Bending and shear reinforcements for timber beams using GFRP plates

Construction and Building Materials 96 (2015) 461–472

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Bending and shear reinforcements for timber beams using GFRP plates M.J. Morales-Conde a,⇑, C. Rodríguez-Liñán b, P. Rubio-de Hita b a Instituto Universitario de Arquitectura y Ciencias de la Construcción, Escuela Técnica Superior de Arquitectura, Universidad de Sevilla, Avenida Reina Mercedes, n° 2, 41012 Sevilla, Spain b Departamento de Construcciones Arquitectónicas 1, Escuela Técnica Superior de Arquitectura, Universidad de Sevilla, Avenida Reina Mercedes, n° 2, 41012 Sevilla, Spain

h i g h l i g h t s  A procedure is developed for the repair or reinforcement of timber beams.  This procedure uses fiberglass and cork plates bonded with epoxy resin.  The proposed experimental technique was tested in two situations.–repairing beam ends: Reinforcement system 1.–repairing and/or reinforcement of

the center of the beams: Reinforcement system 2.

a r t i c l e

i n f o

Article history: Received 11 December 2014 Received in revised form 19 May 2015 Accepted 12 July 2015 Available online 14 August 2015 Keywords: Timber Fiberglass Reinforcement Shear strength Bending strength Rehabilitation

a b s t r a c t The aim of this paper is to present the results from a procedure developed for the repair or reinforcement of timber beams. This procedure uses fiberglass and cork plates bonded with epoxy resin. The proposed experimental technique was tested in two situations: repairing beam ends that had rotted (Reinforcement system 1), and repairing and/or reinforcement of the center of the beam damaged because of a fault or the need to increase the resistance capacity of the timber (Reinforcement system 2). Tests were carried out using Scots pine beams with a size of 50  50  725 mm3 for the reinforcement system 1, and 50  50  1000 mm3 for the reinforcement system 2. In both systems, the reinforcements were tested in various situations, and provided effectiveness similar to that of a healthy beam, and an increased load-carrying capacity of up to 50%. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction A significant part of our building heritage includes timber elements, generally as part of floor or roof structures. The rehabilitation of these buildings usually requires interventions to ensure the stability of their timber structures [1]. Wood as a natural material is vulnerable to attack by biotic agents, such as fungi or insects, and abiotic agents such as fire. The damage caused by these conditions, or new load or design alterations, can require an increase in the load-carrying capacity of the structure. The exact position where reinforcement is required must be taken into account when designing a specific solution for intervention. In most cases, the problem with timber beams is related to the biotic degradation of the supports. However, it sometimes extends to the entire length of the beam. This information is crucial

⇑ Corresponding author. E-mail address: [email protected] (M.J. Morales-Conde). http://dx.doi.org/10.1016/j.conbuildmat.2015.07.079 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

for providing efficient repair or strengthening solutions in order to ensure the safety and serviceability of the structure. The most immediate solution for beam supports consists of attaching a new structural system to the wall to support the existing structure. If the problem extends to the entire length of the beam an immediate solution is to place mullions to shorten the span of the old beams or add struts as intermediate support points. Reinforcements such as applying new timber or steel pieces to the existing beam by mechanical fixings are considered temporary and are only used in emergencies or in cheap repair works [2]. The main disadvantage is that the elements added in these interventions are straight while the original pieces are usually affected by residual deformation. The joint needs some deformation in order for the reinforcement to work, which leads to very low efficiencies. However, a recent study using a steel U-shaped cross-section screwed to the upper side of the beams showed an increase in stiffness of between 45% and 98% and in load-carrying capacity of between 27% and 58% [3]. It should also be taken into

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consideration that steel repairs limit the stability of the structure in the case of fire and present the additional problem of corrosion. Another possible use of wood as a reinforcing material is to splice the two parts. A study showed that splicing the two parts with an oblique cut and using oak pins can raise load-carrying capacity to an efficiency of 0.20 and the stiffness to a factor to 0.24. This solution is increasingly effective when four perpendicular bolts are added, yielding a flexural efficiency of 0.35 and a stiffness factor of 0.27. Optimum efficiency is achieved with an oblique connection at the edge of the pieces, giving a bending efficiency of 0.34 and a stiffness factor of 0.59 [4]. Due to the low-level efficiency of these solutions, they can only be used in areas where bending stress is reduced. Glued joints are an alternative to mechanical fixings as they increase the structural performance of the reinforced elements. Some authors have tested these unions by gluing timber elements with resorcinol adhesives [5,6], and found that the mortise-and-tenon joint system achieves an efficiency of between 0.50 and 0.60. Also, union by oblique cut on the beam face increases the load-carrying capacity to 0.85 and the stiffness factor to 0.97. The splice on a vertical plane is the most effective, with an efficiency that is almost at the unit, added to which is the favorably aesthetic impact of these solutions and respect for the original materials. The use of resins and composite materials dates back to the 1960s [7,8]. There have been several investigations of FRP as wood-reinforcing materials, aimed at improving durability, esthetics and mechanical behavior. Most researchers have used glass fiber-reinforced plastic (GFRP) in various forms parallel to the grain to reinforce solid wooden beams. In 1981, the stiffness and tensile strength increase in reinforced beams tested to bending was analyzed [9] and in 1983, the ‘‘Beta System’’ patent was developed [10]. This system was introduced in the market and is one of the most widely used reinforcement techniques. This consists of replacing the damaged part of the beam ends with an epoxy mortar joined to the sound wood by glass fiber bars, the main advantage being that it can fill cavities without the inconvenience of shrinkage or failure to adhere to certain materials. A variant of the solution is to join the two pieces with steel rods connected by epoxy resin. Two smaller-diameter bars inclined at 45° are inserted into the sides to provide lateral stability, with a consequent load carrying capacity of 0.60 and 2.38 in stiffness [4] (Fig. 1). Another solution is the insertion of the glass-fiber reinforced polymer bars as armour inside the cross-section by making grooves on the surface or sides [11,12]. Research findings indicated that the use of near-surface GFRP bars overcomes the effect of local

