| Load damage directly affects the macro performance and micro structure of concrete.Systematically studying the load damage degree and macro and micro structure variation law of concrete has important theoretical significance and practical engineering value for improving the service life of building structures in coastal areas and improving the durability evaluation system of concrete structures.Concrete permeability has a particularly obvious influence on durability.There are many media about concrete permeability,including free water,pressure water and gas.Micro-cracks often occur during the service of structures.These micro-cracks will lead to increased permeability and improve the transmission performance of the medium inside the concrete.However,the permeability changes caused by the degree of damage are usually unable to be quantitatively analyzed.In view of the negative effect of load damage on concrete permeability,two kinds of water-cement ratio and five kinds of damage stress levels were comprehensively considered in this paper.Ultrasonic pulse test was used to detect the damage degree of concrete under different loads and convert it into damage coefficient.Combined with mercury injection test,the changing trend of pore structural parameters such as porosity and critical pore diameter was analyzed.On this basis,the capillary water absorption test,pressure permeability test and gas permeability test were carried out respectively on the concrete test blocks after load damage.Combined with the existing damage coefficient and pore structure parameters,the evolution law and mathematical model of the permeability performance of load-damaged concrete were analyzed from the macro and micro perspectives.The research work and main conclusions of this paper are as follows:(1)Uniaxial compression test is used to produce load damage to concrete test blocks.By measuring the ultimate compressive strength of concrete test blocks under different working conditions,five stress levels are applied to concrete specimens respectively.Ultrasonic pulse testing is used to detect the degree of internal damage to concrete and to define the damage factor,and the change rule between the load damage degree and damage coefficient is discussed.The test results show that when the stress level exceeds 30%,the damage coefficient shows an almost linear upward trend and the increase rate increases obviously.The increase degree of damage coefficient of water-cement ratio 0.35 is greater than that of water-cement ratio 0.55.(2)The micro-pore structure parameters of concrete with different load damage degrees were tested by mercury injection test(MIP method).Combined with MENGER cavernosum model,fractal dimension was introduced to characterize the complexity of concrete microscopic pore structure.The test results show that there is little difference between 10%and 30% concrete porosity,and the porosity increases significantly when the stress level exceeds 30%.Under low stress level,the specific surface pore ratio can grasp the general distribution of pore structure of concrete material on the whole,but it is not suitable for the analysis of pore structure of concrete with high stress level.With the increase of water-cement ratio,the transition pore increases gradually,especially when the stress level is 10% to 30%,and the content of macropores and gel pores progressively decreases.The maximum fractal dimension of pore volume occurs in the pore size range of 10 to 100 nm,that is,the transition pore occupies the largest proportion in the concrete interior,and the pore structure is the most complex.In the pore size range of 10～100nm,it is easier to form disconnected pores.(3)The capillary water absorption test of concrete under load damage was carried out to analyze the change law of the capillary water absorption process under different damage stress levels,and the capillary absorption coefficient and water absorption test results were summarized and analyzed.The results show that the cumulative water absorption process of concrete is hyperbolic linear,and the water absorption rate is faster in the first 200 minutes.Compared with undamaged concrete,the load damage dramatically improves the capillary water absorption capacity of concrete,and the increase of the capillary absorption coefficient is close to that of 10% and 30% stress levels.When the stress level exceeds 30%,the growth rate of the capillary absorption coefficient surges.Based on the mathematical relationship between the damage coefficient and the relative value of the dimensionless capillary absorption coefficient of concrete,a prediction model of capillary absorption coefficient considering water-cement ratio and damage coefficient is proposed.(4)The moisture permeability test of concrete after load damage is carried out to explore the variation law of moisture permeability height and relative moisture permeability coefficient under different loading conditions,and the influence of different load damage degrees on the moisture permeability of concrete is discussed.According to the theoretical model of equivalent radius and relative permeability coefficient proposed by REINHARDT,combined with damage coefficient and pore structure parameters,the experimental results in this chapter compare and analyze the relative permeability coefficient,and it is found that there is a good polynomial relationship between the empirical and theoretical values of the damage coefficient,porosity and relative permeability coefficient.At the same time,the experimental results also show that the stress level of 30% is the inflection point of the relative permeability coefficient,and the relative permeability coefficient will surge beyond this inflection point.At low stress level(0～30%),a reasonable reduction of water-cement ratio can improve the impermeability of concrete.(5)The gas permeability test of concrete after load damage was carried out.Combining the existing damage coefficient and pore structure parameters,the variation law of damage coefficient and mathematical relationship model,porosity and gas permeability coefficient were analyzed and established from macro and micro perspectives.Based on the prediction model of equivalent pore diameter and gas permeability coefficient proposed by REINHARDT and the experimental data,a prediction model of permeability coefficient considering water-cement ratio-damage coefficient and water-cement ratio-porosity was proposed respectively.The overall results show that the theoretical calculation model of pore equivalent radius and gas permeability coefficient proposed is more suitable for concrete materials with large water-cement ratio and stress level below 50%.The experimental and theoretical values of damage coefficient,porosity and gas permeability coefficient fit smoothly and have good correlation. |
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