Wood decay and resulting strength loss is arguably one the most common issues when evaluating wood and wood-based elements in service. Whenever I set out to inspect a structure one of my first questions: Is there any wood decay present in the elements inspected? If so, then what's the prognosis? The most eye-opening fact about wood decay is the level of strength loss that can result from just 2% to 10% weight loss. Destructive testing is the only method available for determining the actual strength loss associated with wood decay. However, destructive testing is not always an option, for practical and/or economical reasons. The next most practical option is performing microbiological analyses. These analyses can be a very useful and effective as part of a wood pathology study of a structure.
Academically, there are four stages of wood decay: incipient, early, intermediate and advanced. Incipient is simply the "unseen" form of wood decay. This stage is typically considered to occur within the 2% to 10% weight loss level. The next stage is referred to as the early stage of wood decay. This stage generally represents slight changes in the color and texture of the material. Personally, I tend to group the next two stages together: the intermediate and advanced stages of wood decay. These are perhaps the easiest to detect because they are visibly evident when inspected and are represented by distinct changes in color and texture.
Basically, whenever you can see wood decay you can expect significant loss in properties. Photo 1 illustrates examples of intermediate and advanced wood decay. When a structural wood element has intermediate to advanced wood decay the generally accepted practice is to assume there is no wood strength remaining, and replace the element.
For structural assessments of wood elements the most critical stages of wood decay that need to be identified are the incipient and early stages. During the incipient stage when wood weight losses range from 2% to 10% significant reductions in mechanical properties can occur. For example, bending strength can vary anywhere from 15% to as much as 70%. A qualified inspector has the ability to identify these two stages of wood decay using a method referred to as a pick test. The critical issue associated with incipient and early wood decay, once identified, is determining the actual reduction in strength properties associated with the levels present. The only method that can be used to determine actual strength loss is destructive testing. However, this is not often a practical or economical option. This is when extracting increment cores and performing microbiological analyses can be very effective.
An increment core is simply a 0.2" diameter pencil core. The bit used to extract a core is hollow having an inside diameter of 0.2". The outside diameter of the bit is 1/4". The process of extracting an increment core is slightly destructive in that a 1/4" hole is made in the element being examined. However, a 1/4" hole does not cause any appreciable reduction in strength and the hole is always plugged. Once a core is extracted it is examined using a light microscope. First, very small and thin sections of the core are cut using a razor blade along the core which are then placed on microscope slides. The slides are examined with a light microscope at magnifications up to 400X. When the sections are examined the actual wood decay hyphae (i.e. root-like structures) and the effects of their attack are observed, recorded and photographed. Provided in photo 2 below is an example of structural element where incipient wood decay was detected using the pick test. When the element was examined microscopically incipient wood decay was verified. Based on the results and effective cross section was determined for the structural analysis being performed by the engineer of record and a repair was determined.
As mentioned in other blogs, when performing resistance drilling using a Resistograph, microbiological analyses are always extracted to verify the location of sound wood so that an engineer can make the necessary recommendations for any structural repairs.
In summary, microbiological analyses are a simple and effective method for assessing the level of incipient and early decay in wood elements. These analyses are nearly always an integral part of every structural assessment and wood pathology study performed at Wood Science Consulting.