Influence Of Sustained Loading And Cracks On The Transport Properties Of Water And Chloride Within Concrete

The coastal and offshore reinforced concrete(RC)structures in the service process are inevitably subjected to combined actions with various loadings and environment.The synergetic effects of environmental and mechanical loading actions on mass transport process in concrete are the most crucial factor controlling the deterioration of mechanical properties and durability of RC structures.Usually,the drying-wetting zones in coastal concrete structures are always at the highest risk of concrete deterioration and reinforcement corrosion.This reveals that water is either the principal agent responsible for the deterioration or the principal medium by which aggregessive agents(i.e.chloride ions)are transported into the concrete.It has been realized that the different damage and cracking(cracks or microcracks),which are are usually present in concrete caused by mechanical loadings,can significantly accelerate the ingress of water,oxygen and soluble aggregessive agents into concrete,since it provides the preferential flow channels and allow more aggregessive agents to penetrate.Furthermore,it is also believed that cracking plays an important role on the speed of mass transport,resulting in the shortening of RC service life.Therefore,it is of great significance to investigate the transport properties of aggregessive agents in damaged concrete subjected to loading.Based on the above-mentioned considerations,the studies on the influence of loading action and cracks on transport properties of water and chloride in unsaturated concrete are carried out.The specific contents of the research work and main conclusions are summarized as follows:(1)According to the principle of communicating vessels,an improved device for real-time measurement of cumulative absorbed water of concrete subjected to loading was designed.The influence of sustained loadings on water absorption of concrete specimens was experimentally conducted.The correlation between loading mode(tension and compression)or stress level(ranging in 0～50%of the corresponding loading capacity)and sorptivity was quantitatively established.The results show that within the range of a given low stress level.the cumulative water content at a given exposed time and sorptivity of concrete subjected to the sustained loading for 32-hour initially decreases with an increase of compressive stress level and then increases beyond the critical stress level.For the tensile loading case,it increases as the tensile stress level increases,but the effect of tensile stress level on initial sorptivity was not remarkable.Based on the unsaturated flow theory,the prediction models of hydraulic diffusivity and sorptivity under sustained loading were developed.The water content distribution considering the effect of sustained loading was calculated by the proposed method,and the profiles can be in good agreement with the test data.(2)The experimental investigation on chloride transport in unsaturated concrete(with initial saturation degree of 0,50%and 100%)subjected to sustained loading was conducted.The effect of sustained loading and saturation on chloride content and profiles in concrete was discussed,and the relationship between water/chloride diffusivity and sustained stress level(tension and compression)was developed.Based on the transport mechanism within unsaturated concrete,the numerical approach was developed to investigate chloride transport process under sustained loading.Comparisons were made between the results of chloride profiles from experiments and numerical method.It shows that the numerical results of chloride profiles obtained by the proposed model are in good agreement with those of experimental measurement under various stress levels.This study indicates that chloride transport in unsaturated concrete depends on the convection induced by capillary absorption and the diffusion driven by the concentration gradient.The convection effect generally weakens with an increase of saturation,but the diffusion action strengthens.(3)Based on the unsaturated flow theory and the experimental results of capillary absorption,the water content distributions in load-damaged concrete after subjected to higher uniaxial compressive or tensile stress levels(70%,80%and 90%of the corresponding compression and tension loading capacity)were predicted by means of the exponential law and power functions.The prediction approach of water content distribution in damaged concrete was established.The results show that the curves of cumulative water content versus square root of time in damaged concrete exhibits the bilinearity trend.Compared with non-loaded condition,the mechanical loading,whether in compression or tension,could significantly improve the absorbed water content of concrete.The sorptivity and penetration depth of load-damaged concrete can be up to double that of sound concrete sample for the studied ranges of load level.However,the effect of two loading patterns,i.e.sustained loading for 10 min or repeated loading for 25 cycles,on water absorption of concrete,was not obvious.(4)Based on the classical Lucas-Washburn equation and capillary absorbed water of parallel plate,the model of capillary imbibition of water in discrete planar cracks was established.A benchmark study to investigate capillary flow in a series of discrete granite cracks(23.64 μm～240.38 μm),artificially fabricated by means of jointing granites with glue(ignoring the influence of lower aborption of granite),was reported.Combined with the developed theoretical model.the water transport speed in a single crack was calculated.The correlation between transport speed and crack width was developed.The comparison between experimental and theoretical results indicates that the L-W capillary imbibition model of porous capillary tube after taking into account the gravity effect can be applied to the case of discrete crack with a certain range of width.When the crack width exceeds this threshold value,the theoretical model presented should be amended and validated further due to the other potential mechanisms.(5)Considering two main existing forms of water involving both gas and liquid phases and transport hysteresis during drying and wetting periods,the theoretical models of water and chloride transport speed in cracked concrete subjected to drying-wetting cycle were developed.In terms of the mesoscale methodology that the cracked concrete is treated as a four-phase composite material consisting of coarse aggregate,mortar,interfacial transition zone(ITZ)and crack,the two-dimensional lattice network model with a single crack is established by Voronoi diagram meshing and adopted to conduct the numerical simulation of moisture and chloride transport in cracked concrete subjected to drying-wetting cycles.Furthermore,the parametric sensitivity analysis of drying-wetting cyclic mechanism,drying-wetting cycles and crack width is reasonably carried out.Compared with the experimental results,the proposed models of water and chloride transport speed in cracked concrete are reasonably validated.