25th Australasian Conference on Mechanics of Structures and Materials (ACMSM25) Edited by C.M. Wang, J.C.M. Ho and S. Kitipornchai Brisbane, Australia, December 4 – 7, 2018
INVESTIGATION OF THE PROPERTIES OF NATURAL FIBRE REINFORCED POLYMER-CONCRETE COMPOSITE J. CHEN1, Y. LV2, Z.X. LI3 and N. CHOUW1,* 1
Department of Civil and Environmental Engineering, the University of Auckland, Auckland 1142, New Zealand 2 Tianjin Key Laboratory of Civil Structure Protection and Reinforcement, Tianjin Chengjian University, China 3 Tianjin Key Laboratory of Civil Structure Protection and Reinforcement, Tianjin Chengjian University, and Tianjin University, Tianjin, China *(Presenting author:
[email protected])
Abstract. Nowadays, engineers are more aware of the environment and the limited resource. It is also well-known that conventional composites, i.e. reinforced concrete, will likely have corrosion issue with the time. In addition, steel is heavy. Consequently, under dynamic loadings it can cause a large inertia. To avoid this large mass and corrosion issue this research focuses on the usage of natural fibre reinforced polymer-concrete composite. The fibres considered are basalt, flax and coconut fibre. The concrete is enhanced by coconut fibre, and basalt/flax fabric is used to reinforce the polymer. The properties of basalt/flax fabric reinforced polymer enhanced coconut fibre reinforced concrete composite are presented. Keywords: Flax fibre; Basalt fibre; Coconut fibre; Polymer-concrete composite structure; Material properties
1 INTRODUCTION Composite materials have been in use since prehistoric times. The oldest mention is found in Bible, which indicated the difficulty of making bricks without straw. Nowadays, there has been a rapid increase in the production and use of composites. Composite materials have replaced traditional materials in various applications, especially in automotive, aerospace, marine, and process applications. A wide range of concrete composite materials are made in a contribution of synthetic fibre reinforced polymer, such as glass/carbon fibre reinforced polymer (G/CFRP) due to their high strength. However, these synthetic materials have the drawbacks, such as high-cost, non-renewable with a low potential for recycling, and are non-biodegradable. In contrast, natural fibre reinforced polymer composites attract more attention recently not only because they have low-specific mass and corrosion resistance, but also produce less gas emissions and pollutants. A number of natural fibres like sisal, kenaf, hem, flax,
coconut, bamboo and banana have been studied and applied. With regard to the natural fibre reinforced concrete, the coconut fibre has gained popularity because of the cost-effectiveness, and the mechanical properties are comparable to those of bamboo or flax fibres. Reis (2006) reported that coir increased concrete composite fracture toughness and the use of coir showed a better flexural properties than glass and carbon fibres. Baruah and Talukdar (2007) reported that the compressive, tensile and shear strengths of coconut fibre reinforce concrete (CFRC) with 2% fibre increased about 13.7%, 22.9% and 32.7% compared with those of the plain
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concrete (PC) specimens, respectively. CFRC specimens as a whole specimen due to coir bridging effect following a splitting test. Islam et al. (2012) found that an addition of 0.5% volume coconut fibres enhanced the flexural strength of normal PC by 60%, and increased 6% flexural strength of high-strength concrete. Their research also indicated that the ductility and toughness of both normal- and high-strength concrete increased with the volume fraction content of the coconut fibres. Hasan et al. (2012) studied the lightweight concrete structure using coconut fibres as reinforcement. Ali et al. (2012) examined the effects of coconut fibre lengths and fibre contents on the mechanical properties of concrete. Concrete filled fibre reinforced polymer tube (CFFT) composite column is one of the most common composite members that can be used in modern structures, such as bridges, high-rise buildings and warehouses. This is mainly due to the advantages of CFFT composites, i.e. high ductility, high strength and high stiffness. In the last decades, the use of natural fibre reinforced polymer (FRP) sheets for external confinement of concrete members has been a popular topic due to an increasing environment awareness. Dittenber and GangaRao (2012) tested more than 20 commonly used natural fibres as reinforcement of FRP composites for structural applications. They concluded that flax fibre offers the best combination of low cost, light weight, high