| In recent years,due to the shortage of raw materials for concrete mixing,the direct use of seawater and sea sand for concrete mixing has become a popular topic in the research community because of the large amount of available sea resources and the decrease of transportation costs,especially for the marine constructions.However,the rich salt contents in seawater and sea-sand may cause the severe corrosion problems of steel rebars.Thus,it is an attractive way to alleviate the problem by replacing the ordinary steel rebars with the epoxy-coated steel rebars(ECR)in the seawater sea-sand concrete(SSC).Before the applications of the novel ECR-SSC structures in marine and costal constructions,the bonding behavior between the two new materials should be investigated first due to its significant importance.Therefore,the bond properties of epoxy-coated steel bars and seawater sea sand concrete were systematically analyzed in this study through the traditional pullout test.The main research contents and conclusions of the paper are listed as follows:(1)In this paper,a total of 88 axial pullout specimens were prepared and tested with different influenced factors,including the rebar diameter,cover thickness,bond length,concrete strength,and stirrup ratios.Test results show that there are three failure modes in the test: split failure,pull-out failure,and split + pull-out failure.Thus,the bond-slip curves of epoxy coated steel bars and seawater sea sand concrete specimens can be divided into three types: the curves with only monotonically ascending segment,the curves with the post-peak branch and no residual branch,and the curves with horizontal residual portion.(2)According to the test results,the coated epoxy film reduced the bond strength between the ECR and concrete while better corrosion resistance can be achieved.For seawater and sea sand concrete specimens,the bond strength of 16-mm,20-mm,25-mm epoxy coating steel rebars is 91%,95%,and 93% of the bonding strength of traditional deformed steel rebars with the same size,respectively.At the same time,the epoxy coating can also increase the relative slip between epoxy coated rebars and the concrete matrix.For ordinary steel bars and seawater and sea sand concrete specimens,the maximum average bonding stress was increased with the bar diameters.However,the obvious trend was not found for epoxy-coated steel bars.The ultimate bond strength was decreased with the increase of the bond length,but it was increased with the stirrup ratios.The same trend was also observed for black steel rebars and concrete.Based on the results,the parameters of bond strength,concrete cover,diameter,the strength of the concrete,and the stirrup ratios were used to determine the bond strength and slip characteristics of the epoxy-coated steel rebar and the black steel rebar seawater sea sand concrete specimen.The traditional linear model was optimized according to the bond strength and slip key points and a new bond-slip constitutive model was established.The new bond-slip model was proposed for both coated and uncoated rebars embedded in SSC and good agreement can be achieved between the test observations in this study and theoretical predictions based on the proposed model.(3)In this paper,the axial pull-out test specimens with the strain gauges attached on the steel rebar were tested.By analyzing the results of the strain gauges of the steel rebar,the distribution of the bond stress within the embedment length was calculated and the position function was established.It can be observed that the bonding stress distribution was not uniform along the bond zone,and the maximum position of the bond strength was about one-third from the free end.This is because that the concrete constraints at both ends of the bond length are relatively small.Combining the aforementioned modified bond-slip curve model and the effective position function,the resulting bond-slip constitutive model was finally established. |
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