Marie-Claude Biston
email: marie-claude.biston@lyon.unicancer.fr

Timothée Zaragori

Laurent Delcoudert

Marie Fargier-Voiron

Alexandre Munoz

Coralie Gorsse

David Sarrut

Pascal Pommier

Comparison of electromagnetic transmitter and ultrasound imaging for intrafraction monitoring of prostate radiotherapy

Keywords: Image-Guided Radiotherapy, Prostate cancer, Ultrasounds, Electromagnetic Transmitters, monitoring

Background and purpose: To compare two in-beam monitoring devices for prostate radiotherapy: intraprostatic electromagnetic transmitters (EM-T) (RayPilot ® , Micropos Medical) and ultrasound imaging using transperineal probe (TP-US) (Clarity ® , Elekta) used concomitantly on phantom and on patients.

Materials and methods:

The phantom study evaluated accuracy in presence of known translations and rotations. Then intra-fraction motions were analyzed for 10 prostate cancer patients implanted with the EM-T 8 days before the simulation CT (171 sessions). The percent time in which the differences between the systems were 1 to 5mm were scored for each direction.

Results: Experiments on phantom confirmed no interference between the systems and showed deviations of less than 0.5mm when translations were applied progressively. In presence of rotations (5-15°), both systems displayed systematic shifts up to 6.9 and 3.8mm for the TP-US and the EM-T, respectively. Absolute mean differences between displacements observed on patients with EM-T and TP-US were ≤0.55mm in all directions except for one patient (≤1.77mm). With an exception for this patient, a strong correlation was found in left-right direction: differences >2mm were monitored less than 0.22% of the time (mean acquisition time:164 minutes) and never exceeded 5 seconds. Maximum differences were observed in supero-inferior direction with differences>2mm monitored more than 6.5% of the time for 3 patients. Large prostate rotations, the presence of gas and EM-T location in the prostate may explain important differences.

Conclusion:

Apart from the systematic shifts induced by the rotations, the two systems were correlated and represent feasible solutions for monitoring prostate cancer treatment.

Introduction

Hypofractionated radiotherapy protocols in prostate cancer treatment require a better accuracy in dose delivery because of an increased risk of toxicity in the surrounding tissues [START_REF] Clemente | Role of the Technical Aspects of Hypofractionated Radiation Therapy Treatment of Prostate Cancer: A Review[END_REF]. Critical uncertainties are expected on target localizations due to longer treatment sessions. Hence, for accurate dose delivery a robust pre-treatment imaging device combined with a real-time prostate monitoring system for correcting inter and intra-fraction motion is desirable. We studied two monitoring systems: intra-prostatic electromagnetic transmitters (EM-T) (RayPilot ® , Micropos Medical, Sweden) and ultrasound imaging (Clarity ® , Elekta, Sweden). The latter is a 2D US transperineal probe (TP-US) with robotic automated sweeping [START_REF] Lachaine | Intrafractional prostate motion management with the clarity autoscan system[END_REF]. It is an alternative to other US imaging systems since it is fixed on a base plate and use the perineum as the acquisition window, which eliminates operator-dependent and quality image issues encountered with transabdominal devices [START_REF] Fargier-Voiron | Ultrasound versus Cone-beam CT image-guided radiotherapy for prostate and post-prostatectomy pretreatment localization[END_REF]. Finally the probe does not interfere with the treatment beam, which enables monitoring of intrafraction motions [START_REF] Sihono | Determination of Intrafraction Prostate Motion During External Beam Radiation Therapy With a Transperineal 4-Dimensional Ultrasound Real-Time Tracking System[END_REF]. Compared to the TP-US, EM-T is invasive [START_REF] Braide | Clinical feasibility and positional stability of an implanted wired transmitter in a novel electromagnetic positioning system for prostate cancer radiotherapy[END_REF]. A transmitter is temporarily implanted into the prostate and connected via a cable to a receptor plate positioned below the patient, on the accelerator table. The accuracy of the system was evaluated in a study performed on patients and differences of 1.7+/-1mm between EM-T and 2D X-Ray images were reported [START_REF] Kindblom | High precision transponder localization using a novel electromagnetic positioning system in patients with localized prostate cancer[END_REF], which makes the system reliable for intrafraction monitoring.

The objective of this study was to compare the monitoring results obtained with the two devices used concomitantly in a phantom study and on patients. Several studies dealing with monitoring devices dedicated to prostate cancer treatment can be found in the literature but none of them have been used simultaneously for treating patients mainly because of interferences between systems. The phantom study enabled to investigate the behavior of the 2 devices in presence of translations and rotations of known values. Then 171 treatments sessions of ten patients were monitored concomitantly with the two systems, enabling for the first time an in-vivo comparison.

Materials and Methods

Transmitter based monitoring device

The EM-T (RayPilot ® , Micropos Medical) is 3mm in diameter, 17mm in length and connected to a cable which is about 40cm long. It is implanted in the prostate 8 days before the CT-scan by transperineal way on local or general anesthesia guided by intra-rectal US imaging. During treatment, the EM-T is connected via the cable to a receiving plate positioned on the treatment table below the patient. This plate weighting 10kg is composed of a low attenuation area surrounded by two antenna arrays able to detect the EM-T shifts. Thanks to this transmitter, both translations and rotations of the target volume can be monitored in real time. The tracking frequency is 30Hz. After the whole treatment the transmitter is removed.

US monitoring device

The 3D TP-US image-guided radiotherapy (IGRT) system (Clarity ® , Elekta, Stockholm, Sweden) is based on a 2D transperineal US probe tracked by an infrared camera [START_REF] Lachaine | Intrafractional prostate motion management with the clarity autoscan system[END_REF]. For each acquisition, several hundreds of 2D US slices are acquired during an automated probe sweep performed by a step-by-step motor and combined into a 3D image. A specific immobilization device made of a base plate and two cushions for the knees enables the probe to be fixed between the patient's legs. In this work, to avoid interferences with the EM-T system, a dedicated support with no metallic component was manufactured for the US probe and directly fixed to the Raypilot ® sensor plate. During the simulation CT session, a reference US image (USref) is acquired with the same patient set-up as during the CT acquisition. The USref image is superimposed on the CT image through a calibration process, allowing the visualization of the USref and CT images in the same coordinates system. A reference positioning volume (RPV) is then delineated on the USref image. It represents the visible prostate volume on US image. Over the treatment course, a daily US image (USdaily) is acquired at the beginning of each fraction and manually registered on the USref image by RPV projection. The monitoring is performed via continuous 3D automatic rigid registration of the current US image with the TP-US pre-treatment image (0.7seconds per registration). Only translations are currently monitored with this system.

Phantom Study

The prostate phantom consisted in a plastic sphere with EM-T fixed on the edge. The EM-T was deliberately offcentered to reproduce the implantation as it was performed on patients (see patient data section).The sphere was attached to a water-tank support like, that allowed performing translations (0.3mm steps) and rotations (around left-right (LR) axis (pitch) and around antero-posterior (AP) axis (yaw) directions). The phantom was immersed in a parallelepiped tank full of water simulating a pelvic. The tank was positioned on the EM-T detection table. The TP-US probe was then put in contact of the tank and the EM-T connected to the table thus enabling the monitoring of the structure shifts concomitantly with the two devices (Fig. 1). Several movements were studied: translations alone with regular displacements (0.3mm steps); large displacements by thresholds comprised between 0.5-6 mm; translations+rotations by 5° steps, and rotations alone. Mean and standard deviations differences between the two devices were calculated for each configuration.

