Study On Deterioration Behavior Of Concrete Subjected To Sodium Sulfate Under Capillary Suction

Sulfate attack has become one of the main causes of concrete failure in durability.Based on the problem of RC bridge piers in service under the complicated environment in the northwest of China,the paper explored the transfer mechanism of sodium sulfate solution in concrete under capillary action combined with existing research.Some environmental factors,such as salt corrosion,freeze-thaw cycle and large temperature and humidity difference were also be considered.Capillary driving force was applied to consider the effects of physical crystallization resistance of concrete on sulfate.Through experimental research and theoretical analysis,the transmission rules of sodium sulfate solution in cement-based materials at early stage were studied.Meanwhile,the chloride ion diffusion and uniaxial compressive stress-strain of concrete under multi-factor coupling of sodium sulfate attack,drying-weting and freeze-thaw cycles for details were described.The relationship among sodium sulfate solution concentration,half-submerged time,the number of freeze-thaw cycles and capillary rise rate,peak stress of uniaxial compression of concrete were established.Corresponding chloride ion resistance of concrete were mainly emphasized.The research results provided a theoretical basis for the safety judgment of the old bridges under complicated environment in the northwest of China,and the design of concrete material for new bridge.The main conclusions are as follows:(1)The transmission of sodium sulphate solution in cement-based materials under capillary action presents a certain pattern.The capillary transport volume increased rapidly within 12h,then increased slowly until it stabilized at around 168h.The capillary transfer rate reached the maximum at the end of 1 h,then decreased rapidly until it stabilized at around 120h.(2)The 6h electric flux of chloride ion for concrete samples half-submerged in sodium sulfate solution were tested by ASTM Cl202 method.The influence of different sodium sulfate solution concentrations,concrete strength grade,half-submerged age of sodium sulfate and the number of freeze-thaw cycles on the electric flux is analyzed.The relationship between the damage mechanism of sulfate salt to concrete under capillary action and its chloride penetration resistance is also established.(3)At first,the peak point of uniaxial compression stress for concrete attacked by sodium sulfate under capillary action risen and then fallen.Due to the effects of proper temperature and humidity as well as evaporation on the surface of concrete,a small amount of sodium sulfate enter into concrete at early stage,and fill in pores and microcracks inside concrete,which causes the increase of compressive strength.With sodium sulphate continuously entering into inside concrete,the sodium sulphate solution in pores and microcracks become supersaturated and produce crystals with expanded volume.Since the crystals caused cracks inside concrete,compressive strength decreased with the expansion of cracks.(4)Combining the transfer rate of cement-based materials,chloride ion resistance of concrete and the stress-strain curve of uniaxial compressive,the relationship between microscopic and macroscopic property is studied.The relationship among capillary rise,chloride resistance and uniaxial compressive strength also is established.