strength and stiffness. Flax fibre reinforced polymer (FFRP) composites have a stand out advantage when it comes to strength-to-weight and stiffness-toweight ratios. The density of FFRP composites is approximately 1270 kg/m3, i.e. about 1/6 of steel density (Yan et al. 2012). Basalt fibre is another popular natural material to be used to fabricate the FRP composite and has been investigated by a number of researchers, e.g. Sim et al. (2005) and Park et al. (1999). Lopresto et al. (2011) studied the mechanical properties of BFRP and GFRP composites. The authors prepared the specimens using the vacuum bag technology, and the tensile test was carried out by a MTS machine. The test results showed that the BFRP had higher Young’s modulus, compressive and bending strength, higher impact force resistance and energy absorption capacity than GFRP. Wu et al. (2014) investigated the tensile properties of basalt fibres and epoxy composites in corrosive environment. The authors found that the failure at the interface between fibres and resin governed the fracture behaviour of BFRP. Wu et al. (2010) also studied the fatigue behaviors of carbon, glass, basalt and hybrid fibre reinforced polymers. The results showed that the tensile modulus of the fibre affected the failure mode of composite coupons. Colombo et al. (2012) investigated the static material properties of BFRP composite manufactured by using the vacuum infusion process and hand lay-up process. The authors also investigated the effects of different polymer matrices on the mechanical properties. Chen et al. (2017) examined the quasi-static and dynamic tensile properties of BFRP. The authors concluded that the properties including tensile strength, failure strain and elastic modulus increased rapidly with the strain rate. In this work, twelve BFRP tubes with the inner diameter of 100 mm and height of 200 mm were fabricated using a hand lay out process. Eighteen low compression strength PC and CFRC specimens were poured and cured into the steel moulds, 2-layer and 4-layer BFRP tubes, respectively. Compressive tests were carried out to investigate the enhancement of strength and ductility due to the confinement of BFRP tube and an inclusion of coconut fibres. The measured results are compared with that of previous experimental results of FFRP confined CFRC cylinders (Yan and Chouw, 2014). In addition, double FFRP tubes confined CFRC specimens were investigated. The inner FFRP tubes used to further reduce the mass of the concrete member and provided additional inner lateral confinement. To reveal the effect of different ratio of inner FFRP tube diameter and outer FFRP tube diameter (Di/Do) on double FFRP tube confined CFRC (DFFRP-CFRC) composite, different sizes of the inner FFRP tube were considered in this study. The behaviours of double tube confined composite specimens with different Di/Do ratio were presented.
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2 EXPERIMENTS 2.1 BFRP Tube Fabrication and Concrete Specimen Preparation Commercial bidirectional woven basalt fabric (300 g/m2) was used for this study. The fabric has a plain woven structure with a count of 5.5 threads/cm in both warp and weft directions. The epoxy used was Hoxion 35C resin and hardener. The woven basalt fabric was wrapped around the PVC tube using a hand lay-up process. After 24 hours curing in room temperature, the tubes were demoulded and put into an oven for 8 hours with a constant temperature of 80 ℃ to increase the hardening. The weft direction of the fabric was aligned parallel to the axis of the tube. Figure 1 displays the BFRP tubes preparation prior to concrete casting.
Figure 1. BFRP tubes preparation
The coconut fibre had been roughly pre-treated and cut to a length about 50 mm. The coir mass content was 1% of the cement. Two batches of concrete were prepared. Both batches were designed as PC with a 28-day compression strength of 15 MPa to represent the low strength concrete which can be found in a large number of very old buildings. For the second batch, coconut fibre was added in the mixing. The concrete mix ratio by mass was 1:0.58:3.72:2.37:0.00245 for cement: water: gravel: sand: water reducer, respectively. The cement used was 32.5 normal Portland cement. The gravel has a maximum size of 25 mm. The natural sand was used as fine aggregate with a fineness modulus of 2.75. The matrix of the specimens prepared for this study consists of 12 cylindrical specimens, the test matrix of the specimens was given in Table 1. Table 1. Test matrix of cylinders
Specimens
No. of specimens
No. of BFRF layers
PC 3 CFRC 3 2L-BFRP-PC 3 4L-BFRP-PC 3 2L-BFRP-CFRC 3 4L-BFRP-CFRC 3 ∗ 𝑡BFRP is the thickness of the BFRP tube.
