation is during the formation of conical stockpiles by dumping material using a conveyor belt. As the aggregate is transported by a conveyor belt, vibration and motion of the belt causes the fine particles to settle to the bottom of the material stream, whereas coarse particles remain at the top. These coarse particles have a higher velocity at the end of the conveyor, and are thrown a greater distance to the stockpile. In addition, the coarser par- ticles hit the front face of the stockpile with a greater momen- tum and roll down the outer edge of the pile, creating overrun (an accumulation of particles at the pileâs bottom edge or toe). Fine particles, which have settled against the surface of the conveyor belt, tend to cling to the belt and drop to the back face of the pile. The resulting stockpile is segregated, with coarse particles settled at the toes, and fine particles in the center portion of the pile.
41 âMaterial overrun,â particles (regardless of size) moving down the side of the stockpile, is another major source of seg- regation in stockpiles. As the material moves down the side of the stockpile, larger particles tend to move down to the bottom (owing to higher momentum), whereas finer materials tend to settle into the side of the pile. Such spatial distribution of aggregate particles of different sizes at different portions of the stockpile results in pronounced segregation. Figure 24 shows the spatial distribution of different aggregate particle sizes in a segregated stockpile (Nohl and Domnick 2000). Materials with a large variation in particle size usually undergo higher degrees of segregation as a result of improper stockpiling practices. Usually aggregate materials in which the ratio of the largest to the smallest particle size exceeds 2:1 are likely to experience segregation problems during stock- piling (Nohl and Domnick 2000). From in-depth investigation of aggregate stockpiling practices, Miller Warden Associates (1964) observed that flat-mixed piles formed by the use of a crane bucket was the only stockpiling method that resulted in an insignificant amount of segregation. The most commonly used truck dumping method, although economical, was found to cause significant segregation of aggregates. Majidzadeh and Brahma (1969) studied different stages in the aggregate handling process, such as (1) initial material fabrication, (2) producer stockpile, (3) truck transportation, and (4) job- site stockpile, to establish the severity of segregation problem at these different stages and also observed that the segrega- tion problem increased as the material approached the job site from the production plant. Creating stockpiles using the âwindrow conceptâ is one of the alternatives available for storage of materials where segre- gation is a likely problem. Involving the creation of âminiature stockpilesâ in layers, windrow stockpiles can be built effec- tively using a telescoping conveyor that can move laterally as well as along the direction of the conveyor to create the stockpile in layers. Although individual âminiature stockpilesâ in a windrow pile still undergo segregation, such stockpiles are said to have better âsegregation resolutionâ because the seg- regation pattern repeats itself in smaller intervals. Figure 25 shows schematics of (a) the configuration of a windrow pile formed using a telescoping conveyor and (b) segregation reso- lution in a windrow pile (Nohl and Domnick 2000). Stockpiling Practices by Different Agencies Different transportation agencies adopt different stockpiling practices to minimize aggregate segregation. Specifications are often provided to aggregate manufacturers and contractors mentioning the desired storage and stockpiling practices. For example, the New Hampshire Department of Transportation standard specifications on base courses (http://www.nh.gov/ dot/org/projectdevelopment/highwaydesign/specifications/ documents/2010_Division_300.pdf) include the following requirements for aggregate stockpiling: Stockpiles shall be constructed in layers that minimize segrega- tion. The desired optimum thickness of layers is 6 ft. (1.8 m) and in no instance shall the layer be more than 10 ft. (3 m). Each layer shall be completed before the next layer is started. Construction of stockpiles by direct use of a fixed conveyor belt system or by dumping over a bank will not be permitted. Similarly, stockpiling practices recommended by the Ala- bama Department of Transportation (http://www.dot.state. al.us/mtweb/Testing/testing_manual/doc/pro/ALDOT175. pdf) include â¢ Stockpiles need to be placed on firm, well-drained ground that is free of any material that could cause contamination. â¢ Stockpiles should be built in layers of uniform thick- ness and not in cone-shaped piles that result in segrega- tion of piles. â¢ After the first layer of the stockpile is placed, it is impor- tant that heavy transporting equipment not be allowed to run on top of this layer because this tends to degrade the aggregate by grinding the particles together, also con- taminating the aggregate with mud and other deleterious substances from the wheels or tracks of the vehicle. â¢ If the stockpile is to be constructed in more than one layer in height, the aggregate should be dumped in a small pile at the base of the stockpile and then moved over to the stockpiled layer in place by a crane equipped with a clam- shell, front-end loader or bulldozer equipped with large pneumatic tires. The standard operating procedures recommended by the GDOT (http://www.dot.ga.gov/doingbusiness/Materials/ Documents/StudyGuide9_22_04.pdf) provide graphical rep- resentations of the prohibited and recommended practices as far as aggregate stockpiling and aggregate sampling from different types of stockpiles are concerned (see Figure 26). Construction Practices as a Source of Segregation Different construction practices can contribute significantly to aggregate segregation and therefore should be controlled through the agency specifications. White et al. (2004) observed that aggregate trimming operations are likely to contribute the most to the segregation problem as they shake the aggregate, causing fine particles to migrate to the bottom of the layer. Sub- sequent removal of the top aggregate by the trimmer leaves the fine aggregate behind, resulting in uneven spatial distribution FIGURE 24 Spatial distribution of particle gradation in a stockpile (modified from Nohl and Domnick 2000).
FIGURE 25 (a) Windrow configuration and (b) segregation resolution in a windrow pile (Nohl and Domnick 2000). (a) (b) FIGURE 26 Example specifications regarding aggregate stockpiling (Georgia DOT).
43 of aggregate particle sizes in the constructed layer. They also observed that low moisture in the aggregate mostly corre- sponded to increased segregation as a result of poor adhesion between finer and larger particles. Through in situ testing of full-scale unbound aggregate test sections, they suggested changes to construction operations to limit spatial variations in constructed layer properties. These changes include (1) limiting movement of aggregate by pri- marily transporting aggregate transversely, rather than longi- tudinally, and (2) moistening the aggregate before trimming to reduce fines migration. Williamson and Yoder (1967) studied the achieved compaction levels in different rigid pavement subbase layer constructions in the state of Indiana and con- cluded that the lack of compaction could be attributed to non- uniform aggregate gradations in the constructed layer, which is an indicator of segregation during the construction process. Aggregate Degradation and Possible Sources Aggregate degradation is defined as the breakdown of an aggregate into smaller particles (Barksdale 1991). Aggregate degradation can occur during the process of aggregate stock- piling or during the placement and compaction of aggregate base and subbase layers. Formation of stockpiles by pushing of aggregates using dozers and handling of the aggregates during different stages of construction both may result in degradation of the larger particles into smaller fractions. Although the prob- lem of degradation is not as severe for quarries excavating hard rock formations, the problem can be significant for operations dealing with âsofterâ parent rocks. Some quarries compensate for the potential degradation by producing aggregate sizes that are coarser than the target aggregate size. Moreover, different agencies impose different restrictions on the type of construc- tion vehicle allowed to operate on aggregate stockpiles. Compaction of constructed layers may impose heavy loads that cause aggregate degradation. Aughenbaugh et al. (1963) indicated that degradation is dominant in the top lift of an aggregate layer. Thus, the height of a layer during compaction may contribute toward nonuniform aggregate gradation result- ing from degradation of individual particles. This is particularly critical for aggregate layers constructed with large lift thick- nesses. To ensure adequate compaction at greater depths, high compactive energies need to be imparted on the layer surface. Such high compactive energies result in significant crushing of aggregate particles near the layer surface, changing the gra- dation and thus achieved density. Density-based compaction control techniques may give erroneous indications of layer compaction in such cases because density measuring devices, such as the nuclear density gauge, can measure the compac- tion level for only the upper few inches (typically 12 in. for a nuclear density gauge) and do not check the compaction levels for deeper sections in the layer. Thus, it is important to control the amount of energy imparted to the aggregate layer during the compaction process through adjusting the amplitude and frequency of impacts applied by vibratory rollers. CONSTRUCTION LIFT THICKNESS AND ITS EFFECT ON COMPACTABILITY Background One of the primary factors affecting the performance of UAB and subbase layers is the DOC. Aggregate materials received from the source are placed on the prepared subgrade and com- pacted to the design layer thicknesses. Generally, upon compac- tion an unbound aggregate layer loses approximately one-third of its loose placement depth (Barksdale 1991; Saunders 1997). Specifications require the compaction to be carried out imme- diately after placement of the aggregate material while the gradation and moisture content are still at the specified values (NSSGA 1989). Maximum allowable lift thicknesses are usu- ally specified during the construction of unbound aggregate layers to ensure adequate compaction, which is critical to pavement layer performance under loading. Saunders (1997) reports that the maximum lift thicknesses specified by agen- cies most likely were established in the early days of highway construction, when only static rollers or limited vibratory roll- ers were available. Saunders also indicates that in view of the modern construction equipment, these maximum lift thickness thresholds most likely are on the conservative side. Wells and Adams (1997) successfully constructed aggregate base courses in single-lift depths of 10 and 12 in., which were greater than the 8-in. maximum allowed by North Carolina Department of Transportation (NCDOT) specifications. Similarly, Womack (1997) reports successful construction of aggregate base courses in Virginia with 10-in. thick lifts, which were greater than the maximum construction lift thicknesses specified by the Virginia Department of Transportation (VDOT) at that time. Researchers have since focused on evaluating the effective- ness of layer compaction when aggregate layers are constructed with large lift thicknesses. Bueno et al. (1998) constructed test pads in Texas and Georgia using crushed limestone and crushed granite, respectively, with lift thicknesses ranging from 6 to 21 in. Three test strips were constructed in Georgia with differ- ent lift thicknesses. A âtarget test stripâ was compacted in two lifts: one 178-mm (7-in.) compacted lift under a 152-mm (6-in.) compacted lift. Two other test sections (sections 1 and 2) were both compacted in one single lift to a final compacted thick- ness of 330 mm (13 in.). In-place density measurements indi- cated compaction levels of 102.5%, 103.9%, and 103.3% for the target Strip, Test Section 1, and Test Section 2, respectively, Key Lessons â¢ Aggregate segregation and deterioration can be minimized through proper stockpiling and construc- tion practices. â¢ Stockpiling of aggregates using the windrow concept has been proven to be the most efficient practice as far as minimizing segregation is concerned.
44 indicating that adequate compaction levels could be achieved even for higher construction lift thicknesses. Constructing test pads in Texas, Bueno et al. (1998 and 1999) observed that higher densities could sometimes be achieved for 457-mm (18-in.) and 584-mm (23-in.) lifts compared with those achieved for 305-mm (12-in.) lifts. In general, dry densities were found to increase with depth into the compacted base, illustrating that the lower parts of the aggregate base course were being compacted even for greater lift thicknesses. This trend was supported by the shear wave velocity profiles obtained from Spectral analysis of surface waves (SASW) testing. From SASW data, they observed that stiffness of a graded aggregate base (GAB) course was sensi- tive to moisture variations and concluded this sensitivity likely was caused by changes of effective confining stress, which occur when the material is wetted or dried. Overall, findings from this study clearly indicate that thicker single lifts could be compacted equally well or sometimes better than are the thinner aggregate layers commonly constructed by agencies. Allen et al. (1998) conducted a survey of all state trans- portation agencies and found that 12 of 36 responding states allowed a maximum lift thickness of 6 in. or less, one state allowed 7-in. lifts, and 16 states allowed 8-in. lifts. Only three states allowed thicker lifts (Washington, 9-in.; North Carolina, 10-in.; and Maine, 12-in.). The survey of state and Canadian provincial agencies conducted under the scope of the cur- rent synthesis study found that 11 of 46 responding agencies allowed construction lift thicknesses of 8 in. The current lim- its for construction lift thickness in North Carolina and Maine were the same as those reported by Allen et al. (1998). Note that as of 2012, 17 agencies still restricted the maximum lift thick- ness to 6 in. Figure 27 summarizes all the data collected from the survey respondent states and Canadian provincial agencies. From the figure it is evident that despite several research and trial studies demonstrating the effectiveness of aggregate layer construction with larger lift thicknesses, the current practices in state and Canadian provincial transportation agencies still use a conservative approach in this regard. Thus, more research and demonstration projects need to focus on the advantages and disadvantages (if any) of higher construction lift thicknesses. Such studies will also help harmonize the construction prac- tices throughout the United States and Canada. From successful implementation of thick single-lift aggre- gate base and subbase layer construction, Allen et al. (1998) recommended the following changes to unbound aggregate layer construction practices: â¢ Equipment: Mixing is to be accomplished by stationary plant, such as a pugmill, or by road mixing using a pug- mill or rotary mixer. Mechanical spreaders should be used to avoid segregation and achieve grade control. Suitable vibratory compaction equipment should be employed. â¢ Mixing and Transporting: The aggregates and water should be plant mixed (stationary or roadway) to the range of optimum moisture plus 1% or minus 2% and transported to the job site so as to avoid segregation and loss of moisture. â¢ Spreading: The material is to be placed at the specified moisture content to the required thickness and cross sec- tion by an approved mechanical spreader. At the engi- neerâs discretion, the contractor may choose to construct a 500-ft long test section to demonstrate achieving ade- quate compaction without particle degradation for lift thicknesses in excess of 13 in. The engineer may allow thicker lifts on the basis of the test section results. Optimum Construction Lift Thickness As observed from the survey results, no consensus exists among transportation agencies with regard to the maximum allowed construction lift thickness for UAB/subbase layers. From exten- sive review of literature conducted under the scope of this syn- thesis study, it was observed that most research studies and trial implementation projects could successfully compact 12-in. thick aggregate layers while achieving desired compaction levels. Thus, it is suggested that 12-in. aggregate lifts be standardized as âoptimum construction practiceâ for UAB/subbase layers. Given adequate support conditions, construction of unbound aggregate layers in such thick lifts could sufficiently expedite 37% 24% 9% 13% 15% 2% (17) (11) (4) (6) (6) (1) 0 10 20 30 40 0% 20% 40% 60% 80% 100% 6 in. 8 in. 10 in. 12 in. Other (please indicate) No such restrictions Number of Responses Percentage of Respondents 46 survey respondents FIGURE 27 Maximum construction lift thickness allowed for unbound aggregate layers.
