Abstract
This work examined the enhancement of bending strength properties of two wood species, namely fir and pine, reinforced with two carbon fibre fabrics (CFF) of different areal weight, in two different layouts. CFF applied in the bottom surfaces of the specimens (‘F’-type) and in the second wrapped in the bottom surfaces and up to the half of the lateral surfaces (‘U’-type).The results indicated that samples which are reinforced with CCF show higher MOR and MOE values than the controls and this improvement was found to be significantly different. The reinforcement layout affected the bending properties of both pine and fir wood, since the corresponding values are higher in U-shaped reinforcement than in F-type reinforcement, at identical CFF areal weights. In the case of fir wood, the CFF areal weight did not significantly affect the bending properties.
Keywords
Introduction
The progress of humanity has been closely associated with the dependence on wood. The relative ease of working it and its almost universal availability have made wood an essential material for human survival. It varies in colour and density and is considered a primary raw material in building due to high strength in combination with low weight and some durability (Tsoumis 1992). The greater the technological advances, the more diverse and sophisticated the uses that have been found for wood. Nowadays, in spite of the availability of numerous new synthetic materials, even the most highly developed countries would find it difficult to maintain their standard of living if deprived of access to wood products. From the one side, its fabrication and many of its application are so simple that they require no special skills or technical knowledge by the user, from the other side wood, is a substance of a greater complexity than any other major engineering material and its utilization under competitive conditions of modern technology calls for a degree of scientific and technical understanding (Panshin and DeZeeuw 1980; Hoadley 2000).
Wood, however, is an inherent substance because of its origin as a product of metabolism of the living tree. As a result, its properties are subjected to wide variations brought about by the physiology of the trees and the external factors affecting growth. Wood can be also considered as a polymer matrix which consists of cellulose, hemicelluloses and lignin. As a result, wood exhibits considerable heterogeneity both in its structure and its ultrastructure, and this affects its properties, including mechanical properties and its overall performance as a raw material (Haygreen and Bowyer 1996; Hoadley 2000). This heterogeneity can be improved by reinforcement. Reinforcing wood practically means to increase or to recover its loading capacity. The reinforcement of wood or wood structures, in general, traditionally involves the use of nails, steel or screws (Bohannan 1962; Borgin et al. 1968; Curtis 1972). These materials, however, have many disadvantages such as their lower durability, the higher cost of installation and the increased weight of the structure.
These deficiencies can be overcome by using fibre-reinforced plastic (FRP), as a reinforcement material, its high strength and stiffness, the ease of cutting and construction properties and its high strength to weight ratio are some of its advantages which attract the researchers worldwide. Most of the published research studies on FRP-reinforced wood composites have focused on glass fibre-based reinforcements. The first research study into FRP can be attributed to Wangaard (1964), who examined the elastic behaviour of wood-fibreglass beams. A year later, a method which predicted the flexural properties of wood-fibreglass composites was developed by Biblis (1965). The same year, it was reported that the ultimate strength of a wood-fibreglass beam is increased significantly, if the fabric is placed wrapped in U-shape to the beam (Theakston 1965). The technical feasibility of internally reinforcing wood laminates with synthetic fibres has been evaluated (Rowlands et al. 1986). Ten adhesives and numerous reinforcements were assessed. The effects of various processing parameters are included, as is the behaviour of the fibre-reinforced wood under both dry-ambient and severe-weather (aging) conditions. Reinforced laminates were tested in tension and flexure, with and without internal finger joints. The authors reported a significant improvement in strength and stiffness provided by adding glass as a fibre reinforcement (respectively, 50% and 20%). Another study investigated the external bonding of thin FRP sheets onto the tension zones of wood beams and beam columns using epoxy resins. It was found that reinforced wood members seemed to be a promising way of increasing their strength, stiffness and ductility characteristics (Plevris and Triantafillou 1992). Similar results were also reported by Chen and Natterer (2001) who studied the mechanical performance of dowel-type timber joints, reinforced with fibre-glass fabric.
