Internal reinforcement of laminated duo beams of low-grade timber with GFRP sheets L.A. Basterra ⇑ , J.A. Balmori, L. Morillas, L. Acuña, M. Casado Research group of Timber Structures and Wood Technology, University of Valladolid, Avenida de Salamanca, 18, 47014 Valladolid, Spain highlights  We tested low-grade duo glulam GFRP internally reinforced timber beams.  Reinforced glulam beams have higher ultimate load capacity.  Reinforced beams show improvements in stiffness and ultimate moment capacity. article info Article history: Received 22 February 2017 Received in revised form 14 July 2017 Accepted 2 August 2017 Available online 10 August 2017 Keywords: Low-grade timber Glass fibre reinforced polymer (GFRP) Duo glulam Populus  euroamericana I-214 wood abstract The purpose of this work is to study the flexural behaviour of Populus  euroamericana I-214 low-grade glulam timber beams, internally reinforced with Glass Fibre Reinforced Polymer (GFRP). To this end, 30 unreinforced beams and 60 reinforced beams, with two ratios of GRFP reinforcement, were produced and tested. Results show an improvement in the flexural behaviour of reinforced beams, with noticeable increases in stiffness (MOE) and ultimate moment capacity (MOR). By using relatively low reinforcement ratios (1.07%, 1.6%) in the tension zone, we have measured an average improvement of 12.1% and 14.7% in stiffness, and an increase up to 23% in moment capacity. GFRP reinforced seems to reduce the influence of timber heterogeneity and singularities (knots, cracks ...), which translates into less scattered results and better characteristic values. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction The use of fibre-reinforced polymer (FRP) reinforcements in the renovation and repair of civil structures is widespread [1]. How- ever, their use in the manufacture of new wood-based products has been poorly developed. Progress in wood engineering since the 1940s has been based on wood-based composite products, such as laminated beams, microlaminated beams (LVL), rebuilt wood (LSL and PSL) and wood-based laminates (CLT) [2], without resorting to new materials, except in isolated cases such as FiRP Ò reinforced glulam technology. The introduction of reinforcements based on FRPs in the manu- facture of laminated beams allows the use of low-grade woods and fast-growing species for structural purposes, which increases the strength and flexural rigidity of the beams and reduces the vari- ability of mechanical properties [3]. In this way, these lower-cost woods present an economically viable commercial alternative with smaller sections, lower weight and desirable mechanical properties. Of the available reinforcing fabrics, unidirectional glass fibre (E-Glass) reinforced polymers (GFRPs) seem the most appro- priate because of their low cost and suitable mechanical properties [4]. In addition, they are resistant to corrosion, are poor electrical conductors and weigh approximately four times less than steel. In the present study, the duo type of laminated beam is used because it is the most basic industrialisation system. The GFRP reinforcement is introduced into the glue line, inside the section, during lamination of the duo beams. In this way, it improves the fire resistance of the reinforcement [5] compared to FRP external reinforcements because the exposure of the glue line to high tem- peratures limits its stability [6]. In addition, this arrangement allows the manufacture of beams with the same apparent section and different levels of performance by varying the weight of the interior reinforcement without any visual impact. 2. State of the art The use of high elastic modulus FRPs for the reinforcement of pieces of wood subjected to flexion has been studied by many researchers [7]. The first studies, which were conducted by http://dx.doi.org/10.1016/j.conbuildmat.2017.08.007 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: basterra@arq.uva.es (L.A. Basterra). Construction and Building Materials 154 (2017) 914–920 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat