Distributed Sensing to Assess the Shear Behaviour of Reinforced Concrete Beams

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Zhang, Jiachen
Digital Image Correlation , Distributed Fibre Optic Sensor , Shear Behaviour , Stirrup Deterioration , Reinforced Concrete
The long-term durability of infrastructure is a growing concern as assets built in the middle of the last century reach the end of their service lives. One solution to manage these assets is to use distributed monitoring technologies to assess their performance and the effects of deterioration. Distributed fibre optic sensors (FOS) based on measuring Rayleigh backscatter provide strain measurements along the full length of the fibre up to 70 m. Digital image correlation (DIC) can show crack formation over the surface of a structure and assist with the understanding of the strain data from the FOS. An experimental study using these two technologies was undertaken to investigate the shear behaviour of reinforced concrete (RC) beams. In the first test series, seven reinforced concrete specimens with different stirrup spacing and longitudinal reinforcement ratios were instrumented with nylon coated FOS and tested in three point bending to investigate whether FOS could detect differences in behaviour. It was found that distributed strain measurements could detect the peak strain in the stirrups indicating the location of the crack crossing the stirrups as well as providing the strain distribution along the stirrup. The specimens all failed in flexure except the ones that violated the CSA A23.3-14 (2015) stirrup spacing limit. In the second test series, five RC beams with artificial deterioration were instrumented and tested. Artificial deterioration mechanisms included having no bottom or top anchorage as well as pitting corrosion at the mid-height of the stirrup legs. Stirrups without bottom anchorage were found to have the largest impact on the specimen’s performance (the load carrying capacity was reduced by 20% and the failure mode changed from flexural to shear). The other two types of deterioration did not influence the load carrying capacity. Distributed sensing aided in understanding the failure mode. Finite-element models were created using a two dimensional finite-element analysis (FEA) package to evaluate the accuracy of this software for use in structural assessments with distributed sensing. It was found that the results from the FE models were less conservative than the experimental results for most cases. Inaccuracies (i.e. load capacity, failure mode, strain distribution, crack pattern) were due to the use of incorrect modeling methods for the deteriorated stirrups and the use of smeared cracking.
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