Mechanical strength model of engineered cementitious composites with freeze–thaw damage based on pore structure evolution

A freeze-thaw cycle test, nuclear magnetic resonance (NMR) T2 spectrum test, uniaxial tensile test, and uniaxial compression test were designed to study the service conditions of engineered cementitious composites (ECC) in a freeze-thaw (FT) environment in cold areas. The degradation law of the mechanical properties and damage characteristics of the pore structure of ECC with three water-binder ratios (M1: W/B = 0.24, M2: W/B = 028, and M3: W/B = 0.32) are discussed. The results demonstrate that the number of micropores and mesopores in the ECC specimens increases with an increase in the number of freeze-thaw cycles (FTs), thus gradually increasing the porosity. The uniaxial tensile strength (UTS) and uniaxial compressive strength (UCS) decrease gradually with increasing number of FTs. After FTs300, the porosity of the three groups increases by 43.83%, 59.39%, and 59.16%; the loss ratios of UTS are 26.16%, 24.47%, and 19.63%; and the loss ratios of UCS are 31.52%, 27.98%, and 27.30%, respectively. Based on the mechanical properties and pore structure test results of ECC, relation models of the UTS and UCS of freeze-thaw damaged (FTD) ECC with porosity were constructed according to the simplified center hole model of ECC and the elastic mechanics theory. To further disclose the FT evolution laws of uniaxial tensile/compressive strengths (UT/CS), the relationship models of UT/CS and number of FTs with the degree of FTD were deduced. The constructed models and test results were compared with those of some classical porosity-strength models. It was found that the constructed models met the requirements of satisfactory reliability and applications.