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Slab support and thus layer stiffness are major factors in the performance of the pavement and the subbase plays a big role in the success of slab support. The stiffness of the subbase in the field usually is lower than measured in the laboratory because of construction practice and curing. A value two thirds of that measured or deduced from laboratory testing is not uncommon.

 

The combined stiffness of the layers below the subbase is expressed in terms of support stiffness Ee. It is calculated by considering stiffness values E and thickness h of the different layers to arrive at an equivalent stiffness Ee of all layers below the subbase combined. Any void that may develop between the slab and the slab support and primarily between the subbase and the slab, will lead to premature failure. A void can be due to slab curling as a result of moisture or thermal differentials, or it can be due to subbase erosion and pumping, or settlement in the underlying layers.

 

Three layers are being considered in the program, namely

 

•the concrete slab,

•subbase, and

•the layers below subbase including the subgrade.

 

The latter consists of three layers and an equivalent stiffness Ee is calculated to represent these three layers. An equation has been derived to calculate Ee but since this equation is regression-based, it cannot handle unconventional lower layer configurations, and may not be accurate enough for certain application, such as thin overlays. The equivalent stiffness is based on the following relationship:

 

 

where      

 

    E5          stiffness of the subgrade

    E4, h4      stiffness and thickness of Layer 4

    E3, h3      stiffness and thickness of Layer 3.

 

In the event that unconventional lower layer configurations are to be constructed, or a more accurate value of Ee is to be obtained, more sophisticated modeling is needed.