Corrosion of reinforcement bars in steel ibre reinforced concrete structures

Research output: Book/ReportPh.D. thesis – Annual report year: 2014Research

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Steel fibres have been known as an alternative to traditional reinforcement bars for special applications of structural concrete for decades and the use of steel fibre reinforced concrete (SFRC) has gradually increased in recent years. Steel fibres lead to reduced crack widths in concrete formed, among other reasons, due to shrinkage and/or mechanical loading. Steel fibres are nowadays also used in combination with traditional reinforcement for structural concrete, where the role of the fibres is to minimize the crack widths whereas the traditional reinforcement bars are used for structural purpose. Although such, so-called, combined reinforcement systems, are gaining impact within the construction industry, they are only marginally covered by existing guidelines for structural design and the literature concerning their mechanical and, in particular their durability aspects, is sparse.
The aim of the work presented in this Ph.D. thesis was to quantify the influence of steel fibres on corrosion of traditional reinforcement bars embedded in uncracked concrete as well as cracked concrete. Focus of the work was set on the impact of steel fibres on corrosion propagation in uncracked concrete and the influence of steel fibres on initiation and propagation of cracks in concrete. Moreover, the impact of fibres on corrosion-induced cover cracking was covered. The impact of steel fibres on propagation of reinforcement corrosion was investigated through studies of their impact on the electrical resistivity of concrete, which is known to affect the corrosion process of embedded reinforcement. The work concerning the impact of steel fibres on initiation and propagation of cracks was linked to corrosion initiation and propagation of embedded reinforcement bars via additional studies. Cracks in the concrete cover are known to alter the ingress rate of depassivating substances and thereby influence the corrosion process. The Ph.D. study covered numerical as well as experimental studies.
Electrochemically passive steel fibres are electrically isolating thus not changing the electrical resistivity of concrete, whereas electrochemically active (depassivated/corroding) steel fibres are conducting. The impact of electrochemically active (depassivated/corroding) steel fibres on the electrical resistivity of SFRC was studied experimentally and analytically herein. Those studies showed that the addition of electrically conductive steel fibres may potentially reduce the electrical resistivity of concrete. Numerical studies of the correlation between the corrosion rate and the electrical resistivity of concrete were presented to study the impact of conductive steel fibres on the corrosion propagation phase of reinforcement bars. It was observed that under extreme conditions, viz. conductive (depassivated/corroding) steel fibres throughout the concrete volume, the reduction of the electrical resistivity caused by conductive fibres lead to a remarkable increase in the corrosion rate. However it is stressed that the case of corroding steel fibres throughout the concrete volume is somewhat hypothetical due to the very high corrosion-resistance of embedded steel fibres. Thus the investigated case refers to a worst-case scenario.
Numerical and experimental studies on the impact of steel fibres on initiation and propagation of load-induced cracks in concrete showed that the steel fibres restrained the crack width of a bending crack through the concrete cover, once the crack was formed. Moreover the numerical studies showed that the length of separation at the concrete/steel-bar interface (displacement discontinuity perpendicular to the reinforcement bar) was reduced for SFRC compared to plain concrete, whereas there was no clear impact on the slip at the concrete/steel-bar (displacement discontinuity parallel to the reinforcement bar) caused by the steel fibres. Additional experimental and numerical studies concerning corrosion of reinforcement embedded in cracked concrete (plain concrete and SFRC) showed that the time-to-corrosion-intiation was similar for plain concrete and SFRC for the same surface crack width. With regard to the corrosion propagation phase in cracked concrete the numerical studies showed that the corrosion rate and the length of the anodic surface on the reinforcement was comparable in plain concrete and SFRC for the same surface crack width. Thus, based on these observations there apperared to be no impact from steel fibres on the corrosion process of embedded reinforcement.
Finally the influence of steel fibres on. corrosion induced cracking of the concrete cover, was investigated numerically. These simulations covered traditional reinforcement embedded in either plain concrete or SFRC and it was observed that once a crack in the concrete cover was formed, the development of the crack width at the concrete surface was reduced in SFRC compared to plain concrete. This indicates that the fibres can restrain the propagation of corrosion-induced cracks and thereby reduce the detrimental impact of cracks on the corrosion process.
Original languageEnglish
PublisherTechnical University of Denmark, Department of Civil Engineering
Number of pages188
ISBN (Print)978-87-78-77339-5
Publication statusPublished - 2014
SeriesByg Rapport
VolumeR-257
ISSN1601-2917

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