Combination of non-destructive tests for surface and in depth location of decay in timber structures
The work presented in this dissertation of the author Yu Taoyi was carried out at the Civil Engineering Department of University of Minho, Portugal.
The importance of timber constructions
Timber constructions have an important significance in the cultural, architectural and historical heritage in different civilizations, with numerous examples of structures still standing.
As the increase of the rehabilitation market, research topics on timber structures have been increasing recently. However, it should be noticed that topics were more focused on new timber constructions, rather than to existing timber structures, which still remains an extremely important object of research, especially regards of its historical and aesthetic values.
Timber as a construction material
Timber is one of the most used and most ancient construction material used for structural purposes. It is a very efficient construction material regarding its physical and mechanical properties. It has a relatively low density compared to masonry, concrete, and steel, which makes it easy to handle and efficient in terms of load carrying capacity due to its low self-weight. The strength and stiffness of wood vary a lot between different species, but timber has generally a good compression and tension performance. Timber structures also exhibit good earthquake resistance owning to its low weight and stiffness.
Wood structures are prevalent all over the world due to its wide availability. In Portugal, the traditional building construction adopted timber roof and floor structures, and also timber reinforced masonry walls. The most frequently used wood species are Maritime Pine, Eucalyptus and Chestnut. In several timber roof structures were surveyed and more than thirty timber trusses were assessed. It confirmed the importance of king-post and how trusses configurations in Portuguese timber construction, and collected and reported the common values and ranges of geometry, connections solutions and load parameters.
Three important parameters
According to the European standard (BS EN 518 1995), there are three main properties that determine the timber grade: density (mean and characteristic value), bending strength (characteristic value), and bending stiffness (mean value of modulus of elasticity, MoE). When these three parameters are determined, the timber element can be classified to a certain strength grade, which then gives the assumed value for other strength properties and elastic moduli.
The significance of bending properties is due to the fact that bending is the most common loading mode for the structural use of a timber element, such as beams, purlins and roof trusses, among other examples. Consequently, the bending stiffness and strength became the critical parameters that should be determined to assess the serviceability and safety of timber elements. However, it must be noted that many timber elements are also subjected to axial stresses (compression and tension).
Regarding density, as wood is a porous material, it is related to the variation of the cell cavities and the thickness of the cell walls. By measuring the density, one can get an indication of the amount of wood substance contained in the wood sample and, subsequently, get a correlation to its local mechanical properties.
Non-ideal construction material
Timber, as a structural material, presents some specific features that renders the process of determining its physical and mechanical parameters more complex.
Wood is an anisotropic material, meaning that different properties are exhibited along different axes. The three mutually perpendicular axes of a timber element are longitudinal, tangential, and radial axis, as shown in Figure 2. The difference from the different loading orientation can be even greater than that of different wood species. The variations of the parameters, such as the strength and stiffness, are due to the orientation of the wood fibbers and the manner in which a tree increases in diameter as it grows.
Generally, the tension strength perpendicular to the grain (which is often reached leading to the failure mode for most timber elements in practice) is much lower than that in longitudinal direction. Also the MoE perpendicular to the grain is smaller than that of parallel to the grain. In application, the property values are often given only for the longitudinal and tangential axes.
As a natural growing material, a wooden element can present different properties within a same cross section, as well as along its length. Generally, the late wood (the part of wood produced late in the growing season in a growth ring) always exhibits higher density and stiffness. Sapwood (the younger and outmost layer of wood) is less durable than the heartwood.
Moreover, timber elements always contain defects such as knots and misalignment on the grain. The slope of grain (SoG) is defined as the deviation of the grain from the edge of the piece or from a line parallel to the principle geometrical axis. The mechanical properties (bending, tensile and compressive strength) decrease for larger SoG. However, as for the influence to mechanical properties, the SoG is much less significant than the presence of a knot, which as shown in Figure 4 changes a lot the fibre direction around it. Generally, the mechanical strength decreases when a knot is exhibited.
