Flexural Behaviour of Ultra High Performance Fiber Reinforced Concrete (UHPFRC) Under Monotonic Loads and Loading-Unloading Cycles

Ultra High Performance Fiber Reinforced Concrete (UHPFRC) is an innovative material with great mechanical and durability performances, high ductility and toughness. Although the mechanical behaviour of UHPFRC has been extensively studied in the last years, the damage mechanisms and permanent strains of this material when subjected to flexural loads need to be further investigated, in order to quantify and to better predict the performance of UHPFRC structural elements. This work presents the results of an experimental study on the UHPFRC bending behavior. Both static and cyclic loading-unloading bending tests were performed. The effects of brass-coated steel fibers (diameter of 0.20 mm and length of 13 mm) on the flexural behavior of UHPFRC was investigated, varying the amount of fibers up to 2,5% by volume. Four-point bending tests were performed on prisms with different geometries (30 × 70 × 280 mm3 and 70 × 70 × 280 mm3). Particular attention was paid in the UHPFRC post-cracking behaviour, in order to evaluate the strain-softening and/or strain-hardening phases. Damage progress, number and width of cracks were monitored by means of a Digital Image Correlation (DIC) system on both the frontal and bottom surfaces of the specimens. Finally, a phase-field model has been implemented in a FE code and numerical simulations have been performed to better understand the effects of different fiber dosages on the mechanical behavior of UHPFRC specimens under cyclic loads. Concrete matrix and fiber reinforcement have been modeled as brittle and elasto-plastic phases of a mixture, whose internal energies are enriched by non-local damage and plasticity contributions.