Will using a thin sand layer under floor slabs reduce subgrade or subbase support, causing cracking and poor joint performance, especially under repeated loadings such as forklift traffic? To gather data to help answer this question, The Aberdeen Group performed California Bearing Ratio tests on three soil samples with varying dry densities. Each test specimen was tested with no sand, a 1-inch sand layer and a 2-inch sand layer. In addition, 1- and 2-inch sand layers were placed over a steel base and tested to show how the sand would affect subgrade support over a very stiff base. For all the soil samples tested, CBR values decreased dramatically when a thin layer of loose sand was placed over the compacted sample. The decrease was especially large for the sand layer placed over the steel base.

If specifiers require contractors to place concrete over a sand layer, without an increase in the slab thickness, more later-age cracking and poorer joint performance may result, especially for slabs subjected to heavy construction loads, such as cranes or concrete trucks. Contractors should discuss these implications with the architect and engineer before the project begins, and request that the sand layer be replaced with a compactible stone base.