Alternatively, where cracks can 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.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.When plastering is to be continued across different backgrounds, expanded metal fixed across the junction will minimize cracking due to differential movements.It has many large voids and offers an effective key for plastering. A no-fines concrete is, on the other hand, an ideal material to plaster. Commence plastering once it is completely dry.It must be completely absorbed and that no shiny film remains on the surface. Apply Thistle GypPrime thinned with water depending on the absorbency of the background using a suitable roller, brush or spray.GypPrime prevents the rapid dehydration of undercoat plaster that can happen with high suction backgrounds.These conditions can present difficulties when plastering and bonding treatment such as a PVAC bonding agent or Thistle GypPrime may help.Fully compacted lightweight aggregate concrete has a different thermal movement value to plaster and, when hardened, can present a dusty surface with high suction.Plastering on Lightweight and No-Fines Concrete A disadvantage is that the method can be expensive. Probably the most risk-free option of plastering concrete is to apply the plaster to expanded metal lathing plugged and screwed to 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.Plastering should certainly not commence if free water is present on the surface of the concrete. Concrete should not be plastered until it has had a reasonable time to dry out after the shuttering has been struck.Such treatment may help to improve adhesion of the plaster but will not increase surface absorption and alone may be insufficient to prevent failure.Preparing the surface of concrete mechanically by scabbling or shot blasting will remove any contamination of the concrete surface and roughen a smooth surface.To treat this problem, 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. Plastering in Dense ConcreteĪs the dense concrete has low porosity and smoothness, plastering does not adhere to the surface. In areas where greater thicknesses are likely, it will be necessary to apply the plaster over expanded metal lathing fixed mechanically to the concrete.įig 2: Plastering finish on concrete wall. The thickness of the finish coatĪpplied over the undercoat should be 2mm. Used in conjunction with Thistle Bond-it or other bonding agents should notĮxceed 11mm for walls and 8mm for soffits. After the plastered concrete walls have dried, you can paint them or apply wallpaper. Once the plaster is hard and set, curing must be started and continued till 7 days. Remember the thickness of the top coat should be 5 to 10mm. If the plastering is done in only 1 layer then the thickness of the plaster must be not more than 10 to 15 mm. Apply it in small strokes, making sure it is level and uniform. The layer should be in the range of 10 to 15 mm thick. Apply the PlasterĪ base coat of plaster is applied over the layer of dash with a steel trowel. The prepared plaster must be used within two hours. Mixing of PlasterĪ suitable amount of fine aggregate (sand), cement and water are mixed to obtain a required consistency of plaster for the desired work. Slurry or dash is prepared by mixing 1 part cement to 1 and 1/2 parts coarse sand with the quantity of water sufficient to obtain a runny consistency. To achieve this, a slurry is applied on the concrete surface. The texture of the concrete wall must be rough enough to hold the plaster. Prior to the plastering work, the surface of the concrete to be plastered is to be cleaned from peeling paint, flaking bits of old plaster and should be free from paint, oil, and dust so that the plaster can bond properly. The general steps involved in plastering of concrete surfaces are - 1.
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