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Detection method of fireproof coating thickness for steel structures

Apr 09, 2021

Regularly referred to in the short-hand as intumescent paint or intumescent coatings, fireproof coatings are used as passive fire resistance measures in structures and buildings. Whilst the primary use is for “fireproofing” structures, they have added benefit in the way of aesthetic and insulative properties.
Regular readers of our site and blogs will know that the most critical component of thin-film intumescent is that when exposed to extreme heat, for example, in a fire, they significantly expand. The subsequent expansion forms a thick foam boundary – called a char. This char is quite thick and acts as an insulator, “reducing the heating rate of the steel hence preventing it from reaching the failure temperature, hence allowing the occupants to evacuate and emergency crew to take actions.
But here is the thing about the fire protection of structural steel. Not all steel needs to be protected, and the steel which does need protection will not all have the same requirements for fire rating, with the fire rating requirement of a building determined by the design engineer or certifier with reference to building regulations. Some steel will be encased in partition walls and ceilings, some steel will need an FRL of 60/-/- and other steel members may need 120/-/-. But what is what, and how do experts ascertain if the thickness of a coating will actually provide the protection it is designed to? 
Currently, there are a myriad of coating thickness gauges suited for measuring the thickness of the fire rated coating applied to steel. In fact, there are different approaches for different stages of construction:
During the spraying/application when the intumescent coating is still wet, spot checks use coating thickness gauges which measure the wet film thickness to ensure the coating is thick enough. However, the actual measurement used for the purpose of system approval is ultrasonic equipment, which people in the painting industry use frequently. Building surveyors or certifiers do not accept the from the Wet film thickness gauges (see below), they will only approve the system by determining the thickness as measured through ultrasonic measurement. In other words, for all legal documents, ultrasonic is the way to go.

Intumescent Film Thickness Survey

Thickness estimation and measurement utilising a coating thickness gauge in Australia occurs irrespective of whether applied on-site or off-site as long as the material is wet. However, prior to this, a certified structural engineer will have needed to determine the correct thicknesses for the coatings which are to be applied (whether these are products such as vermiculite, Nullifire intumescent or other). With the specifications determined, an applicator can then apply the coatings to the required steel and the exposed sides identified as requiring application. But here is the thing, any coating system has its unique solid volume translation from the Wet Film Thickness to Dry Film Thickness. It is critical to make sure that the right amount of coating is applied after the application job. 
Application inside the specified tolerances is best guaranteed through a process of ongoing and vigilant checking, and perhaps, not surprisingly, through elimination of surface deformities preceding application. For greatest accuracy, it is recommended that no readings are taken inside 25mm of the edge of an Ι-Beam or inside 25mm of the join of rib to web of a Ι-Beam segment when taking dry film thickness readings. However, whilst checking through the process is essential to ensure the materials are likely applied thickly enough, as per above, accurate measurement will only be achieved through an ultrasonic device.  
Considering the above mentioned, readings should then be randomly taken over the remaining areas of a section with WFT gaugues during application and ultrasonic equipment when dry. The frequency of measurements should be taken with regards to:
  • I Sections, Tee Sections, angles and Channels. Webs: There should be two readings for every metre length on each face of web. Flanges: There should be two readings for every metre length on the external face. One reading for every metre length on the inner face of every flange.
  • Square and Rectangular Hollow Sections and Angles: There should be two readings for every metre length on each face.
  • Circular Hollow Sections: Every metre length should have eight readings equitably spread around each segment.
  • Three sets of readings will be taken, where members are under 2m long, one at each end and at the centre point. As above, each set shall comprise the number of readings on each face given by (i), (ii) or (iii) as appropriate.
In ideal circumstances, for the purposes of compliance and to ensure accuracy of the fire protection of completed work, each steel member requiring fire protection would be measured as per the above guidelines. In an ideal world, dry film thickness surveys should be completed by an independent third party, however the applicator should also be undertaking this check throughout the process too. Where thin-film intumescents (such as Nullifire which is designed for structural steel) are applied (as opposed to technologies such as cementitious fire spray) independent signoff is particularly important for the purposes of achieving the warranties and certificates you need, so compliance to guidelines is important for applicators and for the builders/site superintendents.  
To ensure accuracy in the measurement process as well, the following list should act as a guideline:
  • The contractor should give appropriate and sufficient means for access, including to troublesome and/or difficult to reach zones. It is important to schedule the tests for these sections when access is as simple as possible as to allow accurate measurement without assumption.
  • All equipment utilised should be calibrated correctly and often. Where a third party is undertaking the testing using multiple staff, they should ensure they have calibrated and verified the readings across each device.  
  • At least 10% of the steel sections ought to be measured as per the frequency set out in above.
  • These sections ought to incorporate a mix of section sizes, and difficult access segments just as those that are least demanding to get to.
  • Unless the detailed survey (of 10% of sections) identifies recurrent thickness problems, all of the other sections should be measured with reduced frequency.
  • Only in the most dire outcome imaginable, a full and detailed survey might be required.
  • Where no unacceptable patterns are recognized, the rest of the survey should be comprised of random readings taken at a recurrence of 4-5 readings for every metre length.
  • If there are areas with unacceptable low zones, they need to be marked for remedial coating by the inspector.
The field of fire engineering and the related field of intumescent application is an ever evolving, and highly technical one. Our team at Permax exist to take the guesswork away for you, and to help you achieve the best results and optimal outcomes. To learn more about fireproof coating for steel, consult the specialists at Permax today.

This entry was tagged : Char, "Passive Fire Protection", Tips

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