11 Sep Passive Fire Proofing. Are Cracks in Cementitious PFP okay? Maybe…
“If we worked on the assumption that what is accepted as true really is true, then there would be little hope for advance.” Orville Wright
When I was a budding journalist back in the day of typewriters, deadlines and police scanners, we were taught to question everything. There was a saying back then that went, “If your mother tells you she loves you, check it out.”
And that’s how I run my consultancy, Chicago Corrosion Group. Where others make assumptions, we check it out – always from the perspective of what’s in the best interest of the client.
And having been in the coatings, corrosion and associated industries for more than half of my 57 years, it takes a lot to surprise me. But I was recently stunned after meeting with a coating company who said they had some interesting data to share regarding passive fire proofing. They are a material supplier, and I have a healthy skepticism when speaking with well-intended, highly competent individuals who are occasionally overly enthusiastic about representation of their products. But, the bulk of the meeting was spent discussing test results and had little to do with any products or services they were selling.
The individual leading the presentation holds a master’s degree in fire and explosion engineering. Further, he appeared to be one of those rare technical people who was concerned with one thing and one thing only – facts. The credibility he brought to the meeting, and the data, was invigorating.
First – let’s break down what passive fire proofing is within the context of this blog.
For the purposes of this blog, passive fire proofing (PFP) will only consider cement-like materials, that are placed on steel beams. These materials serve one and only one purpose – to keep the steel cool (relatively speaking) until the fire can be put out.
A structural steel beam begins to soften at around 400°C and loses about half of its strength at around 650°C. A column in compression will lose around 70% of its strength at roughly 600°C. PFP is applied to steel beams and columns to keep the heat of the fire away from the steel for a given amount of time. The amount of time is based on the anticipated temperature of the fire, how quickly it can be put out, the type of fire (yes, there are different types, such as pool and jet) among other considerations. The thicker the PFP, the longer the beam can withstand the heat.
As was explained to me, research was carried out on 1.2 and 1.5 meter columns, which would be exposed to the UL1709 temperature curve that is the standard used globally for downstream facilities.
(Designing these systems and installing and maintaining them is fantastically complex and costly. This blog should not be used as a guide.)
PFP systems are absolutely critical for the safe operation of these types of facilities.
It’s one thing to have to battle a fire that’s burning at a refinery where the steel structures remain structurally sound and in place, and quite another if complex frameworks of steel, pipes, pumps and tanks start to buckle and fail, allowing beams to fall and pipes and tanks to rupture.
In fact, and while somewhat open for debate, the most widely accepted reason the World Trade Centers collapsed was not from the initial impact of each plane, but from the heat generated by the subsequent fires, which eventually weakened the steel beams to the point of collapse.
The problem with these types of cementitious PFP systems are manifold. They are difficult to apply, can lead to corrosion under the passive fire proofing (CUPFP) issues, and they also tend to crack – particularly in climates with freeze thaw cycles.
The initial purpose of the fire testing presented was to try to quantify the defect type for existing PFP. Currently, evaluations are predominantly qualitative in nature, that is, educated engineers walk around and poke, prod and visually assess areas of PFP and indicate, in their education opinion, which should be replaced and which are acceptable.
The goal of the testing was to try to quantify and identify these defects. That is, instead of engineers looking at areas that were cracked and assuming all of those areas needed replacing, perhaps there was a way to determine at what point does a crack threaten the efficacy of the PFP system.
The implications of quantification of these cracks and other distress is enormous. If a facility has, say, 500,000 square feet of PFP and 40% is exhibiting some signs of cracking, and an engineer deems that all cracked PFP needs to be replaced, that translates into repairing or replacing 200,000 square feet of PFP. This could take years and cost many millions of dollars.
However, if the same engineer was able to determine, based on testing data, that cracks of a specific width, depth and condition did not detract from the performance of the PFP, the owner might have substantial opportunity to conduct repairs or, as data seems to indicate, that no repairs may ever be required.*
What’s the skinny on the cracks?
The testing, and I’m paraphrasing here, were carried out on newly cast 2” thick concrete which had been intentionally distressed with cracks as large as 709 mils. The testing, so far, indicates that the cracks had little adverse affect on the performance of the overall PFP system.
The data seems to indicate that PFP will still perform acceptably even when exhibiting distress which, heretofore, would have been considered cause for repair or replacement.
This is a very, very big deal for owners.
The most incredible aspect of the testing, from my perspective, is that it’s being carried out at all. And to the best of my understanding, unique.
Further testing will be forthcoming on additional columns of steel with newly applied cementitious materials as well as columns that have been in actual service for various amounts of time. The consequences and results of the testing could change the industry. It may be the case that cracks of a certain type can be repaired with different types and less costly materials, with no degradation of PFP performance.
At a time when companies are constantly searching for new and “innovative” materials and techniques, it’s refreshing when innovation is applied in creative ways to challenge assumptions and provide owners with options they otherwise might not have.
*This does not apply, of course, to the issues associated with corrosion in the presence of a crack.