The failure often takes the form of a progressive de-bonding of the plaster from the surface of the concrete and usually starts with the formation of small hollow areas. This condition usually deteriorates until the plaster on a whole wall becomes unbonded and bulges outwards. On soffits, unbonded plaster may simply fall down in large pieces. The hollowness can normally be detected by tapping the surface with a metal object.
The mechanisms of failure are obscure but normally relate to plaster being unable to form good adhesion to dense concrete due to the very low absorption of the surface. Lightweight concrete may have a high suction which also creates adhesion problems. Stresses set up subsequently, due to differential thermal or moisture movement between the plaster and concrete, will easily break the weak bond between the two materials and cause separation and bulging.
The risk of failure is greater with concrete containing limestone or granite aggregate, as these mixes have a much lower coefficient of thermal expansion than that of plaster. The thermal movement value of concrete mixes with flint aggregates is closer to gypsum plaster and the risk of failure is correspondingly less.
Nevertheless, for all plaster systems the failure is most likely to occur when the building is heated for the first time or shortly after occupation. Smooth concrete surfaces produced by steel or plastic faced shuttering are probably most at risk of failure but it should not be assumed that concrete with a rough surface can be plastered satisfactorily. The photograph in Figure 1 shows a typical adhesion failure:
Figure 1: Bulging and partly detached plaster
RELEASE AGENTS AND MOULD OILS
Adhesion of plaster to concrete can also be inhibited by formwork release agents. Chemical release agents work by inhibiting the setting of cement which can lead to the surface of the concrete becoming dusty.Mould oils, as their name suggests, can leave traces of oil on the surface.
These are applied to the surface of the concrete to reduce water evaporation and thereby assist in the curing process of the concrete. The two main types are resin in an organic solvent and an aqueous solution of metallic silicate.
Plastering over a resin in organic solvent product such as Ritecure from Weber Building Solutions is not recommended as the film deposited may only be weakly bonded to the surface of the concrete. If it becomes necessary to plaster a surface coated with this type of product, the concrete should either be grit blasted (or equivalent) or the plaster applied to expanded metal lath mechanically fixed to the concrete.
The aqueous silicate materials may be slightly less effective in reducing surface evaporation but a hard surface layer forms on the concrete and the adhesion of materials applied subsequently is not be affected.
HEALTH AND SAFETY
Comply with the Health and Safety Manual and ensure that risk assessments are completed. All health and safety recommendations given on the container label and data sheets for Thistle Bond-it or Knauf Betokontakt must be followed. Both products are water based and not classified as hazardous under Chemicals (Hazards Information and Packaging for Supply) (CHIPS) Regulations 1994. Nevertheless, gloves to prevent skin contact, goggles if there is a risk of splashing and standard overalls should be worn.
The following recommendations are applicable to the use of gypsum plasters:
• Gypsum plaster dust can cause short-term irritation of the respiratory system Minimise and control dust when opening bags, mixing or sanding set plasters.
• Avoid prolonged or repeated contact with plasters on the skin or any eye contact. In the event of eye contact, irrigate with plenty of clean water immediately.
• Use correct manual handling techniques.
• Store in dry conditions and on firm level ground. Do not stack pallets more than two high to preserve stability.
Personal Protection for Plastering
• Ventilate the area of work. If dust cannot be controlled, wear a half face mask to EN 149
• Wear impermeable gloves, protective overalls and safety footwear to avoid prolonged or repeated wet contact.
• Apply a barrier cream to the hands to reduce the effect of skin contact.
• Wear safety goggles to BS EN 166 type 2A5 if plaster powder or splashes are likely.
PLASTERING DENSE CONCRETE
Avoid plastering dense concrete where possible but there are ways of minimising the risks. Reference is made in Section 4 of BS 5492: 1990 to the use of bonding treatment on dense concrete backgrounds due to the low porosity, little suction and the absence of a mechanical key on such surfaces.
Preparing the surface of concrete mechanically by scabbling or shot blasting will remove any contamination of the concrete surface and roughen a smooth surface. Such treatment may help to improve adhesion of the plaster but will not increase surface absorption and alone may be insufficient to prevent failure.
Concrete should not be plastered until it has had a reasonable time to dry out after the shuttering has been struck. Plastering should certainly not commence if free water is present on the surface of the concrete.
On most new building sites however it is not practical to wait until the concrete has attained an air dry condition as this may take many months or even years in the case of very thick constructions.
Expanded Metal Lath
Probably the most risk-free option of plastering concrete is to apply the plaster to expanded metal lathing plugged and screwed to the concrete. A disadvantage is that the method can be expensive.
use of polyvinyl acetate (PVAC) bonding agent such as Unibond is useful but varying degrees of success were obtained. British Gypsum now market Thistle Bond-it, which is a ready mixed product used to improve the adhesion of plaster to smooth surfaces. See Figure 2.
Figure 2: Thistle Bond-it
A single undiluted coat is roller applied to the concrete surface and allowed to dry before plastering. The product contains a coarse aggregate to provide a mechanical key and has a light green colour to aid its application. The coverage is approximately 4.5m2 per litre. The base must be clean, free from release agent and frost-free.
