National Academies Press: OpenBook

A Manual for Design of Hot-Mix Asphalt with Commentary (2011)

Chapter: Chapter 2 - Background

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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 2 - Background ." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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4To thoroughly understand the mix design procedures and test methods presented herein, a basic knowledge of construction materials and paving technology is needed. This information is summarized below. Engineers and technicians with a broad range of experience in materials testing and the design of HMA mixtures and flexible pavement structures need not read this chapter in detail. Individuals who are relatively new to asphalt pavement technology will find the information on materials, asphalt pavements, asphalt concrete mixtures, and mix design methods helpful when reading the later chapters of this manual. Materials Used in Making Asphalt Concrete Asphalt concrete is composed primarily of aggregate and asphalt binder. Aggregate typically makes up about 95% of an HMA mixture by weight, whereas asphalt binder makes up the remaining 5%. By volume, a typical HMA mixture is about 85% aggregate, 10% asphalt binder, and 5% air voids. Small amounts of additives and admixtures are added to many HMA mixtures to enhance their performance or workability. These additives include fibers, crumb rubber, and anti-strip additives. Figure 2-1 shows a typical HMA laboratory specimen and the materials used to produce it. Asphalt binder holds the aggregate in HMA together—without asphalt binder, HMA would simply be crushed stone or gravel. Asphalt binder is the thick, heavy residue remaining after kerosene, gasoline, diesel oil, and other fuels and lubricants are refined from crude oil. Asphalt binder consists mostly of carbon and hydrogen, with small amounts of oxygen, sulfur, and several metals. The physical properties of asphalt binder vary tremendously with temperature. At high temperatures, asphalt binder is a fluid with a consistency similar to that of motor oil. At room temperature most asphalt binders will have the consistency of putty or soft rubber. At subzero temperatures, asphalt binder can become very brittle—asphalt samples stored in a freezer will shatter like glass if dropped on a hard surface. Many asphalt binders contain small percentages of polymer to improve their physical properties; these materials are called polymer- modified binders. Much of the current asphalt binder specification used in the United States was designed to control changes in consistency with temperature. This specification and the associated test methods are discussed in more detail in Chapter 3. Because HMA mixtures are mostly aggregate, aggregates used in HMA must be of good quality to ensure the resulting pavement will perform as expected. Aggregates used in HMA mixtures may be either crushed stone or crushed gravel. In either case, the material must be thoroughly crushed, and the resulting particles should be cubical rather than flat or elongated. Aggregates should be free of dust, dirt, clay, and other deleterious materials. Because aggregate particles carry most of the load in HMA pavements, aggregates should be tough and abrasion resistant. Properties of aggregates and the tests that technicians use to evaluate them are discussed in detail in Chapter 4 of this manual. C H A P T E R 2 Background

All HMA mixtures contain small amounts of air voids. In the laboratory, HMA mixtures are usually designed to contain about 4% air voids, with a range of about 3 to 5%, depending on the type of mixture being designed and the design procedure being used. Properly constructed HMA pavements will usually contain about 6 to 8% air voids immediately after placement and compaction. After construction, as traffic passes over a pavement, the HMA in the wheel paths will normally gradually compact to air void levels approaching the design value of 3 to 5%. However, if the pavement is not compacted adequately during construction, compaction under traffic will fail to reduce the air void content to the design value and, as a result, the pavement will be permeable to air and water, potentially leading to moisture damage and excessive age hardening. Asphalt Concrete Pavements Asphalt concrete pavements are not simply a thin covering of asphalt concrete over soil— they are engineered structures composed of several different layers. Because asphalt concrete is much more flexible than portland cement concrete, asphalt concrete pavements are sometimes called flexible pavements. The visible part of an asphalt concrete pavement, the part that directly supports truck and passenger vehicles, is called the surface course or wearing course. It is typically between about 40 and 75 mm thick and consists of crushed aggregate and asphalt binder. Surface course mixtures tend to have a relatively high asphalt content, which helps these mixtures stand up better to traffic and the effects of sunlight, air, and water. Surface course mixtures also are usually made using maximum aggregate sizes less than 19 mm, which helps to provide for a quiet ride. Also, using aggregate sizes larger than 19 mm can make it more difficult to obtain mixtures with sufficient asphalt binder contents to provide adequate durability for surface course mixtures, since the lower aggregate surface area of these aggregates results in a lower demand for asphalt binder. On the other hand, the lower binder content needed for these mixtures can make them more economical than mixtures made using smaller aggregates. Below the surface course of a flexible pavement is the base course. The base course helps provide the overall thickness to the pavement needed to ensure that the pavement can withstand the projected traffic over the life of the project. Base courses may be anywhere from about 100 to 300-mm thick. In general, the higher the anticipated traffic level on a pavement, the thicker Background 5 Figure 2-1. A compacted HMA laboratory specimen and the aggregate and asphalt used to prepare it.

