Plastering Concrete Surfaces – Methods and Procedure

Plastering concrete surfaces is a critical component of construction that involves the application of lean mortar to enhance not only the visual aesthetics but also the durability and structural integrity of the surface. In this comprehensive guide, we delve into the intricacies of plastering concrete surfaces, encompassing various methods applicable to different types of concrete compositions.

Plastering Procedure on Concrete Surface

The plastering process for concrete surfaces can be distilled into a systematic series of steps:

1. Surface Preparation

Before embarking on plastering work, meticulous surface preparation is paramount. To ensure optimal bonding between the plaster and the concrete, the surface to be plastered must undergo a thorough cleaning. This involves the removal of any peeling paint, and flaking remnants of old plaster, and the eradication of contaminants such as paint, oil, and dust. A pristine surface is the foundation for a successful plastering project.

2. Treating with Slurry

To create an ideal substrate for plaster adhesion, the concrete surface must possess a suitable degree of roughness. Achieving this is accomplished through the application of a slurry or dash. This slurry is meticulously concocted by mixing one part cement with one and a half parts coarse sand, blended with an appropriate quantity of water to attain a fluid consistency. This preparatory step establishes an essential base for the forthcoming plaster layers.

3. Mixing of Plaster

The art of mixing plaster entails the amalgamation of precise proportions of fine aggregate (sand), cement, and water to yield the desired consistency of plaster suited for the specific task at hand. This mixture must be deployed within a strict two-hour timeframe to ensure its workability and effectiveness.

4. Applying the Plaster

The application phase necessitates the skilled use of a steel trowel to administer a base coat of plaster onto the slurry-coated surface. This plaster layer should ideally measure between 10 to 15 mm in thickness. Application is performed meticulously in small, even strokes, guaranteeing uniformity and a level surface. In cases where a single-layer application is employed, the thickness should not exceed 10 to 15 mm, while the topcoat should measure 5 to 10 mm in thickness.

5. Curing of Plaster

Once the plaster has been expertly applied and has undergone the process of setting and hardening, curing must commence and endure for a minimum duration of seven days. This critical stage is pivotal in ensuring the longevity and structural soundness of the plastered concrete. Following the drying period, the surface is primed for the application of paint or wallpaper.

Plaster Thickness

The thickness of the undercoat, when used in conjunction with bonding agents such as Thistle Bond-it, should adhere to stringent guidelines, with wall undercoats not exceeding 11 mm and soffit undercoats capped at 8 mm. The finish coat applied over the undercoat should measure precisely 2 mm. In cases where greater thickness is necessitated, plastering over mechanically affixed expanded metal lathing becomes a viable option, albeit one that carries a higher cost.

Plastering on Different Concrete Surfaces

Plastering in Dense Concrete

The unique challenges posed by dense concrete surfaces, characterized by low porosity and smoothness, demand specific attention. To promote adequate adhesion of plaster in such scenarios, reference is made to Section 4 of BS 5492: 1990, which recommends the use of bonding treatments. Mechanically treating the surface through methods like scabbling or shot blasting can enhance the plaster’s grip.

However, it is imperative to refrain from plastering until the concrete has undergone sufficient drying post-shutter removal. Furthermore, plastering should not commence if any residual free water is detected on the concrete’s surface. For optimal results, particularly on new construction sites where time constraints prevail, the application of plaster to mechanically secured expanded metal lathing offers a reliable, albeit relatively expensive, solution.

Plastering on Lightweight and No-Fines Concrete

Plastering on fully compacted lightweight aggregate concrete presents distinctive challenges due to differences in thermal properties and surface characteristics. This type of concrete can exhibit a dusty surface with high suction, necessitating specific measures. The utilization of bonding treatments, such as PVAC bonding agents or Thistle GypPrime, can mitigate these challenges effectively. Thistle GypPrime, in particular, prevents the rapid dehydration of undercoat plaster on high-suction backgrounds. The application process involves thinning Thistle GypPrime with water to ensure complete absorption without leaving a glossy film. Plastering should commence only when the surface is entirely dry. In contrast, no-fines concrete stands as an ideal substrate for plastering, thanks to its abundant voids, which provide an effective key for plaster adhesion.

Plastering on Mixed Concrete Surfaces

When the need arises to plaster across dissimilar backgrounds, strategies to minimize cracking due to differential movements are essential:

  • Expanded Metal Fixation: The installation of expanded metal across the junction between different backgrounds can effectively reduce the likelihood of cracks emerging due to differing material movements.
  • Isolation Measures: For narrow widths where a concrete column separates brick panels, fully bridging the column with expanded metal over building paper can isolate the plaster from differential movement. Alternatively, a straight knife cut through the plaster along the line of the junction can prevent unsightly irregular cracking.

Key Points to Remember in Plastering Concrete

To ensure the success of a plastering project, meticulous attention to detail is essential:

  • Avoidance of Plastering Concrete: When possible, explore alternative construction methods that negate the need for plastering concrete surfaces.
  • Flint Aggregate Mixes: Utilize concrete mixes containing flint aggregate in areas designated for plastering to enhance surface quality.
  • Chemical Release Agents: Prioritize the use of a light application of chemical release agents over traditional mold oils.
  • Contamination Inspection: Rigorously inspect the concrete surface for contaminants, and take appropriate corrective actions as necessary.
  • Surface Dust Removal: On dense concrete surfaces, removal of surface dust is imperative. The application of proprietary bonding agents such as Thistle Bond-it should be considered. On absorbent backgrounds, Thistle GypPrime can prove invaluable.
  • Plaster Application: Apply the plaster with firm pressure, and upon hardening, meticulously inspect for the development of hollowness as the plaster dries and during the initial heating of the building.
  • Gradual Heating: When heating the building for the first time, adopt a gradual temperature increase strategy to prevent abrupt temperature shifts.

Personal Protection for Plastering

The safety of personnel engaged in plastering operations is of paramount concern. Rigorous adherence to personal protective measures is imperative:

  • Adequate Ventilation: Ensure proper ventilation within the workspace. In instances where dust control is challenging, don a half-face mask compliant with EN 149 Class FFP1.
  • Impermeable Protective Gear: Attire yourself in impermeable gloves, protective overalls, and safety footwear to mitigate the risk of prolonged or repeated wet contact.
  • Barrier Cream Application: Apply a barrier cream to your hands as an additional safeguard against skin contact with plaster.
  • Eye Protection: Utilize safety goggles that adhere to BS EN 166 type 2A5 standards when plaster powder or splashes are anticipated during the plastering process.

By adhering to these comprehensive guidelines and safety protocols, you can execute concrete surface plastering with the highest degree of professionalism and precision, resulting in structurally sound and visually appealing outcomes.

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