The known capability for the damping of vibrations establishes wood as more viscoelastic (less elastic) than metals. This always results in potential larger errors in acoustic tests. In general, higher frequency waves are faster in propagation.
Wood is a material that can absorb and lose moisture according to different environment humidity and temperature and, consequently, changes the value of physical and mechanical parameters.
Fungi attack is one of the most common decay which could be observed in both live trees and structural timber elements. Three general causes of fungi attack can be identified: a) the lack of suitable protective measures during storage; b) improper seasoning, storing, or handling of the raw material; c) lack of precautions in using the final product.
Mould fungi and Stain fungi
Mould fungi generate discolorations, which exhibit a fuzzy and powdery surface growth. In softwoods, even when the fungi have penetrated deeply, the discolouring surface can still be easily removed. However, in hardwoods, the stains are always present deeply beneath the surface.
Fungal stains cause the discoloration of wood, which are normally confined to great extent on the sapwood, and cannot be easily removed by surfacing. In principle, it can be classified into sap stain and blue stain. The blue stains vary from bluish to bluish black and grey to brown colour, sometimes also yellow, orange, purple and red. The exact colour depends on the wood species, the fungal species, and the moisture and temperature conditions.
In general, both of them do not result in any significant mechanical change affecting the timber strength. However, they generally make the wood more porous, which results in a higher absorbency. Problems like over-absorption of glue, paint, and wood preservative can happen. Moreover, the absorption of more moisture can lead to the subsequent colonization by wood-decay fungi.
Decay producing fungi
Decay producing fungi can attack either the heartwood (heart rot) or the sapwood. The common types can be recognized as: brown rot (dry rot), white rot (wet rot) and soft rot.
Brown rot fungi colonize both hardwoods and softwoods, yet more commonly observed in softwoods. Under the brown rot attack, the wood cellulose is always extensively under attack. Browner colour is observed, and across-grain crack, shrink, collapse, and powdery crush are exhibited. The brown rot is sometimes called dry rot or water-conducting rot, since although the fungi requires water to grow, the attacked timber elements may after be dried and still have active decay.
White rot fungi also colonize both hardwoods and softwoods, yet more commonly observed in hardwoods. Besides the attack of cellulose, they also remove the lignin of the wood element. Whiter colour can be observed, yet unlike the brown rot attack, little across-grain crack, shrinkage or collapse is exhibited. The attacked timber always retains the outward dimension, and the surface becomes spongy.
Soft rot is caused by fungi more related to moulds. Compared to brown rot and white rot, it is a less important kind of decay since it is primarily very shallow and only affecting the outer surface of wood. Though the attacked part can be greatly degraded and become soft when wet, the very part underneath it still exhibits a firm and hard condition. Overall, in terms of growing conditions, temperature and humidity are the main influencing parameters.
Insects attack can significantly reduce the strength related properties but more specifically the residual cross section of the element. Wood destroying insects, mainly consists of beetles, termites, ants and bees.
Beetles are the most dangerous one among all the insects since they cause serious damage by feeding on the wood.
Bark beetles attack the surface of wood that is not debarked. They are reddish brown to black, approximately 1.5 to 6.5 mm long. Fine brownish-white sawdust-like particles are pushed out while the beetles make their tunnels into the soft inner of the wood, where eggs are laid. High presence of tunnels will result in the loosen or falling-off of the bark and large parts of the section itself.
Termites live mainly inside timber, so that the damages are always well-advanced before being recognized. Subterranean termites excavate galleries throughout their food as they consume it. They make their colonies underground but build the tunnels through earth and around the wood for food supply. They can completely make a honeycomb configuration to wood by feeding along the grain, only leaving a very thin layer of the surface. The damaged wood typically has a layered appearance and soil can always be observed in the excavation as being moved by termites.
Non-subterranean termites do not multiply as rapidly as the subterranean ones, and their destruction is also slow, yet timber can be thoroughly perforated if their tunnelling remain undisturbed for many years.
