Improved test method for intumescent steel coatings
International Fire Consultants Ltd (IFC), based in Princes Risborough, UK, has developed a more accurate approach for assessing and predicting the performance of intumescent coatings on steel.
In order to maintain their strength and elasticity, steel structures must be kept cool and well insulated from heat by using these coatings. Yet current techniques for characterising the performance of the coatings are unreliable. The base layer of the intumescent coating foams when heated, producing a thick insulating char which keeps the underlying steel cool. The thickness of the coating influences that of the char, and therefore tests need to be carried out to determine the optimum depth.
Existing test methods look at how the coating responds to high deflections and the relationship between its thickness and the size and depth of the steel sections. But the transition through heat into char is erratic, and steel temperatures vary over the cross-section, resulting in differing margins of error.
IFC’s 3D interpolation method was developed by Principal Engineer Hans van de Weijgert. Weijgert created the model using satay sticks inserted into a timber board. The sticks represent the performance times of the test specimens, while the board illustrates a combination of steel section factors [the size and thickness] and dry film thickness. The heads of the sticks, which are of varying lengths, form a triangle in this 3D space, with the corners of the triangle used to create a plane from which equations can be drawn.
A paper model of the equations reveals ‘a landscape of rolling hills’, says Weijgert, with the areas of weak performance shown as deep valleys.
‘The 3D interpolation uses the measured performance times from the fire test (the heads of the satay sticks). These times form part of the planes. Hence, they are part of the prediction,’ he explains.
‘The difference between measured and calculated times for the data points is zero. This is unique, no other assessment method can match this. Furthermore, the method reveals information that was previously unknown. Sometimes a thicker coat does not always provide a better char.’
Weijgert has created a computer program to automatically generate the ‘landscape of rolling hills hung on the heads of the satay sticks’. The program can calculate cross-sections through the landscape in order to plot the relationship between the coating thickness, steel temperature and performance time, so the fire resistance capability of structural steelwork can be determined.
The 3D interpolation method can also be used for other processes involving three or more parameters that form a four-dimensional problem, says Weijgert.
International Fire Consultants Ltd, Princes Risborough, UK