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STRUCTURAL PERFORMANCE OF NATURAL FIBERS REINFORCED TIMBER BEAMS

Emanuela Speranzini Professor University of Engineering via Duranti 93, 06125 Perugia, Italy speranzini@strutture.unipg.it* Stefano Agnetti PhD Student University of Engineering via Duranti 93, 06125 Perugia, Italy stefano.agnetti@studenti.unipg.it

Abstract The results of an experimentation on beams in solid wood reinforced with FRP in natural fibers of basalt, flax and hemp are presented. In the first phase, the properties of the materials to be used were studied. Tension tests and pull-out tests were performed on specimens made up of different types of fibers. In the second phase, three series of different size wooden beams were prepared applying natural fiber reinforcements and were then subjected to four-point-bending tests. The results obtained on the beams reinforced with natural fibers were compared with those on the beams without reinforcement or reinforced with traditional glass or carbon fibers. The structural behaviour was satisfactory: the reinforced beams showed higher strength and stiffness than those without reinforcement and a good behaviour when compared to beams reinforced with carbon or glass fibers.

Keywords: basalt fiber, flax fiber, hemp fiber, FRP, natural fiber, timber beam, wood.

1. Introduction The coupling of composite materials with wood is particularly interesting because it gives more in structural strength and stiffness, if compared to the performance that wood can provide alone. There are many advantages in fiber-reinforced composite materials: they are easy to apply and extremely versatile, for both the recovery of the existing and the design of new structures. The lightness, which is one of the most popular features of the wood, is not compromised by the application of the FRP reinforcement. In addition, issues related to the typical defects of wood, such as high mechanical in-homogeneities resulting from the presence of a large number of defects, are mitigated by the synergy with the fiber-reinforced composite.

The joining of wood and FRP is especially good when you consider it in terms of compatibility of features. This would be further confirmed if the fiber-reinforced composite consists of ecologically sustainable natural materials. At present, this purpose can be achieved only partially, because only natural fibers (such as bamboo fiber, flax, hemp, basalt) are available, but no natural resins. In order to achieve these goals, this work studies the properties of wooden structural elements reinforced with natural fibers composites.

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In the past, research related to the reinforcement of wooden structures with FRP was mainly aimed to study the flexural behaviour of the element and the in-the-plan stiffening of two-dimensional structures [1]. Many experiments were performed in the case of CFRP (Carbon-Fiber Reinforced Polymers) [2], [3], [4], [5], [6], or GARP (Glass-Aramid Reinforced Polymers) [7], on samples of different size subjected to bending tests to assess the obtained increases in strength and stiffness. This paper reports the latest results of a study started in the past [8], [9], and concerning the behaviour of wooden beams reinforced with natural fibers of basalt, flax and hemp. The first phase of this experimental investigation has involved small samples of silver fir wood reinforced with unidirectional tapes of natural fibers. The materials used were tested to characterize their mechanical properties. Particular attention was paid to the choice of resin: traction shear tests for bonding and adhesion tests for pull-out were performed in order to test the quality of adhesion. A series of small size beams reinforced with natural fibers were subjected to four-point-bending tests. The results of the samples with natural fibers were compared with those of the samples without reinforcement or reinforced with traditional glass or carbon fibers.

In the second phase of the experiment, two series of beams (medium size of 100x100x2000 mm, large size of 200x200x4000 mm), made up of the same type of wood of the previous phase (but from another supplier) and always reinforced with natural fibers, were tested in four-point-bending tests. Strength and stiffness due to the presence of the reinforcement were then compared with those of samples without fibers. For the large size beams, it is possible to make a comparison with the results of previous experiments, carried out on beams of the same wood species and geometry, but reinforced with CFRP strips.

A finite element analysis was performed and calibrated on the experimental results, in accordance with the existing models of beams reinforced with CFRP [3], [9], [10]. The model reproduces the flexural behaviour of the reinforced elements and allows parametric analysis and comparisons between the different types of reinforcement.

2. Some tests on FRP in natural fibers

2.1 Test on the resin The resin used was chosen on the basis of the results of the previous studies carried out on glued wood and of the tests performed on different types of resins [8], [9]. The aim of traction-shear tests of the bonding was to define the quality of the bonding of the resins used in wooden structures. The sample was subjected to an increasing traction force until the joist broke (Figure 1a). The breaking traction load represents the shear strength of the resin. At the end an epoxy resin was chosen, not only because of its greater transparency, but also because it allows a better bonding between wood and fiber, which has a superior wettability and a consequently better adhesion to the wooden surface.

2.2 Test on the fibers Unidirectional fabrics of natural fibers of basalt, flax and hemp were used for the reinforcement of the wooden elements. Their mechanical properties are shown in Table 1. A series of tests (adhesion tests for pull-out and tensile tests) was performed to mechanically characterize the composites and to study the mechanical behaviour of natural fibers in comparison with that of traditional ones.

