Need for Nondestructive Evaluation (NDE) in the Detection of Decay in Structures

Examples of the need for nondestructive and remote sensing technologies for evaluating early stages of decay are presented. The need is critical to develop methods of analyzing internal decay, at the center of large wood members, and decay behind concealing coverings. Current technologies are reviewed and their inadequacies discussed. Acoustic emission and acousto­ ultrasonics appear to hold the greatest promise among existing technologies. Sometimes, nondestructive evaluation (NDE) is not needed to determine that a potentially serious decay problem exists in a building. If the plywood siding, a hand railing, or a major glulam beam sprouts mushrooms (fig. 1), for example, you can be pretty sure you have a decay problem on your hands. Unfortunately, those are rare occurrences. White mycelium growing on the surface of wooden structural members in moist areas (fig. 2) is also a pretty good clue that you should be concerned about the structural integrity of the member involved, but that, too, occurs relatively infrequently in structures. Once the structure is opened and advanced Figure 1--Mushrooms on exterior glulam (courtesy of W.A. Dost). Presented at the Symposium on Current Research on Wood-Destroying Organisms and Future Prospects for Protecting Wood in Use, September 13, 1989, Bend, Oregon. Professor of Forestry, Forest Products Pathologist, Forest Products Laboratory, University of California at Berkeley, Richmond, California 94804. Figure 2--Mycelial mat on joist and underside of plywood subfloor. decay is revealed, little additional information is necessary to assess the structural impact of the decay in many cases. But, in the majority of cases of wooden structures with water problems, visual indicators are not present and some form of internal assessment must be incorporated. Two broad categories of internal decay are encountered in buildings. One is where wood is exposed to the weather and is sufficiently open that the surfaces of the wood members remain dry enough that any decay occurs inside the member without telegraphing to the surface (i.e., internal decay in exposed members). The second circumstance is where wooden structural components are contained behind surface finishes which effectively conceal any evidence of the decay process. Here are some examples of both such circum­ stances. An A-frame structure supported on glulam beams which extend beyond the perimeter walls and roof-line allowed the glulams to be subjected to internal decay (fig. 3). No external evidence of decay was apparent, but probing revealed sufficient softness to warrant concern for the structural integrity of the members. A combination of increment boring and ultrasonic pulse timing provided sufficient evidence of advanced decay to convince the engineer that reinforcement was required (fig. 4). Elsewhere, the entire Indian Valley College campus was temporarily condemned while exposed glulam beams were investigated and a structural solution was USDA Forest Service Gen. Tech. Report PSW-128. 1991. 7 Figure 3--Glulam and connections exposed to the weather. Figure 4--Internal decay was found by boring and ultrasonic pulse timing. developed. This design proved particularly difficult, since glulams played such an important role in all structural aspects of the building, from lateral and vertical support to floor and roof support, blocking and decorative trim. However, even glulams used as nonstructural elements for their esthetic appeal can constitute life hazard if they fail. Decay initiated in exposed parts of large members may extend past the perimeter wall, through load-bearing areas and into the interior of the structure. Although evaluation of internal decay in exposed members currently presents significant challenges, at least it is possible; evaluation of decay which extends into the structure is often impossible without major demolition. These are two different challenges for NDE. For some time, there was a significant tendency on the part of designers to expose glulams to the weather on the outside of the building for esthetic reasons. We now know that this is not a good idea. Once the major check system develops in the top of a glulam in exterior exposure, it appears to be doomed. Even solid timbers exposed to the weather, with numerous contact points and bolt holes (fig. 5), present a significant hazard, because water is trapped long enough to be absorbed. Figure 5--Solid timber with internal decay around a bolt hole. In both of these cases (exposed glulams, and timbers with exposed joints and fastenings) it might be justifiable to just call for removal in any structure over six years old, and you would not be wrong too much of the time. However, because you would end up removing some perfectly sound wood if you operated that way, and also because the people paying for such treatment usually demand fairly firm evidence of a problem before being willing to expend the large sums of money involved, some sort of internal property evaluation is necessary. As far as destructive sampling goes, if it turns out to be decayed, it does not matter that you have put a rather large hole in it. However, if it turns out to be sound, then it would have been better to have had a nondestructive method of analyzing the interior of the member. Sometimes the life hazard involved in the fail­ ure of an individual wood structural member is so great that a means of continually monitoring its strength, as part of preventive maintenance, would be an extremely valuable tool. This is another role which NDE could play. In all the previous examples, at least portions of the wood structural members were exposed to view. However, often, structural members are covered with a material which effectively conceals any evidence of the existence of a decay problem. Stucco probably is the most troublesome, and common, example of such covering. In one case, glulam beams and solid wood columns were encased in stucco and, in the corners, were affixed with planter boxes containing their own irrigation system. Unfortunately, this system irrigated both the soil and the supporting wood structure, causing decay (fig. 6) which was not at all evident behind the stucco (fig. 7). Although the prior example was inside a structure, stucco is most commonly used as an exterior finish. Both inside and out, stucco is just as concealing and, at the moment, requires actual demolition in order to assess the condition of wood beneath it. Remote, nondestructive assessment is truly needed where stucco covers structural wood. An elastomeric compound, which mimics stucco, failed miserably when applied directly to plywood siding. Because it acted as a membrane, the cracks which de­ veloped let water in but the uncracked portions prevented it from getting back out. Therefore, it was both part of the decay problem and the concealment of the problem. 