Patient data

Ten patients receiving a definitive irradiation of the prostate were included in this study (171 sessions) which was approved by the hospital ethics committee. Patients were implanted with the EM-T and 2 fiducial markers (FM) 8 days before the simulation CT. The EM-T was implanted in the upper base of the prostate to take into account the risk of EM-T migration during the treatment. In addition, it was also positioned laterally to limit the risk of injury of the urethra during the insertion process. During the simulation CT, patients were positioned on the EM-T sensor plate placed on the CT scan table, and immobilized using the above described device for US imaging. A reference US acquisition was performed just before the CT acquisition. Patients were scanned in supine position, with 1mm slice thickness and standard prostate protocol of the Brillance CT Big Bore scanner (Philips medical systems, The Netherlands).

Pre-treatment positioning was first performed with Cone Beam CT (CBCT) imaging+FM/CT registration. During CBCT imaging the monitoring modes of TP-US and EM-T were started to control patients' displacements. During treatment, irradiation was paused for shifts above a threshold of 3mm for at least 15 seconds observed with both devices. If the shift lasted for more than one minute, patient positioning was adjusted based on CBCT acquisition and CBCT+FM/CT registration. For minimizing the delivered dose to the patients and limiting sessions duration, a maximum of 2 intrafraction CBCT per treatment session was allowed. Only one patient (patient 6) needed 2 intrafraction CBCT during one treatment session.

Mean prostate displacements observed with the EM-T device were first plotted against delivery time for each patient to visualize the magnitude and directions of prostate motion during treatment. Then each treatment session was analyzed to determine the difference in displacement observed between the 2 systems. Mean differences and standard deviations over the whole treatment courses were calculated on a patient-by-patient basis. The percent time differences were >1, >1.5, >2, >2.5, >3 and 5mm was scored for each direction and each patient. To check whether differences observed were punctual or not, time thresholds of 3s, 5s, 10s and 15s were secondarily introduced in the analysis. The tracking time representing whole treatment sessions ranged from 90 to 200min. The mean tracking time was 164min, and the mean number of sessions was 17 (range 12-20).

Results

Results of the phantom study showed that, for translations only displacements, a good agreement between TP-US and EM-T was found when progressive shifts (0.3mm steps) were carried out. Maximum differences were less than 0.4mm. When sudden translations varying from 0.5 to 6mm were applied, larger differences were observed (Fig. 2). Similarly, large punctual differences were observed because the TP-US signal was faster than EM-T to detect shifts.

When applying both translations and pitch rotations by steps of 5°, systematic shifts compared to reference displacement appeared for both devices (Fig. 3). For the TP-US, large systematic shifts were observed in supero-inferior (SI) direction in presence of pitch rotation. For the EM-T device smaller systematic shifts were observed but both AP and SI directions were impacted.

When only rotations were applied the magnitudes of the systematic shifts were identical to those obtained when combining translations and rotations (Fig. 4). For the TP-US, the SI direction was more impacted: pitch values of 5°, 10° and 15° lead to systematic shifts of 2.5, 4.9 and 6.9mm, respectively. In other directions the shifts were less than 0.7mm. Similarly yaw rotations had low impact (maximum shifts =1.5mm for a 15°rotation in SI direction). For the EM-T when pitch rotations were applied, shifts were comprised between 0.8-2.3mm and 1.5-3.8mm in AP and SI directions, respectively. Conversely to the TP-US, the EM-T device was also impacted by yaw rotations of 5-15° in the 3 directions with values comprised between 1.2-4.4mm, 1-3mm and 0.5-1.8mm for LR, SI and AP directions,respectively.

The relation of mean prostate displacement over all the sessions with time for each patient measured with the EM-T device is shown in Fig. 5. The prostate displacement increased with longer observation time. The magnitude of the displacements was patient-dependent. They were generally more important in posterior and inferior directions. In particular, mean prostate displacements observed for patient 6 were more important than for other patients. Conversely, the frequency and magnitude of LR displacements was less important for all the patients. The average differences between displacements measured with EM-T and TP-US are listed in Table 1 for all patients during all the treatment sessions. A good agreement was found between the 2 devices for all patients except patient 6, with absolute mean differences being ≤0.55mm whatever the direction. Similarly, there was a low variability (<0.8mm except for patient 5 with 1.45mm in SI direction). The best correlation was found in LR direction: mean differences were between -0.04-0.16mm and variability was ≤0.50mm. Larger deviations were found on patient 6, particularly in SI direction where mean difference was -1.77mm. A large variability was also observed in both AP and SI directions (>3.50mm).

The percent time over all treatment sessions where differences between EM-T and TP-US were >1, >1.5, >2, >2.5, >3 and 5mm was plotted in Fig. 6 for each direction and each patient. In LR direction, differences >1.5mm were monitored less than 2% of the time for all patients, except for patient 6 (10%). Furthermore, no difference >2mm for more than 5s was observed in all cases but patient 6 who had differences >5mm over a period of 15s or more 1.5% of the total treatment time.

In AP direction, differences were >1.5mm less than 5% of the time for 7 patients and less than 10% of the time for 2 patients. Again for patient 6 more important deviations were found: they were >5mm more than 14% of the time and differences >5mm during 15s were observed 13.5% of the time. Furthermore, 6 patients had no difference >3mm for more than 10s and 3 patients had discordances of 3mm less than 0.5% of the time.

In SI direction, deviations were less than 2mm 99.5% of the time for 7 out of 10 patients. Two patients (number 6 and 5) had differences >2mm more than 15% of the time. They were not punctual since differences >3mm were observed for more than 15s 9.5% and 5.3% of the time, for patient 6 and 5 respectively. Finally, 14 intrafraction CBCT for 4 patients were performed to confirm shifts above the alert threshold (3mm for 15s) observed with the 2 monitoring devices. CT/CBCT registration results were compared to the shifts observed with the monitoring algorithm of the TP-US. Mean differences (+/SD) obtained were 0.1mm (+/-0.12), 0.09mm (+/-0.07) and 0.09mm (+/-0.09), in LR, AP and SI directions, respectively. Maximum difference were less than 3mm. Once the corrections were applied, the therapists visually controlled that both monitoring devices showed shifts below the alert threshold.

