Chloride Induced Corrosion of Steel Bars in Fibre Reinforced Concrete

Licentiate thesis, 2015

Chloride-induced corrosion of reinforcement is the most widespread degradation mechanism affecting the durability of reinforced concrete structures. Macro-cracks provide a preferential path for moisture, oxygen and Cl ions to reach the embedded reinforcement, playing a major role in their total transport. Therefore, to effectively control macro-cracking is essential with respect to the service life. Fibre reinforcement, even at low dosages, leads to arrested crack development, also in conventionally reinforced concrete. Thus, it could be advantageous to use fibres in civil engineering structures where their crack limiting effect is of interest. However, despite the increased corrosion resistance of steel fibres, the use of both types of reinforcement in chloride environments raises questions. The present study aimed at investigating the viability of employing fibre reinforcement to improve the durability performance of conventionally reinforced concrete structures with respect to delayed and/or reduced corrosion by controlling the development of cracks. The work includes long-term experiments of naturally corroded concrete elements with and without fibres, in sound and cracked state, subjected to different loading conditions and various crack widths. Complementary material tests to study the influence of fibres on different properties governing the corrosion of steel reinforcement in concrete were also carried out. Additionally, experiments were started to determine the possible formation of galvanic cells between metallic fibres and steel bars. The results showed that while the electrical resistivity of concrete was unavoidably reduced by the presence of steel fibres, the ingress of chloride, assessed through migration and bulk diffusion tests, was not signicantly affected. The analysis of the corrosion initiation period in cracked specimens revealed that, when loaded to reach the same surface crack width, fibre reinforced specimens performed similar or better than their plain concrete counterparts. However, the improvement achieved by adding fibres was, in general, minor compared to the results obtained for uncracked specimens, highlighting the utmost importance of cracks for the initiation of corrosion. Accordingly, corrosion initiated almost immediately in specimens subjected to a sustained load, i.e. with open cracks, regardless of the presence of fibres. This observation indicated the existence of a critical crack width above which the initiation period could be, in practice, disregarded. Questions that remain unclear and that require further research include: (i) the influence of reduced electrical resistivity on the corrosion rate of rebar; (ii) the risk of galvanic corrosion caused by the different steels used for fibres and bars; and (iii) the effectiveness of fibres to control the development of corrosion-induced cracks and spalling of the concrete cover. Forthcoming results from the experiments developed during this project, which are still ongoing, are expected to shed some light on these questions.