PLASTERING

Accelerator

A material that shortens the setting time of gypsum plasters and other cementitious materials.

Admixture

Any substance added to a plaster component or plaster mortar for the purpose of modifying its properties.

Alkalinity

Relating to or containing an alkali having a pH greater than 7.

Basecoat

Any plaster coat applied to a substrate prior to application of the finish coat of plaster.

Bleeding

See: Watering Out

Blistering

A condition characterized by bulging of the finish coat layer as it separates and draws away from the basecoat.

Body

A subjective and often misused term relating to the working properties of a plastic cementitious mass such as gypsum or

Cement plaster

Generally a term used to refer to the relative stiffness or resistance to movement under the trowel.

Bonding Agent

A material applied to a surface to improve the quality of bond between it and succeeding plaster application.

Bond

The connection between two surfaces – the way something ‘sticks’ to something else. A bond can be achieved through adhesion (a liquid that sticks things together as it hardens) or key (the roughness of a surface which allows something to stick to it).

Brown Out

The visual darkening of a plaster surface which indicates the completed setting (hydration) of the plaster.

Bull Nose

A special formed rounded external angle with a radius of 12.5 mm (½”) or more.

Cement Plaster

Variously defined as plaster containing Portland cement.

Coat

A thickness or layer of plaster applied in a single operation.

Cold Joining

The line at which, or the method by which, two separate plaster applications are jointed to provide a continuous plane surface. The first application (set, but not dry) is terminated at a sharp line using the trowel edge. The second application is brought up to the line of set material and finished to provide a smooth continuous surface. This method is used where termination of a plaster application at an edge, angle, control joint or bead is impractical.

Coloured Finish

A plaster finish coat containing integrally mixed color pigments or coloured aggregates.

Consistency

A term denoting the fluidity or viscosity of a plaster or cementitious paste; also refers to the amount of water required to bring a given quantity of dry cementitious material and aggregate to a given fluidity.

Corner Reinforcement

Reinforcement material used at re-entrant or interior angles to provide continuity between two intersecting plaster surfaces.

Corner Tool

A tool consisting of a right angle (90°) metal form with a handle used for application of plaster and finishing of interior angles.

Cove

A concave molding or curved surface used at the junction between a ceiling and a wall.

Craze Cracks

Fine, random fissures or cracks of the finish plaster surface caused by plaster shrinkage. These are generally associated with cement plasters being improperly gauged or troweled and/or rapid-drying conditions, or with second coat plasters subjected to rapid-drying job conditions.

Darby

A flat wooden or metal tool (aluminium straight edge) about 100mm wide and 1200mm long with handles; used to smooth or float the cement plaster; also used on finish coat plasters to give a preliminary true and even surface.

Delamination

The physical separation of plaster coats, or of a plaster coat from the substrate, that results from chemical or physical impairment of plaster bond to previous coats or other substrates.

Dope

A term often used by plasterers for additives of any type that adjust setting, workability or bond characteristics.

Dot

A small lump of plaster placed on a surface (usually scarified basecoat) between grounds to assist the plasterer in obtaining the proper plaster thickness and aid in aligning the surface.

Drag

A workability problem encountered during scratching off the plaster or finish troweling, where excessive pressure is required to draw the trowel across the surface to smooth it. This occurs when the material has not retained sufficient surface moisture to lubricate the trowel; in the extreme, the trowel or straight edge may skip or jump across the surface, creating ripples.

Dry-Out

A condition caused by excessive evaporation or substrate water absorption, when the plaster loses some or all of the water required for hydration (setting) – it appears as a lightcolored soft area.

Efflorescence

A white, powdery surface deposit sometimes found on plaster or masonry. Results from migration of soluble salts to the surface.

Types of Plaster

Wooden float

After applying the plaster to the wall, the plaster is left to set slightly and then screeded off smooth using a straight edge and then wooden floated with water to create a smooth consistent surface. This type of plaster can be left as a one coat or used as a first coat for a two-coat plaster, like Rhinolite or a decorative finish like Cemcrete Satincrete.

Steel float

Same procedure as above except the plaster is polished using a steel trowel after floating with a wooden float, giving a very smooth consistent finish which can be used for paint techniques and is a preferred finish in rooms that require a smooth plaster like bathrooms; without the expense of a second coat plaster. (Can only be achieved if a good quality fine aggregate is used and not recommended for two-coat applications).

Brush plaster

This plaster finish is completed as in wooden floated with the following additional step; a damp brush is then used to brush off in even, vertical strokes leaving a brushed finish.

Bagging

A watery plaster mix is rubbed over the face of brickwork using sacking. This fills in small holes and leaves a rough-textured surface.

Roughcast plaster

This is applied with a trowel and then brushed smooth without any screeding, leaving an undulating, uneven surface.

Material selection

As discussed above the properties of plaster in both the fresh and hardened states depend to a large extent on the properties of the materials used. The following provides guidance in the selection of these materials.

Cement

Use “common” cement complying with SANS 50197-1, or masonry cement complying with SANS 50413-1 strength class 22,5X or higher. The choice of cement should be based on the properties of the sand (see sand and workability improvers below).

