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

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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.

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

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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

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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.

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

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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

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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.

Car Park Refurbishment

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The careful selection of car park decking systems means the right blend of waterproofing, traffic resistance and practical application constrants can be considered to give a long term durable, aesthetic solution which minimises disruption to the users of the car park and maximises revenues for the client, both during the refurbishment and in the long term, by improving the environment for the user.

The decking can be used as part of a long term strategy to protect the structure from degredation along with other possible techniques such as concrete repair, decorative protective concrete soffit coatings, steel corrosion protection systems, corrosion inhibitors, sacraficial anodes and cathodic protection, all in accordance with the Life Care Plan approach suggested in the ICE Document ‘Recommendations for the inspection, maintenance and management of car park structures’ and the management strategies outlined in BS EN1504.

Car Park Waterproofing
Car Park Waterproofing
Car Park Decking
Car Park Decking

Introduction to structural strengthening in the UK

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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.

The Development of Composite Materials for Strengthening Structures – Introduction

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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.

What are the practical benefits of strengthening using carbon fibre plates?

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Thanks to the light weight and low profile materials, carbon fibre plates have the practical benefits of;

  • Reducing the visual impact of strengthening.
  • Acceleration of project times.
  • Resolving difficult access issues.
  • Minimising the disruption to existing services.
  • resolving difficult detailing problems.

What can be achieved by strengthening structures using CFRP plate bonding?

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Structural strengthening using carbon fibre plates bonded to reinforced concrete can;

  • Increased flexural strength
  • Redistribute loads around openings
  • Improve shear/punching shear resistance
  • Increase impact resistance
  • Increase load carrying capacity of structure