Free and Restrained Shrinkage of Hybrid Steel Fibre Reinforced Concrete

Concrete elements are always subjected to a certain degree of restraint caused by internal reinforcement, connected elements, external boundaries, or a combination thereof. The degree of restraint provided may affect considerably how concrete shrinkage cracks develop, thus affecting structural behaviour. The effect of restraint is complex to quantify and still poorly understood. This paper introduces a methodology to simulate and quantify more accurately real life restraining factors for structures. To achieve this, an experimental programme was undertaken to examine the free and restrained shrinkage of Steel Fibre Reinforced Concrete (SFRC). The performance of seven SFRC mixes using various dosages of Recycled Tyre Steel Fibres (RTSF) and Manufactured Undulated Steel Fibres (MUND) is examined. The drying shrinkage was monitored over a period of ten months. The results show that shrinkage strains were very similar in terms of free and restrained shrinkage, whilst the plain concrete exhibited the lowest strains. The variation in shrinkage strain seems to be mostly related to the initial water content and volume of entrained air voids. Specimens subjected to restrained shrinkage showed about half the strain measured on free shrinkage specimens. Free shrinkage tests showed that the resulting strains were lower than the values predicted by fib MC 2010 and EC 2004, possibly due to the use of GGBS as cement replacement. This confirms that more environmentally friendly cementitious materials and fibres can help mitigate shrinkage effects.