Microstructure Parameters In Concrete And The Time-varying Of Chloride Diffusivity

Under the tidal environment,chloride ion penetration will cause the corrosion of steel bars,leading to a reduction of bearing capacity of concrete structures or components and eventually causing the durability damage of concrete structures.In fact,under the natural environment,the durability failure of concrete structures is a long and complicated process.In this process,both the chloride ion diffusivity and microstructure parameters of concrete vary with time,due to that the environmental factors and material properties will change with time.The time-dependent characteristic of chloride permeability in the macroscopic performance of concrete is an intuitive reflection of the changes in internal microstructure.Therefore,it is important to investigate the time-dependent characteristic of permeability and microstructure of concrete,as well as their relationship,in order to reveal the time-varying formation mechanism of concrete diffusion.A long time exposure test of concrete with different water-binder ratios and different admixtures was designed under the natural tidal environment based on the previous studies.The microstructure parameters and pore characteristics of concrete were tested by nuclear magnetic resonance(NMR)after different exposure times.And the time-varying relationship between the chloride diffusivity and the measured microstructure parameters was established.Then,the differences between microstructure parameters tested by mercury intrusion porosimetry(MIP)and NMR were compared.In this paper,the main contents and results are shown as follows:1.The pore microstructure characteristics of concrete after 28 d standard curing were tested by NMR,MIP and X-ray computed tomography(X-CT).Then,the microstructure parameters and morphology of pore characteristics such as porosity and internal morphological images tested by NMR,MIP and X-CT were compared from the aspects of test method,principle and process.2.Under the tidal environment,a penetration experiment of concrete with different water-binder ratios and admixtures was completed for 360 d exposure times.The porosity,pore size distribution and microstructure parameters were tested by NMR after different exposure times.And the time-dependent characteristic of pore parameters in concrete was studied.Results show that the time-varying of microstructure pore characteristic is obvious.3.The influences of water-binder ratio and admixture on the time-dependent characteristic of chloride diffusivity under the long-term natural tidal environment were studied.The time-dependent relationship between the diffusivity of concrete with admixtures and the microstructure parameters was established.Results show that,there is a good correlation between the time-dependent microstructure parameters and chloride diffusion coefficient.The porosity of concrete can be used to verify the apparent chloride diffusion coefficient.4.The test range of pore diameter was analyzed,and the results measured by NMR and MIP were compared.In addition,the reasons for the different test results were analyzed from the aspects of testing principles,test ranges,sample acquisition and sample pretreatment.According to the analysis of the test results,the time-varying microstructure parameters in the concrete by these two methods show a consistent trend.Moreover,the microstructure parameters of concrete measured by NMR may be closer to the reality.