Experimental Study On Mechanical Strengthening And Electrical Conductivity Of Air-Entrained Cement-Based Materials At Low Temperature

When designing or constructing concrete structures in cold regions of northern China,the effects of mechanical properties of concrete at low temperatures must be considered.Since the pore water inside the cement-based material freezes at low temperature,its mechanical properties are quite different from those at normal temperature.To this end,it is necessary to conduct in-depth research on the mechanical properties of cement-based materials at low temperatures,and provide a theoretical reference for the design of concrete structures in low temperature environments.In this paper,270 100 mm cement-based cube specimens and 180 100mm×100mm×400mm cement-based prism specimens were fabricated.Three watercement ratios of 0.35,0.50 and 0.65 were used and the influence of air entraining agent was considered.The compressive strength,splitting strength and dynamic elastic modulus of three cement-based materials of concrete,mortar and cement paste are at 5 °C,-10 °C,-20 °C,-30 °C and-40 °C.difference.At the same time,the conductivity test was carried out on the specimens at normal temperature and low temperature,and the amount of ice content under the low temperature was calculated to establish the relationship between ice content and the mechanical properties,thus revealing the mechanism of mechanical properties enhancement of cement-based materials at low temperature.The main research contents and conclusions include:(1)The results of cubic compression test show that the compressive strength increases linearly with the decrease of temperature.The compressive strength of the air-entrained concrete specimen with water-cement ratio of 0.50 at-10 °C,-20 °C,-30 °C,-40 °C increased by 113%,124%,136%,and 147% relative to 20°C,respectively;the larger the water-cement ratio,the smaller the compressive strength,but the higher the increment at low temperature,for example,the compressive strength of the air-entrained mortar specimen with water-cement ratio of 0.35?0.50?0.65 at-40°C increased by 164%?175%?190% relative to 20°C,respectively.Because of the different water content,the compressive strength of cement paste is the highest at low temperature,followed by mortar,concrete.The compressive strength of air-entrained cement paste ? mortar ? concrete with water-cement ratio of 0.50 at-40°C increased by 189%?175%?147% relative to 20°C respectively.As the porosity is increased after the addition of the air entraining agent,the compressive strength is smaller than that of no air entraining agent at room temperature,but the increment at low temperature is larger than that without air entraining agent.For example,the compressive strength of mortar with water-cement ratio of 0.50 at-40°C with and without air entraining agent increased by 175%?160% relative to 20°C respectively.The results of cubic splitting test show that the temperature decreases and the splitting strength increases gradually,however,the growth trend is not linearly increased and the increase is larger than the compressive strength.The low temperature has a greater influence on the splitting strength of the air-entrained cement-based material,and the splitting strength of mortar with and without air entraining agent at-40°C increased by 199%?166% relative to 20°C respectively.The result of dynamic modulus of elastic test of the prism show that the dynamic modulus of elastic enhancement law is basically consistent with the compressive strength,and basically increases linearly,but the increment is smaller than the compressive strength.(2)The conductivity test results of the cement-based cube show that the resistance value increases with the decrease of temperature;the resistance of concrete is the largest among the three cement-based materials,the mortar is the second,the cement paste is the smallest;the greater the water cement ratio,the greater the influence of temperature on resistance;the resistance with air entraining agent at low tempetature is higher than that without;The resistance and other parameters are substituted for the calculation model of ice content and then the results show that the ice content increases with the decrease of temperature.The cement paste had the largest ice content due to different water content,the mortar was second,and the concrete was the smallest;the larger the ratio,the larger the ice content at low temperature,and the cement-based material with air-entraining agent has a larger ice content than the non-air-entraining agent.Analysis of ice content on the test piece combined with the increment in mechanical properties at low temperature relative to room temperature,and obtain a linear relationship between the increase in compressive strength of the test piece and the ice content on the test piece.The increment of the splitting strength is exponentially related to the ice content in the test piece.In the future,it is expected to estimate the strengthening effect of mechanical properties by detecting the ice content on cement-based materials,and form an effective nondestructive testing method.