Strength and Stiffness Considerations

The strength of the rigid pavement foundation is frequently quantified by the modulus of subgrade reaction (known as the k-value). One of the key assumptions in the Westergaard design equations is that the vertical stress providing support to the PCC pavement is directly proportional to the vertical deflection of the slab. Conceptually, the concrete slabs are supported on a spring-like or dense liquid subgrade foundation. The k-value is determined by measuring the volume of the deflection basin that results from the applied load. By taking the magnitude of the load and dividing it by the volume of the deflection basin, the k-value is determined in units of pounds per cubic inch.

Placing a base or a subbase layer may provide improved protection of the subgrade, a stronger support to the PCC slabs, and result in an increased composite k-value. However, an exact k-value of the foundation is not typically required because the design thickness of the PCC is not significantly affected within the typical k-value ranges achieved by the subgrade and the base layers. The PCC slabs provide most of the structural capacity needed for the pavement.