Effect of Nonuniform Foundation Support on Concrete Slab Responses

The ability to spatially map the stiffness of the foundation layers under concrete pavements with intelligent compaction has created a need to define limits on the allowable stiffness variations in a given construction area as well as the maximum size of a non-complaint area which does not require remedial action. In order to work towards this goal, a two-dimensional finite element analysis was completed to determine the effects of four nonuniform subgrade support conditions (relative to uniform support) on the tensile stresses in a concrete slab under tandem axle loading and temperature curling. The slab responses showed that certain nonuniform subgrade support conditions have a greater impact on the slab tensile stresses relative to the uniform support irrespective if the slab is experiencing curling. A soft support along the longitudinal slab edge and a subgrade with randomly assigned 9 ft2 (0.84 m2) soft and stiff areas were found to decrease the slab performance relative to a uniform subgrade assumption. In the most extreme case, the nonuniform support with soft longitudinal edges had a maximum stress that was 63 percent greater than the uniform soft support. In this analysis, subgrade nonuniformities produced the potential for bottom-up or top-down fatigue cracking depending on the interaction of the axle location, temperature differential, and local support condition.