Some time ago I discussed with professor A.G. Meyers form Rocktest Australia the problem of bushfire on steel mesh.
Prof. Meyers live in Australia where during bushfire the temperatures at ground level can exceed 800oC.
I asked professor Meyers to write something on this topic to be published on the rockfall BLOG......these are his words:
"In areas prone to bushfires, it is possible that at least part of a draped mesh installation may be subjected to a bushfire (see down photos). In such cases, it is prudent to consider how a fire may affect the integrity and longevity of the mesh.
In Australian bushfires, temperatures at ground level can exceed 800oC for periods of up to 120 seconds. The temperatures then tend to decrease rapidly as a flame front passes. Consider how such an event may affect the PVC coated, galvanised wires from which some meshes are woven.
As a bushfire approaches a mesh, the PVC will begin to soften at 75oC and then melt. The PVC will no longer provide any subsequent corrosion protection to the wire.
Zinc is used in a standard galvanising process to provide cathodic protection to the steel. Zinc melts at 420°, significantly less than the temperature of a bushfire. However, free zinc tends to occur only near the surface of the coating. There may not therefore be a significant loss in the total mass of the coating if this free zinc melts.
Most of the coating comprises a zinciron alloy that forms when the zinc reacts with the iron in the wire. The heat of the zinc during the hot-dipping process causes the surface of the steel to become porous at the molecular scale allowing alloying to occur within the pores. The alloy has a melting point of approximately 650oC; a temperature less than that possible in a bushfire. However, as found by Bennets & Thomas (1994), the temperature of 3mm thick steel tends not to exceed 430oC when subjected to a temperature similar to that applicable to a bushfire for a similar time. There may not therefore be a significant loss in the total mass of the coating due to melting of the alloy as the bushfire passes.
Another possible occurrence is that the alloy coating may crack as the volume of the steel increases more than that of the coating due to differences in the thermal expansion coefficients of the two materials. However, cracking may not be too great a concern as the coefficient for the alloy tends to be higher than that for the steel.
Another possible occurrence relates to smoke deposits and humidity in the air that can produce an acidic solution after the bushfire has passed. There can be an increase in the rate at which the coating corrodes if the solution settles and remains on the alloy. However, in most cases, the solution gets diluted rapidly by subsequent rainfall and no longer is a significant problem.
To confirm the above conclusions, samples of wire (see photos down) were obtained from a draped mesh that had been subjected to one of many bushfires that occur in the hills above Adelaide, South Australia. Samples were obtained immediately after the fire and one year after it had occurred. Samples were also obtained from a mesh from the same batch that had not been subjected to a bushfire. The thicknesses of all wires were measured. There was no significant measurable loss of thickness in any wire. The burnt wires were also viewed under a microscope which indicated that there had been no obvious evidence of crack development.
Figure 2. Close up of double twist, PVC coated, galvanised, wire sampled from burnt mesh.
The conclusion was reached that a mesh, woven from PVC coated galvanised wire subjected to a bushfire, is likely to corrode at a rate similar to that for a galvanised mesh without a PVC coating. It is however difficult to ascertain what this rate may be as it tends to vary according to location, environment, thickness and quality of galvanising, characteristics of the steel and mesh installation conditions and details.
Reference: Bennetts, I.D. & Thomas, I.R. 1994. Developments in the design of buildings for fire safety. Preprints of papers in Australasian Structural Engineering Conference (ASEC94), IEAust., Vol 2. Sept. 1994: 640-645."
Tank you to Peter for this interesting text that allows us to increase our knowledge in this field.
Thomas
