Your Comprehensive Guide to Waterproofing with Bituminous Paint
Bituminous coatings are effective at waterproofing and protecting various substrates, particularly metal, from the adverse effects of corrosion; however, as with most protective coatings, proper surface preparation is key to their performance and effectiveness.
The use of natural bitumen as a form of waterproofing can be traced back as far as 5000 to 4000 BC, where crop storage baskets lined with this material were discovered in Mehrgarh, of the Indus Valley Civilization. Further discoveries reveal that bitumen was also used in the same region, around 3000 to 2000 BC, to waterproof the Great Bath, Mohenjo-Daro. Sumerians of the ancient Middle East also used bitumen for several construction purposes, including bricklaying and the caulking and waterproofing of ships. In modern society, increasing demands and harsh industrial environments have highlighted the importance of using bitumen to waterproof and protect various types of structures.
Waterproofing with Bitumen
Waterproofing is the process of applying a material to an object or structure so that it resists the ingress of water or remains relatively unaffected by exposure to moisture under specific conditions. Waterproofing is typically used on structures that under normal circumstances would not be operable in wet or damp environments.
Depending on the formulation and the method of application, bitumen can be used to either waterproof (prevent the penetration of water in its liquid state), or damp proof (increase the resistance to humidity or dampness) objects or structures.
What is Bitumen?
Bitumen, also known as asphalt in North America, is a generic term used to define a classification of naturally occurring minerals that primarily consist of carbon and hydrogen; thus the name hydrocarbon. Bitumen is also comprised of several other elements including nitrogen, sulfur and oxygen. It may also contain solid forms of iron and alumina.
Physically, bitumen is a black, sticky and thick substance with a viscosity similar to that of molasses at room temperature. (Related reading: 6 Ways to Measure Fluid Viscosity.) This liquid to semi-solid form of petroleum is typically found in natural underground deposits or may be a refined product, classified as pitch. Like most petroleum hydrocarbons, bitumen is hydrophobic, i.e., it repels, or does not mix easily with water. This characteristic makes bitumen-based paints and coatings ideal for waterproofing a wide variety of objects and structures.
Over 70% of bitumen consumption is directed towards roadway construction, where it is used as a binder to create asphalt concrete. The remainder is primarily used for sealing, insulating and waterproofing purposes.
Bitumen-coated roofing felt, also known as waterproofing membranes, are used to protect flat and pitched roofs from water ingress and also serve as a base material for roof shingles.
What are Bituminous Paints?
Bituminous paints, as their name suggest, are bitumen-based coating products designed to be applied in a liquid or semi-liquid form. These paints consist primarily of hydrocarbon materials dissolved in another solvent, such as mineral spirits or naphtha. Bituminous paints and coatings are also usually modified with other materials such as polyurethane- or acrylic-based polymers to increase its durability and flexibility, especially when exposed to sunlight.
Since bitumen is naturally insoluble in water, coatings derived from this material serve as an effective sealant or repellent against water ingress. This natural water resistance, combined with strong adhesive properties, allows bitumen to provide excellent barrier protection against corrosion by preventing air and moisture from coming into contact with the coated substrate. Additionally, bituminous paints are also relatively durable, economical and resistant to chemical and ultraviolet (UV) degradation, thus making them well suited for coating structures that operate in harsh environments.
Due to their numerous desirable properties, bituminous paints are a viable solution for waterproofing a wide variety of structures, both above and below ground. In the construction industry these paints are typically used to line reinforced concrete foundations and retaining walls to prevent moisture ingress, which can result in corrosion of the steel reinforcement.
The versatility of bituminous coatings makes them suitable for application on a broad range of materials including metals (iron, steel, zinc, lead, aluminum), concrete, felt, plastics, etc. Stairways, gutters, fences, railings, gates, ladders and water tanks are just some of the other objects that are coated with bituminous paints to increase their longevity in harsh environments.
Surface Preparation for Bituminous Paints
As with all paints and coatings, their adhesion, and by extension performance, is heavily influenced by the degree to which the coated surface is prepared. Before applying bituminous paint, all surfaces should be clean, dry and free of dirt, grease, rust, mill scale and other surface contaminants. Loose or blistered existing paints should also be removed before applying a bituminous paint. (Learn more about surface preparation in Substrate Surface Preparation for Corrosion Prevention.)
Metal substrates, which are susceptible to corrosion, may be cleaned by thoroughly scrubbing the surface with a wire brush or sandpaper. For larger surfaces, light shot blasting, sanding machines or other mechanical methods may be employed, with appropriate precautions taken to ensure that flash rusting does not occur. For harsher conditions, such as marine environments where long-term protection is paramount, the surface may be treated with a suitable anti-corrosive primer. Similar surface preparation techniques can be employed for other substrate materials including wood and concrete.
Residual dust from cleaning operations should then be removed with compressed air before applying the first coat of bituminous paint. The paint should be applied to the entire surface evenly and in as many coats as required.
Applying Bituminous Paints and Coatings
Bituminous paints and coatings are applied to surfaces in liquid form and are used to repel water from a variety of structures. Depending on the environment, coating requirements and coating formulation, bituminous paints can be applied with a brush, roller or sprayer (conventional, airless or hot spray) as required.
Bituminous coatings are effective at waterproofing and protecting various substrates, particularly metal, from the adverse effects of corrosion. Its efficient barrier protection is due mainly to its hydrophobic property, which naturally repels water and prevents moisture from contacting vulnerable substrates.
These types of coatings are also durable, flexible and resistant to chemical and UV attack, making bituminous coatings ideal for protecting objects and structures in harsh, outdoor environments.