Compression Behavior of Basalt Fiber-Reinforced Polymer Tube-Confined Coconut Fiber-Reinforced Concrete

Basalt fiber is cheap and has excellent mechanical performance. In addition, in combination with the environmentally friendly coconut fiber, it can contribute to sustainable construction materials. In this study, a composite material consists of basalt fiberreinforced polymer (BFRP) tube-encased coconut fiber-reinforced concrete (CFRC) is developed. *e 28-day compression strength of the plain concrete is about 15MPa, which represents the low strength and poor-quality concrete widely existing in a large number of old buildings. *e concrete was poured and cured into the steel moulds, BFRP tubes with the inner diameter of 100mm and a height of 200mm, and BFRP tubes with the inner diameter of 160mm and a height of 320mm, respectively. In total, 36 cylindrical specimens were constructed and tested. *e axial compression tests were carried out to examine the strength and ductility enhancement due to the confinement of the BFRP tubes and coconut fibers. Also, the existingmodels were used to predict the ultimate axial compression strength and strain. *e results show that the size of the specimen significantly influences the predicted compression strength.


Introduction
Over the last decades, engineers are more aware of the environment and the limited resource, which has promoted the usage of natural fibers in polymer reinforcement. A huge number of natural fibers like sisal, kenaf, hem, flax, coconut, bamboo, and banana have been studied. Among these natural fibers, coconut fibers have gained popularity because of the cost-effective and high mechanical properties compared with other fibers, for example, bamboo or flax fibers [1]. Coconut fiber is extracted from the outer shell of coconut. More than 500,000 tons of coconut fibers were produced annually worldwide. Reis [2] reported that coconut fibers increased the fracture toughness of concrete composite. e coconut fibers even showed a better flexural property than that of glass and carbon fibers. Baruah and Talukdar [3] reported that the compressive, tensile, and shear strengths of coconut fiber-reinforced concrete (CFRC) increased about 13.7%, 22.9%, and 32.7% compared with those of plain concrete (PC), respectively. CFRC specimens remain intact due to the bridging effect of coconut fibers following a splitting test. Islam et al. [4] found that the addition of 0.5% coconut fibers by volume enhanced the flexural strength of normal concrete and high-strength concrete composite by 60% and 6%, respectively. eir research also indicated that the ductility and toughness of both normal-and high-strength concrete increased with a larger volume fraction content of coconut fibers. Hasan et al. [5] studied the lightweight concrete structure using coconut fibers as reinforcement. Ali et al. [6] examined the effects of coconut fiber lengths and fiber contents on the mechanical properties of concrete.
To overcome possible shortcomings of fibers, fibers can be embedded in a polymer matrix to fabricate a composite called fiber-reinforced polymer (FRP). e most popularly used high-strength fibers are carbon fiber, glass fiber, and basalt fiber. In comparison to other fibers, basalt fiber has superior characteristics, that is, high strength to weight ratio, excellent ductility and durability, high thermal resistance, good corrosion resistance, and cost-effectiveness, and it has been investigated for decades by a number of researchers [7][8][9][10][11][12][13]. Basalt fiber-reinforced polymer (BFRP) composite has also been employed in practice, for example, for postearthquake rehabilitation and strengthening. Lopresto et al. [14] studied the mechanical properties of BFRP and glass FRP (GFRP) composites. eir results show that BFRP has a higher Young's modulus, compressive and bending strength, higher impact force resistance, and energy absorption capacity than GFRP. Wu et al. [15] investigated the tensile properties of basalt fibers and epoxy composites in corrosive environment. ey found that the failure at the interface between the fibers and the resin governs the fracture properties of BFRP. ey [16] also studied the fatigue behavior of different fiber-reinforced polymers made of carbon, glass, basalt, and hybrid fibers. e results have shown that the tensile modulus of the fiber affects the failure modes of composite coupons. Colombo et al. [17] investigated the static properties of BFRP, manufactured by vacuum infusion process and hand layup process. ey also investigated the mechanical properties of different polymer matrices. Chen et al. [18] examined the quasi-static and dynamic tensile properties of BFRP. ey found that the tensile strength, failure strain, and elastic modulus increase rapidly with the strain rate.
In this work, two sizes of BFRP tubes, that is, one with the inner diameter of 100 mm and height of 200 mm and the other with the inner diameter of 160 mm and height of 320 mm, were fabricated using a hand layout process. In total, 36 specimens were poured and cured in the steel moulds and 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 fibers in the concrete. e results are compared with those obtained from analytical confinement models and discussed.

