Study of the bond behaviour between basalt fibre-reinforced polymer bar/sheet and bamboo engineering materials

Abstract

To improve practical application of modern bamboo structures, strengthening the bamboo engineering material is necessary to overcome insufficient stiffness. As an essential step in developing fibre-reinforced polymer–bamboo engineering material composite structures aimed at increasing the structural stiffness, the bonding behaviour at the interface of the fibre-reinforced polymer and bamboo engineering materials should be investigated in detail because currently there is a lack of research. In this article, bonding behaviour is studied between basalt fibre-reinforced polymer bar and bamboo engineering material including laminated and reconstituted bamboo and between basalt fibre-reinforced polymer sheets and laminated bamboo. Failure patterns are categorized, and the load–slip curves are discussed. Based on the failure pattern and strain variation, recommended bond lengths were proposed for the basalt fibre-reinforced polymer bar–bamboo engineering material and basalt fibre-reinforced polymer sheet–laminated bamboo composite specimens, respectively. In addition, a simplified three-phase bond–slip model was proposed for the basalt fibre-reinforced polymer bar–bamboo engineering material composite specimen.

Keywords

basalt fibre-reinforced polymer bonding behaviour bond–slip model laminated bamboo reconstituted bamboo

Introduction

Bamboo has always been used to build local houses in China. It is convenient to obtain raw bamboo material locally, and it has advantages including cost saving, environmental friendliness and recyclability. In general, the tensile and compressive stress of bamboo are twice and 1.5 times those of wood, respectively, and the bamboo strength to weight ratio is higher than wood and steel (Renjie and Yunshui, 2002). However, the mechanical properties of raw unprocessed bamboo material are unstable, with large discreteness (Shan et al., 2012). Many inevitable defects can also be found in unprocessed bamboo material, which results in poor durability (Kaur et al., 2016). To utilize the advantages of raw bamboo and improve its material stability and performance, new types of bamboo engineering materials, including laminated bamboo (Mahdavi et al., 2012; Nugroho and Ando, 2001) and reconstituted bamboo (Qin and Yu, 2009; Yang-lun and Ding-hua, 2006), have been proposed and studied (Li et al., 2019), and they are beneficial for reducing the material discreteness and enlarging the practical applications of bamboo (Sharma et al., 2015; Zhao et al., 2018).

However, the design of modern bamboo structures is controlled by the stiffness based on the previous study (Yang et al., 2010), which limits using the high tensile strength of bamboo engineering material. To broaden practical application of modern bamboo structures, strengthening bamboo engineering material is necessary. Early in 1981, Spaun (1981) studied the strength of a laminated wood beam with glass fibre-reinforced polymer (GFRP) sheet and reported that the stiffness of the laminated wood beam can be significantly increased. Later, wooden beams strengthened by carbon fibre-reinforced polymer (CFRP) sheet and bar were also studied (Gentile et al., 2002).

Inspired by the aforementioned studies on wooden structures strengthened by fibre-reinforced polymer (FRP), insufficient bamboo structure stiffness can also be solved by applying FRP. However, only limited research related to FRP strengthening of bamboo structures has been carried out (Wei et al., 2017). The first step to push the development of FRP–bamboo engineering material composite structures is to fully understand the interfacial behaviour between the FRP and bamboo engineering material. The cooperative working performance and mechanism between the FRP and bamboo engineering material should be investigated. Similar studies on the bonding behaviour between FRP and wood (Wan et al., 2013) and between FRP and concrete (Coelho et al., 2015; Yuan et al., 2004) have been conducted. Compared with the GFRP and CFRP, the elastic modulus of the basalt fibre-reinforced polymer (BFRP) is relatively small (Banibayat and Patnaik, 2014), and it may work better with bamboo engineering material (Gentile et al., 2002).

The common BFRP products include BFRP bar and sheet. Therefore, the bonding behaviour between BFRP bar and bamboo engineering materials including the laminated and reconstituted bamboo and that between the BFRP sheet and the laminated bamboo are studied. A new forming-once processing technology for the BFRP bar–bamboo engineering material composite specimen was proposed, the effectiveness and reliability of which were verified by the pull out test. Also, the double shear test was adopted for testing the BFRP sheet–laminated bamboo composite specimen. Failure patterns of the composite specimen were categorized, and the load–slip curves were also analysed in different stages. Recommended bond lengths were proposed for the BFRP bar–bamboo engineering material and BFRP sheet–laminated bamboo composite specimens.