the pavement must be, and the thicker the base course. Thicker pavements will deflect less than thinner ones under traffic loading, which reduces strains within the pavement and makes them more resistant to fatigue cracking. Traditionally, base course mixtures have been designed using larger aggregate sizes than surface course mixtures, with maximum aggregate sizes ranging from about 19 to 37.5 mm. This helps to produce a lean mixture with low asphalt binder content, which helps keep the cost of these mixtures low. Also, using larger aggregate sizes allows base course mixtures to be placed in thicker lifts, which can reduce construction costs. However, many engineers have recently been designing base course mixtures more like surface course mixtures— with smaller aggregate sizes and higher asphalt binder contents. Using these types of mixtures in base course mixtures can help improve both fatigue resistance and resistance to moisture damage, since increased asphalt binder contents in HMA tends to improve fatigue resistance and will also reduce permeability to water. Sometimes an intermediate course is placed between the surface and base courses of a flexible pavement system. This is sometimes called a binder course. Typically 50 to 100 mm in thickness, it consists of a mixture with intermediate aggregate size and asphalt binder content. The surface, base, and intermediate courses together are referred to as bound material or bound layers, because they are held together with asphalt binder. In a typical asphalt concrete pavement, the bound layers are supported by a granular subbase that in turn lays over the subgrade. Granular subbase is crushed stone or gravel, usually 100 to 300 mm in thickness. The nominal maximum aggregate size varies, but it should always be well compacted prior to placement of the base course. The subgrade is the soil on which the pavement is constructed. If the soil is stable and strong, it may only need compaction prior to placing the granular subbase and remaining pavement layers. However, some soils, including many soils containing clay and clay-like minerals are unstable—that is, they shrink and swell significantly when their moisture content changes, and they can also become very weak when the moisture content becomes too high. Before pavement construction, such a subgrade should be stabilized by blending in lime, portland cement, or other additives, or treating it with asphalt emulsion, and then thoroughly compacting the soil. Sometimes the granular subbase is omitted from a pavement and a relatively thick base course is placed directly on the subgrade soil. Such a pavement structure is called a full-depth asphalt pavement. The advantage of this type of construction is that the overall pavement can be thinner because of the increased strength and stiffness of the supporting pavement. However, it should be remembered that such a base course mixture will be significantly more expensive than granular subbase, since it contains asphalt binder. Figure 2-2 is a cross section of a typical flexible pavement system. 6 A Manual for Design of Hot Mix Asphalt with Commentary Surface course Intermediate course Base course Granular subbase Subgrade 40-75 mm 50-100 mm 100-300 mm 100-300 mm Figure 2-2. Typical asphalt concrete pavement structure. In many cases, the intermediate course is omitted; full-depth asphalt pavements do not include a granular subbase.

