Testing Material that may be Subject to Dry-Rot 3

[dik]

I'm looking for deterioration of Douglas Fir that may have been subject to dry-rot. Wood materials can have a loss of strength with little apparent visable damage after dry-rot.

I know of lab testing devices that impress a small sphere into material to determine the hardness and hence a measure of the strength. The material is approximately 40 years old, so sound material should be relatively hard.

As a quick check in the field, does anyone have any comments about using a Schmidt (Swiss) hammer to test for relative hardness? Can anyone suggest a quick and inexpensive NDT for wood?

Dik

 

[Ron]

dik...you're on the right track.

There are two NDT methods that I know of for mechanical property testing of wood. One is the Pilodyn penetration test which is very similar to a Windsor Probe for concrete. The other is a Janka Indentation test, similar to a Brinell hardness test for metals. I think the Janka test is only available as a lab test, not a field test, but not sure about that.

Since concrete testing equipment is much more readily available than wood testing equipment, I would lean toward correlating a Windsor Probe or Schmidt hammer approach by taking relative readings between known good and expected bad materials.

Another method that I often use is to cut a small, inconspicuous core (1/2" diameter is large enough) and look at it microscopically.

 

[MikeHalloran]

Just stab it with an icepick.


Mike Halloran
Pembroke Pines, FL, USA

 

[Ron]

Stop it, Mike! That makes way too much sense.

Unfortunately we have clients who want something quantified, even if the quantification is ultimately no better than kicking the tires. I'm sure dik would be satified with the ice pick for his purposes, as would I....but the ice pick doesn't play well in court.

 

[berkshire]

Another method that I often use is to cut a small, inconspicuous core (1/2" diameter is large enough) and look at it microscopically.
Lab checks are not cheap or quick but they are definitive. Core samples are the only way of really knowing.
Stabbing the wood will only let you form an educated opinion. If you are dealing with genuine dry rot ( Serpula lacrymans ) even wood that looks good and stands an ice pick or screwdriver can be compromised. The stuff has a nasty habit of going down the center of the beam.
B.E.

"A free people ought not only be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government."
-George Washington, President of the United States----

 

[dik]

Thanks gentlemen... Ron's correct in that I want to be able to quantify the results a bit. I usually use my Swiss Army knife... don't leave home without it! Problem is the glulam arch can be hard and almost inpenetrable with the SAK... and lumber can lose 20%-30% of its strength due to dry-rot (aka brown rot) and still appear to be the same as 'real' wood.
I have an incremental borer to take samples with if needed. There is also a 'core' type drill for a cordless that can be used to take a deeper sample. First step is to see if original design conformed to the code... then onwards.

I've not used an icepick in court... but, I have used spagetti to illustrate compression members and the effect of bracing...

Dik

 

[dik]

Birkshire... there are several types of 'dry-rot' that affect several types of trees... the one you mentioned is one of half a dozen common ones. The term comes from a fungi that 'attacks' the cellulose or hemi-cellulose material of the wood, leaving the harder, more brittle lignin component behind... This causes the area to darken hence 'brown rot'.

Because the brittle lignin crumbles as a 'dry' material, the common term is dry-rot.

Dik

 

[dik]

Ron and Mike... should have added that as soon as an icepick works, you may have lost half the strength... Catch Chapter 17 of Stirlings 'Encyclopedia of Wood'...

Dik

 

[berkshire]

Dik,
I realize that, also the one I mentioned (Brown rot ) is not often found in the USA, it is the name of the stuff I used to find in the UK, and was the first one that popped into my head, but most of this gender of fungus are found where the relative humidity is a little higher than normal. Finding this stuff in Glulam beams or in my case aircraft main spars on wooden aircraft for the most part means the spores were present when the spar/ beam was fabricated. And they are just looking for an opportunity to germinate.
Good luck with this one.
B.E.



"A free people ought not only be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government."
-George Washington, President of the United States----

 

[dik]

What many people don't realise is the 'dry-rot' is generally caused by a brown rot fungus. It's very common in North America... as noted, it gets its name from removing the cellulose cells leaving the darker lignin.

The spores are very common... I think the ones affecting the glulam arches came in on the 'back of the cat'... they are everpresent. They need the proper environment to 'grow'. In this instance, there were problems with the building envelope.

As damage to the spar is an issue, you are likely aware of how much strength and toughness you can lose without significant visual damage or change in hardness.

