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.
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.
The development and use of alternative materials has been a constant process almost since the first use of steel. The installation problems associated with the weight of the steel plates and the potential for corrosion to reduce the durability of the system led to composite materials being considered. In the early 90’s much of the research was carried out at EMPA in Switzerland. In the UK a Dti Link project called ROBUST was established to investigate the use of composite materials for strengthening structures. In 2000 the Concrete Society launched Technical Report 55 ‘Design Guidance for strengthening concrete structures using fibre composite materials’.
The first UK strengthening scheme using composite materials was completed in 1996 at Kings Collage Hospital in London. The addition of a new floor to a building changed the loading requirements of the existing roof to new floor loadings. 1.3Km of Sika CarboDur plates were installed to the soffit of the longitudinal ribs under the slab.
Preparation of the concrete surface for either steel or composite plate bonding is identical. The composite plate is delivered to site in a roll with a diameter of approximately 1.5m. The lightweight nature of the composite material means a roll containing 250m can be easily lifted and moved by a single operative. The roll is cut on site to give the required plate lengths.
The plates are applied to the concrete surface using a similar adhesive to the one used for steel plate bonding. The initial grab of the adhesive is enough to hold the lightweight plate in place during the full cure period of the adhesive, eliminating the requirement for temporary works.
The composite plates are 1.2-1.4mm thick. This means that any residual longitudinal forces in the end of the plate have a much smaller eccentricity to the concrete surface compared to steel plates. In turn this means that peeling forces are lower which generally removes the requirement for anti peel bolts.
As composite materials do not corrode, corrosion protection systems are not required. A decorative coating can be applied to help conceal the strengthening.
I graduated from Surrey University with Civil Engineering Degree which included a sandwich year working for Specialist Piling Contractor, Cementation. My first post graduate role was with Gifford and Partners working mainly within bridge design and assessment, although I did also design the hull of a Hovercraft at one stage! I spend my last year with Giffords seconded to Hampshire CC working as a resident engineer for the bridges on the Bentley Bypass. As the project came to an end I decided to look for a more commercially focused role and started working for the global specialist construction materials manufacturer Sika. Having started at the grass routes level within the company I progressed, via several development roles, to running the UK ‘Contractors’ Business Unit with a gross turnover of approx £16m. In Nov 2013 I left Sika and after being self employed for a few months I established Structural Refurbishment Solutions Ltd (SRS Ltd) as a consultancy specialising in Technical Sales and Marketing Strategy Support, with a focus on structural refurbishment such as concrete repair, structural strengthening, flooring and waterproofing.