07-02-2013, 01:59 PM
FIBER REINFORCED CONCRETE
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ABSTRACT
Concrete is weak in tension and strong in compression .Even though reinforcement is provided in tension zone micro cracks are developed in the tension and compression zone. The propagation of these cracks can be arrested by using fiber reinforcement in concrete. The fiber reinforcement is provided using different materials like steel carbon, glass fibers and polypropylene fibers. The fibers are very small which are distributed over the whole area of concrete .because of this we can not only arrest crack formation but also we can increase flexural ,shear ,torsion, strength, freezing &thawing resistance.
INTRODUCTION
In all countries, the construction industry is rapidly developing based on the invention of different materials and products in engineering fields. Engineers have attempted various types of materials in order to make the task more efficient reducing time, cost, improving durability, quality and performance of structures during their lifetime. Sophisticated analyses on structural Idealization have made a tremendous impact on the development of construction materials. This paper describes the general properties and application of fiber-reinforced concrete used in construction. The promise of thinner and stronger elements reduced weight and controlled cracking by simply adding a small amount of fibers is an attractive feature of fiber-reinforced concrete. The quality of good and durable concrete does not depend only on the quality of raw materials but also on proper mix-design, use of admixtures, placement, vibration and efficient curing. A number of additives are being used with concrete to enhance structural properties. Such additives are different types of fiber, namely steel, carbon, asbestos, jute, glass, polythene, nylon, polypropylene, fly ash, polymer, epoxy, superplasticiser, etc.
ROLES OF FIBRE
Fibers are usually used in concrete to control cracking due to both plastic shrinkage and drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water. Some types of fibers produce greater impact, abrasion and shatter resistance in concrete. The amount of fibers added to a concrete mix is expressed as a percentage of the total volume of the composite (concrete and fibers), termed volume fraction (Vf). Vf typically ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing fiber length (l) by its diameter (d). Fibers with a non-circular cross section use an equivalent diameter for the calculation of aspect ratio. If the modulus of elasticity of the fiber is higher than the matrix (concrete or mortar binder), they help to carry the load by increasing the tensile strength of the material. Increase in the aspect ratio of the fiber usually segments the flexural strength and toughness of the matrix. However, fibers which are too long tend to "ball" in the mix and create workability problems
WHY FIBER REINFORCED CONCRETE IS USED?
Plain and reinforced concrete stuctures are full of flaws such as pores, air voids, shrinkage cracks, etc., even before mechanically loaded.These flaws, especially small in size (micro cracks), grow stably under external loading and unite with existing or newly formed micro cracks until large fracture is formed which causes the collapse of the structure.Concrete is a material weak in tension and its tensile strength approximately ranges from 8 to 15 percent of its compressive strength.The initiation and propogation of these initial cracks and flaws during loading govern the mechanical behaviour of concrete subjected to different loading conditions.For a concrete structure subjected to tension, the cracks propogate in a direction perpendicular to the applied load.On the other hand, for a concrete structure subjected to purely uniaxial copression, the cracks propagate in the same direction as the applied compressive load. Since different mechanical responses of concrete structures under different loading conditions can be explained by fracture process, it is essential to understand when the cracks initiate and how they propagate with increasing load.The presence of micro cracks at the mortar aggregate interface is responsible for the weakness of plain concrete.The weakness can be removed to some extent byinclusion of fibers in the mix.
CONCLUDING REMARKS
Innovations in engineering design, which often establish the need for new building materials, have made fibre-reinforced cements very popular. The possibility of increased tensile strength and impact resistance offers potential reductions in the weight and thickness of building components and should also cut down on damage resulting from shipping and handling. Although ASTM C440-74a describes the use of asbestos-cement and related products, there are, at this time, no general ASTM standards for fibre-reinforced cement, mortar and concrete. Until these standards become available, it will be necessary to rely on the experience and judgment of both the designer and the fibre manufacturer.