Factors Affecting Performance of Flexible Pavements

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Traffic

Traffic has a major effect on pavement performance.Traffic characteristics that affect performance are traffic load, traffic volume, tyre pressure, and vehicle speed. Traffic load produces stresses and strains within the pavement structure and the subgrade, which gradually contribute to the development of pavement distresses. For example, heavier loads result in higher potential for fatigue cracking and rutting. Traffic volume affects pavement performance since larger number of load repetitions increases the chance for fatigue cracking. Also, higher tyre pressure produces higher stress concentrations at the pavement surface that could result in rutting and shoving in the HMS layer. Finally, vehicle speed affects the rate of applying the load. Since asphalt concrete is a visco-elastic plastic material, its response is affected by the rate of load application. Slow or stationary vehicles have more chances of developing rutting and shoving than high-speed vehicles. On the other hand, high travel speeds cause more severe bouncing of vehicles, and result in larger dynamic loading and increased roughness.

Soil and Pavement Materials

Soil and pavement materials significantly affect pavement performance. Of course, high quality materials are needed to provide good support to traffic loads under various environmental conditions. Important material properties include mechanical properties such as elasticity, visco-elasticity, plasticity, temperature susceptibility, durability and aging characteristics. These properties affect how the material responds to traffic loads and environmental conditions such as temperature, freezethaw effect, and rain.

Environment

Environmental conditions that affect pavement performance include moisture, temperature, and

their interaction. For example, moisture may reduce subgrade support and weakens various pavement layers. High temperatures soften asphalt concrete and could create rutting within the

surface layer. Temperatures below freezing have a bad effect on pavement performance, especially cycles of freeze and thaw, if the subgrade is wet and the temperature drops below freezing, ice lenses form. Since these ice lenses have larger volume than water, frost heave will develop and may create bulges in the pavement surface. If the temperature fluctuates above and below freezing points in the same season combined with poor drainage, subgrade support will be significantly reduced and could result in excessive deterioration of the pavement structure within a short period of time.

Construction and Maintenance Practice

In many cases, defects in pavement start during construction and propagate during service. In fact, poor construction procedure will almost always ensure poor pavement performance. For example, poor compaction of subgrade or any pavement layer allows excessive further compaction by traffic, which appears in the form of rutting and surface cracking. Poor placement of HMA during construction may result in weak transverse or longitudinal construction joints that are susceptible to early cracking and deterioration. Excessive air voids in the HMA layer due to poor compaction will result in fast aging followed by cracking. In contrast, too much compaction of HMA will result in too small amount of air voids that could create rutting or bleeding. Lack of smoothness of the pavement during construction increases the dynamic impact of traffic, and consequently, speeds up the rate of developing roughness during service.