BSBG Lead Architectural Technologist, Richard Griffiths (CIAT), presents this handy guide to floor screeds for architects and technologists.
A floor screed is generally a cementitious material made up from either a 1:3 or 1:4.5 ratio of cement to sharp sand. It is often applied to a solid in-situ concrete ground floor slab or to a precast concrete floor unit. There are many proprietary screeds on the market and information about these can be obtained from the manufacturer.
There are particular definitions concerned with specifying screeds. In this article we have used the definitions in BS8204 and BS EN 13318:
It should be noted that pumpable self-smoothing screeds are more commonly known as ‘self-levelling’ screeds.
The screed may be directly bonded to the base, or laid unbonded onto a suitable damp-proof membrane which is placed over the slab. Alternatively it may be applied as a floating finish over a layer of rigid insulation material. This application is suitable for use with cast-in water pipes to provide underfloor heating. If reinforcement is required, this can either be in the form of a fine metal mesh, fibres which are normally polypropylene or a fine glass mesh. The screed may be left as finished, or floated to produce a smooth surface on which to lay the specified flooring or finish. Ready-mixed sand and cement screeds that are factory-mixed and then delivered to site offer additional quality assurance over screeds mixed on site and offer a more consistent screed mix. Some manufacturers provide pumpable flowing screeds which can achieve very level finishes.
Traditional cement sand screeds
These are traditional screeds and are suitable for all applications, provided they are specified correctly. The biggest drawback is the drying time; BS 8203 estimates the drying time for a sand cement screed as one day for each millimetre of screed thickness up to 50mm.
Calcium sulfate pumpable self-smoothing screeds
These screeds can be laid as bonded or unbonded. They can be laid in much larger areas than cement sand screeds, around 2000m2/day. However, they must not be used with reinforcement because the calcium sulfate is corrosive to steel in damp conditions. These screeds are also generally not suitable for use in damp conditions or where wetting can occur. These screeds are all proprietary products and therefore vary from one supplier to another.
Thickness of levelling screed
A levelling screed may be chosen for various reasons. It might be to provide a smoother, flatter surface than can be achieved economically by the structural slab. Levelling screeds are also used to provide falls or to provide a finishing zone in which different types of flooring may be accommodated. A common use these days is for a levelling screed to be used to accommodate underfloor heating/cooling.
Bonded cement sand screed
Recommendations for levelling screeds are given in BS 8204 Part 1, which recommends the minimum thickness of a bonded levelling screed should be 25mm. To accommodate possible deviations in the finished levels of the structural concrete, the specified thickness should normally be 40mm (with a tolerance of ±15mm) this ensures a minimum screed thickness of 25mm.
However, CIRIA report 184 recommends that a tolerance of ±10mm is adopted with a nominal depth of 35mm. This minimises the risk of debonding, but it should be noted that the tolerances specified for the top surface of the base concrete should be compatible. Where the bonded screed needs to be greater than 40mm the following options are available to reduce the risk of debonding:
Recommendations for pumpable self-smoothing screeds are given in BS 8204 Part 7, which recommends the minimum thickness of a bonded screed should be 25mm. Manufacturers quote maximum thicknesses of up to 80mm and therefore there are less restrictions on the overall thickness. A nominal depth of 40mm with a tolerance of ±15mm can be comfortably specified. Recommendations for wearing screeds are given in BS 8204 Part 2, which recommends the minimum thickness of a bonded wearing screed should be 20mm (in contrast to the 25mm given for a levelling screed in Part 1).
To accommodate possible deviations in the finished levels of the structural concrete, the recommended thickness is 40mm. However the guidance in CIRIA report 1843 recommends that a tolerance of ±10mm is adopted with a nominal depth of 30mm. The specification for the base concrete surface should be compatible. In some circumstances the design thickness will have to be increased above 40mm, but it should be noted that there is an increasing risk of debonding. For hollow core units, which often have an upwards camber, especially for longer spans, a nominal thickness of 75mm, rather than 40mm should be specified.
The risk of debonding is mitigated because it is usual to use a concrete of class C25/30 or above and mesh reinforcement. Using concrete rather than sand/cement screed reduces the shrinkage potential and the reinforcement in particular controls the drying shrinkage. This should ensure there is sufficient depth at mid span (i.e. the point of maximum camber) to allow for lapping the reinforcement whilst still maintaining cover to both surfaces. Even so loose bars or mesh reinforcement with ‘flying ends’ may be required to allow lapping of the reinforcement near the point of maximum camber.
Unbonded cement sand screed
The screed thickness should not be less than 50mm; therefore, to allow for deviations in the finished levels, the specified design thickness should be a minimum of 75mm. However, BS 8204-1 emphasises that there is a high risk of screed curling with unbonded and floating levelling screeds. In order to minimise this, the screed should be either reinforced across the joints or made to a thickness of 100mm or more.
Unbonded calcium sulfate pumpable self-smoothing screed
The screed thickness should not be less than 30mm; therefore, to allow for deviations in the finished levels the specified design thickness should be a minimum of 45mm for a tolerance of ±15mm. Thickness of wearing screed (structural topping) should be at least 100mm thick to minimise the risk of curling, consideration should be given to increasing the depth to 150mm to further mitigate this risk/ defect.
When laid on a compressible layer such as a sound insulating quilt, the thickness of the screed at any point should be not less than 75 mm, except for domestic and similar applications where light loading is to be expected, for which a thickness not less than 75 mm should be used. Owing to deviation of the base levels, the specified design thicknesses might have to be 100 mm and 90 mm respectively, or greater, to maintain minimum thickness.
The minimum thicknesses at any point of levelling screeds laid on thermal insulation boards, which offer firmer support for compaction of the screed, should be at least 65 mm, depending on the traffic and loading to be expected. The levelling screed should preferably be of fine concrete. The insulation board should be chosen to bear the imposed loads and should be sufficiently rigid to enable the screed to be properly compacted.
Other design criteria for screeds
The main principals of screed selection is concentrated on selecting the correct thickness of the screed. However this will be dependent on project requirements/ circumstances other criteria may have an impact on the design such as:
For further information on the above, refer to BS 8204 Parts 1,2,3 and CIRIA report 184 for more detailed guidance.
References/ Further Reading