1. Fiber-reinforced concrete:
Fiber-reinforced concrete (FRC) is cement-based composite incorporated with fiber, mainly short and discontinuous fibers. The development of FRC mainly attempts to overcome the two major deficiencies of cement-based composites: a relatively low tensile strength and a rather low energy consumption capacity or toughness. The functions of the fibers in cement-based composites can be classified into two categories: shrinkage crack control and mechanical property enhancement. For shrinkage crack control, usually small amounts of low modulus and low-strength fibers are added to restrain the early-age shrinkage and to suppress shrinkage cracking. And for mechanical property enhancement, fiber reinforcement has been employed in various concrete structures to improve flexural performance to increase impact resistance and to change the failure mode.
The amount of fiber added has a significant influence on the mechanical properties and failure mode of FRC. Based on how much fiber is added, fiber-reinforced cementitious composites can be classified into three groups. FRC employing low fiber volume fractions (
There are four types of fiber-reinforced concrete:
- 1. Steel fiber reinforced concrete.
- 2. Glass fiber reinforced concrete.
- 3. Synthetic fiber reinforced concrete.
- 4. Natural fiber reinforced concrete.
1.1 Steel Fiber-Reinforced Concrete
SFRC is a composite material made of hydraulic cements, water, fine and coarse aggregate, and a dispersion of discontinuous, small fibers. It may also contain pozzolans and admixtures commonly used with conventional concrete. All admixtures meeting ASTM specifications for use in concrete are suitable for use in SFRC. Calcium chloride and chlorides from other sources should be limited to amounts permitted to be added to conventional structural concrete as shown in ACI 318.
1.2 Glass Fiber Reinforced Concrete
Glass fiber-reinforced concrete (GRC) consists basically of a cementitious matrix composed of cement, sand, water, and admixtures, in which short-length glass fibers are dispersed. The effect of the fibers in this composite leads to an increase in the tension and impact strength of the material. GRC has been used for over 30 years in several construction elements, mainly nonstructural ones, like facade panels (about 80% of the GRC production), piping for sanitation network systems, decorative non-recoverable formwork, and other products [ 2 ].
Glass fiber helps insulate the concrete in addition to making it stronger. Glass fiber also helps prevent the concrete from cracking over time due to mechanical or thermal stress. In addition, the glass fiber does not interfere with radio signals like the steel fiber reinforcement does.
1.3 Synthetic Fiber Reinforced Concrete
Synthetic fiber-reinforced concrete uses plastic and nylon fibers to improve the concrete’s strength. In addition, the synthetic fibers have a number of benefits over the other fibers. While they are not as strong as steel, they do help improve the cement pumpability by keeping it from sticking in the pipes. The synthetic fibers do not expand in heat or contract in the cold which helps prevent cracking. Finally, synthetic fibers help keep the concrete from spalling during impacts or fires.
1.4 Natural Fiber Reinforced Concrete
Natural Fibers have been used to reinforce construction materials since time immemorial (the use of straw in sun-dried mud bricks or the strengthening of mortar with horse-hair being typical examples) study into their use in cementitious mixtures has gained popularity only in the last twenty years or so. However, although some progress has been achieved, most notably with roof tiles and roof sheets, the loss of strength due to deterioration of the fibers has given cause for concern. Despite this drawback, the apparent success of natural fibers in economic housing applications elsewhere encouraged investigation of their potential use.
Natural fiber cement composites could be used in almost as many applications as those with man-made fibers. Some of the earlier enthusiastic proposals for their use did not survived application testing, possibly due to the fiber durability problems. The composites in the former group consist mainly of thin roof sheets which have a proportion of fiber up to about 30 percent by volume, the latter consist of cement mortar or concrete matrices with between one and three percent fibers by volume. These latter composites have been used mainly in roofing tiles. Both products are generally pressure cast, although vibrating tables have also been used for roofing sheets with acceptable results .
2. Types of Natural Fibers
Fiber is extracted by a process known as decortication, where leaves are crushed and beaten by a rotating wheel set with blunt knives, so that only fibers remain. The fiber is then dried, brushed and baled. Proper drying is important as fiber quality depends largely on moisture content. Artificial drying has been found to result in generally better grades of fiber than sun drying. Fiber is subsequently cleaned by brushing. Dry fibers are machine combed and sorted into various grades, largely on the basis of the previous in-field separation of leaves into size groups.
