Experimental investigation of deformations of lightly reinforced concrete beams

Abstract

Present research was dedicated to investigation of short-term deformations and tension stiffening in lightly reinforced concrete beams. Experimental results on tests of 8 beams (four couples of twin specimens) having constant reinforcement ratio 0,40 %, but different bar diameter have been reported. All the beams were nominally 3,28 m long and were tested under a four-point loading system. The experimental programme consisted of two series. In the beams of the first series, the tensile reinforcement consisted of four 10 mm bars, whereas the beams of the second series had two 14 mm bars. Prior to the beam tests, measurements on concrete shrinkage and creep were performed. In order to exclude shrinkage influence on cracking resistance of reinforced beams, large amounts of top reinforce-ment were assumed in half of the specimens (second numbers of twins). Concrete surface strains were measured throughout the length of the pure bending zone along four continuous gauge lines located at different depths. The av-eraged strains along each line were employed for curvature assessment. Alternatively, curvature over the constant moment zone was defined from the deflection gauge readings. Moment-curvature diagrams derived from average longitudinal strains and obtained from deflections were in good agreement. The beams having heavy top reinforce-ment possessed slightly higher cracking resistance. Tension stiffening was far more pronounced for the beams of the first series having smaller diameters of tensile reinforcement. These beams at the load corresponding to 50 % of the ultimate bending moment had about 33 % smaller curvatures than the beams of the second series.