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

CFRP Stengthening Plates Technical Data Comparision – Tensile Strength

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.

CFRP Plates - Tensile Strength 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

Martin Richardson – Structural Strengthening Materials Advice

Martin’s experience extents to the following specialist areas;

Flexural Strengthening using CFRP Plates

Near Surface Mounted Reinforcement

Strengthening Cast Iron Using UHM Carbon Fibre Plates

Shear Strengthening Using CFRP L Shaped Links

Structural Adhesives

Martin Richardson SRS Ltd
Martin Richardson SRS Ltd

Innovations in Structural Strengthening – Shear Strengthening with CFRP Shear Links

Strengthening reinforced concrete structures for shear has traditionally been difficult. Early schemes consisted of bonding bolting steel plates to the outside of beams. The development of the use of composite materials for shear strengthening has been researched since 1997. The use of fabric wrapping systems has provided a solution to strengthening columns where it can be applied to all surfaces of the column. The development of a system of preformed L shaped composite links for use on down stand beams has been carried out by the Swiss Federal Laboratories for Material Testing and Research – EMPA. This system is the Sika CarboShear L Link. It consists of a 40mm wide, 1.2mm thick L shaped plate made of carbon fibre.

Providing sufficient composite material to resist the shear force is relatively simple. The technical difficulty is associated with obtaining the required anchorage to the ends of the composite. Initial research was required to quantify the exact capacity of different types of anchorage. At the bottom of the beam the anchorage is provided by the bend of the link under the beam. At the top of the beam the anchorage is achieved by bonding the link into the bottom of the slab. The capacity of the anchorage bonded into the slab can reach the tensile capacity of the Sika CarboShear L link at a bond depth of 120mm. The bend anchorage can only achieve approximately 60% of the capacity of the link, and hence is the limiting factor in any strengthening scheme. The conclusions of these initial tests provided information to allow the design concept to be developed.

Full scale validation testing was then carried out on T section beams at EMPA to confirm the design concept.

The beams used for the validation testing were specifically designed for high shear stresses. Four beams were statically loaded with different shear reinforcement, consisting of combinations of both internal steel links and external bonded Sika CarboShear L Links. Additional beams were used to investigate the effect of preload on subsequent strengthening and fatigue.

The testing concluded that externally bonded shear links could,

Increase shear load capacity of reinforced concrete (ULS), and can also be used to reduce shear defections (SLS).

Preload before strengthening has no effect on the performance of the strengthened beam at ULS.

The composite materials used for shear strengthening can readily bear the applied fatigue stresses.

The first trial project using Sika CarboShear L links in the UK was carried in on the A38 Liskard Bypass in Cornwall. The bridge, which is owned by the Highways Agency, consisted of a heavily skewed reinforced concrete slab. The slab had been propped since an assessment has showed that the deck edge zones had inadequate shear strength to resist the combination of torsional and flexural shear. The links were bonded to the edge of the slab and were anchored into the soffit of the parapet stringcourse at the top and lapped under the slab at the bottom. On completion of the strengthening the propping could be removed.

Innovations in Structural Strengthening – Blast, Impact and Seismic Strengthening.

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.

Introduction to structural strengthening in the UK

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,
  • Shear strength,
  • impact resistance,
  • 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.