Polyurethane waterproofing is a highly effective method used to protect concrete roof slabs from water ingress and damage. This technique involves the application of a two-pack aliphatic polyurethane system formulated with polyether polyols and isocyanate, which not only provides waterproofing but also imparts flame resistance. The system delivers a durable, glossy, and chemically resistant finish that significantly extends the lifespan of concrete roofs.
Materials and Composition
The polyurethane waterproofing system consists of two components: a pigmented or clear resin and a solvent, typically xylene or toluene. These components are mixed in a precise ratio according to the manufacturer’s specifications to prepare the coating. The polyether polyols combined with isocyanate ensure that the cured coating exhibits superior physical and chemical properties.
Key physical attributes of the coating include:
- High resistance to impact, abrasion, and cracking
- Excellent tensile strength, reaching up to 80 Kg/cm² after four weeks of curing at ambient temperature
- A perfectly smooth, dust-free glossy finish that lasts for at least three years
- Resistance to acids and alkalis
- Very low water absorption, limited to 0.5% after seven days at ambient temperature
Step-by-Step Waterproofing Procedure
1. Preparation of the Concrete Surface
The success of polyurethane waterproofing largely depends on proper surface preparation. The concrete roof slab is first cleaned thoroughly using a wire brush to remove all dirt, dust, and loose particles. Any visible cracks on the surface are cut into a “V” shape, cleaned, and filled with a specially prepared paste consisting of a two-component polyurethane-based crack filling compound mixed with white cement in a ratio of 1:2. This ensures a strong and durable repair before the coating is applied.
2. Primer Coat Application
Next, a primer coat is applied to enhance adhesion between the concrete and the polyurethane coating. The primer is also a two-pack polyurethane system mixed according to manufacturer guidelines. It is applied evenly with a brush over the prepared surface. This primer coat must be allowed to dry for at least 8 hours before proceeding to the finishing coats to ensure proper bonding and durability.
3. Application of Finishing Coats
The waterproofing layer is formed by applying three successive pigmented finishing coats of the two-pack polyurethane system. Each coat is carefully mixed per manufacturer instructions and applied with a brush to achieve a smooth, even finish.
Key application details include:
- Achieving an overall dry film thickness of approximately 450 microns
- Ensuring a minimum coverage of 160 grams per square meter per coat
- Allowing a drying time of at least 12 hours between coats
- Prompt application after mixing the two components to avoid loss of pot life (maximum 60 minutes at 30°C)
Additionally, the coating extends beyond the roof surface by a minimum of 150 mm up parapets and walls, and at least 100 mm into rainwater pipes, providing comprehensive waterproof protection.
4. Protective Measures and Finishing Touches
When the final coat is still tacky, a layer of clean 300-micron sand is sprinkled over the surface to improve slip resistance and durability. To further protect the polyurethane coating, a protective screed of plain cement concrete (PCC) is laid on top. This screed consists of a 25 mm thick layer of PCC mixed in a 1:2:4 ratio, reinforced with 24-gauge chicken wire mesh. It is laid in panels sized up to 1.2 m by 1.2 m with joints neatly raked out and filled with an approved elastomeric sealant.
5. Drainage and Junction Details
Proper drainage is critical for waterproofing effectiveness. The concrete screed is sloped appropriately to direct water toward drain outlets and rainwater pipes, which are carefully rounded and sealed as per design drawings. At junctions where the roof meets parapets or vertical surfaces, a cement concrete fillet with a 75 mm radius is formed to prevent water accumulation and ensure smooth transition, thereby reducing the risk of leaks.
Conclusion
Polyurethane waterproofing of concrete roof slabs is a robust and long-lasting solution that combines excellent mechanical strength, chemical resistance, and aesthetic finish. Adhering strictly to preparation and application procedures is essential to maximize the waterproofing benefits. With proper implementation, polyurethane coatings provide superior protection against water damage, prolong roof life, and maintain structural integrity in various environmental conditions.