Study_of_Fire_Resistance_of_Wood-Based_Test_Specimens

3.4.2 Study of Fire Resistance of Wood-Based Test Specimens

The inflammability is a wood property given by its structural and chemical composition. For construction purposes the inflammability is a negative property.

The problems of inflammability (Osvald, 1997), of wood and other inflammable materials are in determination of the factors and conditions that influence the process of burning.

Besides the general physical properties of building materials their chemical properties are also important in case of a fire. Both of them together determine the inflammability, the way of flame propagation and the burning speed. From physical properties we can first of all mention the density, moisture content, material thickness and surface quality. The inflammability of inflammable materials depends also on their surface – volume ratio. The higher the ratio the easier it is to ignite the materials and the faster the flame spreads. Sharp edges as well as porous, rough, fibrous or by cracks corrupted surfaces increase this ratio. Concerning the building material behaviour during a fire the external circumstances such as the heating speed, air suply, heat action time as well as existence or absence of the inflammable source (Neuser, 1974) are important.

3.4.2.1 Description of the Experiment

To study the fire resistance four types of wood-based materials were used:

• chipboard with dimensions of 40 x 40 x 15 mm
• laminated chipboard with dimensions of 40 x 40 x 8 mm
• plywood with dimensions of 40 x 40 x 18 mm
• retarded plywood with dimensions of 40 x 40 x 18 mm

To observe the fire resistance of wood-based test specimens the real-time method of holographic interferometry (see chapter 3.1.2) was applied.

The experimental equipment consisted of a holographic variant of the Mach-Zehnder interferometer adjusted to the infinite fringe width a part of which was also a fixture for location of the test specimen and a flame source located under the test specimen (Fig. 3–61).

The first condition for application of the holographic method was obtaining a hologram. We obtained the hologram by simultaneous illumination of the holographic plate by two light beams at homogeneous distribution of the refractive index in the measuring space, i.e. without insertion of the phase object.

After photochemical analysis of the holographic plate it is possible to reconstruct the original object beam that in the real time interferes with the real beam.

Test specimens were heated for 15 minutes by the flame from below and every minute of heating a record was made. With respect to the continual process of the experiment the CCD camera that works immediately and makes the record in the form of bmp files was used.

After finishing the experimental part it was necessary to analyse the interferograms. The temperature orders were counted at all test specimens above their centers, i.e. in the distance of 20 mm from their boundaries. At randomly selected test specimens we mapped the whole surface keeping the distance of 5 mm between the measured places. The numerical values obtained were statistically analysed and the dependencies of the measured data were graphically displayed.