The application of lightweight plasters to Concrete walls or soffits should be regarded as a High risk operation due to the large number of failures that have occurred.
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:
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Figure 1: Bulging and partly detached plaster
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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.
CURING AGENTS
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
Class FFP1.
• 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.
Bonding Agents
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.
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| 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.
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| 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.
Plaster Thickness
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
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).
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a) Metal mesh on building paper spanning over junctions
|
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| b) Controlled crack line |
Figure 4: Examples of treatments at junctions of solid backgrounds and dissimilar materials
PLASTERING TECHNIQUES
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.
TIPS 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.
REFERENCES
- 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