Canadian Consulting Engineer

Product Specifications: Coatings – Intumescent fireproofing for steel structures

May 1, 2003
By John Witter

Steel is ideal for construction because of its cost, design flexibility and ease of construction. However, it does have one major drawback -- its vulnerability to fire. At high temperatures steel lose...

Steel is ideal for construction because of its cost, design flexibility and ease of construction. However, it does have one major drawback — its vulnerability to fire. At high temperatures steel loses its ability to support itself and, as a result, steel structures must have fire protection. Such protection is essential in order to maintain a building’s structural stability for as long as possible in order to allow the occupants to escape. A side benefit is that if the structural integrity is maintained, once the fire has been put out, repair and rebuilding costs are much reduced.

Active fire protection systems include sprinklers and fire extinguishers. Passive protection systems are also mandatory, and these include sprayed fibre, concrete, gypsum board, and intumescent paint. Of these systems, intumescent paint has the capability of complimenting the design and enhancing the appearance of exposed steelwork. Intumescent coatings are being used extensively in large public buildings simply because they allow the steel designer greater creative freedom without the disruptive element of sprayed fibre or board. Simply stated, intumescent coatings provide the fastest, thinnest and most aesthetically pleasing of all fireproofing methods

An additional advantage to intumescent fireproofing is that it can be applied off-site by the steel fabricator or in a specialized painting shop, reducing costs and minimizing disruption for other trades.

Swelling to save

To intumesce means to swell up. At temperatures above 300 degrees, intumescent coatings expand up to 50 times their original thickness. In a fire, components of the intumescent coating react with each other to produce an expanding char that both reflects and absorbs heat. The result is that the steel behind the coating remains cool, allowing it to support its design load.

The amount of fireproofing applied is determined by the mass of steel divided by the heated perimeter exposed to the fire. The amount is expressed as the W/D ratio in imperial measure (weight in lbs./ft. divided by heated perimeter in inches), or as M/D in metric (mass in kg/m divided by heated perimeter in metres). Manufacturers of intumescent coatings have their products tested by Underwriters Laboratory, which determines the thickness of product to be applied to provide a given hourly rating of fire protection. Each manufacturer is provided with a design number that is exclusive to that manufacturer and thicknesses are published in the Underwriters Laboratory Fire Resistance Directory based upon the size of the steel and the hourly rating required.

Typically intumescent coatings are specified as a system. A system consists of a primer, a basecoat and a finish coat. Of the three components, only the basecoat is an intumescent. However, the importance of the primer and the finish is that they must be compatible with the basecoat. Manufacturers of intumescent coatings recommend acceptable primers and finishes, sometimes of their own manufacture, and sometimes acceptable tested products available from other suppliers. Specifiers should always check with the intumescent manufacturer before specifying any primers or finishes other than those it recommends.

Preparing the surface of the steel before priming is important. Simply stated, the surface should be clean, dry and free of loose rust, dirt and other contaminants. It is always best to reference the surface preparation standards of the Steel Structures Painting Council (SSPC).

New steel should always be abrasive blasted to an SSPC SP 6 standard to remove mill scale. Existing painted steel should be prepared appropriately to ensure proper adhesion of the intumescent base coat. Galvanized steel represents a special challenge as many coatings cannot adhere to the surface, so one should always obtain direction from the supplier.

Intumescent coatings are based either upon solvent-borne or water-borne resin systems. Each has unique advantages and disadvantages. Solvent-borne systems cure at a greater range of temperatures and will also dry to topcoat more rapidly, but they do have a potential for odour. Water-borne systems have less odour, which can be important when several trades are working simultaneously on a particular project. Both systems meet the same codes.

Independent inspections

Although intumescent coatings can be applied by brush and by roller, the results are not as aesthetically pleasing as the finish obtained by spray application. Brush and roller applications are primarily used for touching up damaged areas, or where access by spray is impossible.

An important aspect of intumescent coatings often overlooked is the need for independent, third party inspections. In the world of corrosion resistant coatings and linings such inspection is accepted as a necessity given the importance of the coating in protecting the substrate. Thickness and surface preparation are routinely tested and reported on projects such as bridges and tanks subject to a corrosive environment. In the intumescent fireproofing field, such third party inspection has not been commonplace. It is expected that the applicator will apply the coating in accordance with the specification. However given the life safety issues that are involved with fireproofing, it is important to be certain that the correct thickness of intumescent to meet the stated protection standard is being applied. A trained inspector with the correct inspection tools can verify these factors.

In Canada, intumescent coatings are being used to protect many of our largest and most public structures. The Eaton Centre and Dorval Airport in Montreal, Vancouver Airport and the largest construction project in Canada, the new terminal at Pearson Airport, are all using intumescent coatings for fire protection.

Article supplied by StonCor Group Carboline Coating Division, Whitby, Ont., Tel. 800-263-3112.


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