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n this study, polypropylene fiber and polyolefin fiber were used under varying volume fractions (Vf), 0.5, 1, 1.5 and 2%. The compressive, split-tensile and flexural strength of the hardened M30 concrete at 28 days of standard curing were studied. The compressive, split-tensile and flexural strength have shown, in common, an optimum strength at volume fraction of 1%, and thereafter, there is a reduction in strength. (100 WORDS)

Keywords: Polyolefin, Polypropylene, Hybrid mix (6 SUITABLE WORDS FOR INDEXING)
1. Introduction
In olden days, natural fibers like straw, jute, and horse and sheep hair were used in building materials, but in recent years, inclusion of fibers in concrete has been widely researched. The term fiber reinforced concrete (FRC) can be defined as a concrete containing dispersed randomly oriented fibers (Sahmaran et al., 2005) 1. Artificial steel and polymeric fibers are manufactured for mixing with concrete to achieve improved strength, compared to conventional concrete. Fiber Reinforced Concrete increases the flexural strength and ductility and reduces the crack width ACI 544 2. The inclusion of fibers prevents the propagation of existing and/or initiated cracks by improving the crack tip plasticity that increases the fracture toughness of the composite (Khan et al., 2017) 3.
Hybridization of two types of fibers as reinforcement in concrete is an evolving research study in recent times to obtain superior strength properties. This mixture was suggested to synergistically interact with concrete or cementitious composite matrix to develop a super-hybrid composite (Khan et al., 2014) 4. Different types of synthetic fibers have been incorporated in cementitious composite, such as acrylic, aramid, carbon, nylon, polyester, polyethylene (PE) and PP fibers. Some of these fibers, such as carbon, PE and PP fibers, have been intensively employed; however, few studies have utilized other fibers (Khan et al., 2017) 3. For example, Ahmed (2007) 5 conducted studies using mono fiber composites containing steel and Polyethylene (PE) fiber under Series 1 and 2 respectively; Series 3 and 5 for steel-PE and steel-polyvinyl alcohol (PVA) hybrid fiber composites containing 50% flyash (FA) as partial replacement of cement; and for series 4 is for steel-PVA hybrid composites without FA; and the total volume fraction of fibers was kept constant at 2.5% to maintain a workable mix. Among composites with different volume ratios of steel and PE fibers, the composite with 1% steel and 1.5% PE was found to show the highest flexural strength.
When two fibers of varying properties are combined as reinforcement in concrete, it is expected to give a synergetic effect, thereby giving an increase the strength of concrete, compared to single fiber reinforcement. The properties of fibers that are usually of interest are fiber concentration, fiber geometry, fiber orientation, and fiber distribution. Moreover, using a single type of fiber may improve the properties of FRC to a limited level. However the concept of hybridization, adding two or more types of fiber into concrete, can offer more attractive engineering properties as the presence of one fiber enables the more efficient utilization of the potential properties of the other fiber (Sahmaran et al., 2005) 1. The improvement of mechanical properties such as compressive strength, tensile strength and flexural strength of hybrid fiber reinforced cementitious composites over mono fiber composites and plain concrete has been reported by several researchers (Ahmed et al., 2007) 5.
Alhozaimy et al. (1996) 6 has studied the compression and flexural properties of PP fibre reinforced concrete and pozzolanic material inclusion in concrete. He added PP fibre in 0.05%, 0.1%, 0.2% and 0.3% volume fractions. He concluded that PP fibre addition in concrete does not significantly improve the compression test on concrete. And he also concluded that there are no effects on flexural strength by addition of PP fibre at these volume fractions. From test results also we can see that there is only a low increase in flexural strength. Koksal et al. (2007) 7 has conducted an experimental study to investigate the mechanical properties of concrete when silica fume and steel fibres were used together. Steel fibres of two aspect ratios 65 and 80 were used. Steel fibre was added in two volume fractions 0.5% and 1.0%. With the increase in volume fraction of steel fibre the unit weight of concrete has increased. In no silica fume mix, addition of steel fibre has increased all compressive, split tensile and flexural strength in all aspect ratios and volume fractions. Aspect ratio 80 has given the optimum values at 1.0% volume fraction for all the properties. Compressive strength, split tensile strength and flexural strength has improved 1.19, 1.90 and 1.77 times than the control specimen respectively. Fibers prohibit and interrupt the mechanisms of crack formation and propagation by acting as stress-transfer bridges. The hybrid mixture of fibers can be described as combination of several fibers of diverse dimensions, shapes and mechanical properties.