Material properties of BFRP and bamboo engineering materials

Material tests on bamboo engineering materials including laminated and reconstituted bamboo and BFRP bar and sheet were conducted.

Bamboo engineering materials

The laminated bamboo was composed of bamboo strips, and the reconstituted bamboo was fabricated from bamboo fibres. Based on the production process requirements of both bamboo engineering materials, the moso bamboo with an age of 3–5 years was selected. A total of 24 laminated and reconstituted bamboo specimens with dimensions of 30 mm × 30 mm × 45 mm were tested under compression, and a total of 24 with dimensions of 30 mm × 8 mm × 300 mm were tested under tension per Chinese standards (GB/T1938-2009, 2009). The test results are listed in Table 1.

Table 1.

Material properties of bamboo engineering materials, BFRP bar and BFRP sheet.

Material	σ_cu (MPa)	σ_tu (MPa)	ε_tu (%)	E (GPa)
BSL	97.0 (5.08%)	131.4 (6.43%)	–	11.6 (3.74%)
BCR	78.1 (6.12%)	181.5 (6.93%)	–	14.1 (4.31%)
BFRP bar	–	897.0 (5.51%)	2.75 (6.16%)	32.3 (2.42%)
BFRP sheet	–	1725.4 (9.40%)	2.51 (9.0%)	73.8 (6.50%)

BFRP: basalt fibre-reinforced polymer.

BSL is the laminated bamboo; BCR represents reconstituted bamboo; σ_cu is the ultimate compressive stress; σ_tu is the ultimate tensile stress; ε_tu is the ultimate tensile strain; E is the elastic modulus; and numbers in the brackets are coefficients of variation.

BFRP bar and sheet

Geometric shape affects the bonding behaviour between BFRP bar and bamboo engineering materials. The purposely designed geometric shape of the BFRP bar is shown in Figure 1. The average values of BFRP bar material properties are listed in Table 1. A material test of BFRP sheet reinforced by L-500 epoxy resin adhesive satisfied the requirement of ACI 440 (R-02 A, 2003), and the results are listed in Table 1.

Figure 1.

Geometric shape of the BFRP bar.

Bond behaviour between BFRP bar and bamboo engineering material

In this section, the bond behaviour between BFRP bar and bamboo engineering materials, including laminated bamboo and reconstituted bamboo, was analysed in terms of failure patterns and the bond–relative slip relationship. Different methods of specimen fabrication were evaluated and compared based on bond strength and the distribution of bond stress.

New forming-once processing technology for BFRP bar–bamboo composite specimens

Limited research has been conducted on studying the bond behaviour between BFRP bar and bamboo engineering materials. Based on a literature review of the existing method for processing the FRP bar and wood (a material similar to bamboo) composite specimen aiming at investigating the bond behaviour, the common fabrication method should be as follows: (1) grooving the wood, (2) planting the FRP bar and (3) gluing together the FRP bar and wood with epoxy or phenolic resin (De Lorenzis et al., 2005; Gentile et al., 2002; Madhoushi and Ansell, 2004; Sena-Cruz et al., 2012). However, this planting–gluing method is complicated, time-consuming, high-priced and material-wasting.

A new forming-once processing technology was thus proposed to overcome the aforementioned shortcomings of the traditional planting–gluing method. The forming-once processing technology for BFRP bar–laminated bamboo and BFRP bar–reconstituted bamboo is summarized as follows.

BFRP bar–laminated bamboo

The BFRP bar–laminated bamboo composite specimen was composed of bamboo strips and BFRP bar. The main processes were stripping, drying, dipping, planting, parallel assembling, hot pressing and cutting.

BFRP bar–reconstituted bamboo

The BFRP bar–reconstituted bamboo composite specimen consisted of bamboo fibres and BFRP bar. The main processes were crushing, planting, parallel assembly, cold pressing, thermal curing and cutting.

