Experimental Study On Anchorage Behaviors Of Yield Bar Based On Split Failure

In the theory and method of reinforced concrete structure design,the anchorage of reinforcement is one of the main structural details,and the bond strength is the basic index for the evaluation of bond behaviors.Chinese codes and design guidelines require that the strength-yield-ratio of seismic reinforcement should not be less than 1.25,and rebar pull-down failures are supposed to occur before anchorage damage.However,the correction coefficient of the seismic anchorage length in Chinese codes and design guidelines only considers the negative influence of reciprocating load on the reinforcement bond behaviors and does not explicitly require the additional development length that is required for reinforcement stress to reach 1.25 f_y.After the steel yield,the yielding point moves inward from the loading end,and there is a yield penetration section with the rebar stress higher than the yield strength from the yielding point to the loading end.The length of the yield penetration section is defined as the yield penetration depth.Existing research shows that the reliability of the development length of the original design is reduced,there is a potential safety hazard;and the members in the actual structure often work with cracks near the ultimate limit state,and the existence of cracks will aggravate the bond degradation.There is uncertainty about whether the current design development length can provide sufficient bond strength.Therefore,the anchorage test of 21 beam-end specimens was completed to study the bond behaviors of the steel rebar under the split failure condition.In order to realize the expected damage form,a small protective layer thickness is adopted to ensure the split cracks occurs during the test;the encryption stirrups ensure that the anchorage damage will not occur immediately after the split cracks occur;control the specimen development length,so that the reinforcement can beyond the yield strength but not pull.To determine the position of the reinforced yielding,study the bond strength of the yield penetration depth by employing methods that strain distributions of steel rebars were measured on the rebar surface.And experiment variables include concrete strength,steel strength grade,steel diameter,protective layer thickness,casting position,etc.The relative slip of loading end,steel strain distribution,bond stress distribution and yield penetration depth were studied and analyzed.Based on the test results,the bond strength of pre-yield segment and the existing short anchor test data are verified,which proves that the whole development length can be divided into yield section and pre-yield section to discuss the bond behaviors.The calculation formulas of the bond strength before and after the yield of the steel bar are obtained.The development length calculation formula is based on split failures and considers bond degradation after rebar yielding.The proposed development length and the codes design lengths are compared to evaluate the safety reserve.