EFFECTS OF CONCRETE GRADES ON STRENGTH CHARACTERISTICS OF REINFORCED CONCRETE SLENDER BEAMS

Abstract

The focus of this work is to investigate the effect of reduction in the recommended characteristic strength of concrete, by the consultant structural engineer on the resistance capacities of slender beams. Eight (8) reinforced concrete slender beams were cast in total, two beams for each concrete grade of 5.7 N/mm2, 9.2 N/mm2, 12.5 N/mm2 and 15 N/mm2, which were loaded with a point load at the beam center. The beams produced from 5.7, 9.2, 12.5 and 15 grades of concrete were termed beams B1, B2, B3 and B4 respectively. The shear span to depth ratio of the beam loaded at the center with a point load was 3.8. Each of the beams was reinforced at the top and bottom with 2Y12 mm bars, with shear reinforcement of Y8 bars at 100 mm centers. The cross-sectional dimension of the beams is 100 mm x 150 mm with overall length of 1000 mm. The estimated moment of resistance MER for beams B1, B2, B3 and B4 were 9.41 kNm, 10.28 kNm, 11.10 kNm and 11.71 kNm respectively, while the estimated ultimate loads were 50.19 kN, 54.83 kN, 59.20 kN and 62.45 kN respectively for beams B1, B2, B3 and B4. The estimated shear capacity was 113.19 kN for beams B1, B2, B3 and B4. The actual moment of resistance MAR for beams B1, B2, B3 and B4 were 7.5 kNm, 8.73 kNm, 9.99 kNm and 11.16 kNm respectively, the actual ultimate loads were 40.03 kN, 46.55 kN, 53.27 kN and 59.52 kN respectively for beams B1, B2, B3 and B4. These show that estimated moment of resistance MER and estimated ultimate loads PEUL are greater than actual moment of resistance MAR and actual ultimate loads PAUL by 4% to 20%. The actual shear capacities VAUS were 20.02 kN, 23.28 kN, 26.64 kN and 29.76 kN for beams B1, B2, B3 and B4 respectively. These values of shear capacities show the estimated shear capacity was greater than the actual shear capacities by 70% to 82%. Above results show that the formulae for predicting moment of resistance and ultimate load for rectangular slender beams are satisfactory, while that for the prediction of shear capacity is not satisfactory and needed to be modified, in order to have a model that will give better results. Also from the above, for a drop of 38%, 54.4% and 62% in concrete grade there were corresponding decrease in the strength characteristics of beams by 14%, 25% and 33% respectively.