A review of the current available technical data sheets for Carboplate from Mapei, CarboDur from Sika and weber.tec force from weber, show that there are a variety of different ways that technical information are presented and in general terms highlights how care needs to be taken when checking equivalent performance of CFRP Plate Bonding Products for use in flexural strengthening reinforced concrete structures.
A review of the current available technical data sheets for Carboplate from Mapei, CarboDur from Sika and weber.tec force from weber, show that there are a variety of different ways that technical information are presented and in general terms highlights how care needs to be taken when checking equivalent performance of CFRP Plate Bonding Products.
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.
Where blast impact and seismic loads are been considered a range of materials may be suitable. The increase ductility and toughness of fibres such as aramid and glass fibres has seen them used along with carbon fibre for these types of projects. Some strengthening projects utilise the ease of installation of a plate as discussed previously, but more often than not the use of fabric, bonded in-situ, provides the required solution.
Blast and seismic strengthening are generally a small part of the market place with limited design and practical guidance. However, post September 11, interest in protecting strategic buildings against terrorist attack has been increasing. In many situations the strengthening not only provides an improved resistance to collapse, but also helps to reduce the fragmentation, which can be so dangerous for the users of a building. Seismic strengthening in the UK is at present limited to the nuclear industry. In areas of the world where seismic activity occurs, strengthening against seismic loads becomes the major market for composite materials within the construction industry.
Strengthening columns for vehicle impact loading using composite materials has been common practise within the UK for several years. The installation can be carried out using either the wet or dry application method. The wet method involves saturating the fabric with resin prior to applying it to the concrete surface. The dry method involves applying adhesive to the concrete surface and placing the fabric into the adhesive. Multiple layers of fabric can be applied using either method of application. Due to the unidirectional nature of the majority of fabrics layers can be applied in different orientations to provide strength in different directions when required. Multi direction materials are available, but are often uneconomic.
In late 2002 the Highways Agency published Bridge Directive, BD84/02 Strengthening of Concrete Bridge Supports Using Fibre Reinforced Polymers. This provides design and specification advice on the use of fabric materials in strengthening of columns against impact.
The SikaWrap range of composite fabrics and the Sikadur range of epoxy adhesives have been developed for both dry and wet application methods. The SikaWrap range of fabrics includes carbon, aramid and glass fibre materials.
The composite strengthening is often used in conjunction with additional reinforced concrete elements to improve fixation of columns at supports.
On a project completed in Bristol on the A38 Patchway Viaduct, SikaWrap 300A aramid wrapping system was installed using the dry method of application in August 2003. As part of the installation trial bands were installed above the area where strengthening was required. Defects were deliberately installed in these bands to trial the use of transient pulse thermography in detecting such defects. The system used by the BRE uses short bursts of powerful light to raise the surface temperature of the strengthening and then uses a sensitive infrared camera to monitor the cooling of the surface. Differences in how the surface cools show up possible defects within the layers of the fabric.
For some structures, extended maintenance is a viable option and prolonging the service life of PC44 was commissioned by LUL. This project was delivered on time and budget by LUL and their contractor Clancy Docwra/Concrete Repair JV, culminating in the strengthening of the cast iron bridges using innovative Sika Carbodur UHM carbon fibre plates.
The end product is a structure with an extended design life that reduces expensive and dangerous cyclical inspections and represents excellent value for money for the client and taxpayer. Not only did the chosen delivery method cause no disruption to the underground system, but there was only minimal disruption to the road users.
However, getting to workable solution for this structure with its complex access and environmental conditions was far from simple. PC44 is one of the most technically demanding structural strengthening projects ever carried out using UHM Carbon Fibre Plates applied to a cast iron structure.
Pipe Crossing PC44 is an underline bridge supporting the District & Circle line tracks between Blackfriars and Temple Stations and over the fleet sewer. The asset was constructed in 1886.
The cast iron troughs that support the tracks and form the roof slab gave cause for concern in that they could fail without warning. Carbon fibre plates were bonded to the soffit of these troughs to provide an additional reserve of strength that will remove this concern.
Understanding the challenges presented by the sewer environment below this asset lead LUL to work closely with leading specialist in the field of structural strengthening and to commission significant independent testing and research, to ensure that the highest level of performance was achieved by using the right structural adhesive for the project in the very challenging conditions. Trials were also carried out to ensure the installation method was achievable. Well done to all involved in PC44 and achieving a commendation in the ICE London Civil Engineering Awards 2014.