Shear Behaviour in Reinforced Concrete Beams Monitored with Distributed Sensors

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Authors
Poldon, Jack
Keyword
shear , reinforced concrete , distributed fibre optic sensors , digital image correlation
Abstract
The precise mechanism that provides shear resistance in reinforced concrete has been an unsolved riddle to researchers for decades. This was caused by limitations in the available sensor technology for effectively measuring the response of concrete members. Emerging distributed sensor technologies, namely distributed fibre optic sensors (DFOS) and digital image correlation (DIC), can provide the comprehensive data sets needed to study shear behaviour in reinforced concrete. DFOS and DIC measurements were used to study the load-response of three large slender beams with varying shear reinforcement designs. The beam specimens contained the equivalent of 20,000-point strain measurements and distributed crack width and slip measurements. From the distributed sensor data, differences in the load-response between the three beams relating to serviceability and strength were investigated. The amount of shear reinforcement was found to affect the percentage of total midspan deflections attributed to shear, equalling about 25% at service conditions and up to 42% at ultimate conditions. The DFOS was used to understand the failure modes through studying the longitudinal and transverse reinforcement strains where JP-1 failed in shear prior to longitudinal yield, JP-2 failed in shear at the onset of longitudinal yield, and JP-3 which failed due to anchorage with a plastic hinge near midspan reaching up to 1.6% strain. The distributed sensors were also used to study shrinkage, anchorage of stirrup hooks, bending of stirrups caused by shear cracking, amongst other behaviours. Member equilibrium was studied through constructing free body diagrams (FBD) based on the distributed sensors measurements. This study highlighted how forces across cracks were essential for maintaining equilibrium and that shear forces carried within the flexural compression region of the FBD agreed with past approaches to modelling shear. Consideration of the diagonal compression stress field in members with shear cracks was found to also be necessary, and a method of measuring the angle of principal compressive stress was proposed. The FBD study was used to evaluate the performance of the shear provisions in the ACI 318 code, where V_s was found to be conservative on average. The experimental failure loads were within 14% of the ACI and 11% of the CSA nominal values.
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