This part of BS 8204 gives 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:
a) concrete (see BS 8204-2);
b) polymer modified cement (see BS 8204-3);
c) terrazzo (see BS 8204-4);
d) mastic asphalt (see BS 8204-5);
e) synthetic resin (see BS 8204-6);
f) magnesium oxychloride;
g) pumpable self-smoothing screeds (see BS 8204-7);
Terms and definitions
Base:- Building element that provides the support for a screed and floor finishes
Flooring:-An uppermost fixed layer of a floor that is designed to provide a wearing surface
Direct finished base slab:- concrete flooring element that is suitably finished to provide a wearing surface or to receive directly the flooring to be applied without the need for a leveling screed.
screed:- A layer of material laid in situ, directly onto a base, bonded or unbonded, or onto an intermediate layer or insulation layer, for one or more of the following purposes:
— to obtain a defined level;
— to carry the final flooring;
— to provide a wearing surface
leveling screed:- screed suitably finished to obtain a defined level and to receive the final flooring
wearing screed:- screed that serves as a flooring
bonded screed:- screed that is bonded to the base
unbonded screed:- screed laid either onto a separating layer or onto a base not prepared to achieve bonding
floating screed:- screed laid on an acoustic and/or thermal insulating layer and completely separated from other building elements, such as walls and pipes
fine concrete screed:- screed consisting of concrete in which the maximum aggregate size is 10 mm.
Screed:- screed consisting of screed material containing up to a 4 mm maximum aggregate size.
smoothing compound:- material applied to a base or screed to provide a smooth, even surface suitable for the installation of a floor covering.
departure from datum:- deviation in height of the surface of a flooring layer from a fixed datum plane.
surface regularity:- deviation in height of the surface of a flooring layer over short distances in a local area
NOTE Surface regularity is also known as flatness.
in situ crushing resistance (ISCR):- resistance of leveling screeds to the crushing effect of imposed loads and traffic in service
NOTE ISCR was formerly known as soundness.
screed material:- Mixture comprising cement, aggregates, water, and, in some cases, admixtures and/or additives
Materials
Cement (Portland cement or a composite Portland cement conforming to BS EN 197-1) and combinations
(Portland cement combined at the batching plant with an addition, conforming to BS 8500-2:2006, Annex
A), are treated as being technically equivalent by BS 8500-2. For most applications a cement or combination
of strength class 42,5 N is applicable.
(Portland cement combined at the batching plant with an addition, conforming to BS 8500-2:2006, Annex
A), are treated as being technically equivalent by BS 8500-2. For most applications a cement or combination
of strength class 42,5 N is applicable.
Concretes made with the cement and combinations listed in have different characteristics,
e.g. strength development, susceptibility to poor curing, and therefore the choice of cement or combination
type should depend upon the particular placing conditions.
e.g. strength development, susceptibility to poor curing, and therefore the choice of cement or combination
type should depend upon the particular placing conditions.
Water
Water should be clean and free from materials deleterious to concrete and leveling screeds in their fresh
and hardened states. In general, drinking water is suitable for this use.
and hardened states. In general, drinking water is suitable for this use.
Sub-base
A sub-base should be provided to give uniform support to ground-supported base slabs and the flooring
applied to them. The sub-base should consist of inert well-graded granular material of maximum
size 80 mm, fully compacted and lightly blinded or inert, crushed, fine material to
form a flat surface to within a tolerance of mm. Alternatively, lean concrete with a cement content
in the range 100 kg/m3 to 150 kg/m3 should be used: these are equivalent to nominal proportions in the
range 1:20 to 1:15.
applied to them. The sub-base should consist of inert well-graded granular material of maximum
size 80 mm, fully compacted and lightly blinded or inert, crushed, fine material to
form a flat surface to within a tolerance of mm. Alternatively, lean concrete with a cement content
in the range 100 kg/m3 to 150 kg/m3 should be used: these are equivalent to nominal proportions in the
range 1:20 to 1:15.
Base and screed construction
The floor should be constructed in one of the following ways (see Figure 1 and Figure 2):
a) a structural slab direct finished to receive the applied flooring;
b) a levelling screed bonded to a prepared concrete slab;
c) an unbonded levelling screed or overslab or floating levelling screed thick enough to provide sufficient
rigidity and reduce the likelihood of curling (see 6.4.3).
Levelling screeds laid in large areas can curl as they dry and shrink, and can crack later when loaded if the
bond to the base is insufficient. The use of unbonded and floating levelling screeds might not be
acceptable for some floorings, which would be affected by excessive curling or movement at joints.
Of the three methods of construction described above, method a), which avoids the use of a levelling screed,
is the only one that guarantees that hollowness, curling and cracking of screeds is avoided.
Where a damp-proof membrane is required see 6.10.
a) a structural slab direct finished to receive the applied flooring;
b) a levelling screed bonded to a prepared concrete slab;
c) an unbonded levelling screed or overslab or floating levelling screed thick enough to provide sufficient
rigidity and reduce the likelihood of curling (see 6.4.3).
Levelling screeds laid in large areas can curl as they dry and shrink, and can crack later when loaded if the
bond to the base is insufficient. The use of unbonded and floating levelling screeds might not be
acceptable for some floorings, which would be affected by excessive curling or movement at joints.
Of the three methods of construction described above, method a), which avoids the use of a levelling screed,
is the only one that guarantees that hollowness, curling and cracking of screeds is avoided.
Where a damp-proof membrane is required see 6.10.
Base slabs
The compressive strength class of concrete for base slabs to receive flooring directly should be not less than
that shown in Table 2.
Higher strengths can be required for structural purposes or to resist damaging from early trafficking
during the construction period. Concrete for direct finished base slabs should be in accordance
with BS 8110-1.
Concrete for bases in contact with aggressive soils, e.g. those containing sulfates, should be in accordance
with BS 8500-1.
that shown in Table 2.
Higher strengths can be required for structural purposes or to resist damaging from early trafficking
during the construction period. Concrete for direct finished base slabs should be in accordance
with BS 8110-1.
Concrete for bases in contact with aggressive soils, e.g. those containing sulfates, should be in accordance
with BS 8500-1.
Thickness
The thickness of the base slab should be determined by the loading conditions and, for any
ground-supported slab, by the load-carrying capacity of the ground. The minimum thickness of any slab on
the ground should be 100 mm.
ground-supported slab, by the load-carrying capacity of the ground. The minimum thickness of any slab on
the ground should be 100 mm.
Acceptance limits for in situ crushing resistance test
(after dropping the weight four times)
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