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Flexible pavement is a type of pavement which is constructed by asphalt concrete. This type of road pavement predominantly consists of three layers above the subgrade. These layers are the surface, base course, and subbase. It is possible in some conditions to construct the pavement without having the subbase. The surface layer is made of asphalt concrete while the base course and the subbase are made of aggregate. Base course aggregate can be unstabilized or stabilized with cement or lime. The subbase acts as the foundation of the entire pavement and ensures that the loads are safely transferred to the subgrade.

It is referred to as flexible pavement since it deforms and rebounds each time  traffic load is removed and reapplied.

Fig 1:Flexible pavement (image source)

The thickness of each layer depends on multiple factors such as what materials are used, the magnitude and number of load repetitions, desired service life, and environmental conditions. Usually, the thickness of surface layer ranges from 1 to 10 inches, while the base course ranges from 4 to 12 inches and the subbase from 6 to 20 inches.

Flexible pavements deteriorate rapidly when compared to rigid pavements and their service life usually varies from 10 to 15 years. They should be designed adequately to fulfill their service life before any failure occurs from excessive distress such as fatigue cracking and rutting.

Causes of pavement distress:-

1-Continuous load repetition: Load repetitions generate vibrations that lead to pavement deterioration and service life reduction.

2-Variable load configuration:axle and wheel configuration varies from one vehicle to another. Axles can be single, tandem, tridem, or multiple and wheels are either single or dual. 

Fig-2: Axle Loads

3-Variable tire pressure: the contact stress that tires exert effect pavement durability. The pressure of passenger cars are between 30 to 35 psi and trucks are between 100 to 115 psi.

4-Traffic growth: Engineers must design pavements that can resist future traffic.

Types of distress:-

1-Fatigue cracking: This type of distress is due continuous bending behavior of asphalt layer which is caused by traffic load repetitions that result in tension stresses and cracks. These cracks usually begin at the bottom of asphalt layer then gradually reaches the top of the layer. At first the cracks are short but later they become longer and interconnect with each other. These cracks will significantly affect the performance of the pavement. There is another form of fatigue cracking in which the cracks propagate from the top to the bottom. The mechanical behavior of this form is not generally understood but it can be due to large contact stresses applied on stiff surface layers.

Fig-3:Fatigue cracking

2- Rutting: is another form of distress which is characterized by permanent deformations at the surface layer which can be caused by unstable HMA (hot mix asphalt), densification of HMA, and excessive settlement of the subgrade.

Fig-4:Surface rutting

3- Roughness: is the formation of irregularities in the surface layer. Roughness can severely lead to the deterioration of car parts.

Fig-5:Road roughness

4- Thermal cracking: When temperatures drop during winter, HMA contracts and because it's not permitted to do so, it will experience tensile stresses. If these stresses are larger than the strength of the material, then thermal cracks will be formed.


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Writer Profile

Mohammad Barzan

BSc- Geotechnical Engineering- Koya University.

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