Structural Synthetic Macro Fibre Technology
Replacement of Steel Reinforcement with Fibers in Slab-on-Ground Applications
ASTM C1116/C1116M, "Standard Specification for Fiber-Reinforced Concrete," is the standard specification for all fiber reinforcement. There are four general classifications based upon material type of the fiber used in the concrete mixture:
- Type I, Steel fiber-reinforced concrete
- Type II, Glass fiber-reinforced concrete
- Type III, Synthetic fiber-reinforced concrete
- Type IV, Natural fiber-reinforced concrete
Acceptance of Fiber-Reinforced Concrete shall be based on ASTM C1399/C1399M, ASTM C1550/C1550M, ASTM C1609/C1609M. This indicates that both steel fibres and synthetic macro fibres must adhere to the same testing standards in order to be approved for use in replacement of steel reinforcement.
Field performance of fiber-reinforced concrete has shown that the use of fibers is a viable option as secondary reinforcement to control shrinkage- and temperature-induced cracking in a slab-on-round. The procedure is outlined as follows to offer rational basis for determining the dosage of a particular fiber that is required to provide performance equivalent to that of welded-wire mesh (WWM) or steel rebars.
BASF MasterFiber brand of fibres are tested in accordance with ASTM C 1609/C 1609M "Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam With Third-Point Load)." For the fibre-reinforced concrete with fibres distributed throughout the thickness of the section, the entire cross-section of the section is considered to resist tensile stresses. Hence, the tensile stress that must be resisted to achieve equivalent performance with the steel reinforcement is:
ƒ = stress in the concrete induced by the steel reinforcement, psi (MPa)
As = area of steel, in.2 (mm2)
ƒy = yield strength of the steel, psi (MPa)
b = slab thickness, in. (mm)
h = width of slab, in. (mm)
The tensile strength of a fibre-reinforced concrete, ƒF, must be equal to ƒ for equivalent performance between fibres and the steel reinforcement. ƒF, is obtained from ASTM C 1609 and can be expressed as a percentage (Re,3) of the concrete tensile strength (ƒr) as:
The calculation stops here with regards to steel fibres per ASTM C1609/C1609M. Due to material differences, there are additional safety factors that are incorporated for use of synthetic macro fibres.
Using a strength reduction factor of 1.15 for the steel, and a modification factor of 1.7 to account for the strength of the concrete and any uncertainties related to the long-term performance of the fibres, we can solve for the required Re,3 value required using:
This is derived from equating stress in the concrete induced by the steel reinforcement to the tensile strength required to be provided by the fibre reinforced concrete. We then solve for the tested value within ASTM C1609, Re,3:
For a particular fiber, the dosage required to provide performance equivalent to that of the steel reinforcement is that which gives the relative post-crack tensile strength, Re,3, calculated above.Please feel free to contact the author if you have any questions.