50 mm

Fig. 1. Two-piece splice with steel bars and epoxy formulation.

defects in the timber and enhances the bending strength of the members from to 18% to 46% [11]. Also, reinforcement involving bars placed at an angle is used for large sections in which cracks appear during the process to dry the pieces. These cracks are stitched by using the Beta System with bars inserted at angles ranging from 20° to 30° from the top side, and the epoxy formulation is injected when the reinforcing bars are in place. Some authors have verified the upgrading of beams by using a triangular truss with epoxy resin bars reinforced with fiberglass or steel cables, whose elasticity behavior improved with an increase in the load-carrying capacity of between 32% and 42% and a deformation decrease ranging from 59% to 76%. Nevertheless, in practice this procedure is highly complex [13]. A large part of the research undertaken to date uses glass fiber as reinforcement in composite materials [14–16]. Recently, the appearance on the market of new products such as carbon fiber or aramid and basalt fibers have increased the number of research works on their implementation as reinforcement materials in timber elements [17–21]. These studies have contributed to the usage of FRP and serve as reference for future research. Plevris & Triantafillou [22] reinforced wood beams by placing a thin unidirectional carbon fiber-reinforced sheet on the tension face of the timber beam. It was found that the bending strength increases almost linearly with an increased quantity of fibers up to a critical value, beyond which the moment becomes almost constant. This suggests that the bending failure of beams varies depending on the volume of FRP used. However, the increase in the carrying capacity of reinforced beams also depends on the nature of the timber element (the reinforcement is more relevant for wood with poor resistance [23]), the type of fiber used, the layout of the reinforcement in the element and the integrity of the bonding surface between the FRP and the timber during the test until fracture [21]. Therefore, the results obtained in research works vary according to these aspects and cannot be compared to each other. In general, the works published conclude that the use of FRP reinforcements improves the mechanical properties of reinforced timber elements [24]. In 2002, Fiorelli and Dias [25] focused their research on the experimental and theoretical analysis of timber beams reinforced with GFRP and CFRP. In this study, timber beams were reinforced at their lower face and the adhesive was applied on the surface of the fibers. The final product was a laminated material (fibers + adhesive). The results of this research showed that the flexural stiffness (EI) determined experimentally was greater than the theoretical values. These values favored greater structural safety. For beams reinforced with GFRP there was an increase in stiffness of 1.15. In 2008, Gomez and Svecova [26] studied the behavior of timber stringers reinforced with GFRP sheets. The stringers were reinforced for shear and bending. The proposed reinforcement leads to an improvement in stiffness of between 1.05 and 1.53. In 2009, Alam et al. [27] studied the behavior of different reinforcement materials including GFRP. In this work, grooves were routed into the faces of the fractured beams following straightening, and the reinforcements adhesively bonded onto the top, bottom or both faces of the beams. The results varied widely and the fracture mechanisms in the repaired beams depended on the placement of the reinforcement and the quality of the adhesive for bond reinforcement. All properties were optimized by the bonding reinforcement of both faces of the fractured beams. In 2010, a new work by Yusof and Saleh [28] in which the beams were reinforced with GFRP rods embedded in slots on the lower side. The test on carrying capacity showed an increase of between 1.20 and 1.30 with a stiffness increase of between 1.24 and 1.60. Several research works have studied the reliability of the design model to predict the bearing capacity in the reinforced elements

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without reinforcements. The effect of the placement of the reinforcement and its anchorage length on the mechanical properties of reinforced beams is studied in each situation.

Stress

Compression

2. Test materials

MOEc Strain

Tests were conducted with beams and glass-fiber reinforcement plates. A total of 62 beams were tested. Beams were divided into two groups according to the position of the reinforcement plates to be tested, 35 pieces were tested with reinforcement plates in a support and 27 were tested with the reinforcement plates in the middle of the pieces.

MOEt Tension

Fig. 2. Relationship for wood in tension and compression parallel to the grain (Buchanan, 1990).

2.1. Timber mechanical properties

[29]. For that purpose, the transformed section method and the section conditions of equilibrium are used. When applying calculation models the Buchanan concept [30] is assumed, in which the timber, when subject to tensile force, presents an elastic-linear behavior and when tested by compression the timber presents an elastic-linear behavior and a nonlinear inelastic behavior [31] (Fig. 2). The correct response of the reinforced set is related to the proper functioning of the surface bonding. The glue-line is the weakest ‘‘link’’ in the reinforcement, and this will fail on a critical stress level [27]. In 2000, Dagher [32] explained that the principal problem with the reinforcement is incompatibility between the wood and the reinforcing material. The differences in hygro-expansion and stiffness between the wood and reinforcing materials can lead to separation at the glue-line. Bond integrity may be compromised when there are fluctuations in the moisture content, especially when there is a large stiffness differential between the timber and reinforcement [27]. A study by Wheeler and Hutchinson [33] demonstrated that typical structural epoxy adhesives are well adapted for use in bonding timber. In their research, they found that there was little or no compromise in the bonding strength in moisture content of up to 22%. Similar conclusions were obtained by Broughton and Hutchinson [34]. Beyond this critical moisture content of 22%, a decrease in resistance was reported. After analyzing the existing solutions, the aim of this study is to contribute to the knowledge through the establishment of an experimental technique for strengthening and rehabilitating timber pieces using bonded fiberglass plates. The system is analyzed in two situations: as reinforcement in beam supports and as reinforcement for beam centers. The beams are reinforced by routing out grooves on them and bonding in reinforcement in the form of GFRP plates. The beams are fractured in the laboratory where shear and bending strength are tested. Mechanical properties are compared with the properties of control beams, without reinforcements or grooves, and with the properties of grooved beams but