How Asphalt Concrete Pavements Fail Rutting Rutting (often referred to as permanent deformation) is a common form of distress in flexible pavements. When truck tires move across an asphalt concrete pavement, the pavement deflects a very small amount. These deflections range from much less than a tenth of a milli- meter in cold weather—when the pavement and subgrade are very stiff—to a millimeter or more in warm weather—when the pavement surface is hot and very soft. After the truck tire passes over a given spot in the pavement, the pavement tends to spring back to its original position. Often, however, the pavement surface will not completely recover. Instead, there will be a very small amount of permanent deformation in the wheel path. After many wheel loads have passed over the pavement—perhaps only a few thousand in a poorly constructed pavement, to 10 million or more for one properly designed and constructed for heavy traffic loads—this rutting can become significant. Severely rutted pavements can have ruts 20 mm or more in depth. Rutting is a serious problem because the ruts contribute to a rough riding surface and can fill with water during rain or snow events, which can then cause vehicles traveling on the road to hydroplane and lose control. Rut depths of about 10 mm or more are usually considered excessive and a significant safety hazard. Figure 2-3 is diagram of rutting in an HMA pavement. Other related forms of permanent deformation include shoving and wash boarding. Shoving occurs at intersections when vehicles stop, exerting a lateral force on the surface of the hot mix causing it to deform excessively across the pavement, rather than within the wheel ruts. Wash boarding is a similar phenomenon but, in this case, the deformation takes the form of a series of large ripples across the pavement surface. Rutting, shoving, and wash boarding can be the result of permanent deformation in any part of the pavement—the subgrade, the granular subbase, or any of the bound layers. Excessive permanent deformation in one or more of the bound layers is the result of an asphalt concrete mixture that lacks strength and stiffness at high temperatures. Several problems with a mix design, such as selecting an asphalt binder that is too soft for the given climate and traffic level, can make it prone to rutting and other forms of permanent deformation. Relationships between mixture composition and pavement performance are discussed in detail in Chapter 7. Fatigue Cracking Like rutting, fatigue cracking results from the large number of loads applied over time to a pavement subject by traffic. However, fatigue cracking tends to occur when the pavement is at moderate temperatures, rather than at the high temperatures that cause rutting. Because the HMA at moderate temperatures is stiffer and more brittle than at high temperatures, it tends to crack under repeated loading rather than deform. When cracks first form in an HMA pavement, Background 7 Figure 2-3. Sketch of rutting in a flexible pavement.

they are so small that they cannot be seen without a microscope. The cracks at this point will also not be continuous. Under the action of traffic loading, these microscopic cracks will slowly grow in size and number, until they grow together into much larger cracks that can be clearly seen with the naked eye. Severe fatigue cracking is often referred to as “alligator cracking,” because the pavement surface texture resembles an alligator’s back. These large cracks will significantly affect pavement performance, by weakening the pavement, contributing to a rough riding surface, and allowing air and water into the pavement, which will cause additional damage to the pavement structure. Eventually fatigue cracking can lead to extensive areas of cracking, large potholes, and total pavement failure. Traditionally, pavement engineers believed that fatigue cracks first formed on the underside of the HMA layers, and gradually grew toward the pavement surface. It has become clear during the past 10 years that pavements are also subject to top-down fatigue cracking, where the cracks begin at or near the pavement surface and grow downward, typically along the edges of the wheel paths. It is likely that most HMA pavements undergo both bottom-up and top-down fatigue cracking. However, as HMA pavements have become thicker and as HMA overlays on top of portland cement concrete pavements have become more common, top-down cracking has become more commonly observed than bottom-up cracking. Figure 2-4 illustrates both bottom-up and top-down fatigue cracking. Although fatigue cracking in HMA pavements is still not completely understood, most pavement engineers agree that there are several ways mixture composition can affect fatigue resistance in HMA pavements. One of the most important factors affecting fatigue resistance is asphalt binder content—HMA mixtures with very low asphalt contents tend to be less fatigue resistant than richer mixtures. Poor field compaction also contributes significantly to surface cracking by reducing the strength of the pavement surface. High in-place air void content will also increase pavement permeability, which will then allow air and water into the pavement, both of which can damage the pavement and increase the rate of fatigue cracking. Relationships between fatigue cracking and HMA mix design are discussed in more detail in Chapter 6. Low-Temperature Cracking Temperature has an extreme effect on asphalt binders. At temperatures of about 150°C (300°F) asphalt binders are fluids that can be easily pumped through pipes and mixed with hot aggregate. At temperatures of about 25°C (77°F), asphalt binders have the consistency of a stiff putty or soft rubber. At temperatures of about −20°C and lower, asphalt binders can become very brittle. 8 A Manual for Design of Hot Mix Asphalt with Commentary Figure 2-4. Bottom-up (left) and top-down (right) fatigue cracking.

