In a prior blog about nondestructive testing and the use of a Resistograph I discussed how effective I have found these drills in my line of work. I also touched on the importance of supplementing Resistograph results by extracting increment cores and performing microbiological analyses. These drills are very effective at locating voids and intermediate to advanced wood decay. However, they are not effective at detecting incipient and early wood decay. The reason this is important is because of the strength loss that is associated with the earlier stages of wood decay. For example, reductions in bending strength could be as little as 10-15% or as high as 70-75% at wood weight losses from 2% to 10%. That's not much weight loss compared to the potential strength loss. So as a general rule of thumb, it's always a good to take an occasional increment core to verify that the Resistograph charts which are being collected are indicative of sound wood.
Just recently this philosophy was put to the test and I'm glad I extracted three cores. I was working on a project assessing the condition of glulam arch heels in an older building that was recently renovated. The structure had a long history of uses, but presently an active moisture issue was found to be associated with a few of the glulam arch heels in the structure.
To assess each of the heels, a grid pattern was marked on each one which was used to perform Resistograph testing and to record moisture content levels. The grid was marked across the depth of each element at the 1', 2', 3' and 4' heights. A summary of the results is shown on the element in the photo above. In summary, the results of the testing were found to be fairly consistent in three of the four elements inspected. Moisture content levels capable of supporting wood decay were typically recorded at the 1' height. At the 2' height an occasional MC level capable of supporting wood decay was recorded, otherwise the conditions at 2' height were considered relatively dry. At the 3' height and above all MC levels recorded were dry. All the drilling charts collected at the 1' height indicated the presence of advanced wood decay. At the 2' and 3' heights the charts were fairly consistent (i.e. indicating sound wood), except for the occasional inconsistent chart at the 2' height. At closer examination of the 2' and 3' drilling results there was a slight, but noticeable difference in amplitude levels in the charts. A decision was then made to collect three increment cores: one from a 2' height, one from a 3' height and one from a 4' height in one of the elements drilled. A microbiological analysis was performed on each core from the outer surface to the center of the arch. The results were revealing. Early to intermediate wood decay levels were observed throughout the depth of the arch at the 2' height (see photo below). However, at 3' the wood decay levels diminished significantly. At 4', no evidence of decay was found.
Based on the microbiological analyses and Resistograph testing an accurate representation of the arch heels was developed and a proper engineering repair was implemented. Without performing the microbiological analyses, an inspector may have been inclined to report that the 2' height was sound wood, but at closer examination of the charts and performing a couple microbiological analyses revealed a different and more accurate result.