BFRP Tube Fabrication.
Commercial bidirectional woven basalt fabric (300 g/m 2 ) was used. e fabric has a plain woven structure with a count of 5.5 threads/cm in both warp and weft directions. e epoxy was Hoxion 35C resin and hardener. e woven basalt fabric was wrapped around the PVC tube using a hand layup 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°C to increase the hardening. e weft direction of the fabric was aligned parallel to the axis of the tube. ere were two groups of tubes. One group each had an inner diameter of 100 mm and length of 200 mm. e other group each had an inner diameter of 160 mm and height of 320 mm. e tubes were made of two and four layers of basalt fabrics. Figure 1 displays the BFRP tubes preparation prior to concrete casting. 9 coupons were fabricated and cured in the same condition with the BFRP tubes and the coupons were tested on a universal testing machine. Figure 2 shows the stressstrain relationship of the coupons of 2-layer, 4-layer, and 6layer. e modulus of the BFRP is between 13.5 and 15.0 GPa.

BFRP-CFRC Composite.
e coconut fiber had been roughly pretreated and cut to lengths of about 50 mm. e coconut fiber content was 1% of the cement mass. Two batches of concrete without and with coir were prepared. Both batches were designed as PC with a 28-day compression strength of 15 MPa to represent the low-strength concrete. In the second batch, coconut fiber was added. e concrete mix ratio by mass was 1:0.58:3.72:2.37:0.00245 for cement: water: gravel: sand: water reducer, respectively. e cement used was 32.5 normal Portland cement. e gravel has a maximum size of 25 mm. e natural sand was used as fine aggregate with a fineness modulus of 2.75. e matrix of the specimens prepared for this study consists of 36 cylindrical specimens and the test matrix of the specimens was given in Table 1.

Axial Compression Test.
For compression tests, four strain gauges were used for each cylinder. Two strain gauges were mounted at the midheight of a cylinder aligned along the hoop direction to measure the hoop strain, and two strain gauges were mounted and aligned along the axial compressive direction to measure the axial strain. Two linear variable transducers (LVDTs) were installed between the two surfaces of the compressive machine. Figure 3 shows the compressive test setup of a PC specimen. Each specimen was axially compressed up to failure. Readings of the stain gauges, LVDT, and load cell were taken by a data logging system. Figures 4-7 show the stressstrain curves of the small-size PC and CFRC specimens, the large-size PC and CFRC specimens, respectively. For the small-size specimens, the stress-strain curves can be divided into three regions [19], that is, the linear region, the microcracks-formed region, and the region of activated confinement. In the first region, the stress-strain behavior of the confined specimen is similar to that of the PC and CFRC specimens. e stress increases linearly with the axial strain.

Stress-Strain Relationship.
ere is little lateral expansion of the PC and CFRC core, and the confinement of BFRP tube is not yet activated. When the stress approaches the peak strength of the PC and CFRC, the curves enter the nonlinear transition region. e microcracks begin to propagate in concrete. e lateral expansion gradually increases, and the PC and CFRC specimens are suddenly crushed and lost the axial load-bearing capacity at last. However, for the BFRP tube-confined PC and CFRC specimens, the lateral expansion is impeded by the lateral confinement of the BFRP tube and the stiffness gradually decreases with the axial strain. In the third region, the confinement of the BFRP tube is fully activated, and the stress of the confined specimens increases linearly with the axial strain. e compressive strength and ductility of the confined specimens increase significantly compared with those of PC and CFRC specimens. Meanwhile, the confined stress of the BFRP tube in the hoop direction increases simultaneously with the expansion of the concrete core.
Once the hoop stress exceeds the tensile strength of BFRP, the specimen is suddenly crushed with a loud noise.
In the case of large-size specimens, the microcrack region is not obvious. In the third region, the stiffness is greatly reduced because of the weaker confinement compared with the small-size specimens (the confinement ratio is 160 : 100 for the small-size specimen to the large-size specimen). e difference between PC and CFRC strength is not significant. e ductility of the CFRC specimens are better than that of the PC ones. However, the ultimate compressive strength of the large-size specimen is smaller than that of the small-size one, owing to the weaker confinement of the BFRP tube as mentioned earlier.

