Mechanical properties of FRP materials used for composite strengthening are likely to vary between products manufactured in different facilities. To technically assess similar products to confirm approval for use a specifier must obtainactual properties from the plate manufacturer.
Concrete Society Report TR55 defines that ‘Characteristic rather than mean values should be used for design purposes’ when it comes to material properties. The document defines characteristic property as the mean value less 2xStandard deviation, based on a minimum sample size of 8.
Due to production methods, the mean values can vary from batch to batch,even from the same manufacturer, so the characteristic property is a dynamic figure, depending on the variability of the fibre properties used in production and the volume of continuous fibres in a cross section capable of contibuting to the property of that sample. A knowledgable supplier is unlikely to place a characteristic property on a datasheet, due to the dynamic nature of the calculation of that property, and instead is likely to state a minimum.
With the above in mind the specifier, when assessing equivalence of technically similar looking materials, should check with the manufacturers for the latest figures for a product, to obtain up to date characteristic properties. The method of testing, number of and dates of sample tests should be provided to confirm these calculations.
The normally available characteristic properties from manufacturers are tensile strength, modulus of elasticity and elongation at break.
The project to strengthen Reading Bridge has been secured by Volker Laser, it involves the use of foam concrete to infill some of the approach spans either side of the river, composite CFRP plate bonding to the under side, Near Surface Mounted Reinforcement to the top surface, concrete repair and bridge deck waterproofing.
Strengthening reinforced concrete structures for shear has traditionally been difficult. Early schemes consisted of bonding bolting steel plates to the outside of beams. The development of the use of composite materials for shear strengthening has been researched since 1997. The use of fabric wrapping systems has provided a solution to strengthening columns where it can be applied to all surfaces of the column. The development of a system of preformed L shaped composite links for use on down stand beams has been carried out by the Swiss Federal Laboratories for Material Testing and Research – EMPA. This system is the Sika CarboShear L Link. It consists of a 40mm wide, 1.2mm thick L shaped plate made of carbon fibre.
Providing sufficient composite material to resist the shear force is relatively simple. The technical difficulty is associated with obtaining the required anchorage to the ends of the composite. Initial research was required to quantify the exact capacity of different types of anchorage. At the bottom of the beam the anchorage is provided by the bend of the link under the beam. At the top of the beam the anchorage is achieved by bonding the link into the bottom of the slab. The capacity of the anchorage bonded into the slab can reach the tensile capacity of the Sika CarboShear L link at a bond depth of 120mm. The bend anchorage can only achieve approximately 60% of the capacity of the link, and hence is the limiting factor in any strengthening scheme. The conclusions of these initial tests provided information to allow the design concept to be developed.
Full scale validation testing was then carried out on T section beams at EMPA to confirm the design concept.
The beams used for the validation testing were specifically designed for high shear stresses. Four beams were statically loaded with different shear reinforcement, consisting of combinations of both internal steel links and external bonded Sika CarboShear L Links. Additional beams were used to investigate the effect of preload on subsequent strengthening and fatigue.
The testing concluded that externally bonded shear links could,
Increase shear load capacity of reinforced concrete (ULS), and can also be used to reduce shear defections (SLS).
Preload before strengthening has no effect on the performance of the strengthened beam at ULS.
The composite materials used for shear strengthening can readily bear the applied fatigue stresses.
The first trial project using Sika CarboShear L links in the UK was carried in on the A38 Liskard Bypass in Cornwall. The bridge, which is owned by the Highways Agency, consisted of a heavily skewed reinforced concrete slab. The slab had been propped since an assessment has showed that the deck edge zones had inadequate shear strength to resist the combination of torsional and flexural shear. The links were bonded to the edge of the slab and were anchored into the soffit of the parapet stringcourse at the top and lapped under the slab at the bottom. On completion of the strengthening the propping could be removed.