Bond Characteristics between High-Strength Bars and Ultrahigh-Performance Concrete

A series of pullout tests were conducted to investigate the bond characteristics of deformed high-strength steel bars embedded in ultrahigh-performance concrete (UHPC). This study considered different fiber volume contents, bar diameters, bar embedment lengths, concrete covers, and bar grades. Pullout force, slip at the free end of the bar, and crack patterns were measured, and bond failure modes were investigated. New results are reported regarding bond strength increase with fiber volume content (up to 3%), bar diameter, and bar grade. The thickness of the concrete cover significantly affected the bond failure mode. UHPC had considerable energy absorption capacity due to debonding and pullout of steel fibers from the matrix. A theoretical formula and a simplified formula to evaluate the splitting bond strength are proposed, which are consistent with the experimental results.