No abstract

The aim of this experimental work was to study the porous structure of Ultra-High-Performance Fibre-Reinforced Concretes (UH) made with different fibre volume contents (0%, 1%, 2%) under several curing conditions (laboratory environment, 20 °C, 60 °C, 90 °C), comparing the results with those recorded for ordinary, high strength and very high strength concretes. Scanning electron microscopy, mercury intrusion porosimetry, thermogravimetry, water absorption and oxygen permeability tests were carried out. The results showed a low portlandite content in UH (in the order of 75% lower than in concrete C50) and a low degree of hydration, but they rise with curing temperature. These concretes have a very fine porous structure, with a high concentration of pores on the nanoscale level, below 0.05 µm. Their porosity accessible to water is consequently around 7-fold lower than in conventional (C30), 6-fold lower than in high-strength (C50) and 4-fold lower than in very high-strength (C90) concretes. Their oxygen permeability is at least one order of magnitude lower than in C90, two orders of magnitude lower than in C50 and three orders of magnitude lower than in C30. The percentage of added steel fibre does not affect the UH porous structure.

The present work presents the results obtained with a stainless steel (SS) voltammetric sensor to detect variations in humidity (H2O) and oxygen (O2) availability in concretes. First, studies in solution were run by preparing several solutions to represent the different conditions that can be found in concrete pores. Second, the sensor’s response was studied by varying O2 availability by argon or synthetic air bubbling. Then concrete conditions with different degrees of carbonation were simulated using solutions with a pH between 13 and 8.45. After characterization in solution, a study by means of concrete probes with several water/cement ratios (0.6, 0.5 and 0.4) was performed, in which sensors were embedded and studied under different O2 and H2O saturation conditions. The obtained results revealed that with the voltagram, it is possible to evaluate O2 availability variation from the slopes of the lines identified logarithmically in the voltagram for the obtained cathodic sweeping. All the results obtained with the sensor were correlated/validated by standard assays to characterize porosity in hardened concretes.

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.