Effect of Temperature Amplitude and Cycling on the Shear Behavior of a Concrete–Reconstituted Silty Clay Interface

The behavior of soil–structure interfaces under varying thermal conditions is crucial for the design and stability of geotechnical structures subjected to temperature fluctuations, such as energy piles and underground structures. This study investigates the impact of temperature variation and cycling on the shear strength of the concrete–reconstituted silty clay interface using a custom-designed direct shear apparatus equipped with a temperature control system. Three test series were considered: constant temperatures of 30, 40, 50, and 60 °C; single heating–cooling paths of 25–30–25, 25–40–25, and 25–50–25 °C; and one to three cycles between approximately 25 and 50 °C. Normal stresses of 100–400 kPa were applied, and shearing was conducted at 0.02 mm/min. The interface generally exhibited a strain-hardening response, and its terminal shear stress increased with normal stress but decreased with increasing temperature. In the constant-temperature series, heating from 30 to 60 °C produced a maximum measured reduction of 12.72% in interface shear stress. The apparent interface adhesion decreased from 17.09 to 9.38 kPa, a reduction of 45.11%, while the apparent interface friction angle decreased from 38.66° to 36.50°. Increasing the single-cycle temperature amplitude reduced the fitted apparent interface adhesion and friction angle by 21.65% and 3.05%, respectively. From one to three thermal cycles, these parameters decreased by 18.45% and 9.07%, respectively. The results indicate that temperature level, heating–cooling amplitude, and thermal cycling can modify the measured interface response. The findings highlight the need to consider representative thermal histories when characterizing interfaces for thermally active geotechnical systems.