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Table 1. Mechanical properties of the natural fibers.

Stress [N/mm2] E-moduls [N/mm2] Strain [%] Basalt 2800 89000 3,15

Hemp 1000 40000 2,50

Flax 1500 50000 3,00

Adhesion tests for pull-out allowed evaluating the adhesion force depending on the length of the bond. Specimens were prepared, each one consisting of two wooden elements to which various types of fibers have been applied (with three different anchoring lengths of 10, 20 and 30 mm). As mentioned, the epoxy resin used for bonding was chosen by the tests on the shear strength of the bonding. The segments were then subjected to adhesion test conducted at a constant strain rate of 2 mm/min. After the tests, the transmissible force was observed in order to demonstrate good anchorage for all types of fibers under examination (Figure 1b, 1c). The mechanical properties of the fibers were verified by means of a series of tensile tests in accordance with the norm ASTM D 3039. The tests were carried out by controlling the shifting at a constant speed of 2 mm/min (Figure 1d, 1e). The test sample consisted of a total of 50 specimens made up with an epoxy polymeric matrix and one-way strips of the various types of fibers. All the specimens had the same fraction ratio in weight.

(a) (b) (c) (d) (e)

Figure 1. Traction shear test for bonding (a), pool-out test (b, c) traction test (d, e).

3. Bending test on the reinforced elements Three series of wooden beams of different size (40x50x1000 mm, 100x100x2000 mm, 200x200x4000 mm) were prepared with natural fibers reinforcements and then subjected to four-points-bending tests.

3.1 Small size beams Bending tests were performed on five beams, one for each type of natural fiber (basalt, flax and hemp) and traditional (carbon and glass) reinforcement. The beams had a section of 40x50 mm and a length of 1000 mm and were reinforced at the intrados, by the application of a fiber tape with a two-component epoxy resin. Some beams without reinforcement were subjected to bending test in order to determine the mean value of the wood strength. In the analysis of the non-reinforced beams, it has been observed that the mode of failure depends on: the ratio between the ultimate tensile strength and the compression strength, the non-linear behaviour of wood under compression at its final limit state and the volume of material subjected to the tensile test (a parameter that is directly proportional to the probability of effects deriving from localised defects). In general, it can be

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said that the most common collapse in solid wood occurs when the limit strength value is reached in the tension area. The fibers give a higher resistance to the beam. The ratio between the ultimate tensile strength and the compression strength increases so as to exploit the compressed flap in order to enable the plasticisation of the material and to ensure greater ductility until the collapse. Thanks to the gradual displacement of the neutral axis towards the tension area, the probability of the involvement of local defects in this area decreases: it permits to achieve a higher tensile strength. The graph in Figure 2 shows the significant results of some tests. The reinforced specimens have a different behaviour from those without reinforcement: they have a much higher tensile strength and ductility due to the greater plasticity of the cross-section. Figures 3 and 4 show the modes of failure observed in the beams reinforced with natural fibers.

Figure 3. Failure of a basalt-reinforced

beam.

Figure 2. Load-deflection diagram for reinforced and

unreinforced beams. Figure 4. Failure of a flax-reinforced beam.

The ultimate load is maximum for the carbon fiber reinforcement (+42% approximately) and minimum for basalt, glass and hemp fibers (only +24% approximately), which are all on the same average values. With regard to natural fibers, it has been found a good behavior of the reinforcement with flax fibers (mean increases of 35% of the breaking load), despite these fibers have a characteristic resistance much lower than carbon and basalt ones. It should be noted that, for some beams reinforced with flax and hemp, the recorded resistance values were greater than those of beams with a carbon fiber reinforcement, probably due to a better adhesion of these natural fibers to the wood. As for the modulus of elasticity, the increase recorded in the presence of fibers was poor if compared to samples without reinforcement.

3.2 Medium size beams Three unreinforced beams, three flax-reinforced beams and three basalt-reinforced beams were tested (Figure 5). In addition, it was decided to test two beams for each type of reinforcement, prepared doubling the reinforcement area, in order to evaluate its influence on the flexural strength (Figure 6). Resistance values of reinforced beams were higher than those of unreinforced beams. The basalt fiber reinforcement provided a great increase in strength compared to flax.

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Figure 5. Load-deflection diagram for reinforced

and unreinforced beams. Figure 6. Load-deflection diagram for basalt reinforced and doubling reinforced beams.

The doubling of the reinforcement produced a significant increase of resistance (98% for double basalt area of reinforcement) with regard to elements reinforced with basalt fibers (58% of the increase of resistance), while the values obtained with the use of flax fibers are not significant (the same value of 30%), since there has been a sudden failure due to the presence of defects in the middle of the beam. The Figure 7 and 8 show the failure modes of the reinforced beams.

Figure 7. Failure of a basalt-reinforced beam. Figure 8. Failure of a flax-reinforced beam.

