The using of fiber-reinforced concrete increased in recent years. fibers materials could be steel fibers, glass fibers, natural organic and mineral fibers, polypropylene fibers and other synthetic fibers. studies show that an approximate volume of fibers of 1% to 2% of the concrete mix would improve concrete characteristics. many factors could intensify the tendency of fibers balling and segregation.
The factors are:
- the percentage volume of fibers: increasing the volume of fiber will increase the tendency of balling and segregation.
- Aspect ration(length/diameter): the increase of aspect ration will increase the tendency of balling and segregation.
- Coarse aggregate size and quantity: the increase of coarse aggregate size and quantity will intensify the balling and segregation.
Fiber-reinforced concrete usually will require higher fine aggregate content, smaller coarse aggregate size, and higher cement factor. compacting of fiber-reinforced concrete will require more effort than normal concrete. adding of fibers will alter concrete properties:
- compressive strength: there is no significant improvement in compressive strength.
- modulus of elasticity: modulus of elasticity will increase by adding fibers to concrete. studies indicating that adding 1% of fibers will increase the modulus of elasticity by 3%.
- Flexure: adding fibers will increasing the flexural strength of concrete. some test conducted on fiber reinforced concrete shows an increase of flexural strength by 2.5 times compared to non-fibrous concrete for 4% of fibers.
- Toughness: adding of fibers will increase the toughness by 10 to 40 times of normal concrete.
- splitting tensile strength: adding of fibers to the concrete mix will significantly increase the splitting tensile strength.
- impact resistance: adding fibers will significantly improve the impact resistance. the increase ranges from 5 to 10 compared to non-fibrous concrete.
The size of fibers ranges from 0.25 in(6.3mm) to 3in(76mm). and diameter ranges from 0.01in (0.254mm) to 0.03in(0.762mm). fibers should be hooked and crimped to increase the bond with cement paste. also, the surface of fibers can be chemically modified to improve the bond with cement paste. when concrete cracked. the fibers will bridge the crack and it will prevent the opening and increasing crack width. opening of cracks would require pulling out of fibers first.
Steel and glass fibers are used widely to reinforce concrete. steel fibers used to reinforce concrete used mostly in the pavement, thin shells and precast concrete. glass fibers are using to reinforce shotcrete concrete. Ordinary glass will deteriorate quickly once it is in contact with cement paste. therefore, alkali-resistance glass fibers should be used. the using of fiber reinforced concrete will enhance structural behavior as follows:
- Flexure: using of fibers to reinforce concrete in flexural members will increase moment capacity, tensile strength, and stiffness. also, it will control deflection and persevere and bridge of cracks.
- Torsion: using of fiber reinforced concrete will increase member stiffness, torsional capacity, rotational capacity, ductility and it will eliminate the sudden failure due to torsion.
- Shear: addition of fiber reinforcing will increase the shear strength by 100%. adding of randomly reinforcing fibers will increase shear-friction resistance.
- Cracking and deflection: adding of fibers reinforcement will control cracks and reduce deflection of the structural member.
The cost of producing fiber-reinforced concrete is higher. therefore the use of reinforced concrete is limited. it used mostly for overlays for highway pavements and airport runways