Characterization of restraint from friction tests with slabs cast on ground

External restraint of concrete structures is important for the control of cracking. The horizontal restraint for structures on ground originates from frictional forces, but the knowledge of the frictional behaviour is limited. Friction tests were performed on concrete slabs cast on ground materials such as sand and crushed aggregate. Different levels of uniformly distributed loads on the slab were used and the load–slip relationship and coefficients of friction were examined. Load–slip relationships are non-linear and the coefficient of friction has a peak value for compacted materials and then decreases to an almost constant value. The maximum coefficient of friction, μ_max, decreases with increasing ground pressure, which means that the frictional process can not be described as pure friction. Increasing the size of particles increases the coefficient of friction; conversely, plastic sheeting reduces it. The value of μ_max is about 1.0 for sand and between 1.5 and 3.0 for crushed aggregate.