Research: Furnace Tests

Fire-rated floor and wall assemblies formed with new materials and construction methods are being used increasingly in buildings.

To determine the fire resistance performance of these assemblies, engineers, architects and code officials sometimes require full-scale tests. However, the tests are expensive and time consuming. The National Research Council of Canada (NRC) has been developing an alternative, simpler and less expensive solution. It recently constructed an intermediate-scale furnace that can be used for testing loaded and unloaded wall and floor assemblies.

To ensure that this furnace reflects full-scale test results, it must be characterized. Heat exposure in the furnaces is one of the critical parameters in determining the fire resistance performance of specimens. NRC therefore carried out tests to characterize the heat exposure in both its full- and intermediate-scale fire resistance floor test furnaces.

The tests in each were carried out by exposing the specimens to heat using a propane-fired horizontal furnace. The average furnace temperature in each case followed as closely as possible the CAN/ULC-S101-M89 standard time-temperature curve that is similar to the ASTM E119 curve.

The total heat exposure (radiative and convective) to the test specimen was measured by two Gardon Gauge heat flux sensors, a 2.5-cm diameter and 2.5-cm long copper cylinder, with an accuracy of 63%. The sensors were installed flush to the surface of each specimen and were located at the centre of each half specimen.

Six tests were conducted: three repeat tests using the full-scale furnace and three repeat tests using the intermediate-scale furnace. The duration of each test was two hours and measurements were recorded every minute.

The results (see figure) show that the heat exposure in the intermediate-scale furnace is approximately 15% higher than in the full-scale furnace. The results suggest that the effect of furnace size on heat exposure is significant.

Generally, the heat received by a test specimen in fire resistance test furnaces is by radiation and convection. The radiative part is much greater than the convective part. In full-scale furnaces, the convective heat occurs by natural convection, while in the smaller-size furnaces it occurs by forced convection. Heat transfer by forced convection is greater than by natural convection, which may explain why the heat exposure in an intermediate-scale furnace is higher than in a full-scale furnace.

Development of a non-standard intermediate-scale furnace test is intended to reduce the number of full-scale tests needed during the development phase of an assembly design. It is also intended to lower the cost of testing as the cost of an intermediate-scale test is about 25% that of a full-scale test.

Since heat exposure was found to be higher in the intermediate-scale furnace, tests on floor assemblies using that furnace can be expected to yield more conservative results. However, NRC is now doing further studies to determine the correlation between the results obtained by the two types of furnace on the fire resistance of different floor assemblies.

Dr. Mohamed Sultan is a senior researcher in the Fire Risk Management Program of the National Research Council’s Institute for Research in Construction. Tel. 613-993-9771. E-mail mohamed.sultan@nrc.cnrc.gc.ca