Study on freezing strength characteristics and formation mechanism of frozen soil-concrete interface

Abstract: The mechanical properties of frozen soil-concrete interface have a significant impact on lining stability and long-term service ability of water conservation projects and other infrastructures in cold regions. To investigate characteristic and development mechanism of freezing strength of frozen soil-concrete interface, a series of direct shear tests were conducted on frozen soil-concrete interface under various testing temperatures ((1, (3 and (5 ℃), initial water contents (9.2, 13.1, 17.1 and 20.8%) and normal stresses (50, 100, 200 and 300 kPa). The freezing strength of the frozen soil-concrete interface was divided into two parts, the residual strength and the ice cementing strength. Using the Mohr-Coulomb strength theory, the freezing strength development at the frozen soil-concrete interface was interpreted. Then, cohesion and friction coefficient at the interface of peak strength and residual strength were analyzed. The test results showed that the frozen soil-concrete interface with testing temperature of (5 ℃ performs as strain softening behavior during the shearing. After reaching a peak shear stress, further horizontal displacement increase resulted in post-peak strain softening, causing a reduction in shear stress from peak to residual states. When the testing temperature is (1 ℃, the frozen soil-concrete interfaces with water content of 9.2 and 13.1% showed weak softening behavior. While with water content of 17.1 and 20.8%, the interfaces showed strain hardening behavior. The freezing strength of the frozen soil-concrete interface was affected by the initial water content. The larger the initial water content was, the greater freezing strength of the interface was. This was related to cementing ice increase at the interface with increasing water content. For example, when the testing temperature was (5 ℃, the freezing strength increased from 113 to 399.5 kPa with the initial water content increasing from 9.2% to 20.8%. The ice cementing strength at the interface also increased with testing temperature decreasing. It increased from 4.4 to 111.1 kPa with the testing temperature decreasing from (1 to (5 °C when the initial water content was 13.1% and the normal stress was 100 kPa. With the increase in normal stress, the residual strength of the frozen soil-concrete interface increased. When the initial water content was 20.8% and the testing temperature is (5 °C, the residual strength of the interface increased from 34 to 177 kPa with the normal stress increasing from 50 to 300 kPa. The testing temperature had no obvious influence on the friction coefficient and the cohesion of residual strength. Because the residual strength mainly came from interfacial friction, and the interfacial friction hardly depended on the testing temperature. When the initial water content was 13.1%, the cohesion of residual strength increased from 9.13 to 34.34 kPa and the friction coefficient of residual strength fluctuated between 0.49 and 0.63 with the testing temperature decreasing from (1 to (5 ℃. Relationship between the shear strength and the normal stress followed the Mohr-Coulomb law. A newly formula that describes relationship among the ice cementing strength, the testing temperature and the normal stress was established finally.