45 the construction process while ensuring adequate compaction levels. For aggregate layers being constructed over âfirmâ pre- pared subgrades (often represented by subgrade CBR > 8%), the compaction of 12-in. thick layers may be possible. However, reduced lift thicknesses may need to be adopted for âweakerâ subgrade support conditions (subgrade CBR < 8%). DOCUMENTED AGGREGATE BASE AND SUBBASE LAYER CONSTRUCTION PRACTICES Figures 28 to 30 summarize agency responses to various cur- rently adopted aggregate base and subbase layer construction practices. Figure 28 highlights that 52.2% of the respon- dent agencies allow construction of two functionally differ- ent aggregate layers on top of each other, such as an OGDL underlain by a dense-graded aggregate subbase, in pavements. Furthermore, 66.7% of the respondent agencies do not sepa- rate two unbound aggregate layers by any kind of constructed aggregate separation or filter layers (see Figure 29). Figure 30 indicates that 65.2% of the respondent agencies allow construc- tion of unbound aggregate layers directly over or under pave- ment layers stabilized or treated with lime, fly ash, cement, or bitumen. Note that these findings have implications on some of the domestic and foreign innovative pavement construction practices, such as the construction of âinverted pavements.â INVERTED PAVEMENTS Sustainable application of unbound aggregate structural lay- ers in pavements would improve the designs of low, medium, and moderately high volume roads while reducing the depen- dence on asphalt (and thus crude oil) for pavement construc- tion. Such an alternative is offered by an inverted pavement section that consists of an unstabilized crushed stone base or GAB sandwiched between a lower cement-stabilized layer and a thin upper asphalt concrete surfacing. Conceptual Background Conventional pavement systems rely on a combination of asphalt concrete or PCC and aggregate base components to transfer load to the subgrade. All three components use Key Lessons â¢ No common practice exists among transportation agencies as far as the maximum construction lift thick- ness of UAB/subbase layers is concerned. â¢ Maximum construction lift thickness value for UAB/ subbase layers are best based on project âtest-stripâ sections using the specific materials and equipment. â¢ From extensive review of literature and state prac- tices, this synthesis study suggests an optimum construction lift thickness of 12 in. for UAB/subbase layers. Note that this suggestion is based on the assumption that the UAB/subbase layer to be con- structed is at least 12-in. thick. Moreover, the DOC achieved is contingent upon the use of adequate equipment by the contractor. 52% (24) 26% 22% (12) (10) 0 10 20 30 40 0% 20% 40% 60% 80% 100% Yes No Other Number of Responses Percentage of Respondents 46 survey respondents FIGURE 28 Agency responses to whether multiple unbound aggregate layers are allowed to be placed on top of each other. 17% (4) 67% (16) 17% (4) 0 10 20 0% 20% 40% 60% 80% 100% Yes No Other Number of Responses Percentage of Respondents 24 survey respondents FIGURE 29 Agency responses to whether the two unbound aggregate layers are separated by any kind of constructed aggregate separation/filter layers.
46 aggregates as their primary constituent. Classic pavement design places higher modulus, more durable layers toward the surface. Inverted pavement is a composite system composed of asphalt top layer(s) and a well-compacted unbound crushed stone base layer over a stiffer bound subbase that is usually cement treated. Mechanistically, this configuration provides a stronger reaction platform than unbound subgrades or sub- bases, allowing increased granular base compaction during construction, and it also has the potential to take advantage of the compressive stresses induced in the granular aggregate base owing to the presence of the stiff underlying layer. This pavement design philosophy potentially offers economic advantages by requiring less asphalt concrete and placing the burden of strength and structural performance on relatively less expensive underlying layers. Inverted pavements were first introduced in South Africa and involved the construction of thick crushed stone base lay- ers over stabilized subbase layers. The superior performing crushed stone base layers used in South African inverted pave- ments are also known as âG1â base layers (Horne et al. 1997; Jooste and Sampson 2005; De Beer 2012). These pavements are also called stone interlayer pavements, G1-base pavements, inverted base pavements, sandwich pavements, and upside down pavements (Lewis et al. 2012). Figure 31 shows the layer configuration of a typical inverted pavement structure. As shown in Figure 31, the HMA layers in inverted pave- ment sections often are very thin, so their contribution to the structural capacity of the pavements often is not significant. Primarily, these surface layers provide a smooth ride qual- ity and protect the underlying pavement layers from water infiltration. The unbound aggregate layer is the primary load- bearing layer in inverted pavement structures. Summarizing the construction practices and layer configurations of these pavement systems, De Beer (2012) presented the following definition for inverted pavements: A structural pavement system, where the static modulus of the unbound base layer is lower compared with the supporting (mainly lightly cementitious) subbase layers. Unbound base layer (crushed rock) of extremely high bearing capacity is usually covered with 12 mm to 50 mm asphalt layer for sealing and functional properties. Owing to the reduced thickness of the HMA surface layers, these pavement systems are cost-effective alternatives for high- performance pavement structures. The primary advantages of inverted pavements include (1) better compaction of unsta- bilized materials placed over the stabilized layers; (2) opti- mum use of unstabilized crushed stone; and (3) elimination or significant reduction in reflective cracking in the pavement structures (Barksdale and Todres 1983). Response Mechanism The UAB is primarily a structural load-carrying component in inverted pavement sections. When properly compacted, the UAB causes lateral dissipation of traffic-induced stresses through interparticle contact points. The stiff UAB and the cement-stabilized subbase combined result in a significant reduction in the vertical compressive stress levels on top of the subgrade, thus eliminating chances of pavement fail- ure because of subgrade rutting. However, the UAB in an inverted pavement structure is subjected to considerably higher stresses to make the base layer prone to rutting, a potential failure mechanism for inverted pavement sections. Thus, construction procedures for inverted pavements aim to eliminate rut accumulation within the UAB layers through innovative compaction procedures. The thin HMA surface typically considered in an inverted pavement section induces considerably high stress states within the aggregate base under wheel loading. Owing to the stress-hardening nature of unbound aggregates, these high stress states often lead to the aggregate layer developing high elastic modulus values, often on the order of 689 MPa 65% (30) 22% (10) 13% (6) 0 10 20 30 40 0% 20% 40% 60% 80% 100% Yes No Other Number of Responses Percentage of Respondents 46 survey respondents FIGURE 30 Agency responses to whether the construction of unbound aggregate layers is allowed over or under pavement layers stabilized or treated with lime, fly ash, cement, or bitumen. FIGURE 31 Layer configuration of a typical inverted pavement section.
47 or 100 ksi (Maree et al. 1982a, 1982b; OâNeil et al. 1992). Such high modulus levels achieved in the aggregate bases of inverted pavement sections would help better dissipate the traffic-induced stresses with depth. Moreover, the pres- ence of the stiffer subbase layer in an inverted pavement section causes the neutral axis in bending to fall below the aggregate base layer. This results in the surface layers and the UAB layers performing mainly under compres- sion. Accordingly, the stiffness profile in inverted pave- ment structures prevents the development of tensile stresses in the UAB even if a linear model is used to represent it (Cortes 2010). The stiff aggregate base layer also leads to a reduction in the tensile stresses at the aggregate base course-HMA surface interface, thus significantly reducing the chances for reflective cracking occurring in these pave- ment structures. Material Specifications and Construction Procedure Material Specifications The aggregate material to be used in the base course of an inverted pavement structure is obtained from crushing of hard, sound, durable, and unweathered parent rock. All the faces of the aggregate particles are required to be fractured. South African G1 base specifications allow the material gra- dation to be adjusted only through the addition of fines pro- duced from the crushing of the original parent rock. Table 3 lists the (a) particle size distribution and (b) other material quality specifications used in South Africa for use in G1 base course applications (TRH 1985; Buchanan 2010). Note that the gradation requirements listed in Table 3a are based on restricting the ânâ values in the Fullerâs or Talbotâs equation (as defined in Equation 1) between 0.33 and 0.50. Note that in Equation 1, P is the percentage (%) of material by weight finer than the sieve size being con- sidered; d is the sieve size being considered; D is the maxi- mum aggregate particle size in the current matrix; and n is a parameter that adjusts the gradation curve for fineness or coarseness. P dD n( )= Ã 100 (1) Construction Procedure Compaction of the UAB layer in an inverted pavement struc- ture is the most critical step during its construction to ensure that individual layers perform as desired. The UAB is con- structed on top of a stabilized subbase, which provides a solid construction platform for the placement and compac- tion of the UAB layer and ensures that adequate density lev- els can be achieved. The DOC achieved in the UAB layer is dependent on the energy applied, as well as the initial and final gradations of the aggregate material used. (a) Sieve Size (mm) Sieve Size (in.) Percent Passing G1, 37.5 NMS G1, 26.5 NMS 50 2.0 37.5 1.5 100 26.5 ~1.0 84â94 100 19.0 Â¾ 71â84 85â95 13.2 ~1/2 59â75 74â84 4.75 #4 36â53 42â60 2.00 #10 23â40 27â45 0.425 #40 11â24 13â27 0.075 #200 4â12 5â12 (b) Sources: TRH 1985; Buchanan 2010. Aggregate Material Property Specified Threshold Values Minimum 10% FACTa 110 Maximum aggregate crushing valueb 29% Liquid limit <25 Linear shrinkagec <2% Plasticity index (PI) <4 a 10% FACT (fines aggregate crushing value) is the force in kilonewtons required to crush a sample of aggregate passing the 13.2-mm and retained on the 9.5-mm sieve so that 10% of the total test sample will pass a 2.36-mm sieve. b The aggregate crushing value (ACV) of an aggregate is the mass of material, expressed as a percentage of the test sample that is crushed finer than a 2.36-mm sieve when a sample of aggregate passing the 13.2-mm and retained on the 9.50-mm sieve is subjected to crushing under a gradually applied compressive load of 400 kN. c The linear shrinkage of a soil for the moisture content equivalent to the liquid limit is the decrease in one dimension, expressed as a percentage of the original dimension of the soil mass, when the moisture content is reduced from the liquid limit to an oven-dry state. TABLE 3 RECOMMENDED PARTICLE SIzE DISTRIBUTION RANGE FOR SOUTH AFRICAN G1 BASE
