Investigation of the Bond Behavior of Steel Reinforcement Bars Embedded in Ultra High Performance Concrete under Static Loads using Finite Element Modeling

The bond properties of reinforcement bars (rebar) embedded in concrete depends on the tensile behavior of the surrounding material and its capability of resisting micro cracks. Since the tensile behavior of Ultra High Performance Concrete (UHPC) is dominated by the amount and the orientation of fibers as well as the type of fibers, an effort has been made in this study to investigate the influence of the fiber volume fraction () and the fiber orientation on the bond behavior of steel rebar embedded in UHPC under a static loading condition using finite element simulation. Owing to the inclusion of discrete fibers, the characteristic of UHPC is highly anisotropic even at a macro level and it is important that the material model of UHPC captures that anisotropy properly. While modeling fibers discretely is time consuming and involves a lot of computational power, the present study proposes a computationally efficient way of modeling UHPC. In this approach, UHPC is considered as a composite material with the matrix modeled as a homogeneous material and the fibers modeled as smeared reinforcement. The directional vector of the smeared reinforcement represents the orientation of the fibers inside the matrix. ATENA, a finite element program, is used for this purpose. The material properties of the fibers are calibrated using the stress-stain data obtained from uniaxial direct tensile tests of UHPC. The calibrated fiber properties are then used to model pullout tests of a rebar embedded in UHPC. The bond stress versus slip properties of the rebar are validated with the experimental pullout test results. The calibrated rebar properties along with the fiber properties are then used to model uniaxial direct tensile tests of UHPC with embedded rebar. Once the tensile test model of the composite is validated using the experimental data, parametric studies are conducted to determine the effect of and fiber orientation on the uniaxial tensile behavior of rebar-reinforced UHPC. Based on the parametric studies, the dependence of the structural ductility on is discussed and a recommendation for the minimum strain to attain ductility is made