Bitumen mastic has emerged as a highly effective waterproofing solution for various surfaces, including flat and sloped roofs, tanks, and other critical areas requiring robust protection against water ingress. Its unique properties – fluidity at processing temperatures allowing easy application, the ability to form dense, continuous layers across diverse shapes, and exceptional cracking resistance – make it a preferred choice for many construction projects. This article provides a detailed guide to applying bitumen mastic effectively, covering thickness requirements, remelting procedures, laying methods, and specific considerations for different waterproofing scenarios.
I. Introduction: Bitumen Mastic as a Waterproofing Material
Bitumen mastic is a versatile material gaining prominence in the waterproofing industry. Its success stems from its ability to be easily applied at processing temperatures, forming a dense and continuous layer that conforms to flat, sloped, or curved surfaces without requiring compaction. This adaptability, coupled with its significant deformation capacity, results in exceptional cracking resistance even under low-temperature conditions.
II. Thickness Requirements for Bitumen Mastic Application
Achieving optimal waterproofing performance relies on applying the correct thickness of bitumen mastic. The standard practice dictates either a single or three-coat application, depending on the surface type and angle. Here’s a breakdown:
- Walls & Floors Above Ground Level: A single coat of 10mm is sufficient for these surfaces.
- Vertical Surfaces & Slopes Greater Than 30 Degrees: A total thickness of less than 20mm should be achieved using three coats. This layered approach ensures thorough coverage and enhanced protection against water penetration.
- Horizontal Surfaces & Slopes Less Than or Equal to 30 Degrees: For these surfaces, a maximum total thickness of 30mm is recommended across three coats.
III. Remelting Process of Bitumen Mastic
Bitumen mastic typically comes in block form and requires remelting before application. This process must be carefully controlled to maintain the material’s integrity:
- Location & Temperature Control: Remelting should occur on-site within a mechanical mixer, ensuring the temperature never exceeds 200°C.
- Preparation & Heating Procedure: Begin by breaking the mastic blocks into smaller pieces and layering them in the mixer, starting with the sides and working inwards towards the center. Gradually heat the mastic to approximately 200°C while continuously agitating it to ensure a uniform consistency.
- Duration of Heating: The heating duration should be determined carefully to avoid impairing the bitumen’s properties.
IV. Method of Laying Bitumen Mastic
The application technique varies depending on the surface being waterproofed:
- Three-Coat Treatment – First Layer Function: The initial layer serves as a crucial adhesive, preventing air entrapment (blowing) during subsequent coat applications.
- Application Techniques by Surface Type:
- Horizontal Surfaces: Each coat should be spread evenly and uniformly across the prepared surface using a float.
- Vertical Surfaces: The first coat can be applied with a metal trowel to achieve an even, plastered finish. Subsequent coats are best applied with a wooden float for uniform thickness.
- Timing & Repair During Application: To prevent dust or dirt contamination between layers, each coat should be applied as quickly as possible. Any air or moisture-induced blows (bubbles) must be punctured and repaired while the asphalt is still warm.
- Specific Detail Treatments:
- Chases: Where chases are required, the top of the vertical mastic layer should be formed into a chase measuring at least 25mm x 25mm unless it’s being seamlessly integrated horizontally.
- Fillet Joints: At internal angles (fillets), apply an angle fillet that is at least 50mm wide, using two coats applied to separate planes.
- Construction Joints: Before applying fresh mastic to construction joints, warm the existing layer, cut out any debris with a metal trowel, and pour the new mastic before the warmed surface cools.
V. Waterproofing Above Ground Level
When waterproofing above ground level, specific considerations apply:
- Application Scope: The bitumen mastic should be applied across the full thickness of walls, excluding plaster or the base of cavity walls.
- Continuity with Damp Proofing: Ensure seamless continuity between the bitumen mastic layer and adjacent floor damp proofing.
- Optional Vertical Damp-Proofing Course: A vertical damp-proofing course on the inner surface of the wall can be added for enhanced protection (refer to Figure 2).
VI. Waterproofing in External Tanking
External tanking requires additional precautions:
- Working Space Requirement: Maintain a minimum working space of 0.6m outside the walls.
- Base Concrete Requirements: The base concrete should be structurally sound and extend at least 150mm beyond the wall edges to allow for the formation of an angle fillet at the junction of horizontal and vertical damp-proofing (refer to Figure 3).
- Protective Measures After Application: Immediately cover the applied mastic layer with a 50mm thick screed of cement and sand to prevent damage. The structural slab should be laid as quickly as possible, followed by protection of the wall with a brick wall.
VII. Waterproofing in Internal Tanking
Internal tanking demands careful preparation:
- Pre-Application Precautions: Provide a 300mm space outside the wall during excavation to keep the wall dry while laying the mastic. Ensure an even base slab and build walls to the required height before commencing application.
- Backfilling Restrictions: Backfilling should not be permitted until three coats of vertical mastic have been applied, and the loading coat has hardened (refer to Figure 4).
- Protective Layers: As with external tanking, protective layers of screed should be laid, followed by structural floors and walls.
By adhering to these guidelines for thickness, remelting, application techniques, and specific considerations, you can ensure the effective waterproofing performance of bitumen mastic, safeguarding structures from water damage and extending their lifespan.