Dik

 

[msquared48]

Douglas-Fir has a propensity to form a hard layer on the outside and decay from within, undetected. An ice pick may not be enough here (Sorry Mike and Ron). Structurally speaking, it will retain a good portion of it's strength in bending, but far less in shear, when this happens.

You really need to drill into the member with a 1/8" diameter drill, see what material comes out, and what resistance you engage. If it is soft in the center, you will feel it.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Ron]

Mike...many species have the same propensity....to rot in the center and leave an intact shell. I've seen laminated timber structural members (curved bents) that were rotten on the inside but showed no indication from the ouside. In those cases, I used pulse-velocity techniques (low frequency, generally used for concrete) to determine the voids. I've also tried infrared thermography but all the planets and stars have to be aligned for that to work in this application.

I agree that an ice pick doesn't work for everything, but it does work for advanced conditions and on small cross sections, such as typical wood framing. Your drilling approach is a good one and works, as does a manual core boring tool.

 

[dik]

I want a quick analysis to see if the arch works for specified load... then, I'll consider drilling it... and then 'plugging' with epoxy.

I've not encountered glulam with voids and a hard shell... sawn timber, yes...

Railway ties are often hollow in the middle because of the exterior creosote treatment... sawn timber because the damage occurred while they were still alive and a 'wounded' area was isolated from the rest of the tree by extractives... one of the main reasons for hollow trees... When a tree is damaged, it immediately sends out chemicals above and below the 'injury' as well as to the annual growth rings... these try to prevent organisms from progressing beyond injured site.

Dik

 

[berkshire]

Dik
One of the things with Glue Lam beams, especially the ones done with Phenolic resin glue, is that the glue sometimes stops the migration of the fungus across the section of the beam, resulting in damage in one section only along a part of the length of the beam.
Very hard to find, And with my stuff the only cure is to remove it.
In old wooden aircraft, the mainspars are Scotch pine, Sitka spruce, or Douglas fir, and are quite often laminated construction essentially a gluelam beam.
In your case since your beam is not as highly loaded, can you repair with clear penetrating epoxy? I realise you have to find out the extent of the damage before you can answer that
B.E.

"A free people ought not only be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government."
-George Washington, President of the United States----

 

[dik]

Berkshire: The repair I'm proposing is one I've used in past... A cementitious granular material for bulk and an epoxy binder to secure it to itself and the existing wood. This is because of the volume of material that may be affected to minimise cost. It is also a stiffer 'mix' and can be placed into the cavity created. This is supplemented with steel 'pins' to mechanically secure it to the existing wood.

The approach is from some research work for a dentist about 40 years back with a problem he was having with amalgams failing by what was determined to be a tensile fatigue failure. I had no idea that amalgams were sometimes held in place with steel pins cemented into the remaining tooth.

I've not encountered the glue line as being a barrier, but, it is possible. It's the natural anti-fungal extractives that provide a bit of a barrier for rot for living trees. Most of the issues with 'hollowed' sections have been related to material that was compromised when it was sawn.

Dik

 

[dik]

Berkshire: do you have any information on the loss of strength with the varying degrees of damage caused by rot?

Dik

 

[berkshire]

Dik,
I do not have any quantitative information on this. On an aircraft wing one method of detecting damage or rot is to shake the wing to get it oscilating, the wing will have a known natural frequency, any lower will indicate hidden damage.
Since the object of the exercise in my case is not to have any damage, any rot found, is cut out and replaced with new wood as soon as it is detected. or the entire spar is replaced. I do not know if Forest Products Labs have done any work on this, you might check with them.
B.E.

"A free people ought not only be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government."
-George Washington, President of the United States----

 

[connect2]

I use a wood awl and also a coring device, it's destructive I guess but the bore hole into the wood is 3/16" in dia.and you get a core with it when you extract the bit. you can detect areas to drill by just sounding the member with a hammer but i use a bar because it 'rings' so to speak and you can fell it's vibration in your hand

 

[dik]

Connect2... I have an incremental borer that I use for sampling, it takes a 3/16 approx core and may do that... I prefer my SAK to an awl... it seems to 'slip' between the wood grain better... I hadn't considered using impact from a hammer... I've tried a Schmidt hammer in past... but not for checking for rot, but just to see if is provides a consistent 'strength'... and nothing serious or scientific... just found, out of curiosity, that it seemed to work OK, and may suggest it be used for this project. It provides a consitent 'thump' and can provide a number representative of the hardness... A soft spot will be significantly different. It won't damage the material, either...

Dik