2 4 2 4
∗ 𝑡BFRP (mm)
Mass (kg)
0 0 1.06 1.57 1.04 1.58
3.79 3.63 3.85 3.98 3.78 3.89
2.2 FFRP Tube Fabrication and CFRC Preparation The natural flax fabric is from the Libeco, Belgium. Table 2 gives the mechanical property of flax fabric. The epoxy is SP High Modulus Prime 20LV epoxy system. The physical properties of epoxy resin are given in Table 3. More details about the fabrication of FFRP tube can be found the previous studies by the authors (Chen and Chouw, 2016). 3
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Table 2. Mechanical property of flax fabric Fibre type Flax BL-550
Modulus (GPa) 12
Tensile strength (MPa) 120
Density (g/cm3) 1.3
Elongation (%) 1.6
Table 3. Epoxy system
Mix ratio by weight Density (g/cm3)
Resin: SP Prime 20LV
Hardener: SP PREIME 20 Slow
100 1.123
26 0.936
The coir (extracted from the outer shell of coconuts) was obtained from Bali, Indonesia. The fibres were loosened and separated from one another, and the coir dust removed. The fibres were then manually straightened, combed, and cut into the required lengths (50 mm). The coconut fibres are then added into the concrete during mixing. Table 4 shows the test matrix of the specimens. In total twelve cylindrical specimens were constructed and tested. The inner and outer FFRP tube of all specimens made by four layers flax fabric. The average thickness of 4-layers FFRP is 5.3 mm. Each specimen has an outer core diameter of 100 mm and a length of 200 mm. The inner tube has the core diameter of 25 mm, 30 mm, 35 mm and 40 mm. The sketch of all specimens shown in Figure 2. The compressive tests were performed according to ASTM C39 (2010). Table 4. Specimen configurations of DFFRP-CFRC Configuration DFFRP-CFRC-D25 DFFRP-CFRC-D30 DFFRP-CFRC-D35 DFFRP-CFRC-D40
No. of No. of layer No. of layer specimens inner tube outer tube 3 4 4 3 4 4 3 4 4 3 4 4
Core diameter Douter (mm) 100 100 100 100
Figure 2. Specimens with different Di/Do ratio
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Core diameter Dinner (mm) 25 30 35 40
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2.3 Test Procedure For compression tests, each cylinder used four strain gauges. Two strain gauges were mounted at the mid-height of a cylinder aligned along the hoop direction to measure the hoop strain and two strain gauges were mounted at the mid-height of a cylinder aligned along the axial compressive direction to measure the axial strain. Two linear variable transducers (LVDTs) were installed between the two surfaces of the MST compressive machine. Figure 3 shows the compressive test set-up of BFRP and DFFRP confined concrete specimen. Each sample was axially compressed up to failure. Readings of the stain gauges, LVDT, and load cell were taken by a data logging system. LVD
2L-
Figure 3. Compressive test instrument and set-up.
