Experimental Research On The Durability Of Concrete Exposed To Sulfate Environment

As an important aspect of concrete durability, the damage of concrete due to sulfate attack was attracting more and more attentions. To reveal this erosion characteristic, some fundamental researches should be investigated by experimental method. Therefore, some interesting tests were carried out in this paper to simulate the erosion and damage process of concrete exposed to different sulfate environments under drying and wetting cycles. The deterioration regularity of concrete induced by sulfate attack was discussed comprehensively from the macroscopic and microscopic levels on basis of these experimental results. Firstly, the macroscopic properties of concrete such as strengths, deformation, mass and surface conditions varied with sulfate attack ages were analyzed. Secondly, based on the fracture mechanics, the deterioration regularity of double-K fracture parameters of concrete under different sulfate attack periods was studied by three-point bending beams test. Thirdly, the influences of different solution concentrations and different tensile/compressive stress levels on the contents and ingression rules of sulfate ions were studied, and the erosion depth and the distribution of sulfate ion concentrations from surface to inside were determined. Finally, the microscopic mechanism and damage evolution laws of concrete caused by sulfate attack were revealed based on a modern test technology (SEM).The experimental results indicated that there was a two-stage change pattern both in the axial compressive strength and in the splitting tensile strength of concrete:the strengths increased firstly then decreased with erosion ages, and the latter damage induced by sulfate attack was more sensitive than the former. The erosion coefficients KS & KQ Varied obviously during the stage of 0-60d and 90d-180d, and changed continuously and slowly during other erosion ages. A simulation model was set up in this study on basis of Gaussian function according to the experimental data of unstable fracture toughness as well as the initial fracture toughness. The ratio of the initial cracking load to the unstable fracture load (PQ/Pmax) located between 0.50 and 0.90 approximately, which was larger during the initial erosion ages but smaller at last. The sulfate ion concentrations decreased with the erosion depth and there was a local protuberance, but increased with the erosion ages expect a phenomenon with a sudden drop in then a sudden rise. Meanwhile, with the increase of compressive stresses, the sulfate ion concentrations decreased before 0.3fc and increased after 0.3fc, and the critical level of 0.3fc can be considered as a threshold in this study. However, the tensile stress always had a significantly accelerated effect on the damage of concrete induced by sulfate attack. With the increase of solution concentration, the erosion was much severe and the peak age went ahead because of the faster transmission speed of sulfate ions. In the S1 solution, the specimen was damaged by ettringite expansion, while the specimen in the S10 solution failed due to gypsum expansion. In the late erosion period as 360d, the ultimate compressive strain of concrete became extremely small, and the brittleness increased with the propagation of microcracks, in addition, the surfaces of specimen was sanded seriously, especially when the pit corrosion and salting out phenomena presented significantly on the surface with marked internal spalling and fragments aggregation inside. The microscopic structure of material was studied by the SEM scanning tests, which results showed that the deterioration of the physical and mechanical properties of concrete were controlled by the microscopic changes in the material, and the changing regularity of the microstructure had a good agreement with the macroscopic mechanical properties.