A floor screed is usually a cementitious material made from a 1:3 or 1:4.5 ratio of cement to sharp sand. It may be applied onto either a solid in-situ concrete ground floor slab or onto a precast concrete floor unit. There are many proprietary screeds on the market and information about these can be obtained from the manufacturer.
Application
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 site mixed screeds and offer a more consistent material.
Some manufacturers provide pumpable flowing screeds which can achieve very level finishes. Most of these screeds are anhydrite compounds and are based on a calcium sulphate binder. They are quicker to apply than a traditional sand and cement screed and may be applied to a minimum thickness of 25mm if bonded, 30mm if unbonded, or 35mm if a floating finish is required. They can also be used in conjunction with underfloor heating systems where a minimum 30mm cover to the pipes is needed. Up to 2,000m²/day may be laid using these screeds.
Traditional cement sand screeds
A bonded screed is bonded to the slab or substrate below, and the main way that bonded screeds fail is that the bond between the screed and the substrate fails. This is more likely to happen if the screed is too thick. An unbonded screed is separated from the slab or substrate below, and the main way that an unbonded screed fails is to lift or curl. This is more likely to happen if the screed is too thin. Bonded screeds should therefore be thin, normally less than 50mm. Unbonded screeds should be thick, normally 70mm or more, and 100mm or more if curling must be avoided.
Correctly specifying the depth and type of screed starts early in the design process. The issues that dictate the design of the screed include the architecturally specified floor finishes, the construction tolerances and the provision of falls. There may also be structural requirements such as preventing disproportionate collapse and the development of composite action with the concrete slab below. Sometimes, the use of a screed can be avoided.
This might be achieved by specifying tighter construction tolerances and/or structural finishes that are suitable to receive the flooring materials directly. If a screed is needed it can be either a traditional cement sand screed or more recently developed proprietary pumpable self-smoothing screeds. These types are explained below, together with a list of related definitions and guidance on screed depths.
Screed definitions
There are particular definitions concerned with specifying screeds. In this article we have used the definitions in
BS8204 and BS EN 13318:
● Levelling screed – screed suitably finished to obtain a defined level and to receive the final flooring. It does not contribute to the structural performance of the floor.
● Wearing screed – screed that serves as flooring. This term was formerly known as high strength concrete topping. It is also used to refer to structural toppings as well as wearing surfaces.
● Bonded – screed laid onto a mechanically prepared substrate with the intention of maximising potential bond.
● Unbonded – screed intentionally separated from the substrate by the use of a membrane.
● Floating – screed laid on acoustic or thermal insulation. This is a type of unbonded screed.
● Cement sand screed – screed consisting of a screed material containing sand up to a 4mm maximum aggregate size.
● Fine concrete screed – screed consisting of a concrete in which the maximum aggregate size is 10mm.
● Pumpable self-smoothing screed – screed that is mixed to a fluid consistency, that can be transported by pump to the area where it is to be laid and which will flow sufficiently (with or without some agitation of the wet material) to give the required accuracy of level and surface regularity.
● Curling – an upward deformation of the edges of the screed caused by differential shrinkage
It should be noted that pumpable self-smoothing screeds are often known as ‘self-levelling’ screeds.
Which type of screed?
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 thick. Further guidance on drying times can be found in the Code.
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, the guidance given here is therefore generic and the manufacturer should be consulted before specifying. If they are intended to be used as a wearing (structural) screed then the manufacturer should be consulted.
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.
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:
● Use modified screed or additives to reduce the shrinkage potential.
● Use fine concrete screed, which reduces the shrinkage potential, this has been used successfully up to 75mm.
Bonded calcium sulfate pumpable self-smoothing screed
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.
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 70mm. However, BS 8204-1 emphasizes 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 100mm or more thick.
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)
Bonded screed
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 hollowcore 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 screed
The wearing screed should be at least 100mm thick but to minimise the risk of curling, consideration should be given to increasing the depth to 150mm.
Other design criteria for screeds
Sector guidance is focussed on selecting the correct thickness for the screed. Other criteria may have an impact on the design including:
● Slip, abrasion and impact resistance
● Type of traffic on the floor
● Levels and flatness
● Appearance and maintenance
● Type of flooring to be used or applied
● Drying out moisture in screed
● Location of movement joints
BS 8204 Parts 1,2,3 and CIRIA report 184 give ample guidance and should be referred to.
Base preparation
For all types of bonded screeds (both sand/cement screeds and calcium sulphate screed) preparation of the base is of paramount importance. The structural concrete base should be at least C28/35 concrete with a minimum cement of 300kg/m3. For precast units the surface of the units should be left rough during production and should be thoroughly washed and cleaned e.g. by wire brushing to remove all adhering dirt. Where required, the joints between the units should be grouted at least one day before the screed is placed. Where the levelling screed is designed to act compositely with the units and additional preparation of the units is required, contained shot blasting equipment should be used to avoid damaging the units. Where a screed is required over in-situ concrete then all contamination and laitance on the base concrete should be entirely removed by suitable mechanised equipment to expose cleanly the coarse aggregate. All loose debris and dirt should be removed preferably by vacuuming.