Confinement Analysis.
e average compression strengths of the specimens are listed in Table 2. f c0 ′ and ε c0 are, respectively, the peak compressive strength and axial strain of the unconfined concrete. f cc ′ and ε cc are, respectively, the ultimate compressive strength and ultimate strain of the confined concrete. ε hoop is the rupture strain of the BFRP tube in the hoop direction. f l is the lateral confining pressure produced by the BFRP tube in the hoop direction. f cc ′ /f c0 ′ is the confinement effectiveness. f l /f c0 ′ is the ratio of confinement stress to the peak unconfined compressive strength of concrete and ε c0 /ε cc is the ratio of axial strain at the peak strength of unconfined concrete to ultimate axial strain of confined concrete. As shown in Figure 8, assuming that the volume of concrete is unchangeable, then the lateral confining pressure can be calculated by the following equation: where f BFRP and t BFRP are the hoop tensile strength and the thickness of the BFRP tube, respectively. D is the inner diameter of the tube. E BFRP is the tensile modulus of BFRP and ε hoop is the tensile hoop strain. For the same BFRP tube, the confining pressure of the large-size specimen is 62.5% of that of the small-size one. Table 2 shows that the peak compression strength of PC is higher than that of CFRC of the small-size specimens. In contrast, the peak strength of CFRC is higher than that of PC of the large-size specimen. Compared with PC, the axial strain of CFRC for both small-and large-size specimens is enhanced due to the bridging effect of the coconut fibers. For all specimens, the confinement of BFRP tubes significantly improves the compressive strength of the concrete. e average compressive strength enhancement f cc ′ /f c0 ′ of 2Land 4L-BFRP-PC is 1.48 and 2.20 for small-size specimens, and 1.52 and 2.22 for the large-size specimens, respectively. e average compressive strength enhancement of 2L-and 4L-BFRP-CFRC is 2.15 and 3.03 for the small-size specimens, and 1.22 and 1.63 for the large-size specimens, respectively. As anticipated, the lateral confinement stress f l increases with the tube thicknesses. e lateral confinement stresses of 2L-and 4L-BFRP-PC for small-size specimens are 2.86 MPa and 5.47 MPa. For the large-size specimens, they are, respectively, 2.63 MPa and 4.25 MPa, respectively, while  e ultimate axial strain has a similar tendency; that is, the ultimate axial strains of both BFRP-PC and BFRP-CFRC increase with the BFRP tube thicknesses. e ultimate strain reaches 0.013 and 0.0147 for 2L-and 4L-BFRP-PC, and 0.0112 and 0.0154 for 2L-and 4L-BFRP-CFRC for the smallsize specimens, respectively. In the case of 2L-and 4L-BFRP-PC, the values are 0.0106 and 0.0136, and in the case of corresponding large-size specimens, they are 0.0112 and 0.0139. Although the compressive strength of unconfined CFRC is lower than that of the unconfined PC, the compression of the BFRP-CFRC specimens is the best due to the joined confinement of coconut fiber and BFRP composite.

