FLOOR SCREED READY-MIX
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.
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.
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.
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