3.3 Large size beams This phase of the experimental investigation involved nine beams 4000 mm long, with a square cross-section with sides of 200 mm. The beams, which were of fairly good quality, presented the classic defects of the wood: grain angle, shrinkage cracks and knots in the tension zone. The experimentation was performed in two separate phases: the first three beams were tested after six months of seasoning, while the remaining six after twelve months. A total of three beams reinforced with basalt and three beams reinforced with flax, as well as some beams without reinforcement, were tested.

The area of the reinforcement, the same for each type of fiber, was assumed equal to 28.60 mm2. Therefore, the ratio between the area of the reinforcement (AFRP) and the cross-section area of the wooden beam (Awood) is constant and equal to 0.000715. All the beams were subjected to four-point-bending test. After six months of seasoning, the first three beams (one reinforced with basalt, one with flax and one without reinforcement) showed the behaviour of Figure 9. Both types of reinforcement provided to the beam an increase of tensile strength compared to the

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unreinforced beam. In addition, the beam reinforced with basalt fiber showed a higher resistance together with a more ductile behaviour. After twelve months of seasoning, the second group of beams showed the behaviour represented by the load-displacement diagram in Figure 10, where the reinforced beams present high increases in tensile strength. In this case, the flax-reinforced beams reached the highest loads, but they seem to show a lower ductility compared to the basalt-reinforced ones. Referring to the mean value of all the bending tests, the flax-fibers reinforcement gives to the beam an increase of resistance of 66% on average, while the basalt-fibers reinforcement gives an increase of about 38%. The stiffness does not show significant increases resulting from the reinforcement. In almost all the flax-reinforced beams there has been a sudden shear failure of the reinforcement caused by the breaking of the wood. No one type of FRP has reached the tensile strength limit (Figure 11 and 12).

Figure 9. Load-deflection diagram for reinforced

and unreinforced beams (6 months seasoning). Figure 10. Load-deflection diagram for reinforced

and unreinforced beams (12 months seasoning).

The results of beams reinforced with natural fibers were then compared with those of a previous experiment carried out on beams made up of the same type of wood and reinforced with carbon fiber, with same geometry and AFRP/Awood ratio, equal to 0.000715 [Borri et al. 2001]. This experiment showed an average increase of the maximum load of 42% and an average increase in stiffness of 30%, if compared to unreinforced beams.

Figure 11. Failure of a basalt reinforced beam (12 months seasoning).

Figure 12. Failure of a flax reinforced beam (12 months seasoning).

4. Finite element analysis of reinforced beams A finite element analysis, calibrated on the experimental results, was performed to reproduce

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the behaviour of wooden beams reinforced to bending. This allowed parametric analysis and comparisons between the different types of reinforcement. The wooden beam was modelled with 8-node solid elements with the ability to crack and to pull, while the fibers with 4-node plane elements with membrane-like behaviour and resistant only to traction. Since the phenomenon of delamination is not occurred in any experimental case, it was possible to set the congruence of displacements between FRP and wood for adjacent nodes.

Figure 13. Load-deflection diagram for basalt-

reinforced beam model. Figure 14. Load-deflection diagram for flax-

reinforced beam model.

The numerical analysis reproduces with good precision the behaviour of both reinforced and unreinforced beams, as shown in Figure 13 and 14. The behaviour was studied by changing the FRP resistant area. The ultimate load increases with the increase of the FRP area (Figure 15). The basalt reinforcement allows an ultimate load higher than that produced with flax reinforcement, as shown in the experimental results of medium size beams and in the first set of large beams (less seasoned).

Figura 15. Increase of tensile strength vs FRP area.

5. Conclusions This experimental investigation has allowed testing the structural behaviour of wooden beams of different size, reinforced with natural fibers such as flax, hemp and basalt. For all types of natural reinforcements applied, a considerable increase in strength and a significant increase in tensile deformation were obtained. The maximum stresses in the FRPs during the collapse of the beams were always under the breaking load. The small size beams reinforced with natural fibers showed a high tensile strength and a ductile behaviour due to an interesting plasticization of the section, even compared to conventional carbon and glass fibers. The flax fiber reinforced beams achieved increases of the ultimate load similar to the one obtained with carbon fiber reinforcements.

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For medium size beams, the natural fibers reinforcement provided significant increases in the ultimate load. The basalt fiber reinforcement conferred high resistance to the beam, also providing a good ductile behavior. It has been observed an interesting increase of the ultimate load when the area of the fibers increases. For large size beams, the natural fibers reinforcement showed great increases in the ultimate load, comparable to those of the beams reinforced with CFRP. For less seasoned beams, the basalt fiber reinforcement conferred high resistance to the beam, also providing a good ductile behaviour. For more seasoned beams, the flax fiber reinforcement provided very high load increases, higher than those obtained with basalt fibers and comparable to those of carbon fibers, but showed a less ductile behaviour.

Acknowledgements The authors gratefully acknowledge the support provided by FIDIA S.r.l. of Perugia (Italy).

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