2.2. Reinforcement system 1 (S1): reinforcements at the beam ends (beams reinforced in shear) Thirty-five pieces were used to test the Reinforcement system 1 (S1). These pieces had a size of 50  50  725 mm3 and were divided into 7 series (Fig. 3). The groups had 5 pieces selected at random. A series was tested as control sample (series S1R), and no reinforcement was executed. Three groups of 5 pieces (series S1A, S1B and S1C) were perforated with a groove 6 mm wide and a length of 175, 225 and 275 mm, respectively. No reinforcement plates were inserted in these samples. The purpose was to assess the decrease in resistance caused by the grooves. The three other samples (series S1A0 , S1B0 and S1C0 ) were tested with reinforcements. Grooves were routed into the faces of the pine sections to a width of 6 mm. The reinforce plates were inserted

550

725

50

S1R

The timber used in the study is Scots pine from the demolition of an old building. Each beam was 50  50 mm in cross-section and the length was 725 and 1000 mm, respectively. Before being reinforced, beams were classified according to UNE 56544 standard [35], regarding visual classification of conifer wood for structural use. This standard is set for beams under study since their depth is less than 70 mm. Two quality classes, ME-1 and ME-2, are defined which correspond, in the case of Scots pine, to resistant class C27 and C18, respectively. Mechanical properties for each resistant class are detailed in the UNE-EN 338 standard [36]. The pieces, which were outside the limits set by the standard, were classified as rejected, and no mechanical properties are established for them within this standard. From the 62 beams tested, 23 were classified as ME-1 and 12 as Rejected for the case of beams of 725 mm in length. For beams of 1000 mm in length, 18 pieces were classified as ME-1 and 9 specimens as Rejected. The beams were distributed evenly for each type of reinforcement and the same number of ME-1 and Rejected beams was used for each reinforcement system.

50

535

475 275

215

175

Groove width=6mm

S1A

S1B

550

S1C

535

S1A'

275

215

175

Reinforcement plates 175,215,275x50x4mm

475

S1B'

Fig. 3. Samples to test the Reinforcement system 1 (S1).

S1C'

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Table 1 Number and type of samples to test the Reinforcement system 1 (S1).

Table 2 Number and type of samples to test the Reinforcement system 2 (S2).

Series

Number of specimens

Specimen size (mm3)

Groove: length, width (mm)

Reinforcement: length, depth, width (mm)

Series

Number of specimens

Specimen size (mm3)

Groove: length, width (mm)

Reinforcement: length, width (mm)

S1R S1A S1B S1C S1A0 S1B0 S1C0

5 5 5 5 5 5 5

50  50  725

– 175, 225, 275, 175, 225, 275,

– – – – 175, 50, 4 225, 50, 4 275, 50, 4

S2R S2A S2B S2C S2D S2A0 S2B0 S2C0 S2D0

3 3 3 3 3 3 3 3 3

50  50  1000 50  50  1000

– 500, 600, 700, 810, 500, 600, 700, 810,

– – – –

50  50  725

50  50  725

6 6 6 6 6 6

vertically into the grooves measuring 175, 225 and 275 mm in length, respectively. Each test group is specified in Table 1. 2.3. Reinforcement system 2 (S2): reinforcements at the beam midpoints (beams reinforced in bending strength) Twenty-seven pieces measuring 50  50  1000 mm3 were used to test the Reinforcement system 2 (S2). These were divided into 9 groups of 3 samples each (Fig. 4). A group of nonreinforced pieces was used as reference sample (series S2R). Four samples (series S2A, S2B, S2C and S2D) were grooved to a width of 6 mm with lengths of 500, 600, 700 and 810 mm, respectively. The grooves were centered according to the length of each sample. No reinforcements were incorporated to these pieces. The 4 remaining groups were grooved and reinforced using plates of 500, 600, 700 and 810 mm in length (series S2A0 , S2B0 , S2C0 and S2D0 , respectively). The test groups are shown in Table 2. 2.4. Reinforcing materials Reinforcement plates were manufactured on site in the lab. They were made of cork, as backing material, and two layers of fiberglass fabric, the latter as resistant material. The cork sheets were 1.5 mm thick with a density of between 0.12 and 0.25 kg/dm3. Cork is used due to its low price, and is immune to rot as well as being compatible with wood. The suberin and wax it contains make it almost impermeable, and its appearance adds to the aesthetic of the reinforcement. The fiberglass fabric is produced by the same procedure for textile fibers, and is prepared with an anti-alkaline coating. It has a weight of 300 g/m2 and has great tensile strength, making it suitable for the structural requirements of the reinforcement (Table 3). Unsaturated orthophthalic polyester resin was used to glue the fiberglass to the cork. This material is low in thixotropic styrene (ecological), and was applied by simultaneous projection. The catalyst was Mek peroxide at a rate of 20 cc per kg of resin. Prior to applying the polyester resin, the cork surface was cleaned with a brush, and a primer layer of

250

1000

Groove width=6mm 50

S2R

50

250

Glass-fiber (300 g/m2) Orthophthalic polyester resin Glass – orthophthalic polyester composite [37]

S2A'

Tensile modulus (GPa)

2940 55 –

76 4 26

3.1. Calculating moisture content Before the mechanical tests were carried out, the moisture content was determined by a digital hygrometer following the procedure described in the UNE EN 13183-2 [38] standard.

95

150

700

810

200

600

150

95

700

200

S2B'

95

S2D

S2C 150

250

Reinforcement plates 500,600,700,810x50x4mm

Tensile strength (MPa)

The purpose of the mechanical tests was to check the resistance of the reference pieces, analyze the decrease in resistance caused by the grooves made in the beams and the increase in strength once the reinforcements were in place. Different mechanical tests were performed according to the placement of the reinforcement system tested in each case.

