Study On The Influence Of Saturation On The Deterioration Of Hydraulic Concrete By Freeze-Thaw

Freezing-thawing and temperature fatigue are important factors that affect the strength,stability and durability of concrete water projects in long-term service,and seriously restrict the service life of concrete water projects.The existing literature mainly focuses on the laboratory rapid freeze-thawing test under the condition of water freezing and water thawing for saturated or highly saturated concrete.However,engineering practice shows that the freeze-thawing cycle of saturated or highly saturated concrete occurs only when concrete in water engineering is in frequent contact with water and is in an alternating state of positive and negative temperatures.In actual concrete water project,the back surface,the water position and even the water level change area are all in the unsaturated freeze-thaw state.According to the hydrostatic pressure theory,when the water content of concrete is less than91.7%of the total pore volume,there will be no frozen expansion pressure,that is,saturation is an important factor affecting the freeze-thaw deterioration of concrete.In order to master the real working behavior of freeze-thaw deterioration of concrete in water engineering,this paper,according to the freeze-thaw characteristics of hydraulic concrete structure,divided concrete freeze-thaw into open system and closed system freeze-thaw,carried out freeze-thaw deterioration test of hydraulic concrete with different saturation,and then carried out model and numerical calculation research.The main contents are as follows:（1）Experimental design of freeze-thaw deterioration of unsaturated hydraulic concrete.Firstly,the preparation test of concrete specimens with different saturation was carried out.The research shows that the saturation growth of cube and prismatic specimens slows down gradually with the increase of immersion time.The saturation of the cube specimen after 7d immersion was 85%～91.7%and 91.7%～95%,and that of the prismatic specimen after 21d immersion was 85%～91.7%.Based on this,hydraulic concrete specimens with different saturation were prepared.Then three different sealing methods,namely polyurea sealing,paraffin sealing and vacuum bag sealing,were selected,and the vacuum bag was finally used.Based on this,two freeze-thaw test schemes of closed system and open system were designed.（2）Freezing and thawing deterioration test of unsaturated hydraulic concrete.The freeze-thaw deterioration tests of hydraulic concrete with different saturation and different air entraining agent contents were carried out.The test results showed that:（1）For sealed freeze-thaw specimens with saturation range of 85～91.7%and 91.7%～95%,with the increase of freeze-thaw cycles,the appearance quality and mass loss had little change,and the compressive strength loss was small.However,the surface quality,mass loss and compressive strength of the water-frozen and water-thawing specimens were significantly damaged,and the relationship between the compressive strength loss and saturation was P_85⁹1.7%<P_91.7%⁹5%<P_100%.（2）For different entraining agent contents,the deterioration degree of each damage index is P_0.025%<P_0.05%<P_0%,that is,the addition of entraining agent can increase the frost resistance of concrete,but excessive addition of entraining agent will reduce the frost resistance.（3）For the strain monitoring,the rate of residual strain increase of the specimen with air entrainment agent 0.05%is smaller than that of the specimen without air entrainment agent.The rate of increase of residual strain of the sealed freeze-thaw specimen（saturation85～91.7%）was similar to that of the water-freeze-thaw specimen before 125 freeze-thaw cycles,but after 125 freeze-thaw cycles,the rate of increase of residual strain of the water-freeze-thaw specimen was significantly higher than that of the sealed freeze-thaw specimen.（3）Freeze-thaw deterioration model of unsaturated hydraulic concrete.Combined with the above freeze-thaw test data,the freeze-thaw deterioration model of concrete with single saturation combined exponential,the freeze-thaw deterioration model of concrete with Weibull,and the freeze-thaw deterioration model of concrete with coupling saturation and freeze-thaw frequency were established,and the above different models were compared.The analysis shows that（1）the calculated values of the freeze-thaw damage model of hydraulic concrete with coupling saturation and the number of freeze-thaw cycles are in good agreement with the measured values,which can reflect the changing trend of the damage rate of compressive strength;（2）According to the Weibull freeze-thaw deterioration model established,it can be seen that the scale factorλ₀ varies with the size and saturation of each group in F_85⁹1.7%<F_91.7%⁹5%<F_100%,and the relationship with the amount of air entraining agent is F_0.05<F_0%,that is,the higher the saturation,the lower the frost resistance of concrete.The frost resistance of concrete increases after adding air entrainment agent.（3）Compared with different freeze-thaw damage models,it is found that for sealed freeze-thaw specimens,the two-index model has the best fitting effect,followed by the freeze-thaw damage model based on Weibull theory.For water-frozen and water-thawing specimens,the fitting effect of each model is good,and the complex correlation coefficients are all above 0.9.（4）Numerical simulation and feedback of deterioration process of hydraulic concrete by freeze-thaw.Firstly,the coupling effect of stress field,seepage field and temperature field of hydraulic concrete in freeze-thaw process is analyzed numerically.Then,the calculated temperature and strain are fed back together with the measured temperature and strain data of hydraulic concrete freeze-thaw test.The results show that for the temperature field,the central temperature variation rule calculated by the model is in good agreement with the measured values,and there is an obvious platform around 0℃,which reflects the latent heat phenomenon of water phase transition.For stress field,the minimum vertical strain calculated by the model is similar to that measured.For the seepage field,the pore pressure increases gradually from the surface to the inside of the specimen and changes periodically with temperature,which accords with the general rule.