Another promising reinforcement material is carbon fibre. Despite its higher modulus of elasticity (MOE) and lower strain to failure, carbon fibre reinforcement is not used as extensively as glass fibre reinforcement because of higher costs (Pirvu et al. 2004). Carbon fibre-reinforced composites (CFPR) were first appeared in literature, in 1992 (Meier 1992). An interesting study on glass and carbon reinforcement of glued-laminated bridge girders appeared in the literature in 2000 (Dagher and Lindyberg 1999). They showed that carbon fibres compared similarly to glass in terms of strength and stiffness improvement as a function of reinforcement. Additionally, reinforcement with carbon fibre provided improved strength and stiffness properties at a lower weight. These results pointed out the relative performance advantage of using carbon fibre reinforcement rather than glass fibre reinforcement for glulam beams. In another study, the use of carbon fibres to reinforce wood slabs indicated that the ultimate strength under proper humidity conditions was increased by 100% (Valluzzi et al. 2007). De Jesus et al. (2012) reported a 23% increase in elastic stiffness when wood beams reinforced with carbon fibre and an increase of 12% in load capacity. Similar observation was also made by Neubauerová (2012). Other studies discussed the use of reinforcing FRP pultruded rods. Borri et al. (2005), for example, studied the use of externally adhered CFRP pultruded laminates on the tension zone of wooden beams and they achieved improvements in the ultimate strength of about 42%. The use of one or two pultruded rods introduced in slots made from the outside, within the tension area, was studied by Johnsson et al. (2007) for glulam beams. The load capacity was increased by 44% and some improvements in ductility were mentioned, indicating the advantages of two rods instead of just one.
Li et al. (2009) investigated the flexural performance of wood beams retrofitted by using carbon fibre-reinforced plastic composite sheets. The comparison of unretrofitted wood beams to those retrofitted with CFRP composite sheets revealed that the flexural strength increased in the CFRP–wood beam composites, but the middle vertical displacement decreased. In a recent study, a comparison of different carbon fibre composites in reinforcement layouts of wooden beams was carried out (Rescalvo et al. 2017). Timber elements which were extracted from a recent rehabilitation of the roof of the Faculty of Law building, University of Granada (Spain), were reinforced with carbon fibre with three layouts of reinforcement, namely longitudinal reinforcement, U-shaped reinforcement and longitudinal and U-shaped reinforcement. The results from bending tests show that in all three cases applied, reinforcement provides a clear improvement in terms of bending capacity and stiffness as compared with the control specimens.
As far as the location of reinforcement is concerned, in a recent study, four reinforcement layouts were applied and compared reinforcement only on the tension zone versus sideways wrapping the beam with CFRP fabric, including the compression zone. The improvement varied between 17% and 27% in stiffness, 40% and 53% in flexural strength and between 36% and 68% in shear strength (Buell and Saadatmanesh 2005).
The objective of this work was to examine the enhancement of bending strength properties of two wood species, namely fir and pine, reinforced with two carbon fibre fabrics (CFF) of different areal weight (80 and 130 g m−2), in two different layouts. CFF applied in the bottom surfaces of the specimens (‘F’-type) and in the second wrapped in the bottom surfaces and up to the half of the lateral surfaces (‘U’-type).
Materials and methods
Greek fir (Abies borisii regis) and black pine (Pinus nigra) samples of dimension 20 mm × 20 mm × 360 mm (radial × tangential × longitudinal) were cut from freshly felled kiln dried logs. Physical defects were removed according to EN 385/2001. Samples were conditioned in normal conditions (20°C temperature and 65% relative humidity) until the weight of the wood was stabilized. Both control samples and samples that were reinforced with CFF came from the same tree.
Wood samples were reinforced with CFF. Two types of CFF having different areal weight (80 and 130 g m−2) were used. Both CFF consisted of fibres with the same mechanical properties (tensile strength 4900 MPa, tensile modulus 240 GPa and elongation at break 2%).
