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In previous research a simplified closed-form non-linear hinge model based on the Third Point Bending Test (TPBT) developed by the authors to derive the tensile material properties of Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) was numerically validated. This simplified non-linear hinge model was calibrated for UHPFRC that exhibits strain-hardening constitutive behavior. The aim of this work is the numerical validation of the simplified inverse analysis method to characterize the tensile properties of UHPFRC even with those UHPFRCs which exhibit strain-softening behavior. To get this objective a Finite Element Model (FEM) is carried out. The parameters to characterize the concrete properties from the simplified inverse analysis method by means of TPBT are used in the numerical modelling. The constitutive model for UHPFRC is modelled using a discrete crack approach for the macrocrack position. Different parameters are analysed to quantify the accuracy of the FEM. As a result, the model shows conservative load-deflection response when it is compared to the experimental curves.

Reinforced concrete elements frequently suffer small cracks that are not relevant from the mechanical point of view, but they can be an entrance point for aggressive agents, such as oxygen, which could initiate the degradation processes. Fiber-Reinforced Concrete and especially Ultra High Performance Concrete increase the multi-cracking behavior, reducing the crack width and spacing. In this work, the oxygen availability of three types of concrete was compared at similar strain levels to evaluate the benefit of multi-cracking in the transport of oxygen. The types of concrete studied include traditional, High-Performance, and Ultra-High-Performance Fiber-Reinforced Concrete with and without nanofibers. To this purpose, reinforced concrete beams sized 150 × 100 × 750 mm 3 were prepared with embedded stainless steel sensors that were located at three heights, which have also been validated through this work. These beams were pre-cracked in bending up to fixed strain levels.The results indicate that the sensors used were able to detect oxygen availability due to the presence of cracks and the detected differences between the studied concretes. Ultra High Performance Concrete in the cracked state displayed lower oxygen availability than the uncracked High Performance Concrete, demonstrating its potential higher durability, even when working in cracked state, thanks to the increased multi-cracking response. Author Contributions: Conceptualization, P.S. and M.V.; methodology, P.S., A.M.-I.; formal analysis, A.M.-I. and M.R.-F.; investigation, A.M.-I., E.J.M.-A. and J.L.-F.; resources, P.S. and M.V.; writing-review and editing, M.R.-F. and A.M.-I.; supervision, P.S. and M.V.; project administration, M.R.-F.; funding acquisition, A.M.-I., M.V., P.S. and M.R.-F. All authors have read and agreed to the published version of the manuscript.

Concrete has a natural self-healing capability to seal small cracks, named autogenous healing, which is mainly produced by continuing hydration and carbonation. This capability is very limited and is activated only when in direct contact with water. High Performance Fibre-Reinforced Concrete and Engineered Cementitious Composites have been reported to heal cracks for low damage levels, due to their crack pattern with multiple cracks and high cement contents. While their superior self-healing behaviour compared to traditional concrete types is frequently assumed, this study aims to have a direct comparison to move a step forward in durability quantification. Reinforced concrete beams made of traditional, high-performance and ultra-high-performance fibre-reinforced concretes were prepared, sized 150×100×750 mm3. These beams were pre-cracked in flexion up to fixed strain levels in the tensioned zone to allow the analysis of the effect of the different cracking patterns on the self-healing capability. Afterwards, water permeability tests were performed before and after healing under water immersion. A modification of the water permeability test was also explored using chlorides to evaluate the potential protection of this healing in chloride-rich environments. The results show the superior durability and self-healing performance of UHPFRC elements.

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