Fibre Reinforced Concrete
Random oriented fibre reinforced concrete (FRC) is one of the most potential composite material comprised of Portland cement, aggregate and fibres.
Fibres are generally utilized in concrete to inhibit cracking and improve resistance to material deterioration that is caused due to fatigue, impact, and shrinkage or thermal stresses, manage the plastic shrink cracking and drying shrink cracking. They also lessen the permeability of concrete and therefore reduce the flow of water. Some types of fibres create greater impact, abrasion and shatter resistance in the concrete. The main function of the fibres in the concrete is to increase the concrete toughness and not to raise the flexural concrete strength. The fibres are bonded to the material and allow the fibre reinforced concrete to withstand considerable stresses during the post-cracking stage.
Properties of FRC
The mechanical properties of FRC depends on the type of fibre, fibre length-to-diameter ratio (Aspect ratio), quantity of fibres, strength of the matrix, size, shape and method of preparation of the sample, and the size of the aggregates. The strengthening mechanism of the fibres involves transfer of stresses from the matrix to the fibre by interfacial shear or by interlock between the fibre and the matrix. Fibre’s efficiency is the most important governing factor and is controlled by resistance of the fibres to pull out. The pull out depends on the bond strength at the fibre matrix interface. The pull out resistance is proportional to the interfacial area, therefore, non-round fibres offer more pull out resistance then large diameter fibres.
For a given fibre length, the aspect ratio is considerable. In practice, most of the mixes adopts an aspect ratio of 100 thereby assuring the failure of the composite primarily due to fibre pull out. However, the pull out resistance of the fibres can be increase without increasing the aspect ratio by adopting deformed surface or end anchorage. Below are cited some properties of FRC determined by different researchers.
Compressive Strength –The presence of fibres may alter the failure mode of cylinders, but the fibre effect will be minor on the improvement of compressive strength values (0 to 15 per cent).
Modulus of Elasticity– Modulus of elasticity of FRC increases slightly with an increase in the fibres content. It was found that for each 1 percent increase in fibre content by volume there is an increase of 3 percent in the modulus of elasticity.
Flexure – The flexural strength was reported to be increased by 2.5 times using 4 percent fibres.
Fatigue Strength– The addition of fibres increases fatigue strength of about 90 percent and 70 percent of the static strength at 2 x 106 cycles for non-reverse and full reversal of loading, respectively.
Toughness-For FRC, toughness is about 10 to 40 times that of plain concrete.
Splitting Tensile Strength-The presence of 3 percentfibre by volume was reported to increase the splitting tensile strength of mortar about 2.5 times that of the unreinforced one.
Impact Resistance– The impact strength for fibrous concrete is generally 5 to 10 times that of plain concrete depending on the volume of fibre.
Corrosion of Steel Fibres – A research proves that a ten year exposure of steel fibrous mortar to outdoor weathering in an industrial atmosphere showed no adverse effect on the strength properties. Corrosion was found to be confined only to fibres actually exposed on the surface. Steel fibrous mortar continuously immerse in seawater for 10 years exhibited a 15 percent loss compared to 40 percent strength decrease of plain mortar.
Mechanical Behaviour of FRC
Workability of steel and metakoalin FRC
Researches are being carried out on the mechanical properties of fibrous concrete in terms of materials, workability, compressive strength, modulus of elasticity, split tensile strength, toughness, and impact strength,
A study of the mechanical behaviour of fresh steel fibrous concrete using local materials carried out by Sabale and Ghugal (2014) for workability is presented in Figure 2 for varying Steel fibre (SF) and metakaolinfibre (MK) content and for the compressive and flexure strengths of steel and metakaolin FRC is shown in Figure 3 as carried out by Wafa (1990). The inverted slump test seems to give good results of workability of steel fibrous concrete. The use of 1.5% of fibres (metakaolin and steel) increases the flexure strength by 67%, though compressive strength is not increased significantly.
The other type of fibres available for FRC are, glass fibres, polypropylene fibres, natural fibres, carbon fibres.