3 RESULTS AND DISCUSSION 3.1 Test Results and Confinement Analysis of BFRP confined concrete specimens The average compressive properties of each configurations are listed in Table 5. 𝑓′𝑐𝑜 and 𝜀𝑐𝑜 are the peak compressive strength and axial strain of the unconfined concrete, 𝑓′𝑐𝑐 and 𝜀𝑐𝑐 are the ultimate compressive strength and ultimate strain of the confined concrete. 𝜀ℎ is the rupture strain of the BFRP tube in the hoop direction, 𝑓′𝑐𝑐 /𝑓′𝑐𝑜 is the confinement effectiveness. 𝜀𝑐𝑐 /𝜀𝑐𝑜 is the ratio of axial strain at the peak strength of confined concrete to ultimate axial strain of unconfined concrete. Table 5. Average results of the BFRP confined concrete specimens. Specimens PC CFRC 2L-BFRP-PC 4L-BFRP-PC 2L-BFRP-CFRC 4L-BFRP-CFRC
𝑡𝐵𝐹𝑅𝑃 (mm) --1.10 1.75 1.10 1.75
𝑓′𝑐𝑜 (MPa) 15.93 14.24 -----
𝜀𝑐𝑜 (%) 0.21 0.41 0.21 0.21 0.30 0.30
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𝑓′𝑐𝑐 (MPa) 15.93 14.24 27.90 41.43 30.57 43.21
𝜀𝑐𝑐 (%) 0.21 0.41 1.30 1.47 1.12 1.54
𝜀ℎ (%) --0.99 1.10 1.01 1.20
𝑓′𝑐𝑐 𝑓′𝑐𝑜 --1.48 2.20 2.15 3.03
𝜀𝑐𝑐 𝜀𝑐𝑜 --6.17 7.01 2.73 3.75
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Table 5 shows the peak compression strength of PC is higher than that of CFRC. The axial strain of CFRC is enhanced compared with PC due to the bridging effect of the coconut fibres. For specimens with 2-layers and 4-layers BFRP confinement, the compressive strength significantly improves. The average compressive strength enhancement 𝑓′𝑐𝑐 /𝑓′𝑐𝑜 of 2L- and 4L-BFRP-PC are respectively 1.48 and 2.20, while the confinement effectiveness value of 2L- and 4L-BFRP-CFRC specimens are respectively 2.15 and 3.03. Although the compressive strength of unconfined CFRC lower than unconfined PC, the compression performance of BFRP-CFRC specimens is the best due to the joint confinement of coconut fibre and BFRP composite. The ultimate axial strain of BFRP-PC/CFRC specimen increases with the BFRP tube thickness, i.e. ultimate strain reached 0.013 and 0.0147 for 2L- and 4L-BFRP-PC, 0.0112 and 0.0154 for 2L- and 4L-BFRP-CFRC, respectively. The BFRP-CFRC specimens show la arger ultimate axial strain than that of the corresponding BFRP-PC because of the inclusion of coconut fibre. The BFRP tube confinement allows more deformation and improves the ductility of composites.
(b)
(a)
Figure 4. Comparison of (a) 𝑓′𝑐𝑐 /𝑓′𝑐𝑜 and (b) 𝜀𝑐𝑜 /𝜀𝑐𝑐 between BFRP-PC/CFRC and FFRP-PC/CFRC specimens. Figure 4(a) gives a comparison between the confinement effectiveness of BFRP-PC/CFRC specimens and FFRP-PC/CFRC (Yan et al. 2012) specimens. The 𝜀𝑐𝑜 /𝜀𝑐𝑐 ratio of BFRP-PC/CFRC specimens also is compared with the 𝜀𝑐𝑜 /𝜀𝑐𝑐 ratio of FFRP-PC/CFRC specimens, as shown in Figure 4(b). It clearly displays that the confinement effectiveness of 2L-, 4L-BFRP tubes confined specimens is larger than that of FFRP confined specimens, especially for 2L- and 4L-BFRP-CFRC specimens. In contrast, the 𝜀𝑐𝑜 /𝜀𝑐𝑐 ratio of 2L-, 4L-FFPR-PC/CFRC specimens is larger than the corresponding 2Land 4L-BFRP-PC/CFRC specimens, especially for 2L- and 4L-FFRP-PC.
