Study on effect of Polypropylene and Steel Fiber on the strength of Concrete

Concrete is most widely used material in the construction as it is relatively strong in compression but weak in tension and tends to be brittle. These two weaknesses have limited its use. Fiber reinforced concrete is most widely used for improving tensile and flexural strength of concrete. In this paper an attempt has been made to explore the effects of polypropylene fiber on some hardened properties of PFRC based on its compressive strength, flexural strength and flow ability. In this research, concrete mixes were added with polypropylene fiber of 0%, 0.2%, 0.4%, and 0.8% volume fraction and Steel Fibers were added in a varying dose of 0%, 0.2%, 0.4%, and 0.8%. An experimental result demonstrated a notable increase in flexural, tensile strength was found. However, no significant change in compression strength is observed.


Introduction
Concrete is a tension-weak building material and possess low resistance to the cracking. Also their tensile strength and flexural strength is relatively low compared to their compressive strength. To improve such properties, Fiber reinforced concrete (FRC) has been developed (Banthia N and Sheng J.,1996).The reinforcing fibers are randomly distributed in the matrix. The addition of the fibers enhances the engineering properties of concrete like Flexural strength, Compressive strength, ductility and toughness. The concrete-reinforcing fibers include metal, polymer, glass, asbestos and various others. Among the polymer fibers, the polypropylene fibers enjoy popularity in the domain of concrete. The common forms of polypropylene fibers are smoothmonofilament and have triangular shape. These fibers have low density and are chemically inert and non corrosive. It was reported that application of polypropylene fibers improves the plain concrete properties including splitting tensile strength, first crack strength and impact resistance (Song P.S,.Hwang.S.,andSheu.B.S.,2005) Alhozaimy et al.observed that an additional amount of 0.1% polypropylene fibers in the plan concrete had 44 % increases in flexural toughness of the concrete. ( Preeti A Patel & Arun k Desai.,2012) concluded that the polypropylene fibers do not disperse properly in the mixing water therefore the addition of the fibers to dry mix was found to be more practical. Also in the plain concrete the failure was due to the spalling however the failure mode in the fiber reinforced concrete is due to the bulging in the transverse direction. [Arkan RAdi Ali 2013] demonstrated that the addition of fibers reduces the workability and the slump value. Effect of polypropylene fibers was more dominant in the tension as compared to the compression due to the adhesive and friction forces between the polypropylene fibers and the concrete.

Chemical Admixture
Plasticizer CAC-Super flow 35 U admixture was used having the specific gravity 1.2.

Polypropylene Fibers
Polypropylene Fibers were used in this study .Various properties of the Polypropylene Fibers are as given in the Table 4.  For all mix proportions these components are kept constant while the dosages of polypropylene fibers are Steel Fibers are varied. This can be seen in Table 7 and Table 8.

Properties of Fresh Concrete
Effect of addition of polypropylene fiber on the fresh concrete is measured in terms of slump value of the concrete.
Result indicates that the workability reduces at higher dosage of the fibers as compared to the initial dosage. This is because of increase in the air content due to presence of fibers and thus reduces the workability.
Result shows that at controlled concrete and at 0.2% and 0.4% of fiber content the workability is high However for the dose of 0.6% it is medium.

Effect on the Flexural Strength
Flexural strength at 28 days of curing test was conducted according to the requirements of IS 516 using three 150 × 150 × 700 mm beams under thirdpoint loading on a simply supported span of 600 mm. The flexural strength of the mix at 0.4 % and 0.6% increases by 26.67% and 45.77 % respectively at 7 days and 36.23% and 40 % respectively for 28days as compared to the controlled concrete. The enhancement in the flexural strength is due to the increase in the bond strength between the cement paste and the fibers. The increase in the fiber content also reduces the crack widening. Figure 5 and 6 shows the graphical representation of the flexural strength at 7 and 28 days respectively.

Effect on the Split Tensile Strength
The matrix acted through the stress transfer from the matrix to the fibers and this stress transfer improved the tensile strain capacity. Fiber bridging mechanism is mainly responsible for the increase in the strength.

Conclusion
Based on the test results the following conclusion can be drawn.
1. The inclusion of polypropylene fiber enhances the Flexural strength. 2. Maximum gain of 17% in the split tensile strength at 28 days is being observed. 3. During the investigation it was found that the PFRC has greater crack resistance because of reduction in the width of crack. 4. Workability drops down with the increase in the fiber content as there is increase in the air content due to presence of fiber. 5. Maximum increase in the flexural strength was found to be around 45% as compared to the controlled mix. 6. Result shows that there is no significant increase in the compressive strength with the increase in the fiber content.