S2B 200

4 4 4 4

3. Experimental procedure

250

500

50, 50, 50, 50,

fiberglass fabric was incorporated, then the polyester resin was applied. After the resin dried, a second layer of fiber was applied on the other side of the cork support and the polyester resin added. Epoxy resin was used to bond the reinforcements and timber beams. A formwork was applied to the lower part of the beams to fill the groove, and high pressure jacks were attached to reinforce the hardening of the resin and ensure the adhesion of the plate to the beam. Epoxy resins are two-component adhesives with rapid polymerization. Once hardened, they provide high tensile strength and impact resistance, with no shrinkage problems. The weight ratio of the mixture used was 2:1 (100 g resin per 50 g of hardener). This resin was of low viscosity and was insoluble in water; its composition was bisphenol-A-epichlorohydrin and oxirane mono [(C12-14 alkyloxy) methyl] with a density of 1.15 g/cc.

600

S2A

500, 600, 700, 810,

Table 3 Mechanical properties of the reinforcing materials.

200

500

50  50  1000

6 6 6 6 6 6 6 6

810

150

S2C'

Fig. 4. Samples to test the Reinforcement system 2 (S2).

95

S2D'

465

M.J. Morales-Conde et al. / Construction and Building Materials 96 (2015) 461–472 3.2. Mechanical testing of the Reinforcement system 1 (S1): beams reinforced in shear In the Reinforcement system 1 (S1), timber pieces were reinforced with GFRP in shear. The reinforcement system was tested by mechanical tests conducted with a single point of load application, the span between supports being 625 mm. The load was applied at a distance of 2.5 h from the support where the reinforcement is placed. In this way the reinforcement is subjected to the highest possible load to estimate the maximum shear and moment of rupture in the area tested. This methodology is described in the EN 1168 [39] standard (Fig. 5). It is important to note that these test conditions are more unfavorable than situations in which the load of a slab is subjected to continuous pressure. Maximum shear strength was calculated as follows (1):

Smax ¼

F  b F  ðL  aÞ ¼ ðKNÞ L L

ð1Þ

where; F Total load applied, in Kilonewtons, KN. L Span between supports, in meters, m. In this specific case is 625 mm. a Distance between the point of load application and the closest support (2.5 h), in meters. In this test, 125 mm. And the maximum bending moment was (2):

M max ¼

Fab L

ðKN mÞ

ð2Þ

The three plate lengths tested in the Reinforcement system 1 (S1) were justified by the operating procedure of the trial. The minimum length of the plate at 175 mm covers the distance from the support to the point of application of the load and to the end of the piece. The plates measuring 225 mm cover an additional anchoring of 50 mm, the depth of the plate, and the 275 mm plates an anchoring of 100 mm, slightly longer than twice the depth of the plate. 3.3. Mechanical testing of the Reinforcement system 2 (S2): beams reinforced in bending strength In the Reinforcement system 2 (S2), timber pieces were reinforced with GFRP plates in bending strength. To evaluate the Reinforcement system 2 (S2), specimens were subjected to flexure in four-point bending according to the procedure described in UNE EN 408 [40] standard. This standard specifies tests in which the specimen should be placed by means of a simple support, and the span (distance between supports) must be equal to 18 times the depth of the piece. For the test pieces measuring 50  50  1000 mm3 this amounted to a span of 900 mm. Two loads were placed on the pieces which were equidistant from the supports at 6 times specimen depth (Fig. 6).

F h

b (500 mm)

a (125 mm)

h h

L (625 mm)

F· b L

F· a L

F· b L

S(x) F· a L

Shear stress diagram

M(x) Bending moment diagram

F· a· b L Fig. 5. Schematic of three-point bending test arrangement by UNE EN 1168:2005 [39].

F/2 h

a (300 mm)

F/2 a (300 mm)

a (300 mm)

h

h L (900 mm) F/2

F/2 F/2 Shear stress diagram

S(x)

F/2 Bending moment diagram

M(x)

(F/2)·a Fig. 6. Schematic of four-point bending test set-up by EN 408 standard.

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The pieces were tested to obtain the ultimate fracture load. The load was applied at a constant speed of 0.15 mm/s to the breaking point of the piece estimated at 300 ± 120 s in time. Maximum shear strength was calculated with the following expression (3):

Smax ¼

F 2

ðKNÞ

ð3Þ

where; F Total load applied, in Kilonewtons, KN. And the maximum bending moment was (4):

M max ¼

F  a ðKN mÞ 2

ð4Þ

where; F Total load applied, in Kilonewtons, KN.a Distance between a load point and the nearest support, in meters, m. In this specific case is 900 mm. The tests were performed in a servo-controlled electromechanical press, Suzpecar’s MEM-101/M4 model, equipped with a load cell of 200 KN and operationally controlled by an electronic system for travel or cargo control. Travel control was used in these tests by applying a loading speed of 9 mm/min.

4. Experimental results The data obtained were supplied by the software associated to the testing equipment. The results correspond to the measurement of the loads when applied to the beams up to failure. The procedure for measuring vertical displacements varied according to the test arrangements. With this data, load/displacement graphs for each of the series were displayed. Various modes of fracture were observed in the beams (Fig. 7). Most of beams fractured due to a failure of the lower fibers when in tension, or due to a combination of tension and shear. Where knots were present, they influenced the direction of the propagation of cracks. Also, the grooves caused premature failure when the discontinuity coincided with the load application point. In general, the fracture occurred at the point where the load was applied, for Reinforcement system 1, or in the center of the beams, in Reinforcement system 2. In some cases, the fracture was not appreciated, and there occurred a debonding of the reinforcement plate (Fig. 8). 4.1. Moisture content The moisture content of the pieces tested was within the range specified in the UNE EN 384 standard [41] as reference conditions. The mean value was 12.8% for the pieces tested in the Reinforcement system 1 (S1) and 13.4% for the specimens tested in the Reinforcement system 2 (S2).

that the presence of grooves caused a decrease in the fracture load, the greater the decrease, the longer the groove (series S1A, S1B, S1C). When the reinforcement plates were incorporated, the pieces fractured to a load greater (S1A0 , S1B0 , S1C0 ) than the previous series. The incorporation of reinforcement plates with lengths of 175 and 225 mm (series S1A0 and S1B0 , respectively) presented similar behavior and fractured under a similar load. The insertion of longer plates of 275 mm (S1C0 ) resulted in a more effective increase in the fracture load. In fact, only the S1C0 series fractured in a load significantly greater than the reference sample (S1R). In Table 4, the mean values and the variation coefficients of the rupture loads and displacements are presented. The variation coefficient ranged from 1.71 to 15.52, reflecting the samples’ homogeneity, since most series were below 10%.

