Evaluation of glue line shear strength of laminated timber structures using block and core type specimens

Monitoring and evaluation of glued laminated timber structures in service is essential to warrant its integrity, where non-destructive techniques play an important role. This paper describes the results obtained on glued laminated timber beams of maritime pine, with and without preservative treatment, and of spruce, exposed to natural outdoor environment as well as to artificial weathering. The assessment of glue lines’ shear strength was investigated through the extraction and testing of cores parallel and perpendicular to the glued lines and block shear specimens with 50 mm?×?50 mm and 20 mm?×?20 mm shear area. The results highlight the different performances of the various types of specimens tested, showing that the measured shear strength depends on the specimen size and that the size effect depends on the material strength giving a decreasing modification factor with the increase of strength in opposition to the constant factor suggested by standard EN 14080. They also show that shear testing of cores drilled perpendicular to the glued joints may be a promising tool in the assessment of glulam structures on site.     

Production of glued laminated timber with copper azole treated maritime pine

A study was conducted to evaluate the performance of glued laminated timber (glulam) of maritime pine treated with a copper azole preservative product. Shear strength of glue lines met the requirements of EN 386 (2001) with no influence of clamping pressure and cure temperature. According to the same standard, delamination was satisfactory for higher cure temperatures applied with a clamping pressure of 0.6?N/mm². Finger joints made with treated wood gave satisfactory bending strength. The preservative treatment did not influence the modulus of elasticity of the beams. According to a Monte Carlo simulation and following the requirements of EN 1194 (2002), glulam of class GL 28c can be manufactured if visually graded maritime pine of classes E and EE (NP 4305 1995) is used in the inner and outer lamellas, respectively, and class GL 24h when using both grades in equal proportions randomly distributed through the glulam element.     

Einfluß der Trägerhöhe auf die Biegefestigkeit von Brettschichtholz

The influence of beam height on the bending strength of glued laminated timber was investigated. Tests showed that with increasing beam height the bending strength decreases for glued laminated timber beams without finger joints (wood failure). Theoretical values confirm calculated values of Eurocode 5. There is not a dependence of strength from the beam height by glued laminated timber beams with finger joints in the outer core. In most cases, glued laminated beams have finger joints in the outer core. Therefore, it is not useful to introduce a height factor in design of glued laminated beams in codes.     

Relationship between timber quality and the influence of moisture content above fibre saturation on mechanical properties in beams of fast-growing Argentinean Eucalyptus grandis

The present paper reports on the results of an investigation regarding the relationship between timber quality and the influence of moisture content above fibre saturation on bending strength and stiffness in structural-sized beams of fast-growing Argentinean Eucalyptus grandis. An empirical research project with a sample containing 96 pairs of structural-sized beams (seasoned and unseasoned) was carried out. Results showed that the influence of moisture content on mechanical properties was related to timber quality and it increased with timber quality but this relationship did not clearly emerge from the results obtained by visually strength grading the sample with the most important strength and stiffness reducing growth characteristic (the presence of pith), whereas it was better revealed by machine grading the sample with the most important single parameter (modulus of elasticity). Results also showed that decisions related to structural design with timber of high quality exposed to service class 3 according to European standards or installed near fibre saturation are situated on the unsafe side if the strength and stiffness values are obtained by multiplying those corresponding to seasoned timber by a constant factor representing the average behaviour of this material. Even though this average behaviour compared relatively well with the results obtained by applying the criteria adopted by standards of overseas and Latin-American countries, which do not consider timber quality for this purpose, the reliability required by structural design accounts for the convenience of taking into account the relationship between timber quality and the influence of moisture on mechanical properties where timber of high quality is concerned.     