As a result, HMA pavements in many regions of the United States and most of Canada will become very stiff and brittle during the winter. When cold fronts move through an area causing rapid drops in temperature, HMA pavements can quickly cool. Like most materials, HMA tends to contract as it cools. Unlike portland cement concrete pavements, flexible pavements have no contraction joints and the entire pavement surface will develop tensile stresses during rapid drops in temperature in cold weather. When the pavement temperature drops quickly enough to a low enough temperature, the resulting tensile stresses can cause cracks in the embrittled pavement. These low-temperature cracks will stretch transversely across part or all of the pavement, their spacing ranging from about 3 to 10 meters (10 to 40 feet). Although low-temperature cracking may not at first cause a significant problem in a pavement, the cracks tend to become more numerous and wider with time and cause a significant perfor- mance problem after several years. Low-temperature cracking in HMA pavements can be minimized or even eliminated by proper selection of asphalt binder grade. In fact, one of the main reasons for the development of the current system for grading asphalt binders was to help prevent low-temperature cracking. This grading system, and how it is used to select binders that are both resistant to rutting and low-temperature cracking, is discussed in Chapter 3. Moisture Damage Water does not flow easily through properly constructed HMA pavements, but it will flow very slowly even through well-compacted material. Water can work its way between the aggregate surfaces and asphalt binder in a mixture, weakening or even totally destroying the bond between these two materials. This moisture damage is sometimes called stripping. Moisture damage can occur quickly when water is present underneath a pavement, as when pavements are built over poorly drained areas and are not properly designed or constructed to remove water from the pavement structure. Even occasional exposure to water can cause moisture damage in HMA mixtures prone to it because of faulty design or construction or poor materials selection. The physicochemical processes that control moisture damage are complex and only now are beginning to be understood. Different combinations of asphalt binder and aggregate will exhibit widely varying degrees of resistance to moisture damage. It is very difficult to predict the moisture resistance of a particular combination of asphalt and aggregate, although HMA produced with aggregates containing a large amount of silica, such as sandstone, quartzite, chert, and some granites, tend to be more susceptible to moisture damage. Proper construction, especially thorough compaction, can help reduce the permeability of HMA pavements and so significantly reduce the likelihood of moisture damage. Anti-stripping additives can be added to HMA mixtures to improve their moisture resistance. Hydrated lime is one of the most common and most effective of such additives. The moisture resistance of HMA mixtures is often evaluated using AASHTO T 283 (often referred to as the Lottman procedure). In this test, six cylindrical HMA specimens are com- pacted in the laboratory. Three of these are subjected to conditioning—vacuum saturation, freezing, and thawing—while the other three are not conditioned. Both sets of specimens are then tested using the indirect tension (IDT) test (see Figure 2-5). The percentage of strength retained after conditioning is called the tensile strength ratio (TSR) and is an indication of the moisture resistance of that particular mixture. Many highway agencies require a minimum TSR of 70 to 80% for HMA mixtures. Engineers and technicians should keep in mind that this test is not 100% accurate and only provides a rough indication of moisture resistance. Research is underway on improved procedures for evaluating the moisture resistance of HMA mixtures. The use of moisture resistance testing in HMA mix design is discussed in greater detail in Chapter 8. Background 9 Figure 2-5. Indirect tension test, as used in moisture resistance testing of HMA mixtures.