It is used in conjunction with Thistle Bonding Coat plaster and either Thistle Plaster Finish or Thistle Multi-Finish. Thistle Bond-it may also be used with Thistle Universal One Coat or Gyproc Dry Wall Adhesive. If the surface of the concrete is sufficiently flat and true, apply Thistle Board Finish or Thistle Multi-Finish directly to the Bond-it.
Figure 3: Knauf Betokontakt
The product offered by Knauf for use with their wet plaster systems is Knauf Betokontakt and is red in colour and can be applied by roller, brush or spray and shown in Figure 3.
The thickness of the undercoat used in conjunction with Thistle Bond-it or other bonding agents should not exceed 11mm for walls and 8mm for soffits. The thickness of the finish coat applied over the undercoat should be 2mm.
In areas where greater thicknesses are likely, it will be necessary to apply the plaster over expanded metal lathing fixed mechanically to the concrete.
PLASTERING LIGHTWEIGHT AND NO-FINES CONCRETE
Fully compacted lightweight aggregate concrete has a different thermal movement value to plaster and, when hardened, can present a dusty surface with a high suction. An example of a lightweight concrete is one containing Lytag aggregate. These conditions can present difficulties when plastering and bonding treatment such as a PVAC bonding agent or Thistle GypPrime may help.
GypPrime prevents the rapid dehydration of undercoat plaster that can happen with high suction backgrounds. Apply Thistle GypPrime thinned with water depending on the absorbency of the
background using a suitable roller, brush or spray. It must be completely absorbed and that no shiny film remains on the surface. Commence plastering once it is completely dry. A no-fines concrete is, on the other hand, an ideal material to plaster. It has many large voids and offers an effective key for plastering.
Adhesion tests can be carried out if there is doubt about the adhesion of a plaster system. The usual method is to allow the wall to dry out and dry core or cut 50mm or 75mm diameter circles through the plaster and right down to the concrete surface. Metal discs of the same diameter, and with a threaded hole, are bonded to the plaster circles with an epoxy adhesive. A pull-off machine is then connected to the metal discs using a threaded stud and a direct tension load applied. The load at failure is recorded but ideally, the failure should occur totally within the plaster. There is little information of minimum bond strengths of adhesive type failures but British Gypsum have indicated that bond strengths of bonding plaster to concrete should be around 500kN/m² to 580kN/m².
TREATMENT OF MIXED BACKGROUNDS
Where plastering is to be continued across different backgrounds, expanded metal fixed across the junction will minimise cracking due to differential movements. Where small widths of one material are involved, for example where a concrete column divides brick panels, the column width is best bridged completely by fixing expanded metal over building paper onto the abutting brickwork in order to isolate the plaster from differential movement. Alternatively, where cracks may be expected to form in the plaster over the junction of dissimilar materials, a straight knife cut through the plaster along the line of the junction will prevent the formation of unsightly irregular cracking. Back to back beads may be fixed over the junction instead of making a knife cut (see Figure 4 from BS 8000-10).
a) Metal mesh on building paper spanning over junctions
b) Controlled crack line
Figure 4: Examples of treatments at junctions of solid backgrounds and dissimilar materials
Accurate proportioning and cleanliness on mixing are vitally important for all good plastering. A measuring box is the most accurate method of proportioning and should be used wherever possible. If a mixer is not available, mix by hand in a clean container or gauging board. Use fresh water for gauging, dirty water will quicken the set. Clean tools in other water. Clean the gauging board after every mix. Never re-temper the mix once it has started to set and do not mix a fresh gauging with an old one. An undercoat must not be applied to a frozen background or a finishing coat to an undercoat on which a film of ice has formed.
The undercoat should be ruled to an even surface and lightly scratched to form a key ready to receive the finishing plaster.
Incorrect Use of Plaster
Plasterers have found that browning plaster is easier to work than bonding plaster and there is a temptation for its use on all four walls of a room even though one might be a dense concrete cross-wall.
Another example of misuse is the application of a scratch coat of bonding plaster to dense concrete followed by a floating coat of browning plaster. The materials are not completely compatible, as the bonding coat does not present the right suction or the right degree of key. The slightest differential movement will cause failure between the two layers.
These problems are often brought to light when the plaster is subjected to a thermal shock such as the heating being turned on in the building.
HINTS TO AVOID TROUBLE
Avoid plastering concrete where possible.
Where possible, use concrete mixes containing flint aggregate for areas to be plastered.
Use a light application of a chemical release agent in preference to mould oils.
Make sure the concrete surface is checked for contamination and appropriate action taken.
Remove surface dust from dense concrete surfaces and apply a proprietary bonding agent such Thistle Bond-it. Consider using GypPrime on absorbent backgrounds.
Apply the plaster with firm pressure and, once hardened, check for the development of hollowness as the plaster dries out and when heating the building for the first time.
Try to avoid heating the building suddenly, increase the temperature gradually.
- BS 8000-10: 1995 Workmanship on building sites. Code of practice for plastering and rendering.
- BS 5492: 1990 Code of practice for internal plastering.
- Chemicals (Hazards Information and Packaging for Supply) (CHIPS) Regulations 1994