Machine strength grading of Argentinean<Emphasis Type="Italic"> Eucalyptus grandis</Emphasis>

The present paper reports the results of an investigation regarding the possibility of machine strength grading sawn timber of Argentinean Eucalyptus grandis. In order to assess the usefulness of each parameter for predicting the mechanical properties of this timber species, and to propose strength, stiffness and density profiles, an empirical research project with four samples of beams and one sample of boards subjected to bending, and another sample of boards subjected to tension parallel to the grain, was carried out. The results obtained with specimens in structural sizes reveal the more important single and combined parameters for machine strength grading this timber species, and allow to analyse strength, stiffness and density profiles in comparison with the international strength class system established in the European standard EN 338.     

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.     

Quasi-brittle fracture and size effect of glued laminated timber beams

The load bearing capacity of glued laminated timber beams is size dependent. A material based size effect occurs due to the natural variability of the timber properties and can be explained with the Weibull weakest link theory. The Weibull type size effect has received much attention in the timber research community in the past as it is the dominating size effect regarding the load bearing capacity of timber beams. In this paper, it is shown that a further deterministic size effect, which originates from the timber fracture behavior, influences the load bearing capacity of glued laminated timber beams. It is the size effect associated with quasi-brittle behavior. The paper covers a short theoretical review on fracture related size effects and a study comparing experimentally tested beams of several sizes with detailed numerical simulations considering the natural growth characteristics of timber. Further, an analytical model illustrating the features of the fracture related size effect on the load bearing capacity of glued laminated timber beams is presented. It is demonstrated that the simulated load bearing capacity of the tested glued laminated timber beams distinctively improves if the quasi-brittleness is taken into account by using a softening constitutive law.     

Experimental study on glued laminated timber beams with well-known knot morphology

Nowadays, the impact of knots on the failure behaviour of glued laminated timber (GLT) beams is considered by subjecting the single lamellas to a strength grading process, where, i.a., tracheid effect-based laser scanning is used to obtain information about knot properties. This approach single-handedly defines the beam’s final strength properties according to current standards. At the same time, advanced production processes of such beams would allow an easy tracking of a scanned board’s location, but, at this point, previously obtained detailed information is already disregarded. Therefore, the scanning data is used to virtually reconstruct knot geometries and group them into sections within GLT beams. For this study, a sample of 50 GLT beams of five different configuration types was produced and tested under static four-point-bending until failure. As for each assembled lamella the orientation and position within the corresponding GLT beam is known, several parameters derived from the reconstructed knots can be correlated to effective GLT properties. Furthermore, the crack patterns of the tested beams are manually recorded and used to obtain measures of cracks. A detailed analysis of the generated data and their statistical evaluation show that, in the future, dedicated mechanical models for such timber elements must be developed to realistically predict their strength properties. A potential approach, using fluctuating section-wise effective material properties, is proposed.     

Structural bonding of ash (Fraxinus excelsior L.): resistance to delamination and performance in shearing tests

The utilization of ash (Fraxinus excelsior L.) allows for a significant enhancement of the load-bearing capacity in structural laminated products. However, such applications fundamentally require high-strength and durable bonds between lamellas and in finger joints. Therefore, the aim of the present survey was to evaluate ash bondings in terms of resistance to delamination (EN 302-2) and shear performance (EN 392). Investigations were performed with five adhesives: phenol-resorcinol-formaldehyde (PRF), melamine-urea-formaldehyde (MUF-1, MUF-2), polyurethane (PUR), emulsion polymer isocyanate (EPI) and varying closed assembly times as key bonding parameter. For all tested adhesives and closed assembly times, the shear test showed high wood failure percentages and bond strength values that compare to solid ash. In contrast, for resistance to delamination, significant differences were found between the adhesives as well as between closed assembly times, with improving resistance to delamination for increasing closed assembly times. The best performance was determined for the PRF-adhesive and long closed assembly times. However, standard requirements for resistance to delamination could not be met by any of the adhesives. The resistance to delamination showed no correlation with the shear performance for any of the adhesives. Microscopic examination of the bonded joints revealed that both the penetration behavior of the adhesives and glueline thicknesses clearly correlated with the closed assembly time.     

