Study on flexural behaviour of glulam beams reinforced by Near Surface Mounted (NSM) CFRP laminates

Construction and Building Materials 91 (2015) 23–31

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Study on flexural behaviour of glulam beams reinforced by Near Surface Mounted (NSM) CFRP laminates Weidong Lu a,⇑, Zhibin Ling b, Qifan Geng c, Weiqing Liu a, Huifeng Yang a, Kong Yue a a

College of Civil Engineering, Nanjing Tech University, Nanjing 211816, PR China School of Civil Engineering, Southeast University, Nanjing 210096, PR China c Kingdom Architecture Design, Nanjing 210019, PR China b

h i g h l i g h t s  NSM-CFRP reinforcement was adopted for reinforcing glulam beams made by Douglas fir and Poplar.  The flexural behaviour of the glulam beams was improved significantly by using NSM-CFRP reinforcement.  The glulam beams reinforced by NSM-CFRP reinforcement exhibit pseudo-ductile behaviour.  The reinforcement in the tension zone was seen to lower the location of the neutral axis of the reinforced beams.

a r t i c l e

i n f o

Article history: Received 10 January 2015 Received in revised form 19 March 2015 Accepted 29 April 2015 Available online 16 May 2015 Keywords: Near Surface Mounted CFRP laminates Glulam beams Flexural behaviour Bending test

a b s t r a c t Near Surface Mounted (NSM) reinforcement is an effective strengthening technique for timber engineering. This study investigated the flexural behaviour of glulam beams reinforced by NSM–CFRP laminates. A total of twenty-four glulam beams, including eighteen reinforced glulam beams and six unreinforced glulam beams were tested up to failure to evaluate the reinforcing effect in terms of the flexural strength and the flexural stiffness of the beams. Two types of timber, namely, Douglas fir and Poplar were taken for the tests. The ratio of reinforcement arranged from 0.331% to 1.322%. The experimental results indicate that the flexural strength and the flexural stiffness were significantly improved by 34.2%–52.3% and 8%–28.5%, respectively. Several reinforced beams exhibit the pseudo-ductile behaviour in comparison to the linear elastic behaviour of the unreinforced beams characterized by the brittle tensile failure. The strain profile distribution indicates that the compressive behaviour of the timber can be improved by using NSM–CFRP reinforcement at the tension zone of the beams. Theoretical analysis was adopted for the prediction of the flexural strength of the glulam beams and the predicted results was observed in good agreement with the experimental results of the tested beams. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Fibre reinforced polymer (FRP) material has been increasingly popular in strengthening structural timber members, due to its advantages, such as high strength to weight ratio, excellent resistance to corrosion, good durability and fatigue behaviour. Using FRP as reinforcement can be globally classified as two types [1]: Externally Bonded FRP (EBF) and Near Surface Mounted (NSM) FRP technique. EBF strengthening method has been successfully used in practical applications, since considerable research results are now available and several key aspects of this technique have been interpreted quite well by theoretical models. NSM–FRP technique usually requires slots on the surface of the reinforced elements, ⇑ Corresponding author. Mobile: +86 2558139868; fax: +86 2558169862. E-mail address: [email protected] (W. Lu). http://dx.doi.org/10.1016/j.conbuildmat.2015.04.050 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

and then the FRP reinforcement is bonded into the slot by adhesive. As a novel strengthening technique, it has also been reported in the existing literatures and is trying to be used for reinforcing both the concrete and timber structures. Compared with EBF strengthening technique, NSM–FRP reinforcement has several advantages [2,3]: (a) less prone to debonding from the substrates; (b) more easily installed between adjacent members to prevent debonding failures; (c) less exposure to external environment, so as to avoid the accidental impact and mechanical damage, fire, and vandalism; (d) insignificant changes on the aesthetic appearance of the strengthened structures. 1.1. Timber reinforced with FRP During the last decades, many studies have investigated the flexural behaviour of the timber beams strengthened by fibre