3.2 Compressive Results of DFFRP-CFRC specimens Table 6 lists the average results of DFFRP-CFRC specimens with different size of inner tubes. f' is the ultimate compressive strength. 𝜀𝑎𝑥𝑖𝑎𝑙 is the ultimate axial strain and 𝜀ℎ is the hoop train at the rupture of the external FFRP. f'/w is the ratio of the ultimate compressive strength to the weight of the specimen. The compressive strength roughly shows a declining trend with a larger diameter of the inner tube. The values are 55.68 MPa, 54.64 MPa, 51.86 MPa and 54.33 MPa, respectively for the specimens with 25 mm, 30 mm, 35 mm and 40 mm inner tube diameter. The maximum result is obtained from the specimens with diameter 25 mm inner tube. Simultaneously, the weight of the specimens also reduces with the diameter of the inner FFRP tube. The largest weight reduction is 10.4% obtained from the specimen with 40 mm diameter inner tube. The weight of the material is a vital property, because this double flax FRP tubes confined CFRC material will be used for earthquake-resistant structures in the 6
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future. To obtain optimize size of the inner tube, a ratio between the ultimate compressive strength and the weight have been calculated. The highest ratio is 15.75, which is obtained from the specimens with an inner tube of a 40 mm diameter. This ratio indicates that the DFFRP-CFRC-D40 specimens not only can reduce the mass of the specimen, but also have a satisfactory load bearing capacity compared with that of the DFFRP-CFRC-D25 specimens. Table 6. Average results of DFFRP-CFRC specimens. Configuration DFFRP-CFRC-D25 DFFRP-CFRC-D30 DFFRP-CFRC-D35 DFFRP-CFRC-D40
f’ (MPa) 55.68 54.64 51.86 54.33
𝜀𝑎𝑥𝑖𝑎𝑙 (%) 3.7 4.15 5.48 7.6
𝜀ℎ (%) 0.98 0.93 0.92 1.16
Weight w (kg) 3.85 3.77 3.59 3.45
Weight reduction (%) -2.1 6.8 10.4
f’/w 14.46 14.49 14.44 15.75
The average external hoop strains of 4-layers DFFRP-CFRC specimens are 0.98%, 0.93%, 0.92% and 1.16%, respectively for specimen with a 25 mm, 30 mm, 35 mm and 40 mm diameter inner tube. The specimens with an inner tube of a 40 mm diameter display the greatest expansion in the lateral direction. The axial strain of all specimens clearly shows an increase with a large diameter of the inner tube. The largest axial strain is also obtained from the specimen with 40 mm inner tube due to the less concrete contains. This results show that the DFFRP-CFRC specimen with a large size of inner tube allows a greater expansion in the axial direction.
3.3 Compressive Stress–Strain Relationship of DFFRP-CFRC Specimens The relationship between axial compressive stress and strain of four configurations of DFFRPCFRC specimens is displayed in Figure 5. The compressive modulus of the specimens declines with an increase of the diameter of the inner tube. The axial compressive stress-strain relationship of DFFRPCFRC-D40 shows that the composites with a 40 mm diameter of the inner tube have the highest ductility and lowest compressive modulus. The stiffness of DFFRP-CFRC specimen declines with a decreasing mass of the specimens.
Figure 5. Stress-strain relationship of DFFR-CFRC composites
4 CONCLUSIONS The compressive behaviour of basalt FRP tube confined plain concrete and coconut fibre reinforced concrete was investigated and compared with that of flax FRP tube confined PC and CFRC specimens. In addition, the effect of the double tube confined composites with different inner to outer diameters on the compressive behaviour of double FFRP tubes confined CFRC were studied. The study reveals:
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1. BFRP and FFRP tube confinement enhanced the compressive strength and ductility of both PC and CFRC composites. 2. The BFRP tubes confined specimens has a larger confinement effectiveness and less 𝜀𝑐𝑜 /𝜀𝑐𝑐 ratio in comparison with that of FFRP confined specimens. 3. The weight of DFFRP-CFRC specimens reduced with the diameter of the inner tube, and the compressive modulus decreased with less concrete materials due to the removal of inner concrete core. 4. DFFRP-CFRC-D40 specimens not only have less mass, but also a satisfactory load bearing capacity and high ductility compared with the specimens with smaller inner tube diameter.
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