Sortierverfahren für die kombinierte maschinelle und visuelle Festigkeitssortierung

The aim of this study was to show the possibility of the classification of structural timber into strength class C35 according to EN 338 by using combined visual and machine strength grading techniques. Three different visual-machine strength grading combinations are devised for allocating sawn timber into strength class C35. They are composed on the one hand of the visual grading according to DIN 4074-1 and on the other hand of machine strength grading of green or dry sawn timber by measuring the dynamic MOE or the apparent density of sawn timber. In order to allocate sawn timber into strength class C35 the machine settings for these combinations were estimated. The grading results show that combinations based on the dynamic MOE are suitable for strength grading of lamellae for glued laminated timber and joists. Combinations based on apparent density are also suitable for strength grading lamellae for glued laminated timber.     

Compressive strength and stiffness of Radiata Pine laminated veneer lumber

This paper describes results of experimental testing of compressive strength and stiffness of laminated veneer lumber (LVL) manufactured in New Zealand from Radiata Pine. It evaluates material properties of 45, 63 mm LVL and 36 mm crossbanded LVL in the three different material directions. Testing has been performed according to Australian/New Zealand Standards and European Standards. Results from experimental testing according to both standards are compared and a newly proposed method for strength increase due to stress spreading has been verified. Recommended design values for strength and stiffness are given.     

Effect of laminated structure design on the mechanical properties of bamboo-wood hybrid laminated veneer lumber

The effects of veneer orientation and loading direction on the mechanical properties of bamboo-bundle/poplar veneer laminated veneer lumber (BWLVL) were investigated by a statistical analysis method. Eight types of laminated structure were designed for the BWLVL aiming to explore the feasibility of manufacturing high-performance bamboo-based composites. A specific type of bamboo species named Cizhu bamboo (Neosinocalamus affinis) with a thickness of 6 mm and diameter of 65 mm was used. The wood veneers were from fast-growing poplar tree (Populus ussuriensis Kom.) in China. The bamboo bundles were obtained by a mechanical process. They were then formed into uniform veneers using a one-piece veneer technology. Bamboo bundle and poplar veneer were immersed in water-soluble phenol formaldehyde (PF) resin with low molecular weight for 7 min and dried to MC of 8–12 % under the ambient environment. All specimens were prepared through hand lay-up using compressing molding method. The density and mechanical properties including modulus of elasticity (MOE), modulus of rupture (MOR), and shearing strength (SS) of samples were characterized under loading parallel and perpendicular to the glue line. The results indicated that as the contribution of bamboo bundle increased in laminated structure, especially laminated on the surface layers, the MOE, MOR and SS increased. A lay-up BBPBPBB (B-bamboo, P-poplar) had the highest properties due to the cooperation of bamboo bundle and poplar veneer. A higher value of MOE and MOR was found for the perpendicular loading test than that for the parallel test, while a slightly higher SS was observed parallel to the glue line compared with perpendicular loading. Any lay-up within the homogeneous group can be used to replace others for obtaining the same mechanical properties in applications. These findings suggested that the laminated structure with high stiffness laid-up on the surface layers could improve the performance of natural fiber reinforced composites.     

Sustainable particleboards with low formaldehyde emissions based on yeast protein extract adhesives <Emphasis Type="Italic">Rhodotorula rubra</Emphasis>

Nowadays, there is great interest in developing adhesives that do not generate formaldehyde emissions. The objective of this work was to prepare adhesives based on yeast protein extract from Rhodotorula rubra (Rr) to make particleboards (PBs). The adhesives were characterized, and the physical and mechanical properties of the PBs were evaluated. The results were compared with those obtained using a commercial urea–formaldehyde adhesive (UF). Adhesives viscosity varied from 420 to 600 cP, with pH ranging from 4.35 to 4.73, and solid content of around 50%. The UF had a viscosity of 305 cP, pH of 7.53 and solid content of around 65%. Dynamic mechanical analysis showed that the wood-adhesive using protein extract Rr with reinforcing agent gave a good performance and stiffness, comparable to that using UF. The internal bond strength values of PBs varied from 0.36 to 0.46 N/mm², while swelling thickness reached values of 24 and 38%, respectively. Density values of the PBs fluctuated between 682 and 693 Kg/m³. The formaldehyde emission of the PBs manufactured with Rr extract based adhesive had values between 0.2 and 0.3 mg/100 g, while formaldehyde emission for UF particleboard was 6.8 mg/100 g. Properties of the PB made of Rr based adhesives presented similar properties to particleboard manufactured with UF, but with lower level of formaldehyde emissions. The results from this research validate the manufacture of particleboards with Rhodotorula rubra yeast protein extract based adhesives. These PBs may have the same applications as the traditional PBs, with the advantage of being able to fulfill the high standards of formaldehyde emission demanded by the sustainable market.     

