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.
The technique of bonding external reinforcement to structures was first used in the UK in 1975 on the M5 near Birmingham to strengthen the Quinton Interchange.
The method of strengthening using externally bonded reinforcement, structural strengthening, can be introduced by providing the answers to some simple questions.
What is Structural Strengthening?
Why do we need in?
What can be achieved by using the technique?
Where can we use it?
Structural strengthening involves the bonding of additional reinforcement to the external faces of a structural member. This additional reinforcement can incorporate steel plates, composite plates or composite wrapping systems. The method is attractive because it provides a cost effective solution to increasing load carrying capacity, especially when compared to demolition and rebuilding.
One of the main reasons for the use of the method in the UK is due to the change of use of a structure giving an increased load-carrying requirement. Other reasons such as, inadequate design, poor quality construction, structural damage, fire damage, seismic loading, reinforcement corrosion (If the cause is treated) and loss of prestress force are not uncommon.
Strengthening can improve the load carrying capacity of structures by;
Increasing flexural strength,
punching shear resistance,
redistribute loads around new openings.
Externally bonded reinforcement gives the opportunity to strengthen without having a significant visual impact on the structure. The installation process is fast and can minimise disruption to the function of the structure including the services attached to it.
Structures made from reinforced concrete, steel, cast iron, masonry and timber have all been strengthened to date using a form of the technique. Beams and slabs have been strengthened on both the top and bottom surface for flexural strength. Columns and beams have been strengthened on there side faces for shear. Slabs have been strengthened around columns to increase punching shear resistance. Various other types of structural elements have been strengthened for many different reasons.
Why is it so difficult to get find all the Product Data Sheets for CFRP Plates and Structural Adhesive materials which are available in the UK Market for Structural Strengthening Systems? Many of the major suppliers dont have them readily available on there websites.
Below is a selection of the key products which are generally available in the UK, from Sika, Weber and Mapei. You will probably notice that not all of them are from a UK website, so I hope they are correct. These products are generally for use on concrete structures for flexural strengthening