48 The compaction of unbound aggregate layers in the South African inverted pavement structures involves the following two phases: standard compaction phase and particle inter- locking or slushing phase. The standard compaction phase is carried out using a combination of grid rollers, vibratory roll- ers, and pneumatic tire rollers. Commonly two to three passes of the grid roller are used to gently knead the aggregate layer into shape. Subsequently, the vibratory roller is used to com- pact the layer to 85% of apparent solid density. It is important to note that the amplitude and frequency of vibration need to be strictly monitored during this phase because too much vibration can easily lead to âde-densificationâ of the aggre- gate matrix. Moreover, extreme care is to be exercised to pre- vent the breakage of individual aggregate particles under the vibratory roller. The aggregate moisture content usually is maintained near the âoptimumâ conditions during this phase of compaction to aid the rearrangement of individual aggre- gate particles into a densely packed matrix. The fines fraction in the aggregate matrix plays a critical role during this phase by lubricating the aggregate contact points. Thus, it is impor- tant that the aggregate material used in the construction of these superior performing base layers contain the adequate amount of fines. A rule of thumb used in the construction of South African G1 base course layers is that OMC values less than 4% are indicative of too few fines in the aggregate matrix, whereas OMC values higher than 6% are indicative of too many fines (De Beer 2012). The second phase of compaction involves consolidating the material under saturated conditions by expelling or âslush- ingâ out the excess fines material from the matrix, allowing the larger particles to interlock into a âsuperdenseâ matrix. The fines serve as lubricants to ensure reorientation and interlocking of the larger particles into a superdense matrix. This âwashing outâ of the fines, accompanied by compac- tion, is continued until the water draining from the pavement becomes colorless and does not contain any trace of excess fines (De Beer 2012). A pneumatic tire roller passing over the aggregate layer without leaving any indentations is used as an indicator of the achievement of adequate compaction. South African specifications require achieved density to be greater than 88% of solid particle density (assuming solid rock for the equal volume with no voids). It is usual to place the prime coat and HMA surfacing layer immediately after compaction of the UAB layer. This is primarily because the aggregate base layer is noncohe- sive in nature, and the aggregate matrix may get disturbed upon exposure to direct application of traffic loads and weathering. Previous Findings on the Benefits of Inverted Pavements The first application of inverted pavements in the United States can be traced back to 1954 in New Mexico (Barksdale and Todres 1983). These initial inverted pavement sections involved the overlaying of several badly broken concrete pavements with 152 mm (6 in.) of unstabilized granular base, and 51 mm (2 in.) of asphalt concrete. Johnson (1960) reported that after six years of heavy traffic, no reflection cracking or significant rutting had developed in the test sections. Subsequently, two experimental roads were constructed in New Mexico in about 1960, consisting of a 76-mm (3-in.) asphalt concrete surfacing, 152-mm (6-in.) granular base, and a 152-mm (6-in.) granular subbase treated with 4% cement. U.S. Army Corps of Engineers Experience The U.S. Corps of Engineers studied the behavior of the various layers in flexible pavement structures having lime- stabilized and cement-stabilized subbases: that is, inverted base type structures (Ahlvin et al. 1971; Barker et al. 1973; Grau 1973). The objective of the study was to measure the mechanical response of full-scale pavement structures and compare the results against predictions from layered elastic theory and other available constitutive models. Two inverted base pavement structures were investigated, both composed of a 90-mm asphalt concrete layer, a 150-mm crushed lime- stone base, a 380-mm stabilized clay subbase, and a clay sub- grade (CBR of 4%). The structures were subjected to traffic under controlled conditions while monitoring displacements and stresses at key locations (Ahlvin et al. 1971; Barker et al. 1973; Grau 1973). Linear elastic analyses failed to adequately predict the measured stresses and strains in different layers and the plastic subgrade deformation. The performance of the inverted pavement structures was found to be influenced by the stiffness and tensile strength of the cement-treated base. This study highlighted the importance of a compre- hensive material characterization and numerical implementa- tion through appropriate constitutive models. Furthermore, it urged the development of laboratory tests capable of simulat- ing field conditions and the introduction of nonlinear models in numerical simulations (Barker et al. 1973). Barksdale and Todres Barksdale and Todres (1983) constructed 12 laboratory-scale instrumented pavement structures and cyclically loaded them to failure under controlled environmental conditions. Among conventional flexible pavement and full-depth asphalt pave- ment test sections, they also tested two inverted pavement test sections made of 89-mm thick asphalt concrete layers over 203-mm thick unbound aggregate layers (well-graded gra- nitic gneiss), over a 150-mm thick cement-stabilized subbase, over a micaceous nonplastic silty sand subgrade. One inverted pavement section had a 152-mm (6-in.) thick cement stabilized crushed stone subbase; the other had a 152-mm (6-in.) thick cement-treated, silty sand subbase. It was found that the cement- treated base facilitated compaction in inverted structures lead- ing to denser unbound aggregate layers ( Barksdale 1984).
49 The pavement sections were subjected to a 28.9-kN cyclic load for the first 2 Ã 106 repetitions, followed by cyclic appli- cation of a 33.4-kN load until failure. Monitoring the per- formance of the test sections under loading, Barksdale and Todres observed that the two inverted pavement sections outperformed equivalent pavement structures in terms of lower resilient surface displacements, reduced transferred compressive stress onto the subgrade, and less tensile radial strain at the bottom of the asphalt concrete layer (Barksdale and Todres 1983; Avellandeda 2010). The superior mechani- cal performance of the inverted pavement structures was clearly reflected from the significantly higher number of load cycles to failure (3.6 Ã 106 and 4.4 Ã 106) compared with the best performing conventional flexible pavement section (Barksdale 1984; Tutumluer and Barksdale 1995). Tutumluer and Barksdale Tutumluer and Barksdale (1995) conducted numerical mod- eling of the two full-scale instrumented inverted pavement sections tested by Barksdale and Todres (1983), and made the following observations: â¢ Cement-stabilized inverted sections can successfully withstand large numbers of heavy loadings through â Lower vertical subgrade stresses owing to the âbeam actionâ of the stiff base layer; â Lower tensile strain at the bottom of the asphalt layer; and â Lower resilient surface deflections. â¢ The upper portion of the cement-treated subbase and almost all of the unstabilized crushed stone base near the load were in horizontal compression. The bottom half of the subbase and a thin layer on top of the sub- grade were in horizontal tension. â¢ Presence of the cement-stabilized layer beneath the aggregate base resulted in horizontal compressive stresses of magnitudes ranging from 0 to 110 kPa (0 to 16 psi) in the unstabilized crushed stone base. This was probably a major factor contributing to the lower permanent deformation and higher resilient moduli of these base layers as observed from laboratory testing. â¢ From sensitivity analyses conducted using the GT- PAVE finite element (FE) program, they observed that the optimum and economical inverted pavement sec- tion constructed over a weak subgrade would consist of an unstabilized aggregate base 152 mm (6 in.) thick and a 152-mm to 203-mm (6-in. to 8-in.) thick cement- stabilized subbase. Lafarge Quarry Access Road âMorgan County, Georgia Two 122-m (400-ft.) long inverted pavement test sections were constructed on a new access road at the Lafarge Build- ing Materials quarry near Madison, Georgia, in 2001. Both test sections had a 200-mm (8-in.) thick cement-treated base layer, a 150-mm (6-in.) thick GAB layer, and a 75-mm (3-in.) thick HMA layer. The only difference between the two inverted pavement sections was in the construction of the GAB layer: the first section was constructed using the South African âslushingâ technique, whereas the second section was constructed using conventional construction methods. Terrell et al. (2003a, b) conducted miniaturized versions of traditional cross-hole and downhole seismic tests to deter- mine the stiffnesses of each base layer. Horizontally propagat- ing compression and shear waves were measured under four different loading conditions to determine Youngâs moduli and Poissonâs ratios of the base. An increase in stiffness with an increase in load was measured. In addition, it was found that the Georgia and South Africa sections had similar stiffnesses. Surprisingly, the traditional section was found to be some- what stiffer than the other sections. This higher stiffness was thought to be caused by a prolonged period of compaction before construction of the UAB layer, which essentially trans- forms the traditional section (Terrell et al. 2003b). Comparing the performances of the two inverted pave- ment test sections with a conventional flexible pavement sec- tion subjected to the same loading, Lewis et al. (2012) made the following observations: â¢ The two test sections performed remarkably well for more than 10 years, without needing any maintenance or resurfacing; â¢ No significant rut accumulation was observed in the inverted pavement test sections, whereas the conven- tional pavement section exhibited both âminorâ and âmajorâ rutting problems; â¢ FWD testing conducted in 2007 indicated that the two inverted pavement sections had remaining service lives of 99.34% (conventional compaction) and 94.61% (compacted using the South African slushing method), respectively, whereas the conventional pavement sec- tion had a remaining service life of 67.92%. FHWA International Scanning Tour A scanning study of France, South Africa, and Australia spon- sored by FHWA, AASHTO, and NCHRP investigated innova- tive programs for pavement preservation (Beatty et al. 2002). During the scanning tour, the team observed typical pave- ment structures used by the countries visited to ensure longer- lasting, better-performing pavement systems. Figures 32 and 33 show the typical pavement structures constructed by Australia and South Africa, respectively, as noted by Beatty et al. (2002). As can be seen from the figures, it is common practice in Australia and South Africa to use thick aggregate layers in conjunction with relatively thin HMA surface lay- ers. The practice in Australia involves the use of multiple
50 unbound aggregate layers in conjunction with a thin HMA surface layer, whereas the South African practice involves the construction of inverted pavement sections. Application of Stone Interlayer Pavements in Louisiana Stone interlayer pavement designs were introduced in Loui- siana to reduce the problem of reflective cracking that is often observed in flexible pavements constructed using soil- cement bases (Rasoulian et al. 2000, 2001). Titi et al. (2003) compared the performances of stone interlayer pavements (3.5-in. HMA surface layer; 4-in. crushed limestone inter- layer; 6-in. in-place cement-stabilized base course layer; and 12-in. lime-treated subgrade layer) with conventional flexible pavements with cement-treated bases (3.5-in. HMA surface layer, 8.5-in. in-place cement stabilized base course layer, and 12-in. lime-treated subgrade layer) constructed on State Highway LA-97 near Jennings, Louisiana. Both pavements were monitored for more than 10 years and were evaluated through pavement distress surveys, testing for roughness and permanent deformation, as well as evaluation of pavement structural capacity through dynamic nondestructive testing (NDT). The same two designs were also compared through accelerated pavement testing at the Louisiana Transportation Research Center. Through analyses of the field monitoring and accelerated testing data, Titi et al. (2003) reported that the stone interlayer pavements performed significantly bet- ter than did the conventional pavement designs with cement- treated base. From comparing the performances of the two pavement types, Titi et al. (2003) made the following pri- mary observations: â¢ Both pavement types showed an increasing trend in crack accumulation with time. However, the rate of crack accu- mulation was significantly lower for the stone interlayer pavement sections. â¢ The average International Roughness Index value for the stone interlayer pavement was lower than that for the con- ventional flexible pavement after 10.2 years. This indi- cated smooth surface conditions and better ride quality for the stone interlayer pavement. This was attributed to the lower amount of reflective cracking in the stone inter- layer pavement. â¢ The stone interlayer pavement could withstand about four times the number of load applications (1,294,800 ESALs) under accelerated testing compared with the conven- tional flexible pavement section (314,500 ESALs) before undergoing failure. â¢ From survival analyses of the two accelerated pave- ment sections, Metcalf et al. (1998) concluded that the dominant mode of failure (88%) for the stone interlayer pavement was rutting, whereas that for the conven- tional pavement was cracking. â¢ Through regression analyses of the long-term perfor- mance data of the two test sections along LA-97, it was concluded that the only mode that could lead to the fail- ure of both of the modes was cracking. The regression analyses clearly established the superior performance of stone interlayer pavement sections. FIGURE 32 Typical heavy-duty pavement configuration in Australia (Beatty et al. 2002). FIGURE 33 Typical pavement sections in South Africa constructed with high-quality crushed aggregate base layers (Beatty et al. 2002).
