THE CRACKING BEHAVIOUR OF SFRC BEAMS CONTAINING LONGITUDINAL REINFORCEMENT

Abstract

ABSTRACT Durability of concrete is in a great extent determined by the crack widths that are formed in the structure and that allow water to infiltrate in the concrete so that corrosion of the reinforcement bars can occur. The analysis of the cracking behaviour in concrete has always been a topic of interest for investigation by many scientists. In general it is assumed that the durability of the structure is assured when the crack widths are limited to 0.3 mm. One of the often-used approaches to limit the crack widths is the use of steel fibre reinforced concrete (SFRC). Steel fibre concrete is generally known to reduce crack widths because of its post-cracking tensile strength. At the Department of Civil Engineering of the Catholic University of Leuven, Belgium, a test program has been executed on 19 full-scale SFRC beams containing longitudinal reinforcement. All beams have been tested in four-point bending. The tests were performed in different load steps until failure of the beam. The load steps were chosen so that the beams failed after 10 to 15 steps. At each load step the crack widths and spacing were measured. The results of the test program illustrate the strong beneficial effect of steel fibres on the crack widths as well as on the crack spacing. The addition of fibres to the concrete can lead to a reduction of the crack width of up to 40%. The crack widths of the beams of the test program have been calculated by means of the new Rilem guideline as well as with a newly developed physical cracking model for reinforced SFRC beams. The physical model takes into account the bond between the reinforcement bars and the SFRC matrix as well as the influence of the steel fibres on the stress in the reinforcement bars. A comparison of the calculated results and the experimental results shows that there is a relatively good correlation between the two. The input parameters for the calculation model are the concrete compressive strength, the dimensions of the beam, the position and diameter of the reinforcement bars, the tensile strength and the post-cracking tensile strength of the SFRC material and the bond stress-slip relation. The new calculation model provides a very good understanding of the crack formation process. It also creates the possibility to determine for example the necessary post-cracking strength of the SFRC, given that the crack width of the beam must be lower than a certain value. Furthermore, also the influence of the bond stress-slip relation can be taken into account. This creates the possibility to use the crack model for other types of reinforcement than steel rebars (e.g. GFRP rebars).