By Asst. prof. Dr. Qasim M. Shakir
University of Kufa/Faculty of Engineering
e-mail: [email protected]
Fiber Reinforced Concrete (FRC) is Concrete made of Cement, aggregates, non-continuous fibers that distributed randomly in all directions during the concrete mass, Fig. 1. In order to improve the mixing workability of concrete, superplasticizers are added. Fiber concrete has gained wide popularity in the construction of roofs, paving, machinery foundations, seismic buildings, precast concrete, shotcrete etc.
The main reason for adding fibers to the concrete is to increase Toughness, tensile strength and improve cracking properties of the resulting mixture as shown in Fig. 2. Fiber concrete must meet the requirements of economy and efficiency at the same time. Fiber is made of steel, plastic, glass or natural fibers and is available in shapes, sizes and diameters and different sections with lengths ranging from (6 – 150) mm, with a thickness ranging from 0.005 – 0.75 mm and add to the concrete during mixing.
Features and disadvantages of fiber concrete:
1 – Improve the cohesion of the mixture, and improve the capacity of long – distance pumping
2- Improved resistance against freezing and melting cycles
3 – Improve the resistance of fragmentation due to explosion and fire
4- Improve impact resistance, tensile strength and shear.
5 – Increase the resistance to plastic shrinkage during the curing of the concrete. Thus reduces the plastic cracking and reduce the phenomenon of creep in concrete.
6- Improve structural performance in general. To reduce the air and water spaces in the mixture.
7- Minimize the need for rebar used to control cracking and reduce the need for labor as a result.
8- Improvement ductility of Concrete
9- Prevents the formation of small cracks and reduce the width, and thus improve the durability of concrete better and reduce permeability. In addition, improve the performance of concrete
10-Protection of concrete from edge cracking.
11 – Reduce wearing in concrete.
12 – Reduce the problem of rust and corrosion (except that reinforced with metal fibers) occurring in the concrete reinforced with iron, which can reduce the thickness of the concrete cover.
Disadvantages of using fibers
1 – The process of adding it must be done automatically, raising the cost of production of fibrous concrete.
2- The relatively coarse aggregate should be reduced to obtain good quality fiber (fibrous) concrete.
3- The use of fiber reduces the workability of the concrete, which requires the use of plastic additives (extra cost).
4- The excess cost of materials represented in the price of fiber added to the mixture.
5-In general, the addition of fiber cannot compensate for the steel bar reinforcement used for flexure.
Fiber types used with fiber concrete:
1. Asbestos fibers
Asbestos is a combination of several types of natural silica, Fig. 3. It became popular since the late 19th century. Its fibers are highly resistant to heat, electric, chemical and fire effects, but have moderate tensile strength and high water absorption. Where it was originally used for insulation of buildings and electrical insulation. It was first used with concrete around 1900 AD. However, It was discovered later that asbestos was carcinogenic, and thus its products became completely prevented to be used in construction.
First used in 1986 in the exterior walls of a building in Tokyo. Carbon fiber improves flexibility and makes the tensile properties good, Fig (4).
It is a type of synthetic fibers, Fig. 5, which was first discovered by DuPont as an excellent alternative to asbestos. These fibers are made from organic compounds called aromatic polymers. These fibers are commercially known in many names such as Technora, Nomex and Kevlar.
The Aramid fibers are soft, light-resistant, high strength, flexible and stable. They have excellent resistance to heat (low ignition), cutting and chemical effects, and have weak electrical conductivity and corrosion resistance. The tensile strength is more than five times the strength of steel wire and has high strength (strength properties to excellent weight) but its density is 1/5 of steel wire. Aramid fibers are used as an important base material in other areas as well as concrete such as electronic and chemical industry, military application, and aerospace.
4 – Steel Fibers
Their use of steel fibers began since the 1960s, Fig. 6. There are several forms of steel fiber as shown in Fig. 7.
Steel fibers has a high tensile strength in the range of 200-2600 MPA, and for better bonding it is preferred that the fibers have a high Aspect Ratio (greater than 100). The tangle of fibers with each other reduces the operation and leads to the occurrence of which may adversely affect the mechanical properties of the hardened fibrous concrete so its proportion in the mix should not exceed a maximum (usually not more than 5% weight). Some plasticizers are preferred to overcome this problem. Mineral fibers are affected by chemical solutions and rust in particular, as well as electric and magnetic fields. The loose limbs in the surface of the hardened concrete may cause some damage to vehicle tires and people when moving on them
5-polypropylene fiber, Polyethylene, Nylon
It is a petrochemical and textile product. Propylene can be produced in the form of a bundle of fibers (Monofilaments), fig. 8, or larger diameters (Fibrillated). It can also be Dispersed) or mat form. Polypropylene fibers are characterized by alkali resistance. The disadvantages are poor bonding with matrix and their sensitivity to sunlight and oxygen. This requires special treatment and preservation in special bags. High resistance to alkali and acid resistance. Very high resistance to tensile strength but low elasticity, which reduces its function as a reinforcement material. Thus, it is widely used in non-bearing walls, cladding panels. It is characterized by easy surface finish. Compared with metal ones, it is not affected by rust or alkali nor by concrete additives and easy to use with low cost.
6. Glass fibers
It was first introduced in Russia in the 1940s but was widely used only after the 1970s. Glass fiber is a lightweight, corrosion-resistant, rotting and fire with high flexibility in design and manufacturing, low cost comparing to carbon fiber and friendly environment materials (does not cause environmental damage). However, it is affected by the Alkaline to a certain degree. So, this should be considered when used in concrete. Figure 9 illustrates this type of fiber.
7. Natural Fibers
Examples are horse hair, cellulose fiber, coconut fruit, bamboo, jute and some herbs, form (10). They are less affected by the chemical reactions of iron fig 10. They are less expensive and have many different sources. Some specifications determine the behavior of natural fibers, such as cellulose fibers, which must satisfy the requirements of ASTM D7357. Some natural fibers are affected by alkalis and some have good resistance to them. The regular distribution of fiber in concrete is necessary and super plasticizers can be used to obtain the best possible result.