Discussion

For accurate dose delivery in prostate cancer radiotherapy, a powerful IGRT strategy for correcting interfraction motions based on soft tissue registration is required. However, recent results emphasize that the daily use of a robust interfraction repositioning device may be insufficient to obtain tumor control with reduced CTV-to-PTV margins [START_REF] Engels | Impact of planning target volume margins and rectal distention on biochemical failure in image-guided radiotherapy of prostate cancer[END_REF][START_REF] Lovelock | Continuous monitoring and intrafraction target position correction during treatment improves target coverage for patients undergoing SBRT prostate therapy[END_REF]. Engels et al reported that, in addition with rectal distension, tight PTV margins may also be an independent prognostic factor for disease free survival in prostate cancer radiotherapy, despite the use of daily pre-treatment positioning with CBCT+FM. In addition, Loverlock et al showed that a mean of 1.7 interventions per fraction were required to keep the prostate within 2mm of its planned position [START_REF] Lovelock | Continuous monitoring and intrafraction target position correction during treatment improves target coverage for patients undergoing SBRT prostate therapy[END_REF]. They also demonstrated that, without continuous monitoring of the prostate, the required posterior margin, which was set to 3mm, would need to be increased by 2mm every 5 min starting from the time of the imaging procedure, to meet their clinical coverage requirement (PTV D95>90%). These results confirm that a real-time prostate monitoring system is required for safely reducing margins. Only few modalities allowing continuous intrafraction monitoring are available: electromagnetic transponders (Calypso®) [START_REF] Langen | Observations on Real-Time Prostate Gland Motion Using Electromagnetic Tracking[END_REF] or EM-T [START_REF] Braide | Clinical feasibility and positional stability of an implanted wired transmitter in a novel electromagnetic positioning system for prostate cancer radiotherapy[END_REF], ultrasound imaging and MRI-Linac [START_REF] Kontaxis | A new methodology for inter-and intrafraction plan adaptation for the MR-linac[END_REF]. MRI-Linac is a promising but expensive technology which may only concern a minority of cancer centers. We compared the results obtained with 2 recent modalities used for monitoring prostate intrafraction motion and for which few literatures are provided. The tracking algorithm accuracy of the TP-US has been recently validated on a multimodality pelvic phantom positioned on a motion platform, using an independent optical tracking system [START_REF] Yu | Intrafractional Tracking Accuracy of a Transperineal Ultrasound Image Guidance System for Prostate Radiotherapy[END_REF]. Time delay between the motion phantom and ultrasound tracking system was estimated to be 223±45 milliseconds. They found good correlation between US-measured displacement versus optically measured displacements for both good and poor image quality (R2=0.958 and 0.955, respectively). However, the accuracy in presence of rotations was not evaluated and, to our knowledge, the accuracy of the monitoring algorithm was not assessed on patients.

Contrarily to Calypso ® , Raypilot ® is able to track with submillimeter accuracy in the 3 directions with only one implanted transmitter thanks to its length which is approximately doubled compared to Calypso ® beacons. To our knowledge, there is no published study on the evaluation of the accuracy of the EM-T device on phantom and only 2 articles reporting experiences on patients. While in Kindblom et al the transmitter was inserted in the prostatic urethra [START_REF] Kindblom | High precision transponder localization using a novel electromagnetic positioning system in patients with localized prostate cancer[END_REF], in Braide et al it was implanted by transperineal way, similarly to our study [START_REF] Braide | Clinical feasibility and positional stability of an implanted wired transmitter in a novel electromagnetic positioning system for prostate cancer radiotherapy[END_REF]. While their objective was to assess the pretreatment localization accuracy of the transmitter relative to marker positions, we focused on the accuracy of the monitoring process of the system. This study is original since there is no comparative publication of similar systems on cohorts of patients mainly because the signals of electromagnetic transmitters are easily perturbed by the environment (metallic devices, electromagnetic waves). Thus, the main idea was to co-validate the monitoring algorithms of the 2 devices both in phantom study and on patients.

Experiments on phantom confirmed no interference between the systems, and showed that they were perfectly in accordance with deviations of less than 0.5mm, when translations were applied progressively in a given direction. Contrarily, when larger increments were applied, larger deviations were observed because of the latencies of the TP-US which was shorter than EM-T (approximate difference 10ms). When rotations were applied, both systems responded differently. Although each device compensated for the rotations with systematic shifts, these were different in terms of direction and amplitude. The TP-US showed a large systematic shift in only one direction in presence of pitch rotations while EM-T presented shifts in two directions but of lower amplitude. In addition we observed a linear relationship between the applied angle and the recorded offsets for both devices. For the TP-US, results can be explained because the algorithm used for registration of the RPV on the treatment image is only able to perform rigid translation registration. Hence rotations are compensated by translations of the RPV. Implementation of rigid registration with both translations and rotations would help increasing the accuracy of the system in presence of large rotations. Another likely explanation is that for large rotations the RPV may partly be out of the US acquisition cone which can affect the accuracy. The prostate can display large rotations during treatment (>12° for a pitch rotation) [START_REF] Amro | The dosimetric impact of prostate rotations during electromagnetically guided external-beam radiation therapy[END_REF]. Thus, under clinical conditions the impact of rotations on monitoring would not be negligible.

The reason of the observed systematic shifts with the EM-T device was clearly due to the transmitter that was not centered in the prostate phantom. We deliberately off-centered the transmitter in the phantom because this set up was more representative of our implantation process. We also noticed that the EM-T was subject to large interfractional position instability, as also observed in Braide et al [START_REF] Braide | Clinical feasibility and positional stability of an implanted wired transmitter in a novel electromagnetic positioning system for prostate cancer radiotherapy[END_REF]. Indeed, in their study they found that important transmitter shifts (>5mm even 9mm) could happen during the treatment course, as also observed in our institution. We also did some tests with the transmitter centered and observed that applied systematic shifts were less than 2mm whatever the direction even for large rotations (>15°). This raises the importance of transmitter implantation close to the center of the prostate in order to minimize the impact of rotations on the accuracy of the system.

Results on patients showed a strong correlation in 2 directions between the 2 devices, except for one patient, with maximum differences being less than 1.5mm in more than 98% and 90% of the time, considering LR and AP directions, respectively. In SI direction deviations were generally more important but less than 2mm 99.5% of the time for 7 out of 10 patients. Note that we showed on the experiment on phantom that the accuracy of both devices may be more impacted in SI direction in presence of large prostate rotations on the LR axis. When analyzing pretreatment CBCT, we observed that patient 5 was the one having the most important prostate rotations in the LR direction over the treatment sessions (mean -4.5°±1.70° against <1.6° for other patients), which could explain the observed difference between the 2 devices in SI direction for this patient.

Results obtained on patient 6 may also be explained by prostate rotations induced by frequent gases and also because he was moving and relaxing frequently (which could also explain results in AP direction). Thus, sudden large displacements were observed and because of the difference in latency between the 2 devices, large differences over long periods could be observed.

In conclusion, apart from the systematic shifts induced by the rotations, the two systems were correlated and represent feasible solutions for monitoring prostate cancer treatment. Compared to Calypso ® , the EM-T device has the advantage of being removable after treatment and maybe in the future able to perform real time in vivo dosimetry. Ultrasound imaging represents a promising alternative to other modalities since it is non-invasive, non-irradiating and inexpensive. It enables visualization and registration of both prostate and organs-at-risks during the whole treatment session. However the accuracy needs to be confirmed in presence of large rotations.

Fig1. Set-up for the phantom study. The prostate phantom consisted in a plastic sphere with EM-T fixed on the edge, immersed in a water phantom. The plastic sphere was attached to a water-tank like support. The EM-T detection plate was positioned below the phantom. The US probe was fixed on a dedicated support, with no metallic component. The probe was in contact of the water phantom.

Fig 2 .

 2 Fig 2. Shifts recorded by the EM-T and TP-US devices when applying displacements by threshold in all directions (A and B), and absolute differences observed between the devices (C and D). Positive values are inferior, anterior and left directions.

Fig 3 .

