means that deep sections of the timber element can still increase their temperature as the thermal wave continues progressing after self-extinguishment. Consequently, mechanical properties and structural stability can change after the flame has extinguished, with timber elements often most vulnerable to structural failure in the decay phase and beyond into the cooling phase of fires. Smouldering combustion Smouldering is a slow, low heat release rate and low- temperature type of combustion, where oxygen reacts with a solid, eg wood 20 . The reaction of oxygen with the solid matrix of the smouldering timber produces the characteristic glowing in the char. Smouldering usually appears at corners, connections, interfaces, fissures and similar localised areas. Recent research conducted at Imperial College 21 , has focused on the smouldering phenomenon happening on large timber structures after flaming combustion has been extinguished. This degradation process took place for several hours and days after flaming combustion had stopped. In consequence, localised smouldering penetrated through the CLT soffit, and on some occasions, it created the right condition for the re-ignition of local flaming combustion. On other occasions the smouldering self-extinguished. The work conducted by Crielaard 22 indicates that self- extinguishment of smouldering combustion in timber elements will happen below certain threshold values of external heat flux and airflow. These thresholds are orders of magnitude lower than for flaming combustion. Given the complexities associated with the construction of modern buildings, eg tolerances, installation of services, etc, conditions can exist that support continued smouldering in mass timber buildings. Therefore, designing on the basis of smouldering self-extinguishment is often impractical and challenging to evidence. Heat induced delamination (HID) During a fire, heat may propagate deeper into a timber member, such as cross laminated timber (CLT), and heat a glue-line. At this point, delamination at the glue line is possible. This is a phenomenon that consists of the
mechanical failure in the glue line of an engineered timber product like CLT, where the external lamella detaches and exposes preheated virgin timber to the fire. It is very likely that the newly exposed surface will ignite and burn at a faster rate as there is no longer a char layer to regulate the energy reaching the pyrolysis front. Its occurrence increases the average charring rate of timber elements and may challenge self-extinguishment. It also reduces the timber cross section faster, affecting its ability to support loads. representing commercial enclosures have demonstrated that self- extinguishment is possible despite delamination for the tested configurations 23,24 averting delamination is often imposed as a limitation on the design of CLT elements, especially where interactions between combustible elements can occur. Although, large-scale experiments Delamination is a complex phenomenon, and to date, it is known that several variables will affect its occurrence. These variables are the type of glue, lamella thickness, heating conditions, moisture content and imposed loads on the elements 25,26,27,28 . Currently, the most typical glues used in Europe are polyurethane (PUR) glues and modified PUR glues (ie with added fire retardant). Microscale experiments showed that the commonly used polyurethane adhesive experiences softening at 220-240 ⁰ C which is not the case for modified
Pyrolysis
Char Oxidation
Drying
100%
75%
50%
25%
0%
100
200 300 400 500 600
Temperature (°C)
Figure: Thermal decomposition of timber as a function of temperature
Thermal decomposition of timber as a function of temperature
66
F I R E
Made with FlippingBook - professional solution for displaying marketing and sales documents online