Experimental study on bond performance of staggered lap rebars in ultra-high performance concrete (UHPC)

Abstract

Ultra-high performance concrete (UHPC) has received extensive attention worldwide due to its ultra-high mechanical properties and durability. The bond performance between steel bars and UHPC is crucial for the design of Reinforced-UHPC structures. To study the bond performance of overlapping steel bars in UHPC, a total of 13 groups of 39 pull-out tests were conducted with overlapping lengths, cover thickness, steel bar diameter, and clear spacing of steel bars as influencing variables in this study. According to the results, when the lap length increased from 4 times the bar diameter (4db) to 5db and 6db, the maximum bond strength decreased by 10% and 20%, respectively. Moreover, with an increase in the cover thickness from 25 mm to 35 mm and 45 mm, the maximum bond strength increased by 6% and 14%, while the residual bond strength increased by 15% and 48%, respectively. When the diameter of the reinforcing bars increased from 16 mm to 20 mm and 25 mm, the ultimate load capacity increased by 50% and 70%, respectively, while the bond stress decreased by 16% and 27%. Additionally, the bond strength showed less sensitivity to the clear spacing of steel bars, with a decrease of 3% and 10% in bond strength when the clear spacing increased from 1.5db to 2db and 3db, respectively. The ductility of the specimen decreases when the bar diameter or the clear distance between bars is too large. A theoretical calculation formula for splitting bond strength was established, which yielded accurate results. Furthermore, a bond-slip model was fitted based on the test results, providing reference for finite element analysis of steel bar-UHPC structures.