 3 Fig 3. Shifts recorded by the EM-T and TP-US devices when applying rotations along LR axis and displacement toward anterior direction only. Positive values are inferior, anterior and left directions.

Fig 4 .

 4 Fig 4. Systematic shifts recorded by the EM-T and TP-US devices in LR, SI and AP directions when applying rotations only along LR (Pitch) and SI (yaw) axis.

Fig 5 .

 5 Fig 5. Average (+/-standard deviation) prostate displacement observed in LR, SI and AP directions over all the sessions in function of the treatment session time for each patient measured with the EM-T device. Positive values are left, superior and anterior displacements.

Fig 6 .

 6 Fig 6. Percent time over all treatment sessions where difference between EM-T and TP-US exceeds specific threshold (mm).

Table 1 .

 1 Average differences between displacements measured with EM-T and TP-US

			Average difference (± SD)
		LR (mm)	SI (mm)	AP (mm)
	Patient 1	0.16 (0.29)	-0.11 (0.54)	-0,02 (0,69)
	Patient 2	0.09 (0.38)	0.55 (0.77)	0.13 (0.69)
	Patient 3	0.05 (0.43)	0.09 (0.56)	0.04 (0.69)
	Patient 4	0.01 (0.50)	-0.03 (0.33)	0.05 (0.62)
	Patient 5	-0.04 (0.33)	-0.09 (1.45)	0.24 (0.74)
	Patient 6	0.41 (1.41)	-1.77 (5.73)	1.24 (3.51)
	Patient 7	0.11 (0.42)	-0.53 (0.50)	0.30 (0.43)
	Patient 8	0 (0.42)	-0.19 (0.34)	0.17 (0.45)
	Patient 9	0.02 (0.25)	-0.10 (0.34)	-0.33 (0.75)
	Patient 10	0.01 (0.26)	-0.04 (0.29)	0.38 (0.57)

Acknowledgements

This work was performed within the framework of the SIRIC LYriCAN Grant INCa-INSERM-DGOS-12563, and the LABEX PRIMES (ANR-11-LABX-0063) of Université de Lyon, within the program Investissements d'Avenir (ANR-11-IDEX-0007) operated by the ANR. We are grateful to the Micropos ® team for providing the device and transmitters.


Francisco J Rescalvo
email: rescalvo@ugr.es

Robin Duriot

Guillaume Pot

Antolino Gallego

Louis Denaud

Enhancement of bending properties of Douglas-fir and poplar laminate veneer lumber (LVL) beams with carbon and basalt fibers reinforcement

Keywords: Wood, composites, Douglas-Fir, Poplar, LVL, retrofitting

This paper presents the experimental results of the bending mechanical behavior (static and dynamic moduli, and maximum stress) of LVL (Laminated Veneer Lumber) beams reinforced with fiber polymer material (FRP). Different variables were studied: i) Wood species (Douglas fir or poplar); ii) Type of reinforcement (bidirectional carbon, unidirectional carbon or basalt); iii) Veneer quality and iv) Veneers orientation in the beam (flatwise or edgewise). The reinforcement percentages respect to the total cross-section was of 2.17%, 0.89% and 1.74% for the unidirectional carbon, bidirectional carbon and basalt, respectively. A clear improvement provided by unidirectional carbon has been demonstrated (up to 40% more in the elastic modulus for the flatwise layout and more than 20% of the maximum stress, for both wood species). The influence of the quality of the veneers of the panel was also clearly demonstrated: the weakest wood material obtained the greatest improvements in their mechanical properties when reinforced, allowing to obtain stiff second quality poplar LVL, or strong second quality Douglas fir LVL.

Introduction

The building, understood as the use of buildings, the production and transport of materials, construction, maintenance and the end of the useful life of construction systems, represents up to 36% of the greenhouse gas emissions of the European Union [START_REF] Jefree | Building the bioeconomy[END_REF]. Throughout the twentieth century, reinforced concrete has allowed a competitive and rapid development of our built environment. Today we know that an average reduction of 2.1 t of CO2 emissions can be achieved substituting 1 t of concrete by 1 t of wood in the building process. [START_REF] Jefree | Building the bioeconomy[END_REF]. Wood, a building material widely spread on our planet is renewable and biologically based. Compared to other materials, it has environmental advantages in all its phases. Wood also has certain limitations with respect to other materials, such as its durability, fire resistance or lack of homogeneity, among others. However, there is a growing social interest for a construction with less environmental impact. Likewise, the growing awareness in the sector is evident by reducing its carbon footprint and its ecological backpack, being the use of wood, in combination with other materials widely validated through hybrid solutions, one of the most effective and profitable strategies in hands of the architects and prescribers of the sector. The growing social interest in a construction of almost zero energy consumption and the numerous added values of wood, are a great opportunity for this material in the construction sector, which is a huge opportunity for many tree species such as planted poplar of development based on technological products.

Wood can contribute to the market with technological products that respond to the current needs of construction, and thus revalue poplar plantations and establish a processing industry in the area with added value and sustainability over time, much greater than the current poplar wood processing industry. New technologies and scientific advances offer poplars the possibility of moving from traditional products (paper, chipboard panels, biomass, pallets, furniture, plywood, fruit boxes, etc.) to technological products of high added value in wood construction (Oriented Strand Board panels, I beams, trusses, Cross Laminated Timber wood, high quality structural plywood panels, or Laminated Veneer Lumber (LVL) wood, among others).

LVL is made of several wood veneers bound together, where for standard LVL all are placed in grain direction. Although less used, it can be found crossbanded LVL which has veneers in the perpendicular grain direction. LVL is used in many structural applications because of its high mechanical properties [START_REF] Rahayu | Ten new poplar cultivars provide laminated veneer lumber for structural application[END_REF]. The modulus of rupture in bending (MOR) of LVL is higher than solid wood with less variations because of the even distribution of natural defects such as knots, slope of grain or splits [START_REF] Laufenberg | Parallel-laminated veneer: processing and performance research review[END_REF]. In general, LVL is manufactured by using secondary qualities (i.e. presence of many knots, lower density and mechanical properties). LVL can be made of very different wood species, such as beech, birch, poplar, spruce, Douglas fir, etc.. The effects of ply organization on the mechanical properties of LVL elements have been studied, demonstrating that poplar and beech LVL beams offer higher bending strength and modulus of elasticity (MoE) compared to solid elements [START_REF] Burdurlu | The effects of ply organization and loading direction on bending strength and modulus of elasticity in laminated veneer lumber (LVL) obtained from beech (Fagurs orientalis L.) and Lombardy poplar (Populus nigra L.)[END_REF][START_REF] Strickler | Tension proof loading of finger joint for laminated beams[END_REF]. Some authors found that a first-grade veneer LVL beam can have a bending strength improvement of 6% respect to LVL beams manufactered with second or third-grade veneers [START_REF] Viguier | An innovative method based on grain angle measurement to sort veneer and predict mechanical properties of beech laminated veneer lumber[END_REF]. Other authors found, by grading beech veneers thanks to the measurement of fiber orientation, that the difference in MoE can reach 29% [START_REF] Marx | Effects of lumber width and tension laminated quality on the bending strength of four ply laminated beams[END_REF]. Meanwhile, the width of the layers used for the lamination process does not affect the flexural behavior and MoE of LVL elements [START_REF] Youngquist | End jointing of laminated veneer lumber for structural use[END_REF][START_REF] Wolf | Bending strength of vertically glued laminated beams[END_REF], defects such as knots, their diameter and the distance between them decrease the mechanical properties of the LVL beams [START_REF] Purba | The influence of veneer thickness and knot proportion on the mechanical properties of laminated veneer lumber (LVL) made from secondary quality hardwood[END_REF][START_REF] Theakston | A feasibility study for strengthening timber beams with fiberglass[END_REF]. By testing secondary quality beech LVL [START_REF] Purba | The influence of veneer thickness and knot proportion on the mechanical properties of laminated veneer lumber (LVL) made from secondary quality hardwood[END_REF] obtained mean bending strength of 63 MPa while beech LVL could reach more than twice this value if there was no defect in it [START_REF] Burdurlu | The effects of ply organization and loading direction on bending strength and modulus of elasticity in laminated veneer lumber (LVL) obtained from beech (Fagurs orientalis L.) and Lombardy poplar (Populus nigra L.)[END_REF].

