Study On The Influence Of Different Temperatures On The Mechanical Properties And Microstructure Of UHPC At Room Temperature

Ultra-High Performance Concrete(UHPC),as a new building material,has good mechanical properties and durability,can fully meet the requirements of the structure for light weight and high strength,and has broad application space and prospects.Among them,the maintenance method of high temperature and pressurization is a more commonly used maintenance method,which can improve the strength of UHPC in a short time,but the maintenance method of high temperature and high pressure will produce microcracks due to the thermal expansion of the matrix,which is not conducive to the development of long-term strength of UHPC;At the same time,hightemperature curing will also increase the brittleness of UHPC,resulting in worse resistance to crack growth in the later stage,which affects the durability of UHPC.In order to solve the problem of high temperature curing adversely affecting strength and reduce the production energy consumption in steam/autoclave curing,this paper uses a combination of experimental research and theoretical calculation to analyze the mechanical properties and microstructure development of UHPC at room temperature.The main research contents of this paper are as follows:(1)Perform compressive and tensile tests on UHPC specimens at 5 curing temperatures at room temperature to obtain the strength variation pattern of UHPC under room temperature curing conditions.The results show that the compressive strength of UHPC increases most significantly at a curing temperature of 25℃,with a compressive strength increase of 21.86%at 25℃ compared to 20℃.Before 20℃,the growth rate of compressive strength for 3-7 days is significantly lower than that for 7-28 days.When the temperature is higher than 20℃,the opposite phenomenon occurs.The compressive strength growth rate increases by 10%to 16.84%for every 5℃ increase in temperature,while the tensile strength does not increase significantly.(2)From the microscopic point of view,the effects of different temperature gradients on the hydration,pore size distribution and microstructure of UHPC at room temperature were discussed,and XRD technology,electron microscopy scanning and mercury pressure test were used to evaluate the hydration products,micromorphology and porosity of UHPC to explore the mechanism of UHPC microstructure change,which provided a theoretical basis for the study of macroscopic mechanical properties.(3)Based on compressive test data and tensile test data,a concrete maturitystrength model is established for predicting concrete strength.Combined with fractal dimension theory,the macroscopic mechanical properties are linked with the microstructure,and the porosity,average pore size and maturity influencing factors are considered,and a multi-parameter fractal dimension model is established to describe the relationship between microstructure and macroscopic mechanical properties,which can provide a theoretical basis for UHPC strength optimization design.