The potentials of our concrete

As Bendt mentioned in his latest post, CRC i2 elements have been used in projects that experienced fire on three occasions, luckily all during the construction phase without any serious injury to people, and in all cases, the CRC i2 elements could be used after the fire with only minor repairs or cleaning.

In addition, a more comical fire related incident: An apartment tenant once got the bright idea to place a charcoal chimney starter directly on a CRC i2 balcony … why I do not know!

However, let us begin with the three actual projects that experienced fire during the construction phase:

• November 26, 2012: Sydhavnsskolen, Copenhagen (DK)
• September 22, 2016: Heilig Harn, Den Helder (NL)
• September 29, 2016: Strandingsmuseet, Thorsminde (DK)

Of these, only the fire at Heilig Harn resulted in significant fire exposure of the CRC i2 elements – so I’ll begin with the Heilig Harn fire, and the rest will follow in next week’s blog post.

Den Helder, September 22, 2016

During the late stages of the construction phase of the very large renovation project Heilig Harn, foam insulation in a high gate opening was accidently ignited and developed rapidly into a fierce fire. The fire was located under CRC i2 balconies with parapets (covered completely by the smoke in the image above) exposing the balconies to the full effect of the fire for a duration of app. 10 minutes before the fire dissipated.

After the fire, Rogier van Nalta from the main engineering company on the project Pieters Bouwtechniek inspected the site, and he could observe that the aluminum spacers used to fix the foam insulation had melted during the fire. The melting point of aluminum is 660°C, so the fire reached at least this temperature.

  Molten aluminum spacers (l) and over-all gate roof impression after the fire (r), Images by Rogier van Nalta, Pieters Bouwtechniek

Despite the intense and rapid heat development, the fire-affected underside of the CRC i2 elements had only suffered minor surface spalling a few mm deep (not explosive, mind you) and could remain in use after cosmetic surface repairs and cleaning.

Underside of fire affected CRC i2 Balcony showing superficial (few mm) spalling – the reinforcement is not visible despite the very low cover. Image by Rogier van Nalta, Pieters Bouwtechniek.

This outcome of the fire was in line with the results from the full-scale fire test in Finland, mentioned by Bendt in his latest post. In the lab test, the balcony underside also experienced spalling, but again not explosive and to a limited depth, despite exposure to a standard fire for more than 120 minutes, with temperatures reaching more than 1000°C in the furnace:

     Underside of a CRC i2 balcony after testing at TUT in Finland. Furnace temperature curve and test duration embedded. Spalling is limited (reinforcement not generally visible despite very low cover) even after the 2 hours and 4 minutes test at full design load, without indications of explosive spalling.

At the Heilig Harn, the high conventional concrete columns in the gate opening also caught the full effect of the fire, experiencing significant spalling:

Regular concrete column after the fire. Image by Rogier van Nalta, Pieters Bouwtechniek

After cleaning away damaged material and reassessing the retained structural capacity against the design loads, it was determined, that with the casting of a new cover layer the columns could remain in use.

   Regular concrete columns after cleaning away damaged material in preparation of repairs. Notice the exposed main reinforcement. Images by Rogier van Nalta, Pieters Bouwtechniek.

So in the end, the actual damages to the structural parts of the building was luckily – and by good design – very limited. You can see images of the completed Heilig Harn project at our website.

To summarize it is – luckily – not often that I and the other great engineers I work with get a chance to see how the concrete elements we design, UHPC or conventional, perform during fire. So on those rare occasions it is important that we learn from the experience.

In the Heilig Harn fire, molten aluminum proved that the fire temperature reached close to 700°C within 10 minutes, matching the standard fire curve quite well. This of course is a complete coincidence but underlines that knowledge gained in full scale testing under realistic conditions is very important and relevant – especially when using novel materials such as UHPC – Because general codes based on a long proven track record of previous events and accidents does not exist for UHPCs yet.

In the fire at Heilig Harn, the CRC i2 balconies performed well – without explosive spalling. This combined with the knowledge from the laboratory test that the elements could have withstood even higher temperatures and for a much longer fire duration, suggest an answer to the question in the headline of Bendt’s latest post: Fire resistance – is it a problem for UHPC?:

With sufficient knowledge and documentation, the answer is NO!

Next week I will present two more fire cases from actual projects, but invite you to continue the discussion of fire and UHPC below.

Tommy Bæk Hansen
Group Product Development Manager
tbh@hi-con.dk

Read about Tommy’s 20 years of experience with UHPC right here