In order to overcome the lower mechanical properties of second quality wood with presence of many knots, lower and more scattered mechanical properties, the combination of wood with fiber reinforced plastics materials (FRP) has been discussed in numerous research papers from the early 1960s [START_REF] Biblis | Analysis of wood-fiberglass composite beams within and beyond the elastic region[END_REF][START_REF] Laufenberg | Economic feasibility of synthetic fiber reinforced laminated veneer lumber (LVL)[END_REF]. [START_REF] Tichy | Flexural properties of glued laminated lodgepole pine dimension lumber[END_REF] focuses on the use of this type of material on LVL elements, studying its economic feasibility by using Eglass, S-glass and Kevlar-49 as reinforcing material, asserting the enhancement of the longitudinal strength and stiffness when unidirectional reinforcement is used. Furthermore, previous works have demonstrated the importance of the quality of the wood used in laminated lumber under bending stresses [START_REF] Shihong | Experimental study on poplar LVL beams reinforced with CFRP sheets[END_REF]. The use of carbon and basalt fiber reinforced polymers (CFRP and BFRP, respectively) has been studied in sawn wood and technological products such as glulam and LVL elements, demonstrating the improvement of mechanical properties under different reinforcement configurations [START_REF] Wei | Mechanical properties of poplar laminated veneer lumber modified by carbon fiber reinforced polymer[END_REF][START_REF] Gilfillan | The use of FRP composites in enhancing the structural behavior of timber beams[END_REF][START_REF] Hng | Bending properties of laminated veneer lumber produced from keruing (Dipterocarpus sp.) reinforced with low density wood species[END_REF][START_REF] Pengyi | Strengthening mechanical properties of glulam with basalt fiber[END_REF][START_REF] Solarov | Glulam beams reinforced with frp strips and their application in architecture[END_REF][START_REF] Ansell | Hybrid wood composites-integration of wood with other engineering materials[END_REF][START_REF] Rescalvo | Experimental and analytical analysis for bending load capacity of old timber beams with defects when reinforced with carbon fiber strips[END_REF][START_REF] Rescalvo | Experimental comparison of different carbon fiber composites in reinforcement layouts for wooden beams of historical buildings[END_REF][START_REF] Raftery | Low-grade glued laminated timber reinforced with FRP plate[END_REF][START_REF] Bekir | Flexural properties, bonding performance and splitting strength of LVL reinforced with woben glass fiber[END_REF].

In this paper, Douglas fir and poplar LVL beams have been manufactured and tested in edgewise position and flatwise and edgewise positions, respectively. The enhancement of the CFRP reinforcement was firstly evaluated in Douglas fir LVL beams, gluing the reinforcing material at the external faces of the beams. After this evaluation, a more complex manufacturing process was carried out, embedding the reinforcement in poplar LVL beams and comparing different types of FRP in order to evaluate the enhancement of the mechanical properties and its commercial feasibility comparing carbon and basalt, which is about 1/3 cheaper than carbon based materials. In addition, the effect of the use of two qualities of veneer has been evaluated.

Materials and Methods

FRP properties

Carbon and basalt based reinforced fibers (CFRP and BFRP, respectively) were used as reinforcing material. In edgewise bending, the lateral reinforcement may prevent shear failure in wood, and it is expected that bidirectional layout is more resistant to shear than unidirectional, this is why two types of CFRP were compared, unidirectional CFRP Mapewrap® C-Uni-Ax and bidirectional CFRP Mapewrap® C-Bi-Ax. For the case of basalt, the unidirectional Mapewrap® B Uni-Ax was used, in order to compare basalt and carbon unidirectional. For the CFRP-wood adhesion, the epoxy resin Mapewrap® 21 was used, ensuring a moisture content of 10±2% of the wood during the manufacturing process.

Douglas LVL samples

The Douglas fir used in this study came from Correze, a French department located in the western part of Massif Central, which is one of the main reserves of this species in France. The peeled logs were representative of the Douglas fir that can be supplied in this region, this Douglas fir having large growth rings and large knots in comparison with spruce (which is the main softwood species used in Europe to produce LVL). The veneers obtained were visually knotty. All bolts were soaked in hot water at 50±2 °C for 48 hours before peeling. Afterward, bolts were cut into veneers using an industrial rotary peeling lathe. The thickness of each veneer was set to 3 mm. In Douglas fir, the color is highly differentiated between heartwood (salmon pink) and sapwood (white cream, yellow) and allows for visual sorting. The veneers from the middle class containing too much sapwood were thus discarded. Only purely heartwood was kept for the present study. Then, the veneers were dried using an industrial air drying machine to reduce veneers moisture content to 6% MC. The dimensions of each veneer were 2600 mm x 1300 mm x 3 mm. The veneers were glued together with the grain orientated parallel to panel length. The glue used was a thermosetting phenplast resin or phenolic glue, with glue spread of 190 g/m². Then, the glued veneers were pressed under between 1 and 1,1 MPa for about 30 minutes in a stage press at 200°C. After gluing and pressing, the LVL were stacked and stabilized for three days before cutting in several 120 mm beams. Among several beams representative of the different panels, smaller 850 mm x 45 mm x 45 mm beams were cut out as Figure 1 shows with a specific nomenclature for each particular beam extracted (Table 1). All the beams were always taken cut by couple of beams, taken from the location showed in Figure 1. Locating the cut-outs of the daughter beams is mainly justified by the following reason: the material provides better integrity at the ends, away from the predominantly central fracture area and in the upper part of the mother beams. The number of beams and different testing conditions are summarized in Table 2. In particular, the right couples of beams were used to quantify the effect of unidirectional CFRP while the left couples of beams were used to quantify the effect of bidirectional CFRP. Indeed, a layer of FRP was glued to one beam from each couple, with epoxy resin during 24 hours at room temperature (Figure 2).

Poplar LVL samples

Poplar wood from the cultivar I-214 was used for LVL manufacturing. All the raw material was extracted from the same tree divided in 6 logs with a log length of 900 mm, in an attempt to avoid as much as possible the heterogeneity of the results during comparisons. However, due to the presence of knots in upper logs, differences in veneer quality were clearly visible (Figure 3). Thus, the veneers obtained from the logs above 2.5 m from ground were assembled together and called "Q2" quality, while the others (obtained from logs below 2.5 m) were called "Q1" quality (better quality).

