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.
Steel plate bonding has been used in both buildings and civil structures in the UK since 1975 using first generation epoxy adheives. In 1994 the Highways Agency published BA 30/94 ‘Strengthening of Concrete Highway Structures Using Externally Bonded Plates’. This provided information on application, design and specification of the technique. The application of steel plates is still the best solution to some strengthening problems that occur today.
Steel plate bonding provided the basis for the establishment of strengthening using externally bonded reinforcement. The process involves the bonding of a mild steel plate with a minimum thickness of 4mm (for handling purposes) to a prepared concrete surface.
The steel plates are fabricated off site to the required dimensions and specification, including holes for anti-peel bolts.
To prevent any corrosion of the steel plate a primer system needs to be applied to the prepared steel surface during fabrication. This primer also provides the critical function of transferring forces from the structure to the steel plate and is hence a crucial part of the system.
Holes for anti-peel bolts also need to be inserted in the steel plate during fabrication. These bolts are required to provide additional resistance to peel forces applied to the bond line due to any residual force in the end of the plate. The bolts have to be positioned carefully to avoid damage to the existing reinforcement in the concrete surface.
Temporary works are required to support the heavy steel plates while the 2-part epoxy adhesive is curing. The curing period is dependant on ambient conditions but is likely to be a minimum of 3 days.
A fillet of adhesive is generally placed around the edge of the plate, this provides additional protection to the bond line but also allows the application of the final corrosion protection system to the steel plates to be lapped out onto the concrete surface. The corrosion protection system is likely to provide a life to first maintenance of 8 years and to major maintenance of 16 years in an exposed environment. However, the first project carried out in 1975 has only recently come to the end of its service life over 35 years after its first installation. Whilst the limited exposure conditons that these plates were exposed to may have extended the life span, current understanding of the performance of corrosion primers and adhesives could have possibily extended the life span.
Interestingly the steel plates have been replaced with a Carbon Fibre (CFRP) plate bonding solution.
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.
The technique of flexural strengthening of reinforced concrete structures using externally bonded composite reinforcement has been in use since the mid 1990’s. The method of strengthening by bonding steel plate, which had been used in the UK since 1975, has now generally been replaced by the use of composite plates.
The increased acceptance of the method has led to different types of structure and problems being considered for strengthening using composite materials. The flexural capacity of metallic structures such as cast iron and steel can be improved. The resistance of reinforced concrete or masonry against impact, blast and seismic loads can be increased. Reinforced concrete structures deficient in shear can be strengthened. The use of pre-stressed composite plates can relieved both dead and live loads at the same time as overcoming difficult detailing problems.
The success of any new techniques depends on detailed research and development and the long term durability of all the materials used in the strengthening system. The other main system components that should be included in this development are the adhesives used to bond the composite to the structure.