4.3. Reinforcement system 2 (S2) Specimens were subjected to flexure in four-point bending. In this case, the data about the loads applied were supplied by the software connected to the testing equipment, and vertical displacements were measured manually using an LVDT displacement transducer to monitor the center point deflection. Fig. 10 shows the results for the nine series tested. Each graph shows the separate specimens according to the lengths of the grooves and the reinforcement plates used. Comparing the diagrams, it can be seen that for the S2A, S2C and S2D series, the grooves caused a similar decrease in the fractured load regardless of length. The S2B series fractured at a load significantly smaller than the other three series. When the reinforcement plates were incorporated, the fractured load increased. However, unlike what happened previously with the Reinforcement system 1, the results did not correlate to the length of the reinforcement plates. The pieces with the shorter reinforcement plates (S2A0 series) experienced the greatest increase in fractured load, while the series with the longest plates (S2C0 and S2D0 ) fractured under a similar maximum load, lower than that of S2A0 . The S2B0 series fractured under the lowest load. In Table 5, the mean values and the variation coefficients of the rupture loads and displacements are presented. The variation coefficient ranged from 5.25 to 18.20. The series with grooves but without reinforcement plates showed the most homogenous results with variation coefficient values below 10%. The series with the longest reinforcement plates (S2C0 and S2D0 ) showed a similar and greater variation coefficient of 16.87% and 18.20%. The S2A0 series showed the most homogenous fractured loads with a variation coefficient of 3.37%.

4.2. Reinforcement system 1 (S1) The loads applied up to failure and the vertical displacements measured on the beams were supplied by the software associated to the testing equipment. Fig. 9 shows the graphs for the seven series tested (S1R, S1A, S1B, S1C, S1A0 , S1B0 , S1C0 ). Each graph shows the separate specimens according to the lengths of the grooves and the reinforcement plates used. In general, it should be noted

5. Analytical study The analytical study is presented separately for each reinforcement system. In both systems, it is assumed that the cross-section of the pieces works as a unique element. So, the shear stresses at the bonding interface are not analyzed.

Cross grain tension Knot influenced Through failure Simple tension No visible fracture

a

b

Fig. 7. Fracture modes for the tested beams. (a) Reinforcement system 1 (S1). (b) Reinforcement system 2 (S2).

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14

S1R

14

12

S1A

12

10

Load (KN)

Load (KN)

Fig. 8. Fracture modes for the tested beams. Images of the mechanical tests.

S1A'

8 6 4

0

5

10

15

20

25

S1B'

8 6 4

14

0

0

5

10

15

Displacement (mm)

Displacement (mm) S1R

12

Load (KN)

S1B

10

2

2 0

S1R

S1C

10

S1C'

8 6 4 2 0

0

5

10

15

20

Displacement (mm) Fig. 9. Load/displacement graph of the Reinforcement system 1 (S1). Beams reinforced in shear.

20

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Table 4 Experimental results for the Reinforcement system 1 (S1). Sample type

Rupture load (KN) Mean value CoV (%)

Displacement (mm) Mean value CoV (%)

Maximum shear (KN) Mean value CoV (%)

Maximum bending moment (KN m) Mean value CoV (%)

Sample Sample Sample Sample Sample Sample Sample

12.37 (6.93) 10.21 (7.55) 8.28 (12.58) 6.97 (6.95) 10.53 (2.36) 10.86 (15.52) 13.45 (1.71)

16.10 12.96 12.32 11.01 14.33 13.51 17.90

9.89 6.93) 8.17 (7.55) 6.62 (12.58) 5.57 (6.95) 8.42 (2.36) 8.69 (15.52) 10.76 (1.71)

1.24 1.02 0.83 0.69 1.05 1.09 1.34

10 9 8 7 6 5 4 3 2 1 0

(6.17) (27.15) (14.24) (6.30) (30.60) (14.24) (7.71)

S2R S2A

Load (KN)

Load (KN)

S1R S1A S1B S1C S1A0 S1B0 S1C0

S2A'

0

5

10

15

20

25

30

35

40

45

10 9 8 7 6 5 4 3 2 1 0

S2R

0

5

10

S2C S2C'

5

10

Load (KN)

Load (KN)

S2R

0

15

20

25

30

35

15

20

25

30

35

40

45

Displacement (mm)

Displacement (mm) 10 9 8 7 6 5 4 3 2 1 0

(6.93) (7.55) (12.58) (6.95) (2.36) (15.52) (1.71)

40

45

10 9 8 7 6 5 4 3 2 1 0

S2R S2D S2D'

0

5

Displacement (mm)

10

15

20

25

30

35

40

45

Displacement (mm)

Fig. 10. Load/displacement graph of the Reinforcement system 2 (S2). Beams reinforced in bending.

