Steel Fibre Reinforced Concretes
Steel Fibre Reinforced Concretes
Steel fibre reinforced concretes are structural materials that are gaining importance quite rapidly due to the increasing demand of superior structural properties. These composites exhibit attracti...
Steel fibre reinforced concretes are structural materials that are gaining importance quite rapidly due to the increasing demand of superior structural properties. These composites exhibit attractive tensile and compressive strengths, low drying shrinkage, high toughness, high energy absorption and durability. This is due to the tendency of propagating micro-cracks in cementitious matrices to be arrested or deflected by fibres, which is guaranteed by the local bond between fibres and matrix. Studies show that fibre-matrix interfacial bond is provided by a combination of adhesion, friction and mechanical interlocking (Li, 2007). Thus fibre reinforced concrete has superior resistance to cracks and crack propagation. The net result of all these is to impart to the fibre composite pronounced post- cracking ductility which is unheard of in ordinary concrete (Nguyen Van,2006). These properties of SFC can be enhanced by the addition of a suitable polymer into it. The properties of which has been overlooked based on the studies conducted by Gengying Li and Xiaohua Zhao, Dept. of civil engg, Shantou university, China.
Polymer cement concretes have high tensile strength, good ductile behavior and high impact resistance capability due to the formation of a three dimensional polymer network through the hardened cementitious matrices. Because of the void-filling effect of this network and its bridging across cracks, the porosity decreases and pore radius are refined. Furthermore, the transition zone may be improved due to the adhesion of a polymer. A styrene butadiene rubber emulsion is incorporated to improve the ductile behavior and flexural strength of steel fibre reinforced cement concretes (SFC). Silica fume and fly ash are also used to enhance the densification of cementitious matrix. The mechanical properties, microstructure, porosity and pore size distribution of polymer modified steel fibre reinforced concrete are studied.
Plain, unreinforced concrete is a brittle material, with a low tensile strength and a low strain capacity. Steel fibre reinforcement is widely used as the main and unique reinforcing for industrial concrete floor slabs, shotcrete and prefabricated concrete products. It is also considered for structural purposes in the reinforcement of slabs on piles, tunnel segments, concrete cellars, foundation slabs and shear reinforcement in prestressed elements. In tension, SFC fails only after the steel fibre breaks or is pulled out of the cement matrix. The role of randomly distributed discontinuous fibres is to bridge across the cracks that develops and
provide some post- cracking ductility. The real contribution of the fibres is to increase the toughness of the concrete under any type of loading. When the fibre reinforcement is in the form of short discrete fibres, they act effectively as rigid inclusions in the concrete matrix.
MIX DESIGN OF SFC
As with any other type of concrete, the mix proportions for SFC depend upon the requirements for a particular job, in terms of strength, workability, and so on. Several procedures for proportioning SFC mixes are available, which emphasize the workability of the resulting mix. However, there are some considerations that are particular to SFC. In general, SFC mixes contain higher cement contents and higher ratios of fine to coarse aggregate than do ordinary concretes, and so the mix design procedures the apply to conventional concrete may not be entirely applicable to SFC. Commonly, to reduce the quantity of cement, up to 35% of the cement may be replaced with fly ash (Nguyen Van, 2006). In addition, to improve the
workability of higher fibre volume mixes, water reducing admixtures and, in particular, superplasticizers are often used, in conjunction with air entrainment.
PROPERTIES OF SFC Compressive strength
Fibres do little to enhance the static compressive strength of concrete, with increases in strength ranging from essentially nil to perhaps 25%. Even in members which contain conventional reinforcement in addition to the steel fibres, the fibres have little effect on compressive strength. However, the fibres do substantially increase the post-cracking ductility, or energy absorption of the material.
Fibres aligned in the direction of the tensile stress may bring about very large increases in direct tensile strength, as high as 133% for 5% of smooth, straight steel fibres. However, for more or less randomly distributed fibres, the increase in strength is much smaller, ranging from as little as no increase in some instances to perhaps 60%, with many investigations indicating intermediate values, as shown in Fig. 2.1. Splitting-tension test of SFRC show similar result. Thus, adding fibres merely to increase the direct tensile strength is probably not worthwhile. However, as in compression, steel fibres do lead to major increases in the post- cracking behaviour or toughness of the composites.