Last reviewed 16 Sept 2026 · 10 min read
Components of a rigid pavement
| Layer | Function / typical specification |
|---|---|
| Pavement quality concrete (PQC) | Main structural slab carrying loads by flexure; high-strength concrete (commonly M40, design flexural strength about 4.5 MPa at 28 days) |
| Separation membrane / debonding layer | Polythene sheet or bitumen layer between PQC and sub-base to reduce friction and restraint cracking |
| Dry lean concrete (DLC) sub-base | Uniform, non-erodible support; commonly about 150 mm thick with a minimum compressive strength (e.g. 10 MPa) |
| Granular sub-base / drainage layer | Drains water, prevents pumping, provides construction platform |
| Subgrade | Compacted soil; strength expressed by modulus of subgrade reaction |
Unlike flexible pavements, the thickness of a rigid pavement depends mainly on the flexural strength of concrete; subgrade support has a relatively smaller effect.
Design factors
- Wheel load and axle configuration — axle load spectrum, tyre pressure, contact area.
- Traffic — number of repetitions of different axle loads over the design period (commonly 30 years for rigid pavements).
- Modulus of subgrade reaction — from plate load tests (or from CBR correlations), increased for the effect of DLC/granular sub-base (effective k).
- Concrete properties — flexural strength (modulus of rupture), modulus of elasticity (about 30 000 MPa), Poisson's ratio (0.15), coefficient of thermal expansion (about per °C).
- Temperature differential between top and bottom of the slab — causes warping.
Radius of relative stiffness
= modulus of elasticity of concrete; = slab thickness; = Poisson's ratio; = modulus of subgrade reaction.
It measures the stiffness of the slab relative to the subgrade — the distance over which the slab spreads the load.
Equivalent radius of resisting section
For a load of contact radius on a slab of thickness (thin-plate theory correction):
Load stresses (Westergaard)
Westergaard considered three critical positions of the wheel load:
- Interior — load well away from edges and corners.
- Edge — load at the edge, away from corners.
- Corner — load at the corner of the slab.
( = wheel load in kg; in cm; , , in cm; stresses in kg/cm²)
Interior:
Edge:
Corner:
- Interior and edge stresses are tensile at the bottom of the slab; corner stress is tensile at the top.
- Edge load stress is generally the most critical of the load stresses (without load transfer).
Temperature stresses
Warping stresses
A temperature differential between the top and bottom of the slab makes it curl (warp); self-weight and subgrade restraint resist this, inducing stresses.
- Day (summer, mid-day): top hotter → slab tends to curl down at edges → tension at the bottom (adds to wheel load tension at the edge).
- Night (winter): top cooler → edges curl up → tension at the top (adds to corner load stress).
Interior:
Edge: (or with , whichever is greater)
Corner:
= coefficient of thermal expansion; = temperature differential; , = Bradbury's coefficients depending on and (slab length and width ÷ radius of relative stiffness), read from Bradbury's chart.
Frictional stresses
Uniform temperature change makes the slab expand or contract; friction with the sub-base restrains it.
( in kg/cm²; = unit weight of concrete, kg/m³; = slab length, m; = coefficient of friction, about 1.5.) This governs the spacing of contraction joints.
Critical combinations
| Condition | Critical combination |
|---|---|
| Summer, mid-day | Edge load stress + warping stress at edge (bottom tension) − frictional stress |
| Winter, mid-day | Edge load stress + warping stress + frictional stress (contraction) |
| Night | Corner load stress + corner warping stress (top tension) |