Durability of sprayed concrete : steel fibre corrosion in cracks

Sammanfattning: A combination of sprayed concrete technique and steel fibre technology gives obvious advantages when saving the work needed to place conventional reinforcement. In rock strengthening applications this is most accentuated. Sprayed concrete in general, made by skilled workmen, will generally be of high quality and good durability. Durability requirements can also be found in today’s regulations with demands on service-life of more than 100 years. Since steel fibre reinforcement in wet-mix sprayed concrete has been common practice only since the late 1980s questions could be raised regarding its resistance to corrosion. It has previously been proved that steel fibres show an excellent durability against corrosion in homogenous concrete. In conditions where conventional reinforcement shows high rates of corrosion the steel fibres may still be unaffected. Fibres have a smaller size than conventional reinforcement and they seem therefore to be better protected by the alkaline environment provided by the concrete. A smaller cathode area compared to the anode area is another argument for better resistance to corrosion. However, the high quality combined with relatively thin layers applied in sprayed concrete structures give rise to deformations imposed by shrinkage, which is a common cause of cracks. In the design of steel fibre reinforced sprayed concrete (SFRSC) for e.g. rock strengthening purposes the fibres are used both to minimize crack widths from shrinkage and to obtain a sufficient post-crack behaviour. A system with bolts and SFRSC is dependent on a long- term residual strength capacity. Therefore, the purpose of this thesis is to investigate the mechanisms governing initiation and propagation of corrosion for cracked sprayed concrete. Field inspections performed on old, cracked, SFRSC show that the amount of corrosion is limited after 5-15 years of exposure. Even in the presence of high chloride concentrations the attack seemed limited. Moreover, it was noticeable that the amount of fibres crossing the cracks was very small in all the inspected structures. Two different approaches to studying the corrosion of steel fibres in cracks have been tested. Cracked beams of SFRSC have been exposed in field at three different sites. Crack width, fibre length, mix-composition, accelerators and spraying technique (wet-/dry-mix) are parameters that have been tested. After 5 years of exposure the samples exposed along a motorway with direct splashing of water containing de-icing salts show heavy corrosion on fibres crossing the crack. A loss of 15-20% of the fibre diameter in the outer 25 mm is common. Samples with longer fibres (+10 mm) show almost a doubled attack. Samples on the other sites start to show corrosion, but to a much more reduced extent. Except for the samples exposed in a tunnel environment, freeze-thaw damages may also be seen. In the river environment there seems to be an effect on the residual strength with reduction due to decreased concrete matrix strength. Differences in frost resistance could also be seen between samples with and without addition of water glass accelerator. According to an air void analysis the samples with water glass addition receive a more coarse air void system and therefore lowered frost resistance. Laboratory studies with accelerated exposure tests have also been performed. The purpose is to develop a technique for isolating parameters in a better way than in field and to perform exposure tests in a more controllable environment. In addition a useful technique combined with a correlation to the field exposures could make it possible to imitate longer real exposures in a shorter period of time and in this way estimate the long-time behaviour. Mainly the same behaviour as in field, with increased corrosive attack with increased crack width and fibre length, could be seen in the laboratory exposures. The influence of fibre length accentuates the importance of the anode- /cathode ratio for the rate of corrosion which has also been noticed for conventional reinforcement. In addition to the parameters tested in field exposures, different steel qualities are also tested in the laboratory exposures. Stainless steels seem to give full protection (at least for approximately 50 years), whilst galvanized fibres give temporary protection. A very rough estimation is that the laboratory exposures accelerate the exposure by about 50 times compared to the motorway environment (1 year in lab. corresponds to 50 years in field). As mentioned the steel fibres are supposed to be able to carry load during their entire service-life. The ductility of fibre reinforced concrete is given from the pullout strength achieved by the interaction between fibre and concrete matrix via bond-strength, friction and fibre deformation. If corrosion is initiated, the corroded fibres give ductility as long as the fibre strength is greater than the pullout resistance. An analytical model developed indicates that the fibre reinforced concrete shows substantial residual strength a long time after corrosion is initiated. Traditional service-life criteria are not valid for steel fibre corrosion in cracks. Instead, the service-life prediction should be based on an acceptable reduction of load bearing capacity. Measures taken at the design stage to ensure the load bearing capacity can be addition of extra amount of fibres, increased thickness of the structure or change of fibre material to a more corrosion resistant materials. A parameter influencing the residual strength of steel fibre reinforced sprayed concrete is the fibre distribution in the concrete. Commonly used methods (e.g. manual counting in a cross-section) for estimating the fibre amount in sprayed samples could be questioned and should be further investigated. Homogenous fibre distribution is important as results from standardised tests and the inventory of old structures point in the direction that cracks occur where the amount of fibres is smallest.