Strengthening Systems for Cast Iron Structures

The use of adhesively bonded UHM carbon fibre plates to strengthen cast iron structure has been established for many years.  Essentially it appears a very similar installation to bonding plates to concrete structures.  However, the loads carried by the strengthening mean the bond line interfaces work much closer to the limits of the adhesive properties when strengthening cast iron.

There are examples where the selection of installers with experience in application of strengthening systems to concrete have had problems installing plates on cast iron structures.  These have usually been caused by operatives following widely acceptable procedures on concrete and using them on cast iron, and adhesion issues have been exposed due to the higher performance required from the bond lines.

Adhesives for use in bonding UHM CFRP plates to cast iron should be considered carefully.  Selection of adhesives that may be suitable for application to concrete, can also lead to problems when used on cast iron.

Designers and clients need to take responsibility to select and insist on the correct materials and installer being used to avoid issues.

Clients who have gone through the learning curve on these issues, will make sure that designers and installers all have a proven track record for installing systems.  This track record needs to present at both management and operative level, to ensure correct installation procedures are followed at all times.

A classification system for the acceptable performance of adhesives for use in bonding UHM CFRP plates to cast iron was established by Oxford Brookes University as part of a DTI funded project called CompClass.  All the established and experienced designers, material suppliers and contractors had an input into this project.

The minimum any adhesive system should have is the following classification certificate showing that it is suitable for the use of bonding CFRP onto cast iron. If there is a significant interval between preparation and application, then this system needs to include a primer suitable for cast iron. In this situation the performance of the primer, not only as a corrosion protection system, but also its ability to transfer loads, then becomes the critical. An example of material classification certificate can be seen in the link below.

Material Classification Certificate

Failure to use experienced installers, designers and adhesive systems with the correct performance can potentially premature system failure.

CFRP Plates for Strengthening Concrete Structures Technical Data – Tensile Modulus

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.


CFRP Plates - Flexural Modulus Comparision
CFRP Plates – Flexural Modulus Comparision

Development of Structural Strengthening – Pultruded Carbon Fibre Plates (CFRP)

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.

Development of Structural Strengthening – Steel Plate Bonding

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.

Kings College Strengthening of Building

The first use of carbon fibre plates to strengthen a concrete building at Kings College

Carbodur Carbon Fibre Plate Bonding
Structural Strengthening to Building

Concrete Society Technical Report TR55 Design Guidance for strengthening concrete structures using fibre composite materials 2nd edition

TR55 Concrete Society

Concrete Society Technical Report TR55 Design Guidance for Strengthening Concrete Structures using fibre composite materials Third Edition

Concrete Society TR55

M3 Strengthening Hawley Bridge

Concrete Bridge on M3 Strengthened using CFRP Plates

CFRP Strengthening
Structural Strengthening to Bridge

Martin Richardson

Martin Richardson, founder of Structural Refurbishment Solutions Ltd

Martin Richardson
Martin Richardson