Veneers with a thickness and length of 3 mm and 850 mm were peeled, and then dried in a climate chamber at 30.7 °C and 70% of relative humidity during 48 hours. The differences between the two qualities and different size are detailed in Table 3. The width of the veneers, the amount of adhesive and the distance between adhesive lines varied during the gluing process (Table 3). Regardless of the quality of the veneers, the elaboration process was divided in two steps (Figures 4 and5): I) 13 veneers were glued with the PU adhesive Purbond® HB S309, applying a total pressure of 0.5 N/mm 2 at room temperature (20 °C during 4 hours). II) Application of the three different kinds of FRP and the last 2 veneers of poplar, gluing them with the epoxy resin ensuring constant pressure during 24 hours at room temperature.

Once the panels were cured, 10 and 19 beams were extracted from the Q1 and Q2 panels, respectively. Each beam was prepared with a cross-section of 45 mm x 45 mm and a total length of 760 mm. To be fair in comparison, due to the two types of bending tests performed (flatwise and edgewise), the beams extracted from each panel were used alternately for each type of bending tests, i.e. the odd beams were used for FW beams and the even beams for EW ones (Figure 6). Table 2 summarizes the samples used in the whole study.

2.4.

Vibration test [START_REF] Brancheriau | Natural vibration analysis of clear wooden beams: a theoretical review[END_REF] showed that the Timoshenko bending theory can be applied to determine the dynamic longitudinal MoE and the shear modulus from the flexural vibration frequencies in free-free boundary condition. Indeed, they gave the following solution of the equation of motion of a vibrating beam at the first order:

𝑀𝑜𝐸 𝑑𝑦𝑛-𝑋𝑊 𝜌 - 𝑀𝑜𝐸 𝑑𝑦𝑛-𝑋𝑊 𝐾𝐺 𝑋𝑊 𝑥 𝑛 = 𝑦 𝑛 (1)
where 𝑀𝑜𝐸 𝑑𝑦𝑛-𝑋𝑊 is the longitudinal dynamic MoE when bending is in edgewise or flatwise direction, "XW" being replaced by "EW" or "FW", respectively; 𝜌 is the density; 𝐾 is the shear factor (𝐾 = 5/6 for a rectangular cross-section); 𝐺 𝑋𝑊 is the dynamic shear modulus when bending is in edgewise or flatwise direction, "XW" being replaced by "EW" or "FW", respectively; 𝑥 𝑛 and 𝑦 𝑛 are parameters that depend on the vibrational mode frequency (see [START_REF] Brancheriau | Natural vibration analysis of clear wooden beams: a theoretical review[END_REF] for details). By plotting 𝑦 𝑛 against 𝑥 𝑛 for different vibration modes, a linear regression can be performed and the dynamic MoE and shear modulus can be found. The deviation of this equation is generally less than 1% if the length-todepth ratio is between 10 and 20 (about 19 in the present work). Based on this theory, the BING device (Beam Identification by Non Destructive Grading, [START_REF] Cirad | Non-destructive testing of wood[END_REF]) was used to test all the samples in both EW and FW flexural vibrations, and thus obtain 𝑀𝑜𝐸 𝑑𝑦𝑛-𝐸𝑊 and 𝑀𝑜𝐸 𝑑𝑦𝑛-𝐹𝑊 .

Bending test

A four-point bending test has been performed on every specimen following the [START_REF]Timber structures -Structural timber and glued laminated timber -Determination of some physical and mechanical properties[END_REF] standard, using a distance equal to 810 and 750 mm, 18 and 17 times the specimen's height between the support for Douglas fir and poplar LVL beams, respectively. The distance between a loading point and the nearest support (a) was set as 285 and 255 mm for Douglas fir and poplar LVL beams, respectively. All the tests were made with a bending machine composed of a 100 kN electric actuator, equipped with a load sensor, and a global deflection rotary potentiometer sensor. The upper and lower supports were made by 4 cm wide metal plates, fitted on pivot. Figure 8 shows a picture of a particular test. Maximum bending stress was calculated according to the Equation 2.

𝜎 𝑚𝑎𝑥 = 𝑎 𝐹 𝑚𝑎𝑥 ℎ 4 𝐼 𝐺𝑧 (2)
where:

• Fmax is the maximum bending effort.

• a is the distance between a loading point and the nearest support.

• h is the height of the beam.

• IGz is the moment of inertia for a rectangular cross section beam.

Global MoE was calculated according to the Equation 3.

𝐸 𝑚,𝑔 = (3𝑎𝐿 2 -4𝑎 3 )(𝐹 2 -𝐹 1 ) 48 𝐼 𝐺𝑧 (𝑉 2 -𝑉 1 ) (3)
where:

• L is the beam length supports (mm) • V2-V1 is the increment of displacement measured by the rotary potentiometer corresponding to F2-F1 (mm).

Results and Discussion

This section shows the results obtained in terms of dynamic modulus, MoEdyn, global modulus MoEg,m, and maximum bending stress. Firstly, for the case of Douglas fir LVL, a comparison was made between the non-reinforced configuration (NR) and unidirectional (CU) and bidirectional (CB) carbon reinforcement configurations. All Douglas fir samples were tested in edgewise position. Then, for the case of poplar LVL, results were divided in three groups: Group I, in which a comparison between the nonreinforced (NR) and unidirectional carbon (CU) for the high-quality panel (Q1) was carried out; Group II, in which a comparison of the high-quality panel (Q1) and lowquality panel (Q2) was carried out for the case of unidirectional carbon (CU) reinforcement; Group III, for which a comparison between the non-reinforced (NR), unidirectional carbon (CU) and unidirectional basalt (BU) for the low-quality panel (Q2) is shown. For all the three groups the edgewise and flatwise configurations are shown and compared to each other.

Douglas LVL

Table 4 summarizes the average values of the main mechanical properties of the LVL Douglas fir tested beams in edgewise position. The dynamic modulus (MoEdyn), the global modulus (MoEm,g) and the maximum stress are presented comparing the nonreinforced beams with each type of reinforcement, CU and CB (carbon unidirectional and bidirectional, respectively). Figure 9 depicts the stress-deflection curves for each LVL Douglas fir beams. It can be clearly seen that the two types of reinforcement behave differently. The unidirectional carbon reinforcement (CU-DR) enhanced all the mechanical properties with remarkable improvements of 19.5%, 20.4% and 40.9% for the MoEdyn, MoEm,g and maximum stress, respectively. With respect to the bi-directional CFRP, no improvement is obtained, as the values are within the COV. This fact can be associated with the edgewise position and the external application of the CFRP. However, an increase in the maximum stress of 16.4% was achieved for this type of reinforcement. Regarding the stress-deflection relationship shown in Figure 9, an improvement in the ductile behavior of the reinforced beams can be noted, regardless of the type of CFRP.