Sand

Sand is by far the major constituent of a plaster mix and has a significant influence on its performance and material cost. In South Africa, natural sands, i.e. pit, river and dune sands, are almost invariably used for plaster mixes. An essential requirement is that sand should be free of organic matter such as roots, twigs and humus.

Note: “Karoo” sands, which consist mainly of disc shaped dark-coloured particles, should not be used for plastering. This is because they exhibit excessive swelling and shrinkage with increasing and decreasing moisture content.

Only a small proportion of clay can be tolerated in plaster sand. Sands with high clay content may generally be recognized as follows:

• The fraction that passes a 0,075 mm sieve1 can, after being moistened, be rolled into a thread about 3 mm or less in diameter.
• Plaster mixes made with such sands are:
  • Very “fatty” and tend to cling to a trowel
  • Have a high water requirement

For a field test, place a few handfuls of dry sand in the foot of a nylon stocking and tie closed. Shake the sand and collect the dust in a bowl.

Ideally, the sand should have a continuous grading, from dust to the largest particles. The fractions passing the 0,15 mm and 0,075 mm sieves (“fines”) are important because they significantly influence the water requirement, workability and water retentivity of the mix. Increasing these fractions results in increased water requirement (with consequent lower strength and higher shrinkage), but improved workability and water retentivity. The optimum fines content is therefore a compromise between these properties. A sand lacking in fines may be used with hydrated builder’s lime, mortar plasticizer, or masonry cement – See improving workability.

Note: Some coarser material may be acceptable, or desirable, for textured decorative work.

A workable plaster will squeeze out from under the trowel and it will be possible to push the trowel to within a few millimetres of the underlying surface. An unworkable mix will “lock up” once the trowel has moved a few millimetres and prevent further downward movement of the trowel. If the mix appears to be workable, pick up some of the plaster on a trowel then tilt the trowel. The plaster should slide off easily. If it clings to the trowel the mix is too “fatty”, an indication of excessive clay content of the sand.

Improving workability

These materials may only be used in cases where a mixture of sand and Portland cement does not have satisfactory workability, usually because of a deficiency in the sand. Workability is improved by increasing the amount of very fine material in the mix, entraining air in the mix, or a combination of these. The following materials are used to improve workability:

Hydrated builder’s lime

This is in the form of very fine plate-like particles. The amount added to the mix may be as much as the amount of cement (by volume, see mix proportion table). Only hydrated lime complying with SANS 523: 2007 Limes for use in building should be used. Use type A2P if possible for better plasticity. Note: that the limes used in South Africa do not have cementing properties. They cannot therefore be used to replace cement but are used in addition to Portland cement.

Material selection

Materials for screeds
Sand

The quality of the sand, i.e. its concrete-making properties, has a large influence on the quality of the resulting screed. It should be a “concrete” sand – not a “plaster” sand – but the largest particles should be removed by sieving the sand through a sieve with openings about 5 mm wide to facilitate finishing. Where possible, the sand should be tested in a laboratory beforehand. It should then, in a mix of 3, 5 parts of dry sand and 1 part of cement by mass, produce a plastic, easy working, cohesive mortar of plastering consistence (i.e. a slump of about 40 mm) with a water content per cubic metre of not more than 320L but preferably not more than 300L. (The higher the water requirement, the lower the strength of the hardened screed and the greater the drying shrinkage and tendency to crack.) To produce a mix that is easy to finish to a smooth surface, it may be necessary to blend two or more sands. Commonly a blend consisting of 4 parts of crusher sand, sieved as above, and 1 part of clean “plaster” sand, gives good results.

Note: Sieving must be done on horizontal sieves which are shaken, or on cylindrical sieves which are rotated. The practice of throwing the sand onto a sloping sieve with a shovel is unacceptable because it is inaccurate, unreliable and wasteful.

Cement

Use cement complying with SANS 50197-1 type CEM I or CEM II A. To use other cements, first obtain expert advice.

Admixtures

Commercially available admixtures, especially of the water reducing type, may be used, but preferably only on the recommendation of the laboratory which tests the sands, and only where adequate control of dosage on site can be guaranteed.

Technical know-how

Surface finish
Screeds

The surface of the screed should be finished according to the type of wearing surface or flooring that is to be laid. For mastic asphalt, wood block and strip, and some textile floor coverings, a non-slip finish is appropriate, while thin sheet and tile floor coverings usually require a smoother, steel trowelled surface. If a designer specifies the use of a thin sheet or tile floor, then the use of a smoothing compound needs to be specified in certain circumstances.

Screeds are essentially light-duty flooring elements and are suitable for:

• Wearing surfaces of floors of utility rooms in domestic premises (e.g. store rooms, garages)
• Floors covered with carpets, plastic tiles or linoleum, etc. and subjected to relatively light traffic such as in offices, shops and hospitals.

Screeds are generally not suitable as wearing surfaces in commercial buildings, schools etc. or in industrial premises. Preferred methods of floor construction for such premises are full-thickness trowelled concrete or a topping on a concrete base.

Toppings

The surface of the topping should be finished to meet the abrasion requirements of the facility.

