Slab thickness in IRC 58 is found iteratively: a trial thickness is checked so the cumulative fatigue damage (bottom-up + top-down) stays ≤ 1 for the design traffic, on a foundation characterised by the effective modulus of subgrade reaction k. k is enhanced by the sub-base (DLC/GSB) over the subgrade — the foundation stiffness and the slab thickness are coupled, not independent.
Key Requirements
•Characterise the foundation by the effective modulus of subgrade reaction k (subgrade CBR → k, enhanced by the sub-base)
•Adopt a trial slab thickness; compute load + temperature stresses for the axle spectrum
•Iterate thickness until cumulative fatigue damage ≤ 1 for both bottom-up and top-down checks
•Provide the specified sub-base (DLC/GSB) — it raises effective k and provides a stable construction platform
•Account for the design flexural strength, k, axle spectrum and temperature differential together (coupled design)