Research Of Freeze-thaw Mechanism And Durability Evaluation Of Concrete And Prestressed Concrete

Freeze-thaw damage is one of the most serious diseases for structures in cold regions.Damages induced by freeze-thaw cycles(FTCs)influence the serviceable performance and long-term safety,which cause huge economic losses.FTCs will damage the meso-scopic structure of concrete,and the change in meso-scopic structure determines the macro-scopic material deterioration and failure,further the damaged material cast shadow on the service life of concrete structures.In this paper,the meso-scopic damage,macro-scopic deterioration of concrete material and the loss in serviceable performance of concrete structures were analysed through experimental,theoretical and finite element methods(FEM).(1)The pore structure of the hardened concrete and the meso-scopic changes of a few selected pores throughout the freeze-thaw test were investigated by a method combining RapidAir,digital metalloscope and Image Pro-Plus(IPP).Traditional tests were also performed to evaluate the macro-scopic change caused by FTCs and its relationship with meso-scopic change.The investigation shows that the concrete material with large spacing factor and small air content is still frost-resistable.Meso-scopic cracks tends the occur early between cluster of pores to release the concentrated stress.The development of these meso-scopic cracks corresponds well with the loss in compressive strength in the early stage of FTCs.Isolated pore and interfacial transition zone(ITZ)are not sensitive to freeze-thaw damage.If sufficient FTCs is achieved,these areas will be damaged and the compressive strength will drop non-linearly,indicating the coming macroscopic failure.Comparing with the relative dynamic modulus of elasticity(RDME),the compressive strength is a more stable and reliable evaluation indicator when assessing the performance of concrete material throughout its life,especially when the material is severly damaged by FTCs.The compressive strength loss versus FTCs could be divided into three parts.At first,the meso-scopic cracks increase the defects inside the concrete,the compressive strength dops quickly.Then,these meso-scopic cracks develop steadily,and some pore edges begin to scale due to FTCs,the loss of compressive strength slows down.At last,the meso-scopic cracks connect with each other forming wider cracks,the compressive strength drops more quickly.(2)A modular hybrid programming of Python language and Fortran language was adopted in the Abaqus analysis system.The concrete damaged plasticity model was modified to delete the failed element.The code of automatic analysis program containing geometric generation,material definition,element mesh and restart analysis was unveiled.This program has a good user interface and a good versatility.If necessary,user could change the size of geometry model and material properties.All of the time-consuming process will be done by the program,which improves the efficiency considerably.The model shows quite similar meso-scopic freeze-thaw damage with the experimental research.As FTCs are repeated,cracks tend to occur early between group of pores,edge broken appears late when sufficient pore extension is reached.Compressive strength loss of the model coincide with the test result,could be divided into three parts too,but more smooth.Moreover,the envelope diagram of the simulated compressive strength covers the test result points.(3)It was the first time that the prestressed concrete members were tested throughout its freeze-thaw durability life.Prestress forces have a remarkable impact on the failure pattern under FTCs.External forces will change the stress distribution,which might enhance or weaken the meso-scopic freeze-thaw damage,so cracks inside the member develop quite different from that inside the concrete cube.In addition,the test and analysis show that the measured prestress losses of bonded specimen are larger than that of unbounded specimen under the attack of FTCs due to the duct grouting effect.The ultimate freeze-thaw prestress loss is about 5%of ?con for both the bonded and unbonded specimens because the grouting cement paste will eventually be completely destroyed.The freeze-thaw prestress loss is a long-time prestress loss,as time goes on,this kind of prestress loss will increase constantly.When evaluating the concrete structures in cold regions,the minnum suggested freeze-thaw prestress loss should be larger than 5%of ?con.According to the meso-scopic and macro-scopic research about concrete material under FTCs and the test of prestressed concrete members under FTCs,reliable FEM simulation techniques and simplified calculation methods for concrete structures under FTCs were proposed.Compared with the experimental results,the proposed techniques and methods are accurate if the compressive strength remains more than 50%.From the experimental,analytical results and the life prediction technique based on performance degradation,a serviceable life prediction technique for concrete structures based on compressive strength degradation was proposed.