Shear strength of glued laminated timber made from European beech timber

This paper describes an investigation into the shear strength of glued laminated timber (GLT) made from European beech. Special consideration was paid to the possible strength influence of a frequently occurring discolouration of the timber, termed red heartwood, which is inherent to the species. The beech lamellae were visually graded according to German hardwood grading standard DIN?4074-5. Grade LS13, conforming to European hardwood strength class D40, was split into two sub-grades LS13? and LS13+. Additionally, modulus of elasticity (MOE) was determined by ultrasound pulse, longitudinal vibration and static tension tests. Sub-grade LS13+ showed a mean density and MOE of 690 kg/m³ and 14,800 N/mm², respectively. The GLT shear strength was evaluated by means of four-point bending tests on structural sized I-shaped beams with a depth of 0.6?m and a span to depth ratio of 5:1. The slightly inhomogeneous build-up of the cross-section conformed to glulam strength class GL42c. Two beam samples were investigated, each with seven specimens, where one grouping had no red heartwood and the other with a high red heartwood in the web laminations. Additionally block shear tests on bond line strength were performed with standardized small specimens according to EN?392. Neither the beam shear capacity tests nor the bond line block shear tests revealed an influence of the red heartwood discolouration on strength. The fifth-percentile value of shear strength of all beams was 3.5?N/mm². The results of the block shear tests indicate that the present requirements on minimum block shear strength are set too low in the European standard EN 386 with regard to beech GLT.     

Delaminierungseigenschaften und Scherfestigkeiten von verklebten rotkernigen Buchenholzlamellen

The paper deals with investigations on dry shear strength and delamination behavior of gluelines between beech wood laminations showing no or distinct red heartwood. The investigations covered 36 parameter combinations comprising besides the red heartwood two widths of the laminations, two different adhesive families and two delamination procedures related to verification of adhesive type I or II. The tests revealed no influence of red heartwood on shear strength which for all parameter combinations exceeded almost throughout 10 N/mm². In contrary thereto the delamination tests revealed a distinct dependency of the glueline integrity on red heartwood and width of the adherents. No parameter combination conformed to the requirements of adhesive type I. In case of laminations without red heartwood the requirements of adhesive type II were met. Gluelines between two laminations with red heartwood showed throughout very pronounced delamination above 50%. The integrity of gluelines versus delamination represents one of the essential normative requirements for adhesives used in glued load bearing wooden elements. This proof was not possible for gluelines of beech wood laminations with red heartwood when applying today’s European delamination test procedures and performance requirements developed and established for gluelines of softwood laminations.     

Comparing dry bond strength of spruce and beech wood glued with different adhesives by means of scarf- and lap joint testing method

Dry bond strength of different adhesives on spruce and beech wood was tested by means of the former DIN standard 53253 (1964)scarf joint and the operative standard lap joint testing method according to EN 302-1 (2004). The failure mode in standard lap joint tests was mainly wood failure, whereas scarf joints failed in the bond line. Due to the high proportion of wood failure, no clear effect of the different adhesives, but a clear effect of wood species on the bond strength was observed for lap joint samples. By contrast, scarf joint testing was far less dependent on wood species and delivered clear differences in the strength of different adhesive systems.     

Beitrag zur Prüfung der Leimbindefestigkeit von schutzmittelgetränktem Holz

A specimen for testing shear strength under compression was dimensioned to evaluate the glue bonding strength of impregnated timber. Upon investigating the influences of the course of annual rings, thickness and height of the specimen and the amount of glue used, optimum dimensions for the specimen were fixed. One oil-borne and one water-borne preservative were tested for their effects on the glue bonding strength in pine and spruce specimens. For pine values for shear strength under compression were the same for impregnated and control specimens; failure in the wood also showed corresponding values in all three test groups. For spruce impregnated specimens had considerably lower shear strength values, whereas failure in the wood remained constant. This suggested an evident reduction in shear strength in spruce wood due to impregnation.     

Influence of processing parameters and wood properties on the edge gluing of green <Emphasis Type="Italic">Eucalyptus grandis</Emphasis> with a one-component PUR adhesive

The predominant plantation hardwood species in South Africa Eucalyptus grandis is, despite relatively good strength and stiffness properties, seldom used for lumber production due to low dimensional stability, splitting, and cracking of the wood. As critical defects often develop or aggravate during the drying process, the edge gluing of Eucalyptus grandis lumber in its wet state, before kiln-drying, was considered a potential inhibiting factor for this behaviour. The objective of this study was to determine how certain material and processing variables influence the bond quality of unseasoned, edge-glued, Eucalyptus grandis, using a moisture-curing one-component polyurethane adhesive. The study considered the effect of four parameters, namely wood density, moisture content, adhesive spread rate and pressure, on the shear strength of bondlines. Micro computed tomography scanning was used to observe adhesive penetration behaviour. Overall, the bonding quality was satisfactory proving the feasibility of edge gluing of this wood in the wet state. Multiple ANOVA showed that generally better results were obtained for samples with a higher moisture content of roughly 60% compared to specimens with lower moisture content around fibre saturation point. Results indicated that the lower spread rate tested (150 g m^?2) should be preferred to the higher one (250 g m^?2), since it will likely give more stable shear strength results and it would be preferable from an economic point of view. Increasing pressure did not increase the shear strength significantly.     