8 USDA Forest Service Gen. Tech. Report PSW-128. 1991. Figure 6--Decayed beam which was behind stucco as in Fig. 7. Figure 7--Stucco wall showing no external evidence of decay behind it. At the time of this writing, the most reliable field test available for diagnosing decay is the "pick test." Unfortunately, it does not begin to become reliable until the equivalent of approximately 10-15 percent weight loss, at which point a significant amount of wood strength has already been lost. Also, it is effective only when decay is present on the surface, which is frequently not the case with major wood structural members in exposed service. An effective way of examining the inside of a large member is to use an increment borer. This instrument produces a core of actual wood from the interior of the member, which can be either visually examined or microscopically evaluated. Microscopical examination can effectively diagnose decay in its earliest stages, before significant loss in strength. However, it is an arduous, time-consuming and highly specialized technique which cannot be applied to a large number of wood members, and still leaves behind a hole. Probably the most commonly used device for detecting decay in the field is the screwdriver, or some other type of penetrating probe. Probing allows evaluation of both the hardness of the wood (a strength property not particularly sensitive to early stages of decay), and its toughness (which is very sensitive to early stages of decay) through application of the "pick test." When a building is undergoing evaluation for the presence of decay in structural members the screwdriver is likely to be present. However, information gained from probing may be misleading. Properly evaluated and maintained, wooden build­ ings are capable of providing long service. Around 250 years is about the best that we can show in this country. But elsewhere, over 800 years can be demonstrated. The big problem is that most of the strength of wood is lost in such an early stage of decay that we have no current field-applicable technology capable of evaluating it with reliability. Professional judgment and experience may be our most effective current means of evaluating decay in structures. Actually, the intuitive expertise we've developed, out of necessity, may be an impediment to the development of electronic technology. A fairly recent survey by Barry Goodell and Oregon State University (Goodell and Graham, 1983) found that a majority of utility pole inspectors are confident that they can detect early stages of decay by hammer soundings! Moisture meters are used in the field, and can indicate whether wood is in a moisture condition capable of supporting decay. Units with long insulated pins are capable of giving such information, even from the inside of a member. This technique, however, does not allow evaluation of strength loss. A sonic device has been in use, by utilities, on poles for many years and appears to be effective in locating advanced decay and voids. However, there is no information to suggest that it can detect early stages of decay. Experiments have shown that ultrasonic pulse transit time measurement is capable of detecting early stages of decay in some circumstances, but not others (Wilcox, 1988). Acoustic emission under compression appears to be extremely sensitive to early stages of decay, but is not nondestructive (Beall and Wilcox, 1987). Acousto-ultrasonics appears to be able to detect early stages of decay under nondestructive appli­ cation, but has not yet been adequately researched to know for sure. (The primary difference between acoustic emission and acousto-ultrasonics is that acoustic emission testing involves analysis of sound being generated within a material being stressed, whereas acousto-ultrasonics involves analysis of sound which has traveled through the material but is induced by a source external to the material being tested.) Resonant vibration and electrical resistance methods may bear promise for nondestructive evaluation of wood structural members, but are just beginning to be researched for this application. There is a need for more sensitive tools for inspecting wood structural members for damage zones and wood decay. Significant efforts are in progress to apply emerging technologies to the evaluation and inspection of wood members. These tools are being developed considering the need to correlate NDE measurements with member strength and stiffness. Whether such technology can be developed for nondestructive application in structures remains to be seen. REFERENCES Beall, F.C.; Wilcox, W.W. 1987. Relationship of acoustic emission during radial compression to mass loss from decay. Forest Products Journal 37(4):38-42. Goodell, B.S.; Graham, R.D. 1983. A survey of methods used to detect and control fungal decay of wood poles in service. The International Journal of Wood Preservation 3(2):61-63. Wilcox, W.W. 1988. Detection of early stages of wood decay with ultrasonic pulse velocity. Forest Products Journal 38(5):68-73. USDA Forest Service Gen. Tech. Report PSW-128. 1991. 9 Termites and Forest Management in Australia