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.