Headed bars in simulated exterior beam-column joints of Ultra-high-performance fiber-reinforced concrete

摘要

Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) exhibits improved compressive and tensile strengths considerably superior to those of conventional concrete. While these characteristics can significantly reduce the cross sectional area of the member, the space for accommodating development lengths of straight or hooked bars is deficient A headed bar can be a viable option for this problem. With the enhanced tensile strength of UHPFRC, the anchorage strength of a headed bar is expected to be improved. The anchorage strengths of headed bars with 4d_b or 6d_b embedment length were evaluated by simulated exterior beam-column joint tests where 1he headed bars were used as beam bars and the joints were cast of 120,150, or 180 MPa UHPFRC. In all specimens, the actual yield strengths of the headed bars of more than 600 MPa were developed. Some headed bars were fractured due to the high anchorage capacity in UHPFRC. Therefore, a headed bar with only 4d_b embedment length in 120 MPa UHPFRC can develop a yield strength of 600 MPa. A previous model derived from tests with normal concrete and a design equation of ACI 318-14 underestimated the anchorage capacity of the headed bar anchored in UHPFRC because the previous model and the design equation do not consider the effects of the high tensile strength of UHPFRC. In this study, the effects of embedment length, bar diameter, and compressive strength of UHPFRC on the anchorage strength of headed bars are investigated. From a regression analysis assuming that the anchorage strength is proportional to (f_c )~α, a model for predicting the anchorage strength of headed bars in UHPFRC is developed. The average and coefficient of variation of the test-to-prediction values are 1.02 and 6.95%, respectively.