Experimental Study On Anchorage Performance Of Headed Bars Reinforcement

Presently,research on mechanical anchorage for steel reinforcement in concrete is mainly focused on welded short bars or welded steel plates on the end of the reinforcement,while little research has been done on straight-threaded donut（STD）headed bars.The anchorage strength（τ_u）was used as the index for evaluating the anchorage performance,and the distribution of anchorage force（F_y）between the bonding force（F_b）of the straight anchor section and the bearing force（F_p）of the headed bars was not explained in detail.Based on this,130 headed bars specimens with STD were fabricated and tested in the pull-out test.The anchorage performance and anchorage force distribution were explored with the variables of reinforcement strength,reinforcement diameter,anchorage length,concrete strength,protective layer thickness,and whether stirrup was configured,and the following conclusions were drawn.Based on the behaviors of specimens,three main damage patterns of the STD headed bars were obtained,among which yielding damage of reinforcement and concrete fracture damage occurred in the specimens with stirrup;splitting damage of concrete occurred in the specimens without stirrup.After analyzing the load-slip curve,it is obtained that the headed bars bearing effect is an important factor affecting the slip between the reinforcement and concrete in the STD headed bars,and the greater the headed bars bearing effect,the smaller the slip.The slip between reinforcement and concrete in the stirruped specimen was greater than that in the specimen without stirrup under the same conditions;the slip of the headed bars specimen with longer anchorage was greater than that with shorter anchorage under the same conditions.Based on the experimental data,the influencing factors of anchorage strength（τ_u）were investigated,and the formulae for calculating anchorage strength（τ_u）applicable to HRB400 and HRB500 bars with STD headed bars were suggested.τ_u increases with concrete tensile strength（f_t）,cover thickness（c/d）,and decreases with relative anchorage length（l_a/d）.Based on the experimental data,the anchorage distribution between headed bars（F_p）and bonding force（F_b）was investigated,and the formula applicable to HRB400and HRB500 STD headed bars reinforcement（F_p/F_b）was established.F_p/F_y decreases with the relative anchorage length（l_a/d）,stirrup ratio(ρ_sv),and the ratio of yield strength（f_y）of steel reinforcement to tensile strength（f_t）of concrete（f_y/f_t）.Based on the experimental phenomenon,the recommended value of anchorage length(l_ae)for STD headed bars reinforcement is proposed in combination with the F_p/F_y calculation formula.The proposed value(l_ae)is about 11%lower than that specified in the Code for the Design of Concrete Structures（GBJ50010-2010）and the Technical Specification for the Application of Headed bars（JGJ256-2011）.Based on the reliability analysis and the anchorage strength（τ_u）calculation formula,the critical anchorage length values were determined for HRB400 and HRB500 STD headed bars reinforcement.The critical anchorage length values are about 5%lower than the recommended anchorage length(l_ae).It shows that the recommended value(l_ae)has proper reliability.