24

W. Lu et al. / Construction and Building Materials 91 (2015) 23–31

reinforced polymers, including FRP sheets, FRP plates and FRP bars. The possible feasibility and practicality of timber reinforced by FRP was evaluated by Theakston [4]. Bulleit [5] reported an early review on the reinforcement of wood materials. It was commonly reported that the flexural stiffness and the load capacity of timber beams reinforced by FRP materials can be improved significantly [6–11]. Several researchers also reported that using FRP reinforcements, the effects of local defects could be reduced effectively and the failure modes of reinforced timber beams changed from tensile failure to compression failure [12–14]. Raftery [15] adopted a nonlinear numerical model to predict the mechanical behaviour of reinforced low-grade laminated spruce. Raftery [16] investigated the strengthening in flexure of low-grade glulam beams using bonded-in glass fibre reinforced polymer (GFRP) rods. Johnsson et al. [17] reported bonded-in rods as reinforcement for glued laminated timber elements and proposed an analytical model to predict the anchorage length. Although considerable studies have been undertaken on timber beams strengthened by FRP reinforcement. It seems that limited studies have been found related to the timber beams strengthened by NSM–FRP reinforcement, especially NSM–CFRP. Alam et al. [18] investigated the effects of geometry, material properties and reinforcement location on the flexural properties of laminated veneer lumber strengthened by adhesively bonded plate and rod reinforcements. Jankowski et al. [19] examined CFRP reinforced wooden beams by 4-point bending tests and photo-elastic coating technique. D’Ambrisi et al. [20] conducted an experimental investigation on the flexural behaviour of timber beams repaired with CFRP laminates. The results showed that using CFRP laminates is an effective method for strengthening both the new and old timber beams and it allowed to completely restore and to increase their flexural strength and ductility. Micelli et al. [21] investigated the possibility of taking CFRP rods as glued-in reinforcement of glulam beams and as glued-in connectors for glulam timber head joints that should transfer flexural moment between two adjacent beams. Alhayek and Svecova [22] carried out an experimental program on timber stringers strengthened with glass fibre-reinforced polymer (GFRP) laminates. The results indicated that the strengthening with GFRP laminates increased the strength of the beams significantly, while little increase contributed by GFRP laminates was observed on the stiffness of the beams. Rosa García et al. [23] investigated the use of basalt fibre materials for the reinforcement of timber. The results indicated that the timber beams reinforced by basalt fibre materials exhibit good behaviour. Raftery and Kelly [24] examined the flexural behaviour of low-grade glue laminated timber reinforced by bonded-in basalt FRP rods. It was shown that the use of basalts FRP rods is seen to be highly effective as a repair technology for damaged timber elements. 1.2. Objectives of the present study The objective of this study is to investigate the flexural behaviour of the glulam beams reinforced by NSM-CFRP reinforcement. The reinforcing procedure was finished in a glulam factory environment during the glulam manufacturing. Test programme involved in the fabrications and the bending tests on both the unreinforced and reinforced glulam beams. The test results were discussed in terms of load–deflection behaviour, failure modes, strengthening effects, and strain profile distributions. 2. Materials 2.1. Timber The species of timber used in this study are North America Douglas fir and fast-growing Poplar. The material properties of timber obtained in accordance with Standard for Test Methods of Timber Structures [25] were reported in Table 1.

Before the procedure of glulam beams, the lamellas were stored for 3 months in a conditioned environment of 65 ± 5% relative humidity and 20 ± 2 °C temperature once delivered to the laboratory. The average moisture content of timber was measured by the oven-drying method according to the British Standards (BS EN13183-1:2002) [26] after the curing period. The average moisture content and average density of Douglas fir were recorded as 490 kg/m3 with a standard deviation of 10 kg/m3 and 11.5% with a standard deviation of 0.5%, respectively. The average moisture content and average density of Poplar were measured as 458 kg/m3 with a standard deviation of 19 kg/m3 and 12.0% with a standard deviation of 0.9%, respectively.

2.2. CFRP reinforcement CFRP laminates with 1.2 mm thickness and 100 mm width were used as reinforcement in this study, due to its high strength-to-weigh ratio and good resistance to corrosion. This material was manufactured using the pultrusion process and incorporated carbon fibres aligned unidirectionally. The mechanical properties of the CFRP laminates obtained by directly tensile testing were presented in Table 1.

2.3. Adhesive Previous research [27–29] showed that the phenol resorcinol formaldehyde (PRF) was appropriate for the bond between timber laminations, and certain epoxy resins were suitable at the FRP wood adhesive depending on the FRP material which was being bonded. Two-component epoxy resin was chosen for the bond between CFRP laminates and the timber laminates. Table 2 reports the mechanical properties of the epoxy resin provided by manufacturer.

3. Experimental program 3.1. Beam fabrication and configurations The glulam beams were made by 32 mm thick lamellas glued together in a conditioned environment with resorcinol resin adhesive. The initially ranking of the lamellas was conducted according to mechanical grading [30]. Subsequently, visual grading of the lamellas was undertaken with reference to IS 127 [31]. Lamellas with excessive margin knot area ratios, excessive total knot area ratios or other critical strength reducing defects such as fissures were not used in both the top and bottom of the beams. A novel composite lamella reinforced by NSM-CFRP laminates was proposed in this study and then glued with other lamellas to form the reinforced glulam beams, shown in Fig. 1. Instead of slotting at the bottom of the beam, this type of strengthening technique only involved in gluing between timber lamellas and FRP plates, which is more convenient and efficient for glulam industry. The gluing of glulam beams was finished within two hours, which was suggested by the manufacturer of the adhesive. And during gluing, a pressure of 0.7 N/mm2 was applied and kept constant for 24 h in an environment of 65 ± 5% relative humidity and 20 ± 2 °C temperature. All the beams were assembled as 75 mm wide, 300 mm deep and 6000 mm long. The beam configurations are shown in Fig. 2 and Table 3. The unreinforced beams were considered as the reference beams for the reinforced ones. Four types of reinforcement ratio were involved in this study: 0.331%, 0.661%, 0.992% and 1.322%. Three replicates for each configuration were

