Background.

The type and location of the spacers and chairs used to position the steel reinforcement are important for three reasons:

1. Structural If the reinforcement is not in the position where it was designed to be, the strength (and safety) of the structure can be seriously affected. This is crucially important for cantilevers where the reinforcement is designed to be near the top of the concrete, but sometimes ends up in the middle or at the bottom due to inadequate support before or during concreting. Collapses can occur as a result.
2. Durability When the specified cover is not achieved the durability of the reinforced concrete isgreatly reduced. The steel reinforcement starts to corrode, spalling off the face of the concrete and weakening the structure. As an example, for external concrete sheltered from the rain 30mm of cover will give 135 years of protection to the reinforcement, but 10mm of cover will give only 10 years of life. In marine locations any deficiency in the specified cover can greatly reduce the life of the structure in this demanding environment.
3. Fire In a fire the time before the heat reaches the reinforcement is dependent on the cover. When the reinforcement heats up, the steel softens and can no longer take the stresses for which it was designed. The structure can collapse, possibly with the firefighters and / or the occupants still inside a building.

Currently some buildings do not achieve their intended design lives due to deficiencies in the cover achieved. Some have had to be demolished prematurely, or have undergone extensive repairs, both of which can adversely affect their value. A key part of sustainable construction is the prudent use of resources. Structures can be built with the specified cover achieved, and this will ensure that they achieve their design life and do not fail prematurely from this cause. Examples of the lack of cover or misplaced reinforcement include spalling of the concrete face, wall reinforcement with too little or too much cover, reinforcement outside the concrete itself („negative cover‟), cantilever reinforcement in the bottom of a slab, and reinforcement laid on the ground with no cover to the bottom of the slab. These are only a small selection of the problems that continue to be found, even on current projects.
There is another reason why this is important. Many buildings such as offices, warehouses etc. are used by pension funds, building societies and banks as investments. The value of the investment and the income from it is based on the building being properly constructed for its purpose. If the specified cover has not been achieved then the building may fail prematurely, the investment value is greatly reduced, and the income can be lost. Sometimes the building has to be demolished so the investment becomes worthless, and possibly a liability. The consequential losses in such cases are sometimes far more than the original building costs.

When something goes wrong with a building the owners look for someone to blame. This can be the designer, the contractor, or both. In the UK, clients and their designers have a direct responsibility under the Construction (Design and Management) Regulations (CDM) and Health & Safety legislation for the design, construction, maintenance and demolition of a building or structure. Designers also have a duty of care, a duty to use Standards and Codes of Practice where they exist, and to comply with their respective Institution‟s Code of Professional Conduct. Not achieving the specified cover can lead to spalling of the concrete, and pieces falling to the ground with possible injuries to people underneath. If the reinforcement is not in the correct position demolition contractors could face unnecessary hazards as they would have no way of knowing that the strength or behaviour of a structure during demolition was not what they would have expected from the design drawings.

The correct use of spacers and chairs reduces the potential liability of this aspect of the construction.

Inspecting existing buildings.

 

Before 1989 there was no published national guidance on how to achieve the specified cover. Reinforcement was positioned using site made mortar blocks, pieces of brick, tile, wood, or other materials, if anything at all. Some published literature suggested placing the reinforcement into the wet concrete after it had been poured. It is therefore very likely that reinforced concrete in buildings built before 1989 will not have the specified cover to the reinforcement. They should be carefully inspected for signs of spalling, cracking, and / or excessive deflection. Checking with a cover meter can determine the actual cover achieved, although in areas of congested reinforcement this can be difficult. Radiography can also be used to „see‟ the position of the reinforcement (and other defects) in the concrete. Physical investigation by drilling and measuring is a useful but semi-destructive method of locating the reinforcement.

For buildings built between 1989 and 2001 the question “Was the reinforcement positioned in accordance with Concrete Society report CS101?” should be asked. Usually there will be no response, often because the construction records are no longer available. In the absence of a positive confirmation it may be presumed that the building was not constructed in accordance with the requirements of CS101. If a positive reply is received then the question “What specific spacers and chairs were used?” should be asked. In the absence of a positive reply caution should be exercised. If a positive reply is received then the details should be checked to determine whether the products used were compliant with the requirements of, and used in accordance with, CS101. Site record photographs are useful for this. Often, inferior products were used.

