Study On Damage Properties Of Concrete Subjected To Loading History And Sulfate Attack

Sulfate attack is one of the main factors causing damage and failure to concrete, leading to concrete structure performance degradation o and decrease of serviceability. The damage mechanism of concrete structure is rather complicated when subjected to coupling effect of load and sulfate erosion environment. In order to determine whether the safety performance of concrete structure meets the requirements of engineering design, construction or use, the long-term mechanical properties and damage mechanism of concrete need to be studied. At present, study on concrete damage properties under multi-factor coupling effect of load and sulfate is insufficient, still at the start-up stage, which should be gradually improved.In order to investigate the damage evolution law of concrete mechanical properties and strain-stress relationships of concrete under coupling effects of loading history, sodium sulfate solution with long-term erosion or drying-wetting cycles, experimental study and theoretical research are carried out. Applying uniaxial compressive loading history is proposed to be done at first in test, followed by the coupling the action of sodium sulfate attack by long-term soaking or drying-wetting cycles, measuring the ultrasonic velocity and quality in the process of the whole test, finally obtaining uniaxial compressive strength and collecting the strain-stress curves of concrete after 80 days erosion age. Test includes a variety of sulfate solutions(1%, 5% and 10% sulfate concentrations), different loading history(four kinds of monotonic stress levels of 0%, 10%, 25% and 50% and one kind of cyclic stress level 50%), and two immersion ways of long-term erosion and drying-wetting cycles. Taking mass change, relative dynamic elastic modulus and compressive strength change as the evaluation index of concrete performance degradation, prediction model of concrete's relative dynamic elastic modulus changing over time is established under loading history and sulfate attack. On the basis of concrete mechanical damage theory and characteristics of sodium sulfate attack, a concrete damage constitutive model under the combining effect of load and multiple environmental factors is established, and compared with concrete strain-stress curves in test. The following conclusions are obtained:(1)The coupling effect of multi-factors has delayed or aggravated influence on concrete mechanical performance. Mechanical properties of concrete under sodium sulfate attack are increased in the early days of erosion. Due to the influence of wetting-drying cycles, the epidermis, corner spalling of concrete and internal crack extension and increase with the development of the surface erosion process, which results in relative weight, dynamic elastic modulus and compressive strength decrease or reduced.(2)The shapes of stress-strain full curves of damaged concrete tend to be flat, with slope of curves rising stage reduced, descent stage being flat; Concrete peak stress is reduced, peak strain increased, elastic modulus decreased. All these show that the plastic deformation of concrete after sodium sulfate erosion decreases, and ductility declines.(3)A uniaxial compression damage statistical model of concrete constitutive are derived based on strain equivalent assumption in damage mechanics and concrete strength following Weibull distribution assumption. Compared the model curve drawn by MATLAB programs with the experimental one, it is found that the deviation is within the scope, and the model coincides well with experimental results. Thus, it is suitable for describing damage stress-strain relationship of concrete.