The minimum thickness of concrete members is dependent on a few variables: the type of component (such as beams or slabs), the size of the aggregate used within the mix, whether there is any steel reinforcement, and standards for fire resistance. In the UK, the design of concrete is covered by Eurocode 2: Design of Concrete Structures (BS EN 1992-1-1:2004+A1:2014), which specifies minimum levels of thickness for various elements. Below you will find a quick summary of what the Eurocode says.
(Note that the Eurocode may be updated, changed or withdrawn, so always check what standard applies before using any figures.)
However, if there is any reinforcement included within the slab, this thickness will increase to ensure there is adequate concrete cover to the reinforcement (see the Reinforcement section for how this is calculated). If you have more than one row of reinforcement, you’ll need to include the spacing between the bars in your calculation of the minimum thickness as well – see the diagram below.
The strength of your concrete will also affect the minimum thickness. Stronger grades of concrete can provide thinner slabs – up to a point. This is because they will be able to resist the loads on them better than weaker grades of concrete.
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Another factor in the minimum thickness of slabs is how the slab will be supported – i.e. is it a slab cast directly onto the ground, simply supported on beams or columns, continuously supported on one or both ends, or cantilevered. These support conditions affect the load-bearing capacity of the concrete. For example, if your concrete slab is cantilevered and is too thin, a heavy load applied to the unsupported end will break the cantilevered section. This is because the load-bearing capacity of the slab reduces the further away from the supported section it is. On a simply supported slab, the load-bearing capacity of the slab is lowest in the middle. See the diagrams below for how these two support conditions differ.
Typical SituationsWhile there many variables in the minimum thickness of slabs, there are some standard thicknesses used for typical situations. It’s important to remember that these may not be appropriate for your loading conditions, however. See Table 7.4N in Eurocode 2 for more information. N.B. Some of these situations, such as footpaths and patios, don’t contain structural members and are generally not designed by engineers, hence they are not subject to the Eurocode 2 regulations. These examples, however, are meant as ‘real world’ applications with suggestions as to what you could potentially use without expecting any problems. With that being said, and to avoid too much ambiguity, we’d advise a minimum of 125mm for all situations.
|Use||Thickness (mm)||Thickness (in.)|
|Patios (foot traffic only)||100mm||4”|
|Driveways and parking areas||150mm||6”|
|Ground-bearing concrete floor slabs (residential construction)||100mm||4”|
|Simply supported slab, one- or two-way spanning||Length/20 (lightly stressed) Length/14 (highly stressed)|
|One-way continuous slab||Length/26 x 1.3 (lightly stressed) Length/18 x 1.3 (highly stressed)|
|Slab supported on columns without beams (flat slab)||Length/24 x 1.2 (lightly stressed) Length/17 x 1.2 (highly stressed)|
|Cantilevered slab||Length/8 x 0.4 (lightly stressed) Length/6 x 0.4 (highly stressed)|
|Simply supported beam||Length/20 (lightly stressed) Length/14 (highly stressed)|
|End span of continuous beam||Length/26 x 1.3 (lightly stressed) Length/18 x 1.3 (highly stressed)|
|Interior span of beam||Length/30 x 1.5 (lightly stressed) Length/20 x 1.5 (highly stressed)|
|Cantilevered beam||Length/8 x 0.4 (lightly stressed) Length/6 x 0.4 (highly stressed)|
The density, quality and thickness of cover to reinforcement as well as the extent of cracking will all affect the corrosion protection and fire resistance of the concrete.
How to Calculate Cover Minimum Thickness
What we would normally think of as the cover “minimum thickness” is actually called the cover nominal thickness and is indicated on structural drawings. It is calculated by determining the minimum thickness plus a tolerance for any deviations.
Cnom = Cmin + ∆Cdev
Typically, the absolute minimum that this value will be is 20mm, which is made up of 10mm of minimum cover and 10mm of allowance for deviation. However, this can vary depending on the situation – if you are casting concrete direct onto the ground or using prestressed tendons, for example, then the cover will likely be increased.
The Minimum Cover Cmin
The minimum amount of cover (Cmin) is determined by comparing the minimum cover required to achieve a good bond between the concrete and reinforcement, and the minimum cover required for the environmental conditions (such as if the reinforcement is a prestressed tendon, the desired design working life, quality control of the concrete, etc.) The highest value of these two values is used, unless they are both less than 10mm, in which case 10mm is the minimum. And if there is an uneven surface that the concrete is being cast onto, such as exposed aggregate, the minimum cover should be increased by at least 5mm for a total of 15mm minimum cover. Similarly, if large aggregates are used in the concrete mix (32mm or greater nominal aggregate size) then the cover will also need to be increased by 5mm to allow for this.
The Cover Allowance ∆Cdev
Added to the minimum cover is an allowance for any deviation when the concrete is installed (∆cdev), much like a tolerance. This allows for any low spots in the concrete as it cures. The recommended value of cover allowance is 10mm according to Eurocode 2. However, in some situations, this value can be reduced or increased where the accepted deviation will be more or less than 10mm.
Reducing Cover Allowance
A good example for when the cover allowance would be reduced would be when precast concrete elements are being used, as the factory will typically have a high standard of quality control. In this case, the finished surface of the concrete might be accurate to within a few millimetres, so the cover allowance may be reduced to anywhere between 0-10mm. Similarly, if there is a rigorous quality management system in place that monitors the cover depth during fabrication, the cover allowance can be reduced to between 5-10mm.
Increasing Cover Allowance
On the other hand, if concrete is being cast against an uneven surface – such as onto the existing ground for example – the cover allowance needs to be increased to allow for greater differences in the surface. The recommended values in this instance are:
- For concrete cast against prepared ground, including blinding: 40mm
- For concrete cast directly against soil: 75mm
Other reasons why you might increase cover include the finish that needs to be achieved, such as ribbed finishes or leaving aggregates exposed.
Typical Minimum Cover Thicknesses
For concrete that isn’t designed by an engineer, the minimum cover for typical residential situations is given in the NHBC Standards. The current standards (2019 version) use these values:
Minimum Cover (mm)
|In contact with the ground||75|
|Cast against a DPM on sand blinding||40|
|Against adequate blinding concrete||40|
|Protected or internal conditions||25|
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