Chemical Attack, Salt Crystallization And Stress Attack To High Performance Concrete And The Microstructural Evolution During Corrosion Process

Durability of concrete is a complicated, important and major scientific issue concerned at homeand abroad. Effect of chemical attack caused by sulfate, chloride, magnesium etc, salt crystallizationattack caused by dry-wet cycles, stress attack caused by applied loading, and combined action of saltcrystallization attack and stress attack on concrete durability is very important. In this study, based on anumber of reference, optimization proportion design and incorporating with synergistic effect ofmineral admixture, water reducing and air-entraining properties of concrete admixture, shrinkagecompensation functions of expansion agent and reinforcing effects of fibers, fly ash concrete (FAC),high performance concrete (HPC) and green high durability concrete (GHDC) were prepared. The lawsof the changes of relative dynamic modulus of elasticity (Erd) and surface spalling, corrosion productsand evolution of microstructure under the chemical attack, slat crystallization, stress attack and thecoupling effect of slat crystallization and stress attack were investigated. Changes of visual rating of thedeterioration, damage evolution equation, a theoretical model of density of cracks and a method forservice life of concretes based on density of cracks were established. The main contents and results ofthis paper are as follows:In Chapter1, the research significance and latest development of durability of concretes exposedto chemical attack, slat crystallization attack, stress attack and the coupling effect of slat crystallizationattack and stress attack and service life predicting methods were summarized. Problems remaining tobe investigated were also proposed, and outline of this contribution was sketched.In Chapter2, raw materials, mix design, major experimental equipment and method ofexperiment were described in detail and equipments on durability experiment for applying tensilestress and compressive stress were designed. Four types of C50concrete were prepared. They areFAC incorporated with20%FA by mass of cement, HPC incorporated with fly ash (FA), slag (SG)and silica fume (SF), in which dosage of supplementary binder is10%FA+20%SG+20%SF by massof cement, and GHDC based on HPC and then incorporated fibers, expansion agent and Air-entrainedagent. For different fibers added in, GHDC was denoted as GHDC1that was incorporated steel fiberand polypropylene fiber and GHDC2that was incorporated steel fiber, polypropylene fiber andpolyester fiber.In Chapter3, the chemical attack to concrete exposed to chloride, magnesium sulfate andsulfate-magnesium-chloride was investigated. There is no chemical attack to concrete exposed tochloride. Chemical attack is severe to concrete exposed to magnesium sulfate and that is more severe toconcrete exposed to sulfate-magnesium-chloride. Chemical attack to concrete exposed to magnesiumsulfate is accelerated by chloride. GHDC2shows good durability tunder chemical attack caused bymagnesium sulfate and sulfate-magnesium-chloride.In Chapter4, the salt crystallization attack to concrete exposed to chloride, magnesium sulfate andsulfate-magnesium-chloride was investigated. There is no salt crystallization attack to concrete exposedto chloride. Salt crystallization attack is severe to concrete exposed to magnesium sulfate and that ismore severe to concrete exposed to sulfate-magnesium-chloride.. Chemical attack to concrete is accelerated by salt crystallization and salt crystallization attack to concrete exposed to magnesiumsulfate is accelerated by chloride. HPC shows good durability under salt crystallization attack causedby magnesium sulfate and sulfate-magnesium-chloride.In Chapter5, the stress attack to concrete exposed to chloride, magnesium sulfate andsulfate-magnesium-chloride was investigated. There is no stress attack to concrete exposed to chloride.Stress attack is severe to concrete exposed to magnesium sulfate. The effect of stress state is veryimportant to stress attack. When in the same stress level but different stress state, the value of thedamage velocity to concrete is tensile stress> compressive stress> bending stress. Stress attack is moresevere to concrete exposed to sulfate-magnesium-chloride than that exposed to magnesium sulfate.Chemical attack to concrete exposed to magnesium sulfate is accelerated by compressive stress andbending stress attack to concrete exposed to magnesium sulfate is accelerated by chloride. HPCexposed to sulfate magnesium shows good durability under tensile stress attack and compressive attackand GHDC2exposed to sulfate magnesium and sulfate-magnesium-chloride displays good durabilityunder bending stress attack.In Chapter6, the coupling effect of slat crystallization attack and stress attack on concrete exposedto chloride, magnesium sulfate and sulfate-magnesium-chloride was investigated. There is no couplingeffect of slat crystallization attack and stress attack on concrete exposed to chloride. The couplingeffect of slat crystallization attack and stress attack is severe to concrete exposed to magnesium sulfateand that is more severe to concrete exposed to sulfate-magnesium-chloride. The coupling effect of slatcrystallization attack and stress attack brings superposition effect that means accelerating the chemicalattack to concrete more quickly. Slat crystallization accelerates the bending attack to concrete andbending stress accelerates the salt crystallization attack to concrete. The coupling effect of slatcrystallization attack and stress attack on concrete exposed to magnesium sulfate is accelerated bychloride. Exposed to sulfate magnesium, GHDC2shows good durability under the coupling effect ofslat crystallization attack and stress attack. Exposed to sulfate-magnesium-chloride, FAC and HPCdisplay good durability under the coupling effect of slat crystallization attack and stress attack.In Chapter7, corrosion products and the evolution of microstructure were investigated by usingXRD and SEM-EDAX. Results show that the major corrosion products are gypsum, AFt, AFm,C3A·CaCl2·10H2O and magnesium hydroxide. Base on a number of SEM pictures about differentconcrete exposed to different corrosion condition, the new concept of “Microcrack Initiation” is presentand along which the microcrack grows as a star, develops and connects to form a net of cracks. Thepozzolanic reaction of mineral admixture compact the structure and improve the corrosion resistance ofconcrete. The spherical pore for addition of air-entrained agent releases surface developing energy ofmicrocracks that near the spherical pore, decreases stress concentration of tip of microcracks, reducesthe expansion stress and crystal stress during corrosion process, and improves further the corrosionresistance.In Chapter8, based on the results of concrete experiments on durability and experiment dataanalysis, a damage developing equation of Erd was established, and then, a quick service life predictingmethod based on density of cracks were established further. The laws of evolution of micro-cracksduring corrosion process under different corrosion environment. Results show that the conclusions ofdamage developing equation and service life predicting method match the experiment results and fit thepractical engineering. The service life predicting method based on density of cracks show more well results.In Chapter8，a summarization of this contribution was presented and the shortcomings of thisstudy and proposition for further researches were listed.