Raveling Raveling occurs when tires dislodge aggregate particles from the surface of an HMA pavement. Many of the same factors that contribute to poor fatigue resistance will also contribute to raveling, including low asphalt binder contents and poor field compaction. Because the pavement surface is exposed to water from rain and snow, poor moisture resistance can also accelerate raveling in HMA pavements. Asphalt Concrete Mixtures Asphalt concrete mixtures can be classified in many different ways. Perhaps the most general type of classification is by whether or not the mix must be heated prior to transport, placement, and compaction. HMA concrete, or simply HMA, must be thoroughly heated during mixing, transport, placement, and compaction. The asphalt binder used in HMA is quite stiff at room temperatures, so that once this type of asphalt concrete cools it becomes stiff and strong enough to support heavy traffic. Cold mix asphalt, on the other hand, is normally handled, placed, and compacted without heating. This material can be handled cold because it uses liquid asphalts in the form of emulsions and cutbacks that are fluid at room temperature. Asphalt emulsions are mixtures of asphalt, water, and special chemical additives called surfactants that allow the other two materials to be blended into a stable liquid. When blended with aggregate, the emulsion “breaks,” meaning the asphalt separates from the water and thoroughly coats the aggregate. Cutback asphalts are blends of asphalt binder and petroleum solvents. Once placed, cold mix made with cutback asphalts gradually cure as the solvent evaporates from the asphalt concrete. Many engineers now avoid the use of cutback asphalts because of environmental concerns. Cold mix is economical because it does not require large amounts of energy to heat the mix during production and placement. However, it is difficult to compact thoroughly and in general is not as durable as HMA. Cold mix is sometimes used for base course construction and is also commonly used for patching and repairing pavement. A new, third type of mix—called warm-mix asphalt (WMA)—has recently become increasingly popular. In this type of mixture, various different methods are used to significantly reduce mix production temperature by 30 to over 100°F. These methods include (1) using chemical additives to lower the high-temperature viscosity of the asphalt binder; (2) techniques involving the addition of water to the binder, causing it to foam; and (3) two-stage processes involving the addition of hard and soft binders at different points during mix production. WMA has several benefits, including lower cost (since significantly less fuel is needed to heat the mix), lower emissions and so improved environmental impact, and potentially improved performance because of decreased age hardening. There is some concern that WMA might in some cases be more susceptible to moisture damage, but this has yet to be clearly demonstrated. This manual deals exclusively with HMA of which there are three different major types— dense-graded mixtures, gap-graded mixtures, and open-graded mixtures. Dense-graded mixtures are the most common HMA mix type. The term dense-graded refers to the dense aggregate gradation used in these types of mixtures, which means that there is relatively little space between the aggregate particles in such mixtures. Historically, dense-graded mixtures were popular because they required relatively low asphalt binder contents, which kept their cost down. However, experience has shown that HMA with binder contents that are too low can be difficult to place and compact and may be prone to surface cracking and other durability problems. Therefore, many “dense-graded” HMA mixtures do not use a true maximum density gradation, but use somewhat “open” gradations that deviate slightly from maximum density; such mixtures have more space between the aggregate particles and can be designed to contain more asphalt binder. Mixtures that are somewhat coarser than the maximum density gradation are called coarse- 10 A Manual for Design of Hot Mix Asphalt with Commentary

graded mixtures, while mixtures somewhat finer than the maximum density gradation are called fine-graded mixtures. This terminology can be somewhat confusing, since both coarse- and fine-graded mixtures should be considered variations of dense-graded HMA. A more appropriate terminology is to refer to the three types of dense-graded HMA as dense/dense-graded, dense/ coarse-graded, and dense/fine-graded mixtures. When engineers and technicians first began developing mix designs using the Superpave system in the 1990s, there was a clear trend toward dense/coarse-graded mixtures, in order to increase the rut resistance of HMA pavements. However, in the past few years, many agencies have shifted back toward finer mixtures (dense/dense or dense/fine), to help improve the durability of surface course mixtures. Also, recent research has suggested that dense/fine HMA mixtures can, in most cases, be designed to have just as much rut resistance as dense/coarse mixtures. The procedure for designing dense-graded HMA mixtures given in this manual (in Chapter 8) suggests that a range of gradations be evaluated during the mix design process and that the gradation most effective in meeting the given mixture specifications should be selected. The suggested volumetric requirements do include a slight increase in the allowable range for dust-to-binder ratio and an optional table for high-durability mixtures that includes an even higher dust-to-binder ratio and an increase in minimum VMA; both of these changes will probably reduce the number of dense/coarse-graded HMA mixtures being designed under this system. During the past 20 years, stone-matrix asphalt (SMA) has become increasingly common in the United States and Europe. SMA is a special type of HMA designed specifically to hold up under very heavy traffic. SMA is composed of high-quality coarse aggregate, combined with a large amount of mastic composed of a high-performance asphalt binder, mineral filler, and a small amount of fibers. The aggregate used in SMA contains a large amount of coarse aggregate and a large amount of very fine material (called mineral filler), but not much sand-sized material. For this reason, such aggregates are called gap-graded, and SMA and similar HMA types are referred to as gap-graded mixtures, or gap-graded HMA (GGHMA)—the term used in this manual. A well-developed coarse aggregate structure in combination with a relatively large volume of high performance binder helps ensure that a properly designed SMA mixture will exhibit excellent per- formance. SMA is usually only used on very heavily trafficked roadways, where its excellent per- formance makes it cost-effective despite the high initial investment required to construct SMA pavements. The design of GGHMA mixtures is discussed in detail in Chapter 10 of this manual. Figure 2-6 shows an SMA surface course on a dense-graded HMA base. Background 11 Figure 2-6. SMA surface course on dense-graded HMA base.

