Research On Performance Of Concrete Under Combined Action Of Sulfate Attack And Freeze-thaw Cycles

Sulfate attack and freezing are important environmental factors causing the degradation of the durability of concrete. More serious damage will be produced when concrete were under freeze-thaw cycles and sulfate attack condition compared with under a single bad condition. Therefore, carrying out the study about the durability of concrete under sulfate attack and freeze-thaw cycles condition systemic,explaining the degradation mechanism of concrete when they were under freeze-thaw cycles and sulfate attack condition are significant to enhance the durability of concrete and predict its lifetime.Based on those problems, this paper investigates that the impact on mechanical properties and deformation of concrete under water to cement、gas content and other internal factors, and frost and thaw condition and other external factors in sulphate erosion environment, and then conduct the analysis of pore solution and microscopic tests, probe the transmission mechanism of sulfate ion and concrete micro-structural degradation mechanism under sulfate attack and freeze-thawing complex conditions.The main contents and results are as follows:(1) The experiment studied the law of the mechanical properties of the concrete blocks when they were under the sulfate corrosion and freeze-thaw cycles conditions.Two factors influence each other and produce additive effects much more serious than the average freeze-thaw damage when the concrete were subjected to freeze-thaw damage and sulfate attack. Compared with 5% magnesium sulfate salt freeze,5%sodium sulfate salt freeze will cause more serious damage to the concrete, this phenomenon is mainly concerned with the type of damage: in addition to causing chemical erosion,5% sodium sulfate can also cause physical damage to the concrete by the crystallization of the salt, which is the main reason causing the 5% sodium sulfate salt freeze damage to the concrete.5% magnesium sulfate is mainly cause the chemical attack to the concrete, physical damage caused by salt crystals inferior to 5%sodium sulfate salt freeze damage.(2) The experiment studied the law of deformation of the concrete blocks when they were under the sulfate corrosion and freeze-thaw cycles conditions. Putting the air entraining and non-entraining concrete blocks in the 5% sodium sulfate solution,5% magnesium sulfate solution, water and carrying a fast freeze-thaw treatment on them, at the same time compared with the concrete blocks, which were immersed in5% sodium sulfate solution and 5% magnesium sulfate solution. Measuring strain development laws of the specimen by the static strain gauge in the process. The results show that the strain growth of air entraining concrete is one times of non-entraining concrete in 1200 min time, specimen in sodium sulfate solution have the fastest strain growing, followed by specimen in the water, in the magnesium sulfate solution, in sodium sulfate and magnesium sulfate solution.(3) Adding entraining agent, Analysis from the micro-structure we can increase porosity,the several apertures and critical aperture by adding in air entraining agent.The using of air entraining agent can effectively prevent the destruction of the concrete due to the hydrostatic pressure generated. We can discover that the sulfate ion diffusion coefficient of entrained concrete is lower and sulfate ion content of deeper inside places of entrained concrete is higher than that of non-entrained concrete from the results of sulfate ion diffusion test under freezing conditions. The introduced closed pores blocking the seepage channel of pore solution which contains sulfate ion.(4) Concrete more vulnerable to damage under freezing conditions. Freeze-thaw cycles can accelerate sulfate’s transfer to internal concrete. Sulphate produce chemical reaction after after entering the interior concrete, so hydration products are consumed and generat a product which has expansibility. Most of the products are ettringite and gypsum. Different types of sulfate solution has different damaging effects on concrete.Corrosive effect of magnesium sulfate is smaller than that of sodium sulfate on concrete. The higher water-cement ratio is, the more pores concrete has.And degree of density is worse too. So that sulfate ion transmiss into the interior of concrete. Which is means that sulphate resistance capability of higher water-cement ratio concrete areinferior than low water-cement ratio concrete. Adding entraining agent can increase tdurability of concrete.