Macroscopic And Mesoscopic Research Of The Effect Of Temperature On Thermal And Mechanical Properties Of Concrete

Infrastructure connectivity is a priority area for development in the "One Belt and Road".The route crossed many countries and regions,and the natural climate,topography and geology,economy and culture are characterized by diversity,complexity and uncertainty,so the impact of climate change on infrastructure in the countries along the route should be fully taken into account in the construction.The variation of environment temperature affects the hydration rate and the development of various properties of cement.The heterogeneity of meso-composition makes the strength and deformation of concrete more complex,and the discrete of variation range and performance index become bigger.Moreover,the thermal and mechanical properties of concrete materials will vary with time and environment conditions.Therefore,the corresponding researches such as the influence of environment temperature on the thermal and mechanical properties of concrete and the development law are carried out.It is of great theoretical and practical significance for the infrastructure construction of "one belt and one road".Taking the temperatures of Lanzhou city in Gansu Province as an example,this paper studies the evolution law and influence mechanism of thermal and mechanical properties of concrete in the whole process(pouring-curing-maturing)under complex environment temperature from the macro and meso levels through laboratory experiments and numerical simulations.The influence of environment temperature on the concrete strength formation is analyzed by mechanical test,and the prediction model of concrete strength considering temperature is established.With the help of industrial CT,the evolution and strength development characteristics of concrete internal structure at different temperatures,ages and loads are revealed from mesoscopic level.Then,MatchID analysis system and the algorithm of computer graphics principle are combined and used to transform the CT scanning images at different ages into displacement contour and strain contour,and the internal deformation of concrete specimens before failure is visualized.Through the adiabatic temperature rise test of concrete,the thermal parameters of each component material are obtained by BP inversion analysis,and the influence of thermal parameters on the spatial-temporal distribution of internal temperature is studied in depth.The main research contents of this paper are as follows:(1)According to the multi-year temperature data in Lanzhou City,Gansu Province,it is generalized into three typical curing temperature histories of concrete,and the concrete specimens are respectively cured under these three environments.The uniaxial compression tests of various characteristic ages(1d,3d,7d,14 d,28d)are carried out with MTS universal testing machine to study the deformation process and failure mode of the specimens.According to the test results,the influence of curing temperature on the mechanical properties of concrete is analyzed,the relationship between temperature history and concrete strength is studied,and the prediction model of concrete compressive strength considering temperature is established based on the equivalent age.(2)The concrete specimens cured in the three generalized temperature environments and standard temperature environment are scanned by industrial CT.According to the CT images,the three-dimension model of concrete specimen is reconstructed and spatial distribution and characteristics of pore are carried out.And the influence of temperature on the microstructure and its evolution during the process of concrete strength growth is analyzed.Then the relationship of temperature-internal microstructure-compressive strength is revealed.Finally,the strength evolution mechanism of concrete under complex temperature environments is further studied.(3)Based on the principle of computer graphics and the MatchID software,the CT scanning images of the concrete specimens are processed,the displacement field and strain field of the concrete during the loading compression test can be obtained more accurately,and a new method about the internal deformation of the concrete is explored.According to the displacement contour and strain contour of the concrete obtained by MatchID,the evolution process of the internal structure stress,deformation and failure of the concrete under axial load is deeply analyzed,and the influence of the environment temperature on concrete mechanics performance is analyzed.(4)An adiabatic temperature rise test of concrete is carried out to study the spatiotemporal evolution of temperature in the hydration process and analyze the temperature changes in different parts through a number of temperature sensors placed inside the concrete.Furthmore,the relationship between the spatial gradient of temperature and the hydration degree is analyzed,and the hydration heat model based on the hydration degree is obtained.The thermal parameters of the concrete are inverted by BP method based on the temperatures of multiple measurement points.Finally,the internal temperature field of the concrete is simulated in detail and verificated by the finite element analysis.(5)Aggregate,cement mortar,interface transition zone and pore are extracted respectively from CT images of the concrete specimen,and the three-dimension numerical model which reflecting the concrete structure is established by 3D reconstruction technology.The numerical simulation of temperature field and uniaxial compression process are carried out by commercial finite element analysis software.Compared with the experiment results,the feasibility of the modeling method is verified.and a new approach for the analysis of thermal and mechanical properties of concrete by numerical simulation is provided.