Table 5 Experimental results for the Reinforcement system 2 (S2). Sample type

Rupture load (KN) Mean value CoV (%)

Displacement (mm) Mean value CoV (%)

Shear (KN) Mean value CoV (%)

Maximum bending moment (KN m) Mean value CoV (%)

Sample Sample Sample Sample Sample Sample Sample Sample Sample

7.07 4.37 3.12 4.46 4.51 9.03 6.69 7.45 7.03

28.00 17.00 12.00 16.00 13.00 34.00 30.67 29.33 23.00

7.07 4.37 3.12 4.46 4.51 9.03 6.69 7.45 7.03

2.12 1.31 0.94 1.34 1.35 2.71 2.01 2.24 2.11

S2R S2A S2B S2C S2D S2A0 S2B0 S2C0 S2D0

(9.25) (6.05) (10.66) (5.25) (6.22) (3.37) (11.74) (16.87) (18.20)

(6.17) (7.15) (14.24) (26.30) (29.52) (7.78) (13.58) (27.56) (30.43)

5.1. Reinforcement system 1 (S1): theoretical and experimental shear capacity The wood specimens of Reinforcement system 1 (S1) are assumed to be reinforced with an epoxy-bonded matrix of GFRP and Polyester Resin (PR) in the shear-critical zones. The pieces have

(9.25) (6.05) (10.66) (5.25) (6.22) (3.37) (11.74) (16.87) (18.20)

(9.25) (6.05) (10.66) (5.25) (6.22) (3.37) (11.74) (16.87) (18.20)

the cross-section detailed in Fig. 11. The wood sections in Fig. 11 have a height hw and a width bw, and the GFRP + PR composite reinforcement has a thickness tc and a height hc. By transforming the GFRP + PR composite reinforcement into a wood equivalent, the maximum shear stress (which occurs in the middle of the section) in the wood is given by the Collignon expression as (5): (see Fig. 11)

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Table 6 Mean fracture loads of the reinforced series for the Reinforcement system 1 (S1), experimental and theoretical values.

bt

hw = hc = h

bw

ðN=mm2 Þ

ð5Þ

where; F Fracture load applied in Newtons, N. Mz Maximum static moment of the area in cubic millimeters, mm3. bt Section width in millimeters, mm. It Moment of inertia in mm4. Relating these parameters to the transformed section:

bt ¼ 2bw þ tc 

Ec ¼ 2bw þ tc n Ew

3

It ¼ 2 

3

3

ð6Þ 3

bw hw nthc bw hw nt c hc þ ¼ þ 12 12 6 12

ð7Þ 2

M static ¼ 2  bw  n¼

2

hw hw hc hc bw hw ntc hc  þ nt c   ¼ þ 2 4 2 4 4 8

Ec Ew

ð8Þ

eth

Fexp/Fth

– 14.159

– 1.144

– 0.743 0.767 0.949

3  F max 2  bt  h

ðN=mm2 Þ

ð10Þ

2  bt  h  Cd 3

ðNÞ

ð11Þ

S1R S1A0 S1B0 S1C0

14 13 12 11 10 9 8 7 6 5 4 3 2 1

eth

eexpC

S1A

S1A'

S1B

S1B'

S1C

Fth S1R

S1C'

Series Fig. 12. Theoretical and experimental rupture loads. Theoretical and experimental effectiveness. Reinforcement system 1 (S1).

Table 6 shows the theoretical fractured load values, Fth, of the reinforced series calculated from the expression (11) defined above. Experimental loads, Fexp, correspond to the mean of ultimate load obtained for each reinforcement system. The second column indicates the coefficient of effectiveness, eexp, of each reinforcement system series regarding non-reinforced beams (series S1R), observing that the best results are obtained with the longest reinforcement plates (series S1C0 ). This coefficient is obtained by defining the GFRP + PR area fraction, qc, as follows (12):

qc ¼

Failure of the wood in shear will occur when the maximum shear stress sw.max becomes equal to the shear strength of the wood in horizontal shear sd. That value has been obtained from the mean value of the reference sample (S1R). Hence, the theoretical shear capacity, Fth, of GFRP + PR – reinforced cross-section is obtained as follows (11):

F th;max ¼

Fth (KN)

– 0.851 0.877 1.087

ð9Þ

where n = ratio of GFRP + PR matrix composite elastic modulus Ec (26 GPa) [37] to wood elastic modulus Ew in the longitudinal direction. Wood elastic modulus Ew was determined experimentally and the mean value was 4900 N/mm2 (4.9 GPa). This value was obtained by testing to compression thirty prismatic specimens and following the established model (Fig. 2), where the value of the elasticity modulus is considered the same under tension and compression. This value largely coincides with the value given by UNE-EN 338 [36] for a C14 strength class. Although the visual grading results provided a C27 strength class for the specimens classified as ME-1, others were rejected as structural pieces. Since hw = hc = h, it can be assumed as a rectangular section and the expression (5) is simplified as (10):

Cw;max ¼

eexp

12.37 10.53 10.86 13.45

Rupture Load (kN)

Fig. 11. Cross-section of the reinforced pieces.

F max  M static bt  It

Fexp (KN)

Series Series Series Series

bw bc

Cw;max ¼

Sample type

t c  hc tc ¼ 2  bw  hw 2  bw

ð12Þ

It can be written (13):

  F max 2 ð1 þ nqc Þ  ð1 þ nqc Þ 2 ¼ e ¼  ð1 þ nqc Þ 3 b  h  Cd 3

ð13Þ

The factor e in Eq. (13) defines the ratio of shear capacity of GFRP + PR – reinforced section to shear capacity of unreinforced section, and represents the effectiveness of the GFRP + PR reinforcement. The theoretical effectiveness of the reinforcements, eth, is shown in the fourth column. Finally, the last column shows the coefficients relating the theoretical and experimental values (Fexp/Fth) (Fig. 12). Since the cross-section is identical for all series tested, the theoretical fractured load, Fth, coincides in all cases. However, the different anchorage length provides different experimental fractured loads, Fexp. 5.2. Reinforcement system 2 (S2): theorical and experimental bending strength The specimens used in the Reinforcement system 2 (S2) have the cross-section detailed above in Fig. 11. These pieces are assumed to be reinforced with an epoxy-bonded matrix of GFRP + Polyester Resin (PR) in the flexural-critical zones. In this case, the maximum

