1. Dowel-laminated timber from reclaimed boards : bending properties and stiffness prediction from board-level NDTMohammad Derikvand, Olga Srejić, Matthew Schwarzkopf, Michael Nicolas Mrissa, 2026, original scientific article Abstract: Reusing timber in load-bearing applications is a strategy for extending material lifecycles and reducing the environmental impact of construction. However, uncertainty in mechanical properties of reclaimed timber due to the absence of standardized strength grading is a key barrier. The aim of this study was to produce and evaluate the bending properties of dowel-laminated timber (DLT) from reclaimed boards and to assess whether its bending stiffness can be predicted from board-level non-destructive testing (NDT) data. For this, several reclaimed spruce beams with ~200 years of service life were obtained from a heritage building. The beams were 3D-imaged, sawn into boards, and the recovery yield was calculated. The relationship between the dynamic (Edyn) and static (Em,g) modulus of elasticity of the boards was evaluated and benchmarked against virgin boards (strength class C24). DLT specimens with beech dowels were then produced using a stiffness-informed lamination process in three configurations: (i) DLT from reclaimed boards, (ii) DLT from virgin boards, and (iii) hybrid DLT combining reclaimed and virgin boards. The DLT specimens were tested in bending until failure. Based on the results, the recovery yield after initial sawing was 79%, calculated as the volume of the sawn boards relative to the volume of the beams. After defect removal and planing, the recovery yield decreased to 32.1% for the final full-length boards plus about 1.9% shorter pieces. Service-related biological and physical degradations had a greater in- fluence on the recovery yield than the sawing process itself. A strong relationship (R² = 0.823) was found be- tween Edyn of the reclaimed boards before defect removal and planing and their Em,g after full processing. The DLT specimens from the reclaimed boards exhibited bending strength and stiffness comparable to those of DLT from the virgin boards. A simplified analytical model based on board-level stiffness (Edyn or Em,g) predicted the average DLT bending stiffness within ±2%. Overall, the findings demonstrate that stiffness-informed lamination based on board-level NDT can be used to produce DLT from reclaimed timber with predictable performance
Keywords: reuse, repurpusing, reclaimed timber, recovered timber, beech dowels, DLT, non-destructive testing, bending test, dynamic modulus of elasticity, failure analysis
Published in RUP: 14.09.2026; Views: 321; Downloads: 23
 Full text (12,93 MB)
This document has more files! More... |
2. Reusing timber for a circular, low-carbon future : challenges and the path forwardMohammad Derikvand, 2026, other scientific articles Abstract: Load-bearing timber is usually not reused in a second structural application after its first service life, not due to proven inadequacy, but largely because prevailing standards recognize only first-use material. When buildings are dismantled, the recovered timber is typically routed toward energy recovery, re- or down-cycling, or disposal, while functionally equivalent components for new construction are manufactured from newly harvested logs. The current regulatory framework favors virgin timber, even where reused elements could technically perform the same structural functions. Therefore, one of the primary barriers to structural timber reuse is the absence of codified pathways for qualification, certification, and market acceptance.
Keywords: timber, reuse, reclaimed timber, circular economy, standards and certification
Published in RUP: 03.02.2026; Views: 896; Downloads: 11
Full text (267,35 KB)
This document has more files! More... |
3. Moisture-activated set-recovery of thermo-hydro-mechanically densified hardwood dowels for high-tolerance timber connectionsLei Han, Alexander Scharf, Mohammad Derikvand, Matthew Schwarzkopf, Bogdan Mitkovski, Dick Sandberg, Andreja Kutnar, 2026, original scientific article Abstract: Conventional wooden dowel connections in timber structures rely on tight press-fit installation, which requires high insertion forces and often loosens over time due to stress relaxation. This study investigates an alternative approach that exploits the moisture-activated set-recovery of thermo-hydro-mechanically (THM) densified hardwood dowels to enable slip-fit assembly followed by self-tightening in service. To this end, European beech and black poplar were densified radially and tangentially at different compression ratios. They were then evaluated for swelling kinetics, swelling pressure, bending performance, and moisture-activated expansion using in-situ X-ray CT in water at 20 °C and 100 °C. Results show that activation kinetics can be controlled by temperature. Expansion was rapid within minutes in hot water and slower but equivalent in magnitude at room temperature. Beech outperformed poplar, with radial densification at 35 % compression ratio producing a peak swelling pressure of 5.7 MPa and a modulus of rupture of 268 MPa after activation. Poplar generated higher free expansion but significantly lower pressure due to its lower stiffness. Radial densification was consistently more effective than tangential, enhancing both expansion magnitude and pressure generation. Capillary uptake triggered expansion along the dowel length (∼30 mm in 1 h) and produced an elliptical expansion profile. Importantly, mechanical strength was retained post-activation, which confirms structural suitability. These results demonstrate that THM-densified beech dowels can offer a robust self-tightening mechanism, combining low-force installation with durable pressure generation and stable mechanical performance. This provides a viable path toward adhesive-free, metal-free, high-tolerance timber connections.