51 â¢ The initial material cost for the stone interlay pavement was approximately 20% higher than that for the conven- tional pavement. However, considering the significantly higher number (300% higher) of load applications until failure, the stone interlayer pavement alternative indi- cated considerable savings when life-cycle costs were analyzed. LaGrange Bypass Project, Troup County, Georgia Encouraged by the positive results from the inverted pavement test sections in Morgan County, Georgia, in 2009 GDOT con- structed another inverted pavement test section on the South LaGrange Loop in Troup County. The constructed inverted pavement test sections had (1) 150-mm (6-in.) thick stabilized subgrade; (2) 250-mm (10-in.) thick cement-treated base; (3) 150-mm (6-in.) thick GAB; (4) 50-mm (2-in.) thick Super- pave binder course; and (5) 37-mm (1.5-in.) thick Superpave surface course (Lewis et al. 2012). The GAB was constructed using standard construction techniques at a moisture content of 100% to 120% of the OMC. Figure 34 shows a schematic of the inverted pavement sections constructed as part of this project. Buchanan (2010) compared the life-cycle costs for the LaGrange Bypass inverted pavement sections with a rigid pavement designed to carry the same amount of traffic (the rigid pavement had a 9.5-in. thick PCC slab over a 10-in. thick GAB over a 6-in. thick prepared subgrade with a minimum soil support value of 5). Table 4 lists the comparative cost estimates over a 30-year life cycle as presented by Buchanan (2010). As can be seen from the table, the inverted pavement section results in net savings of $139,000 over a 30-year period. Avellandeda (2010) developed new field test methods to characterize the stress-dependent stiffness of UAB layers in these inverted pavement test sections and found that inverted pavement sections could exceed the structural capacities of flexible pavement designs and result in savings to 40% of the initial construction costs. Lewis et al. (2012) reported that the test sections showed excellent structural capacities and long remaining lives upon FWD testing immediately after construction. Cortes (2010) conducted precompaction and postcompaction sieve analyses of aggregate samples collected from the GAB and reported inconclusive data about the extent of particle crushing. By digging trenches through the HMA layers to expose the GAB and subsequently processing the grain skeleton photographs through digital image analysis, Cortes found evidence of compaction-induced anisotropy in the GAB as the coarse aggregate particles were found to preferentially align their major axis parallel to the horizontal plane. Through FE analy- ses of the test sections, Cortes observed that both vertical and radial stresses in the UAB layer remained in compression throughout the layer depth. Luck Stone Bull Run Project, Virginia An application in 2010 of inverted pavement in Virginia involved a relocated road (Virginia Highway 659) bypassing the Luck Stone Bull Run Quarry. The project included collab- oration between Luck Stone, Texas A&M University (ICAR), FHWA Office of Infrastructure R&D, Virginia DOT, and the Virginia Transportation Research Council. This section was designed using the ICAR model, and the materials charac- terization protocol was carried out at Texas A&M Univer- sity and the Texas Transportation Institute and instrumented heavily through FHWA sponsorship. Discussing the benefits of this inverted pavement trial application, Weingart (2012) reported a potential for 22.3% cost savings compared with the construction of a conventional flexible pavement with equiv- alent structural and functional capacities; the estimated cost for construction of the conventional flexible pavement section was $21,311 per 100 linear ft, whereas that for the inverted pavement section was $16,555 per 100 linear ft. FIGURE 34 Schematic of inverted pavement section constructed in LaGrange Bypass Project, Troup County, Georgia. Event Cost ($/Lane-Mile) Inverted Pavement PCC Pavement Installation cost 342,000 584,000 10 years of maintenance 101,000 20 years of maintenance 123,000 20â30 years of maintenance 121,000 30-year life-cycle cost 566,000 705,000 Net savings 139,000 TABLE 4 LIFE-CYCLE COST COMPARISON FOR LAGRANGE BYPASS INVERTED PAVEMENT SECTION WITH A RIGID PAVEMENT SECTION DESIGNED TO SUSTAIN THE SAME TRAFFIC LEVEL OVER A 30-YEAR PERIOD
52 Summary of Past Experience on Inverted Pavements From extensive review of literature covering inverted pave- ment applications internationally as well as within the United States, it was observed that almost all applications of inverted pavements have resulted in favorable performance compared with conventional pavement structures. In addi- tion to resulting in superior performance, inverted pavement sections often have led to significant cost savings over the life cycle of the pavement. Although the construction of pave- ments using thick unbound aggregate layers appears to be a common practice in countries such as Australia and South Africa, projects involving such pavements have been con- fined primarily to trial studies in the United States. Moreover, these trial projects have been confined to a limited number of states, with most other states showing resistance to the adop- tion of such innovative pavement construction practices. Possible explanations of why inverted pavements have not been constructed in the United States include (1) traditional pavement designs and construction practices using rather thick asphalt or concrete surface courses were still afford- able; (2) details of foreign technology related to UAB com- paction, such as the South African slushing technique, were not readily available; and (3) cement-treated subbase used in inverted pavements was considered a potential risk for pavement cracking, especially in northern climates. A con- scious effort needs to be made to encourage the construction of inverted pavements in the United States to fully study any potential disadvantages, such as pavement distresses occur- ring as a result of cracking of the cement-treated subbase, perhaps as a result of shrinkage and exposure to freeze-thaw conditions. In addition, in colder climates further evaluation of related pavement design considerations is needed. Accord- ing to the Portland Cement Association, it is possible to limit the percent cement used in inverted subbases (such as to 2% to 3%) to potentially mitigate these cracking problems. The successful construction, ongoing documentation, and tech- nology transfer of the superior performances of the inverted pavement trials no doubt will have a positive impact on such sustainable alternatives to pavement design. Current State of Practice on Alternative Base Course Construction One of the objectives of the current synthesis study was to gather information on the state of practice in the United States and Canada regarding the application of alternative UAB/ subbase layers, such as inverted pavement sections. Accord- ingly, the survey of state and Canadian provincial transpor- tation agencies included questions regarding construction practices such as the South African slushing technique. Only two states (New Mexico and Rhode Island) reported the use of alternative construction techniques. New Mexico DOT reported an ongoing project involving the construction of inverted pavement sections that will use the South African slushing technique for compaction of the UAB layer. Rhode Island DOT indicated that the agency used the âtest stripâ method to determine the maximum achievable density value for an UAB/subbase layer through repeated compaction of the same spot until no noticeable increase in the density was achieved. The DOC achieved during construction of UAB/subbase layers was then compared with the maximum density values obtained from the test strips. No other state reported the use of innovative construction practices. SUMMARY This chapter presents an overview of current practices as far as material handling and construction practices for UAB and subbase layers are concerned. Extensive review of published literature was conducted to gather information on differ- ent procedures and practices identified by researchers to be adequate/inadequate for pavement layer construction. Aggregate segregation and degradation are identified as two major concerns affecting aggregate gradation, and different practices that magnify these problems are listed. A survey of state and Canadian provincial transportation agencies indicated that only 37% of the responding agencies (46 total respondents) currently have specific guidelines governing aggregate storage, transportation, and stockpiling practices. The current state of the practice regarding construction lift thicknesses indicates a significant gap between the knowl- edge gained through research and trial projects and current agency specifications. Different research studies establishing the effectiveness of greater lift thicknesses during construc- tion are summarized in this chapter and the need for har- monizing such practices among transportation agencies was established. Key Lessons â¢ Inverted pavements involve the construction of a âhigh qualityâ crushed stone base layer over a stabi- lized subbase course. â¢ With the aggregate base layer functioning as the primary structural component, inverted pavements offer a long-lasting, economical alternative to con- ventional pavement construction. â¢ Construction of inverted pavements and similar pave- ments utilizing thick crushed aggregate base layers is a common practice in countries such as South Africa, Australia, and France. â¢ All inverted pavement applications in the United States have resulted in equal or better performance compared with equivalent conventional pavement sections. â¢ A conscious effort is required in the United States and Canada to encourage the construction of alterna- tive pavement structures with thick UAB layers as the primary structural component.
53 Finally, this chapter discusses the concept of inverted pavements as an alternative application of UAB layers. The concept behind this application was described, as were the response mechanism and construction procedures. Different research studies emphasizing the effectiveness of inverted pavements are highlighted, and the need for further explo- ration in this area was established. The next chapter dis- cusses the different methods used for the characterization of unbound aggregate materials and layer design. REFERENCES Ahlvin, R.G., W.J. Turnbull, J.P. Sale and A.A. Maxwell, Multiple-Wheel Heavy Gear Load Pavement Tests, U.S. Army Corps of Engineers, Vicksburg, Miss., 1971, 216 pp. Allen, J.J., J.L. Bueno, M.E. Kalinski, M.L. Myers, and K.H. Stokoe II, Increased Single-Lift Thicknesses for Unbound Aggregate Base Courses, ICAR Report No. 501-5, Inter- national Center for Aggregate Research, The University of Texas at Austin, 1998. Aughenbaugh, N.B., R.B. Johnson, and E.J. Yoder, Degrada- tion of Base Course Aggregates during Compaction, School of Civil Engineering, Purdue University, West Lafayette, Ind., 1963. Avellandeda, D.D.C., Inverted Base Pavement Structures, Ph.D. Thesis, School of Civil and Environmental Engi- neering, Georgia Institute of Technology, Atlanta, 2010. Barker, W.R., W.N. Brabston and F.C. Townsend, An Inves- tigation of the Structural Properties of Stabilized Layers in Flexible Pavement Systems, U.S. Army Corps of Engi- neers, 165, Vicksburg, Miss., 1973. Barksdale, R.D. and H.A. Todres, A Study of Factors Affecting Crushed Stone Base Performance, School of Civil Engi- neering, Georgia Institute of Technology, Atlanta, 1983. Barksdale, R.D., âPerformance of Crushed Stone Base Courses,â Transportation Research Record 954, Trans- portation Research Board, National Research Council, Washington, D.C., 1984, pp. 78â87. Barksdale, R.D., The Aggregate Handbook, National Stone Association, Washington, D.C., 1991. Beatty, T.L., et al., Pavement Preservation Technology in France, South Africa, and Australia, Office of International Programs, Federal Highway Administration, U.S. Depart- ment of Transportation, and the American Association of State Highway and Transportation Officials, Alexandria, Va., 2002. Buchanan, S., Inverted Pavements-What, Why, and How? AFTRE Industry Education Webinar, Aggregates Foun- dation for Technology, Research, and Education, Alexan- dria, Va., June 1, 2010. Bueno, J.L., K.H. Stokoe, and J.J. Allen, âA Study on the Fea- sibility of Compacting Unbound Graded Aggregate Base Courses in Thicker Lifts that Presently Allowed by State Department of Transportation,â Report No. ICAR 501-2, International Center for Aggregates Research, University of Texas at Austin, 1998. Bueno, J.L., K.H. Stokoe II, J.J. Allen, and M.E. Kalinski, âEval- uation of Constructing Increased Single-Lift Thicknesses of Unbound Aggregate Bases Case Study in Georgia,â Trans- portation Research Record: Journal of the Transportation Research Board, No. 1673, 1999, pp. 95â102. Cortes, D.D., âInverted Base Pavement Structures,â Ph.D. Dissertation, School of Civil and Environmental Engi- neering, Georgia Institute of Technology, Atlanta, 2010. De Beer, M., âSouth African G1 Base Course-Inverted Pave- ment,â 2012 Transportation Research Board Mineral Aggregates Committee (AFP70) Meeting Presentation, 91st Annual Meeting of the Transportation Research Board, Jan. 22â26, 2012, Washington, D.C. Grau, R.W., âEvaluation of Structural Layers of Flexible Pavements,â Miscellaneous Paper, S-73-26, Waterways Experiment Station, 1973. Horne, D., et al., South African Pavement and Other Highway Technologies and Practices, Federal Highway Adminis- tration, U.S. Department of Transportation, Washington, D.C., 1997. Johnson, V.W., âComparative Studies of Combinations of Treated and Untreated Bases and Subbase for Flexible Pave- ments,â Bulletin 289, Highway Research Board, National Research Council, Washington, D.C., 1960, pp. 44â61. Jooste, F.J., and L. Sampson, âThe Economic Benefits of HVS Development Work on G1 Base Pavements,â Department of Public Transport, Roads, and Works, 2005. Lewis, D.E., K. Ledford, T. Georges, and D.M. Jared, âCon- struction and Performance of Inverted Pavements in Geor- gia,â Paper No. 12-1872, Poster Presentation in Session 639, 91st Annual Meeting of the Transportation Research Board, Jan. 22â26, 2012, Washington, D.C. Majidzadeh, K. and C.S. Brahma, Statistical Analysis of Aggregate Size Distribution, The Ohio State University Department of Civil Engineering and Transportation Research Center, 1969. Maree, J.H., N.J.W. van zyl, and C.R. Freeme, âEffective Moduli and Stress Dependence of Pavement Materials as Measured in Some Heavy-Vehicle Simulator Tests,â Transportation Research Record 852, Transportation Research Board, National Research Council, Washington, D.C., 1982a. Maree, J.H., C.R. Freeme, N.J.W. van zyl, and P.F. Savage, âThe Permanent Deformation of Pavements with Untreated Crushed-Stone Bases as Measured in Heavy Vehicle Sim- ulator Tests,â Proceedings of the 11th Australian Road Research Board Conference, Melbourne, 1982b. Metcalf, J.M., S. Romanoschi, L. Yongqi, and M. Rasoulian, Construction and Comparison of Louisianaâs Conven- tional and Alternative Base Courses under Accelerated Loading, Interim Rep. 1, Phase 1, Louisiana Transporta- tion Research Center, Baton Rouge, 1998. Miller Warden Associates, âEffects of Different Methods of Stockpile Sampling Interim Report,â Proceedings of Highway Research Board, Highway Research Board, National Research Council, Washington, D.C., 1964.