Another type of HMA mixture is open-graded friction course (OGFC). OGFC mixtures contain very large amounts of coarse aggregate, with very little fine aggregate or mineral filler. The air void content is much higher than in conventional HMA, typically 15 to 20%. A large amount of high-performance binder is needed to provide adequate stability in these mixtures. OGFC mixtures are usually used as thin overlays, where they can help control noise and limit spray on wet pavements. OGFC mixtures are discussed in detail in Chapter 11 of this manual. HMA Mix Design Methods When HMA pavements first began to be constructed, mixture composition was determined based on the judgment and experience of the contractor or used proprietary mix designs. Some of these pavements performed well, others did not. In the 1930s, Bruce Marshall, an engineer working for the Mississippi Highway Department, developed a more rational system for designing HMA mixtures, which became known as the Marshall mix design method. This method of mix design became common by the 1950s and continued to be widely used through the 1980s. It was adopted for use by the U.S. Army Corps of Engineers (USACE) during World War II and was modified by that agency and by the many state highway departments that eventually chose the Marshall method of HMA mix design. Briefly, the Marshall mix design procedure relies on compacting specimens using a standard drop hammer over a range of asphalt binder contents. The binder content is selected to produce proper air void content and voids in the mineral aggregate (VMA). An essential part of the Marshall design method is the stability and flow test, which is an empirical procedure used to evaluate the strength and flexibility of the HMA mixture. Figure 2-7 shows a mechanical Marshall drop hammer and several Marshall specimens prepared in the laboratory. Francis Hveem, an engineer working for the California Division of Highways at about the same time Bruce Marshall was developing his procedure, developed an HMA mix design method adopted 12 A Manual for Design of Hot Mix Asphalt with Commentary Figure 2-7. Marshall compactor and laboratory specimens for use in the Marshall mix design method.

by many agencies in the western United States. The Hveem method of mix design is unique in its use of the centrifuge kerosene equivalent (CKE) test to determine an initial estimate of the asphalt binder content for a given aggregate. Laboratory specimens are prepared using a kneading com- pactor and then evaluated using a stabilometer test and a swell test. At one time, a cohesiometer test was also used to evaluate HMA properties in the Hveem method. As with the Marshall mix design method, evaluation of mixture volumetrics was an important part of the Hveem procedure. In the late 1980s and early 1990s, the Strategic Highway Research Program (SHRP) was con- ducted by engineers and researchers at various universities and research organizations throughout the United States. The SHRP Asphalt Research Program was a 5-year, $50 million-dollar research program to develop improved test methods and specifications for asphalt binder and aggregates and an improved procedure for HMA mixture design and analysis. The performance grading system now used to specify binders in the United States and other countries was one of the products of SHRP. Another SHRP product was the Superpave system of mix design and analysis, often simply referred to as Superpave. This method is similar to the Marshall system in its use of volumetrics, but laboratory specimens are prepared using a gyratory compactor rather than a drop hammer. Superpave also includes a comprehensive set of requirements for aggregate gradations and property requirements. The Superpave method of mix design was meant to include mixture test methods and an associated computer program that would predict the performance of HMA pavements, as an aid in the design and analysis of mixtures. However, this computer program never produced reliably accurate predictions of rutting and fatigue cracking, and this aspect of Superpave was never implemented, although the Mechanistic-Empirical Pavement Design Guide (MEPDG) is in many ways a continuation of the SHRP efforts for flexible pavements. Figure 2-8 shows the Superpave gyratory compactor (SGC). In many ways, the Superpave system was a success. The performance grading system appears to do a better job of ensuring that asphalt binders have adequate stiffness at high temperature Background 13 Figure 2-8. Superpave gyratory compactor (Courtesy of Pine Equipment Company).