During the fabrication process, the BFRP bar was planted parallel to the bamboo strip or the bamboo fibre, and its position was controlled at the centre of the bamboo engineering materials. To avoid local failure of the composite specimen because of surface roughness, high strength non-shrinkage cement-based grouting material (CGM) was employed with a thickness of l₂ at the loaded end, as shown in Figure 2. A polyvinyl chloride (PVC) tube was applied around the BFRP bar outside the bamboo engineering material to avoid bonding between the BFRP bar and grouting material. The reliability of the proposed forming-once processing technology is checked in the following sections.

Figure 2.

BFRP bar–bamboo composite specimen.

Testing setup and loading pattern

As shown in Figure 3, tests of the BFRP bar–laminated and BFRP bar–reconstituted bamboo composite specimens were carried out on the electronic universal testing machine with a load capacity of 600 kN. Test data were collected by TDS-530. It is difficult to monitor the displacement variation in the loaded end of the BFRP bar, and therefore, the displacement of the BFRP bar position depicted in Figure 3 was measured. Based on the measured certain displacement, δ_c, the slip at the loaded end, δ_s, can be calculated by subtracting the elastic elongation, δ_e, of the BFRP bar from δ_c, as expressed in equation (1)

δ_{s} = δ_{c} - δ_{e} = δ_{c} - \frac{F_{b}}{A_{b} E_{b}} l_{1}

(1)

where F_b is the load applied to the BFRP bar; A_b is the cross-sectional area of the BFRP bar; E_b is the elastic modulus of the BFRP bar; and l₁ is the distance between the loading point and the displacement transducer at a certain position A, as depicted in Figure 3. A uniaxial monotonic loading pattern was adopted with a loading rate of 1 mm/min, and 2 kN of pre-loading was applied to the tested specimen to check the workability of the testing setup and proper specimen installation. Details of the tested specimens and selected bonding lengths are listed in Table 2. Due to limitations of laminated bamboo fabrication, the thickness of the BFRP bar–laminated bamboo composite specimen was restricted to 40 mm, similar to the laminated bamboo thickness in a previous study (Gentile et al., 2002).

Figure 3.

Testing setup.

Table 2.

Details of specimens and test results.

Type	Label	l _0,d	l ₀	Size (b × h)	l ₁	l ₂	F_u	δ_max	τ_max	Failure mode
BFRP bar–laminated bamboo	BCL-1	90	92	40 × 60	670	50	18.14	2.24	5.43	A
	BCL-2		92	40 × 60	640	50	16.45	2.09	4.75	A
	BCL-3		89	40 × 60	630	50	15.86	2.33	4.73	A
	BCL-4		90	40 × 60	640	50	16.90	2.16	4.98	A
	BCL-5		91	40 × 60	730	50	19.96	2.37	5.82	A
	BCL-6	300	300	40 × 60	750	50	49.53	3.69	4.38	B
	BCL-7		301	40 × 60	760	50	27.76	3.38	–	C
	BCL-8		302	40 × 60	740	50	45.96	–	–	D
BFRP bar–reconstituted bamboo	BCR-1	124	125	150 × 147	664	–	22.56	1.68	4.79	A
	BCR-2		124	150 × 147	664	–	25.74	2.25	5.51	A
	BCR-3		123	150 × 147	750	80	23.99	2.44	5.18	A
	BCR-4		124	150 × 150	660	60	26.72	3.12	5.72	A

BFRP: basalt fibre-reinforced polymer.

l _0,d is the designed bond length; δ_max is the slip at the loaded end corresponding to the ultimate bond strength based on equation (1); the unit for l_0,d, l₀, b, h, l₁ and l₂ is mm; the unit for F_u is kN; the unit for δ_max and τ_max is MPa.

Test results and discussion

Failure patterns

Typical test phenomena are summarized as follows: for composite specimens with 90 and 124 mm bond length, the slip values at the loaded and free ends increased slowly at the initial loading and a crackle was heard at about 95% of the ultimate load. The BFRP bar was finally pulled out until the ultimate load. For composite specimens with 300 mm bond length, relative slip was observed with increasing load, and then the specimen failed accompanied by crushing the CGM. In particular, the CGM crushed in advance but the specimen did not fail.

