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Background 7 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, Figure 2-3. Sketch of rutting in a flexible pavement.

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8 A Manual for Design of Hot Mix Asphalt with Commentary 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 150C (300F) asphalt binders are fluids that can be easily pumped through pipes and mixed with hot aggregate. At temperatures of about 25C (77F), asphalt binders have the consistency of a stiff putty or soft rubber. At temperatures of about -20C and lower, asphalt binders can become very brittle. Figure 2-4. Bottom-up (left) and top-down (right) fatigue cracking.

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Background 9 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 Figure 2-5. Indirect this test is not 100% accurate and only provides a rough indication of moisture resistance. tension test, as used Research is underway on improved procedures for evaluating the moisture resistance of HMA in moisture resistance mixtures. The use of moisture resistance testing in HMA mix design is discussed in greater testing of HMA detail in Chapter 8. mixtures.