Prediction of Ultimate Compressive Strength.
Models for predicting the ultimate compressive strength of concrete cylinder can be divided into two categories, that is, the design-oriented and analysis-oriented models [20]. e design-oriented models have a closed form equation and are  directly based on the interpretation of experimental results. ese models consider the confined concrete as a single composite material and are thus simple and convenient to apply. e analysis-oriented models are generated using an incremental numerical procedure to predict an increase of the strength. ey separately deal with the contribution of confinement and core concrete. e compressive strength prediction is based on the radial displacement compatibility and equilibrium between the confinement and concrete core. In this study, similar approach according to Yan and Chouw [20] is used. e most common form of design-oriented models can be represented by the following equation: where k is the effectiveness coefficient and m is the power coefficient of the confinement ratio. e most frequently used design-oriented models are listed in Table 3.
Meanwhile, the often used analysis-oriented models are listed in Table 4. Most of the analysis-oriented models are developed from the Mander [41] model. is model was derived from the William−Warnke failure surface for triaxial compression state with equal effective lateral confining pressure.
Based on those models, the ultimate strength of the specimens is predicted and listed in Tables 5 and 6, respectively.
To verify the prediction precision of each model, the prediction error is defined as where v test and v prediction are the experimental and predicted results, respectively. e summation of the absolute errors of each specimen is used to judge the effectiveness of the model. Design-oriented and analysis-oriented models are listed from the best to the worst, respectively. Table 5 shows that the predicted ultimate compressive strengths of the BFRP tube-confined specimens are very different. For the small-size specimens, the design-oriented model of Samaan et al. [26] predicted the ultimate compressive strength very well. e errors are, respectively, about 8.24% and −6.91% for the 2L-and 4L-BFRP-PC and −0.61% and −6.35% for the 2L-and 4L-BFRP-CFRC. e model proposed by Toutanji [28] can also precisely predict the ultimate compressive strength. e maximum error is −14.56%. However, some models are not suitable for predicting the compression strength of BFRP-PC/CFRC. e prediction errors are more than 30% compared with the experimental values, such as the models proposed by Wu et al. [24] and Lam and Teng [25], Saafi [27], Karbhari and Gao [29], and Cheng et al. [31]. e analysis-oriented models can somehow better predict the ultimate compressive strength, especially the model proposed by Marques et al. e prediction errors are about 6.58% and −3.95% for the 2L-and 4L-BFRP-PC and −0.99% and 1.38% for the 2L-and 4L-BFRP-CFRC, respectively. e prediction errors of the models proposed by Fam and Rizkalla [32], Saaman et al. [26], Saadatmanesh et al. [33], Restepol and De Vino [34], Spoelstra and Monti [35], and Chun and Park [36], and Binici [38] are within 15%.
In the case of large-size specimens, among the designoriented models, Lam and Teng's model [23] predicts the compression strength best. e prediction errors are 6.76% and −9.01% for the 2L-and 4L-BFRP-PC, and 0.73% and −17.00% for the 2L-and 4L-BFRP-CFRC, respectively. e model proposed by Teng et al. [40] is the best among the analysis-oriented models, and the prediction errors are 9.25% and −6.22% for the 2L-and 4L-BFRP-PC and 1.87% and −14.98% for the 2L-and 4L-BFRP-CFRC, respectively.
Based on the experimental data in this study, it should be noted that, the prediction precision depends significantly on the size of the specimens. e model proposed by Samaan et al. [26] is the best to predict the ultimate compressive strength of the small-size specimens, but it is the worst to predict the larger ones. In contrast, the model proposed by Marques et al. [39] is the best to predict the ultimate Advances in Materials Science and Engineering Table 3: Coefficient of design-oriented confinement models.

Prediction of Ultimate Axial
Strain. e most frequently used models to predict the ultimate axial strain are listed in Table 7. Based on those models, the ultimate strains of smallsize specimens and large-size specimens are predicted and listed in Tables 8 and 9, respectively. e error of all 4 specimens is used to evaluate the effectiveness of the model and is listed in the best to the worst order. Tables 8 and 9 show that the model proposed by Miyauchi et al. [30] fits the experimental ultimate strains very well for all specimens considered. For the small-size specimens, the errors are about 0.62% and 7.12% for the 2L-and 4L-BFRP-PC, and 21.41% and 9.57% for the 2L-and 4L-BFRP-CFRC,  To evaluate the effectiveness, it is assumed that (1) there is no gap between the tube and concrete, and the expansion of the concrete can directly transverse to the confinement of the BFRP tube; (2) the volume increase due to the cracks created during the compression process is ignored; and (3) BFRP tube experiences homogeneous expansion in the hoop direction. Based on these assumptions, the maximum ultimate axial strain of the concrete can be obtained: Table 7: Prediction models of the ultimate axial strain.