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HMA: In certain situations where high base stiffness is desired, base courses
can be constructed using a variety of HMA mixes. In relation to surface course HMA mixes, base course mixes usually contain larger maximum aggregate
sizes, are more open graded and are subject to more lenient specifications.
c. Sub Base Course
The subbase course is between the base course and the subgrade. It functions primarily as structural support but it can also:
1- Minimize the intrusion of fines from the subgrade into the pavement structure.
2- Improve drainage.
3- Minimize frost action damage.
4- Provide a working platform for construction.
The subbase generally consists of lower quality materials than the base course but better than the subgrade soils. A subbase course is not always needed or used.
For example, a pavement constructed over a high quality, stiff subgrade may not need the additional features offered by a subbase course so it may be omitted from
design. However, a pavement constructed over a low quality soil such as swelling clay may require the additional load distribution characteristic that a subbase course
can offer. In this scenario the subbase course may consist of high quality fill used to replace poor quality subgrade Haffman, 2008.
2.1.2 The Cause of Failure
The key to a useful evaluation is identifying different types of pavement distress and linking them to a cause. Understanding the cause for current conditions is extremely
important in selecting an appropriate maintenance or rehabilitation technique. The causes of pavement distresses and deterioration are environmental and
structural. Environmental induced distresses are due to weathering, moisture, and aging. Loading causes structural induced distresses. Pavement deterioration usually occurs from
both loading and weathering Lavin, 2003. Pavement deterioration is usually caused by a combination of factors such as traffic
load, environment, initial design, and quality of construction. Therefore, pavement
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deterioration may result from traffic -induced distress, environmentally associated distress, and the interaction of these two. For example ,rutting and alligator cracking are regarded as
traffic -induced distresses, whereas longitudinal and transverse cracking are viewed as environmental or non -load-related distresses.
Several factors are responsible for the degradation of pavements over time, affecting the service life of the pavement. The initial design of the pavement, based on
anticipated traffic volumes and loads, is a major factor influencing its life. Cumulative traffic volume, especially truck traffic, is another major factor in the life of pavements.
Finally, environmental factors such as moisture infiltration into the supporting base, and heat and cold cycles, affect how well the subsurface is able to support the pavement. The
routine maintenance effort applied to a pavement also affects pavement life. The rate at which pavement deteriorates depends on its environment, traffic loading
conditions, original construction quality, and interim maintenance procedures. Poor quality materials or poor construction procedures can significantly reduce the life of a pavement.
As a result, two pavements constructed at the same time may have significantly different lives, or certain portions of a pavement may deteriorate more rapidly than others. On the
other hand, timely and effective maintenance can extend a pavement’s life. Crack sealing
and seal coating can reduce the effect of moisture in aging of asphalt pavement. With all of these variables, it is easy to see why pavements deteriorate at various
rates and why we find them in various stages of disrepair. Recognizing defects and understanding their causes helps us rate pavement condition and select cost-effective
repairs. The pavement defects shown on the following pages provide a background for this process. Periodic inspection is necessary to provide current and useful evaluation data
Walker, 2002. Environmental due to weathering and aging, and structural caused by repeated
traffic loadings. The temperature is one of the most important factors affecting the design and performance of both flexible and rigid pavements. Temperature variations within
pavement structure contribute in many different ways to distress and possible failure of that structure. Knowledge of temperature effects is essential for the determination of the
design and maintenance requirements especially in the desert climates.