Bonding performance of heat treated wood with structural adhesives

Bonding of untreated, intermediate (hydro-thermolysed) and heat treated wood with melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF) and polyurethane (PUR) adhesives was studied. An industrial heat treatment process (Plato ^®) was used, which included two separate heat treatment stages and a drying stage in-between. Laminated beams having four lamellas were prepared from untreated and treated timber for mechanical testing of the bond lines. The results of the tests showed that heat treatment affected the shear strength and the delamination of the laminated wood depending on the adhesive system used for bonding. The PUR and MUF adhesives performed in a rather similar way, and better than the PRF adhesive. The shear strength of laminated wood bonded with the waterborne MUF and PRF adhesives decreased for the specimens made of hydro-thermolysed timber and decreased further for the specimens made of fully heat treated timber. The difference in adhesive bond shear strength between untreated, intermediate and fully treated wood was less obvious in the case of the PUR adhesive. Delamination of the PRF bond line decreased drastically for all the specimens made of heat treated timber.     

Hydroxymethylated resorcinol (HMR) priming agent for improved bondability of wax-treated wood

The effects of hydroxymethylated resorcinol (HMR) on the tensile shear strength of wood joints treated with montan ester wax were studied with polyvinyl acetate (PVAc) and melamine formaldehyde (MF) adhesives. Untreated specimens which were bonded with MF and PVAc had similar bond strength properties under dry conditions, but MF bonded specimens’ tensile shear strength values were higher than that of PVAc glued specimens under wet conditions. With both adhesives tested, dry tensile shear strengths of wax-treated specimens were in a comparable range to those of untreated wood, while under wet conditions the strength values of wax-treated specimens were significantly lower than those of the controls. HMR priming increased the shear strength of PVAc bonded specimens under wet conditions and of MF-bonded specimens under dry and wet conditions. The effect on MF-bonded specimens, however, was much more pronounced under wet conditions.     

Some investigations concerning density profile,internal bond and relating failure position of particleboard

Testing machines for the measurement of density profiles of wood based panels have become a wide-spread tool in production laboratories. New equipment for the in-line measurement of density profiles was presented recently. The high significance of the density profile for the properties of particleboard ticleboard is well known for some years. Maximum densities in the outer zones of the profile are closely related with MOE, bending strength, water uptake and swelling. Minimum density is closely related with shear modulus (G _(xz , (G _(yz ), shear strength and compression strength. However, results about the correlation of the minimum density with internal bond are contradictory, and statements concerning the failure positions relating to the internal bond-test are published rarely. For five different thicknesses of furniture grade particleboard the relation between internal bond, parameters derived from the density profile and the respective failure position were determined. Indeptendent of panel thickness the failure position was found to be in a range of the 25-75%-panel thickness. A correlation between the failure position and parameters derived from the density profile could not be found. There appear to be three predetermined failure lines (35, 50, 65% of panel thickness). The correlation between mean density and internal bond is high for the total sample (r=0.81). The correlation between minimum density and internal bond is lower, especially with regard to results relating to a single panel thickness.     

Integrity check of structural softwood glue lines: correspondence between delamination and block shear tests

In factory production control (FPC) of glued-laminated timber the integrity of glue lines according to product standards has to be checked by carrying out either delamination or block shear tests. The chosen test method should provide reliable and reproducible results, and from a practical point of view should be as cheap and easy to perform as possible. An extensive experimental study aiming at systematically comparing the outcomes of delamination and shear tests on structural softwood glue lines using different wood species and adhesives was carried out. In the delamination tests, the length of open glue lines was assessed after subjecting the test specimens to vacuum- and pressure-soaking and to subsequent drying. In the block shear tests, the shear strength and wood failure percentage were determined. Only if the benchmark values in the product standards were met or exceeded, the respective specimen passed the test and the glue lines were regarded as sound. As a main overall result it turned out that the outcomes of both test methods generally agree well, but will not necessarily lead to the same pass/fail conclusion in any case. Therefore, for FPC the choice of their application should be defined more specifically.     

