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A floor screed is usually a cementitious material made from a or 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. 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.
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 mm 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. 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. There are particular definitions concerned with specifying screeds. These are traditional screeds and are suitable for all applications, provided they are specified correctly.
The biggest drawback is the drying time; BS 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. These screeds can be laid as bonded or unbonded. 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. 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. Recommendations for levelling screeds are given in BS Part 1, which recommends the minimum thickness of a bonded levelling screed should be 25mm.
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 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.
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 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 mm or more thick. Recommendations for wearing screeds are given in BS 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. 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. This should ensure there is sufficient depth at mid span i. The wearing screed should be at least mm thick but to minimise the risk of curling, consideration should be given to increasing the depth to mm.
Sector guidance is focused on selecting the correct thickness for the screed. Other criteria may have an impact on the design including:. For precast units the surface of the units should be left rough during production and should be thoroughly washed and cleaned e.
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. British Standards Institution, BS Screeds, bases and insitu floorings — Part 1: Concrete bases and cement sand levelling screeds to receive floorings — Code of practice.
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A floor screed is usually a cementitious material made from a or 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. 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.
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Old version of document Newer versions. Provides recommendations for constituent materials, design, work on site, inspection and testing of concrete bases that are to receive in situ wearing screeds of the following types: concrete; polymer modified cement; terrazzo; mastic asphalt; synthetic resin; magnesium oxychloride; pumpable self-smoothing screeds; and for bases and levelling screeds that are to receive flexible floor coverings such as: textiles; linoleum; polyvinyl chloride; rubber; cork; and rigid floorings such as: wood block and strip; laminate floor coverings; ceramic tiles; natural stone. Applies to ground-supported and suspended concrete floor bases. Their committees work with the manufacturing and service industries, government, businesses and consumers to facilitate the production of British, European and International standards.