Shrinkage cracking of steel fibre reinforced self compacting concrete overlays: test methods and theoretical modelling test methods and theoretical modelling

Abstract: Steel fibres are often applied as crack reinforcement in overlays even though methods for the design with respect to crack widths are not yet available. Thus, the intention was for the research to form a basis for future design of overlays with respect to cracks. A main ambition was to give guidance on e.g. type of steel fibre reinforced concrete (SFRC) required to limit crack widths and how to prepare the substrate surface in order to produce a high and even base restraint. Test methods and theoretical analysis has been applied to fulfil the aims. The experimental part consisted of end-restrained shrinkage tests, bond tests and half scale overlay tests. End-restrained shrinkage and half scale overlay tests were conducted in order to evaluate the effect of steel fibres for different restraint situations while the bond tests were intended to give information regarding the appropriateness of different substrate treatments from a bond strength perspective. Test results showed that the situation of restraint has a significant influence on the cracking response. A single crack developed if the overlay was restrained only at the ends for both un-reinforced concrete as well as for SFRC. However, the crack width was found to be reduced due to the addition of steel fibres. In case of a continuous restraint provided by bond to the substrate on the other hand, numerous well distributed, fine cracks were observed. For this situation there was no measurable influence of fibres on the width and distribution of cracks. A conclusion is thus that reinforcement is not required in case of thin overlays (depths of 50 mm have been studied) if a high and even bond strength is obtained. Test results verified that high bond strength can be achieved by pre-moistening the substrate prior to overlaying, in combination with thorough cleaning. However, the substrate should be allowed to dry back prior to overlaying, as a wet surface was shown to be deteriorating for the bond strength. Caution is also recommended if the overlay is cast onto a dry substrate. In case of insufficient bond strength, with an apparent risk for partial debonding, i.e. parts of the overlay debond while other parts are still bonded, reinforcement is however required to limit crack widths. A theoretical method has been proposed for the design of SFRC for such situations and predictions have been verified through comparisons with experimental results.