Impact Resistance Characteristics And Mechanism Of Cement-Based Composites Cured At High Temperature Under Cyclic Impact Load

With the increasing buried depth of tunnels,the high temperature construction environment has become an unavoidable problem in tunnel excavation.As an indispensable part of tunnel engineering,lining structure is crucial to construction safety.In deep buried tunnels constructed by drilling and blasting,the lining concrete used for support often faces the coupling effect of high temperature and cyclic explosive loads.Aiming at the high temperature geological environment of deep buried long tunnels,conducting research on the dynamic mechanical properties and action mechanisms of cement based composite materials at different temperatures can provide important support for the sustainable development of tunnel lining concrete and its structures,and is of great significance for ensuring the steady construction of major deep buried tunnel projects in China.Based on the background of a deep buried tunnel in Sichuan and Tibet,three curing temperatures of 20℃,40℃,and 60℃were designed according to the actual working conditions.Using static/dynamic mechanical performance testing techniques,hydration kinetics theory,and microstructure evolution analysis,experiments were conducted to study cement based materials The dynamic mechanical response and mechanism of steel fiber cement based composites and polypropylene fiber（PP fiber）cement based composites under high temperature,single and cyclic impact loads are summarized as follows:（1）Through the study of hydration kinetics,it was found that the maximum exothermic peak value of the water mud based material at 40℃was 0.0027 w/g,which was 74.1%higher than that at 20℃.That is,when the curing temperature was 40℃,it was beneficial to promote the hydration reaction.Under standard curing conditions of 20℃,the cumulative heat release of steel fiber cement based composites is 11.45%lower than that of PP fiber,which can reduce the risk of tunnel lining cracking.（2）Through a single impact test,it was found that the steel fiber cement based composite material had the slowest crack growth and the smallest number of cracks under the curing condition of 40℃.At the same time,the dynamic compressive strength of different cement-based composites is improved at this temperature,and the strength value of the steel fiber system is the highest.In addition,when the strain rate is 105s^-1 at this temperature,the toughness of the steel fiber system is 40.3%higher than that at 20℃.（3）Through cyclic impact test research,it was found that macroscopic cracks first appeared on the local weak surface at the edge,and expanded with the increase of impact pressure and impact times,resulting in increased cumulative damage.At 40℃,the two fiber systems have fewer cracks and flaking,and have strong resistance to cyclic impact;The average toughness of PP fiber system at medium and high strain rates(80s^-1,90s^-1)at this temperature is the highest,with an average increase of 10.4%compared to room temperature curing.（4）As the strain rate increases,increasing the ductility of the material is more important for improving the toughness index.The average toughness of PP fiber cement based composites with low elastic modulus increases more significantly.With the increase of strain rate,the average toughness of PP fiber system increases more significantly than that of steel fiber system by enhancing the overall strength of cement based composites.