Two layouts of reinforcement were considered and compared, as shown in Figures 1 and 2. In the first case, CFF was applied in the bottom surfaces of the specimens (‘F’-type) and in the second case, was wrapped in the bottom surfaces and up to the half of the lateral surfaces (‘U’-type). The latter is less often used as reinforcement than F-type due to the fact that lateral faces are not always accessible. U-shaped reinforcement was bonded on wood surfaces with an epoxy-based adhesive. The fibre direction of the CFF adjusted to be the same as the fibre direction of the wood. After the application process, a second layer of epoxy-based adhesive was brushed onto the CFP fabric, and specimens were dried for a week at room temperature. Five specimens for every parameter were prepared and tested, and this corresponded to 25 pine samples and 25 fir samples, 50 samples in total.
Reinforcement layouts: (a) longitudinal reinforcement, full width (F-type) and (b): U-shaped reinforcement. Reinforcement layouts: (left) control, (centre) longitudinal reinforcement, full width (F-type) and (right): U-shaped reinforcement.
Modulus of rupture (MOR) and MOE tests were performed in accordance with DIN 52186:1978 standards with a Zwick material testing machine. The rate of crosshead movement was adjusted at 0.12 mm s−1, so that the maximum load was reached within (1.5 ± 0.5) min throughout the test. The loading was continued until a break occurred on the surface of the test samples. The flow diagram of the experimental procedure is depicted in Figure 3.
The flow diagram of the experimental procedure.
Results and discussion
Mean density values of pine and fir samples, at 20°C temperature and 65% relative humidity, were 0.54 and 0.43 g cm−3 and mean moisture content values were 10.4% and 10.9%, respectively.
The MOR and MOE values are summarized in Table 1, both for pine and fir wood, whereas the stress–strain curves are depicted in Figure 4, for every parameter tested in this study. MOR values for pine were varied from 102.5–115.5 N mm−2, whereas for fir from 73.9–89 N mm−2. MOE values for pine were varied from 11,395 to 12,147 N mm−2, whereas for fir from 9683 to 10,510 N mm−2. The results indicated that samples which are reinforced with CCF show higher MOR and MOE values than the controls, this improvement in bending properties is found to be significantly different as depicted in Table 1 and this was the case for both pine and fir wood. However, the magnitude of improvement was more pronounced in fir wood. This may be attributed to the lower density values of fir compared to pine wood.
Stress–strain curves for every parameter tested in this study. Bending strength properties (MOR–MOE) of pine and fir wood samples reinforced with carbon fibre fabric (CFF). Notes: Different letters show which values are statistically different at the 5% level. Each value is a mean of five replicates. aStandard deviation. bCoefficient of variation (%). cPercentils at 5% and 95%.
A closer inspection of the data depicted in Table 1 reveals that the reinforcement layout affected the bending properties of both pine and fir wood, since the corresponding values are higher in U-shaped reinforcement than in F-type reinforcement, at identical CFF areal weights, and this is in line with the observations made by Rescalvo et al. (2017). This behaviour was slightly greater in MOR than in MOE values. In fir samples, the difference between U-shaped and F-type reinforcement was significant, for both properties. The greater improvement was observed in U-shaped reinforced fir samples. In pine samples, however, the bending values obtained for the F-type reinforcement at 130 g m−2 areal weight were significantly lower than all the other treatments. In the case of fir wood, the CFF areal weight is not significantly affected the bending properties.
Conclusions
This work examined the enhancement of bending strength properties of two wood species, namely fir and pine, reinforced with two CFF of different areal weight, in two different layouts. CFF applied in the bottom surfaces of the specimens (‘F’-type) and in the second wrapped in the bottom surfaces and up to the half of the lateral surfaces (‘U’-type). The results indicated that samples which are reinforced with CCF show higher MOR and MOE values than the controls and this improvement was found to be significantly different. The reinforcement layout affected the bending properties of both pine and fir wood, since the corresponding values are higher in U-shaped reinforcement than in F-type reinforcement, at identical CFF areal weights. In the case of fir wood, the CFF areal weight is not significantly affected the bending properties.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the author(s).