For buildings built from 2001 onwards the question “Was the reinforcement positioned in accordance with the requirements of British Standard 7973 „Spacers and chairs for steel reinforcement and their specification‟?” should be asked. The Standard was published in 2001 and is the result of over 30 years of research, development, and application of the requirements in actual structures. Usually there will be no response to the question, sometimes because the construction records are no longer available. In the absence of a positive confirmation it may be presumed that the building was not constructed in accordance with the requirements of BS 7973. If a positive reply is received then the question “What specific spacers and chairs were used?” should be asked. The recipient should be asked for details of the products used, their reference numbers, the manufacturer / supplier‟s details, and copies of the Specification, a typical reinforced concrete drawing, and / or record photographs of the work under construction. In the absence of a positive reply caution should be exercised. If a positive reply is received then the details should be checked to determine whether the products used were compliant with BS 7973 -1:2001, and used in accordance with BS 7973 -2:2001. Often, non-compliant products are used, and used incorrectly, even on current projects. The use of the questions reflects the importance of this aspect of the construction on the safety and value of the building. Additional questions relating to other aspects of the concrete work, such as the types of cement and aggregates used, may also need to be asked.

Design of new buildings.

Reinforced concrete, both in-situ and pre-cast, for new buildings should comply with British Standard 8110, (BS 8110), the „Structural Use of Concrete‟, at least until such time as it is replaced by Eurocode 2 and its National Annexe. BS 7973 is a freestanding Standard and will not be affected by the introduction of the Eurocode. Compliance with BS 8110 is often quoted in support an application under Part A of The Building Regulations. BS 7973 is incorporated by reference in Section 7.3 of BS 8110, so compliance with BS 8110 requires compliance with BS 7973 as well in order to be valid.  However, it is possible to achieve the specified cover to the reinforcement, first time, every time, through the application of the requirements of British Standard 7973 “Spacers and chairs for steel reinforcement and their specification”. The Standard consists of two parts. Part 1 gives the requirements for the products i.e. the spacers and chairs, and Part 2 shows how they need to be used in order to achieve the specified cover.
It is also necessary to inspect the reinforcement prior to concreting to ensure that the specified requirements have been met. Too often non-compliant products are used, and in a non-compliant manner.

The process chain.

So why does it go wrong? The construction process involves many people. Observation has shown that there are still many structures where reinforced concrete is being used but the specified cover is not being achieved. The participants of a study generally agreed that the defects could be roughly equally divided between operatives (53%) which included steel fixers, formwork fixers, and placing and compaction operatives; and management (47%) which included architects, structural engineers, designers, contractor‟s management, reinforcement suppliers, and site engineers. The process chain is a long one and the omission can occur at any one or more of the stages for the following reasons:

• The client is probably not aware of the problem, and assumes that their consultants will design and the contractor will build the structure in accordance with all of the Standards.
• The project manager takes an overview of the project and does not usually get involved in the actual design.
• The designer concentrates on the calculations, and passes these to the detailer who may be some distance away in another office, organisation or country.
• Having not received instructions on the spacers and chairs to use the detailer does not include them on the reinforcement drawings.
• The contractor‟s estimator does not see anything about spacers or chairs specified on the drawings or in the Specification, so does not include for them in the price for the work.
• The contractor often sub-contracts the fixing of the steel reinforcement entirely to another contractor, who receives no instructions regarding the spacers and chairs from the drawings, Specification, or contractor. Research has shown that steel reinforcement sub-contracting fixing companies are sometimes not even aware of the requirements in the British Standards.

The end result is that all too often the reinforcement does not end up in the correct position, or in accordance with the Standards.

This can be remedied by:-

• The requirement to comply with BS 7973 being specifically written in to the Specification.
• Including on the reinforced concrete drawings „The specified cover shall be achieved by the application of BS 7973 “Spacers and chairs for steel reinforcement and their specification”‟.
• Raising awareness of the problem and the solution, throughout the process chain. This is becoming increasingly important for clients with the current revisions to the CDM Regulations.

