Plaster defects can significantly impact both the durability and appearance of a building. Fortunately, most of these defects are easily avoidable when the right precautions are taken during the construction phase. From proper surface preparation to selecting suitable materials and following standard procedures, this article highlights essential measures to prevent common plaster defects and ensure a smooth, long-lasting finish.
Importance of Surface Preparation
One of the most critical steps in plastering is preparing the wall surface. Removing all loose materials, dust, and oil ensures the plaster bonds effectively with the substrate. Without proper cleaning, the plaster can delaminate or debond, causing unsightly and structural issues. Applying a cement slurry or spatterdash coat before plastering further improves adhesion and reduces the risk of plaster separation from the wall.
Environmental Control During Plastering
Environmental factors like wind and direct sunlight can cause the plaster to dry too quickly, leading to plastic cracks and shrinkage. To prevent this, it’s crucial to time the plastering work carefully and protect freshly applied plaster from harsh weather conditions. This protection helps maintain moisture levels necessary for proper curing and prevents rapid drying that damages the plaster surface.
Material Selection and Mix Proportions
The choice of materials and their proportions play a significant role in preventing defects such as crazing cracks — fine, shallow cracks that form hexagonal patterns. Using well-graded sand helps avoid these cracks, while avoiding mortar mixes rich in cement prevents excessive shrinkage. A recommended mix ratio is one part cement to four parts sand, although this can vary based on sand type. Additionally, using fly ash blended cement can reduce heat of hydration and minimize crack development compared to ordinary Portland cement.
Moisture Control and Curing
Before plastering, wetting the masonry wall with sufficient water is vital to prevent the bricks from absorbing moisture from the mortar. Excessive absorption increases the likelihood of plastic shrinkage cracks. After application, prompt and adequate curing is necessary—ideally for at least ten days. Curing keeps the plaster moist, allowing it to gain strength and preventing drying shrinkage cracks that appear when moisture loss happens too quickly.
Plastering Techniques
Careful workmanship can further reduce defects. Avoid overworking the cement finishes, which can lead to surface weakness. Maintaining the correct plaster thickness is also important: plaster should ideally be 1.5 cm or less. Plaster thicker than 2 cm is prone to drying shrinkage cracks and may delaminate due to excessive shrinkage stresses. For thicker plaster requirements, it is better to apply two layers about 1.2 cm thick each, with a gap of approximately three days between applications.
Prevention of Specific Plaster Defects
- Cracks: Plastic cracks develop from rapid drying, while crazing cracks are caused by poor sand grading and cement mix.
- Efflorescence: This whitish crystalline salt deposit arises from soluble salts in materials like bricks or sand. To remove efflorescence, brush wash the surface with a diluted acid solution (one part hydrochloric or sulphuric acid to five parts water) and dry thoroughly.
- Popping: Caused by impurities such as seeds or dead burnt lime in the plaster, popping weakens the surface.
- Grinning: Visible masonry joints through plastering can be prevented by applying an undercoat and two coats on a firm surface.
- Rust Stains: When plaster is applied on metal surfaces, rust stains may appear, requiring extra care or protective coatings on the metal.
Recommended Sand Properties
Using sand with a fineness modulus between 2.4 and 2.6 is ideal. Sand with a fineness modulus lower than 2.2 often leads to drying shrinkage cracks due to its finer particles and higher water demand.
Conclusion
Preventing plaster defects during construction demands attention to detail, from cleaning and preparing surfaces to selecting the right materials and applying proper curing methods. By following these best practices, builders can avoid common problems such as cracks, efflorescence, and delamination, ultimately achieving a durable, aesthetically pleasing plaster finish that stands the test of time.