Distributed sensing to assess the effect of axial restraint on reinforced concrete slabs
Current design codes do not account for effects such as arching and membrane action in reinforced concrete slabs, which can increase capacity, and allow for a reduction of construction materials to be used. Research conducted to date has been unable to measure stress distribution throughout reinforced concrete slabs to fully understand the impact of arching and membrane action, because a method for capturing distributed strains was previously unavailable. One solution is to use distributed monitoring technologies to evaluate slab behaviour. Distributed fibre optic sensors (FOS) based on measuring Rayleigh backscatter provide strain measurement along the full length of the fibre up to 70 m. An experimental study using this technology was undertaken to investigate the behaviour of reinforced concrete slabs subjected to axial restraint. In the first test series, eleven one-way reinforced concrete slabs with varying depths, reinforcement ratios, and boundary conditions were instrumented with nylon coated FOS and tested in three-point bending to investigate the effects of axial restraint on slab behaviour. It was found that axial restraint increased the load carrying capacity of the specimens up to 45% and reduced reinforcement strains at a given applied load prior to yielding. An increase in capacity and decrease in reinforcement strains at a given load were also seen as reinforcement ratios and slab depths were increased. All specimens failed in flexure. In the second test series, four two-way reinforced concrete slabs with varying reinforcement ratios and boundary conditions were instrumented and tested under a central point load. Increasing the reinforcement ratio led to an increase in slab capacity up to 45%. Two specimens failed in flexure and two failed due to punching shear. Finite-element models were created for the one-way specimens to evaluate the accuracy of the models for use in structural assessment with distributed sensing. Plastic collapse analysis was completed for the two-way specimens in order to determine the accuracy of the analyses. Results from the FE model were accurate for the control specimens but showed stronger and stiffer responses compared to the restrained specimens. The plastic collapse analyses estimated the experimental control capacity within 13%.