Interface between construction materials and soils plays an important role in many geotechnical systems, including retaining walls, shallow and deep foundations. Interface behavior is studied using split samples consisting of concrete block at the bottom half and granular soil at the top half of the direct shear test. The objective of this research is to assess the effect of aggregate crushing at the concrete-soil interface. To be able to control the large number of parameters affecting the interface behavior, granular soil was manufactured using fly ash by cold-bonding pelletization technique with predetermined shape, size, grain size distribution, surface roughness, water absorption, unit weight and crushing strength. The physical and mechanical properties of these aggregates were investigated by using conventional soil mechanics tests. Interface test results between lightweight aggregates and concrete showed that grain crushing, particle shape, level of normal stress and the surface roughness of the grains play an important role in the interface behavior between granular soils and concrete. At low normal stress values, the crushing strength of aggregates did not affect the interface behavior. For the synthetic aggregates studied, the ratio of interface to internal friction angle decreased with increasing crushing strength.