关键词: 喷射混凝土, 初始损伤, 硫酸盐侵蚀, 相对动弹模量, 劣化机理, 损伤演化模型

Abstract: In order to reveal the performance degradation law of shotcrete with initial damage under sulfate attack, the initial damage degree was defined according to the change of ultrasonic wave velocity. The initial damage of concrete specimens was prefabricated by pressure testing machine, and the initial damage presented five different gradients: 0.10, 0.15, 0.20, 0.25 and 0.30. The dry-wet cycle sulfate attack test was carried out on shotcrete, and the appearance, mass change rate and relative dynamic elastic modulus were used as evaluation indexes. The effect of initial damage on the sulfate attack resistance of shotcrete was systematically studied by means of micro-morphology analysis and chemical composition analysis. The mechanism of damage effect was discussed and the damage evolution model of initial damaged shotcrete under sulfate attack was established. The results show that with the extension of erosion time, the mass and relative dynamic elastic modulus of shotcrete specimens increase first and then decrease in the whole erosion process. When the initial damage degree is 0.10,the sulfate attack process has little effect on concrete, which can be basically ignored. After the initial damage exceeds 0.10, the deterioration speed of specimens under sulfate attack is accelerated. After 180 dry-wet cycles, compared with reference group, the mass of specimens with initial damage of 0.15, 0.20, 0.25, and 0.30 decreases by 2.9%, 3.4%, 4.0%, and 4.7%, respectively. The relative dynamic elastic modulus drops to 0.42, 0.34, 0.28, and 0.16, respectively. The mortar microcracks and aggregate displacement caused by the initial damage of prefabrication provide favorable conditions for the intrusion of sulfate. There is a small amount of erosion on the surface of the specimens with a damage degree of 0.10. And the internal microcracks of specimens are slightly more than reference group, but the erosion deterioration is not serious. The epidermis of specimens with a damage degree of 0.30 almost fall off. There are a large number of through cracks in the interior, which are wide and deep. Among the reaction products, the diffraction peak intensity of gypsum and ettringite crystal is the highest, and the erosion damage is the most serious. The erosion damage factor is defined in terms of relative dynamic elastic modulus changes. The established damage model curves are in good agreement with the test results, and the fitting correlation coefficients are all above 0.94. The model can better reflect the damage deterioration law of shotcrete with different initial damage.

Key words: shotcrete, initial damage, sulfate attack, relative dynamic modulus, deterioration mechanism, damage evolution model