Study On High Temperature Damage And Mass Transport Properties Of High-strength Self-compacting Lightweight Aggregate Concrete

The application of lightweight aggregate concrete(LWAC)not only effectively alleviates the crisis of resources and environment caused by the over-exploitation of natural aggregates such as rocks,but also has significant advantages for thermal insulation and fireproofing of building structures.However,the problems of low strength and large porosity of LWAC are the main reason for limiting its application and development.The emergence of self-compacting concrete(SCC)not only solves the problem of low strength but also facilitate the construction of high-rise and super high-rise buildings.In recent years,the frequent occurrence of building fire accidents has put forward higher requirements on the performance of concrete materials,which requires not only the characteristics of light weight and high strength,but also the characteristics of high temperature resistance and good insulation effect.Therefore,if the LWAC is combined with self-compacting technology to prepare a high-strength self-compacting lightweight aggregate concrete(HSSC-LWAC),it is an effective and feasible way to solve the problems of poor mechanical properties of LWAC.In order to make LWAC meet the requirements of modern concrete structural engineering,it is necessary to conduct an in-depth study on the high temperature damage mechanism and mass transport properties of HSSC-LWAC.Based on this,the experimental and numerical analysis methods were used to investigate the durability deterioration of high-temperature damaged HSSC-LWAC.The specific work and the main conclusions are shown as follows:(1)Two different forming methods of lightweight aggregates,namely the cloud concrete stone aggregate(CCS)and fly ash ceramic(FAC)aggregates were chosen to prepare HSSC-LWAC was prepared by replacing natural coarse aggregates(the replacement rates of 0,30%,50%,100%,i.e.NC,CCSC30,CCSC50,CCSC100,FACC30,FACC50,FACC100),and the tested results of workability meet the requirements of SCC.Further,the experimental studies on the damage deterioration and microstructural characterization of HSSC-LWAC before and after high temperatures were carried out.The mechanical properties and damage evolution of HSSC-LWAC after exposure to different high temperatures(20°C,200°C,400°C,600°C,800°C)were also investigated.Combined with microscopic test methods,the physical-chemical reactions and phase transformation characteristics of high-temperature damaged specimens were analyzed during the internal heating.The study shows that the compressive strength of specimens first increased and then decreased as the temperature was elevated,and the compressive strength increases obviously at 200°C.The high temperature resistance of CCS concrete was integrally better than that of FAC concrete.It indicates that the combination of LWA and mortar matrix embedding makes ITZ denser and high temperature leads to secondary hydration and "steam curing" phenomenon.The increase of temperature causes the products decomposition and crystalline transformation,the connection of microcracks and loosened microstructure to lead to interface separation,but the interface performance of concrete with 100% replacement of LWA was better.When the temperature reaches 800°C,the residual compressive strength of CCSC100 and FACC100 was respectively increased by10.9% and 10.6% compared to NC group.(2)In order to further analyze the high temperatures damage mechanism of HSSCLWAC,a mesoscale model of two-dimensional HSSC-LWAC with random distribution of circular LWA was developed by Monte Carlo method considering the aggregate size effect and random distribution characteristics.Based on the Fourier heat conduction equation,the internal heat transfer process of HSSC-LWAC at high temperature and the distribution of stress damage under axial compressive loads were simulated to achieve the visualization of internal temperature field distribution inside concrete.Combined with the plastic damage intrinsic model of concrete,a coupled heat-force damage model of HSSC-LWAC was established to analyze the degradation mechanism of mechanical property of HSSCLWAC after high temperature damage.The study shows that the temperature gradient inside specimens is increased and the LWA effectively decrease the overall thermal conductivity of the concrete with the increase of LWA replacement rate.Both stress damage distribution and axial compression state show a more consistent "ring hoop" damage mode.With the increase of temperature and replacement rate,the crack damage belt of concrete along the 45° was widened significantly and showed a shear damage mode.When the temperature reaches 800°C,the stress damage at the interface of CCSC100 is the lowest,indicating the good interfacial bonding performance and high temperature resistance of CCS.(3)In order to evaluate the durability performance of HSSC-LWAC after high temperatures damage,the properties of capillary water absorption and chloride transport were carried out after different high temperatures.The influence mechanisms of different temperatures damage and replacement ratio of LWA on the mass transport properties of HSSC-LWAC were analyzed.The experimental results show that the cumulative mass of capillary absorbed water for CCSC100 and FACC100 was 34.3% and 93.1% higher than that of NC at ambient temperature for 8d.At the same temperature level,the water absorption rate increased with the increase of LWA replacement rate.With the increase of temperature,the cumulative mass of capillary absorbed water tended to saturation more quickly due to the loose microcrack structure inside specimens.The increase of LWA replacement rate enlarges the internal porosity and provides the convenient pathway for chloride transport into concrete.Compared to NC at 600°C,the chloride content of CCSC100 and FACC100 at 2 mm-depth was increased by 36.8% and 41.8%,respectively.The increase of temperature promotes the pore connectivity and crack development inside microstructure.Considering the relationship between HSSC-LWAC damage degree,temperature and chloride diffusion coefficient,a chloride transport model for high temperature damaged unsaturated concrete was established by combining the basic principle of heat transfer and unsaturated fluid diffusion theory.And the distribution of chloride content at different temperatures and LWA replacement rate were obtained through the mesoscale numerical simulation.The accuracy of established mesoscale model of chloride ingress into concrete damaged by high temperature was verified by the comparative analysis of the obtained experimental results.