Reinforced concrete is one of the mostly widely used structural materials in the world. The introduction of CARES (Certification Authority for Reinforcing Steels)
…has ensured that quality problems for reinforced steel in the UK are rare. Most of the concrete now in the UK comes from quality assured ready mixed concrete suppliers and the biggest problem is failure to position the concrete within accurate cover which affects the concrete durability as a result. This type of failure costs the UK £550 million/ year in the UK alone.
The type and location of the spacers and chairs used to position the steel reinforcement are important because if they’re not in the correct position it can cause for the strength of the structure to be compromised. This is especially important for cantilevers where the reinforcement is designed to be near the top of the concrete but can end up in the middle or bottom if the support is inadequate. This can lead to collapse. When spacers aren’t correctly placed, steel reinforcement can begin to corrode and the structure is significantly weakened.
As an example; for an external concrete structure sheltered from the rain 30mm of cover will give approx 135yrs of protection to the reinforcement, but 10mm of cover gives only 10yrs. In marine locations, deficiency in the specified cover can also reduce the life of the structure. During a fire the time before the heat reaches the reinforcement is dependent on the cover. When the reinforcement bars heat up the steel softens and can no longer take the stresses it was designed for which may lead to collapse.
Concrete spalling is when the chips or fragments of a material is broken off a bigger object. The most common reason for spalling in bricks is due to excess moisture. In concrete, especially older concrete structures spalling is usually caused by corrosion of the steel reinforcement bars. The rebar is used to offer strength to a material that is extremely high in compressive strength but has very limited flexible or tensile strength. One of the properties of reinforced concrete is that the high alkaline content of the concrete protects the embedded steel from corrosion. The correct cover should reduce the chances of the rebar corroding in reinforced concrete.
Surveying existing buildings
There isn’t any published national guidance on how to achieve the specified cover before 1989. Back in the day reinforcement was positioned using site made mortar blocks, pieces of brick, or anything else found. Sometimes the rebar was placed into the wet concrete so its highly likely that buildings built before 1989 won’t have the specified cover for reinforcement. Therefore, they should be checked for cracking, spalling or excessive deflection. Using a cover meter makes it possible to check the actual cover however this can be difficult when there is congested concrete. You can also use radiography and even investigation by drilling and measuring. The latter however is semi-destructive.
Spacers and chairs
Spacers are used to create the cover for the rebar in reinforced concrete. They can come in plastic, cement or metal.
Single cover plastic ‘A’ spacers can be used for most purposes, including foundations, columns, beams, slabs and walls. They can be used for up to 20mm sized rebar’s. The spacer clips on to the rebar and is the cheapest option because it doesn’t need tying on to the wire (cost and time effective). They can be used for covers from 20-90mm.
Soft substrate ‘A’ spacers are used when there is a soft substrate involved, like insulation. The spacer has a spreader base which clips to the bottom of the A spacer which spreads the load carried by the spacer into the insulation.
End spacers are used at the ends of the wires or the rebar to ensure the correct end cover.
Circular spacers have been used on vertical concrete members such as walls and columns. They contain more plastic than necessary so are not the best resource.
Cementitious spacers are used where the surface of the concrete may be subject to abrasion. They should be wired on to the reinforcement. The wire is usually 16 or 18 gauge soft iron wire but in marine environments stainless steel should be used.
Cementitious line spacers are used to support rebar which is 25mm or above. They are manufactured in 1m lengths and in sizes from 25mm to 60mm. They should be used in small lengths not exceeding 350mm.
Chairs are used to support the top reinforcement from the bottom reinforcement. They are manufactured as continuous, individual or circular.
Reinforcing mesh bars should be clean, free from rust and placed in accordance to the structural drawings.
The appropriate cover is shown below as according to LABC standards;
Concrete in direct contact with the ground – 75mm
External applications, shuttering – 50mm
Floor slabs, and other applications where concrete is cast onto membrane – 40mm
Concrete over blinding concrete – 40mm
Internal conditions 25mm