Poplar LVL

Beams mainly failed in two patterns, tensile and shear. Figure 10 shows some examples for each failure pattern. NR beams and all reinforced beams tested in edgewise position had a tensile failure. Thus, the maximum EW bending stress could be considered as the maximum bending strength. However, all the beams with FRP tested in flatwise position had shear failure. The reason lies in the fact that the flexural strength contribution of the FRP exceeds the shear strength of the poplar veneer itself, causing this type of failure. Thus, the maximum FW bending stress was not the maximum bending strength, but the bending stress reached when the shear failure occured. This may explain why the maximum FW bending was only slightly higher than maximum EW bending stress. As a result, the shear resistance becomes a key aspect for both the design and a proper application of the reinforcement. Figure 11 depicts a comparison between the dynamic modulus for each beam obtained for flatwise and edgewise position, MoEdyn-FW and MoEdyn-EW, respectively. Due to the layup, reinforced LVL was stiffer in flatwise than in edgewise configuration, while it was not the case for non-reinforced LVL. It can be seen the clear influence of the reinforcement of the CU_Q1 beams compared to the non-reinforced. The variation of bending moduli appeared to be much higher for CU-Q1 than CU_Q2 and other types of reinforcements, and the enhancement of the dynamic modulus was higher in flatwise than edgewise configuration. Regarding Group I (NR/CU -Q1), it can be clearly observed that the CU_Q1 beams reached higher values for all the mechanical properties respecting to the nonreinforced ones, making especially relevant the enhancement for the case of elastic modulus of flatwise beams. The improvement of MoEdyn, MoEm,g and maximum stress for flatwise beams was of 48.5%, 42.2% and 24.1%, respectively, while for the case of edgewise ones, the improvement was of 19.1%, 22.1% and 13.0%, respectively. Both enhancements in EW elastic moduli were fully in accordance with those obtained in the case of Douglas fir LVL (19.5% and 20.4% for the MoEdyn and MoEm,g, respectively). However, the improvement in EW maximum bending stress was higher for Douglas fir LVL, 40.9% instead of 13%, but the CU reinforced values were similar (67.7 MPa and 69.0 MPa for poplar and Douglas fir, respectively). Thus, the relative improvements of the same CU reinforcement were different, and this may be explained by different veneer qualities. Figure 12 shows a remarkable improvement in the ductility behavior for the CU_Q1 beams compared to the common brittle failure of NR beams. This ductile behavior can be explained by the ductile behavior of poplar under compressive stress, the LVL being compressed while the FRP resisted under the tensile stresses.

Reinforcement

Table 6 summarizes the results obtained for the beams of Group II, comparing the same type of reinforcement for different qualities of the panel. The importance of considering veneer quality as a key parameter to evaluate the mechanical results is clearly demonstrated. In particular, the improvements of the MOEdyn, MoEm,g and maximum stress for flatwise testing in the case of CU_Q1 beams were of 11.5%, 14.6% and 12.3%, respectively, while for the edgewise testing they were of 6.0%, 14.6% and 10.5%, respectively. As expected, reinforced flatwise beams achieved higher values than edgewise regardless of the quality of the panel. The improvement in ductility for both the FW and EW layouts (Figure 12) is evident when compared with non-reinforced beams, no matter the quality of the panel is. Group III compares the three types of FRP, CU, CB and BU, in a low-quality panel (Q2). It can be observed that the beams reinforced with carbon unidirectional FRP reached the highest values of mechanical properties for both flatwise and edgewise testing, the latter always providing lower ones. Similar results were obtained for carbon bidirectional and basalt unidirectional reinforcements. This fact becomes relevant considering that the cost of the BU reinforcement is one third of CB. The cost of the CU reinforcement is 70% and 400% higher than the CB and BU reinforcement, respectively. Comparing the elastic modulus of the CB reinforcement for LVL Douglas fir and poplar beams, an improvement respect to non-reinforced beams of the latter was obtained, which may be due to the fact that the FRP material was embedded. This behaviour can be associated with the fact that the external veneers when CB reinforcement is embedded help to reduce the transversal deformation of this type of fabric. Note that in all cases, MoEdyn, MoEm,g and maximum stress values were higher than those of the NR_Q1 beams. This translates directly into a greater use of wood as a base raw material, and therefore, of a better use of second quality wood characterized by a higher presence of knots, lower density and thus lower and more scattered mechanical properties. As Figure 12 shows, regardless of the type of FRP reinforcement, the beams show a ductile behavior, especially in the case of FW beams.

Conclusions

An evaluation of the bending properties of Douglas fir and poplar LVL beams reinforced with different fibers has been carried out, studying the effects of different variables as the veneers quality and orientation in the beam. A clear improvement provided by unidirectional carbon is demonstrated (up to 40% more in the elastic modulus for the flatwise layout and more than 20% of the maximum stress, for both types of wood). However, a modest enhancement of the mechanical properties was provided by the bidirectional carbon and the unidirectional basalt.

For the case of low quality Douglas fir LVL beams, placing the reinforcement at the external part of the beam, a clear enhancement of the main mechanical properties MoEdyn, MoEm,g and maximum bending stress was achieved when unidirectional carbon reinforcement was used (19.5%, 20.4% and 40.9%, respectively). On the opposite, bidirectional reinforcement only improves the maximum bending stress (16.4%).

During the manufacturing process of the LVL poplar beams, the reinforcement was embedded, allowing it to be hidden from view. Furthermore, two quality of wood-panel manufacturing was analyized, Q1 and Q2 for high and low quality, respectively. The analysis was divided in 3 groups. For group I, Q1 panel, non-reinforced versus unidirectional carbon reinforced beams, a remarkable improvement of the mechanical properties was achieved in flatwise bending (48.5%, 42.2% and 22.1% for MoEdyn, MoEm,g and maximum stress, respectively). Comparing the edgewise tests from both species, results were in totally accordance, suggesting that these results can be transferable to other types of wood. Furthermore, the influence of the quality of the panel has been proven, comparing the same type of reinforcement for Q1 and Q2 panels: reinforced Q2 poplar LVL exhibited better mechanical properties than non reinforced Q1 LVL. This fact could lead to a better use of wood material and for example the upper parts of the tree (usually lower quality).

When comparing different types of reinforcements for low quality panels, as expected, the CU reinforced beams achieved the higher values, obtaining similar results for carbon bidirectional and basalt unidirectional. It is important to note that basalt fiber costs is 1/3 of carbon fiber. Comparing the cost difference against mechanical properties enhancement, the application of additional layers using basalt fiber could lead in a better solution than a single CU layer at a lower cost. The additional manufacturing costs due to the application of reinforcements should also been evaluated. To evaluate that, a deep analysis is needed. A comparison between the dynamic modulus in flatwise and edgewise position for each beam has been carried out, remarking the enhancement of the reinforced beams in both positions. The relationship between MoEdyn-FW / MoEdyn-EW has been calculated, obtaining a value of 0.99 for the non-reinforced beams, meanwhile this value was of 1.22, 1.20, 1.10 and 1.07 for CU_Q1, CU_Q2, CB_Q2 and BU_Q2 beams, respectively. The bidirectional carbon fiber reinforced beams provided an increase of the MoEdyn in both positions due to its braiding with a possible shear benefits. More studies are needed on this topic.

Figure 1 .

 1 Figure 1. Sampling Douglas specimens scheme (non-representative number of plies). Distances in mm Test Reinforcement Location BT: "Bending Test" NR: "Non-Reinforced" UL: Up Left EW: "EdgeWise" UR: Up Right CB: "CFRP bidirectionnal" DL: Down Left CU: "CFRP Unidirectionnal" DR: Down RightTable 1. Douglas specimens designations.

Figure 2 .