54 National Stone, Sand and Gravel Association (NSSGA), Stone Base Construction Handbook, NSSGA, Alexandria, Va., 1989. Nohl, J., and B. Domnick, âStockpile Segregation,â Techni- cal Paper T-551, Superior Industries, Morris, Minn., 2000. OâNeil, D.J., J.P. Mahoney, and N.C. Jackson, An Evaluation of Granular Overlays in Washington State, Final Techni- cal Report, Report No. FHWA-SA-92-042, Washington State Department of Transportation, Olympia, 1992. Rasoulian, M., B. Becnel, and G. Keel, âStone Interlayer Pavement Design,â Transportation Research Record 1709, Transportation Research Board, National Research Coun- cil, Washington, D.C., 2000, pp. 60â68. Rasoulian, M., H. Titi, M. Martinez, B. Becnel, and G. Keel, Long Term Performance of Stone Interlayer Pavement, Louisiana Transportation Research Center Report 29, Baton Rouge, 2001. Saunders, C.H., âIncreasing the Single Lift Thickness for Aggregate Base: Base Placement and Compaction of Increased Lift Thickness of Aggregate Base on Test Proj- ects,â Paper Presented at the 5th Annual Symposium of the International Center for Aggregate Research (ICAR), 1997. Terrell, R.G., B.R. Cox, F.Y.U.H. Menq, J.J. Allen, and K.H. Stokoe, II, âStiffness of Unbound Aggregate Base Lay- ers in Inverted Flexible Pavements,â International Cen- ter for Aggregates Research 11th Annual Symposium: AggregatesâAsphalt Concrete, Bases and Fines, Austin, Tex, Apr. 27â30, 2003a. Terrell, R.G., B.R. Cox, K.H. Stokoe II, J.J. Allen, and D. Lewis, âField Evaluation of the Stiffness of Unbound Aggregate Base Layers in Inverted Flexible Pavements,â Transportation Research Record: Journal of the Trans- portation Research Board, No. 1837, Transportation Research Board of the National Academies, Washington, D.C., 2003b, pp. 50â60. Titi, H., M. Rasoulian, M. Martinez, B. Becnel, and G. Keel, âLong-Term Performance of Stone Interlayer Pavement,â Journal of Transportation Engineering, Vol. 129, No. 2, 2003, pp. 118â126. TRH, Guidelines for Road Construction Materials, Draft Technical Recommendation for Highways (TRH) 14, Pre- toria, South Africa, 1985. Tutumluer, E. and R.D. Barksdale, âInverted Pavement Response and Performance,â Transportation Research Record 1482, Transportation Research Board, National Research Council, Washington, D.C., 1995, pp. 102â110. Weingart, R., âInverted Base: The Virginia Experience,â 2012 Transportation Research Board Mineral Aggregates Committee (AFP 70) Meeting Presentation, 91st Annual Meeting Transportation Research Board, Jan. 22â26, 2012, Washington, D.C. Wells, G.E., and R.C. Adams, âIncreased Single-Lift Depths for Aggregate Base Course in Highway Construction: Sec- ondary Road 1117 Relocation, Richmond County, North Carolina,â presented at the 5th Annual Symposium of the International Center for Aggregate Research (ICAR), 1997. White, D.J., P. Vennapusa, and C.T. Jahren, Determination of the Optimum Base Characteristics for Pavements, Iowa DOT Project TR-482, Center for Transportation Research and Education, Iowa State University, Ames, 2004. Williamson, T.G., and E.J. Yoder, An Investigation of Com- paction Variability for Selected Highway Projects in Indi- ana, Publication FHWA/IN/JHRP-67/05, Joint Highway Research Project, Indiana Department of Transportation and Purdue University, West Lafayette, 1967. Womack, W.W., âIncreasing the Lift Thickness for Aggregate Base: Virginia Department of Transportation Perspec- tive,â presented at the 5th Annual Symposium of the Inter- national Center for Aggregate Research (ICAR), 1997.
TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 445: Practices for Unbound Aggregate Pavement Layers consolidates information on the state-of-the-art and state-of-the-practice of designing and constructing unbound aggregate pavement layers. The report summarizes effective practices related to material selection, design, and construction of unbound aggregate layers to potentially improve pavement performance and longevity.
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Serving the needs of liquid asphalt manufacturers and suppliers worldwide since 1919.
Asphalt Pavement Construction
While there are an infinite number of questions that can be asked, we compiled a list of those questions that have been directed to us the most.
These FAQs are categorized into subject areas listed in the contents drop-down box below.
We tried to keep both the questions and answers concise. Additional information is referenced where applicable for those seeking more in-depth information on a given subject.
For further information see the other engineering areas , Asphalt magazine and APA websites, Asphalt Institute online store and the links page for other information related to these topic area.
We also recommend that you attend our Asphalt Academy courses at sites throughout the country to obtain expert instruction on asphalt topics.
An application of a low viscosity asphalt to a granular base in preparation for an asphalt surface course.
- To coat and bond loose material particles on the surface of the base.
- To harden or toughen the base surface to provide a work platform for construction equipment.
- To plug capillary voids in the base course surface to prevent migration of moisture.
- To provide adhesion between the base course and the succeeding course.
For a prime coat to be effective it must be able to penetrate into the base course. Usually a light grade of medium curing cutback such as an MC-30 will work well. However, in a lot of areas air quality is of concern and the EPA has restricted or eliminated the use of cutbacks. In such areas the use of an emulsified asphalt is necessary. There are several ways to accomplish a prime when using an emulsion:
- Most emulsion manufacturers make proprietary products, one of which is an emulsion specifically designed for use in prime coats.
- If the granular base material has a gradation that is somewhat porous, placing a prime coat can often be affected by placing a slow-setting emulsion (SS-1, SS-1 h, CSS-1, CSS-1 h) diluted 5 parts water to 1 part emulsion. By applying several (4 or 5) light applications (0.10 gal/sy), a waterproof surface can be obtained on the base course.
- Incorporate an emulsion into the compaction water while placing the last 2 to 3 inches of the base course. Use a dilution and application rate which will provide 0.1 to 0.3 gallon per square yard (3:1 dilution; 4 applications; 0.15 gal/sy rate).
- Complete placement of the base course material, then scarify up about 3/4 inch. Apply about 0.20 gal/sy 2 of straight emulsion (undiluted) and blade mix it with the scarified material. Then relay the mixed material and compact.
At one time it was thought that a prime coat was an essential element of good pavement construction. However, in recent years some engineers have eliminated the use of a prime, especially when asphalt layer(s) (surface and/or base) is 4 inches or more in thickness. In many instances, prime coats have not been used even when surface thickness have been as thin as 2 inches. Over the past 20 years, few, if any, pavement failures can be attributed to the lack of prime coat.
To ensure a bond between the succeeding layers of a pavement.
A slow-setting emulsion, either SS-1, CSS-1, SS-I h, or CSS-1 h, works well when diluted 50/50 with water.
You want to accomplish a very uniform application of about 0.03 to 0.05 gal/sy of residual asphalt on the layer to be tacked (a paint job, so to speak). Slow-setting emulsions generally have a residual asphalt content of about 2/3. Therefore, an application rate of 0.10 to 0.15 gals/sy of the diluted material will give you the 0.03 to 0.05 gals/sy.
- Caution #1 : Once the tack coat is applied, time must be allowed for emulsion to break (turn from brown to black) prior to placing hot mix on it. The length of time required for this to happen will depend on the weather. In good paving weather, it will take only a few minutes. In marginal weather it may take several minutes.
- Caution #2 : Never apply an emulsion tack coat to a cold pavement (below the freezing point). The emulsion will break, but the water and emulsifying agents will freeze and remain in the layer that has been tack coated.
If either of these cautions is violated, there is a good chance that upper layer will not bond to the under layer and a slip plane will develop.
Almost always! On rare occasions when a pavement is being constructed which is not being used by traveling public and each succeeding lift is placed in rapid succesion, a tack coat may not be necessary. However, a good cheap insurance policy is to always use tack coats.
No rule of thumb answers your question, but two issues should be considered:
- Is the pavement structure (subgrade, subbase, base, and all asphalt layers) adequate to support the loads? You need to purchase our MS-23 Manual, Thickness Design of Asphalt Pavements for Heavy Wheel Loads .
- Is the hot mix asphalt surface stiff enough to resist deformation (ruts or indentations)? This is dependent on many factors, such as stiffness of the original mixture, age of the mix (gets stiffer over time), temperature of the mix during loading, loading itself, duration of applied load, etc. While not usually a problem, when it occurs it can typically be resolved by placing some steel (or other rigid material) plates below the point load to distribute the load across a wider area.
Yes. However, since Superpave mixes tend to be coarser and contain modified binders than conventional mixes, good construction practices are more important than ever. Segregation is more likely to occur with coarser mixes if proper equipment and techniques are not used. Density can also be more difficult to achieve with Superpave mixes. Proper rolling techniques and adequate equipment are essential to achieve sufficient compaction. Breakdown rolling for Superpave mixes is normally done right behind the paver when the mix is hottest. Some contractors have found that additional and/or heavier rollers are sometimes needed. Pneumatic rubber-tired rollers work well, but tend to stick to the mat when polymer modified asphalt is used. Hand-working should be minimized. Sufficient well-graded (not segregated) material should be supplied by the paver augers to the joint to facilitate a low-void, low-permeability seam.
Generally speaking, there are no unique problems with using polymer modified mixes as RAP. Some individuals express environmental concerns about running millings containing ground tire rubber (GTR) through a drum plant. Florida uses a small percentage of GTR on most of their highway surface mixes. California and Arizona also use GTR frequently.
Mix temperature is dependent on the grade of asphalt used in the mix: Less viscous asphalt requires lower temperatures, while more viscous asphalt requiers higher temperatures. At the start of a mix design, target temperatures are specified for proper mixing and compaction. These temperatures should be adjusted for project conditions (weather, haul distances, etc.). If at all possible, avoid discrepancies from the mix design temperature of more than 25 degrees. Note: When working with modified binder, the binder supplier should provide mix temperature recommendations.
Mixes must be placed and compacted before they cool to 185 o F, so the minimum temperature will depend on the temperature of the layer upon which it is being placed as well as ambient conditions. Generally, agency specifications will spell out a minimum acceptable temperature for the mix. Some specifications will use 225 o F, and others may use 250 o F.
Conventional mixes should be impervious to water as long as the total in-place air void content is below 7 to 8%. Mixes with higher void contents can be pervious to air and water leading to premature aging and raveling.
The Asphalt Institute strongly endorses the use of RAP in asphalt mixtures. RAP has a history of positive performance. The specifying agency or owner will set the limit for RAP content. Almost all state highway departments now allow the use of RAP. A few restrict its use in wearing courses; even fewer (one or two) prohibit its use completely. Most agencies have developed a means of accomodating the stiffness of the reclaimed asphalt from the RAP by the selection of the particular grade of the virgin binder. The FHWA Asphalt Mixture Expert Task Group developed recommendations that are being considered by the Association of State Highway and Transportation Officials (AASHTO) to provide guidance in asphalt binder grade selection when using RAP. These recommendations are summarized below.
- When 15% or less RAP is used: “The binder grade for the mixture is selected for the environment and traffic conditions the same as for a virgin mix. No grade adjustment is made to compensate for the stiffness of the asphalt in the RAP”.
- When 16 to 25% RAP is used: “The selected binder grade for the new asphalt is one grade lower for both the high and low temperature stiffness than the binder grade required for a virgin asphalt. For example, if the specified binder grade for the virgin mix is a PG 64-22, the required grade for the recycled mix would be a PG 58-28”.
- When more than 25% RAP is used: “The binder grade for the new asphalt binder is selected using an appropriate blending chart for high and low temperature. The low temperature grade is one grade lower than the binder grade required for a virgin asphalt”.
