Splice length and bond behaviors of doubled-row lapped bars in ultra-high performance concrete (UHPC)

This study investigated the bond behavior of hot-rolled ribbed bar (HRB) 500 in ultra-high performance concrete (UHPC) through direct tension tests on 11 groups of double-row spliced specimens. The variables included splice length, cover thickness, matrix type, fiber volume content, spacing between lapped bars, bar diameter, and bar grade. In addition, seven groups of anchorage specimens were designed to examine the differences between the two specimen types. Results indicate that the bond strengths of the spliced test ranged from 0.45 to 0.86 times those of the central pullout tests. The bond strength in spliced specimens was less affected by embedded length compared to anchorage specimens; however, it increased more significantly with a greater cover thickness. A size effect related to rebars was observed in the spliced specimens, whereas it was absent in anchorage specimens. The bond strength of UHPC with 2 % fiber was approximately twice that of ordinary concrete, but increased slowly when the fiber content increased to 3 %. A splice length of 12d led HRB 500 rebar (20 mm diameter) to yield, and a cover thickness of 3.8d was recommended to ensure yielding of a bar with a 3d splice length. A theoretical equation for calculating the UHPC bond strength was proposed that consider the bundle effect of bar lapping. Finally, a calculation model for predicting the yield splice length derived from the nonuniform bond stress distribution model, which aligned well with the experimental results of UHPC bond from existing studies.