2.1.1 Application in Concrete
The use of sisal fiber as reinforcement in cement paste and concrete have been reported by Swift and Smith. Their results on the flexural static strength and toughness of beams made of cement based matrices reinforced indicated that remarkably high strengths can be achieved using suitable mixing and casting techniques with optimum fiber volume fraction, although in the modulus of rupture is found for different ages. They also found that impact resistance can be improved by the addition of sisal fibers. Several application of this material was suggested for low-cost housing and they produced corrugated sheets of 2140 x 690 x 7 mm in different ways to optimize the processing technique.
Coir is a natural fiber extracted from the husk of coconut and used in products such as floor mats, doormats, brushes, mattresses, etc. Technically, coir is the fibrous material found between the hard, internal shell and the outer coat of a coconut. Other uses of brown coir (made from ripe coconut) are in upholstery padding, sacking and horticulture. White coir, harvested from unripe coconuts, is used for making finer brushes, string, rope and fishing nets.
2.2.1 Application in Concrete
The addition of coconut-fibers significantly improves many of the engineering properties of the concrete, notably torsion, toughness and tensile strength. The ability to resist cracking and spalling can also be enhanced. However, the addition of fibers adversely affected the compressive strength, due to difficulties in compaction which consequently led to increase of voids. Coir is very durable for natural weathering and also increases modulus of rupture of concrete.
Bamboo is one of the fastest growing plants, has got a great economic potential. Bamboo has been used in constructions of bridges and houses for thousands of years in Asia. Bamboo takes less energy to harvest and transport. Therefore, bamboo has low manufacturing costs compared with steel; bamboo is widely expected to be possible even in countries and regions that have no advanced manufacturing technology and construction techniques.
2.3.1 Application in Concrete
Elasticity Modulus of bamboo is very similar to that of concrete. It is Susceptible to volume changes in water. Increases ultimate tensile strength and increases modulus of rupture of concrete.
Jute is known as the ‘Golden Fiber’ due to its golden brown color and its importance. In terms of usage, production and global consumption, jute is second only to cotton. It is the fiber used to make hessian sacks and garden twine. Jute is environmentally friendly as well as being one of the most affordable fibers; jute plants are easy to grow, have a high yield per acre and, unlike cotton, have little need for pesticides and fertilizers. Jute is a bast fiber, like flax and hemp, and the stems are processed in a similar way.
2.4.1 Application in Concrete
Compressive and flexural strengths of chemically treated jute fiber reinforced cement concrete can be improved by 60 and 66% respectively than that of the concrete without jute fiber reinforcement. Chemical modification of jute fiber can improve tensile strength and elongation at break about 41 and 34 % respectively[ 7 ].
1. Li, Z., Advanced concrete technology. 2011: John Wiley & Sons.
2. Ferreira, J. and F. Branco, THE USE OF GLASS FIBER-REINFORCED CONCRETE AS A STRUCTURAL MATERIAL. Experimental Techniques, 2007. 31(3): p. 64-73.
3. Stephens, D. Natural fibre reinforced concrete blocks. in Proceedings of 20th WEDC Conference: Affordable Water Supply and Sanitation, Colombo, Sri Lanka. 1994.
4. Swift, D. and R. Smith. Sisal fibre reinforcement of cement paste and concrete. in Proc. Int. Conf. on Materials of Construction for Developing Countries. Asian Inst. of Tech., Bangkok. 1978.
5. Filho, R.D.T., et al., The use of sisal fibre as reinforcement in cement based composites. Revista Brasileira de Engenharia Agrícola e Ambiental, 1999. 3(2): p. 245-256.
6. Yalley, P. and A.S.K. Kwan, Use of coconut fibre as an enhancement of concrete. Journal of Engineering and Technology, 2009. 3: p. 54-73.
7. Aggarwal, L., Bagasse-reinforced cement composites. Cement and Concrete Composites, 1995. 17(2): p. 107-112.