Both opportunities and challenges exist in scaled fire tests

Fire tests in e.g. scale 1:5 for can certainly tell us a lot about the tested structures, but there are also differences in relation to full scale tests. This was shown at a technical meeting at IDA Brandteknik (The Danish Society of Engineers, Fire Engineering) in October 2014.

Yet another small piece of the large puzzle known as scaling down of fire tests was described at a meeting of IDA Brandteknik in Copenhagen on October 7, 2014.

Here, civil engineer Martin Sørensen from If P&C Insurance told about his Master’s thesis from the Technical University of Denmark on scaled experiments with sandwich panels. The thesis was completed in cooperation with the insurance company that has great interest in more knowledge about whether scaled down experiments can provide results which can be used in connection with insurance work.

Several fires where sandwich panels with combustible insulation were involved have formed the background of the project. The fires cost If a large amount, and the company therefore decided to examine the fire properties of the sandwich panels in more detail.

– My project is only a small part of the effort. I am building upon a series of full-scale experiments with sandwich panels to see if one can achieve the same results with scaled down fire tests, said Martin Sørensen at the meeting.

Save on costly full-scale tests
The interest in scaled tests is due to the fact that full-scale tests require large test facilities and are expensive to conduct. If the same results can be achieved through significantly smaller tests, then there will be a lot of money to be saved.

And the principle of scaled fire tests is simple. By making everything smaller, both the structures that are to be tested, as well as the fire effect, the temperature sequence and the development of the fire ought in principle to be the same as with a full-scale test in principle. In practice, however, this is not the case in all areas, as Martin Sørensen’s project shows.

The full-scale tests were performed in compliance with the ISO 13784-1 standard Reaction-to-fire tests for sandwich panel building systems, which affects a constructed room of panels with 100 kW for 10 min., 300 kW for 10 min. and 0 kW (burner off) for 10 minutes. However, the standard was extended by the following three “additions” in order to get closer to the situation in reality:

• The tested structures were built by craftsmen and not by fire technicians.
• After the first 20 minutes, the output was increased to 600 kW per minute in order to correspond better to the heavy fire load which is often found in companies.
• The panels were given various major and minor damages to assess the significance of these.

The scaled-down tests were conducted in a corresponding room built up of panels one-fifth of the length and width. Tests were conducted with two types of panels with rock wool and PIR insulation (polyisocyanurate) respectively, which isolates significantly better than rock wool, but is a combustible cellular plastic. In both cases, the insulation was enclosed between two steel plates.

– The 1:5 scaling-down of the test room did not, however, include the thickness of the panels, which is 100 mm. It has a significant influence on the results, says Martin Sørensen.

The gas burner was also physical scaled down to 1:5, but the fire affect must be significantly reduced if it is to correspond to the full-scale experiments. In fact, the effect must only be 1.8 kW in the first period, 5.4 kW in the second period and 10.7 kW in the last period, instead of 100 kW, 300 kW and 600 kW. Nor are the periods 10 minutes long in the scaled experiments, but 4.5 minutes. The measurements included temperature and loss of mass.

Both similarities and differences
– The results clearly show that the thickness of the panels is important for the results in the scaled experiments. The panels absorb more of the heat, so the temperature increases more slowly. However, the final temperature is about the same, said Martin Sørensen.

At the same time, the scaled experiments confirmed a result from the full-scale experiments, which showed that minor damages to the panels such as screw holes do not have a significant impact in the event of fire. The loss of mass of the panels does not increase when the panels have minor damages, so the insulation does not contribute additionally to the fire. This consistency between large and small scales can perhaps pave the way for scaled experiments to assess damaged panels and any repairs.

– However, here it is important to emphasize that the influence of minor damages may not be as significant in relation to the total size of the fire, but on the contrary very important in relation to how a fire can occur, and how quickly it can develop, said Martin Sørensen.

He ended his presentation by concluding that, in general, there is good consistency between full-scale experiments and scaled experiments with regards fire development and temperatures in the room, but that there are differences when it comes to the temperature development in the core of the panel and heat transfer through the panel. In this way, heating­inertia does not follow the scaling.

Scaled tests are good with development work
Fire safety adviser Anders Dragsted from DBI found the presentation interesting, in the light of the fact that DBI also works with scaling fire tests.

– Scaling is an interesting possibility, which we would like to find out how we can use. For example, it would be a good idea to use scaled tests in connection with development work so that our customers can find out if their ideas work before they invest in a full-scale test for approval, he says.

DBI is currently investigating the possibilities for scaling in the project ‘MAT – risk assessment of new materials’.