Specifying tolerances in levels and surface regularity When specifying departure from datum and surface regularity, taking into account the types and thicknesses of the flooring and the screed or topping, the designer needs to consider:

• the finished floor surface
• the screed/topping/direct finished slab surface
• the base slab to receive a screed or topping

Thicknesses & Tolerances

Monolithic screeds & toppings

As defined above, these are screeds and toppings that are applied at the time when the underlying concrete floor is placed. Screed thickness should be approximately 25 mm but not less than 15 mm or more than 40 mm. The minimum thickness of a monolithic topping at any part should be 20 mm. The actual thickness required will depend on structural requirements. In some circumstances, the design thickness of a topping may have to be increased to more than 40 mm, but then there will be an increasing risk of loss of adhesion to the base, due to differential shrinkage stresses.

In attempting to achieve good adhesion between screed/ topping and base concrete, it is important to take cognizance of the phenomenon of bleeding of fresh concrete. Bleeding causes water to accumulate on the top surface of the fresh concrete and the presence of this water layer may impair adhesion unless suitably dealt with.

The screed/topping should therefore be laid at one of two stages:

• either immediately the concrete has been compacted and levelled off and before bleed water appears on the surface (“immediate placing”);
• or after bleeding of the concrete has ceased and bleed water has evaporated or has been removed completely by mopping up (“delayed placing”).

Immediate placing requires careful timing but has the advantage that no preparation of the concrete surface is required. Timing of delayed placing is not as critical but the surface of the concrete does require some preparation: laitance should be loosened by light brushing, with a wire brush or a brush with stiff bristles, and thoroughly removed by sweeping, or preferably, by vacuum cleaning. Delayed placing should however be done within an hour or two after the end of the bleeding period.

How to lay a screed

Screeds
Batching

• Mix proportions should be:
• Sand, measured in the moist, loose state: 130 ℓ
• Cement: 50 kg
• Water: sufficient to achieve a plastic, workable consistence.

Material quantities

The net approximate quantities of materials required to manufacture 1 m² of compacted screed mix 25 mm thick are:

• Sand in moist, loose state: 28 ℓ
• Cement: 11 kg

An allowance of 10% over and above these quantities should be made for wastage. The capacity of a builder’s wheelbarrow is 65 ℓ so batches consisting of two barrow loads of sand and one bag of cement are convenient for mixing by hand or in a sufficiently large concrete mixer. The volume of compacted screed mix produced by such a batch is approximately 115 ℓ. The size of the batch should never exceed the amount that can be used up within 45 minutes of mixing. Apart from wheelbarrows, containers such as buckets, boxes or drums of known volumes may be used for batching. Batching containers should always be filled flush to the rim. Sufficient containers for a complete batch should be provided to avoid the possibility of errors in counting.

Mixing

Machine mixing is preferable and each batch should be mixed for not less than three minutes if this method is used. With hand mixing, the sand and cement should be mixed without adding water until the colour is uniform. Then only may the water be added; the quantity used being just sufficient to produce a mix of the desired consistence. Hand mixing should be carried out with shovels on a smooth concrete floor or a steel plate. Mixing directly on the ground should not be permitted as this results in contamination of the mix with earth and/or organic matter.

Placing and compacting

Control of levels

Narrow strips of screed mix, laid 3 to 4 m apart and compacted to finished level, should be used as guides to establish the level of the screed. The screed should be placed and compacted immediately after laying the guide strips. Where the edge of a guide strip forms a day work joint it should be formed or cut to produce a vertical joint.

A topping should have a characteristic 28-day strength appropriate to the desired abrasion resistance or designed compressive or flexural strength, but of at least 20 Mpa where abrasion resistance is not a consideration.

1. Ordinary non-slip – The surface is left as finished with wooden floats, except that if it is too open or too coarse it may be given a few passes with either a Perspex or aluminium float to close the surface without smoothing it. Over-working should be avoided.
2. Steel-trowelled – If a hard, smooth finish is required, the surface will have to be steel-trowelled, using the delayed trowelling method. After the screed or topping has been spread, compacted and screeded to level, it should be bull floated. Bull floating should immediately follow screeding and should be completed before any excess moisture or bleed water is present on the surface. The purpose of bull floating is to eliminate ridges and fill voids resulting from straight edging.
3. Finishes that are not to be exposed to severe service conditions – The finish (after it has been spread, compacted, screeded to level, and floated) should be left undisturbed until bleeding has ceased and the finish has stiffened to the extent that a footprint will barely show. Surface water should then be removed from the surface, floated and steel-trowelled at intervals until the desired texture is achieved. The texture of concrete finishes can be varied from a fine matt to a glossy surface, depending on the number of trowelling’s applied. A coarse non-slip texture may be produced by brushing a freshly floated surface with a broom, but the resulting ridges may become subject to abrasion.

A number of screed and topping admixtures and finishes are available in the market place which are typically used in industrial type applications like factories, workshops, hospitals etc. and include products like Abrasive resistant screeds, anti-static screeds, epoxy screeds, polyurethane screeds and self-levelling screeds. We have not covered these items in this publication as these applications are seldom used in domestic and light commercial building projects.