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

Or Similar Approved Material Specifications – CFRP Strengthening Plates

Mechanical properties of FRP materials used for composite strengthening are likely to vary between products manufactured in different facilities.  To technically assess similar products to confirm approval for use a specifier must obtainactual properties from the plate manufacturer.

Concrete Society Report TR55 defines that ‘Characteristic rather than mean values should be used for design purposes’ when it comes to material properties.  The document defines characteristic property as the mean value less 2xStandard deviation, based on a minimum sample size of 8.

Due to production methods, the mean values can vary from batch to batch,even from the same manufacturer, so the characteristic property is a dynamic figure, depending on the variability of the fibre properties used in production and the volume of continuous fibres in a cross section capable of contibuting to the property of that sample. A knowledgable supplier is unlikely to place a characteristic property on a datasheet, due to the dynamic nature of the calculation of that property, and instead is likely to state a minimum.

With the above in mind the specifier, when assessing equivalence of technically similar looking materials, should check with the manufacturers for the latest figures for a product, to obtain up to date characteristic properties. The method of testing, number of and dates of sample tests should be provided to confirm these calculations.

The normally available characteristic properties from manufacturers are tensile strength, modulus of elasticity and elongation at break.

Martin Richardson – Structural Strengthening Materials Advice

Martin’s experience extents to the following specialist areas of strengthening using composite materials;

Flexural Strengthening using CFRP Plates

Near Surface Mounted Reinforcement (NSM)

Strengthening Cast Iron Using Ultra High Modulus (UHM) Carbon Fibre Plates

Shear Strengthening Using CFRP L Shaped Links

Structural Adhesives

Materials advice can be provided to both designers and contractors who have a structure that needs strengthening.

Structural Strengthening Materials Advice
Structural Strengthening Materials Advice

Pipe Crossing PC44 – Structural Strengthening of Cast Iron with UHM Carbon Fibre Plates

For some structures, extended maintenance is a viable option and prolonging the service life of PC44 was commissioned by LUL. This project was delivered on time and budget by LUL and their contractor Clancy Docwra/Concrete Repair JV, culminating in the strengthening of the cast iron bridges using innovative Sika Carbodur UHM carbon fibre plates.

The end product is a structure with an extended design life that reduces expensive and dangerous cyclical inspections and represents excellent value for money for the client and taxpayer. Not only did the chosen delivery method cause no disruption to the underground system, but there was only minimal disruption to the road users.

However, getting to workable solution for this structure with its complex access and environmental conditions was far from simple. PC44 is one of the most technically demanding structural strengthening projects ever carried out using UHM Carbon Fibre Plates applied to a cast iron structure.

Pipe Crossing PC44 is an underline bridge supporting the District & Circle line tracks between Blackfriars and Temple Stations and over the fleet sewer. The asset was constructed in 1886.

The cast iron troughs that support the tracks and form the roof slab gave cause for concern in that they could fail without warning. Carbon fibre plates were bonded to the soffit of these troughs to provide an additional reserve of strength that will remove this concern.

Understanding the challenges presented by the sewer environment below this asset lead LUL to work closely with leading specialist in the field of structural strengthening and to commission significant independent testing and research, to ensure that the highest level of performance was achieved by using the right structural adhesive for the project in the very challenging conditions. Trials were also carried out to ensure the installation method was achievable. Well done to all involved in PC44 and achieving a commendation in the ICE London Civil Engineering Awards 2014.

Innovations in Structural Strengthening – Cast Iron and Steel Structures using UHM Carbon Fibre Plates

Interest in strengthening metallic structures was first led by the off shore industry. The civil engineering industry soon realised the opportunity for its use and major bridge owners such as London Underground Limited and Network Rail expressed interest.

In 1996 a DETR/PiT program was set up to develop the use of composite materials in strengthening metallic structures. Detailed areas for investigation included design, specification, materials performance and installation best practise. The main output from this program was the Institution of Civil Engineers design and practise guide ‘ FRP Composites Life Extension and Strengthening of Metallic Structures’, which was published in 2001.

Both cast iron and steel structures have been strengthened with bonded composite plates. Wrought iron is also a material being considered for strengthening.

Another major part of the DETR/PiT program was to carry out full-scale validation of the technique. A London Underground Ltd bridge called D65A was selected for this trial. The bridge consisted of 3 main longitudinal beams with transverse beams spanning between them. Two of the transverse beams were selected for the trial. A target of a reduction in peak strains in the beams of 25% was chosen and designed for. A known load was pasted over the bridge both before and after strengthening and a reduction of peak strains of 23% was recorded. The discrepancy between the target and the actual reduction in strain was investigated and found to be due to the stiffened beams attracting additional load from other parts of the structure.

The plates for strengthening metallic structures are generally considerably larger than those used for strengthening reinforced concrete. The requirement for increased stiffness of the strengthened element means larger cross section areas are required and the fibres used within the plates, although still carbon, have a higher stiffness than those used for strengthening reinforced concrete. These requirements mean that plates are generally manufactured to the exact dimensions and properties required for the project, making them bespoke plates.

To provide plates that have a high level of control of fibre alignment, and hence final plate properties, a pre-preg method of manufacturing is adopted. This involves unidirectional fibres being pre-impregnated with an epoxy resin to form a sheet. These sheets are then laid up individually in the required direction and a vacuum used to consolidate the layers and obtain the maximum fibre fraction content. The whole component is then cured in an autoclave. This method of production also allows tapers to be built into the plate, reducing the thickness at the end of the plates to approximately 1mm and in turn reducing forces on the bond line.

The finished plates are then delivered to site flat in a similar way to steel plates. However, they are considerably lighter than the equivalent steel plate. The substrate needs to be prepared to remove all contamination and corrosion products and provide the maximum levels of adhesion of the adhesive. Although the plates are lighter than steel plates they are too heavy for the adhesive to hold in place whilst still curing. Temporary supports are required to maintain pressure between the plate and the substrate while curing takes place. Once cured a final protective coating is applied over the plate and on to the metallic substrate to prevent corrosion of the substrate.

In 2002 the LUL bridge MR46A was strengthened using the Sika CarboDur DML System. The bridge is a single 28m span bridge consisting of mild steel main and transverse beams. Strengthening was required for the bridge to carry the full LUL load requirements. Various Sika CarboDur UHM plates up to 13m in length, 30mm thick and 250mm wide were installed.

In more recent times production of plates using aramid fibres has been carried out. The aramid fibre is less conductive than carbon fibres and this is believed to be beneficial in reducing induction forces when strengthening bridges over high voltage cable, a common situation on electrified railways.