Research On The Deterioration Process And Mechanism Of Reinforced Concrete On The Basis Of Non-destructive Monitoring

The coupling action of freeze-thaw cycles and de-icing salt is a typicaldeterioration mode in cold regions. It will result in severe surface scaling and internaldamage of the concrete matrix. However, measurement of existing durabilityevaluation mainly focuses on weight loss and retention value of dynamic elasticmodulus. The development of assessment method fall behind the actual durabilityissues obviously. Research on durability of concrete under combined actions mainlyfocuses on the degradation of concrete macro properties, while internal damage anddeterioration behavior of pore structure in mesoscopic scale as well as the relationshipbetween pore structure deterioration and permeability were neglected so far. Morethan that, cracks induced by freeze-thaw cycles and external load increase the hazardof steel corrosion and chloride penetration when concrete subject to freeze-thawcycles, chloride salt attack and external load. Therefore, corrosion behavior ofreinforced concrete subjects to combined action deserve to research systematically.To solve the above problems, the deformation and resistivity behavior of cementbased materials were monitored by using strain gauges and7-1/2digital multimeter,respectively. Relationships between traditional evaluation parameters and residualstrain as well as test method and evaluation basis of resistivity to characterize thedurability of cement based materials were also studied. The deterioration behavior ofconcrete pore structure under the action of freeze-thaw cycles in mesoscopic scalewas investigated. Further, given that the possible effects of freeze thaw cycles andchloride salt attack combined with load, corrosion behavior of steel bar were studied.A deterioration process model of concrete exposed to freeze thaw cycles wasproposed according to the results of strain, resistivity and pore structure. Thefollowing research findings were obtained:Test method of strain to characterize the damage of concrete expose to freezethaw cycles and chloride salt attack were established. Results show that deformationof concrete show nonlinear. The150με residual strain of saturated concrete with thewater to cement ratio of0.65were generated after10freeze thaw cycles.Strain-temperature hysteresis phenomenon was found. With regard to the driedconcrete, coefficient of thermal expansion decrease with the increase of freeze thawcycle numbers. Thermal expansion coefficient of concrete with the water to cementratio of0.51and0.61decrease by0.87(10^-6/℃) and0.23(10^-6/℃) respectively after 45heating and cooling cycles for non-air entrained concrete, and air-entrainedconcrete with the water to cement ratio of0.51decrease by0.79(10^-6/℃).Critical residual strain during the process of freeze thaw damage was found.Concrete only suffer slight damage before the critical residual strain for specimenswith the w/c of0.61. Weight loss increases obviously and relative dynamic elasticmodulus decreases rapidly after exceeding the critical residual strain. Bothair-entrained concrete and concrete with the w/c of0.39have two critical residualstrain points, namely, initial critical residual strain SICRSand critical residual strainSCRS.Concrete can be unacted on the environmental load before SICRS. Then concretesuffer slight damage until it reaches the SCRS.Test method and evaluation basis of resistivity to characterize the freeze thawdamage of cement based materials were established. Results show that resistivity isproportional to freeze thaw damage. Resistivity reflects the variation of pore solution,pore connectivity and pore sinuosity as well as cracks and defects of cement basedmaterials. Resistivity-temperature hysteresis phenomenon was also found. Pearsoncorrelation coefficient of ρmaxto slope of fitting curve P and Y-intercept A are0.926and0.958respectively. Further, ice nucleation temperature, depecolation point andrepecolation point were obtained from the resistivity-temperature curve. Furthermore,chopped wave of resistivity curve of unsaturated cement based materials wasobserved.Internal damage of concrete caused by freeze thaw attack was confirmed by flatscanning method. Results show that average crack number and average crack lengthincrease with the progress of freeze thaw damage. Average crack number can reach0.25and0.5（average crack length can reach3.5μm and7μm） for concrete subjectedto50and100freeze thaw cycles, respectively. Moreover, per area increases with theongoing of freeze thaw damage. Fractal dimensions of pore sectional are between1.09and1.14and average roundness is between1.6and2.Effects of external load, freeze thaw cycles combined with chloride slat attack oncorrosion behavior of steel bar were clarified. Results show that steel bar will keeppassivation state after200freeze thaw cycles combined with chloride salt attack withthe absence of external loads. depassivation time of steel bar moved up with the stressratio of0.4, the cathode reaction was restrained and resistance of anode reactiondecreases after the depassivation of steel bar. Steel corrosion damage may occur inreinforced concrete during10¹5years.Deterioration model of concrete exposed to freeze thaw damage was established. The deterioration process of concrete can divide into three phases, namely, inductionphase, accelerated phase and stabilization growth phase, according to the difference ofchanging rates of residual strain. Saturation degree of concrete is increasing ininduction period, but weight loss is not obvious. Internal crack numbers and crackdensity increase with the growing of residual strain, and the deterioration degree wascontrolled by the saturation degree in accelerated phase. With regard to thestabilization growth period, long cracks connect to each other and the connectivityincrease. Paste and mortar scaling from the surface of concrete steadily.