Based on experimental observations, four failure modes were categorized in Table 2, including (A) BFRP bar pull out shown in Figure 4(a), (B) bamboo engineering material split depicted in Figure 4(b), (C) bamboo engineering material local compression failure depicted in Figure 4(c), and (D) the high strength non-shrinkage CGM being crushed. In failure mode A, the BFRP bar was pulled out of the composite specimen and bamboo sawdust was found on its surface. This failure mode was observed in the composite specimens with 90 and 124 mm bond length. In failure mode B, the bamboo engineering material was split. The transfixion crack passed across the composite specimen and the BFRP bar was pulled out. However, no transfixion crack was observed in failure patterns C and D. The local compression failure observed in the BFRP bar–laminated bamboo composite specimen with 300 mm bond length was mainly caused by the limited thickness of the laminated bamboo engineering material. Therefore, to fully utilize the bamboo engineering material, the recommended bond length for the BFRP bar–laminated bamboo composite specimen is less than 300 mm because of good bonding between the BFRP bar and laminated bamboo and avoiding local compression failure.

Figure 4.

Failure patterns: (a) A—pull out of the BFRP bar, (b) B—split of the bamboo engineering material and (c) C—local compression failure of the bamboo engineering material.

Analysis of load–relative slip curves

The test results of all specimens are listed in Table 2. The average maximum bond stress within the bond length at the ultimate bond strength, τ_max, is calculated by the following equation

τ_{\max} = \frac{F_{u}}{π d l_{0}}

(2)

where F_u is the ultimate bond strength; d is the nominal diameter of the BFRP bar, as shown in Figure 1; and l₀ is the measured bond length.

As listed in Table 1, the average maximum bond stress of the BFRP bar–laminated bamboo composite specimens designed with 90 mm bond length was calculated to be 5.14 MPa, and with 300 mm bond length, it was 4.38 MPa, which demonstrates a decrease in the average maximum bond stress with increasing bond length. In addition, the average maximum bond stress of the BFRP bar–reconstituted bamboo composite specimens designed with 124 mm bond length was calculated to be 5.30 MPa, which is larger than the average maximum bond stress of the BFRP bar–laminated bamboo composite specimens. It is shown that in terms of the average maximum bond stress, the bonding between the BFRP bar and reconstituted bamboo is better than between the BFRP bar and laminated bamboo. The ultimate bond strength increased almost linearly with increasing bond length, and a similar variation was found in bonded-in CFRP bars in glulam timber (De Lorenzis et al., 2005).

Comparisons of the average maximum bond stress between different processing technologies are listed in Table 3. It is found that the average maximum bond stress obtained from forming once was more stable and larger than that from the planting and gluing, which proved the effectiveness and reliability of the proposed forming-once processing method. In addition, both the laminated and reconstituted bamboo materials are suitable for the forming-once processing technology.

Table 3.

Comparisons of average maximum bond stresses from different processing methods.

Processing method	Material A	Material B	d (mm)	l _0,d (mm)	τ_max (MPa)
Forming-once	BFRP	BCL	12	90	5.14 (4.73–5.82)
	BFRP	BCL	12	300	4.38
	BFRP	BCR	12	124	5.3 (4.79–5.72)
Planting–gluing	CFRP	Glulam	12	150	4.40
Planting–gluing	GFRP	Glulam	8	63	5.27 (4.47–5.63)

BFRP: basalt fibre-reinforced polymer; CFRP: carbon fibre-reinforced polymer.

BSL is the laminated bamboo; BCR represents reconstituted bamboo. The numbers in brackets represent the range of the average maximum bond stresses under the same designed bond length. Data related to the planting–gluing processing method were from other tests (De Lorenzis et al., 2005; Gentile et al., 2002; Madhoushi and Ansell, 2004; Sena-Cruz et al., 2012). The italicized values are obtained from other tests.

The typical load–relative slip curves obtained from the BFRP bar–laminated bamboo composite specimen with 90 mm designed bond length and the one with 124 mm designed bond length are shown in Figure 5. It is obvious that the load–relative slip curve for both types of composite specimens can be divided into five stages: (1) microslip, (2) ascending, (3) slip adjustment, (4) strengthening and (5) descending stages. No or only extremely small relative slip between the BFRP bar and the bamboo engineering materials occurred at the first microslip stage. Then, the load almost linearly increased with increasing relative slip at the ascending stage. Similar to the yield plateau of steel, no significant increment in the load was found with increasing relative slip at the slip adjustment stage. After the load–slip plateau, the load gradually grew with increasing relative slip, similar to the steel strengthening stage after the yield plateau. Finally, the load decreased with further relative slip development after the ultimate load, and the residual anti-pulling strength was provided by friction.

