Flexural behaviour of externally post-tensioned two-span lightweight concrete beams

This study examined the effect of the partial prestressing ratio on the flexural behaviour and degree (β) of moment redistribution of two-span lightweight aggregate concrete (LWAC) beams post-tensioned using external strands. The total length of the T-shape beam, between two exterior supports, was 10 600 mm. The unbonded prestressing three-wire strands were arranged with a harped profile of variable eccentricity, using two deviators at the sagging zone in each span and a deviator over the intermediate support. To simulate a uniform load, the top loads were uniformly distributed at four points in each span. Test results revealed that two-span externally post-tensioned LWAC beams typically exhibited smaller incremental stresses (Δf_ps) in the unbonded strands at the ultimate state and a smaller displacement ductility ratio than those of corresponding internally post-tensioned beams. The safety level of ACI 318-14 equations for Δf_ps was abruptly reduced for two-span externally post-tensioned beams, indicating non-conservative results for beams with partial prestressing ratios exceeding 0·66. The fib model code significantly overestimated the β values of post-tensioned LWAC beams. The ACI 318-14 code also tended to overestimate the β values of externally post-tensioned LWAC beams, although it yielded a relatively conservative estimate for externally post-tensioned LWAC beams.