Experimental Research On The Bond Performance Of FRP Bars In Seawater And Sea Sand Concrete

Concrete is the basic structural material in civil engineering.However,producing concrete needs to consume a large number of fresh water,river sand and other natural resources,and then causes heavy environmental burden.Using seawater,sea sand,coral reefs and other marine resources to mix concrete is an important way to solve the dilemma.As seawater and sea sand contain a large amount of salt,steel bars in seawater and sea sand concrete（SWSSC）will corrode,which will seriously affect the safety of structures.It is a fact that the reinforced materials with excellent corrosion resistance should be used in SWSSC.Fiber reinforced plastic（FRP）bar is an ideal substitute considering it can not corrode by salt.In the FRP reinforced SWSSC structure,the bond behavior between FRP bars and SWSSC is the m ost basic mechanical properties.However,existing investigations are mainly focused on the single parameter and lacks systematicness due to the complexity of this problem.In current study,a series of bond tests of FRP bars and SWSSC were designed and carried out.The test parameters include d bond length,the diameter of FRP bars,the surface treatment of the bars,the type s of FRP bars,the strength grade of SWSS C and the types of concrete.Based on the test data and analysis,the main conclusions can be drawn,as follows:（1）The failure modes,bond strength,and bond stress-slip curves were obtained.Results showed that the failure modes could be divided into pull-out failure,splitting failure of SWSSC and FRP bars fracture failure.The bond stress-slip curve of FRP bars in SWSSC was similar to the normal concrete（NC）specimens,corresponding to three stages:microslip stage,slip stage,pull-out stage,softening stage and remaining stage.The bonding force between FRP bars and SWSSC mainly consists of three parts:chemical bonding force,friction force and mechanical bite force.（2）The bond strength of FRP bars in SWSSC decreases as the embedment length increases,which is approximately related to（l_d/d_b）^-0.41.The bond strength increases as increasing of the compressive strength of SWSSC and decreases as increasing of diameter of FRP bars.The surface treatment of FRP bars with coated sand significantly improved the bond properties of FRP bars with SWSSC.The enhancement factor could be taken as 1.76.The bond strength of BFRP and CFRP bars in SWSSC has little difference.On the other hand,the bond strength of GFRP bars in SWSSC was obviously lower than that of other two kinds bars.Compared with NC specimens,the bond strength of CFRP bars in SWSSC showed a certain degree of reduction.The bond strength ratio was between 0.87～0.91.It indicates that salt ions in seawater and sea sand will cause the degradation of bond strength.（3）ACI 440-1R-06,CSA S6-06,CSA-S806-12 and Japan（JSCE,1997）codes were adopted to predict the bond strength of FRP-SWSSC specimens.It showed that the predicted results were obviously higher than experimental results.These methods are not suitable for directly estimating the bond strength of FRP bars in SWSSC.Combined with the test data,a new formula for calculating bond strength was proposed based on the modification of ACI 440-1R-06 specification,and the accuracy of the predicted results was evaluated by the average absolute error（AAE）and average ratio（AR）.The results show that the predicted results are very close to experimental results.AAE is close to 5%,and AR is close to 1.The applicability of this method still needs further verification.（4）Based on the test data,the development length calculation formula was established.The required development lengths were given,and the minimum development lengths of FRP bars in C35 SWSSC were proposed.According to the test results,the results of development length in specification formula were calculated and evaluated.AR are all greater than 1,indicating that the development length determined by the specification formula is safe.The results of this study provide basic data for the design of SWSSC reinforced by FRP bars.