Evaluation of the Durability of the Fiber-Reinforced Concrete Beam Under Long-Term Pure Bending and Local Creep

Based on the energy approach formulated earlier by the authors, the calculation model for determining the durability of a fiber-reinforced concrete beam under long-term pure bending and local creep was developed. The model relied on the first law of thermodynamics regarding the energy balance and the balance of rates of energy variations in the fiber-reinforced concrete beam with initial volumetric damage after such loading. The following provisions and assumptions were introduced. Matrix and fiber materials were homogeneous and isotropic. The process of cracking in concrete occurred after the stresses attained the strength value of concrete. Opening of the generated microcracks and pull-out of fibers from concrete was treated as the main mechanism of its creep. The tensile diagram of fiber concrete was taken as piecewise linear. For simplicity of calculations, the area of the tensile diagram corresponding to the second stage of deformation was approximately represented by a straight line, similar to the compression case. During long-term bending of the beam element, the loss of its carrying ability was determined mainly by the stress-strain state of the tensile zone. Its rheological characteristics in the first approximation were assumed to be equal to the compressed ones. Fibers (of the same circular section and length) in the tensile zone were evenly distributed in all directions and were subjected to tension only. There was a complete adhesion between the fibers and the concrete, so their strains were identical. To improve the computation efficience, we used the plane section hypothesis, simplified models of fiberreinforced concrete deformation under tension, compression, and bending, as well as the creep condition of its creep at fracture (loss of performance properties of the fiber-reinforced concrete beam). The model feasibility was proved by the durability assessment of a real fiber-reinforced concrete beam with specified characteristics from literature.