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bending strength is given by the expression (14), defined in UNE-EN 408 [40] as follows:

rw;max ¼

a  F max 2W

ðN=mm2 Þ

ð14Þ

where; a Distance between a load point and the nearest support, in millimeters, mm. Fmax Fracture load of the specimen, in Newtons, N. W Section modulus, in cubic millimeters, mm3. The section modulus of the transformed section, Wt, defined in Fig. 11, is calculated by transforming the GFRP + PR composite section into a theoretically corresponding wood section using a modular ratio technique. The hypothetical transformed section of the reinforcement to an equivalent section of Scots pine is obtained according to the Eqs. (15)–(17):

Wt ¼

It ðh=2Þ

ð15Þ

where It is the transformed second area moment in mm3 defined in (7);

bt ¼ 2bw þ tc  3

It ¼ 2 

Ec ¼ 2bw þ t c n Ew 3

ð16Þ

3

3

bw hw nthc bw hw nt c hc þ ¼ þ 12 12 6 12

ð17Þ

Failure of the wood in flexion will occur when the maximum flexural strength, rw.max, becomes equal to the bending strength of wood, rd. That value has been obtained from the mean value of the reference sample (S2R). Hence, the theoretical rupture load of the GFRP + PR – reinforced cross-section is obtained as follows:

F th;max ¼

2  W t  rd a

ðNÞ

ð18Þ

Table 7 shows the theoretical fractured load values, Fth, experimental loads, Fexp, corresponding to the mean of ultimate load obtained for each reinforcement system. Also, the experimental

Table 7 Mean fracture loads of the reinforced series for the Reinforcement system 2 (S2), experimental and theoretical values. Fexp (KN)

eexp

Fth (KN)

eth

Fexp/Fth

Series Series Series Series Series

7.07 9.03 6.69 7.45 7.03

– 1.277 0.946 1.053 0.994

– 8.09

– 1.144

– 1.116 0.826 0.920 0.868

Rupture Load (kN)

Sample type S2R S2A0 S2B0 S2C0 S2D0

10 9 8 7 6 5 4 3 2 1

eexpA

S2A S2A'

eth

S2B S2B'

S2C S2C'

Fth S2R

S2D S2D'

Series Fig. 13. Theoretical and experimental rupture loads. Theoretical and experimental effectiveness. Reinforcement system 2 (S2).

and theoretical coefficient of effectiveness, eexp and eth, of each reinforcement system series regarding non-reinforced beams (series S2R) are shown (Fig. 13). It should be noted that although the tests were carried out using samples with a medium size, the related correlations (Eq. (14)) can be applied to structural beams as the results were corrected with the correction factor, kh, specified in UNE EN 384 [41] due to beam size. The value of this factor is calculated by the following expression:

kh ¼

 0:2 150 h

ð19Þ

where; h Height of the specimen in millimeters, mm. In this case, 50 mm. So, the 5-percentile for bending strength, rw.max, is corrected dividing by the factor Kh provided. 6. Results analysis The results analysis is given separately for each system: 6.1. Reinforcement system 1 (S1): reinforcements at the beam ends To verify the effectiveness of the Reinforcement system 1 (S1), an experimental program was carried out involving several samples with three different anchorage lengths (S1A0 , S1B0 , S1C0 series). Previously, three additional series (S1A, S1B, S1C) were tested without reinforcements. In these series, the beams were perforated with grooves of different lengths in order to test the decrease in fracture loads caused by the discontinuities. One series, S1R, was taken as a control sample. In all cases identical specimens were used to test each situation. Comparing the results displayed in Table 4, it can be observed that the grooved beams reached lower ultimate fracture loads in relation to the control sample. The lower the load, the longer the groove. In sample S1A, which had beams with grooves of 175 mm in length, the fracture load decreased by 18%, whereas in sample S1C with grooves of 275 mm, the ultimate fracture load experienced a decrease of 44% in relation to the reference sample. The S1B sample with grooves of 225 mm length experienced a decrease of 33% in ultimate load. These results were achieved with variation coefficients of 7.55, 12.58 and 6.95, respectively, reflecting the samples’ homogeneity in the case of S1A and S1C. On the other hand, the deformation was significantly different in all samples. For series S1A and S1B, the deflection was approximately 25% lower, and 32% for the S1C series. For the deformation, the variation coefficients varied between 6.30% and 27.15%, showing a higher heterogeneity. The tests conducted on the reinforced samples provided a general increase of the fracture loads. However, that increase was not uniform. S1A0 experienced a slight increase of 1% compared to S1A. The mean fracture load in reinforced samples of the S1B0 series was 31% higher than the fracture load in specimens in S1B, although the ultimate load of the control sample was not reached. Finally, the S1C0 series experienced a big increase of around 93% in relation to the grooved sample, S1C. In this case, the fracture load of the control sample was exceeded and the effectiveness of the system, eexp, was above 1 (1.087), as detailed in Table 6. The deformation in the S1A0 and S1B0 series was similar, and was 10% higher compared to the S1A and S1B samples, respectively. The deflection reached in these cases was lower than the reference sample. The displacement logged in the specimens of the S1C0 series increased 62% and exceeded the deformation of the reference sample by 11%.

