Asphalt Pavement
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Asphalt consists of a mix of bitumen, the sticky binding material derived from crude oil, and gravel. The consistency of the asphalt mixture is important because too much bitumen in the mix lowers the strength, and too much gravel in the mix reduces wearability.
An asphalt pavement system is constructed on a foundation of a base course and properly compacted soil. In certain applications, the asphalt may be constructed directly on top of the soil that is treated with moisture or chemicals.
(a) The surface course also referred to as the wearing course or surface layer, is the upper layer in a concrete roadway and is composed of asphalt.
(b) The main asphalt layer, also referred to as the main supporting asphalt layer, holds the materials above and below it together. It is coarser than the surface layer.
(c) Base course, made up of high-quality aggregates such as crushed gravel and crushed stone mixes, provides support, allows for drainage, and provides resistance to freezing.
(d) Subgrade is compacted soil or dirt that supports the pavement structure. It can become harder and more stable over time.
Common Asphalt Pavement Conditions
Asphalt experiences the effects of climate along with wear and tear in the form of cracks and surface deterioration and requires maintenance over its lifetime. Asphalt pavement failures can present in the form of cracks, ruts, depressions, and potholes.
However, failures in the form of large cracks and deterioration shortly after placement can be attributed to construction defects. Construction defects related to asphalt can be due to deficient engineering design, methods, or materials.
SURFACE SLOPE
Due to the surface characteristics of asphalt pavement, design criteria dictate that asphalt pavement be installed at a minimum slope of 2%, or 1/4-inch per one foot. 3% is preferable. Asphalt pavement sloped less than 2% is hard to construct without the formation of flat areas and drainage low points.
Bird baths (shallow standing water on pavement) are caused by inadequate pavement slope, preventing the surface from properly draining. Low points holding water of only a depth more than the thickness of a coin are hazardous and can lead to slippery or icy conditions. It is often difficult for pedestrians to notice or gauge how deep a shallow depth of standing water is and take the proper precautions when traveling on such a surface.
DRAINAGE
Water is the greatest enemy of asphalt pavement, which is supported by layers of gravel and soil.
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Typically, water enters the subgrade through cracks and edges from adjacent areas. Infiltration of water into supporting materials weakens the bearing capacity of the soil and can result in failure.
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More information about maintaining proper drainage around pavement >>
LONGITUDINAL CRACKS
Longitudinal cracks run parallel to the edge of the roadway and often occur along the centerline. A common cause of this type of crack is shrinkage of asphalt material as it cools.   If not sealed, these openings allow water to infiltrate into the soil and gravel below the asphalt. As a result, these supporting materials lose strength.
TRANSVERSE CRACKS
Transverse cracks form perpendicular to the edge of the roadway and typically occur between sections of asphalt caused by shrinkage of asphalt material as it cools.
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If these cracks are not properly sealed or if they become wider and wider, water infiltrates into the subgrade and weakens the soil underneath, which supports the asphalt layer.
LINEAR STRESS CRACKS
Linear stress cracks on asphalt are a common occurrence as asphalt ages, and requires sealing during regular maintenance.  If not sealed, these openings allow water to seep through and worsen the damage.
BLOCK-SHAPED FATIGUE CRACKS
Possible causes of block-shaped fatigue cracks are:
Inadequate asphalt pavement section (asphalt and/or supporting gravel is not thick enough).
Weak supporting subgrade materials (due to soil type, water intrusion, or inadequate compaction).
Excessive loading (such as heavy trucks, for example).
Another term for this is "alligator cracking."
CRESCENT FATIGUE CRACKS
Crescent cracks occur at the edge of asphalt pavement. Possible causes are:
Inadequate asphalt pavement section (asphalt and supporting gravel are not thick enough)
Weak supporting subgrade materials (due to soil type, water intrusion, or inadequate compaction)
Temperature
COMBINATION CRACKS
Crescent and block cracks often appear together.
SURFACE DISTORTIONS
Surface distortions are caused by loading and/or a weakened subgrade. Asphalt pavement can be weakened by temperature or wetting of the soil and gravel below the asphalt layer.
POTHOLES
Potholes typically form at locations of fatigue cracks. As water infiltrates into the subgrade, the supporting soils are weakened and eventually give out over time.
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Potholes can increase in size very rapidly.
DELAMINATION
Water intrusion between layers of asphalt can cause delamination, which is the separation between layers of asphalt.
SEEPAGE
Seepage is caused by drainage water infiltrating in or flowing out of the soil and gravel below the asphalt layer.
SHRINKAGE
Wide separations between asphalt and edges of concrete are due to the shrinkage of asphalt. If not sealed, these openings allow water to infiltrate into the soil and gravel below the asphalt. As a result, these supporting materials lose strength.
CONSTRUCTION MEANS AND METHODS
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Placement of asphalt in adverse weather (too hot or too cold) can cause the concrete to be weak and can result in damaged.
Asphalt producers that mix and deliver asphalt can add chemicals to the mix to make it stronger or hold up to placement in adverse temperatures. However, these chemicals add cost, so the contractor has an incentive to omit them.
If the subgrade of asphalt pavement is not at the proper depth or not adequately compacted by the contractor, cracks can appear along with the formation of potholes.
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DESIGN STANDARDS FOR ASPHALT
Engineers design asphalt pavement per applicable criteria of a specific Jurisdiction or Agency. These criteria, which dictate required strength and design life, vary; however, a typical target design life is approximately 20 years. The thickness of the asphalt layer and the depth of the supporting subgrade depends on weight loading and the amount of traffic.
The National Asphalt Pavement Association (NAPA) is the only trade association that exclusively represents the interests of the asphalt producer/contractor on the national level with Congress, government agencies, and other national trade and business organizations.
Each state also has its own asphalt association that provides the same type of information. However, the State of California Department of Transportation is a leading source of asphalt design criteria and field studies due to its varying climates.