Table 1 Mechanical properties of timber and CFRP. Material

Ultimate tensile strength (MPa)

Ultimate compressive strength (MPa)

Modulus of elasticity in tension (MPa)

Modulus of elasticity in compression (MPa)

Douglas fira Poplara CFRP

88.5

36.0

–

10760

87.6 2104

35.2 750

– 151,600

9760 –

a For Douglas fir and Poplar, the mechanical property values are at the direction of parallel to the grain.

25

W. Lu et al. / Construction and Building Materials 91 (2015) 23–31 Table 2 Mechanical properties of the epoxy resin provided by manufacturer. Compressive strength (MPa) Tensile bending strength (MPa) Tensile modulus of elasticity (MPa)

75.4 55.5 2640

prepared with the consideration of the variability of the test results. B0

3.2. Test set up

B1

B2, B3

B4

Fig. 2. Beam configurations (dimension in mm).

Testing was conducted according to ASTM Standard D198–99 [32]. All beams were simply supported with a span of 5700 mm and tested in four-point bending using a hydraulic actuator with a 250 kN capacity. The loading rate was set as 7 mm/min in order to ensure failure occurred in 6–10 min. Five Linear Voltage Displacement Transducer (LVDT) were employed to record the deflection of the beams. Lateral supports were provided at both sides of the beams. Before loading, a pre-load of 300 N was applied for the adjustment of data recorded systems. The strain profile distribution was recorded by five strain gauges attached along the depth of the beam. The test set up and the layout of the strain gauges are illustrated in Fig. 3.

Table 3 Beam configurations. Group

Timber

Layout of reinforcement

Reinforcement ratio (%)

Replicates

DB0

Douglas fir Douglas fir Douglas fir Douglas fir Douglas fir Poplar Poplar Poplar

Unreinforced

0

3

Only in tensile zone

0.331

3

Only in tensile zone

0.661

3

Only in tensile zone

0.992

3

Half in tensile zone and half in compressive zone Unreinforced Only in tensile zone Only in tensile zone

1.322

3

0 0.331 0.661

3 3 3

DB1 DB2 DB3 DB4

4. Experimental results and discussion 4.1. Typical failure modes Typical failure modes of the tested beams are presented in Fig. 4. Fig. 4(a) illustrates the failure mode of the unreinforced beams characterized by brittle tensile failure in the tension zone of the beams. It should be noted that the tensile failure in timber was still observed although the beam was reinforced by NSM-CFRP reinforcement in the tensile zone, as shown in Fig. 4(b). Fig. 4(c) shows the fracture of CFRP reinforcement prior to the tensile failure of the timber in tension zone, which was observed frequently during the testing, especially for those beams reinforced only in the tensile zone. Fig. 4(d) exhibits the compressive failure of timber without failure of the bond layer, which indicated that the reinforced timber beams exhibited pseudo-ductile behaviour and the bonding performance between the timber and the reinforcement was perfect. 4.2. Load–deflection behaviour Fig. 5 illustrates the comparisons of the load–deflection curves between the reinforced beams and the unreinforced beams. It should be noted that the load–deflection curves of specimen DB1-1, DB2-1 and PB2-1 are not shown in Fig. 5(a), (b) and (f), respectively. This is owing to those three specimens failed in out-of plane failure, rather than flexural failure. The deflection in Fig. 5 is in the mid-span of the beams. the recordings of the two transducers located at the loaded points are not reported here because of the limited space for this article. Moreover, during the

Fig. 1. Assembly of reinforced glulam beams.

PB0 PB1 PB2

testing, the deflection recordings of the beam at the two loaded points was basically symmetric and less than that of the mid-span transducer. In general, the ultimate load capacity of the reinforced Douglas fir beams was significantly improved compared with the unreinforced ones. However, it seems that no significant increase of the ultimate load capacity was observed between the reinforced and unreinforced Poplar beams. It can be seen from Fig. 5(a)–(d) that the initial stiffness of most of the Douglas fir beams was just slightly improved after reinforcing. While Fig. 5(e) and (f) show that the initial stiffness of Poplar beams was significantly improved by using NSM reinforcement compared with the Douglas fir beams. It is interesting to note that several reinforced beams exhibited pseudo-ductile flexural behaviour in comparisons of the linear elastic flexural behaviour of the unreinforced beams. Such a conclusion was also confirmed by the previous research [33]. 4.3. Global stiffness According to Standard for methods testing of timber structures (GB/T50329-2012) [34], the global stiffness can be calculated from the linear-elastic stage of the load–deflection curve of each beam by using the following equation:

EI ¼

aDP ð3L2  4a2 Þ 48Dd

ð1Þ

where DP refers to the given range of the applied load; Dd means the range of the deflection corresponding to DP; E and I represent the modulus of elasticity and moment of inertia respectively; L is the span of beam between two support; a is the distance from the support to the loading point. Fig. 6 presents the global stiffness of both the reinforced and unreinforced beams obtained by Eq. (1). As shown in Fig. 6, for Douglas fir beams, the global stiffness of the reinforced group DB1, DB2, DB3 and DB4 was improved by 11.5%, 8.0%, 11.1% and

26

W. Lu et al. / Construction and Building Materials 91 (2015) 23–31

P

SG1 SG2 SG3 SG4 SG5

LVDT 1900

1900

1900

5700 Fig. 3. Test set up.

Fig. 4. Typical failure modes: (a) unreinforced beams; (b) tensile failure in reinforced lamella; (c) fracture of CFRP; (d) timber failure in compression.

28.8%, respectively compared with the unreinforced group DB0. Fig. 6 also shows that the global stiffness for the reinforced poplar beams PB1 and PB2 was significantly improved by 23.0% and 39.2%, respectively compared with the unreinforced poplar beams PB0. It is interesting to note that the global stiffness of group DB4 is higher than that of other groups, which indicates that the global stiffness of the beam can be improved more effectively by using reinforcement both in the tensile zone and the compressive zone of the beam. 4.4. Strain profile distribution Fig. 7 exhibits the strain profile distribution at different load level for each group of beams respectively. It can be seen from Fig. 7 that the strain profile distribution basically obeys the plain section assumption in the elastic region. Raftery et al. [16] indicated that the reinforcement in tensile zone can lower the location of the neutral axis. It is also observed in Fig. 7 that the reinforcement in the tensile zone can lower the location of the neutral axis

in the elastic region, especially for Poplar beams, such as PB2. While the neutral axis just slightly lower for Douglas fir beams with the reinforcement in tensile zone. The non-linear strain behaviour exhibited on Poplar beams more obviously than Douglas fir beams. This can be explained as firstly, Poplar used in this study is one kind of fast-growing species, which is usually characterized by low wood quality and modulus of elasticity compared with Douglas fir beams. The maximum compressive strain of the reinforced Poplar beams can be improved by 90.9% for group PB1 and 106.8% for group PB2, respectively compared with the unreinforced one. 4.5. Modulus of rupture Modulus of rupture (MOR) is a measure of the strength of specimens before rupture, which can be calculated by Eq. (2):

MOR ¼

Mu Sg

ð2Þ

27

90

90

80

80

70

70

60

60

50 40 DB0-1 DB0-2 DB0-3 DB1-2 DB1-3

30 20 10 0

0

50

100 Deflection (mm)

150

Load (kN)

Load (kN)

W. Lu et al. / Construction and Building Materials 91 (2015) 23–31

50 40

20 10 0

200

80

80

70

70

60

60

50 DB0-1 DB0-2 DB0-3 DB3-1 DB3-2 DB3-3

30 20 10 0

0

50

100 Deflection (mm)

150

0

20 10 0

200

0

40

40

30 PB0-1 PB0-2 PB0-3 PB1-1 PB1-2 PB1-3 100 150 Deflection (mm)

200

(e) PB0 ( =0%) VS PB1 ( =0.331%)

250

Load (kN)

Load (kN)

50

50

50

100 Deflection (mm)

150

200

(d) DB0 ( =0%) VS DB4 ( =1.322%)

50

0

200

DB0-1 DB0-2 DB0-3 DB4-1 DB4-2 DB4-3

30

60

0

150

40

60

10

100 Deflection (mm)

50

(c) DB0 ( =0%) VS DB1 ( =0.992%)

20

50

(b) DB0 ( =0%) VS DB2 ( =0.661%) 90

Load (kN)

Load (kN)

(a) DB0 ( =0%) VS DB1 ( =0.331%) 90

40

DB0-1 DB0-2 DB0-3 DB2-2 DB2-3

30

30 PB0-1 PB0-2 PB0-3 PB2-2 PB2-3

20 10 0

0

50

100 150 Deflection (mm)

200

250

(f) PB0 ( =0%) VS PB2 ( =0.661%)

Fig. 5. Comparisons of load–deflection behaviour between different groups of beams.

