Reconditioning of Collapsed/Washboard Timber
Collapse of sawn lumber during drying is a rare event. Steaming may help prevent the problem, and to some extent cure it. January 12, 2015
Question (WOODWEB Member) :
I am a third year student of Wood -Tech at NMMU Saasveld in George - South Africa. I have found the information on this site very helpful in my studies. I am looking for information on the reconditioning process of collapsed timber. I have searched online for a while but can only seem to find limited information on the subject. I assume that it is executed on a large scale commercially with susceptible species such as western red cedar, hemlock, redwood, and eucalyptus species. Any information regarding the technical aspects of the process (the type of steam chambers used, MC's, schedules, or scientific data available) would be much appreciated. Have any in-depth studies been done on this subject?
Forum Responses
(Commercial Kiln Drying Forum)
From Gene Wengert, forum technical advisor:
Presently, the recovery of collapse is done by aggressive steaming (wet steam rather than steam with super heat, so the steam is boiled through water after it reaches atmospheric pressure). I have seen operations that use six to eight hours of steaming for redwood, but I do not recall any scientific studies. In KILN DRYING OF TIMBER by Koehler and Thelen, 1926, it states "It has been found possible to restore some pieces of collapsed wood to their full size by steaming them at 100 per cent humidity, or nearly so, and 180 to 200 degrees F until they have reached about 25 per cent moisture and then refrying them. For individual shingles the steaming must be carried on for 48 hours, but for shingles in packages and 1-inch or thicker lumber considerably more time is required."
From Contributor V:
Prevention is better than cure. Depending on the species, your drying schedule can almost prevent it. If you end up with less washboard, less steaming is required. Unfortunately, drying schedules recommended for South African Eucalyptus species is so slow that the economic viability of it is questionable.
From Gene Wengert, forum technical advisor:
I have seen several species (including redwood, cottonwood, aspen, hemlock, and eucalyptus - cannot recall the species) collapse during air drying. Sometimes this collapse has been related to bacterial wetwood. The theory about the cause of collapse was that the internal tension created by evaporation from small capillaries was large enough to result in cell collapse. So, slow air drying was attempted as well as slow kiln drying, but without a great deal of success. The hotter and drier the kiln was, the worse collapse was, up to a point. There is some literature that indicated that steaming at 100% RH and 212 F (or close to that) was able to greatly reduce collapse. However, this steaming seemed to affect several strength properties on the long term, so it was stopped. On the other hand, collapse and then steaming at the end to recover collapse probably also affects strength properties, but apparently not severely.
From the original questioner:
Like you said prevention is better than cure, and of the many successful prevention methods including chemical and microwave treatments, none of them were found to be viable commercially. From the information I found on collapse I could not see a difference in strength properties except for the contribution of collapse to severe internal checking. This occurred to high degrees in woods with large differences between earlywood and latewood density. Internal checks close up after steaming, although the hairline cracks will remain thus affecting strength. Collapse prone species weren’t recommended for high stress bearing structural applications because of this. Apparently only drybulb sensors are required to monitor steam chamber conditions. These coupled to the steam source and vents located at the bottom of the chamber via a PLC unit gives control over the process. Though the process itself was found to be fairly simple, the theory and reasons why heat causes collapse recovery was somewhat unclear.
From Gene Wengert, forum technical advisor:
In my previous posting, the steaming I was referring to was done prior to drying in an effort to prevent collapse, rather than try to recover collapse. I am not sure if this was clear. Steaming after drying has been done, but such a treatment is very tough on the building's structure. As you have already found out, the research with eucalyptus is probably the best source of information. There is not much steaming for collapse recovery done in North America, mainly because collapse is not very common in most commercially dried species here.
