Flexible pavement design

A flexible pavement spreads wheel loads through layers of decreasing stiffness from the surface down to the subgrade. It deflects under load and recovers, unlike a concrete slab that acts as a stiff plate. Design aims to keep two critical strains within limits over the design traffic: horizontal tensile strain at the bottom of the bituminous layer, which causes fatigue cracking, and vertical compressive strain at the top of the subgrade, which causes rutting.

Indian design follows IRC 37, a mechanistic-empirical method. The designer establishes design traffic in msa, the subgrade resilient modulus, and the materials and layer types proposed. A multi-layer linear elastic analysis (IITPAVE software accompanies the guidelines) calculates strains under a standard dual-wheel load for a trial section. Fatigue and rutting models give the number of repetitions each strain can withstand, using reliability factors of 80 or 90 percent depending on traffic. The trial section is adjusted until both checks pass, and the guidelines include catalogue sections for common cases.

Layer options include granular sub-base and base, cement-treated sub-bases and bases with stress-absorbing interlayers, reclaimed asphalt bases and bituminous layers of DBM and BC or SMA. Drainage layers protect the structure, and minimum thicknesses apply. Good design is wasted without construction quality: compaction, layer thickness, bitumen content and temperature control decide actual life.