Concrete containing cement, water, aggregate, and discontinuous, uniformly dispersed or discrete fibers is called fiber reinforced concrete. It is a composite obtained by adding a single type or a blend of fibers to the conventional concrete mix. Fibers can be in form of steel fibers, glass fibers, natural fibers , synthetic fibers, etc.
Why fibers are used
Main role of fibers is to bridge the cracks that develop in concrete and increase the ductility of concrete elements. There is considerable improvement in the post-cracking behavior of concrete containing fibers 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 abrasion and shatter resistance in concrete. Imparts more resistance to Impact load.
Necessity of Fiber Reinforced Concrete
- It increases the tensile strength of the concrete.
- It reduce the air voids and water voids the inherent porosity of gel.
- It increases the durability of the concrete.
- Fibers such as graphite and glass have excellent resistance to creep, while the same is not true for most resins. Therefore, the orientation and volume of fibers have a significant influence on the creep performance of rebars/tendons.
- Reinforced concrete itself is a composite material, where the reinforcement acts as the strengthening fiber and the concrete as the matrix. It is therefore imperative that the behavior under thermal stresses for the two materials be similar so that the differential deformations of concrete and the reinforcement are minimized.
- It has been recognized that the addition of small, closely spaced and uniformly dispersed fibers to concrete would act as crack arrester and would substantially improve its static and dynamic properties.
Types of Fibers:
- Steel fibers
- Glass fibers
- Carbon Fibers
- Cellulose Fibers
- Synthetic Fibers
- Natural Fibers
- High modulus of elasticity for effective long-term reinforcement, even in the hardened concrete. Does not rust nor corrode and requires no minimum cover.
- Ideal aspect ratio (i.e. relationship between Fiber diameter and length) which makes them excellent for early-age performance.
- Easily placed, Cast, Sprayed and less labour intensive than placing rebar.
- Greater retained toughness in conventional concrete mixes.
- Higher flexural strength, depending on addition rate.
- Can be made into thin sheets or irregular shapes.
- FRC possesses enough plasticity to go under large deformation once the peak load has been reached.
- Greater reduction of workability.
- High cost of materials.
- Generally fibers do not increase the flexural strength of concrete, and so cannot replace moment resisting or structural steel reinforcement.