 2 Figure 2. Elaboration process of the reinforced Douglas fir LVL beams. Distances in mm.

Figure 3 . 1 Table 3 .

 313 Figure 3. Veneers used in the paneling process. Left: Low quality veneers for Q2 panels. Right: High quality veneers for Q1 panels. Q1 panels (high quality) Q2 panels (low quality) Width of the veneer (mm) 500 1000 Amount of adhesive (g/m 2 ) 200 150 Distance between adhesive lines (mm) 5 10 Part of the tree where the veneers come from (m) 0.5-2.5 2.5-6.1Table 3. Main characteristics of the two types of poplar panels.

Figure 4 .

 4 Figure 4. Manufacturing process of the reinforced LVL poplar panels. Distances in mm.

Figure 5 .Figure 6 .

 56 Figure 5. Images of the manufacturing process of the reinforced LVL poplar panels. a) Gluing application. b) Panels ready to be pressed. c) Application of the FRP. d) Placement of the external veneers.

Figure 7 . 4 -

 74 Figure 7. 4-point bending tests configuration for Douglas fir and poplar LVL beams.

Figure 8 .

 8 Figure 8. LVL poplar beam during bending test.

Figure 9 .

 9 Figure 9. Stress versus global deflection for the LVL Douglas fir tested beams.

Figure 10 .

 10 Figure 10. Failure patterns.

Figure 11 .

 11 Figure 11. Dynamic modulus for the tested poplar beams: flatwise versus edgewise. Red dotted line: y=x line. Tables 5, 6 and 7 show the average values of the dynamic elastic modulus (MoEdyn), global modulus (MoEm,g) and maximum stress for the beams tested in bending in both flatwise and edgewise configurations. Due to the shear failure mode for the reinforced beams tested in flatwise (Figure 10), it was not possible to determine the real MOR for this particular case. Figure 12 represents the stress versus the global deflection for all the beams in each group.

Figure 12 .

 12 Figure 12. Stress versus global deflection for all the LVL poplar beams tested in flatwise and edgewise.

UNI- AX MAPEWRAP C BI- AX MAPEWRAP B UNI- AX Type of FRP

  Table 1 summarizes the main properties of the three types of used commercial FRPs.

		Carbon	Carbon	Basalt
	Fiber orientation	Unidirectional	Bidirectional [0/90]	Unidirectional
	Specific weight (g/m 2 )	300	238	400
	Thickness (mm)	0,166	0,064	0,140

MAPEWRAP C

Resistant area per width unit (mm 2 /mm) 166

	,6	64,2	142,5

Max. tensile stress (MPa) 4830 4800 4840

	Tensile elastic modulus (GPa)	230	230	89
	Max. elongation (%)	2,0	2,1	3,2

Table 1 .

 1 Main properties of the FRPs.

Table 2 .

 2 Test matrix of LVL beams.

	Wood Species / source	Quality	Type of FRP	Bending test orientation	Nomenclature	Number of samples
	Douglas fir heartwood /	-	Non-reinforced	EW	Doug-NR-(UR/UL)	18 / 23
	Industrial	-	CFRP-uni	EW	Doug-CU-DR	18
	source	-	CFRP-bidi	EW	Doug-CB-DL	23
			Non-reinforced	Flatwise (FW)	Pop_NR_Q1	13
		Q1 (high quality)	Non-reinforced	Edgewise (EW)	Pop_NR_Q1	13
			CFRP-uni	FW	Pop_CU_Q1	5
	Poplar I-214 /		CFRP-uni	EW	Pop_CU_Q1	5
	One tree		CFRP-uni	FW	Pop_CU_Q2	8
			CFRP-uni	EW	Pop_CU_Q2	8
		Q2	CFRP-bidi	FW	Pop_CB_Q2	13
		(low quality)	CFRP-bidi	EW	Pop_CB_Q2	13
			BFRP-uni	FW	Pop_BU_Q2	13
			BFRP-uni	EW	Pop_BU_Q2	13

Reinforcement MoEdyn (GPa)±COV MoEm,g (GPa)±COV Max. Stress (MPa)±COV

	NR-UR	13.1±6.8%	12.1±7.6%	48.9±20.6%
	CU-DR	15.6±11.4%	14.5±5.5%	69.0±13.4%
	Var. respect NR-UR	19.5	20.4	40.9
	(%)
	NR-UL	12.3±11.1%	11.8±9.7%	47.7±19.7%
	CB-DL	12.4±9.6%	11.3±10.6%	55.6±14.7%
	Var. respect NR-UL	0.8	-4.2	16.4
	(%)

Table 4 .

 4 Main mechanical properties of the LVL Douglas fir beams tested.

Table 5 .

 5 Main mechanical properties of the LVL poplar beams tested for Group I: Evaluation of the enhancement provided by reinforcement CU.

	/Quality	MoEdyn (GPa)±COV	MoEm,g (GPa)±COV	Max. Stress (MPa)±COV
		FW	EW	FW	EW	FW	EW
	NR_Q1	9.4±4.1% 9.5±2.6% 8.9±4.6% 8.6±3.3% 58.1±12.9% 59.9±4.2%
	CU_Q1	13.9±11.0% 11.3±4.8% 12.7±7.2% 10.5±5.6% 72.1±5.0% 67.7±4.4%
	Var. respect NR_Q1 (%)	48.5	19.1	42.2	22.1	24.1	13.0

Table 6 .

 6 Main mechanical properties of the LVL poplar beams tested for Group II: Evaluation of the influence of the panel quality.

	Reinforcement/Quali ty	MoEdyn (GPa)±COV FW EW	MoEm,g (GPa)±COV FW EW	Max. Stress (MPa)±COV FW EW
	CU_Q2	12.5±8.5%	10.7±2.4 %	11.0±6.1 %	9.8±3.1%	63.2±5.8 %	60.6±3.2 %
	CU_Q1	13.9±11.0 %	11.3±4.8 %	12.7±7.2 %	10.5±5.6 %	72.1±5.0 %	67.7±4.4 %
	Var. respect CU_Q2 (%)	11.5	6.0	14.6	6.9	12.3	10.5
	Reinforcement/Quali	MoEdyn (GPa)±COV MoEm,g (GPa)±COV Max. Stress (MPa)±COV
	ty	FW	EW	FW	EW	FW	EW
	CU_Q2	12.5±8.5% 10.7±2.4% 11.0±6.1% 9.8±3.1% 63.2±5.8%	60.6±3.2 %
	CB_Q2	11.0±2.0% 10.0±2.6% 9.4±4.2% 8.8±4.0% 62.7±8.5%	58.9±5.3 %
	BU_Q2	10.6±5.9% 9.9±3.7% 9.5±6.0% 8.9±5.6% 64.6±8.4%	57.6±6.1 %

Table 7 .

 7 Main mechanical properties of the LVL poplar beams tested for Group III: Comparison of the three types of FRPs.

Acknowledgments and funding sources

Funding: This work has been possible thanks to financial support of the COMPOP_Timber project "Desarrollo de productos de ingeniería elaborados a base de tablones y chapas de chopo con inserciones de material compuesto para su uso en construcción", BIA2017-82650-R, Acknowledgements to the region of Burgundy Franche-Comté, France Douglas, and Thebault Group. This work is a consequence of the postdoc stay carried out by Dr. Rescalvo at LaBoMaP.