Category Archives: Concrete

Reinforcing concrete & cover

It is important when inspecting reinforcing bars that they are clean and free of loose rust. They should be placed in accordance with the structural engineers drawings, usually made up of main and secondary bars. When inspecting reinforcement you have to make sure that the laps are sufficient, and it’s usually 40x the diameter of the bar being used. Continue reading Reinforcing concrete & cover

Ground floor concrete slab prep and pour

Your ground floor concrete slab will be ready to be poured once the foundations have been laid and the walls have been built up to damp proof course. Before the epic pour, there are a few key stage preparations that need to take place.

1) Compression  If hardcore (or more commonly a Type 1 floor seal) is in place, this must be compressed before work can be carried out.

2) Sand blinding – If hardcore has been used, sand blinding will need to take place next. Building sand is shovelled into the area where the slab is being poured, then its distributed and levelled off. The sand will fill any bumps and hollows, and a thickness of around 50mm is usual once tamped down.

3) DPM -Damp Proof Membrane goes down. Thicknesses of DPM’s vary, but 1200 gauge is a common thickness. It is preferable to cover the area with one large sheet, but in cases where more than  one sheet is required, joins should overlap by at least 150mm, and be sealed to prevent any moisture coming up through the concrete slab. The edges of the DPM will go up the walls and wrap over the brick walls, and eventually sit under the next course of bricks.

This is poorly shown below. In the first picture, you can see the DPM scrunched up, but eventually it will be pulled out, cavity cleaned and then wrapped around the brick wall. In the second picture again, the detail is poor and not a good example of how it should be. Constructions never straight forward.

4) Pour & reinforcement – Then comes the concrete, which, on larger sites is pumped directly into place, typically 100mm. It will take around 3 days to cure. The metal mesh which can be seen in the picture is used to reinforce the concrete, & as it’s a ground floor slab should be sitting on plastic/concrete spacers giving a minimum of 75mm cover.


5)  Insulation – Once the concrete is  levelled and has set, the insulation goes down, which is usually 75-100mm of rigid foam board.

6) Vapour barrier – Finally, many people choose to put down a vapour barrier on top of the insulation before screed is poured. Any pipe work for underfloor heating is then laid out and secured before the screed is put down. The screed usually takes around 7 days to cure

The importance of of concrete cover


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

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.

concrete spalling

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 bars

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