Rigid pavement design

A rigid pavement is a slab of paving quality concrete, usually 250 to 350 mm thick on highways, resting on a dry lean concrete or cement-treated sub-base over a prepared subgrade. Because concrete is stiff, the slab spreads load over a wide area, so stresses on the subgrade are low. The governing stresses are in the slab itself: flexural tension from wheel loads at critical positions, combined with curling stresses as the top and bottom of the slab are at different temperatures.

Westergaard's analysis of a plate on a dense liquid foundation introduced the radius of relative stiffness, which depends on slab thickness, concrete modulus and the modulus of subgrade reaction k. It identified interior, edge and corner as critical load positions. Modern Indian design, in IRC 58, checks cumulative fatigue damage under the expected axle load spectrum: for each axle load group, the flexural stress ratio to the concrete's flexural strength sets an allowable number of repetitions, and the sum of expected over allowable must stay below one. Bottom-up cracking (edge loading with daytime curling) and top-down cracking (with night-time curling and axles straddling a transverse joint) are both checked.

Joints control cracking: contraction joints at regular spacing, longitudinal joints between lanes, and expansion or isolation joints at structures. Dowel bars transfer load across transverse joints; tie bars hold lanes together. Concrete grade, flexural strength (often around 4.5 MPa) and good curing are central.