Flexural tests on pre-tensioned lightweight concrete beams

Eight pre-tensioned beams made using lightweight concrete with a design compressive strength of 35 MPa were tested under two symmetrical third-point concentrated loads. The main variables investigated were the eccentricity and effective prestress of the strands and the partial prestressing ratio. The flexural capacity of the beam specimens was compared with predictions obtained from simple equations using the equivalent stress block combined with different formulas proposed for the stress of bonded tendons at the ultimate strength of beams and from non-linear strain compatibility analysis. Test results showed that the normalised flexural capacity measured from pre-tensioned lightweight concrete beams with a reinforcing index below 0·15 was comparable to that of pre-tensioned normal-weight concrete beams with a similar reinforcing index, while that of a pre-tensioned lightweight concrete beam with a reinforcing index of 0·207 was lower than that of pre-tensioned normal weight concrete beams with a reinforcing index of 0·2. As a result, the increasing rate of normalised flexural capacity against the reinforcing index in pre-tensioned lightweight concrete beams was slightly lower than that observed in pre-tensioned normal weight concrete beams. The flexural capacity of pre-tensioned concrete beams predicted using an equivalent stress block was marginally influenced by the values determined from different formulas for the stress of bonded tendons at the ultimate strength of beams, although the ratio of the measured and predicted flexural capacities was commonly lower in pre-tensioned lightweight concrete beams than in pre-tensioned normal weight concrete beams.