Pore Evolution Of Hydraulic Concrete Under Low Temperature And Low Humidity Curing And Its Influence On Physical And Mechanical Properties

With the improvement of dam construction technology,the construction of water conservancy projects has gradually shifted to the northwest region with high altitude and complex environmental conditions.However,the climate conditions of low temperature and low humidity in this area all the year round bring severe challenges to the construction and operation of hydraulic concrete structures.The main problems are as follows:Low-temperature and low-humidity curing conditions are not clear about the law of pore change process of hydraulic concrete;In actual operation,hydraulic structures will be affected by both environmental conditions and external loads,and the relationship between pore structure and concrete performance is not clear.The hydraulic heterogeneity model of concrete does not consider the influence of actual environmental conditions on its parameters.It is of great significance for the planning and development of hydraulic concrete structures in northwest China to improve the above problems and study the influence of pore evolution of hydraulic concrete on its performance under low temperature and low humidity curing.In this paper,the laboratory test,theoretical analysis and numerical simulation are used to study the influence of low temperature and low humidity climate conditions on concrete performance in northwest China.The main contents include:considering different curing conditions,water-binder ratio and curing age,dynamically observing the influence of various factors on the evolution law of different pore types in the hydration and hardening process of concrete.The relationship between strength,pore structure and fractal dimension of concrete under different curing conditions is discussed.Considering the theoretical curves of concrete strength and porosity,the relationship model between compressive strength and indirect tensile strength is established.The freeze-thaw cycle test of hydraulic concrete under low-temperature and low-humidity curing conditions was carried out to establish the relationship between damage degree and pore structure.Based on the micro-mechanical test,the thickness of concrete interface transition zone under different curing conditions is determined,and the freezing-thawing process of hydraulic concrete is numerically simulated by combining the pore structure characteristics and freezing-thawing damage model in the early stage,so as to reveal the freezing-thawing damage mechanism of hydraulic concrete under low temperature and low humidity curing conditions.The main research contents are as follows:(1)The pore structure of concrete under different curing conditions was tested by nuclear magnetic resonance,and the influence of low temperature and low humidity curing conditions on the pore structure of hydraulic concrete was revealed.Considering curing conditions,curing age and water-binder ratio,the pore structure composition and distribution characteristics of concrete specimens were quantitatively analyzed.Divide that pore size range obtained by test.The porosity of concrete specimens at different ages was determined by the conversion method of nuclear magnetic relaxation time and pore radius.Combined with the test results of pore structure and fractal theory,the fractal dimensions of different pores are calculated by fitting the corresponding pore size range,and the multifractal properties of concrete internal pores under the influence of water-binder ratio and curing conditions are characterized.(2)Based on the indoor macro-mechanical test,the influence of pore structure on the mechanical properties of hydraulic concrete is expounded.Based on the test results of mechanical properties of concrete under low-temperature and low-humidity curing conditions and fractal theory,the relationship between pore structure and strength under different curing conditions was studied.An extended model of the relationship between concrete strength and porosity under different curing conditions is put forward,and the relationship between the ratio of compressive strength to indirect tensile strength and porosity is clarified.Based on the theory of continuous damage mechanics,Ottosen failure criterion of concrete test data calibration under different curing conditions is established.Combined with the tension and compression meridian equation,the change of concrete failure surface is visually expressed.(3)Based on low-temperature and low-humidity curing conditions,the relationship between pore structure characteristics and damage degree of hydraulic concrete during freezing and thawing is established.Through the freeze-thaw cycle test,the apparent damage,mass loss rate and relative dynamic elastic modulus of concrete are tested,and the influence of curing conditions and water-binder ratio on the frost resistance of concrete is explained.The internal pore parameters of concrete under freeze-thaw cycles are quantitatively analyzed,and the evolution law of pore distribution of concrete with freeze-thaw cycles is expounded.Taking the relative dynamic elastic modulus as the damage variable to determine the cumulative damage,and combining the test results and Boltzmann equation,the freeze-thaw damage models of concrete under different curing conditions are constructed.(4)Considering the influence of actual environmental conditions on the microstructure of hydraulic concrete,combined with the theory of multi-field coupling,the freeze-thaw damage mechanism of hydraulic concrete under different curing temperature and humidity is revealed.Based on nanoindentation technology,the micro-mechanical properties of concrete under different curing conditions were tested.The micro-mechanical properties and thickness of interface transition zone of concrete under low temperature and low humidity curing are determined.Using Monte Carlo method and Fuller curve of aggregate used in the test,combined with the porosity of concrete,freeze-thaw damage model and thickness of interface transition zone,a two-dimensional meso-model of concrete containing three phases of aggregate-interface transition zone-mortar was established.By constructing the freeze-thaw damage control equation of concrete considering the coupling action of temperature,seepage and stress,the freeze-thaw damage process of concrete under different curing conditions is numerically simulated,and the variation law of pore water pressure,maximum principal stress and temperature field in concrete during freeze-thaw cycle is revealed.