[Univ of Cambridge] [Dept of Engineering]

Concrete beams with external prestressed 

carbon FRP shear reinforcement

Gyamera Kesse


Abstract:
 
An increasing number of reinforced concrete (RC) structures are being assessed as having inadequate shear capacity.  Reasons for these strength deficiencies include an ageing infrastructure, deterioration due to corrosive environments and greater loads being applied to existing structures.  There could be significant economic consequences if these structures cannot be strengthened as the only other options are weight restrictions or complete replacement.

One possible solution is to use external carbon fibre reinforced polymer (CFRP) shear straps to strengthen existing RC beams.  The CFRP material is advantageous since it is non-corrosive and light weight.  However, the material is brittle and does not yield.  The unbonded CFRP straps are formed by wrapping layers of carbon tape around a beam.  The straps are installed at discrete locations within the shear span and prestressed.  In the design of a CFRP strap strengthening system, the number of tape layers/stiffness, the strap spacing and the level of prestress must be specified. The level of enhancement must be estimated numerically and the mode of failure predicted.

This thesis describes an investigation into the use of prestressed CFRP straps to strengthen beams weak in shear.  In the first instance, a series of beam experiments were performed where the strap stiffness, spacing and initial prestress were varied.  The experiments gave insight into the beam behaviour in terms of the peak load and the modes of failure.  The influence of the straps on diagonal shear cracks was also clearly identified.

The modelling of cracks in finite element (FE) formulations was studied since the experimental results suggested a strong interaction between the straps and the cracks.  Two FE packages were used;  DIANA a commercial package, and CamFEA developed as part of the current work.  This study provided insight into the use of complicated and simplified crack models and the possible effects of these models on the FE results in a full-scale beam analysis.  A detailed FE analysis was then performed on the experimental beams.  The FE results confirmed the observed behaviour and the peak loads and modes of failure were predicted fairly accurately.  The earlier crack modelling predictions proved helpful in explaining the FE results.

The experimental results and FE analysis confirmed that the modes of failure were primarily governed by the strap spacing, the stiffness and the initial prestress level of the straps.  A high initial prestress level is undesirable as the reserve strain capacity of the strap becomes limited and strap failure becomes imminent.  However,  a minimum prestress level is also required in order for the strap to be effective.  The strap spacing interacts with the strap stiffness and initial prestress level to influence the mode of failure.  If the spacing is inadequate, then a shear failure is bound to occur. While if the strap spacing is sufficient, then the stiffness and prestress will affect the behaviour and subsequent mode of failure.

Two analytical models suitable for hand calculations were developed to predict the level of resistance.  Unlike many design methods which assume ductile behaviour, these models rely on a compatibility condition which ensures that the actual strain in the brittle elastic strap can be predicted and the possibility of a strap failure captured.  The two models differed in that one included aggregate interblock whilst the second model ignored this component.  In general, the peak load predictions were fairly good and the models could be used as a tool to aid practising engineers in estimating the required number of layers (stiffness) and prestress levels required once a strap spacing has been specified.

The project has shown that prestressed CFRP straps can be used to enhance the shear strength of RC beams and change the brittle shear mode of failure to a ductile flexural mode. 


[Cambridge University | CUED | Structures Group | Geotechnical Group]

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