The design of bituminous pavements, using mechanistic-empirical design method, involves the selection of thicknesses of the bituminous layer and the granular layers to keep the strains at critical locations of the pavement structure within the allowable limits arrived from performance equations. The tensile strain under the bituminous layer defines the fatigue life, and compressive strain on top of the subgrade defines the rutting life of the pavement based on the performance equations adopted. It is generally considered that increasing the thickness of the bituminous layer decreases the strain at the bottom of bituminous (binder) layer. So, most of the pavement designers increase the thickness of the bituminous layer to restrict the strains. However, this is not always true. In fact, the tensile strain at the bottom of the bituminous layer increases with the increase in the thickness of bituminous layer up to a critical thickness and then slowly starts decreasing as the thickness further increases. The critical thickness of bituminous layer ranges between 50 mm and 100 mm. In the majority of the cases, the critical tensile strain under the bituminous layer corresponding to the thicknesses of the bituminous layers of 25 mm and 200 mm are very close. Apart from the economy, the problem of mix rutting is greatly minimized in the case of thin bituminous surfacing (TBS). The issues in rehabilitation and recycling of pavement materials are very less as the granular layer can be reused without much processing requirements.
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