Concrete will not adhere effectively to surfaces that lack natural bonding agents or create a physical barrier between the concrete and the underlying material.
Materials Concrete Resists Bonding With
Understanding what concrete won't stick to is crucial for various applications, from construction to mold making. Several types of materials inherently resist concrete adhesion, primarily due to their chemical composition or the formation of a non-stick film.
1. Urethane Rubbers (Molds)
Many concrete molds are specifically designed from materials like urethane rubbers because they naturally resist concrete bonding. This property is vital for ensuring that cured concrete can be easily released from the mold without damage, allowing for the precise formation of various shapes and structures. The inherent qualities of urethane rubber prevent the chemical or mechanical interlock necessary for concrete to stick.
2. Paint
Painted surfaces generally pose a challenge for concrete adhesion. This is because paint lacks natural bonding agents that would allow concrete to create a strong, lasting bond. Instead, the paint layer acts as a separator, preventing the concrete from directly interacting and adhering to the underlying substrate. While some specialized primers might exist for specific applications, standard paint coatings are not conducive to concrete bonding.
3. Oil and Oiled Surfaces
One of the most effective ways to prevent concrete from sticking is by applying a layer of oil or using pre-oiled surfaces. The oil creates a thin, non-adhering film that acts as a release agent. This film prevents the wet concrete from making direct contact with the surface, thereby inhibiting any bonding. This method is commonly employed in formwork for pouring concrete, ensuring that the forms can be easily removed once the concrete has cured.
Practical Implications
Knowing these non-stick properties is highly beneficial:
- Formwork Release: Oils are widely used as release agents on timber, steel, or plastic forms to ensure easy demolding without damaging the concrete structure.
- Mold Making: The selection of urethane rubbers for concrete molds is a direct application of their non-stick properties, enabling the creation of complex and reusable molds for decorative or structural elements.
- Surface Preparation: When concrete adhesion is desired (e.g., for repairs or overlays), surfaces must be thoroughly cleaned of paint, oil, and other contaminants to ensure a strong, durable bond.
