Hybrid reinforcement solutions in chloride environment: durability, safety and life cycle costs

Research Project, 2018 – 2020

Civil engineering structures like bridges and harbours have strict crack width limitations, due to the risk for chlorides to cause reinforcement corrosion. This leads to large reinforcement amounts, causing difficulties in production. But with traditional bar reinforcement and the concrete covers required in chloride environments it is not possible to meet the requirements on surface crack widths which often leads to costly repair work during and after construction (guarantee period). For example, AMA Anläggning 17 requires that all surface crack widths larger than 0.20 mm must be injected. The cost for crack injections depends on conditions and can vary from 1000 up to 20 000 SEK per meter of crack. As this types of cracks usually are caused by restrained deformations (shrinkage and temperature) they tend to form with a spacing that can vary between 0.3 m up to 10 meters depending on type of structural element (e.g. edge beam, slab, wall, etc.) but which means that the accumulated crack length needing injection can be very large and thus may lead to substantial costs! If not injected, the cracks may cause reduced service life. Hence, it would be beneficial to use hybrid reinforcement solutions, in which fibres and traditional reinforcement are combined for achieving an improved crack control.

The hybrid reinforcement technology proposed has been studied in an industrial PhD project in cooperation between Thomas Concrete Group and Chalmers University of Technology; the PhD defence was on September 14, 2017, Berrocal (2017). This work concludes that hybrid reinforcement solutions is a very promising solution in chloride environment. Before this work, several question marks about steel fibres possibly affecting corrosion of rebars in a negative way were raised, e.g. steel fibres reducing the resistivity and risks of galvanic corrosion. The potential risk in reduced resistivity could by this thesis work be shown not to be relevant; instead, the use of fibres reduced the corrosion rate. Furthermore, fibres supressed the crack width openings and decreased the interfacial damage between rebars and concrete at loading. There was also a tendency for internal crack branching and increased tortuosity when fibres were present, which can potentially decrease the permeation of concrete and promote crack self-healing. This in turn leads to a slower ingress of detrimental substances, with the consequent delay in corrosion initiation. The use of fibres also delayed the appearance of corrosion-induced cracks, significantly improved the local bond behaviour between concrete and corroded bars and supressed the risk of cover spalling, thereby improving structural resilience and reliability. A qualitative investigation of the corrosion characteristics revealed a more distributed corrosion pattern and larger remaining cross-sectional area of rebars in Fibre Reinforced Concrete (FRC), which resulted in the mitigation of the structural impact of corrosion damage. All these effects lead to an extended service life of the structures while increasing the structural safety. Initial observations on specimens specially designed to investigate the risk of galvanic corrosion indicate that this seemed to be negligible; these results are yet preliminary and still unpublished. We have kept these galvanic corrosion specimens together with 22 beams for further research, as described in this proposal.

The positive findings from our work agree with other recent work internationally. In a PhD thesis on the subject of reinforcement corrosion, Pease (2010) investigated the influence of cracking on chloride ingress and corrosion of reinforcement in both ordinary and FRC. Pease proposed that debonding along the concrete-reinforcement interface is more important for depassivation and corrosion of reinforcement than surface crack width. In another PhD project at DTU, the influence of steel fibres on resistivity and corrosion initiation of reinforcement has been investigated, Solgaard et al. (2014). Grubb et al. (2007) investigated the effect of steel microfibres on corrosion of reinforcing steel. They found that steel microfiber-reinforced cement based materials had lower measured electrolytic resistance values, but this did not lead to a higher corrosion rate. Conversely, the measurements indicated that the steel microfiber-reinforced mortars were more resistant to corrosion than the control mortars, despite higher chloride concentrations. Also in Trondheim in Norway, work is currently ongoing where an application of the concept is being investigated, see e.g. Skare (2016). Hybrid reinforcement will also be beneficial to control early-age thermal and shrinkage cracking, as it is superior to traditional reinforcement in limiting the surface crack width, see Norwegian studies in Sandbakk (2007) and Persson et al. (2017), and also work supervised by the main applicant, Antona and Johansson (2011).

The main aim of this suggested project is to take the positive results of the mentioned projects further to practical use, by further removing doubts and substantiate the conclusions, and by quantifying the long-term effect of fibres on structural safety and resilience. The successful cooperation between Thomas Concrete Group and Chalmers on FRC will continue through this suggested project; the thesis of Berrocal (2017) is in fact the third in a row: Löfgren (2005) and Jansson (2011) were two precursors. Some of the positive results from the ongoing project are already published in well-reputed international journals, see Berrocal et al. (2015), Berrocal et al. (2016a), Berrocal et al. (2016b), and Berrocal et al. (2017). In combination with a long row of successful projects on structural effects of reinforcement corrosion, e.g. Lundgren (2002), Zandi Hanjari (2010), and Tahershamsi (2016), the applicants can guarantee the best possible competence and environment for the suggested work.

To summarise: It has been shown that fibres can effectively control cracking, reduce rebar corrosion and have positive structural effects. This suggested project will further certify this, and provide tools to quantify the long-term effect of fibres on structural safety; thus bringing the positive results into practical use.