Carpenter ant and bee
Carpenter ant always make tunnels through the naturally soft or decay-induced softwood. They use the wood for shelters rather for food. The tunnels can reach to the point where replacement or extensive repairs are needed if their colonization remain undisturbed. High humidity is needed for the young ants to fully develop. Carpenter bees make large tunnels around 13 mm in diameter into unfinished soft wood for nests. They partition the hole into cells, each of which are provided with pollen and nectar for a single egg. Since the nests are always reused, they can extend to several feet and have multiple branches. They prefer the surfaces of the element or environment sheltered from the sunlight.
Bacteria can be observed in most of the wood that have been wet for a considerable long time. Usually, they have little effect on wood properties but generate some sour smells. Moreover, they can increase the absorption of moisture, adhesive, paint, or preservative.
Damage by marine-boring occurs to wood structures which are in salt or brackish water. The progression velocity of decay depends on the local conditions and the borer type. The main marine borers are the shipworms, especially the species of Teredo and Bankia. They bury themselves in the wood and grow from 0.3 to 1.2m long. The entrance holes do not change their size and show only slight perforations while the interior of the wood element may be completely honeycombed and ruined (Laboratory 2010).
Weathering is the outdoor physical and chemical degradation of wood, caused by the ultraviolet radiation of the sunlight, the moisture and temperature change, the freeze-thaw cycles, the abrasion by wind- blown particles, or the growth of micro-organisms. It can result in colour change and checking of the timber element.
Mechanical and Non-destructive tests – Analysis
Non-destructive tests refer to the tests that generate no or little damage to the tested specimens. Though some of the timber properties such as the strength and stiffness can only be determined by the destructive tests (DTs) which, however, cannot be applied to historical structures due to their historical value and preservation requirements, the NDTs are serving as the indirect methods for prediction of these parameters.
Generally, according to the target testing area, the NDTs can be divided into two groups: the global test method (GTM) and local test method (LTM). GTM includes: visual inspection, ultrasonic stress wave method, ultrasound test, ultrasonic array imaging, sonic stress wave test, acoustic emission test, IR thermography, among others. The LTM is consist of Resistograph®, Pilodyn®, hardness test, small non- standard sampling, X-Ray, micro-CT, among others.
Non-destructive tests have shown their advantages of maintaining the integrity of the structures, yet the complexity for interpreting their results increases especially when decay is presented in the tested element. Hence, the influence of surface and in-depth decay on the timber properties and on the non- destructive test results were analysed in this work.
Non-destructive tests (Pilodyn®, Resistograph®, ultrasound test, acoustic emission test) and mechanical tests (density and bending test) were applied on three dismantled elements from a roof truss of an old building. Results were then compared with the defect map and visual grade which were generated based on a preliminary visual inspection. A general idea of how the defects (knot, fungi attack, insect attack, ringshake) influenced the test values were summarized.
Furthermore, linear correlation and multiple regression models between non-destructive test results and the physical and mechanical properties were built. Specific attention was paid to the influence of segment size and the presence of decay. Finally, a methodology of how to interpret the onsite timber assessment data was proposed, and applied to a timber roof truss from an old timber building.
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This work intended to give an overview about how different type of decay would influence the timber properties and the results of NDTs. However, since the deterioration features by different types of insects and fungi can vary significantly, the conclusion found in this work can only serves as a reference yet may not be applicable directly to other wood species and decay types.
With the limit number of fungi and ringshake presented samples, the comparison for ability of NDTs to predict density was only carried out in terms of insect decay.
Within this scope, future work should be considered regarding collecting of data from the samples that exhibit sufficient amount and levels of other type of decays. In that case, a more thorough methodology could be completed, providing guide on which NDTs to carried according to the inspection of decay level for the test target.
Moreover, the case study carried in this work stopped at predicting the density and bending stiffness since the objective of this dissertation was to provide a methodology to infer on the mechanical properties of a timber element based on different levels of indepth and surface decay. Nevertheless, further structural analysis could be made to determine the truss behaviour under external loading. Then by comparing the results with previously made full scale tests, the validity for the methodology could be checked.