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CROSS Safety Report

Intumescent paint application

Report ID: 1113 Published: 6 June 2022 Region: CROSS-UK

This report is over 2 years old

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A report has been received regarding issues of detailing by following the relevant advice provided from an intumescent paint supplier. The reporter was incorrectly advised by a supplier that the paint would still be effective if it was applied to steel and then closely covered, leaving no gap for the expansion of the intumescent material.

Key Learning Outcomes

Structural and Fire Engineers:

  • Engineers should give consideration to and understand the limitations of the materials, products, and systems they specify

For manufacturers and suppliers:

  • Ensure that the people providing advice and guidance to clients are operating within the limits of their knowledge
  • Give thorough consideration to how the product might be used and provide appropriate guidance

For the construction team:

  • Be aware that passive fire protection components should be installed in accordance with the manufacturer’s specifications
  • Consider introducing a quality assurance process that covers the correct use and installation of fire protection products and components

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Intumescent coatings

Thin film intumescent coatings are paint-like materials used on steel sections to provide fire protection to the structure. At ambient temperatures they are inert, but at elevated temperatures they follow a series of chemical reactions that leads them to expand, forming a layer of low conductivity char that has the ability to insulate the steel section. This insulating function protects the structure by “keeping it cool”, ensuring that the degradation of material properties that occurs at higher temperatures is reduced and the structure can carry its design load successfully during and after a fire.

It follows that for an intumescent paint to be successful in its role, it needs space to expand when heated in order to form that protective layer. If the boundary condition is such that facilitates a local or global increase in the steel section’s temperature, then its ability to carry the design load can be compromised.

Cause for concern

Engineers need to consider infill solutions between steel frames and eventually detail how these may be connected to the intumescent painted steels. A reporter informed CROSS that an architect sought advice on detailing from a supplier who claimed that an intumescent paint does not need space to expand. The supplier responded to the architect confirming that (a) the steels will start to intumes[ce] up at 70 degrees and be fully active at 120 degrees, and (b) there is no scientific evidence to suggest a space between the steels and a covering is relevant. 

It should be highlighted that the supplier incorrectly stated that 'the steels' will intumesce, however this is a property of the paint. The reporter is also of the mind that this is inaccurate and incorrect advice, citing a publication from the Steel Construction Institute which has the highlighted statement that 'finishes must be detailed to allow room for expansion of the intumescent coating' (page 22). Any seller 'guidance' can only be given on the basis of appropriate technical justification or physical testing, otherwise any answer is ‘poor advice’. 

The reporter considers that this example should be picked up by structural engineers, who need to be aware of this issue. They should then be advising and clarifying to architects why the intumescing zone should remain free of infill materials, except when a formally recognised heat sink, protective board, or any other tested solution of proven efficacy is present. This way, any careless detailing that could render the intumescent non-operational will be avoided early on in the design process and highlight areas where a combination of fire protection solutions can be more appropriate.

Expert Panel Comments

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In the panel’s experience, it is considered common knowledge that an intumescent paint must have room to expand significantly in order to be effective in insulating the structural element. One such description of an intumescent coating is that it is 'swelling up when heated, thus protecting the material underneath in the event of a fire'. If anything stops it from doing so, it will be ineffective. Such remarks are also commonly found in the market literature.

Covering boards, and any kind of secondary structure, need of course to connect to the primary structure in some way. Brackets of many forms may be connected to the primary steel member, and their effect on the intumescent paint expanding will depend on the nature and size of the clip fittings. It is expected that any responsible manufacturer should have a tested detail for this scenario, and sharing that information with clients will allow the design team to give appropriate consideration on how products can be used. The manufacturers’ guidance needs to be clearer than what is presented by the reporter, and it seems challenged when the logic seems at fault.

A similar issue which also is encountered frequently is where there is a junction between, for example, a boarded fire protected column and a beam protected through an intumescent coating. Testing of assemblies that incorporate different manufacturers’ products is rare, and common practice is to overlap the protection systems; this also requires careful detailing to ensure that every solution performs its intended function successfully.

The panel also has recent experience in which the uncertainty around a specific chosen solution was resolved through testing, since no other form of information on its expected performance was available.

In parallel to this case, a very common issue seen in the retrospective use and application of intumescent paints is that they are very often seen as an easy solution and an easy way to achieve the necessary fire resistance requirements. However, their application can often be misunderstood in regards to:

  • The prescribed application method e.g., number of coats, base layers, mid coats, topcoats, and drying times.
  • Humidity and moisture levels that dictate when intumescent coatings can be applied.
  • Suitable substrates of application.
  • UVA/UVB degradation and any required reapplication times.
  • Physical damage and the need to reapply a coating.
  • What any test certification means and what an intumescent coating’s performance in fire actually is.

Good practice comes down to following manufacturers' instructions and ensuring the correct product is specified. There is, however, a need for the supplier to sell responsibly as well.

Once in situ, there is a need for the Responsible Person to be provided the Regulation 38 information under the Building Regulations (as amended). This will assist in the production of a suitable and sufficient Fire Risk Assessment under the Regulatory Reform (Fire Safety) Order 2005, with these products identified and managed appropriately.

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