Effect of Polyethylene Terephthalate Fibres on the Structural Performance of Beams with Openings in the Shear Region

Abstract

The provision of transverse openings in beams to allow passage of services reduces the cracking and ultimate load capacities of the beams and increases deflection under the openings. Strengthening of the pre-planned opening regions by use of special steel reinforcements causes reinforcement congestion and difficulties in compaction, calling for alternative ways of strengthening beams with openings. In this study, concrete material was modified by the use of Polyethylene Terephthalate (PET) fibres produced from waste plastic bottles to improve the tensile properties and hence reduce the cracking around the openings and improve the overall performance of the beams. To evaluate the performance of PET fibres in beams with openings, eight (8) reinforced concrete beams with and without fibres of dimensions (150*250*2000 mm) were cast with varying opening sizes. The horizontal position of the openings was fixed at 300 mm from the support while the vertical position was maintained at the mid-depth of all beams. This performance was evaluated in terms of ultimate and first cracking loads, mid-span deflections, ductility, crack patterns and failure modes, and strain behavior. Test data showed that incorporating PET fibres in beams with openings resulted in a slight increase in ultimate load of 4.1% and 5.82% for 0.25 h and 0.35 h beam opening sizes respectively, beyond which the strength was reduced by 9.57% for beams with 0.45 h opening size, where h was the overall depth of the beam. The first cracking loads increased by 44.12%, 48.48%, and 9.38% for 0.25 h, 0.35 h, and 0.45 h opening sizes, respectively. In addition, a slight improvement in the ductility of PET fibre beams for all opening sizes was observed. The PET fibre beams exhibited a slight change in the mode of failure from dominant shear failure to combined shear and flexure failure characterized by multiple cracks in the flexure region with minimal spacing. An increase in the concrete compressive and tensile strains accompanied by a reduction in concrete and steel shear strains was observed because of the tension stiffening effects of the fibres. The incorporation of PET fibres, therefore, showed a significant improvement in strengthening beams with openings. Keywords— Beams with openings, first cracking load Polyethylene Terephthalate fibres, ultimate load.