| Hydraulic concrete structures are often subjected to different water pressure conditions during the service,and the difference in water content will affect the mechanical properties of concrete.In the study of concrete moisture content,the dry-based concrete specimens are obtained firstly.When the dry-based specimens are obtained by using different drying methods,the water mass loss ratio,drying time and strength change of concrete are different.In this paper,based on physical experiments and numerical simulations,the drying laws of concrete with different strength grades,cube size and curing age were studied systematically at different temperatures.The microscopic detection method was used to reveal the effect of drying on the interior of concrete,and the reasons of the change of strength were analyzed.Finally,the optimal drying mechanism with the least effect on concrete was explored,which provides a theoretical basis for the selection of the optimal drying mechanism of concrete.The main work and conclusions of this paper are as follows:(1)The concrete specimens with different strength grades of C20,C30,C40 and C50 were dried in a thermostatic electric drier at constant temperatures of 60,80,105,120 and150 ℃,and a modified Page model II was obtained for describing the drying and dewatering process of concrete with different strength grades.The difference in water mass loss ratio of concrete at different drying temperatures is caused by the type of lost water,when the temperature is below 80 ℃,the lost water is mainly free water and adsorption water,when the temperature is above 80 ℃,some of the crystal water started to be removed,and the drying temperature was higher,the more crystal water was removed.The concrete specimens before and after drying were tested for compressive and splitting tensile strength.At the same time,the concrete specimens were analyzed by ultrasonic testing and scanning electron microscopy.The results show that the the change in concrete strength is the result of mutual competition between the damage caused by water evaporation and the density increase caused by drying,which shows the increase of concrete strength in different degrees.(2)Concrete specimens of C30 with different sizes are oven-dried at constant temperatures of 60,80,105,120 and 150 ℃,the drying characteristics and change of strength size effect before and after drying were analyzed.The results show that the maximum drying rate and water loss ratio of the test specimens of different sizes at the sametemperature are basically the same,but the larger the size,the longer the drying time.After isothermal drying,the strength of C30 concrete with different sizes has increases in some extent compared to those before drying.The smaller the size,the greater the effect of temperature on the boundary layer effect and time-dependent effect enhancement,and the greater the strength increase,which shows the obvious size effect of concrete strength.The effective thickness of the concrete crack boundary layer was effect by the temperature,causing the boundary layer coefficient change in the size effect law.By introducing the temperature coefficients,the modified size effect law of C30 concrete was obtained.(3)The C30 concrete specimens with curing ages of 28 d,60 d,90 d and 180 d were isothermal dried at 105 ℃ until no mass changes,and the drying laws were analyzed.It was found that the time required for drying increases with the increase of the age,but the total water loss rate is basically the same.Before and after drying,the specimens were tested for the compressive and splitting tensile strength,combining with the results of ultrasonic testing,the results show that after drying,the concrete strength increases with different curing ages,and the growth rate decreases with the increase of the curing age.(4)The concrete macroscopic model was established.Considering the hydrothermal coupling effect,Fick’s Second Diffusion Law and Fourier Heat Conduction Equation were used to simulate the wet heat conduction in the concrete drying process.The simulated results are in good agreement with the experimental conclusions,which reveals the nature of the water and heat change,and further verifies the accuracy of the tests.(5)Considering the drying and dehydration process of different strength grades and sizes of concrete at different drying temperatures,their compressive and splitting tensile strength and the numerical simulation results,and the drying temperatures and drying time were analyzed.Finally,the optimal drying mechanism with the least effect on concrete with different strength grades and sizes was obtained.For C20,C30,C40 and C50 concrete,the optimum drying temperatures for cubic concrete specimens with a side length of 100 mm are105 ℃,105 ℃,120 ℃ and 125 ℃,respectively,and the corresponding time are 72 h,96 h,74 h and 86 h,respectively.Regardless of cube size,the optimum drying temperatures for concrete with the same strength grade of different sizes are substantially the same.The optimum drying temperature for C30 concrete is still 105 ℃,and the time required for drying increases with the cube size.Among them,the drying time for drying the C30 concrete cubes with side lengths of 70.7 mm,100 mm and 150 mm are 60 h,96 h and 145 h,respectively. |