Die Biegefestigkeit von Keilzinkenverbindungen aus Brettern der Buche (Fagus silvatica L.)

The bending strength of glulam depends on both board strength and finger joint strength. In the case of softwood glulam this fact is well known from experience. It was recently reported by the authors that visual strength grading of beech provides a characteristic glulam bending strength of 36 N/mm² and mechanical strength grading of 48 N/mm². Therefore, adequate values for the characteristic finger joint bending strength have to be met. At the time those values were unknown. It was the aim of the following project to study the influence of visual or mechanical strength grading techniques on the finger joint bending strength. Bending tests on finger joints were carried out. The specimens were manufactured from visually strength graded boards according to the German standard DIN 4074-5 and from mechanically strength graded boards according to the dynamic MOE. The test results confirm that visual strength grading provides strength class GL36 and mechanical grading provides GL48.     

Physical,mechanical and biological properties of thermo-mechanically densified and thermally modified timber using the Vacu<Superscript>3</Superscript>-process

Densification and thermal modification change wood properties in different ways depending on the treatment conditions and the wood species. In the presented investigations, densification and thermal modification were applied consecutively. The primary objective of this treatment combination was the compensation of reduced mechanical properties due to the thermal modification by densification. The combined processes were applied to five European wood species: poplar (Populus nigra L.), beech (Fagus sylvatica L.), Norway spruce (Picea abies Karst.), English oak (Quercus robur L.) and European ash (Fraxinus excelsior L.). Depending on the mean density of the species, a thermo-mechanical densification of 43 or 50% was imposed to improve mechanical strength parallel to the grain. Subsequently, the densified material was thermally modified in the so-called Vacu³-process at 230 °C and 20 or 80% vacuum and at 240 °C and 20% vacuum. The thermal modification resulted in changing wood colour, mechanical strength, hardness, dimensional stability and durability. All the wood modification processes were carried out at industrial scale after pre-tests at laboratory scale. The modified material was characterized regarding flexural properties, static and dynamic hardness, structural integrity, abrasion resistance, moisture dynamics, dimensional stability, and durability against white, brown and soft rot fungi. In summary, the test results showed that the consecutive application of thermo-mechanical densification and thermal modification leads to significantly improved durability whilst mechanical properties at least for beech, ash and poplar remained and the material is dimensionally stable.     

Verklebung von Buchenholz für tragende Holzbauteile

The load bearing capacity of glulam could be enhanced by using lamella of European beech (Fagus sylvatica??L.). Essential requirements for the production of those beams are adhesives which allow for a durable und reliable bonding of lamella. The paper shows to what extent commercially available MUF and PU adhesives and adapted bonding parameters ensure gluing of European beech timber for load-bearing timber structures. Both beech wood containing heartwood and beech wood without heartwood were included in the study. With regard to technical approval, the chosen test methods were resistance to delamination according to EN??302-2 (2002) and shear test of glue line according to EN??392 (1995). From the two tested MUF systems it was evident, that delamination decreased by increasing the closed assembly time. The requirements of EN??301 (2006) were fulfilled for adhesive type??I and??II. Without any limitations beech wood containing heartwood could be glued using MUF-1. On the other hand, the results of the PU adhesive were insufficient. Light-optical micrograph examinations show that long assembly times secure the formation of a glue line. It was assumed that the curing process is retarded by beech wood. No relationship was found between the results of the shear strength test and the delamination test. For quality control of glulam the delamination test is recommended. Based on the positive results an application for technical approval of glulam consisting of beech was made. For this reason beech-glulam can be generally utilized in service class??1.     

