National Transport Library Research Database

Sprickors effekt på armeringskorrosion (Effects of cracks on rebar corrosion)

  • Lundgren, Karin
  • Chalmers tekniska högskola AB, Universitet eller högskola, 556479-5598
Sponsors, duration, budget: Trafikverket ; 2019-11-01 -- 2022-12-31 Registration number:
  • Trafikverket 2019/108016
Subject(s): Abstract: För att få mer valuta för insatta resurser kan upphandling utgå från funktionskrav som öppnar upp för optimala lösningar. Dessa funktionskrav måste vara verifierbara för att kunna regleras i kontrakt mellan beställare och utförare. Detta projekt syftar till att öka kunskapen för att kunna förbättra formuleringen av tekniska funktionskrav. För att erhålla beständiga armerade betongkonstruktioner ställs idag krav på tillåten ytsprickvidd. Trots mycket forskning är det dock osäkert hur stark koppling det egentligen finns mellan sprickvidd och armeringskorrosion. I detta projekt kommer effekten av sprickor på armeringskorrosion att undersökas genom att sammanställa hur dokumenterad sprickbildning korrelerar med uppmätta korrosionsangrepp. Provkroppar från både befintliga konstruktioner och väldefinierade laboratorieprover kommer att användas. På detta sätt kommer erfarenhet från befintliga konstruktioner att återföras till tekniska funktionskrav för både nya och befintliga konstruktioner. Projektet kommer genomföras i samverkan mellan Chalmers tekniska högskola, Lunds tekniska högskola och Norges teknisk-naturvitenskapelige universitet. Därigenom kombinerar vi den expertis som finns på Chalmers om korroderade konstruktioners bärförmåga med den i Norge om material, specifikt armeringskorrosion, och den i Lund om sannolikhetsbaserade metoder. Vi har stor erfarenhet av liknande arbete tidigare. Arbetet börjar med att sammanställa resultat om sprickor och korrosion som vi redan har samlat in i tidigare projekt. Svenska resultat kommer från kantbalkar på två broar och provkroppar som har exponerats i laboratoriemiljö, och från norska projekt finns resultat från fler konstruktioner. Dessa resultat, tillsammans med övriga från litteraturen, bildar bas för att ta fram korreleringsmetoder. Ytterligare provkroppar kommer tas från befintliga konstruktioner. Dessa kommer användas för att verifiera metoderna med oberoende data. Resultaten från ett framgångsrikt projekt är ökad kunskap om hur viktiga sprickor är, vilket säkerställer säkerheten på sikt men undviker alltför stränga krav. Därigenom bidrar resultaten till en minskad användning av resurser och lägre livscykelkostnader för betongkonstruktioner. Abstract: As described in the call for this proposal, it is of priority for Trafikverket to increase productivity in construction as well as in maintenance and upgrading of existing facilities. One strategy for improvements is to establish more open functional requirements instead of specifying restricting technical details. These functional requirements should be at a level that allows freedom for relevant parties to offer alternative and innovative solutions to optimize construction projects. To achieve this, verifiable functional requirements that can be regulated in contracts between the client and the supplier are necessary. Thus, adequate formulation of technical functional requirements are needed. These can then be further elaborated to performance indicators, in line with the findings in Cost action TU1406, Strauss et al. (2016), where a framework and database for performance indicators for roadway bridges are given. The major part of the bridges in Sweden are made of reinforced concrete. For these, the most common cause of deterioration is reinforcement corrosion, Bell (2004). In current codes, the crack width on the concrete surface is limited, as a way to try to avoid or at least delay reinforcement corrosion and thus obtain durable structures. Depending on environment class, defined e.g. by the presence of chlorides and wet or dry conditions, different limits for the crack widths apply. This requirement can however be questioned, as the effect of cracks on durability is debated among researchers. During the latest decades, several researchers have worked on the subject. Schiessl and Raupach (1997) found that thickness and quality of concrete cover influenced the corrosion rate much more than the crack width, while Yoon et al. (2000) found that cracking influenced the corrosion initiation time. Ballim et al. (2003) tested beams with simultaneous load and corrosion and found that cracking influenced the corrosion rate. This finding was confirmed by Mohammed et al. (2001); however only at the very early age of exposure. Also El Maaddawy et al. (2005) concluded that sustained load and cracks initially increased the corrosion rate; however, as time progressed, no correlation was