Fiber hybridization has been implemented using various kinds of fibers or mono-fiber with different sizes. Numerous investigators show the favored usage of fiber hybridization compared to single fibers, which have been attributed to the synergetic interactions of hybrid fibers (Khan et al., 2017) 3.
Yao et al. (2003) 8 have conducted experiments on three types of hybrid fiber-reinforced concretes with carbon-steel, steel-PP, and PP-carbon fiber combinations in terms of compressive, splitting-tensile, and flexural tests. Among the three types of fibers, carbon fibers gave the highest compressive strength, PP fibers the lowest. When the fibers used in a hybrid form, it obviously increased strength in the case of carbon-steel fibers and carbon-PP fibers; and among the three hybrids, carbon-steel gave the highest strength and steel-PP fibers gave the lowest. As far as splitting-tensile strength is concerned, fiber addition increased strength with carbon and steel fibers, but decreased with PP fibers when the fibers were used in an individual form. Carbon fibers gave the highest splitting tensile strength, while PP fibers gave the lowest splitting tensile strength. In hybrid form, carbon-steel fibers gave the highest splitting tensile strength, which was much higher than that of either carbon fiber-reinforced concrete or steel fiber-reinforced concrete. Among the three types of fibers, steel fibers gave the highest flexural strength and PP the lowest. When the fibers were used in hybrid form, it increased MOR in the case of carbon-steel fibers and carbon-PP fibers compared to any of the simple fibers. The main advantage of carbon fiber addition is the resulting high compressive and splitting tensile strengths, while the main advantage of steel fiber addition is the resulting high flexural strength. The carbon-steel hybrid was the most beneficial for the improvement of strength; and improvement of 31.4% in compressive strength, 36.5% in splitting tensile strength, and 32.9% in flexural strength.
Pliya et al. (2011) 9 have investigated by steel and polypropylene fibres individually and as a combination. PP fibres were added under 2 volume fractions 0.11% and 0.22% which is equivalent to 1kg/m3 and 2kg/m3. Steel fibres were also added in 2 volume fractions 0.38% and 0.50% which is equivalent to 30 kg/m3 and 40kg/m3. Compressive strength was increased to a maximum of 13.40% at hybrid combination of 0.11% PP and 0.5% Steel fibre. The maximum increase of flexural strength is less than 7% only at the maximum volume fraction combination.
Yazici et al. (2007) 10 have used varying volume fraction (Vf) of steel fiber on the strength of concrete using hooked-end bundled steel- fibers of different fiber volume fractions at 0.5%, 1.0%, 1.5% by volume of concrete. The flexural strength at 28 days of steel fiber reinforced concrete significantly improved with increasing Vf. The workability of fiber reinforced concrete has decreased for Vf=1.0 and 1.5%. Yao et al. (2003) have used various types of hybrid fibers at the same volume fraction 0.5%. Three hybrid combinations are polypropylene (PP) and carbon, carbon and steel, and steel and PP fibers. Individually, carbon fibers have given the highest compressive and splitting-tensile strength and steel fibers the highest flexural strength. In hybrid form, carbon-steel fibers have shown the highest compressive strength and splitting-tensile strength. The moment of resistance (MOR) increased in the case of carbon-steel and carbon-PP fibers. In another study, Sivakumar (2011) 11 investigated on concrete reinforced with steel fibers and hybrid combinations of steel with glass, polyester and polypropylene, and found that there is no appreciable improvement in compressive strength at 28 days for all combinations; but the split tensile strength of hybrid fiber reinforced concrete is found to be higher than reference and mono steel fiber concrete. Steel-polypropylene combination at 0.5% (with 30% polypropylene fibers) has given higher strength than mono-steel fiber concrete.
The objective of the present study is to compare the performance of two different types of fibers, namely the polypropylene and polyolefin fibers in mono form as well as in combined form, in terms of compressive, splitting tensile and flexural tests.