2

Average global stiffness (N·mm )

2.40E+012 2.00E+012 1.60E+012 1.20E+012 8.00E+011 4.00E+011 0.00E+000 DB0 DB1 DB2 DB3 DB4 PB0 PB1 PB2 Fig. 6. Average global stiffness of each group of beams.

where Mu represents the ultimate bending moment at failure; Sg is the section modulus of the transformed section. Fig. 8 shows the relationship between the modulus of rupture (MOR) and the modulus of elasticity (MOE) for Douglas fir beams. It should be noted that Poplar beams are not involved in Figs. 8 and 9. This is because the experimental results of Poplar beams is relatively variable compared with the Douglas fir beams. The MOE was calculated from the slope of the linear elastic stage of the load–deflection curves by using the classical beam theory. It can be seen from Fig. 8 that the MOR value varied between 40 MPa and 55 MPa as the MOE increased from around 9300 MPa to 12,000 MPa, which indicates that no significantly proportional relationship is obverse between the MOR and the MOE. Previous research [14,22] indicated that MOR is greatly affected by the MOE and an approximately linear relationship between the MOE and the MOR was observed for the unreinforced timber beams. This was not confirmed by the conclusion drawn in this section.

28

W. Lu et al. / Construction and Building Materials 91 (2015) 23–31 300 5kN 15kN 25kN 35kN 45kN 50kN

200

250

Depth (mm)

150 100

200 150 100

0 -4000

-2000

0 Strain (μ ε)

2000

0 -4000

4000

(a) DB0 ( =0%)

150 100 50

-2000

0 2000 Strain (μ ε)

200 150 100 50

-4000

-2000

0 2000 Strain (μ ε)

1 x 10

4

(c) DB2 ( =0.661%)

200 150 100

4000

6000

0 -6000

-4000

-2000

0 2000 Strain (μ ε)

Depth (mm)

100 50

200 150 100

200 150 100 50

0 -8000

(f) PB0 ( =0%)

5kN 15kN 25kN 35kN 45kN 55kN

250

50

0 2000 4000 6000 8000 Strain (μ ε)

6000

300 5kN 15kN 25kN 35kN 45kN 50kN

250

150

4000

(e) DB4 ( =1.332%)

300 5kN 15kN 25kN 35kN 45kN

0 -6000 -4000 -2000

0.5

50

300

200

0 Strain (μ ε)

5kN 20kN 35kN 50kN 65kN 80kN

(d) DB3 ( =0.992%)

250

-0.5

250

Depth (mm)

Depth (mm)

0 -1

6000

300 5kN 15kN 30kN 45kN 60kN 70kN

250

0 -6000

4000

(b) DB1 ( =0.331%)

300

Depth (mm)

200

50

50

5kN 20kN 35kN 50kN 65kN 80kN

250

Depth (mm)

Depth (mm)

250

300 5kN 15kN 30kN 45kN 60kN 70kN

Depth (mm)

300

-4000

0 Strain (μ ε)

4000

0 -6000

8000

(g) PB1 ( =0.331%)

-4000

-2000

0 2000 Strain (μ ε)

4000

6000

(h) PB2 ( =0.661%)

Fig. 7. Strain profile distribution of different groups of beams.

60

50 40 30 20 10 0 0.9

ρ=0% ρ=0.331% ρ=0.661% ρ=0.992% ρ=1.322% 0.95 1 1.05 1.1 1.15 1.2 4 Modulus of Elasticity (Mpa) x 10

Modulus of Rupture (Mpa)

Modulus of Rupture (Mpa)

60

50 40 30 ρ=0% ρ=0.331% ρ=0.661% ρ=0.992% ρ=1.322%

20 10 0 0

0.3

0.6 0.9 1.2 Reinforcement ratio (%)

1.5

Fig. 8. Relationship between MOR and MOE for both the reinforced and unreinforced beams.

Fig. 9. Relationship between MOR and MOE and the ratio of reinforcement.

Fig. 9 illustrates the relationship between the MOR and the reinforcement ratio for Douglas fir beams. It is shown in Fig. 9 that the MOR increased initially as the reinforcement ratio increased

from 0% to 0.661%, while it began to fluctuate after the reinforcement ratio of 0.661%. Generally, it concludes that the ratio of reinforcement affected the MOR of the reinforced timber beams,

29

W. Lu et al. / Construction and Building Materials 91 (2015) 23–31

while the MOR value just fluctuated in a certain range as the reinforcement ratio varied.

f

f ftu

5. Theoretical analysis The predictions for the ultimate moment of the reinforced timber beams were conducted based on the constitutive laws for timber and CFRP laminates shown in Figs. 10 and 11, which were proposed by Buchanan [35] and Lindyberg [36], respectively. As shown in Fig. 10, the tensile behaviour of timber parallel to the grain is assumed to be linear-elastic. f wtu is defined as the ultimate tensile strength of the timber, corresponding to the strain ewtu . A bi-linear relationship is assumed for the compression behaviour, with a linearly ascending branch up to the compressive strength f wcy and a corresponding strain ewcy followed by a descending branch until to failure. The slope of the descending branch is a constant fraction, m, of the modulus of elasticity of the timber [14]. Fig. 11 shows that CFRP laminates was considered as linear-elastic material with the ultimate tensile strength f ftu and a corresponding strain eftu . It should be noted that this predicting model is only for static behaviour and does not consider for creep or fatigue. The following assumptions were adopted for the simplification of the calculations: (1) Timber was considered as orthotropic material with two orthogonal planes of symmetry. (2) Bonding between CFRP laminates and timber was assumed as perfect. (3) Shear effect was neglected, only bending was considered. (4) The assumption of plain section was always satisfied. The failure modes of the reinforced beams were mainly characterized by timber tensile failure or CFRP fracture combined with timber failure in compression. The equilibrium of the cross section of the reinforced beams were shown in Fig. 12. For the glulam beams reinforced in tensile zone, there are two types of failure modes: timber tensile failure combined with timber failure in compression and CFRP fracture combined with timber failure in compression. For the first failure mode, the equilibrium of the beam cross section was presented as Fig. 12(a). According to the assumption of plain section, Eq. (3) can be easily found:

