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Fiber strands due to their flexibility, high aspect ratio, cross-section varieties and degree of crystallinity are adequately strong to be used as reinforcement in composites such as concrete. Newly introduced fiber reinforced concretes (FRC) are the cementitious materials that exhibit reinforcing features in all directions. FRCs due to their interesting properties are enormously favored by civil and structural engineers. Natural and synthetic fibers can be employed in concretes, shutcretes and mortars. The interface between the added fibers and the cementitious matrix fundamentally influences the properties of the FRCs. Fibers are classified into hydrophobic and hydrophilic. The former fibers have negligible moisture absorbent capacity while exhibiting acceptable mechanical properties. Hydrophobic fibers are incapable of forming adequate adhesion with cementitious matrix. Properties such as low weight, strength parity in wet or dry conditions and inertness in acid or alkaline environments are among the salient properties of polypropylene (PP) fibers. PP as a hydrophobic fiber has gained wide acceptance as concrete reinforcement. The hydrophobicity of fibers, such as PP, has been always been disadvantageous for the use of these fibers in concrete structures. Treatments such as chemical surface modification imparts hydrophilic property to PP fibers. Thus the modified PP fibers can successfully adhere to concrete matrix. In this research melt-spinning technology as the most widely used manufacturing technique for production of the PP fibers was used. Pure and grafted anhydride maleic PP granules were used to produce both hydrophobic and hydrophilic PP fibers. The produced fibers were characterized according to relevant standards prior to be added to concrete samples at identical fiber volume fraction. The results pointed to the positive effect of the induced hydrophilic properties in the fibers as far as the fiber-matrix adhesion was concerned. The ability of the chemically modified fibers to absorb water when wetted with the moisture present in the concrete, greatly improved the adhesion of the added fibers with the concrete matrix. The effect of hydrophilicity of PP fibers on mechanical properties of reinforced concrete was investigated by comparing concrete samples prepared by modified and unmodified fibers. Results showed that in comparison to control concrete sample, addition of modified hydrophilic fibers to concrete enhances compressive, tensile and flexural strength of concrete by 11%, 43% and 75% respectively. It was found that compressive, tensile and flexural strength of concrete samples containing the chemically modified fibers were respectively higher by 5%, 8% and 5% in comparison to the concrete samples containing unmodified hydrophobic fibers. Addition of fibers is more effective in enhancement of flexural strength of resultant concrete. This is due to the fiber bridging phenomena that prevent both crack formation and propagation. Addition of fibers also improves load bearing capacity of the resultant concrete, which in turn leads to enhancement of flexural strength of the concrete. Results also showed that addition of hydrophobic polypropylene fibers leads to 66% increase in the flexural strength of the samples. The increase in flexural strength of the concrete samples containing hydrophilic fibers in comparison to the control sample was found to be 75%.
 

Keywords: Fiber reinforced concrete (FRC) Macro fibers Hydrophilic polypropylene Concrete mechanical properties

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