Optimum Shear Strengthening of Reinforced Concrete Beams

Hiran Deshantha Yapa 2011

External prestressed carbon fibre reinforced polymer (CFRP) straps can be used to strengthen
shear deficient reinforced concrete (RC) structures. The strengthening system is associated with
a number of parameters including the number of straps, strap locations, strap stiffness, and strap
prestress. The initial goal of this research was to identify the optimum values for these
parameters in order to design an efficient and effective shear retrofitting system.

The shear friction theory (SFT) and modified compression field theory (MCFT) were identified
as potential predictive theories to model the shear behaviour of RC beams retrofitted with CFRP
straps. Possible modifications to the theories to reflect CFRP prestressed straps were
investigated. Two popular optimisation algorithms namely the genetic algorithm (GA) and
particle swarm optimisation (PSO) were coded and tested with six test functions. These
algorithms were used to find the optimum shear retrofitting configurations and also to reduce
the computational cost associated with the SFT and MCFT evaluations.

An experimental investigation was carried out to validate the SFT and MCFT predictions for
various CFRP strap configurations. The investigation consisted of an unstrengthened control
beam and five CFRP strengthened beams. The shear behaviour of the beams was significantly
influenced by the CFRP strap configurations. A critical load level where the beam stiffness
started to deteriorate significantly was identified. It was found that there was a correlation
between this load level and the yielding of the internal shear links and a rapid increase in crack
opening.

The SFT and MCFT were validated using the experimental results. The peak shear capacities
predicted using the SFT were more consistent with the stiffness deteriorating loads identified in
the experimental investigation than with the ultimate loads of the beams. The reinforcement
forces and crack opening values found from the SFT were consistent with the experimental
results. The MCFT predicted the total shear response, ultimate shear capacity, crack opening,
and internal and external reinforcement forces in the beams. The accuracy of the MCFT
predictions reduced slightly when either the strap configuration was highly nonuniform or the
initial prestress level in the straps was relatively low. The shear link yielding load levels
predicted by the MCFT were found to be similar to the SFT predictions.

By using the coded optimisation algorithms in combination with the SFT or MCFT, the
optimum CFRP strap configurations were found for a selected case study. Both theories
predicted an offset for the optimum strap locations from the locations associated with equal
spacings along the shear span. A reasonable agreement between the SFT and MCFT predictions
for the optimum shear strengths and strap locations was observed. A parametric study
demonstrated that the concrete strength, internal shear link locations, beam depth, and shear
span to depth ratio of the beam do not significantly influence the optimum strengthening
configurations for the CFRP strap system.