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OCR for page 7
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 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.
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.
