Fibers in Concrete

Fibers in concrete do not act in uncracked concrete. At the start of crack development process, when crack develops, the tensile stress of the fibers is used to stop cracking. So,  the fibers provide post cracking strength or residual strength. The residual strength of the fibers depends on physical properties  (such as shape, length, thickness) and chemical properties ( such as types of material ) and the dispersion of the fibers in the concrete or concrete composite.

The post cracking behavior of FRC is assessed  and applied to design application for ground rested slabs or for structures subjected to bending.

(Reference  ASTM C 1609/1609 M-Flexural test)

The designer should suggest the specifications after laboratory testing and suggest the type of fibers and its physical and chemical properties and the flexural test and compressive strength values to be achieved. Quality control on mix operation is important to have proper dispersion of the fibers in concrete mass.

The steel fibers conforming to ASTM C 1116/ 1116 M (Type 1) and as per manufacturer’s instructions may be used.

The polymeric fibers may be used as per manufacturer’s recommendation but conforming to ASTM C 1116/1119M (Type 3)

Furthermore, information can be obtained by the following standards:

• EN 14889-1:2006 Fibers for Concrete. Steel Fibers. Definitions, specifications & conformity

• EN 14889-2:2006 Fibers for Concrete. Polymer Fibers. Definitions, specifications & conformity

• EN 14845-1:2007 Test methods for fibers in concrete

• ASTM A820-06 Standard Specification for fibers in Fiber Reinforced Concrete

• ASTM C1018-07 Standard test methods for flexural toughness & first crack strength

Steel Fibre Types

The types of steel fibres are defined by ASTM A8201 91

(1 PSI= 0.0069 MPa)

• Type I: cold-drawn wire; [145,000 psi (1000 MPa) to 445,000psi (3070MPa)]; round wires with deformed ends and in varying diameter

• Type II; cut sheet (rectangular or square shapes with dimples- depressions)

• Type III: melt-extracted

• Type IV: mill cut

• Type V: modified cold-drawn wire

Types II, III, IV, and V have tensile strength as low as 50,000 psi (345 MPa).

Other fiber shapes range from round, triangular, rectangular, twisted as well as other shapes and in  lengths, ranging from 6 mm to more than 50mm.  The shapes and sizes may be determined as per  availability in the market,  in conformance to  the standards as mentioned above or as per the country specific standards.

The fibers are defined about the aspect ratio (length / diameter or thickness). The higher the aspect ratio, the better is the performance.

Effect of fibers in concrete

(Fiber effects in concrete will depend on the quality production (mixing most important), transportation, placement and curing operations of the matrix for different structural members )

The effects of the fibers in composite are given below:

Control of cracking in concrete due to shrinkage and temperature stresses (also freeze and thaw) – Post cracking strength or residual strength of the fibers is used at the initiation of cracking process to check the crack developement

Reduces permeability – FRC controls cracking and thus reduces permeability

Control impact and abrasion – Concrete is non ductile or quasi ductile. Use of fibers increases ductility and make the concrete tough

Control bleeding – Reduces slump and checks bleeding

Reduces slump – Slump maintenance requires more water or the use of admixtures

Polypropylene fibers – enhance  pumpability

Too long fibers – reduce workability and pumpability

Improve structural strength

Reduce steel requirements

Developments in SFRC

The quantity that was in use for control crack due to temperature variations  was 1.0 percent. Now it is used in increased volume  1% to 2 % or even more as per the requirement of mix design, and this has controlled not only shrinkage effect but also has enhances  the properties of the concrete by increasing the flexural strength and ductility.

SFRC can be produced with the same precautions that are necessary to produce NSC, however following additional steps becomes necessary:

The fibers with higher aspect ratio are preferred in use.

The quantity needs to be optimized, otherwise the fibers with higher aspect ratio can impair the workability of concrete.

The performance of hardened and stressed concrete is better with the use of higher aspect ratio fibers.

It must be noted that the optimum quantity of fibers is well mixed and dispersed uniformly without segregation or grouping.

The fibers stored  may be clumped and it should be ensured that they are dropped in the drum in non-clumpy state.

It is advantageous to add water after mixing the  other matrix contents including fibers.

Addition of  glued fibers is another alternative.

Glued Steel Fibbers

Generally these are made by drawing the low carbon steel rods / bars to achieve the required shape. They are considered as replacement of rebars in concrete