19.7. Failure of flexible pavements

The major flexible pavement failures are fatigue cracking, rutting, and thermal cracking.

Flexible pavements can face three key types of failures: fatigue cracking, rutting, and thermal cracking. Fatigue cracking occurs due to repeated stress from the weight of vehicles, leading to horizontal tensile strain at the bottom of the asphalt. Rutting manifests as permanent deformations or grooves that develop along the wheel paths due to the continuous compression of the pavement. Thermal cracking arises from temperature changes affecting the materials in the pavement.

Imagine a rubber band that you stretch repeatedly. Over time, the repeated stretching will cause it to weaken and eventually break, similar to how fatigue cracking occurs in pavements. Additionally, think of a road that has grooves or dips where cars consistently drive over – this is akin to rutting where the surface has permanently deformed. Lastly, consider how water bottles left in a cold car can crack if the temperature drops – this reflects thermal cracking as materials expand and contract with temperature fluctuations.

The fatigue cracking of flexible pavement is due to horizontal tensile strain at the bottom of the asphaltic concrete.

Fatigue cracking is specifically tied to the stress caused by the cyclic loading of vehicle traffic. As vehicles pass over the pavement, they exert loads that create tensions in the asphalt layers. Over time, this repeated tension leads to cracks forming at the bottom of the asphalt layer. Understanding this failure helps engineers design pavements that can withstand specific volumes and types of traffic over their lifespan.

Think of a favorite pair of jeans that you wear often. Over time, the repeated stress of bending and sitting can cause the fabric to wear down and eventually tear. Similarly, the constant pressure from vehicle loads on asphalt eventually results in visible cracks, indicating the pavement's fatigue.

Rutting occurs only on flexible pavements as indicated by permanent deformation or rut depth along wheel load path.

Rutting is characterized by the permanent indentations that form in the pavement surface due to the weight of traffic. These ruts can become hazardous, as they can lead to water pooling on the pavement, reducing friction and increasing the risk of accidents. Two primary design methods are used to control rutting: limiting the vertical compressive strain at the top of the subgrade and maintaining the rut depth to tolerable levels, typically around 12 mm.

Picture a garden path made of soft soil. If you step on it repeatedly in one area, your foot creates a groove in the ground. This is similar to how vehicles create ruts on roads – just as repeated pressure on the soil leads to a visible indentation, heavy traffic causes ruts on flexible pavements.

Thermal cracking includes both low-temperature cracking and thermal fatigue cracking.

Thermal cracking happens when the pavement material expands in heat and contracts when it cools. Low temperature cracking occurs when the temperature drops significantly, causing the asphalt to become brittle and crack. Thermal fatigue cracking comes from repeated cycles of heating and cooling. Both types of thermal cracking should be considered during the design phase to choose materials that can accommodate such changes to reduce the likelihood of damage.

Consider a chocolate bar that you leave in the sun; once you take it out and put it in the fridge, you notice it cracks. This represents thermal cracking in pavements. Just as the chocolate changes state and becomes brittle with temperature changes, asphalt behaves similarly under various thermal conditions.

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