Studying the Influence of Cracking on The Behaviour of Reinforced Concrete Beams using Distributed Fibre Optic Sensors

Abstract

This research investigated the post-cracking behaviour of reinforced concrete members in flexure and how the shape and pattern of the cracking affected the behaviour. Two experimental campaigns were conducted, the first involving five slender continuous beams and the second three lightly reinforced one-way slabs. All of the specimens were instrumented with distributed fibre optic sensors (DFOS) to measure reinforcement and concrete surface strains. The continuous beams were one degree statically indeterminate and had two asymmetric span lengths. Each span was loaded by an independent actuator so that the beams could be loaded both symmetrically and asymmetrically. Five specimens were constructed, four were nominally identical full-scale beams, while the fifth beam was a scaled down version of the other specimens preserving the shear span to depth ratio and the percentage of longitudinal reinforcing steel. All four full-scale beams failed in shear at a lower load than predicted, while the fifth scaled down beam developed a plastic collapse mechanism as expected. This difference in behaviour with depth suggested a size effect that has only previously been observed clearly in beams without shear reinforcement. The shear failures were believed to be caused by the shifting inflection point which caused extended flexural cracks to form before turning over into diagonal shear cracks. As a result of this cracking behaviour, fewer stirrups were crossed by the critical shear crack resulting in a lower steel contribution than predicted by design codes. The slabs had a novel instrumentation setup whereby surface DFOS were installed after an initial load cycle to cause cracking. During the second load cycle polyimide coated fibre optic cables enabled surface strains in between cracks to be measured and these strains were used to infer the effects of crack formation on tension stiffening. The results indicated that crack spacing and bar curvature had a significant impact on the amount of tension stiffening developed.