Damage And Deterioration Mechanism Of Lightweight Ultra-high Performance Concrete Under The Coupling Action Of Load And Corrosion Medium

The lightweight ultra-high performance concrete(LUHPC)is an advanced cement-based material prepared by partially or completely replacing the quartz sand in the ultra-high performance concrete(UHPC)with lightweight aggregate(LWA)based on the light-high-strength low-shrinkage collaborative design concept.LUHPC is inevitably affected by loads and corrosive media during its service.Load often lead to damage in the interior of LUHPC(such as crack initiation and propagation in the interface transition zone).This increases the transmission channel of corrosive media,and the accelerated invasion of corrosive media leads to the aggravation of LUHPC damage.However,the damage of LUHPC under high stress level is not limited to the interface transition zone,and porous LWA is also a high-risk area for damage.The high porosity of LWA leads to its low strength,which has a great impact on the formation and propagation of microcracks in LUHPC under load and the transmission of corrosive media.Therefore,it is necessary to discuss the microstructure evolution and performance degradation mechanism of LUHPC under the coupling action of load and corrosion medium,so as to lay a foundation for the life cycle assessment of LUHPC.This thesis relies on the joint fund project of the National Natural Science Foundation of China “The microstructure modification and performance improvement mechanisms for ultra-high performance concrete(UHPC)material under the bridge dynamic effects”(U21A20149)and the general project “Preparation of lightweight and low-shrinkage ultra-high performance concrete and its microstructure formation mechanism”(51878003).The stress corrosion device suitable for LUHPC structure is designed by selecting high-precision and high-durability four-point bending loading equipment,so as to study the damage and deterioration law of LUHPC under the action of bending load and corrosion medium(chloride or sulfate).By analyzing the damage and deterioration mechanism of LUHPC,the damage evolution equation reflecting the stress corrosion effect is established.The innovative achievements of the thesis are as follows:(1)The damage and deterioration mechanism of LUHPC under bending load is explored.The macroscopic morphology evolution,mechanical performance degradation and microstructure damage rate of LUHPC are positively correlated with the stress level and loading age.According to the stress difference of different areas of LUHPC under bending load,it is divided into compression area and tension area.As the loading progresses,the corrosion resistance coefficients of the compression zone and the bending zone decrease to 63% and 66% respectively,with the difference gradually decreasing as the loading progresses.The internal damage of LUHPC is gradually aggravated under load,which promotes the release of water in LWA,fills defects and slows down the damage rate of LUHPC under load.Finally,a reliability evaluation method based on Weibull distribution function is proposed to predict the fatigue life of LUHPC under bending load.(2)The mechanism of microstructure damage and performance degradation of LUHPC under the coupling action of bending load and chloride salt was revealed.A response surface model was built to study the variation of chloride ion content in LUHPC under the coupling action of bending load and chloride.With the increase of stress level and loading age,the content of chloride ions in LUHPC increases,reaching a maximum of 0.366%,and the content of chloride ions in LUHPC compression zone is lower than that in tension zone.The change law of mechanical properties of LUHPC under the coupling of bending load and chloride is basically the same as that of chloride ion content,both of which are external manifestations of microstructure damage of LUHPC.LWA reduces the possibility of steel fiber corrosion by absorbing chloride ions that invade the interior of LUHPC.Finally,the fatigue life equation of LUHPC is established by using Weibull distribution function to evaluate the reliability of LUHPC under the coupling action of bending load and chloride.(3)The damage and failure mechanism of LUHPC under the coupling of bending load and sulfate was revealed.The damage rate of LUHPC increases with the increase of stress level and loading age,which is manifested by the increase of defects,the increase of sulfate ion content and the deterioration of mechanical properties.In the tensile area of LUHPC,the bending load accelerates the sulfate attack of LUHPC,while in the compression area,the bending load increases the resistance of LUHPC to sulfate attack.This effect becomes more obvious with the increase of stress level.The porous nature of LWA provides space for the formation of corrosion products and avoids the expansion and cracking of the matrix.Finally,the reliability of LUHPC under the coupling of bending load and sulfate is evaluated by constructing the fatigue life equation of LUHPC.Figure 49 Table 29 Reference 181...