Keywords: THM densification, dowel laminated timber, CT scanning
Published in RUP: 28.01.2026; Views: 1127; Downloads: 3
Full text (13,97 MB)
This document has more files! More... |
4. Experimental and analytical evaluation of shear capacity of reclaimed timber connections with thermo-mechanically densified wooden dowelsMohammad Derikvand, Donya Bazyar Khoshroodi, 2025, original scientific article Abstract: The reuse of reclaimed timber in structural applications is increasingly recognized as a strategy to extend ma- terial life cycles and reduce waste. Particular attention has recently turned to reusing reclaimed timber in adhesive-free laminated products with wooden dowels, where connection performance is critical. The aim of this study was to investigate the mechanical performance of double-shear connections assembled from reclaimed spruce timber using thermo-mechanically densified wooden dowels. The dowels were first evaluated in bending and embedment tests on lamellae made from both reclaimed and virgin spruce timber. The shear capacity of the connections was then evaluated in three layups: two homogeneous (all-reclaimed lamellae and all-virgin spruce lamellae) and one hybrid (reclaimed side lamellae with a central virgin spruce lamella). Analytical predictions were performed based on the draft of the new Eurocode 5, however, with two alternative definitions of dowel bending resistance (ultimate moment Mu and yield moment My), and three alternative definitions of embedment strength (f 0.05d h calculated using the 0.05 d offset method, f Fmax h calculated from Fmax, and f 0.8Fmax h calculated from 0.8 Fmax). The three lamellae layups exhibited relatively similar mean capacities (7.3–8.3 kN). The all-reclaimed connections were on average 13 % stronger than the other assemblies, but this was not statistically significant. Among the model input variants, Mu with f 0.05d h showed the closest agreement with the experimental mean values, while My with f 0.05d h led to more conservative predictions preferable for design. In both cases, excluding outliers from the embedment test results reduced overestimation in the predictions. All other input variants overestimated the shear capacity. Overall, both homogeneous and hybrid connections with reclaimed timber lamellae performed comparably to virgin timber connections and exhibited predictable mechanical behavior.
Keywords: reclaimed timber, wood densification, structural reuse, embedment strength, dowel connections, European yield model, Eurocode 5
Published in RUP: 17.12.2025; Views: 1062; Downloads: 11
Full text (23,08 MB)
This document has more files! More... |
5. Size effect on bending strength and failure modes of finger-jointed timberMohammad Derikvand, Michael David Burnard, Donya Bazyar Khoshroodi, Rok Barbič, Marko Vouk, Andreja Kutnar, 2025, original scientific article Abstract: Finger-jointed timber boards for cross-laminated timber production are typically assigned the same characteristic bending strength (fm,j,k) if produced from the same strength class, regardless of differences in their cross-sectional dimensions. To validate the relevance of this approach, this study investigated the effects of cross-sectional dimensions on the bending performance of finger-jointed timber produced from spruce (strength class: C24). A large industrial dataset of 1100 specimens, with seven thicknesses (ranging 20 to 40 mm) and variable widths, were statistically analyzed. The bending tests were performed with a constant span-to-depth ratio (l/h = 18), meaning thinner specimens had a shorter test span. Bending strength was modeled with Bayesian multilevel linear model, and the proportions of three quality-control failure modes (joint interface failure, joint base failure, and outside-joint failure) were analyzed with a zero-one-inflated Dirichlet regression. Based on the results, all groups with n ≥ 100 exceeded the declared fm,j,k of 27.6 MPa, with characteristic strengths of 43.7 MPa (+58.3 %), 40.0 MPa (+45.0 %), and 38.3 MPa (+38.8 %) for the 20-, 30-, and 40-mm thickness groups, respectively. Thinner specimens demonstrated higher bending strength with convincing evidence (pairwise contrasts PD = 100 % with 95 % HDPIs entirely below zero), while width had no credible effect (PD < 95 %). Dirichlet regression revealed shifts in failure mode proportions with varying strength. Higher bending strengths were associated with a higher proportion of joint interface failure. Outside‑joint failure was observed with a higher proportion in lower-strength and thicker specimens. Overall, assigning uniform fm,j,k to various cross-sectional dimensions proved to give adequate safety margins. Beyond the uniform fm,j,k, however, statistical evidence of a size effect on both bending strength and failure modes was observed. The magnitude of the observed size effect reflects the combined influence of increasing thickness and test span under the current quality control bending test regime, which means a coupled change in stressed volume and geometry rather than a pure cross-section scaling effect. These findings are relevant to flatwise four-point bending tests on finger-jointed boards from industrial production made with visually graded C24 spruce with thickness ranging 20 to 40 mm.
Keywords: wood, finger joint, strength
Published in RUP: 18.11.2025; Views: 1232; Downloads: 8
Full text (4,20 MB)
This document has more files! More... |
6. Push-out resistance of threaded wooden dowels in cross-laminated timber with hardwood cross-layerLei Han, Mohammad Derikvand, Jakub Michal Sandak, Martin Hladnik, Dick Sandberg, Andreja Kutnar, 2025, original scientific article Abstract: Europe is experiencing a shift in timber resource availability, with a decline in softwoods and an increase in hardwoods, creating a need to explore the utilisation of hardwoods, particularly in construction, where softwoods have traditionally been predominant. This research aimed to assess the potential of using hardwood in hybrid, doweled cross-laminated timber (DCLT) fabricated with pre-dried threaded beech dowels. The objective was to evaluate the push-out resistance of beech dowels in a three-layer DCLT with spruce in the outer longitudinal layers and beech, poplar, or spruce in the cross-layer. The DCLT specimens were conditioned at various temperatures and relative-humidity (RH) levels before the push-out test. The specimens with beech cross-layer exhibited a higher push-out resistance than those made entirely of spruce. In contrast, using poplar in the cross-layer showed no considerable difference in push-out resistance compared to using spruce. Exposure to 70 °C and 10% RH for three days decreased the push-out resistance in all groups due to extreme shrinkage of the lamellae, whereas exposure to 20 °C and 65% RH for either 60 days or 240 days resulted in higher push-out resistance for DCLT with a beech cross-layer.
Keywords: DCLT, underutilised species, hardwood, dowel
Published in RUP: 12.09.2025; Views: 1217; Downloads: 13
Full text (1,60 MB)
This document has more files! More... |
7. |