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The calculation model used for analyzing the reinforced beams allows to predict the theoretical fracture load, Fth, Since the reinforcement system provided an identical cross section in the three series, S1A0 , S1B0 and S1C0 , the section modulus, W, and the theoretical fractured load. Fth coincided in all cases. However, the experimental results, Fexp, varied widely, as mentioned in Tables 4 and 6. This fact showed the importance of the anchorage length of the reinforcement used. The anchorage length of the three series tested was proportional to the height of the samples. No anchorage length was tested in series S1A0 . A length equal to the height of the specimens, h, was used in series S1B0 and series S1C0 was tested with an anchorage length of 2h, which turned out to be the most effective length. This fact showed that the anchorage length of the reinforcement plays a key role in the reinforcement system. 6.2. Reinforcement system 2 (S2): reinforcements at the beam midpoints Four different reinforcement lengths (series S2A0 , S2B0 , S2C0 , S2D0 ) were tested to study the most effective anchorage length in bending strength. Also, four additional series (S2A, S2B, S2C, S2D) were tested by perforation, and no reinforcements were added in order to test the decrease in fracture loads caused by the discontinuities. Series S2R was taken as a reference. As can be observed in Table 5, the results showed a decrease in the fracture load in the grooved beams (series S2A, S2B, S2C and S2D) when compared to the reference sample. For these series, the results were heterogeneous and there was no clear pattern to explain them since the decrease was not in line with the groove lengths. The fracture loads experienced a decrease from 36% for the S2D series with a groove length of 810 mm to 56% for the S2B series whose grooves of 600 mm in length. These results were achieved with variation coefficients of 6.05, 10.66, 5.25 and 6.22 respectively, reflecting the samples’ homogeneity. Also, comparing the deformations reached, the results showed a decrease which ranged from 40% for the S2A series to 57% for S2B. For deformation, the variation coefficients varied between 7.15% for the series with the shortest grooves, series S2A, and 29.52%, showing greater heterogeneity. When the samples were reinforced (series S2A0 , S2B0 , S2C0 and S2D0 ), the fracture load experienced an increase, although it was not uniform. The best results were obtained with the shortest reinforcements. Series S2A0 with reinforcements of 500 mm experienced a net increase of 103% compared to series S2A without reinforcement. The mean ultimate fracture load exceeded the value of the control sample, S2R, by 28% with an experimental effectiveness, eexp, of 1.277, as detailed in Table 7. It should be noted that the anchorage length of the reinforcements used in this series was 2h, twice the height of the specimens. The variation coefficient was 3.37%, showing greater homogeneity in the results of the samples tested. The increase in the fracture loads in the rest of series S2B0 , S2C0 and S2D0 was less significant and only the specimens in series S2C0 reached a greater rupture load than that of the control sample. In these samples, the variation coefficients were higher than that of the previous S2A0 sample, with values of 11.74%, 16.87% and 18.20%. Analyzing the displacements that occurred, the deformation was higher than in the reference sample in all reinforced samples, except the sample with the longest reinforcements, S2D0 , with a decrease of 18%. The greatest displacement was reached in series S2A0 with an increase of 21% in relation to the reference sample. The calculation model used for analyzing the reinforced beams allows us to predict the theoretical fracture load. Since the section modulus, W, was identical for all series, the theoretical fractured load, Fth, coincided in all cases. However, the experimental results, Fexp, varied widely, as mentioned in Table 7. This showed the importance of the anchorage length of the reinforcement used.

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As in the Reinforcement system 1 (S1), the most effective anchorage length was 2h, h being the specimens’ height. 7. Conclusions This paper presents the testing of a new procedure for reinforcement and/or repair of wooden beams comprised of GFRP plates with cork support as base material. Based on the results presented the following conclusions are drawn: The use of GFRP composites can be applied to timber beams as an effective solution for the repairing or reinforcement of beam ends and/or beam midpoints. In contrast to other studies presented, the system developed inserts the GFRP plates inside the timber element in order to protect them in case of fire. This solution improves the aesthetic appearance of the repaired element by minimizing the visual impact. The repairing or reinforcement of beam ends could restore the pieces to their original state and even reach fracture loads higher than that of the healthy beams, showing the effectiveness of the proposed system to shear strength of 1.087. Previously, pre-damaged beams were tested in order to quantify the decrease in shear capacity produced by the grooves. The samples tested experienced a mean decrease of between 18% and 44%. Then, the reinforced beams provided a net increase of between 1% and 93% on average. The repairing or reinforcement of beam midpoints showed that this procedure could increase the load-carrying capacity of the healthy beam by an efficiency factor equal to 1.277. In this case, the pre-damaged beams decreased their bending strength between 36% and 56% in relation to the healthy specimens. All tests were carried out using plates with 2 layers of GFRP with varying anchorage lengths in order to determine the most appropriate length. The greater effectiveness of the procedure for shear and bending strength, 1.087 and 1.277 mentioned above, was reached using plates with an anchorage length of 2h, h being the specimens’ height. The analytical study showed a theoretical effectiveness of 1.144. This effectiveness was amply attained in the repairing or reinforcement of beam midpoints. However, the tests conducted on the beams reinforced in shear were less effective. According to these results, future research should develop tests to improve the connection between the plate and beam, for example, by tightening the two elements together as the resin hardens. Likewise, additional tests should be considered by incorporating a greater number of GFRP layers on the reinforcement plates. Finally in future research, tests on real beams should be developed to adapt the system (number of plates and lengths) to a real work situation with beam supports and beam centers. It should also be noted that this is a versatile solution for works in various situations (with accessibility above or below the element). In addition, its visual impact is minimal and the cost is more feasible than other options where these materials are used since the amount of resin needed is reduced. Acknowledgements The author Ma Jesús Morales-Conde acknowledges the financial support of the V Research Plan of the University of Seville. References [1] A. Feio, S.J. Machado, M.V Cunha, Reabilitação Estructural: Análise de Técnicas de Reforço em Estruturas de Madeira, 1° Congresso Ibero-LatinoAmericano da Madeira na Construçao, CIMAD-2011, Coimbra. Portugal., 2011. [2] F. Arriaga, F. Peraza, M. Esteban, I. Bobadilla, F. García, Intervención en estructuras de madera, AITIM, 2002 (464 págs. ISBN: 84-873-8124-3).

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