Your Complete 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, 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 B.C., where crop storage baskets lined with it were discovered in Mehrgarh of the Indus Valley Civilization. Further discoveries reveal bitumen was also used in the same region, around a thousand years later, to waterproof the Great Bath at Mohenjo-Daro.
Sumerians of the ancient Middle East also used bitumen for several construction purposes, including bricklaying and caulking and waterproofing 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.
So, here's your comprehensive guide to getting started:
Waterproofing with Bitumen
Waterproofing is the process of applying a material to an object or structure so 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.
However, waterproofing should not be confused with damp proofing. The former refers to preventing water, in its liquid state, from penetrating a material; the latter involves increasing a material's resistance to humidity or dampness.
Depending on the formulation and the method of application, bitumen can be used to either waterproof or damp proof objects and 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 black, sticky and thick with a viscosity similar to that of molasses at room temperature. (For more on this topic, see: 6 Ways to Measure Fluid Viscosity.)
This liquid-to-semi-solid form of petroleum is typically found in natural underground deposits—but it can also be a refined product, classified as "pitch." Like most petroleum hydrocarbons, bitumen is hydrophobic: 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.
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 suggests, 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 their durability and flexibility—especially when exposed to sunlight.
Since bitumen is naturally insoluble in water, coatings derived from it serve as an effective sealant or repellent against water ingress. The material's natural water resistance, combined with its strong adhesive properties, means it can prevent air and moisture from coming into contact with the substrates it coats—thus providing excellent barrier protection against corrosion. Additionally, bituminous paints are relatively durable, economical and resistant to chemical and ultraviolet (UV) degradation, making them well-suited for coating structures that operate in harsh environments.
In short, bituminous paints are a viable solution for waterproofing a wide variety of structures both above- and below-ground. In the construction industry, they are are typically used to line reinforced concrete foundations and retaining walls to prevent moisture ingress, which can result in the steel reinforcement corroding. (For more on this topic, see: 7 Things to Know and Understand About Steel Corrosion.)
But, since bituminous paints are so versatile, they can also increase the longevity of metals—such as iron, steel, zinc, lead and aluminum—concrete, felt and plastic. This makes them able to fortify structures like stairways, gutters, fences, railings, gates ladders against 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. Surface preparation for bituminous paints can be executed in two steps: surface cleaning and surficial preparation.
Step One: Surface Cleaning
Before applying bituminous paint, make sure all surfaces are clean, dry and free of dirt, grease, rust, mill scale and other contaminants. You may need strong cleaning agents or detergents to remove grease, oil or organic materials. Finally, remove any loose or blistered existing paints before applying a bituminous paint. (For more on this topic, see: 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. Sand blasting and rubbing with a coarse-grit abrasive stone are two common practical approaches to prepare a surface for painting.
For larger surfaces, you might try light shot blasting, sanding machines or other mechanical methods. However, in these cases, make sure to take appropriate precautions taken to ensure flash rusting does not occur.
For harsher conditions such as marine environments—where long-term protection is paramount—you may treat the surface with a suitable anti-corrosive primer. You can employ similar surface preparation techniques can for other substrate materials, including wood and concrete.
Step Two: Surficial Preparation
After you've cleaned the surface, remove residual dust with compressed air before applying the first coat of bituminous paint.
Also, you should properly clean and prepare the surficial coating base before applying the paint. You can do this by repairing or filling any holes and cracks to be flush with the surface, smoothing irregularities with a group mix made of cement and sand and applying a cement-based initial paint before the bituminous paint. (For more on this topic, see: 4 Types of Concrete Floor Coatings (And What You Should Know About Each).)
Once you've gone through both steps, apply the bituminous paint to the entire surface evenly and in as many coats as required—but note two coats is the recommended minimum. For best practice, it is also advisable to maintain an ambient temperature (minimum 10 degrees Celsius) in warm and dry conditions.
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, trowel or sprayer (conventional, airless or hot spray).
Typically, brushing is used on areas you can't reach by spraying and/or on projects for which setting up spray equipment isn't practical. When brushing is used, coating thicknesses range from one to 65 mils.
However, spraying is the most popular method for applying cold coatings. When spraying, coating thickness ranges from one to 250 mils; and coating consistency is key to obtaining the desired thickness. You can use conventional paint-spray equipment for coatings up to six mils thick; but heavier coatings require mastic spray guns fed from pressure pots or heavy-duty pumps. You might also try heated vessels and feed lines to decrease viscosity, permit faster application and allow thicker films to build up in one application.
Troweling is usually used in inaccessible areas—or where it is necessary to produce a very heavy coating in one application. Trowel coats are usually applied in thicknesses above 250 mils.
From the heating requirement perspective, prior to the application, coating applications can fall into two classes:
- Hot-Applied Coatings. These are either 100% bitumen or bitumen blended with selected fillers. A common loading for coatings employing fillers is 10% to 20% filler. Hot-applied coatings, as their name would suggest, are heated to the desired application viscosity. The majority of buried pipelines are coated with this type of bituminous coating.
- Cold-Applied Coatings. These employ both solvents and water to attain the desired application viscosity. A wide range of solvents is used; and the choice depends mainly on the drying characteristics desired and the solvent power required to dissolve the particular bitumen being used. Various fillers are also used in cold-applied coatings to obtain specific applications and end-use properties.
Common Scenarios In Which Bituminous Coatings Are Used
Bituminous coating is a protection against corrosion and is typically used on steel pipelines, iron, concrete masonry, cladding, corrugated metal sheets, corrugated fiber cement/steel roofing panels and storage drinking water tanks.
It is also used for waterproofing comprised of elastomeric bitumen, aluminum pigments, fibers, mineral fillers and volatile solvents. It can cover and enhance old waterproofing coats.
Bituminous paint can be used on both ferrous and non-ferrous metals as well as concrete, wood and masonry. It's frequently used on metal guttering, fire escapes, stairways, ladders, railings and fences.
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. All in all, bituminous coatings are ideal for protecting objects and structures in harsh, outdoor environments.
Written by Krystal Nanan | Civil Engineer