Factors affecting bond development between Ultra High Performance Concrete (UHPC) and steel bar reinforcement

The advent of Ultra High Performance Concrete (UHPC) has led to strong interest in developing new structural applications for the material. While UHPC's tensile and compression behaviors are relatively well understood, an in-depth analysis of UHPC's behavior at the component level, specifically the bond strength between UHPC and steel bar reinforcement is lacking and the published data shows large scatter. In the presented study, a series of bar pull out tests was performed in order to characterize the bond strength of a non-proprietary UHPC blend. The tests were conducted using plain and epoxy-coated grade 60 bars with nominal diameters of 13 mm, 16 mm, and 19 mm. Other experimental parameters include three development lengths (50, 75 and 100 mm), two casting orientations (longitudinal and transverse to the steel bar reinforcement), two steel fiber volume contents (1% and 2%) and early age bond strength at 1, 3 and 7 days curing. Results from pull out testing show that bond strength decreases with increased embedment length suggesting a non-uniform distribution of bond strengths. Differences in steel fiber content yielded significant differences in bond strength of up to 36% when using 1% vs. 2% fiber volumes. Early age testing showed that 75% of compressive and bond strength in UHPC is developed within 7 days of casting.