Study on Deterioration Characteristics and Fracturing Mechanism of Concrete Under Liquid Nitrogen Cold Shock

High-pressure water jet crushing concrete has significant advantages in safety, quality and environmental protection, which has a broad application prospect in the maintenance and reconstruction of concrete building. Nevertheless, it still has some problems such as high threshold pump pressure and large specific energy consumption. Water jet breaking concrete with liquid nitrogen (LN₂) cold shock assistance combined with the low-temperature-induced fracturing and hydraulic impact can effectively reduce the working pressure of water jet and improve the energy utilization rate. On account of the unclear cracking characteristics and mechanism of concrete under the LN₂ cold shock, this research carried out the following systematic research focusing on the key scientific issues above based on mechanical tests, scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR). Results indicate that the total mass of concrete exfoliated blocks after compression failure increases as the LN₂ cold shock time and the number of shock cycles goes up, and the uniaxial compressive strength decreases from 8.27 to 21.96%. Through SEM and NMR analysis, it is found that LN₂ cold shock can cause more micro-cracks to develop inside the concrete, and the pore development increases as the cold shock time and the cycle number increase. Additionally, under the condition of water jet pump pressure of 150 MPa, the maximum width and depth of crater for cold shock of 5 min increase by 41.79% and 20.48%, respectively, and those for cold shock of 10 min increase by 76.72% and 40.43%, respectively, compared with the original sample.