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Getting all wrapped up

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There are some exciting new possibilities in strengthening building structures with carbon-fibre composite wraps

In recent years there has been an increasing emphasis on the need to extend the potential service life of existing structures. Rehabilitation of structures using new materials and strengthening techniques can provide an economic alternative to demolition and reconstruction.

Strengthening may be required to return a structure to its original condition or to achieve the performance for which it was designed. For example, structural elements may have been inadequately reinforced in the past due to design or construction flaws, or be no longer serviceable due to an unacceptable level of deterioration from environmental or mechanical damage. Alternatively, strengthening may be required to enhance the capability of a structure for a new purpose, such as the refurbishment of old bridges to carry increased traffic loads.

The use of externally bonded reinforcement is now recognised as an effective and economic method of stiffening and strengthening existing concrete structures. The standard engineered response is the use of steel-plate reinforcement bonded to the concrete members by means of an epoxy-resin adhesive. There have been many applications of the technique to bridges and buildings, both in the UK and overseas.

The first reported application of the plate-bonding technique was in France during the 1960s on a bridge strengthening scheme on the Autoroute du Sud using steel-bonded plates. During the 1970s plate bonding began to gain wider recognition as an effective strengthening technique. For example, by 1975 some 240 bridges in Japan had been strengthened using epoxy-bonded steel plates to cope with increased traffic loading.

The load-carrying capacity of floor slabs has been enhanced using steel-plate bonding for a number of refurbished office buildings in Canada, Germany, Belgium, Poland and the UK. For example, during the refurbishment of a multistorey office building in Leeds, the reinforced concrete floor slabs were found to be cracking near the support columns.

The floor slabs were strengthened by bonding 6m 2horseshoe-shaped steel plates to the top surface of the slab next to the columns. Although steel-plate bonding is generally associated with concrete structures, the technique was adopted to strengthen a 110-year-old cast-iron road bridge over the River Stour, at Bures, Suffolk, and Thomas Telford's cast-iron Mythe Bridge over the River Severn.

Although steel-plate bonding has been proven to be an efficient means of strengthening, there are nevertheless certain disadvantages:

steel plates are heavy and can be difficult to install, requiring support until the resin has cured sufficiently; due to their weight, steel plates are generally restricted in length and often require further lapped plates to be installed; and steel must be protected from the effects of corrosion by priming and painting. The steel-resin interface is especially at risk and requires particular attention.

During the mid 1980s researchers began to investigate the use of fibre-reinforced plastic; composite laminates as external reinforcement to replace mild steel plates. It was not many years ago that the use of carbon fibre was confined to the aerospace and automobile industries. The first use of carbon fibre in construction was in the US and Japan for seismic retrofitting of structures. Now in Europe, the concrete repair and strengthening industry is enthusiastically embracing carbon-fibre technology. However, to date applications have largely been on engineering structures.

Rigid and flexible systems

The advantage of carbon fibre over traditional steel-plate bonding techniques is the very impressive strength/weight ratio; typically these materials have tensile strengths up to 10 times greater than steel and a density of only one-fifth. Pultruded carbon- fibre plate systems are available which are applied in a similar manner to conventional steel plate, that is the stiff plates are bonded to the surface of the concrete with a thixotropic epoxy resin.

The advantage is that this system is easily applied and requires few, if any, lapped plates. However, pre-formed plates are generally too stiff to strengthen elements such as columns to resist shear, where the carbon fibre needs to be wrapped around the columns.

Carbon-fibre wrap systems are available for just this situation. The carbon fibre is supplied on a roll either as a unidirectional mat of fibres or as a woven bi-directional mat. The carbon-fibre mat is applied rather like wallpaper, it can be readily cut with sharp scissors and 'pasted' onto the surface of the element with a paste-like consistency epoxy resin. A roller is then used to ensure that all air pockets are drawn out and the epoxy resin has saturated the individual fibres. As many layers of carbon-fibre mat as are necessary can be built up on top of each other and may be applied in different directions, for example, to strengthen a two-way spanning slab to ensure that best use is made of the tensile properties of the carbon fibre.

Wrap and pultruded plates are available that can be used to strengthen virtually any shaped member - be it a beam, slab, column, floor or wall. Carbon fibre has been successfully employed to strengthen steel, reinforced and prestressed concrete, masonry, cast iron and timber structures. All this can be achieved with only minimal access requirements, minimum disruption to users of the structure.There is usually very little residual evidence that any strengthening has actually taken place.

The use of carbon-fibre strengthening in the UK has principally been applied to civil structures and bridges. However, there is great potential for its use in building refurbishment and strengthening for new use. Carbon-fibre technology has finally made the transition from aerospace to civils. The potential to rescue 'inadequate' structures, or to reduce the specification requirements of new-build elements is definitely worth consideration, by clients and architects alike.

Dr Mark Roberts is a project manager at Maunsell. Tel 0121 643 6711 or e-mail mbr2@maunsell.co.uk

Cast Iron Bridge, Tickford

Constructed in 1810, this is the oldest operational cast-iron bridge in the world and is designated a scheduled ancient monument.

Carbon fibre was adopted because of its low visual impact. The bridge was strengthened using a layer-up carbon-fibre system to enable the bridge to carry traffic with heavier 40-tonne loads under new European legislation.

Oakland House car park, Manchester

Under-strength concrete columns were externally reinforced using a carbon-fibre wrap system. Carbonfibre strengthening was chosen in preference to more overcasting in concrete because it offered benefits of reduced time for the works, minimal change in appearance and an overall cost saving.

ECONOMICAL CORROSION PREVENTION WITH ELECTRIC CONCRETE

In coastal areas or in high chloride environments such as swimming pools, a system of 'cathodic protection' has been devised to slow down the corrosion of concrete structures.

Due to the uniquely corrosive atmosphere in swimming pools and surrounding facilities, structural concrete is prone to developing accelerated concrete reinforcement corrosion, especially in the dry ducts behind the pool walls. The problem is attributed to the high levels of chloride found in these warm and humid environments.

Cathodic protection involves negatively charging the steel reinforcement enabling it to repel negatively charged chloride ions. An anode is applied to the concrete topping together with conductive paint applied to the walls. The resultant electrochemical reaction generates alkaline hydroxyl ions at the steel surface which promote repassivation - that is, corrosion protection.

Concrete Repairs of Mitcham in Surrey successfully applied a discreet anode in dummy tiles at a swimming pool in Tenby, Wales. When the pool is not in use, the cathodic protection system is energised to initiate the electrochemical process, although by using movement detectors to control the switching, a full 24-hour protection facility has been afforded.

The external walls in the dry duct are protected by anodes located in holes drilled into the beams and columns. This is the first example in the UK of discreet water anodes being used for corrosion control.

Variations on this theme include, for example, a titanium mesh set into the screed in the duct floor at a swimming pool in Hove, Sussex.

John Drewett of Concrete Repairs, which also worked on this project, notes that 'this method of corrosion protection is effective and economical and avoids the cost and disruption of intrusive maintenance programmes'.

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