h  xc ewtu ¼g¼ xce ewcy

ð3Þ

xce þ xcp ¼ xc

ð4Þ

ftu

f

Fig. 11. Constitutive law for CFRP laminates.

where h is the depth of the beam; xc , xce and xcp represent the total depth, the depth of elastic zone and the depth of plastic zone of timber in compression, respectively; ewtu and ewcy are the ultimate tensile strain and the peak compressive strain of timber. The equilibrium of force should be satisfied strictly as follow:

F wc1 ¼

1 bðf wcy þ f wcu Þxcp 2

ð5Þ

F wc2 ¼

1 bfwcy xce 2

ð6Þ

F wt ¼

1 1 bfwtu ðh  xc Þ ¼ bgf wcy ðh  xc Þ 2 2

ð7Þ

F ft ¼ rf Af ¼ gf wcy kAf

ð8Þ

1 1 bfwcy xcp þ bfwcy xce ¼ bgf wcy ðh  xc Þ þ gf wcy kAf 2 2

ð9Þ

where, b is the width of beam; f wcy , f wcu and f wtu represent the peak compressive strength, the ultimate compressive strength and the ultimate tensile strength of timber; k ¼ Ef =Ew , where Ef and Ew are the elastic modulus of CFRP and timber; rf and Af represent the axial stress and the cross section area of CFRP. Then, the position of neutral axis can be determined by solving the above four equations:

xce ¼

2ðhb  gkAf Þ ðg þ 1Þ2 b

ð10Þ

w

xc ¼ h 

f wtu

2gðhb  gkAf Þ

xcp ¼ h 

wcu

wcy

2ðg  1Þðhb  gkAf Þ ðg þ 1Þ2 b

ð11Þ

ð12Þ

Once the position of neutral axis was obtained, the ultimate moment capacity of the reinforced timber beams can be predicted as follows:

Ew wtu

w

Mu ¼

mEw

ðg þ 1Þ2 b

f wcy

Fig. 10. Constitutive law for timber.

  1 1 1 bðf wcy þ f wcu Þxcp xc  xcp þ bfwcy x2ce 2 2 3  zf  1 2 þ bgf wcy ðh  xc Þ þ gf wcy kAf  h  xc  3 2

ð13Þ

where Mu is the ultimate moment capacity of the timber beams and zf is the position of CFRP plates along the depth of the beam.

30

W. Lu et al. / Construction and Building Materials 91 (2015) 23–31

f wcu

wc

(a)

Fwc1

xcp wcy

xc

Fwc2

xce

h

h

NA

f wcy

zf

M Fwt Fft

zf

b

h-xc

f wtu

wt

b

f wcu

wc

(b)

Ffc Fwc1

xcp wcy

f wcy

Fwc2

xce

h

NA

xc

M Fwt Fft

zf

h-xc

f wtu

wt

b

Fig. 12. Equilibrium of the reinforced beam cross section for different reinforcing configurations.

xce1 ¼

  2 hb  ðg  1ÞkAf

xc1 ¼ h 

ð14Þ

2

ðg þ 1Þ b   2g hb  ðg  1ÞkAf

xcp1 ¼ h 

ð15Þ

ðg þ 1Þ2 b   2ðg  1Þ hb  ðg  1ÞkAf

ð16Þ

ðg þ 1Þ2 b

where xc1 , xce1 and xcp1 represent the total depth, the depth of elastic zone and the depth of plastic zone of timber in compression, respectively. Once the location of the neutral axis was obtained, the ultimate moment capacity of the reinforced timber beams can be obtained by the following equation:

M u ¼ 12 bðf wcy þ f wcu Þxcp1 þ 13 b

xc1  12 xcp1




þ 13 bfwcy x2ce1 þ f wcy kAf

  gf wcy ðh  xc1 Þ þ gf wcy kAf  h  xc1  z2f





z xc1  2f



2

ð17Þ The mechanical properties of timber used for the prediction of the ultimate moment capacity was reported in Tables 1 and 4. The mechanical properties for Douglas fir was obtained base on the material testing and the mechanical properties for Poplar was recommended by Yang [37].