Normally, the above guidelines would be applied to both new and existing pavements. If a warranty was applied to a project, a more conservative approach – such as the use of blending charts – might be taken.
It is suggested that you contact the local state highway agency and/or asphalt binder supplier for the prevailing local practices.
Rice (Gmm) is typically not run on material from cores as it is not the preferred method of material collection for this test. In fact, ASTM D5361, Standard Practice for Sampling Compacted Bituminous Mixtures for Laboratory Testing, does not include Rice testing in its Significance and Use section.
Note paragraph 3.1 from the standard reads: 3.1 Samples obtained in accordance with the procedure given in this practice may be used to measure pavement thickness, density, resilient or dynamic modulus, tensile strength, Marshall or Hveem stability, or for extraction testing, to determine asphalt content, asphalt properties and mix gradation. There are a couple of reasons for this. First, coring is naturally a destructive process which alters the gradation. The level to which the gradation shifts varies with the nature of the parent gradation and material. i.e., a half-inch SMA is likely to see a greater gradation shift then say a fine, dense-graded three-eighths mix. Secondly, and more importantly, by coring you are creating aggregate that is not coated with asphalt. This lack of coating can then allow for water absorption into these non-protected surfaces. Naturally, the more absorptive the aggregate the greater the potential issue with this situation. The AASHTO standard for Rice is T-209. It addresses absorption in part 15 of the standard entitled, “Supplemental Procedure for Mixtures Containing Porous Aggregate.” This is also known as the “dry-back procedure.” It is used on mixes produced with aggregate who’s water absorption is greater than 1.5%.However, while collection of Rice material via cores is not the preferred method, it is an acceptable method when more preferred alternatives (plant or lab produced samples) are not available. I am unaware of any state that does not allow for cores to be used for Gmm when no good alternative is an option. With the previous discussion in mind, one should do what they can to minimize any potential problems that may arise from field-cut specimens. What this leads to is a bigger is better mindset. A 6-inch core will have a smaller percentage of its aggregate affected by the coring than would a 4-inch core from the same road. Therefore, it is highly recommended that if alternative methods of producing materials for Rice are not an option, to use at least a 6-inch core. If a bigger specimen can be collected, such as saw-cutting, then it should be considered. Judgment, and locally acceptable practice, will certainly need to come into play.
Lift thickness governs aggregate size. Minimum lift thickness should be at least 3 times the nominal max. aggregate size to ensure aggregate can align themselves during compaction to achieve required density and also to ensure mix is impermeable. The maximum lift thickness is dependent also upon the type of compaction equipment that is being used. When static steel-wheeled rollers are used, the maximum lift thickness that can be properly compacted is three (3) inches. When pneumatic or vibratory rollers are used, the maximum thickness of lift that can be compacted is almost unlimited. Generally, lift thicknesses are limited to 6 or 8 inches. Proper placement becomes a problem in lifts thicker than 8 or 8 inches. For open-graded mixes, compaction is not an issue since it is intended that these types of mixes remain very open. Therefore, the maximum size aggregate can be as much as 80 percent of the lift thickness.
This common question can mean different things to different people because of the wide range of precipitation encompassed by the word “rain.” On one end, occasional light sprinkles should not be cause to shut down operations. However, a steady downpour, either light or heavy, should result in cessation of paving activities. To avoid waste, some states have verbiage in their specifications stating that trucks in route to the project when rain begins can be laid at the contractor’s risk. Also keep in mind that the surface on which you are paving may influence your decision. Paving on a firm, stable, well-draining crushed aggregate base might be given more leeway than a thin asphalt overlay. Raining or not, new pavement must be placed on a firm, unyielding base. Critical ideas to keep in mind when dealing with rain:
- Rain will cool the asphalt mix and could make obtaining proper compaction more difficult
- the asphalt lifts must be able to properly bond together and moisture can be a hindrance to that bond
- puddles overlaid with HMA turn to steam, which may cause stripping (separation of the asphalt binder from the aggregate) – never pave over puddles whether it is raining or not
If you temporarily suspend paving operations due to rain, don’t forget to:
- keep all trucks tarped
- construct a vertical-faced construction joint
- properly dispose of all material left in the hopper
- be careful not to track mud and dirt onto the project
Asphalt pavements are designed to last for many years, so don’t let a sense of urgency to get the job done quickly allow you to make decisions which could strip years away from the pavement life.
Information on fuel-resistant asphalt sealers can be found at www.aaptp.us with Report 05-02.
Minimum lift thickness should be at least 3 times the nominal max. aggregate size to ensure aggregate can align themselves during compaction to achieve required density and also to ensure mix is impermeable. The maximum lift thickness is dependent also upon the type of compaction equipment that is being used. When static steel-wheeled rollers are used, the maximum lift thickness that can be properly compacted is three (3) inches. When pneumatic or vibratory rollers are used, the maximum thickness of lift that can be compacted is almost unlimited. Generally, lift thicknesses are limited to 6 or 8 inches. Proper placement becomes a problem in lifts thicker than 8 or 8 inches. For open-graded mixes, compaction is not an issue since it is intended that these types of mixes remain very open. Therefore, the maximum size aggregate can be as much as 80 percent of the lift thickness.
Mix temperature will be dependent on the grade of asphalt used in the mix. The less viscous the asphalt, the lower the temperatures should be. The more viscous the asphalt, the higher the temperature can be. During mix design temperatures are specified for proper mixing and for compaction. These are good targets with which to start a project. However, they will have to be adjusted for the project conditions (weather, haul distances, etc.). If at all possible, avoid discrepancies from the mix design temperature of more than 25 degrees. Note: When working with modified binder, the binder supplier should provide mix temperature recommendations.
Mixes must be placed and compacted before they cool to 185 o F, so the minimum temperature will depend on the temperature of the layer upon which it is being placed as well as ambient conditions. Temperature session charts are shown on Page 6-6, Fig. 6.03 of the new MS-22 and Page 234 of the old MS-22. Generally, agency specifications will spell out a minimum acceptable temperature for the mix. Some specifications will use 225 o F, and others may use 250 o F.
When all the aggregate particles are coated with asphalt. The large aggregate particles are always the last to be coated. If the large aggregate particles are completely coated, the mix is properly mixed. Generally we see mixing problems only with batch plants. The producer is trying to mix each batch as quickly as possible (probably in about 30 seconds) which may or may not be adequate mixing time. Typical specifications set minimum coated particle percentages at 90 to 95 percent. The Ross Count procedure for determining these percentages (ASTM-D2489 or AASHTO T195) is outlined on pages 4-41 to 4-44 of the new MS-22 and pages 162 and 163 of the old MS-22.
Minimum mixing times to meet the specified requirement should carefully adhered to in order to avoid excess oxidation of the asphalt films on the aggregate particles as it is exposed to air (oxygen) during the mixing process.
As a general rule we do not see this problem with drum mixes. The mix remains in the mixing portion of the drum for much longer periods of time (maybe 2 to 3 minutes) than in the pugmill of a batch plant, so the aggregate particles get very well coated. Keep in mind that we are not as concerned about oxidation in drum mixes as the mixing portion of the drum mixer is essentially an oxygen-free atmosphere.
Another way to look at it is this: In a 6000 lb. batch of mix, there are about 5600 lbs. of aggregate and about 400 lbs. of asphalt. Dense-graded aggregate has about 35 sq. ft. of surface area per pound, or 196,000 sq. ft/6000 lb. batch; 400 pounds of asphalt is about 48 gallons. The mixing process has to take 48 gallons of asphalt and paint about 3.8 football fields. When the aggregate particles are coated, it’s mixed.
Far too often we still see diesel fuel used as a mix release agent. Diesel fuel is a solvent. Any excess amount will dissolve the asphalt films on the aggregate particles, thus contaminating the mix. Commercial mix release agents are readily available and should be used. They generally are soap or emulsified wax or other stick-resistant materials that do not contaminate the mix. A couple of suggestions are a bag of hydrated lime mixed with 1000 gallons of water or a bottle of dish soap (Joy) mixed with water. The portions depend on the water with which it is mixed. Soft water won’t need nearly as much as hard water.
It has been our experience that a special release agent is required for modified asphalts. Contact your local State Department of Transportation for a list of approaved release agents.
Paver speed should be geared to mix production and delivery. Every effort should be made to maintain a constant paver speed. Several factors effect that constant speed. With a consistent production and delivery flow, the speed of the paver will vary with lift thickness and width of paver pass. Thicker lift – slower speed; thinner lift – faster speed. Wider pass – slower speed; narrower pass – faster speed. Most equipment manufacturers will give a suggested maximum speed for their paver. A lot of agency specifications will specify a maximum speed, such as 30 or 40 feet per minute.
The paver screed has too much lead crown in it.
The paver screed does not have enough lead crown in it. Note : Paver screeds should have slightly more crown in the leading edge than in the trailing edge – usually about 1/8 inch. This may very with equipment manufacturer and/or width of paver pass. Even if the trailing edge of the screed is to place a flat or straight grade, the leading edge must still have the increased crown.
The Asphalt Institute strongly endorses the use of RAP in asphalt mixtures. RAP has a history of positive performance. Regarding limiting the RAP content, that is the decision of the specifying agency or owner. Almost all of the state highway departments now allow the use of RAP. A few restrict its use in wearing courses; even fewer (one or two) do not allow its use at all. Most agencies have developed a means of accomodating the stiffness of the reclaimed asphalt from the RAP by the selection of the particular grade of the virgin binder. The FHWA Asphalt Mixture Expert Task Group developed recommendations that are being considered by the Association of State Highway and Transportation Officials (AASHTO) to provide guidance in asphalt binder grade selection when using RAP. These recommendations are summarized below.
- rain will cool the asphalt mix and could make obtaining proper compaction more difficult
Here’s the process:
- Calculate the number of cubic feet to be paved. (Remember to convert the thickness to feet – by dividing by 12 inches per 1 foot). 10′ x 25′ x (4/12)’ = 83.3 cubic feet of HMA
- Asphalt Mixture typically weighs from 142 to 148 pounds per cubic foot (PCF) in-place. Use 148 PCF.
- Calculate the tonnage needed. (remember to convert from pounds to tons; 2000 pounds per ton).
83.3 cubic feet x 148 PCF = 12328 pounds of mix = 12328 / 2000 tons = 6.1 tons
Yes. However, since Superpave mixes do tend to be coarser and contain modified binders than conventional mixes, good construction practices are more important than ever. Segregation is more likely to occur with coarser mixes if proper equipment and techniques are not used. Density can also be more difficult to achieve with Superpave mixes. Proper rolling techniques and adequate equipment are essential to achieve sufficient compaction. Breakdown rolling for Superpave mixes is normally done right behind the paver when the mix is hottest. Some contractors have found that additional and/or heavier rollers are sometimes needed. Pneumatic rubber-tired rollers work well, but tend to stick to the mat when polymer modified asphalt is used.Hand-working should be minimized. Sufficient well-graded (not segregated) material should be supplied by the paver augers to the joint to facilitate a low void, low permeability seam.
Paver speed should be geared to mix production, delivery and compaction; with emphasis placed on compaction. Every effort should be made to maintain a constant paver speed. Several factors effect that constant speed. With a consistent production and delivery flow, the speed of the paver will vary with lift thickness (thicker/slower; thinner/faster) and width of paver pass wider/slower; narrow/faster). Most equipment manufacturers will give a suggested maximum speed for their paver. A lot of agency specifications will specify a maximum speed, such as 30 or 40 feet per minute. Most compaction manufacturers recommend a maximum roller speed of 3 mph and most often more than one roller pass is needed to get compaction. Therefore, the number and type of rollers being used is very important.
We do not recommend spraying water on freshly laid hot mix asphalt (HMA) in order to cool the mat faster and open to traffic sooner. First, spraying water on the hot mat is not very effective since the water should drain properly on a new surface and only cools the crust temporarily, with the internal HMA temperature not being affected much. In addition, there is a concern that the water could cause a foaming effect with the hot asphalt binder, making the HMA less stable under traffic. We believe it is best to let the hot mat cool naturally.
The Asphalt Institute recommends a transverse slope of between 1.5 to 3.0% on all pavement surfaces, and an even steeper slope of 3 to 6% on shoulders. Maintaining a slope of at least 1.5% on parking lots will ensure proper surface drainage (no ponding or birdbaths) and minimize infiltration, hydroplaning and the detrimental effects of water.
