The Mechanism Of Damage Evolution And The Model Of Internal Expansion Force Of Concrete Under Sulfate Attack

Sulfate attack is one of the main factors to reduce the durability of concrete structures in marine environment. When sulfate ions diffuse in pores of concrete, the chemical reaction between the hydration solution of concrete and sulfate ions may take place, therefore, ettringite crystal may nucleate and grow in these pores, and this kind of ettringite is also called as"delayed ettringite". Usually, volume of delayed ettringite is about 2.5 times bigger than the volume of reactant hydrated calcium aluminate. Therefore, the expansion forces will be induced by delayed ettringite crystal, and the expansion strain of concrete will be produced by the delayed ettringite. If the tensile stress around the voids of concrete caused by the internal expansion force is greater than the tensile strength, the nucleation of damage will take place. Hence, the mechanism of damage nucleation of concrete under sulfate attack is studied, and the model of internal expansion force is also suggested.In chapter 1, problems of concrete durability are summarized, which includes three parts: progress of researches on concrete in marine environment; researches on the damage theory of materials; researches on the constitutive relation of concrete under impact loading.In chapter 2, we assume that concrete is a two-phase composite, and the constitutive relation concrete matrix may be approximately described by the standard linear solid model. The stress analysis about the surface of microvoids in concrete is performed. It is found that the tensile stress may appear at the void surface even the concrete is under the action of uniaxial compressive load. If that tensile stress is greater than the tensile strength of concrete, the nucleation of micro-cracks may take place.In chapter 3, damage evolution of concrete immersed in solution of sodium sulfate of different concentrations is investigated by means of the experiment results of the variation of density and wave velocity of ultrasonic propagated in samples of concrete. The variation of wave velocity of ultrasonic is mainly caused by three factors. The first factor is the continuation of hydration of concrete, and the second one is the effect of delayed ettringite to filling concrete. These two factors may lead to the increase of average modulus of the material and wave velocity of ultrasonic. The third factor is damage evolution, and the wave velocity of ultrasonic will decrease because of the decrease effective modulus due to the erosion damage. The results show that the Poisson's ratio is significantly affected by the concentration of sulfate solution and water-cement ratio of concrete.In chapter 4, theory of micromechanics is adopted to obtain the relation between the internal expansion stress and expansion strain of concrete under the sulfate attack. The result indicates that Boltzmann foundation can preferably character the evolution of expansion strain and internal expansion stress.In chapter 5, theory of internal variable is used to establish a rate-independent non-liner viscous-elasticity constitutive model, and this model is given by an iterative form. The condition of convergence of the iterative form is analyzed. Based on this model, a new 3-D non-linear viscoelastic constitutive relation of concrete is suggested by considering the damage evolution. It is found that this constitutive model can perfectly describe the experimental results. It is also found that the strain rate will significantly affect the relaxation time and initial modulus of concrete.In chapter 6, the conclusions of this research are summarized, and the further research work is analyzed.