Table 4 Mechanical properties of timber. Timber species

ewtu (%)

ewcy (%)

ewcu (%)

m

Dougas fir Poplar

0.41 0.6

0.35 0.349

1.2 1.2

0.108 0.00825

100 Predicted results(kN.m)

If Af = 0, Eq. (13) represents the ultimate moment capacity of the unreinforced timber beam. For the failure mode of CFRP fracture combined with timber failure in compression, similar derivations as mentioned above can be adopted for the prediction of the ultimate moment capacity of the reinforced timber beams. The only difference is that g in Eq. (3) should be changed to g ¼ f cu =efu , where efu is the ultimate tensile strain of CFRP. For those beams reinforced in both the tensile and compressive zone, the equilibrium of the beam cross section was shown as Fig. 12(b). Base on the similar derivations from Eqs. (3–9), the neutral axis position of timber beams reinforced in both the tensile and compressive zone can be determined by the following equations:

80

Douglas fir beam Poplar beam y=x

60 40 y=x R2=0.83

20 0 0

20

40 60 Test results (kN.m)

80

100

Fig. 13. Comparisons between the predicted ultimate moment capacity and test results.

Fig. 13 presents the results of the predicted ultimate moment capacity compared with the test results. It can be seen that the predicted ultimate moment capacity fits the test results well with a R-square of 0.83, which confirmed that the above mentioned theoretical derivations can be used to predict the ultimate moment capacity of the reinforced timber beams effectively. Actually if the predictions are good or not, which is related to the failure modes of the beam. For those beams, which failed only in tensile failure without timber failure in compression. The predictions are good, such as the reinforcement ratio of 0–0.661%. For the reinforcement ratio of 0.992% and 1.322%, the reinforced glulam

W. Lu et al. / Construction and Building Materials 91 (2015) 23–31

exhibit pseudo-ductile behaviour obviously. The timber in compression zone got into plastic stage, which increased the variability of the predictions for the beams. As is shown in Fig. 13, the variability between the predicted results and the test results is increasing gradually as the reinforcement ratio increased from 0% to 1.322%. 6. Conclusions The paper reports the experimental and theoretical study on the flexural behaviour of timber beams with NSM-CFRP reinforcement. The key conclusions are drawn as follows: (1) NSM-CFRP reinforcement is an effective way for enhancing the flexural strength and the flexural stiffness of glulam beams. Compared to the unreinforced beams, the load capacity of the reinforced Douglas beams was significantly improved, while the reinforcing efficiency on the Poplar glulam beams is insignificant, compared with the Douglas fir beams. (2) Some reinforced glulam beams exhibit the pseudo-ductile behaviour in comparison to the linear elastic behaviour of the unreinforced beams characterized by tensile failure of timber. The reinforcement ratio of 0.992% in tensile zone was suggested as the most effective reinforcement ratio for glulam beams. (3) The strain profile distributions indicate that the compressive strain of the reinforced glulam beams can be fully developed by using NSM-CFRP reinforcement. (4) No obvious proportional relationship between the modulus of rupture and the modulus of elasticity was observed. The modulus of rupture increased initially as the reinforcement ratio increased from 0% to 0.661%, while it began to fluctuate after the reinforcement ratio of 0.661% (5) The predicted results was found in good agreement with the test results with an R-square value of 0.83, which indicated that the flexural behaviour of the reinforced glulams beams can be predicted effectively by the prediction model.

Acknowledgment The financial support from the National High Technology Research and Development Program of China (Grant No. 2012AA03A204) is much appreciated. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.conbuildmat. 2015.04.050. References [1] Ceroni F, Pecce M, Bilotta A, Nigro E. Bond behavior of FRP NSM systems in concrete elements. Compos Part B: Eng 2012;43(2):99–109. [2] Lee D, Cheng LJ, Hui JY. Bond characteristics of various NSM FRP reinforcements in concrete. Compos Struct 2012;17(1):117–29. [3] Novidis DG, Pantazopoulou SJ. Bond tests of short NSM-FRP and steel bar anchorages. Compos Struct 2008;12(3):322–33. [4] Theakston TH. A feasibility study for strengthening timber beams with fiberglass. Can Agric Eng J 1965;1:17–9.

