Parameters’ Influence on Measured Behavior of Rebar Reinforced UHPC in Tension

Abstract

Direct tensile testing serves as the principal approach for assessing the mechanical performance and cracking behavior of rebar reinforced ultra-high-performance concrete (R-UHPC). However, the measured outcomes are notably sensitive to specimen dimensions and rebar configuration. This study investigates the effects of specimen thickness (50 and 100 mm), gauge length (70 and 170 mm), reinforcement ratio (1.1–6.2%), and rebar layout (a single rebar and distributed rebars) on the tensile response of R-UHPC through direct tensile tests. A total of 18 R-UHPC dog-bone specimens were tested. The UHPC had a steel fiber volume fraction of 2.0% and tensile strengths ranging from 10.9 to 13.7 MPa, while Digital Image Correlation (DIC) was used to capture the full-field crack evolution and deformation process. The test results reveal that R-UHPC exhibits an effective influence zone around rebar. In thick specimens, edge regions beyond the effective influence zone suffer from intensified strain localization, reducing effective tensile strength of the UHPC component to approximately 0.69 times that of plain UHPC. Within the effective influence zone, the reinforcement ratio influenced the UHPC component strength contribution, with the effective tensile strength decreasing from 0.99 times that of plain UHPC at a 1.1% reinforcement ratio to 0.82 times at 3.1%, and further to 0.53 times at 6.2%. Regarding failure modes, R-UHPC demonstrates a consistent transition from pre-peak multiple cracking to post-peak strain localization at the main crack. Increasing the gauge length averages this localized deformation and reduces the measured elongation at fracture. For specimens with the same gauge length, the yielded rebar length (Lp), which governs the extent of localized deformation, increases with rebar diameter and can be expressed as Lp = 0.011 fy db. The coefficient 0.011 is approximately half the empirical value of 0.022, which is commonly adopted for strain penetration in plastic hinge regions of conventional reinforced concrete members. The distributed rebar layout in thick specimens enlarges the effective influence zone, improving cross-sectional strain homogeneity and enhancing the UHPC component effective tensile strength relative to a single rebar layout. At the same time, the associated reduction in rebar diameter intensifies strain localization and decreases elongation at fracture.