Outdoor weathering performance parameters of exterior wood coating systems on tropical hardwood substrates

Wood coating research almost exclusively focuses on softwood as substrate despite the fact that coatings applied on tropical hardwoods show a different weathering behaviour. The objective of this study was to quantify the impact of wood substrate and coating type on the weathering behaviour of finished wooden joinery in outdoor exposure using a set of statistical methods. Therefore, a broad range of white opaque paints and mid-oak semi-transparent stains were used. The coatings were industrially applied on window frames made of frequently used commercial tropical hardwood species. The general appearance of the wood/coating systems was assessed visually using a rating scale that takes into account the anatomical characteristics of hardwood. For the evaluation, a required minimum exposure time of 2.5 years outdoor weathering for joinery in vertical position and facing south-west was determined. At 30 months best performances were recorded for waterborne alkyd primers with an acrylic topcoat when dry coating film was sufficiently thick. The dry layer thickness also affected the impact of the hiding power on the performance of the wood/coating system. Within hardwood species the degradation rate of coatings on species with a fine texture was the lowest. The significant influence of the substrate’s texture on the weathering behaviour of coated hardwood stressed the need to evaluate the wood/coating system in total.     

人工林杉木胶合工艺优化研究

采用SAS全因子试验设计,研究了胶粘剂类型、涂胶量、单位压力、加压时间和胶合面纹理五个因子对人工林杉木木材胶合性能的影响,并对其胶合工艺进行了优化。研究结果表明:胶粘剂类型对杉木木材常态胶合剪切强度和木破率的影响不显著,但是老化处理后API和PF的胶合剪切强度和木破率比PVAc和UF的要高;在以API为胶粘剂时,涂胶量、单位压力、加压时间和胶合面纹理对杉木胶合剪切强度和木破率的影响显著,优化的胶合工艺为涂胶量250g/cm2,单位压力1.5 MPa,加压时间50 min和胶合面纹理为弦切面-弦切面。     

Influence of steaming on selected wood properties of four hardwood species

Influence of steaming on various mechanical and physical properties of two European (Robinia pseudoacacia L., Quercus robur L.) and two tropical (Intsia bijuga, Hymenolobium petraeum) hardwood species were investigated. Each of these wood species requires adequate adhesive systems for the use as dimension stock because of their highly reactive surface chemistry. In order to optimize the gluing behaviour of the timbers involved, steaming processes with five different sets of parameters (steaming time and temperature) were carried out. In addition to the adhesive test, bending strength, hardness, and colour of the modified timbers were examined. The result of steaming highly depends on the wood species. For black locust, steaming is a suitable method of colour homogenization and colourization. Short term, low temperature treatment improves the adhesion performance also, whereas the colour change value reaches its maximum in the case of long term, high temperature steaming. Hue shift of oak and sapupira was inconsiderable for any applied set of parameters, only a small L* decrease was observed at higher temperatures. The colour of merbau samples shifted slightly during the treatment. Bending strength and hardness of wood samples in all of the four wood species decreased during the treatment. However, steaming time is more important than temperature while aspect of colour change both time and temperature has the same significance. Adhesive properties of sapupira can be greatly improved by hydrothermal treatment.     

Environmental effects on the bond at the interface between FRP and wood

The advantages of fiber reinforced polymer (FRP), such as high specific strength, resistance against corrosion and formability, have made it a more acceptable alternative to conventional materials regarding repairing and retrofitting of structures. Although investigations in recent years have proved the concern of civil engineers about the environmental effects on the bond between FRP and concrete or masonry (especially moisture and temperature), only few researches have been reported on FRP-wood interfaces. This research investigated the effect of five different environments on the bond at the interface between FRP and wood. A series of pull-out tests were performed on 375 wood specimens bonded with aramid, carbon and glass fibers (wet lay-up) and one type of epoxy resin. Then, these samples were immersed in chemical solutions with pH of 2.5, 7, 10 and 12.5 and substitute sea water for 2, 4, 6, 8, 10 and 13 weeks to estimate bond strength reduction. The effect of dry heat was also investigated. The results of such experimental tests illustrated that an acidic solution had the most effect on the bond between carbon-FRP and wood by 57% reduction. Sea water and fresh water had almost the same effect on the specimens bonded with glass and aramid–FRP (40% reduction), and they had the least influence on the bond between carbon-FRP and wood. Further, alkaline solutions had the most effect on the specimens bonded with glass-FRP. In addition, dry heat did not affect the bond between FRP and wood considerably.     

Hydroxymethylated resorcinol (HMR) priming agent for improved bondability of silicone modified wood glued with a polyvinyl acetate adhesive

A hydroxymethylated resorcinol (HMR) was used as a coupling agent to improve glueability of silicone (aminofunctional polydimethylsiloxane) modified pine sapwood (Pinus sylvestris) with polyvinyl acetate (PVAc). Priming with HMR did not significantly increase tensile shear strength of unmodified and silicone treated wood under dry conditions. Under wet conditions, tensile shear strength of the control specimens was increased by 37% and of the silicone modified specimens by 13%. Despite of higher shear strength under wet conditions, the primed silicone treated specimens displayed lower degree of wood failure than the unprimed treated specimens.     