Contrary to popular belief, the number of rollers required for proper compaction is based on the square yardage placed rather than the production or delivery tonnage. Roller speed should be limited to 3 mph. With this speed and the width of the roller, the coverage rate can be calculated. The width of paver pass and speed can give you the square yardage placed. The number of required coverages will then tell you the total area in square yards the roller must be able to cover. On very small jobs, one roller may be adequate. On very large projects, six or eight rollers may be needed. A lot of projects are compacted with three rollers: a breakdown roller, a compaction roller, and a finish roller. On most average projects, two rollers are used – a vibratory steel-wheeled roller for breakdown and compaction, and a heavy static steel wheel for finish rolling.
Occasionally, agency specifications will require a light (65 to 75 psi contact pressure) pneumatic roller to be used to knead or seal the surface prior to the finish rolling.
Efforts should be made to control compacted air voids between 7% and 3%. At 8% or higher, interconnected voids which allow air and moisture to permeate the pavement, reducing its durability. On the other hand, if air voids fall below 3%, there will be inadequate room for expansion of the asphalt binder in hot weather. When the void content drops to 2% or less, the mix becomes plastic and unstable.
Air voids are a reverse proportion of the density of the compacted mix. By specifying a density requirement, the voids are inversely controlled. Keep in mind that density is a relative term, compared to a target density of either lab compacted mix, a maximum theoretical density, or a control strip density. Procedures for using the three methods are spelled out on Page 7-17 to 7-21 of the new MS-22 and Page 241 of the old MS-22.
Testing should be done on a random sampling basis with a minimum of five tests per lot (agency requirements define a “lot” as “A day’s or full day’s production”). The average of the five density determinations should be equal to or greater than:
- 96% of lab density with no test less than 94%
- 92% of maximum theoretical with no test less than 90%.
- 99% of the control strip density
Nuclear gauges are generally used for density testing because of the ease and speed with which the testing can be done. This allows for many more tests – more than the five minimum for a better statistical result. Caution : The nuclear density gauge needs to be correlated to core densities that are taken from the same location as was nuclear gauge tested. This should be done for each different mix that might be used.
There is not a predictable value or ‘rule-of-thumb number’ for the difference in air void content of original and reheated samples. The general trend would be for the reheated samples to have higher air voids than the original, compacted specimens. Absorption and hardening or stiffening of the asphalt binder in the reheated samples likely causes this difference. Reheated samples can be utilized to give an overall check of the original sample results. Before any significant precision is attributed to reheated sample results, a correlation should be developed for reheated sample air voids and original sample air voids by performing a series of comparative tests.
Without knowing what the surface cracking looks like, it is hard for us to identify the problem. Could the “surface cracking” be checking cracking from the rolling operation? It is shallow hairline surface cracks spaced an inch or two apart from each other and running transverse to the direction of rolling. The cause is rolling on the mat too hot and/or too tender of a mix. You can reference page 6-9 of the new MS-22 and page 219-220 of the old MS-22 manual if you are not sure what checking is.
There are several ways to establish density targets. Some of the more common approaches include:
- Specifying a percentage of the unit weight from the laboratory mix design. Example: 96% of the Marshall unit weight
- Establishing a value based on results achieved on a project-site test strip. Example: 98% of test strip density.
- Specifying a percentage of the maximum unit weight. Example: 94% of the maximum unit weight.
Specifying some minimum percent of the maximum unit weight has gained acceptance with many specifying agencies. The maximum unit weight is sometimes called the “solid density”. This value is based on the asphalt mixture’s maximum specific gravity – also known as the Rice value or G mm in Superpave. The maximum unit weight is determined by multiplying the Rice value by 62.4 pounds per cubic foot (PCF). For example, 2.500 is a typical Rice value. 2.500 X 62.4 = 156.0 PCF. Then, if 95% compaction is specified, the minimum acceptable unit weight is: 0.95 X 156.0 = 148.2 PCF. If 93% of solid is specified, or a maximum of 7% air voids are allowed in the compacted mat, then the minimum target value would be 145.1 PCF (0.93 X 156.0).
The thickness of the course being compacted does influence its compactability. Too thin a mat does not have sufficient workability, and too thick a mat may be unstable. In order to be compacted, the mixture must have controlled workability. Typically, for dense-graded mixes, a lift thickness of 3 to 4 times the nominal maximum size (NMS) of the aggregate is needed. For example, a mix containing ½-inch NMS stone should be placed at a compacted depth of at least 1-½ to 2 inches. If a ½ -inch top-size mix is placed at 1 inch compacted depth, the mat may pull and tear and the stones may be broken by the rollers. Thus, the “depth of paving” does influence the ability to obtain proper compaction. The target value for compaction, based on a materials property – the maximum specific gravity – does not change but the likelihood of meeting the target density is changed.
Yes. However, since Superpave mixes do tend to be coarser and contain modified binders than conventional mixes, good construction practices are more important than ever. Segregation is more likely to occur with coarser mixes if proper equipment and techniques are not used. Density can also be more difficult to achieve with Superpave mixes. Proper rolling techniques and adequate equipment are essential to achieve sufficient compaction. Breakdown rolling for Superpave mixes is normally done right behind the paver when the mix is hottest. Some contractors have found that additional and/or heavier rollers are sometimes needed. Pneumatic rubber-tired rollers work well, but tend to stick to the mat when polymer modified asphalt is used. Hand-working should be minimized. Sufficient well-graded (not segregated) material should be supplied by the paver augers to the joint to facilitate a low void, low permeability seam.
Minimum lift thickness should be at least 3 times the nominal maximum aggregate size to ensure aggregates can align themselves during compaction to achieve required density and also to ensure mix is impermeable. The maximum lift thickness is also dependent on the type of compaction equipment being used. When static steel-wheeled rollers are used, the maximum lift thickness that can be properly compacted is 3 inches. When pneumatic or vibratory roller is used, the maximum lift thickness that can be compacted is almost unlimited. Generally, lift thicknesses are limited to 6 or 8 inches. Proper placement becomes a problem in lifts thicker than 6 or 8 inches. For open-graded mixes, compaction is not an issue since it is intended that these types of mixes remain very open. Therefore, the maximum size aggregate can be as much as 80 percent of the lift thickness.
It is not advisable to start paving if it is raining. If rain starts after paving has begun, the work can continue as long as there is no standing water and the rain is not too hard. The primary concern is achieving adequate compaction, as the mix will cool much faster due to evaporative cooling if laid on a wet surface or rain falls on an uncompacted mat. Additional compactive effort will be needed and monitoring temperatures is key to achieving adequate density.
When all the aggregate particles are coated with asphalt. The large aggregate particles are always the last to be coated. If the large aggregate particles are completely coated, the mix is properly mixed. Generally we see mixing problems only with batch plants, where the producer mixes each batch as quickly as possible (probably in about 30 seconds), which may or may not be adequate mixing time. Typical specifications set minimum coated particle percentages at 90 to 95 percent. The Ross Count procedure for determining these percentages (ASTM-D2489 or AASHTO T195) is outlined on pages 4-41 to 4-44 of the new MS-22 and pages 162 and 163 of the old MS-22.
Minimum mixing times to meet the specified requirement should be carefully adhered to in order to avoid excess oxidation of the asphalt films on the aggregate particles as it is exposed to air (oxygen) during the mixing process.
The tools now exist to gain improved performance from HMA intersections. Well-designed, properly constructed HMA intersections provide an economical, long-lasting pavement with minimal disruption to traffic.In order to achieve these benefits, we must recognize that intersection pavements are subject to extreme stresses. Ordinary materials and techniques may not be sufficient. There must be adequate pavement structure, select materials, appropriate construction techniques, and careful attention to detail in the process.To learn more about how to design and build high performance HMA intersections see the following series of ASPHALT magazine articles.
- Intersection Strategy 1: Developing a Strategy for Better Performing Intersection Pavements
- Intersection Strategy 2: Ensuring Structural Adequacy - A Key Step to Intersection Strategies
- Intersection Strategy 3: Materials and Construction Concerns for Improved Intersection Performance
- Intersection Strategy 4: Three Examples of Implementing the Plan
- World's Strongest Intersection
No rule of thumb answers your question. There are two issues:
- Is the hot mix asphalt surface stiff enough to resist deformation (ruts or indentations)? This is dependent on many factors, such as stiffness of the original mixture, age of the mix (gets stiffer over time), temperature of the mix during loading, loading itself, duration of applied load, etc. This is generally not a problem, but if it is, can typically be resolved by placing some steel (or other rigid material) plates below the point load to distribute the load across a wider area.
There is not a predictable value or “rule-of-thumb number” for the difference in air void content of original and reheated samples. The general trend would be for the reheated samples to have higher air voids than the original, compacted specimens. Absorption and hardening or stiffening of the asphalt binder in the reheated samples likely causes this difference.
Reheated samples can be utilized to give an overall check of the original sample results. Before any significant precision is attributed to reheated sample results, a correlation should be developed for reheated sample air voids and original sample air voids by performing a series of comparative tests.
Generally speaking, there should be no unique problems with using polymer modified mixes as RAP. There have been some individuals express environmental concerns about running millings containing ground tire rubber (GTR) through a drum plant. Florida uses a small percentage of GTR on most of their highway surface mixes. California and Arizona also use GTR frequently.
Mix temperature will be dependent on the grade of asphalt used in the mix. The less viscous the asphalt, the lower the temperatures should be. The more viscous the asphalt, the higher the temperature can be. During mix design temperatures are specified for proper mixing and for compaction. These are good targets with which to start a project. However, they will have to be adjusted for the project conditions (weather, haul distances, etc.). If at all possible, avoid discrepancies from the mix design temperature of more than 25 degrees. Note : When working with modified binder, the binder supplier should provide mix temperature recommendations.
A liquid asphalt, such as a Rapid Setting Emulsion (RS-1,2 or CRS-1,2 includes modified) 1 Cutback asphalts in some areas depending on EPA regulations which would include RC-250, 800 or 3000, are normally used. Highly skilled crews could also use an AC-5 or 10.
The amount of asphalt applied depends on three factors:
- The existing surface condition
- The amount of traffic
- The average particle size of the chips.
Allowance should be made for surface conditions – dry, pocked, badly cracked, flushed, bleeding, etc. Lower traffic volumes require higher asphalt applications than higher traffic. The average particle size should be embedded 60-75% into the asphalt. Higher traffic should be closer to the 60% and lower traffic should be closer to the 75% embedment factor. The average particle size is the average size of chip in the gradation, the 50% passing size can be used for this number.
Yes – AASHTO T-11 Dust ratio should be less than 0.75
Several factors can lead to this appearance; improper distributor nozzle sizes, pump pressure, spray bar height, angle of nozzle, and cold asphalt.
Mix Release Agents
Far too often we still see diesel fuel used as a mix release agent. Diesel fuel is a solvent. Any excess amount will dissolve the asphalt films on the aggregate particles, thus contaminating the mix. Commercial mix release agents are readily available and should be used. They generally are soap or emulsified wax or other stick-resistant materials that do not contaminate the mix. It has been our experience that a special release agent is required for modified asphalts. Contact your local State Department of Transportation for a list of approved release agents.
While not widely used, there are ways to color an asphalt pavement other than the common blacks and greys. The second and third options are considered specialty products and more information can be obtained by contacting individual manufacturers.
- Use a naturally colored aggregate. As the asphalt binder wears way from the surface with traffic, the color of the aggregate is exposed.
- Use an additive in the asphalt binder. Various iron compounds can impart a red, green, yellow or orange tint to a pavement, while other colors can be achieved using different metal additives. A special “synthetic” binder that contains no asphaltenes has been used because it takes color more readily. This method of tinting the mix allows color to permeate the entire depth of the material, so there are no surface wear-off concerns.
- Coat the surface with a material that penetrates the voids and bonds well to asphalt pavement, such as an epoxy-fortified acrylic emulsion. Many colors are available. Care should be taken to ensure that surface friction is not compromised, especially if the pavement is used for vehicular traffic. One possible disadvantage of this method is that the surface may wear off with time and need to be renewed.
