Design guidance for strengthening concrete structures using fibre composite materials

Design guidance for strengthening concrete structures using fibre composite materials

Clarke, John

An overview ibre composites (or fibrereinforced polymers, generally known as FRPs) have been used successfully for many years in the aerospace and automotive industries. They are used in construction, for example as structural elements and for cladding. The new Technical Report 55 does not consider such applications but deals only with a recent development, strengthening concrete structures by bonding fibre composites to the surface.

Suitable fibres are made from carbon, aramids* or glass. These may be used in the form of:

composite plates, made from fibres and epoxy resins which are fixed with epoxies to the soffits of beams and slabs

sheet materials, which are wrapped round columns and similar members

preformed shells, bonded round columns.

Advantages

The principal advantages of using composites over steel plates are their high strength and light weight; typical properties are given in the Report for commercially available materials. This makes installation simple and quick and eliminates the need for temporary support. The materials can be easily cut to length on site. The availability of long lengths and the flexibility of the materials also simplify installation because:

Laps and joints are not required. The material can take up irregularities in the shape of the concrete surface and can follow a curved profile.

The material can be readily installed behind existing services.

Overlapping, required when strengthening in two directions, is not a problem because the material is thin.

These various factors in combination lead to a significantly simpler and quicker strengthening process than when using other methods. This is particularly important for bridges because of the high costs of lane closures and possession times on major highways and railway lines.

An additional advantage of FRPs over some other types of strengthening is that the weight of the structure and the dimensions of the member are not significantly increased. The latter may be particularly important for bridges, tunnels and other structures with limited clearance.

Disadvantages

One disadvantage of FRP strengthening is the risk of fire, vandalism or accidental damage. For bridges over roads the risk of soffit reinforcement being hit by over-height vehicles should be considered. In general, some form of protection will be required.

Examples of FRP strengthening There are many concrete structures around the world which have been externally strengthened with FRP. The Report concentrates on applications in the UK. The floors of various buildings have been strengthened to carry additional loads and FRP has been used in structural alterations. Columns have been strengthened in several multi-storey car parks by wrapping with carbon fibre sheet.

Several major highway bridges and a large number of small bridges have been strengthened using FRPs to increase their load capacity. Most applications have been on soffits but some bridges have had FRP bonded to the upper surface or around the columns. Other strengthening applications in the UK include lighthouses and cooling towers; elsewhere in the world almost every type of concrete structure, from chimneys to tunnels, has been strengthened.

Design approach

Fibre composites have a straight-line stress-strain response to ultimate with no yielding. Thus elastic methods of analysis with no redistribution are appropriate. For members in bending, the traditional design assumptions are still valid. However, further checks are required to avoid peeling failure at the ends of the laminate and debonding from the concrete. If failure occurs, it will be in the outer layer of the concrete; the proposed, conservative, approach is to limit the longitudinal shear stress in the concrete at ultimate to 0.8N/mm^sup 2^. To minimise the risk of debonding, the strain in the FRP should not exceed 0.8% when the applied load is uniformly distributed, and 0.6% if combined high shear forces and bending moments are present. A minimum anchorage length of 500mm is recommended.

FRP strips may be used to strengthen members in shear. The material may be treated as an external stirrup, again using traditional design assumptions but the strain in the FRP should be limited to 0.4%.

Wrapping circular columns with FRP increases the axial load capacity as well as the bending and shear capacities. (Only limited increases are possible with square and rectangular columns.) Approaches are given which relate the enhanced ultimate stress and strain in the concrete to the degree of confinement.

Workmanship and installation The installation of FRP materials must be carried out correctly, to ensure good long-term performance. Detailed guidance is given, including the selection of the appropriate material and adhesive, adequate preparation of the concrete surface, application of the composite and correct curing of the adhesive. It is important that the work is carried out by a suitably qualified contractor with trained staff.

Inspection and maintenance

As strengthening with FRP is a relatively new technique, regular inspection and maintenance regimes should be set up. This is particularly important for buildings, which, unlike bridges, are not generally subject to any form of routine inspection. Where practical, additional material should be installed, which can be removed at a later stage for testing. Information on the materials used, along with information on the actions to be taken in the event of damage to the FRP, should be included in the Health and Safety File.