Some designers and contractors are known to be using this approach, and benefiting from the commercial advantage that results from its use. Consultants and designers specifying and using the Standard have reduced their potential liability for Professional Indemnity Insurance (PII) claims being made against them. Such claims, successful or otherwise, are inevitably costly and best avoided whenever possible. Contractors using the Standard benefit from getting the work right first time every time. They do not then need to become involved in disputes regarding the actual cover achieved, costly remedial works and delays whilst the matter is sorted out, or, in the worst case, demolition and rebuilding of the affected work.

Development of the British Standard.

The publication of British Standard 7973 in October 2001 was a landmark towards achieving the specified cover. It was the result of many years of research into the problem and the development of
measures to overcome it. The author‟s recognition of the problem started in 1968 and by 1979 a system had been devised and tested on actual construction projects for the first time. The first comprehensive nationally published guidance was the Concrete Society Report CS101 in 1989. Other publications followed and it was then decided that this proven system warranted its own British Standard which was published as BS 7973 in 2001. A Paper on cover was presented at the 6th International Congress on Concrete in Dundee in 2005 and the author is now recognised worldwide as the leading authority on this subject.

The products.

Spacers provide the specified cover between the reinforcement nearest to the surface of the concrete and the surface itself. The surface may be horizontal (e.g. slabs), vertical (e.g. walls), or inclined. Chairs are used to support the top (usually horizontal) reinforcement from the bottom reinforcement or to separate layers of vertical reinforcement in walls. Historically, spacers and chairs have been the poor relation of the concrete industry. Pieces of wood, brick, broken concrete, tile and slate have all been seen used as spacers, even on recent projects. At one time site made sand / cement blocks with pieces of tying wire cast in them were a preferred option. Factory made spacers have now replaced site made ones, which are excluded in British Standard 8110-1 the “Structural Use of Concrete”.

BS 7973 contains the performance requirements for both plastic and cementitious spacers, and steel wire chairs. These have been developed to carry the normal construction loads associated with their particular applications. The selection of the relevant category partly depends on the size of the reinforcement to which they are to be fixed. Although spacers and chairs complying with BS 7973 are available there are also many non-compliant products on the market.

Spacer and chair types.

There are three types of spacers and chairs included in the Standard.

1. Plastics spacers.
2. Cementitious spacers
3. Wire chairs.

Each has its correct use. Most buildings and structures can be built using a total of about four types of spacer and chair. Many of the designs of spacer and chair currently on the market do not comply with the requirements of the Standard, and have been observed to fail in practice in various ways. Spacers manufactured to provide a single cover value should always be used. Spacers with more than one cover value are often placed with the incorrect cover. For example, triple cover cementitious spacers have been found placed in any one of four cover positions, so there is only a 25% likelihood that the correct cover position will be used. These spacers are also prone to fracture under load.

Spacer and chair categories and applications.

Three categories of spacers and one of chairs are included in the Standard, and the applications are specified in Table 1 of BS 7973 -1. The categories for spacers are light, normal and heavy.

Light category

Light category spacers provide the cover in vertical members to the reinforcement nearest to the surface of the concrete, or to horizontal reinforcement in small sections not subject to foot traffic, e.g. pre-cast concrete products. They are used on reinforcement up and including 16mm in size.

Normal category

Normal category spacers are used for most in-situ concrete work and can be used for larger pre-cast products. They provide the cover where the size of the reinforcement to which they are fixed is 20mm or
less.

Heavy category

Heavy category spacers provide the cover where the size of the reinforcement is greater than 20mm. This is typically in bridge decks and heavily reinforced foundations.

Chairs

Chairs are used to support the top reinforcement from the bottom reinforcement in slabs to provide the required top cover. They are also used to separate layers of reinforcement e.g. between the reinforcement in opposite faces of a wall. They are manufactured as continuous, individual, or circular. Continuous chairs are widely used for separating layers of reinforcement. Individual chairs with protective tips can be used to support the top reinforcement in slabs where there is no bottom reinforcement provided e.g. cantilevers.

Plastic Spacers