Wood colour of central European wood species: CIELAB characterisation and colour intensification

In this project, 24 central European wood species have been characterized spectrophotometrically using the CIELAB-colour-system. Special focus has been put on the analysis of colour intensification of wood surfaces due to surface wetting. The analysis shows how the various wood species react to wetting and how the colour is changed. Especially darker hardwood species are susceptible to the colour intensification effect, resulting in a more intensive colour appearance. Additionally, an idealised model to describe the appearance of the wood colour is proposed: The recognized wood colour is a function of: first the reflected wavelength of the visible light, second the surface roughness and third the inner structure of the specific wood piece and the resulting refraction properties of the interacting substances. This model is supported by colour and gloss measurements of the wood species and selected coating investigations with oil and clear lacquer. Furthermore, a novel characteristic value, namely the light-reflection-depth (LRD), is introduced to describe wood properties according to light penetration. All this helps to understand how the wood colour impression is created for a standard observer. Particularly, the intensification effect can be described and explained by structural changes, which affect a change in colour. Understanding the reasons for the colour intensification supports innovative natural colour engineering of specific wood species.     

Withdrawal and lateral resistance of wood screw in beech, hornbeam and poplar

Withdrawal and lateral resistance of wood screw in three hardwood species of beech (Fagus orientalis Lipsky.), hornbeam (Carpinus betulus) and poplar (Populus deltoides 75/51) were studied. Measured values of ultimate withdrawal resistance of wood screws were compared with predicted values. Moreover, shear strength parallel to the grain of wood as an input parameter of the expression given by Eckelman was measured. Measured values of lateral resistance of wood screw connection were compared with predicted values of yield limit equations, and dowel bearing strength of wood as the input parameter of yield limit equations was measured. Results show that the expression by Eckelman gives a more accurate prediction than the expression given in NDS. Furthermore, yield limit equations accurately predicted observed yield mode, and in the three species lateral resistance was predicted lower than the measured values.     

A semi-automatic method to determine the wood failure percentage on shear test specimens

An alternative method to quantitatively determine the wood failure percentage on shear test specimens was developed, experimentally tested and successfully applied to specimens with typical combinations of adhesive types and wood species used by the glulam industry in Switzerland. The method consists of a staining technique for a better differentiation of wood fibers and adhesive, and an image processing procedure for a standardized estimation of the fractured surface ratio covered by wood fibers and adhesive, respectively. The semi-automatic method allows for a more objective determination of wood failure percentage and is suitable for quality control in the glulam industry.     

Moment resisting connections composed of friction-welded spruce boards: experimental investigations and numerical strength prediction

Research on friction welded wood-to-wood connections has shown high potential for further development regarding the industrial application of timber construction. This paper addresses the question to which extent welding of wood can be applied to fabrication of cross-laminated timber panels (CLT). Those panels can be used as wall elements, in which load transfer occurs by transmission of moments and shear stresses. For this purpose, experimental investigations on friction welded L-shaped specimens were carried out. The welded bond between the boards, arranged perpendicularly to their fibre directions, represents points of intersection of those CLT panels. The results allowed for (a) the determination of a fracture mechanism of welded timber connections under torsion, (b) the definition of a set of design parameters influencing the resulting joint strength and (c) the comparison between the performance of welded timber joints and those of commonly glued alternatives. The experimental investigations were compared to results obtained by numerical modelling, thus allowing a probabilistic strength prediction algorithm for predicting the joint capacity as a function of the surface size.     

Untersuchungen über die Zug- und Druckscherfestigkeiten verklebter Hölzer

Investigations in tension and compression loading have shown, that all glue lines in climate condition have nearly the same strength like solid timber. The absolute value has been found in relation to density of wood and dimension of test specimens. The result of comparisons with strength properties in shear by tension and compression loading is, that compression loading of solid wood is nearly 40% higher than tension loading in tests. The variance of test results from glued and water treated specimens doesn't allow an accurate statement. Shear tests by compression loading of specimens from two different wood species are to suppose species with lower density have a considerable influence on the test results.     

Strength and stiffness of ring and shear-plate connections

The development of Eurocode 5, the future European timber design code which is based on the partial safety coefficient method, means a change of timber design code formats in a number of European countries. Because of the change in the code format from allowable values towards partial safety coefficients, characteristic values of strength and stiffness of materials and connections are needed. In order to establish characteristic values of the strength and stiffness of ring and shear-plate connections, existing test data have been evaluated. The results of the evaluation correspond well to a mechanical model assuming a shear block failure of the wood as the governing failure mode.