Research On High Temperature Damage Mechanism Of Alkali Activated Slag Concretes Based On Digital Image Correlation

With the rapid economic development,China has not only accelerated the construction of traditional infrastructure such as transportation,but also made new breakthroughs in the construction of new infrastructure.The high temperature of fire can lead to deteriorat ion of concrete materials and deterioration of components,resulting in insufficient concrete bearing capacity and building collapse,which poses a serious threat to the safety of people’s lives and property.Ordinary silicate cement in the production process of high energy consumption,high pollution;alkali-inspired slag cementitious materials in the production process of low carbon and low consumption,and can make full use of solid waste resources,is an environmentally friendly material,in line with China’s basic national policy of building an "environmentally friendly,resource-saving" society.As one of the most promising new construction materials,most of the existing research on the high temperature mechanical properties of alkali slag concrete is focused on the level of alkali slag net slurry and mortar,and relatively little research is done on the level of concrete.On the other hand,due to the inconsistent deformation of the aggregate and hardened body of concrete under the action of heat,resulting in more serious damage to concrete at the hardened body-aggregate interface,however,there are fewer studies in this area,therefore,the study of its mechanical properties after high temperature and the exploration of the deformation law of high t emperature alkali slag concrete aggregate and hardened body,to explore the high temperature mechanical properties of alkali slag concrete and its damage evolution mechanism,can broaden the slag concrete,provide reference for improving the high temperatu re resistance and ratio design of alkali slag concrete,and provide theoretical support for structural safety assessment and reinforcement of alkali slag concretes.In this project,alkali slag concrete materials were prepared using water glass as alkali exciter.After 90 d of standard curing,axial compression experiments were conducted on alkali activated slag concretes specimens to investigate the mechanical properties(axial compressive strength,modulus of elasticity,stress-strain relationship)of alkali slag concrete in uniaxial compression at room temperature and after experiencing different high temperatures effects(200°C,400°C,600°C,800°C,1000°C),to investigate the effects of water glass alkali concentration,modulus,and the rate of temperature increase and The effects of the water glass alkali concentration and modulus,as well as the rate of temperature increase and the time of constant temperature at the target temperature on the mechanical properties of alkali activated slag concretes were investigated.In addition,the apparent images of concrete specimens in high temperature were taken based on DIC tec hnique for different water-cement ratios(0.40,0.45,0.50)of alkali slag concrete and normal concrete,and the expansion coefficients of aggregates and hardened bodies and the crack lengths at the aggregate-hardened body interface were calculated to investigate the mechanism of concrete damage due to the inconsistent deformation of aggregates and hardened bodies in high temperature.The following conclusions were obtained in this paper:(1)The axial compressive strength and modulus of elasticity of alkal i activated slag concretes gradually decreased with the increase of temperature after high temperature.The decreasing trend gradually slowed down with the increase of temperature after 400℃.The stress-strain curve tends to level off with increasing temperature,and the peak point decreases and shifts to the right.1000°C later,due to the decomposition of most of the hydration products C-S-H gel,the number of cracks inside and on the surface of concrete increases,leading to the deterioration of concrete specimens,and the relative residual axial compressive strength of alkali activated slag concretes after high temperature has disappeared.(2)Before 600°C,the alkali concentration of 6% promotes the hydration of alkali activated slag concretes cementitious material to form a dense matrix structure to resist the damage of alkali slag concrete at high temperature,and the alkali activated slag concretes specimens can maintain a good relative residual axial compressive strength at this time.Under hot temperature conditions,higher water glass modulus and alkali concentration will promote the generation of C-S-H gel of alkali slag cementitious material and lower hydration of matrix,which will intensify the dry shrinkage of alkali slag cementitious material and the loss of mechanical properties of alkali slag concrete after high temperature is more obvious.(3)Faster heating rate can shorten the heating time of alkali activated slag concretes,the internal thermal stress of concrete increases,and the short er heating time is conducive to the maintenance of the residual axial compressive strength;the extension of the constant temperature will aggravate the deterioration of the internal aggregate-hardened body interface of alkali slag concrete,resulting in lower residual axial compressive strength of alkali activated slag concretes.(4)200℃～600℃ range,the water-cement ratio of alkali activated slag concretes and ordinary concretes high-temperature expansion coefficient of the influence of larger.The phenomenon of aggregate expansion and slurry shrinkage is more obvious,which intensifies the deterioration effect of matrix-hardened body and leads to serious structural damage of concrete and a sharp decrease in axial compressive strength.(5)With the increase of temperature,the cracks at the interface of alkali activated slag concretes and ordinary concretes matrix and hardened body gradually increase.Normal temperature ～ 200℃,due to the low temperature,not enough to make a large number of C-SH gel decomposition,the crack length is not obvious.200℃ ～ 600℃ range,the aggregate,hardened body deformation inconsistent and lead to the rapid development of cracks at the interface of the aggregate-hardened body inside the concrete.(6)The DIC technique and displacement extensometer were applied to simultaneously measure the axial compressive strength,modulus of elasticity,and stress-strain relationships of alkali activated slag concretes after the action of room temperature and high temperature,and the numerical results obtained from the two differed very little,which verified the feasibility of the DIC technique to measure the deformation of concrete materials.The images of heated specimens in hot temperature were taken in real time by using a high temperature image acquisition system to obtain the expansion coefficients of aggregates and hardened bodies as well as the crack length changes at the aggregate-hardened body interface in the high temperature test of concrete specimens.