Collaborative project

The Concrete Society has recently completed a collaborative project to develop appropriate design guidance on-the use of FRP for strengthening, based on British design codes. The project was supported financially by three major owners of structures (Highways Agency, Railtrack and London Underground) and the leading suppliers of composite materials in the UK (DML Composites, Dupont de Nemours International, Exchem Mining & Construction, MBT Feb, SBD Weber & Broutin, Sika Ltd, Sumitomo Corporation Europe Ltd and Toray Europe Ltd). In addition, a number of specialist consultants, contractors and research organisations participated in the Steering Group and provided valuable information.

Launch seminar

The project has resulted in the publication of the new Technical Report 55, Design guidance for strengthening concrete structures using fibre composite materials, which was launched during a highly successful half-day seminar at the Royal Institution of Chartered Surveyors in London on 5 December 2000, attended by about 80 delegates. The seminar was introduced by Neil Loudon, senior technical advisor at the Highways Agency and chair of the Steering Group. He spoke of the importance of wide dissemination of the available information on composites, particularly to those professionals who have little or no knowledge of the subject.

John Clarke of The Concrete Society presented the background to the subject, starting with an outline of the main reasons for strengthening, such as the current requirement to take heavier lorries on roads in the UK. There was a brief overview of the relevant fibre types, their properties and the forms in which they are available, together with their advantages and disadvantages for strengthening. Chanakya Arya, University College London, who was responsible for the design chapters in TR 55, emphasised that the approach was in line with current British design codes. However, while the basic methods were unchanged when using fibre composite strengthening, additional checks were required to avoid local bond failure between the laminate and the concrete. He also introduced the more detailed approach to partial safety factors, including an allowance for the construction method of construction as well as the fibre type.

Site operations were dealt with by John Drewett of Concrete Repairs Ltd. In his presentation he covered the various stages in preparing concrete surfaces and applying materials, showing the type of environmental protection required for correct curing of the adhesives. Throughout his presentation, he emphasised the importance of good workmanship and the need for operative training. John Keble of SBD Weber & Broutin gave examples of bridge strengthening, focussing on Bible Christian Bridge, Cornwall and Barnes Bridge, Manchester. The latter included the use of steel plates and through-thickness shear strengthening in addition to externally bonded fibre composites. Some examples of strengthening buildings were presented by Martin Richardson of Sika Ltd. He started with Kings College Hospital, the first significant application of fibre composites in the UK. Other applications were used to illustrate some of the benefits of fibre composites over other strengthening techniques, such as the ability to install the composites behind existing services and minimised disruption to users of the building. The examples included the installation of new escalators and lifts through existing concrete slabs in two Allders department stores.

The final presentation was by Neil Loudon who gave an owner’s viewpoint. He reiterated his support for the project and outlined the Highway Agency’s involvement with various strengthening projects, some of which had been described in more detail by earlier speakers. He concluded with a slide of a proposed foot and cycle bridge, the deck of which is built entirely from composites. Currently under construction, it will be installed over a busy road in Cornwall in the Spring.

This was a successful launch for TR 55 which, judging by the reaction of the audience, will be welcomed by the industry. However, the technique is developing rapidly and The Concrete Society is now considering a second phase, which will look at the important aspects of inspection and maintenance of fibre composite repairs.

Copies of TR 55 Design guidance for strengthening concrete structures using fibre composite materials may be obtained from the Society, priceL (English pound) 80 (members’ price L (English pound) 40) plus post and packing. Send publication orders to: Natalie Ball, Publications Department, The Concrete Society, Century House, Telford Avenue, Crowthorne, Berkshire, RG45 6YS, UK. Tel: +44 (0)1344 466 007; Fax: +44 (0)1344 466 008; e-mail: ball@concrete.org.uk. Pay by Visa or Mastercard or by cheque payable to The Concrete Society.

* Aramids are manufactured fibres in which the fibre-forming substances consists of a long-chain synthetic aromatic polyamide.

John Clarke, The Concrete Society, introduces the newly published

Concrete Society Technical Report 55

Copyright The Concrete Society Jan 2001

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