31

[5] Bulleit WM. Reinforcement of wood materials: a review. Wood Fiber Sci 1984;16(3):391–7. [6] Van de Kuilen JWG. Theoretical and experimental research on glass fibre reinforced laminated timber beams. In: Proceedings of the international timber engineering conference, London, England, vol. 3; 1991. p. 226–33. [7] Moulin JM, Pluvinage G, Jodin P. FGRG: fibreglass reinforced glulam – a new composite. Wood Sci Technol 1990;24(3):289–94. [8] Tingley DA, Cegelka S. High strength fiber reinforced plastic reinforced wood. In: Proceedings of the international wood engineering conference, USA, vol. 3; 1996. p. 57–66. [9] Tingley DA. The stress–strain relationships in wood and fiber-reinforced plastic laminae of reinforced glue laminated wood beams [Ph.D. thesis]. Corvallis (OR): Oregon State University; 1996. [10] Dagher HJ, Kimball T, Abdel-Magid B, Shaler SM. Effect of FRP reinforcement on low-grade eastern hemlock glulams. In: Proceedings of national conference on wood transportation structures, Madison, WI; 1996. [11] Hernandez R, Davalos JF, Sonti SS, Kim Y, Moody RC. Strength and stiffness of reinforced yellow-poplar glued laminated beams. Res pap FPL-RP-554, Madison, WI; 1997. [12] Fiorelli J, Alves Dias A. Analysis of the strength and stiffness of timber beams reinforced with carbon fiber and glass fiber. Mater Res 2003;6(2):193–202. [13] Issa CA, Kmeid Z. Advanced wood engineering: glulam beams. Constr Build Mater 2005;19:99–106. [14] Gentile C, Svecova D, Rizkalla SH. Timber beams strengthened with GFRP bars: development and applications. J Compos Constr 2002;6(1):11–20. [15] Raftery GM, Harte AM. Nonlinear numerical modeling of FRP plate reinforced glued laminated timber. Compos Part B: Eng 2013;52:40–50. [16] Raftery GM, Whelan C. Low-grade glued laminated timber beams reinforced using improved arrangements of bonded-in GFRP rods. Constr Build Mater 2014;52:209–22. [17] Johnsson H, Blanksvard T, Carolin A. Glulam members strengthened by carbon fibre reinforcement. Mater Struct 2006;40:47–56. [18] Alam P. The reinforcement of timber for structural applications and repair [PhD thesis]. Dept. of Mech. Eng. University of Bath; 2004. [19] Jankowski LJ, Jerzy J, Tomasz PN. Experimental assessment of CFRP reinforced wooden beams by 4-point bending tests and photoelastic coating technique. Mater Struct 2010;43(1–2):141–50. [20] D’Ambrisi A, Focacci F, Luciano R. Experimental investigation on flexural behavior of timber beams repaired with CFRP laminates. Compos Struct 2014;108:720–8. [21] Micelli F, Scialpi V, La Tegola A. Flexural reinforcement of glulam timber beams and joints with carbon fiber-reinforced polymer rods. J Compos Constr 2005;9(4):337–47. [22] Alhayek H, Svecova D. Flexural stiffness and strength of GFRP-reinforced timber beams. J Compos Constr 2012;16(3):245–52. [23] de la Rosa García P, Escamilla AC, Nieves González GMN. Bending reinforcement of timber beams with composite carbon fiber and basalt fiber materials. Compos Part B: Eng 2013;55:528–36. [24] Raftery GM, Kelly F. Basalt FRP rods for reinforcement and repair of timber. Composites Part B: Eng 2015;70:9–19. [25] GB/T50329-2012. Standard for test methods of timber structures; 2012 [in Chinese]. [26] BS EN 13183-1. Moisture content of a piece of a sawn timber – Part 1: determination by oven dry method; 2002. [27] Raftery GM, Harte AM, Rodd PD. Bond quality at the FRP wood interface using conventional wood laminating adhesives. Int J Adhes Adhes 2009;29(2):101–10. [28] Raftery GM, Harte AM, Rodd PD. Bonding of FRP materials to wood using thin epoxy gluelines. Int J Adhes Adhes 2009;29(5):580–8. [29] Raftery GM, Harte AM, Rodd PD. Qualification of wood adhesives for structural softwood glulam with large juvenile wood content. J Inst Wood Sci 2008;18(1):24–34. [30] EN 14081-4. Timber structures – strength graded structural timber with rectangular cross section. Part 4: Machine grading – grading machine settings for machine controlled systems. CEN, European Committee for Standardization; 2005. [31] IS 127. Specification for stress grading of softwood timber. Dublin, NSAI; 1990. [32] ASTM D198-99. Standard test methods of static tests of lumber in structural sizes. West Conshohocken, PA; 1999. [33] Raftery GM, Harte AM. Low-grade glued laminated timber reinforced with FRP plate. Compos Part B: Eng 2011;42(4):724–35. [34] GB/T50329. Chinese standard for tensile tests of metallic materials; 2012 [in Chinese]. [35] Buchanan AH. Bending strength of lumber. J Struct Eng 1990;116(5):1213–9. [36] Lindyberg, RF. A nonlinear probabilistic model for the analysis of reinforced glulam beams in bending [Ph.D. thesis]. University of Maine; 2000. [37] Yang HF. Study on flexural behavior of wood composite beams made from fast-growing timber [Ph.D. thesis]. Nanjing Tech University; 2007.