Curing behavior and bonding strength of two-component fast-setting adhesives prepared with phenol-liquefied <Emphasis Type="Italic">Cryptomeria japonica</Emphasis> wood

Honeymoon adhesive is a special adhesive system that contains two components which are applied onto two substrates separately for glulam manufacturing to provide a fast-setting capability. It has been confirmed that phenol-liquefied wood can be used as a raw material for preparing phenolic resins. In this study, liquefied wood (LW) was obtained by liquefying Cryptomeria japonica wood in phenol with sulfuric acid as a catalyst. Resorcinol–formaldehyde resin (RF), phenol-resorcinol–formaldehyde resins (PRF), LW–PF and LW–PRF (synthesized using LW as a raw material), and RF/LW and RF/LW–PF (blended RF with LW and LW–PF) were prepared. The properties such as non-volatile content, pH, viscosity gelation time and DSC curing behavior of these resins were measured. The results show all resins can be cured at room temperature when paraformaldehyde is added as a hardener, except for the LW–PF. The honeymoon adhesive was prepared by using RF and PRF that contains 10% paraformaldehyde as the A-component and LW–PRF, RF/LW and RF/LW–PF without paraformaldehyde as the B-component. After mixing the A-component and B-component, the gelation time of RF and PRF can be shortened. The glued lumber was prepared by separately applying the A-component and B-component on the surface of two wood strips and combining them. The result shows using RF/LW–PF as the B-component has faster bonding strength development than others. When PRF and RF are used as the A-component, they have a bonding strength of over 5.4 MPa, the requirement of CNS 11031 standard for the glued lumber of C. japonica, at pressing times of 2 and 4 h, respectively.     

Evaluation of strength properties of parallel splint lumber (PSL) and its comparison with laminated veneer lumber (LVL), rubber wood and teak

Heavea brasiliensis ) and an exterior type of adhesive were evaluated in air-dry condition. Block shear tests were also carried out in wet and dry condition. Comparisons of these strength values were made with those of laminated veneer lumber (LVL), solid rubber wood and standard teak. The results show that PSL has slightly lower values of strength properties compared to teak, solid rubber wood and quite inferior ones to those of LVL. On the basis of the data obtained in the present study, PSL made from rubber wood can be utilised as a structural material for different purposes like door and window frames, rigs and trusses.     

Evaluation of strength properties of parallel splint lumber (PSL) and its comparison with laminated veneer lumber (LVL), rubber wood and teak

( Heavea brasiliensis ) and an exterior type of adhesive were evaluated in air-dry condition. Block shear tests were also carried out in wet and dry condition. Comparisons of these strength values were made with those of laminated veneer lumber (LVL), solid rubber wood and standard teak. The results show that PSL has slightly lower values of strength properties compared to teak, solid rubber wood and quite inferior ones to those of LVL. On the basis of the data obtained in the present study, PSL made from rubber wood can be utilised as a structural material for different purposes like door and window frames, rigs and trusses.

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Production and characterization of particleboards from cork-rich <Emphasis Type="Italic">Quercus cerris</Emphasis> bark

Single-layered particleboards were produced from granulated Quercus cerris bark containing cork and phloem granules using standard hot-press equipment and phenol–formaldehyde resin. The experimental boards were tested for thickness swelling, mechanical strength and thermal properties. Scanning electron microscopy observations were carried out to analyze the panel structure. The results showed that Q. cerris bark particleboards had low thickness swelling in water, high resistance to thermal degradation and high calorific values but their mechanical strength was below that of commercial wood particleboards. The produced Q. cerris bark particleboards were adequate for exterior applications where mechanical strength is not the key factor. Potential for process and feedstock optimization was acknowledged.