Study On Deterioration Of Concrete Under Sulfate Attack

Sulfate exists widely in our country, such as in seawater, groundwater, salt lake andsulfate soil. Sulfate attack is one of the most widespread and common forms of damageto concrete. The deterioration of concrete constructions resulting from sulfate attack is asevere problem affecting the durability and service life of concrete structures. Thedeterioration of concrete caused by sulfate attack is a process from outside to inside,which can take the form of expansion, cracking, loss of strength of concrete, and thedamage layer of concrete is formed from the outside to the inside, eventually lead todamage of concrete. In sulfate environment, the elements of concrete constructionsituated in water table fluctuation, tidal zone and splash zone will suffer from morecomplex attack because of the drying-wetting cycles, which can accelerate thedeterioration of concrete. In cold regions, concrete structures are severely deteriorateddue to sulfate attack and freeze-thaw damage, and the degradation mechanism ofconcrete is more complicated. Therefore, the research of the damage mechanism ofconcrete and the degradation rule of concrete durability exposed to drying-wettingcycles and freeze-thaw cycles under sulfate environment is significant for the structuraldurability assessment.In this paper, research on the performance of concrete subjected to drying-wettingcycles and freeze-thaw cycles in sulfate solutions was conducted based on thecombination of theoretical analysis and laboratory rapid test method, the macroscopicand microscopic test. The damage mechanism and the deterioration law of concreteunder sulfate environment are analyzed. Based on the research, the compressivestrength degradation model of concrete and the constitutive model of concrete under uniaxial compression are established, and the mechanical properties degradation of thedamage layer of concrete is analyzed. Furthermore, the damage failure model ofconcrete under sulfate environment is established on the basis of the damage mechanicsand Ottosen failure model, and the damage constitutive model under multi-axial stressstates is also investigated. The main contents in this paper are as follows:(1) The microscopic analytical techniques, including scanning electron microscopy,X-ray diffraction and thermal analysis are adopted in test. Microstructural changes ofconcrete and characteristics of corrosion product are investigated, and the corrosionproducts in concrete were distinguished and quantitatively compared under sulfateenvironment. Moreover, the damage mechanism of concrete subjected to drying-wettingcycles and freeze-thaw cycles in sulfate solutions is revealed from the microscopicview.(2) Experiment of concrete exposed to sulfate attack and drying-wetting cycles iscarried out. The affections of the water-binder ratio, content of fly ash, concentrationand types of sulfate solution to weight loss, relatively dynamic modulus elastically andcompressive strength of concrete are analyzed. The compressive strength degradationmodel of concrete considering the water-binder ratio, content of fly ash andconcentration of sulfate solution is established. The influence of concentration and typesof sulfate solution on deterioration of concrete under freeze-thaw cycles and sulfateattack is also investigated. Furthermore, the effect brought by sulfate attack tofreeze-thaw cycles and the coupling effect under freeze-thaw cycles and sulfate attackare analyzed.(3) A one-side non-destructive testing method is used to detect the surface damageof concrete. Research on the surface damage layer thickness of concrete subjected todrying-wetting cycles and freeze-thaw cycles in sulfate solutions is conducted. Thedamage mechanism of concrete under sulfate environment is analyzed by theview of the damage layer of concrete. When subjected to drying-wetting cycles andsulfate attack, the damage layer thickness prediction model and the damage rateprediction model of concrete considering the water-binder ratio, content of fly ash andconcentration of sulfate solution are established. The damage layer thickness predictionmodel and the damage rate prediction model of concrete exposed to freeze-thaw cycles and sulfate attack are also established. Furthermore, the mechanical propertiesdegradation of the damage layer of concrete is analyzed, and the average compressivestrength in damage layer is obtained.(4) Experiment of stress-strain relationship of concrete subjected to drying-wettingcycles and freeze-thaw cycles in sulfate solutions under uniaxial compression is carriedout. The performance of concrete under uniaxial compression and the features ofstress-strain curves of concrete and the damage layer of concrete under differentcorrosion time. The relationship between peak stress, peak strain and modulus ofelasticity of concrete with corrosion time are built. Based on the test results, thecomplete compressive stress-strain curve equations of concrete and the damage layer ofconcrete subjected to drying-wetting cycles and freeze-thaw cycles in sulfate solutionsare built.(5) Based on the damage mechanics and Ottosen failure model, the damage failuremodel of concrete under sulfate environment is established. The nonlinear index isobtained on the basis of the damage failure model of concrete. Furthermore, the secantmodulus of concrete is obtained based on the complete compressive stress-strain curveequations of concrete under sulfate environment. The damage constitutive model ofconcrete exposed to sulfate environment under multi-axial stress states is investigated.