observed. Francois et al. (2006), who carried out tests on 17 year-old beams, found no correlation with the initial presence of cracks and the long-term corrosion level. Papakonstantinou and Shinozuka (2013) assumed that self-healing can explain why cracks with small widths appear not to have any effect on long-term corrosion. Weiss et al. (2017) found that chloride transport in relatively small cracks can be influenced by the mixture composition (w/c and aggregate size), while the mixture is of less importance for wider cracks. Blagojevic (2016) exposed beams to wetting and drying cycles in a chloride environment for two years, and found no direct relationship between crack width and corrosion. Instead, he suggested that maximum steel stress is more important for the corrosion behaviour, rather than the maximum surface crack width. Tammo and Thelandersson (2009) showed that the steel stress is closely related to the crack width in the vicinity of the reinforcement bar, but not to the surface crack width. Thus, cracks appear to promote the initiation of the corrosion process, as they facilitate the ingress of chlorides or carbon dioxides. Cracks are also likely to increase the corrosion rate for early age exposure. Only few studies exist on the effect of long-term exposure, and they indicate that small cracks have minor impact on the corrosion rate. Due to the limited number of such studies, there is however not sufficient background for a general statement, Hornbostel and Geiker (2017). It should be noted that most studies have been carried out on specimens exposed to laboratory conditions. However, in a recent study by the co-applicant, Mette Geiker, five reinforced concrete structures from Norway exposed to either de-icing salts or seawater were investigated, Danner and Geiker (2018). Results from other field investigations are being evaluated, and will be published the coming year. In a recent review, it was concluded that “The current state-of-knowledge does not permit general recommendations for critical crack widths, neither for corrosion initiation nor for corrosion propagation. This clearly illustrates the urgent need for more research”, Käthler et al. (2017). The main applicant has already studied the structural behaviour of corroded reinforced concrete structures in a number of projects. This work has earned international recognition, e.g. through the “Achievement Award for Young Engineers” by fib (International Federation for Structural Concrete). In recent work, edge beams from two bridges of different age have been structurally tested, see Tahershamsi et al. (2017), and Robuschi (2019). Additionally, well-defined laboratory exposed beams have been tested, Berrocal et al. (2018). From all these specimens, the reinforcement bars were brought out after structural testing, carefully cleaned and 3D-scanned. This data constitutes a very valuable database, which so far has not been utilized to its full potential. We have investigated possible links between corrosion level and spalling cracks due to corrosion, Tahershamsi et al. (2017); however, we have not explored possible links between transverse cracks and corrosion. This will be done in this suggested project. Thus, we will compile all our data from previous projects and investigate how the location and the size of corrosion pits correlate with the existence of cracks transversely crossing the reinforcement bars, and their crack width. Further, we will compile additional data by taking and investigating specimens from more structures. To summarise: Adequate formulation of technical functional requirements, instead of technical details, are needed to open up for optimal and innovative solutions. Thus, this area has been pointed out by Trafikverket to be of high priority. Surface crack widths are today strictly limited in codes; however, in spite of large research efforts, the impact of cracks on long-term corrosion is still debated. This project will address this gap, by compiling unique data from both naturally deteriorated samples of varying age and well-defined laboratory specimens, thereby investigating how the location of corrosion pits correlate with the existence of cracks crossing the reinforcement bars, and their crack width. In this way, we will provide knowledge for improved formulation of technical functional requirements. The combination of competence of the co-applicants, from both the structural and the material side, and with major competence in reinforcement corrosion and on probabilistic methods, forms the best possible group for targeting this issue. This is a clear opportunity to take a significant step beyond current state-of-the-art, and also limits the risks of the project to a minimum.
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