The fibers in various forms, such as natural fibers, etc. were primarily used to increase the load carrying capacity of members. Their use date back to the Roman period (300 BC–476 AD), wherein the ancient concrete was found to contain fibers. Straw reinforced mud bricks were found at a number of ancient sites in the Middle East dated back to approximately 10,000 years ago. The indigenous inhabitants in the USA were using the sun dried adobe bricks, believed to be made using a mixture of sand, clay and the straw. In recent years, the first patent for the fiber reinforced concrete was filed in California (1874) by ( A. Bernard ). A patent by H. Alfsen in France (1918) was followed by G.C. Martin in California (1927) for the pipes manufactured using the steel fiber reinforced concrete. A process was patented by H. Etheridge (1931) regarding the use of the steel rings to address the anchorage of steel fibers. The widespread applications of the material in the routine construction practice however were hindered by their high costs, poor testing facilities and most importantly, the rapid parallel development of using concrete reinforced with the steel bars and cables. It was not until the experiments of James Romualdi (1962) that a clearer understanding of properties of the steel fiber reinforced concrete (SFRC) emerged.
The development of SFRC continues and in more recent years, extensive research efforts in this direction are aimed to frame guidelines regarding the testing and design methods using the steel fibers alone or along with the synthetic fibers. The following section briefly describes the established reinforcing mechanism of the steel fibers in concrete.
The first modern and sophisticated example of a fiber-reinforced construction material produced with a controlled and predictable level of quality was asbestos-cement, a material manufactured in thin-section forms such as flat or corrugated sheet and pipe by combining asbestos fibers with a slurry of cement and water, sometimes with finely divided silica, sand or other additives included. Asbestos-cement manufactured by the Hatschek, Magnani or Mannville process. As shown in Fig.(1.1)
Fig.( 1.1) Asbestos-cement manufactured by the Hatschek, Magnani or Mannville process
The process has been effectively and widely utilized in sheeting, roofing and cladding panels and in pipe since about 1900. In its final dewatered for the fiber content by volume is 6–8% I sheeting, 8–10% in pipe, and 14–21% in fire-resistant boards, with the fibers typically less than 5mm in length. Shown in Fig ( 1. 2) .
Fig.( 1.2) Asbestos cement used for cladding
From about 1970, its use has declined because of the hazard to human health now associated with breathing asbestos fibers, and considerable effort has been directed towards finding alternatives that are comparably effective for thin-section prefabricated applications in terms of engineering properties and cost. Like asbestos-cement, these newer fiber-cement composites with fibers such as glass, carbon and aramid are characterized by relatively high fiber content, more than 2% by volume, and by a production process that blends the fibers into a cement-based slurry without coarse aggregate thus avoiding the possible damage that fragile fiber types suffer in a conventional concrete mixing process with coarse aggregate included.
A quite different category of fiber-reinforced cementitious materials is the range of fiber-reinforced concretes made possible by including more robust discontinuous fibers as an ingredient of concrete in a conventional mixing process along with other ingredients like aggregate and admixtures. The fiber content in these composites is much lower than in fiber cement composites, typically no more than 1.5% by volume and sometimes as little as 0.1% by volume, and the fiber length is longer, 15–65 mm. Apparently this idea originated with a French patent in 1918 based on uniformly mixing small longitudinal bodies (fibers) of iron, wood or other materials into concrete . The patent also suggested that fiber elements must be rough, or be roughened, and that the ends should be bent, features that are used today to improve the pullout resistance of fibers from concrete. Other patents followed , although the concept did not really take hold until the 1960’s when smooth straight steel fibers produced by cutting wire or sheet metal became more widely available commercially (U.S. Patent Office, 1969, 1970,1972).
In addition to steel, other types of fiber that have since emerged specifically for use with conventionally mixed concrete containing coarse aggregate include polypropylene, polyethylene and various types of polyester .The reason for using fibers in both categories of composite is to enhance the properties of an inherently weak, brittle and crack-prone cementitious matrix. Depending on fiber type and fiber content, this enhancement may include in varying degrees improvements in tensile or flexural strength, ductility, toughness or energy absorption capability, impact resistance,fatigue resistance, resistance to cracking, permeability, and durability.
The idea of reinforcing brittle building materials with various forms of fiber has been known since ancient times.Mud huts made using baked clay reinforced with straw as shown in Fig.(1.3 ) , and masonry mortar reinforced with animal hair are early examples of fiber-reinforced materials in construction.
Fiber Reinforced Concrete (FRC) defines as a “composite material characterized by a cement matrix and discrete fibers (discontinuous)”. In its first state-of-the-art report in 1973 the American Concrete Institute (ACI) defined fiber-reinforced concrete as “concrete made of hydraulic cements containing fine, or fine and coarse aggregate, and discontinuous discrete fibers”, The matrix is made of either concrete or mortar. Fibers can be made of steel, polymers, carbon, glass or natural materials, it is well known that concrete is a quasi-brittle material with a low strain capacity, especially under the tensile stress conditions. The presence of the cracks both in the interfacial transition zone and the mortar matrix of concrete is a single most factor responsible for the insignificant tensile strength exhibited by it.
The development and the transformation of the micro cracks existing in the transition zone of the concrete into a system of major cracks that led to the strain softening behavior at higher strains and finally, the failure of the concrete when it is stressed to about 90 % of its ultimate compressive strength.