No rule of thumb answers to your question. There are two issues:
Yes. However, since Superpave mixes do tend to be coarser and contain modified binders more often than conventional mixes, good construction practices are more important than ever. Segregation is more likely to occur with coarser mixes if proper equipment and techniques are not used. Density can also be more difficult to achieve with Superpave mixes. Proper rolling techniques and adequate equipment are essential to achieve sufficient compaction. Breakdown rolling for Superpave mixes is normally done right behind the paver when the mix is hottest. Some contractors have found that additional and/or heavier rollers are sometimes needed. Pneumatic rubber-tired rollers work well, but tend to stick to the mat when polymer modified asphalt is used. Hand-working should be minimized. Sufficient well-graded (not segregated) material should be supplied by the paver augers to the joint to facilitate a low void, low permeability seam.
Without knowing what the surface cracking looks like, it is hard for us to identify the problem. Could the “surface cracking” be check cracking from the rolling operation? “Checking” is the development of shallow hairline surface cracks spaced an inch or two apart from each other and running transverse to the direction of rolling. The cause is rolling when the mat too hot and/or the mix is too tender. You can reference our page 6-6 of the new MS-22 manual and page 219 & 220 of the old MS-22 if you are not sure what check cracking is.
Railroad information can be found in our Engineering section .
You can also visit a web page on the University of Kentucky website where you can download papers, PowerPoints and also the computer program called KENTRACK, which is computer program for hot mix asphalt and conventional ballast railway trackbeds.
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Composition of Road Structure:
Road Structure Cross Section is composed of the following components:
- Surface/Wearing Course
- Base Course
1. Surface/Wearing Course in pavement cross section:
The top layers of pavement which is in direct contact with the wheel of the vehicle. Usually constructed of material in which bitumen is used as binder materials.
a. Bituminous Pavement:
Consists of combination of mineral aggregate with bituminous binder ranging from inexpensive surface treatment ¼ in or less thick to asphaltic concrete . For good service throughout the full life bituminous pavement must retain following qualities.
- Freedom from cracking or raveling.
- Resistance to weather including the effect of surface water heat and cold.
- Resistance to internal moisture, particularly to water vapors.
- Tight impermeable surface or porous surface (if either is needed for contained stability of underlying base or subgrade).
- Smooth riding and non skidding surface.
The design should be done so that to meet the above requirements for considerable number of years (need proper design and construction supervision). Pavement meeting all the requirements above have been product if six distinctly different construction processes as follows:
- Heat a viscous bituminous binder to make it fluid, then in a plant mix it with heated aggregate place and compact the mixture while it is hot.
- Use fluid bituminous binder, mix it with aggregate at normal temperature. Mixing may be done at a plant (plant mix) or on the prepared roadway base (road mix). Spread and compact the mixture at normal temperature.
- Add solvent such as naphtha or kerosene to a viscous bituminous binder to make it fluid with aggregate at normal temperature by either plant or road mix methods. Spread and compact at normal temperature before solvent evaporates.
- Use fluid emulsion of viscose bituminous binder in water, mix it with aggregate at normal temperature by either plant or road mix method. Spread and compact at normal temperature before the emulsion breaks down with its components.
- Spread and compact clean crushed aggregate as for water bound macadam. Over it spray heated dissolved or emulsified bituminous binder which penetrates open areas of the rock and binds the aggregate together. Thus is commonly called “Penetration Method”.
- Spread bituminous binder over the roadway surface then cover it with properly selected aggregate. This is commonly called the “Inverted Penetration Method”.
Selections based on the requirements and economy, large volume of heavy vehicles, low traffic volume etc.
2. Base course
It is the layer immediately under the wearing surface (Applies whether the wearing surface is bituminous or cement concrete and or more inch thick or is but a thin bituminous layer). As base course lies close under the pavement surface it is subjected to severe loading. The material in a base course must be of extremely high quality and its construction must be done carefully.
Types of Base Course
- Granul ar Base Course
- Macadam Base
- In-water bound Macadam
- Treated Bases
3. Sub Base:
It is layer of granular material provided above subgrade generally natural gravel. It is usually not provided on subgrade of good quality. It is also called granular subbase.
a. Function of Sub base in Road Cross Section
- It enables traffic stresses to be reduced to acceptable levels in sub-grade in the Road Cross Section so that excessive deformation is prevented.
- It acts as a working plate form for the construction of upper pavement layers.
- Acts as a drainage layer, by protecting the sub grade from wetting up.
- It intercept upward movement of water by capillary action.
- It acts as a separating layer b/w subgrade and road base. By this it prevent the two layers from mixing up.
b. Characteristics of materials used in Sub Base:
The subgrade material should be clean and free from organic matter and should be able to be compacted by roller, to form stable sub-base. The material should have following characteristic.
- Well graded uniformity coefficient (D60/D10) should not be less than 3.
- Fraction passing sieve #200 shall not be greater than 2/3rd of the fraction passing sieve #40.
- Should have a L.L not greater than 25%.
- P.I not greater than 6
- In coarse grain, aggregate retained by #10 sieve, %age of wear shall not be greater than 5%.
- The max dia of any particle shall not be greater than 2/3ed of the layer thickness of sub-base.
- Typical particle size distribution for the sub-base (granular) when will meet strength requirement are:
* To avoid intrusion of silt and clay material in sub-base from subgrade
D15 (sub base) < 5 D15 (sub grade)
- Recommended plasticity characteristic for granular Sub Base (Road Note 31) are;
4. Sub Grade:
Consists of the naturally occurring material on which the road is built, or the imported fill material used to create an embankment on which the road pavement is constructed. Subgrades are also considered layers in the pavement design, with their thickness assumed to be infinite and their material characteristics assumed to be unchanged or unmodified. Prepared subgrade is typically the top 12 inches of subgrade.
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ChatGPT is getting an upgrade that will make it more up to date
- ChatGPT users will soon have access to more up-to-date information.
- The OpenAI chatbot will soon be trained on information up to April 2023.
- The announcement was made by OpenAI CEO Sam Altman at its first developer day.
ChatGPT is about to be more up to date than ever.
The OpenAI chatbot will now have knowledge of the world up to April 2023, CEO Sam Altman said at OpenAI's first developer conference on Monday.
When ChatGPT was launched in November 2022, the chatbot could only answer questions based on information up to September 2021 because of training limitations. That meant that the AI couldn't respond to prompts about the collapse of Sam Bankman-Fried's crypto empire or the 2022 US elections, for example.
"We are just as annoyed as all of you — probably more — that GPT-4's knowledge about the world ended in 2021," Altman said during his keynote . "We will try to never let it get that out of date again."
The update will only be available to paying users of GPT-4 Turbo model — OpenAI's latest, most advanced large language model to date.
The update is different from ChatGPT's web-browsing feature that was introduced in September. That feature, called "Browse with Bing," allowed ChatGPT Plus users to use the AI to search the web in real time. GPT-4 Turbo, however, is trained on data up through April 2023, which means it can generate more up-to-date responses without taking additional time to search the web. The "Browse with Bing feature, which searches the web in real-time, may still prove more useful for information since April.
The expansion of ChatGPT's knowledge base is just one of many new features Altman announced around OpenAI's GPT-4 Turbo model.
GPT-4 Turbo will be able to digest more context — up to 300 pages of a standard book — to produce answers with higher accuracy, accept images as prompts, and write code in a specific language.
Users of GPT-4 Turbo will also be able to create customizable ChatGPT bots known as GPTs that can be trained to perform specific tasks.
A preview of GPT-4 Turbo is available for paying developers, and the final model will be available in the coming weeks.
Watch: What is ChatGPT, and should we be afraid of AI chatbots?
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Flexible pavement composition and structure.
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Composition and Structure of Flexible Pavement
Fig 1: Layers of Flexible Pavement
1. Surface Course
Fig 2: Surface Course
2. Binder Course
Fig 3: Binder Course
3. Base Course
Fig 4: Laying Base Course
4. Subbase Course
Fig 5: Laying Sub-base Course
5. Frost Protection Layer
Fig 6: Compacting Sub-grade
a. Seal Coat
Fig 7: Seal Coat
b. Tack Coat
C. prime coat.
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Drones are messing with training at Fort Worth military installation
Naval air station joint reserve base leadership wants to educate the public., by tahera rahman • published november 15, 2023 • updated on november 15, 2023 at 6:20 pm.
A statistic climbing at the Naval Air Station Joint Reserve Base Fort Worth that leadership doesn't like: More encounters with unauthorized drones in their airspace.
Captain Mark McLean, Commanding Officer of the base, said they've gone from seeing about 100 of them a month to upwards of 300, some even going through the area multiple times, pushing incidents up to 700 a month.
Military planes have already had to maneuver twice to avoid a crash.
"We do not for any reason believe that there is hostile intent in flying a drone within our airspace," McLean said. "The majority of the ones we're seeing are just the younger crowd that's owning and operating these that don't know any better."
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He said they're typically 18 to 24-year-olds flying drones recreationally and might not even know they're in controlled airspace.
"We have a tower here on the airfield, we have aircraft departing and arriving, and they're talking to a controller at that tower to make sure they're safe in the most critical phase of flight; when they have their landing gear and their flaps down, and they're vulnerable to a bird that might strike them or some other kind of foreign object damage that they might encounter in the air," he explained.
He said the unauthorized objects are hazardous for single-engine aircraft, pointing to the September 2021 incident when a bird flew into the engine on a training flight.
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The pilots of the single-engine aircraft had to eject, and the aircraft damaged three houses as it crashed.
"Birds are made up of about 70% water. That investigation revealed that that was a four-and-a-half-pound buzzard that went down the intake of that engine," McLean said. "Knowing that it's made up of mostly water and drones are not made up of mostly water, you can parallel those two potential scenarios and understand that a drone in a single-engine fighter aircraft can put not only the pilot in harm's way but the public that resides in that airspace."
McLean said they have about 15-20 training flights a day.
He said base leaders are trying to educate the public about drone rules for the area.
McLean said yellow areas on the map are still part of the base's controlled airspace, and drone operators must request permission from the FAA's Drone Zone website .
"So that between us and the FAA, we can either approve or disapprove flight inside the Class Delta airspace," he said.
McLean said air traffic controllers watch for unauthorized drones and communicate that with pilots. Then, they try to track down the operators to figure out why they were flying there in the first place.
"We'll work with our federal law enforcement here on the base; that's NCIS. And NCIS will work with local law enforcement to go seek out where that operator is," McLean explained.
He said recent state legislation allows them to ticket a drone operator who flies in their controlled airspace. That became enforceable in September, but to the base's knowledge, no citations have been issued yet.
This article tagged under:
KLST San Angelo
Goodfellow to conduct emergency response training Thursday
Posted: November 15, 2023 | Last updated: November 15, 2023
SAN ANGELO, Texas ( Concho Valley Homepage ) —The San Angelo Fire Department, Goodfellow firefighters and other first responders will participate in an emergency response training on Thursday, November 16 at Goodfellow Air Force Base.
In a press release from the base, Goodfellow explains that San Angelo citizens near and around the base may hear sirens and the “big voice” announcing safety conditions of and around the base over the loudspeaker.
Gate delays can be expected during training.
For the latest news, weather, sports, and streaming video, head to ConchoValleyHomepage.com.
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Taylor Swift entrepreneurial course to be offered at UH in spring 2024
Each session of the specially designed course for Swifties will focus on a particular album or "era" of her career. Other features include friendship bracelets as gifts for students, unexpected songs played during breaks, and "Easter eggs" hidden in the material.
HOUSTON, Texas (KTRK) -- An entrepreneurial course centered around business teachings from Taylor Swift will be available at the University of Houston in the spring of 2024.
It's worth mentioning that Taylor Swift's mother, Andrea, is a University of Houston alumnus.
The course called "The Entrepreneurial Genius of Taylor Swift" will be focused around a particular album or era in the pop singer's career. Kelly McCormick, a professor of practice and managing director of Red Labs, will be teaching the course.
"You definitely don't have to be a hardcore fan - a Swiftie - to learn and appreciate the entrepreneurial genius that has made Taylor Swift an international phenomenon," McCormick said.
In fact, McCormick has been a fan of Swift since the early days of megahits "Our Song" and "Love Story" in which he began to see the entrepreneurship lessons in her 17-year career after attending one of the Houston stops on the record-breaking Eras Tour in April.
"The number one business lesson students can learn from Taylor is the way she treats her fans," McCormick said. "She is beloved because she truly does so much to make sure they are happy, appreciated, and feel like they are important to her. If every company acted that way about their customers - they'd have way more customers."
Overall, each session of the specially designed course for Swifties will focus on a particular album or period of her career. Other features include friendship bracelets as gifts for students, unexpected songs played during breaks, and "Easter eggs" hidden in the material.
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