2. Experimental Investigation
2.1 Materials and Methods
Experiments were conducted to evaluate the mechanical strength of concrete when fibers are added in mono form and in hybrid form. Polyolefin and Polypropylene have been added in concrete in volume fraction, Vf=0.5, 1.0, 1.5 and 2.0% in individual form; and in hybrid form, for the same Vf%, 50-50% is added and strength determined at 28 days of standard curing. For each mix, 3 replicate specimens of cube of specimen size 150 mm × 150 mm × 150 mm, cylinder specimens of 150 mm × 300 mm (height), and prismatic beams of size 100 mm × 100 mm × 500 mm are cast to determine the compressive, splitting-tensile and flexural strength of hardened conventional concrete and fiber reinforced concrete for 2 fiber types in mono form and hybrid mix, in fiber volume fraction, Vf=0.5, 1.0, 1.5 and 2%, as shown in Table 1.

Ordinary Portland Cement (OPC) 53 Grade conforming to IS 12269-1987 12 was used for the concrete. In the present experimental study, two types of synthetic fibers, polypropylene and polyolefin have been used in mono and in hybrid (combined) form in concrete to determine the flexural strength, when fiber volume fraction (Vf%) is varied between 0.5 to 2.0% (with 0.5% interval) have been presented in Table 1. Coarse aggregate passing through 20 mm sieve of specific gravity 2.72, conforming to IS have been used. The properties of Polyolefin and polypropylene fibres used in concrete mixes are given in Table 2. Mix design for M30 grade of concrete done as per IS 10262 – 2009 12. The mix details are given in Table 3. The properties of Polyolefin and Polypropylene fibers are given in Table 1. Polyolefin (PO) fiber is of 54 mm length and diameter of 0.53 mm an ultimate tensile strength of 640 N/mm2, Young’s modulus of 10 GPa with an aspect ratio of 101.89 has been used in this study.

2.2 Casting of Specimens
The cube, cylinder and prismatic beam specimens were cast (3 replicate specimens for 28 days) as per the combinations shown in Table 2, and average results taken and presented in the next section.

The control specimens were cast with plain cement mortar and test specimens with polyolefin fibers. Initially, dry cement mortar was prepared and then polyolefin (or stainless steel) fiber is evenly spread to the dry mortar and thoroughly mixed, and then water was gradually added to the dry polyolefin fibrous mix and repeatedly mixed, and wet concrete is ready for casting. Proper compaction of the concrete was done with compaction of the ingredients together. The specimens are finished properly. After 24 hours of hardening, the samples were cured for 28 days and tested for mechanical strength

The hybrid fibres have shown high improvement comparing other two individual fibres in all compressive strength, split tensile and flexural strength. All individual and hybrid show an increase in compressive strength, split tensile strength and flexural strength till 1.0% of volume fraction. After that increase in fibre content has shown decrease in the properties of concrete. The compressive strength of fibre reinforced concrete has increased about 14.4% in hybrid fibre concrete at 1.0% of volume fraction. The split tensile strength of fibre reinforced concrete has increased about 33.6% in hybrid fibre concrete at 1.0% of volume fraction. The flexural strength of fibre reinforced concrete has increased about 65.6% in hybrid fibre concrete at 1.0% of volume fraction.
Yazici et al. (2007) 10 have studied three different aspect ratios of steel fibres under three volume fractions, compressive strength was found to be improving about 19% at 1% volume fraction in both 65 ; 80 aspect ratios. Split tensile strength was found high at 1.5% volume fraction of 65 aspect ratio about 54% and flexural strength was found high at 1.5% volume fraction of 80 aspect ratio about 81%. In our study 1% volume fraction of hybrid fibre was found to be optimum. Compressive strength, split tensile strength and flexural strength has shown an improvement of 14.44%, 33.8% ; 65.6% comparing control concrete. Sivakumar (2011) 11 show that steel-polypropylene(75%-25%) of 0.5% volume fraction was found to be perform better in all aspects of study. Compressive, split tensile and flexural strength was found to be increasing about 15.3%, 34.1% and 37.8% respectively. In our study 1% volume fraction of hybrid fibre was found to be optimum. Compressive strength, split tensile strength and flexural strength has shown an improvement of 14.44%, 33.8% ; 65.6% comparing control concrete.

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