Durability And Service Life Prediction Of Concrete Under Complex Environmental Conditions

In the field of transport infrastructure,concrete materials are widely used in roads,bridges,culverts,tunnels,and urban overpasses,etc.They are directly exposed to the external environment,in addition to withstand loads,and also subject to the erosion of complex environmental conditions,then their using function and bearing capacity gradually decrease.At present,many standard test methods of concrete durability are to simulate the single environmental factors,which cannot reflect truly the durable characteristics of concrete going through multiple factors under complex environmental conditions.Although the amounts of researches on concrete durability considering multiple environmental factors are increasing rencently,the environmental factors are still not considered fully.The related research achievements,such as the change law of durability,damage mechanism and service life prediction of concrete under complex environmental conditions,are still not formed systematically.Carbonation,drying-wetting cycles,freezing-thawing cycles and chlorine salt erosion were selected to constitute the complex environmental conditions,and the experiments on concrete durability exposed to the above conditions were conducted in lab.The relative compressive strength,relative dynamic modulus of elasticity,mass change and carbonation depth were measured as the macro performance indexes,and the microscopic pore structure parameters were obtained by the mercury injection test.The change law of concrete durability under complex environmental conditions was investigated via these indexes.The compressive strength attenuation model,the damage model and the prediction model of carbonation depth of concrete under complex environmental conditions were established,respectively.Also,the service life prediction of concrete structure under complex environmental conditions was discussed preliminarily.The main research works and research results in this article are as follows:The indoor tests of concrete durability exposed to complex environmental conditions were designed and conducted,including the test of alternate carbonation and freezing-thawing actions,the test of alternate carbonation and drying-wetting and freezing-thawing actions,and also some tests of single factors,such as the test of carbonation actions,the test of drying-wetting actions and the test of freezing-thawing actions,which were used as the contrast tests.The above tests were conducted in water environment and salt water environment,respectively.Macroscopic test results show that for water environment,the relative compressive strength of concrete under the two alternate actions is greater than that for the freezing-thawing actions.Also it's great for alternate carbonation and drying-wetting and freezing-thawing actions compared with alternate carbonation and freezing-thawing actions.However,for salt water environment,the relative compressive strength of concrete under the two alternate actions is less than that for the freezing-thawing actions,and also it's low for alternate carbonation and drying-wetting and freezing-thawing actions compared with alternate carbonation and freezing-thawing actions.Different from the change law of relative compressive strength of concrete,for both water environment and salt water environment,the relative dynamic modulus of elasticity of concrete under the two alternate actions is less than that for the freezing-thawing actions,and the mass loss is greater than that for the freezing-thawing actions.Also the loss of dynamic modulus of elasticity and mass of concrete exposed to alternate carbonation and drying-wetting and freezing-thawing actions is greater than that for alternate carbonation and freezing-thawing actions.In general,the durability of concrete exposed to complex environmental conditions is inferior to that exposed to the freezing-thawing actions.And the more complex environmental factors,the more serious deterioration of concrete durability is.For both water environment and salt water environment,the amount of capillaries and macropores inside concrete under the two alternate actions increases with the increase of alternate action times,and the total pore volume and total porosity of concrete increase,which is consistent with the change of macroscopic performance of concrete,i.e.the compressive strength reduces.However,the amount of gel pores and transition pores is different,and the total pore area and average pore diameter change irregularly.Microscopic test results show that the alternate actions of multiple environmental factors contribute to the destruction of micro-pore structures of concrete,while the pore morpholohy of concrete is random.A new index,pore sinuosity of concrete was also put forward.The pore sinuosity of concrete under the two alternate actions decreases with the increase of alternate action times,the pore structures of concrete become short and thick.And the pore sinuosity of concrete is also consistent with its compressive strength attenuation law,which indicates that pore sinuosity can reflect the change law of concrete durability better.The compressive strength attenuation models of concrete under complex environmental conditions,i.e.alternate carbonation and freezing-thawing actions,or alternate carbonation and drying-wetting and freezing-thawing actions were established,respectively,based on the data of relative compressive strength from the carbonation,drying-wetting and freezing-thawing actions.Combining the theory of damage mechanics,the damage values of concrete were calculated via the relative dynamic modulus of elasticity,and the environmental influence coefficient ? of concrete damage was put forward.The damage models of concrete under complex environmental conditions,i.e.alternate carbonation and freezing-thawing actions,or alternate carbonation and drying-wetting and freezing-thawing actions were established,respectively,based on the calculated results of damage values from the carbonation,drying-wetting and freezing-thawing actions.The carbonation depth of concrete under the two alternate actions,i.e.complex environmental conditions was measured and compared with that for the carbonation actions.Regardless for water environment or salt water environment,the carbonation depth of concrete under the two alternate actions is greater than that for the carbonation actions.After the same carbonation time,the carbonation depth of concrete exposed to alternate carbonation and drying-wetting and freezing-thawing actions is greater than that for alternate carbonation and freezing-thawing actions.Test results indicate that the more complex environmental factors,the larger carbonation depth of concrete is.Based on the above data,the environmental influence coefficient ? of concrete carbonation depth was introduced,and the prediction models of concrete carbonation depth under complex environmental conditions were established,which took into account the influence of multiple environmental factors.Combining the damage model of concrete under complex environmental conditions and the environmental influence coefficient ? of concrete damage,the service life prediction model of concrete structure under complex environmental conditions was established preliminarily.According to the minimum limit values of durability factors from Chinese standard “Code for durability design of concrete structures”,the suggested values of maximum freezing-thawing numbers of concrete were given.The service lives of concrete structures which employed concrete materials in this article were analyzed,which indicate that the more complex environmental factors,the lower service life of concrete structure is.The combined actions of multiple environmental factors should be considered especially when designing concrete durability.The research on concrete durability under complex environmental conditions in this article clarified further the mechanism and change law of concrete durability,and the related research achievements have an important guiding significance for improving the experimental methods of concrete durability and perfecting the evaluation standards of concrete durability in the future.