while remaining flexible at low temperatures for a wide range of applications, helping to provide resistance to both rutting and low-temperature cracking in HMA pavements. Very few pavements using mixtures designed using the Superpave system have exhibited excessive rutting. However, recently some highway agencies have expressed concern over surface cracking and high perme- ability in pavements using HMA designed using the Superpave system. Many highway agencies have modified the Superpave system to address these problems and others associated with local materials and conditions. A major research effort, continued since the implementation of SHRP, is under way to refine various aspects of this system, including volumetric requirements, com- paction levels, and specifications for aggregate properties and gradation and finally implement practical mixture performance tests and methods of analysis. This mix design manual is largely based on the Superpave system of mixture design and analysis, but an attempt has been made to address the perceived performance problems associated with some mixtures designed using this system. Also, the results and recommendations of some recent research projects have been adopted where such results have been published and presented to the paving technology community and favorably received. Because local conditions and materials vary significantly throughout the United States and Canada, an attempt has been made to provide more flexibility in the procedure used to design dense-graded HMA mixtures, compared to the Superpave sys- tem. This manual is also much more comprehensive than the procedures given in the original Su- perpave system; it includes design procedures not just for dense-graded HMA, but also for gap- graded or SMA-like mixtures and OGFC types. Because of these changes, the term “Superpave” is not used to describe the procedure, tests, and requirements used in the mix design methods presented in this manual. Bibliography Referenced AASHTO Standards T 283, Resistance of Compacted Asphalt Mixture to Moisture-Induced Damage Other Publications The Asphalt Institute (2007) The Asphalt Handbook (MS-4A), 7th Ed., 832 pp. The Asphalt Institute (2001) Introduction to Asphalt (MS-5), 8th Ed., 74 pp. The Asphalt Institute (2001) Superpave Mix Design (SP-2), 128 pp. The Asphalt Institute (1997) Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types (MS-2), 6th Ed., 141 pp. Brown, E. R., et al., (2004) NCHRP Report 531: Relationship of Air Voids, Lift Thickness, and Permeability in Hot- Mix Asphalt Pavements, Washington, DC, Transportation Research Board, National Research Council, 37 pp. Hot-Mix Asphalt Paving Handbook (1991) AASHTO, FAA, FHWA, NAPA, USACE, American Public Works Association, National Association of County Engineers, James Sherocman (Consultant), USACE Publication UN-13 (CEMP-ET), July. McNichol, D. (2005) Paving the Way: Asphalt in America, NCAT, Auburn, AL, 304 pp. NCAT (1996) Hot-Mix Asphalt Materials, Mixture Design and Construction, NAPA, Lanham, MD, 585 pp. 14 A Manual for Design of Hot Mix Asphalt with Commentary

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary incorporates the many advances in materials characterization and hot-mix asphalt (HMA) mix design technology developed since the conclusion of the Strategic Highway Research Program (SHRP).

The final report on the project that developed NCHRP Report 673 and Appendixes C through F to NCHRP Report 673 were published as NCHRP Web-Only Document 159. The titles of the appendixes are as follows:

• Appendix C: Course Manual

• Appendix D: Draft Specification for Volumetric Mix Design of Dense-Graded HMA

• Appendix E: Draft Practice for Volumetric Mix Design of Dense-Graded HMA

• Appendix F: Tutorial

The companion Excel spreadsheet HMA tool and the training course materials described in NCHRP Report 673 are available for download from the Internet.

In January 2012, TRB released NCHRP Report 714: Special Mixture Design Considerations and Methods for Warm Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot Mix Asphalt with Commentary. The report presents special mixture design considerations and methods used with warm mix asphalt.

In January 2012, TRB released an errata to NCHRP Report 673: Page 41, Table 4-7, and page 123, Table 8-10, respectively, should be replaced with a new table.

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