Experimental characterization of sliding and shear mechanisms in EPS-insulated foundations for seismic loading conditions

Thermally insulated foundation systems with expanded polystyrene (EPS) layers placed beneath reinforced concrete slabs are increasingly used in practice. However, their mechanical behavior under combined normal and cyclic shear loading is not sufficiently documented for structural and seismic analyses. This study presents an experimental investigation of the frictional, shear, and compressive response of EPS-insulated foundation assemblies. Monotonic and cyclic shear tests were performed on single-layer and double-layer EPS configurations, including button–groove connections. The specimens were placed over a waterproofing membrane either freely or bonded with adhesive. The results showed that sliding at the EPS–waterproofing interface governs the response of the assembly. The shear capacity of button–groove connections (approximately 80–100 kPa) and of homogeneous EPS layers (approximately 170 kPa) exceeded the shear stress at sliding initiation. This indicates that failure within EPS material or at the interlayer connection is not expected prior to interface sliding. Static and kinetic friction coefficients were determined as 0.61 and 0.58 for freely placed insulation, and 0.96 and 0.65 for bonded insulation. Limited cyclic tests provided qualitative information on the hysteretic response of the interface. The results suggest that the envelope response may be approximated by a simple elasto-plastic relationship. Compressive tests confirmed that EPS400 panels reached strengths between 440 and 487 kPa at 10% strain and exhibiting progressive densification without brittle failure.