Figure 5.

Typical load–relative slip curves obtained from: (a) BFRP bar–laminated bamboo composite specimen BCL-1 (l_0,d = 90 mm) and (b) BFRP bar–reconstituted bamboo composite specimen BCR-3 (l_0,d = 124 mm).

In particular, the stress within the bond length was expected to be redistributed in the slip adjustment stage, which was proven by the stable load amplitude versus varied slip values. After redistribution, the bond stress can be viewed as evenly distributed within a certain bond length, and therefore, it is recognized as a constant value within that bond length. A similar idea is also observed in the study by De Lorenzis et al. (2005). Based on the test result, the bond stress in the composite specimens with a bond length of less than 300 mm was redistributed. Therefore, the ultimate bond strength was proportional to the bond length and the application of equation (2) is acceptable.

The traditional bond–slip models including the CMR (Cosenza et al., 1997) and BPE models (Eligehausen et al., 1982) can be effectively used to investigate FRP-concrete bonding behaviour; however, they are not suitable for BFRP bar–laminated and BFRP bar–reconstituted bamboo composite specimens. For ease of application, the bond load–relative slip relationship is simplified to a three-phase model, expressed as equation (3)

{\begin{matrix} F = F_{s} \frac{δ}{δ_{0}} δ \leq δ_{0} \\ F = F_{s} δ_{0} < δ \leq δ_{s} \\ F = F_{u} + (F_{s} - F_{u}) {(\frac{δ - δ_{\max}}{δ_{s} - δ_{\max}})}^{2} δ_{s} < δ \leq δ_{\max} \end{matrix}

(3)

where F is the bond strength and δ is the slip value; F_s is the bond strength at the slip adjustment stage; and δ₀ and δ_s are slip values at the initiation and end of slip adjustment stage, respectively. All these parameters can be calibrated by experimental data, as listed in Tables 2 and 4. For BFRP bar-laminated bamboo composite specimens, good agreement between the theoretical model and test results can be obtained at the microslip, ascending, slip adjustment, and strengthening stages for both the free and loaded ends. However, the descending stage obtained from the theoretical model is relatively underestimated for both the free and loaded ends compared to the test results. For BFRP bar–reconstituted bamboo composite specimens, the theoretical model is in good accordance with the test results at all stages for both the free and loaded ends. In general, the accuracy of the proposed three-phase simplified model is acceptable, which demonstrates that the theoretical model can be conveniently adopted to predict the bond behaviour of BFRP bar–laminated and BFRP bar–reconstituted bamboo composite specimens.

Table 4.

Calibrated parameters for equation (3).

Specimen	End	δ ₀ (mm)	δ_s (mm)	δ_max (mm)	F_s (kN)
BCL-1	Free	0.078	0.316	1.458	12.322
BCL-1	Loaded	0.622	0.869	2.240	12.300
BCR-3	Free	0.160	0.480	1.623	21.238
BCR-3	Loaded	0.866	1.267	2.441	21.156

Bond behaviour between BFRP sheet and bamboo engineering material

The bond behaviour between the BFRP sheet and bamboo engineering materials is fundamental to research for strengthening bamboo engineering material with BFRP sheet. However, research related to BFRP sheet–bamboo interface behaviour is extremely limited. Existing studies on the bond behaviour of the FRP sheet–wood interface inspired the following studies on the interface behaviour between BFRP sheet and bamboo engineering materials (Barbero et al., 1994; Davalos et al., 2000).

Specimen preparation and experimental programme

As shown in Figure 6, a double shear test was adopted where two laminated bamboo blocks were bonded together by two layers of BFRP sheet at each surface. The width of the BFRP sheet was 50 mm, and the bond length was 300 mm in the shear zone. The BFRP sheets and the laminated bamboo block at the left end were bolted together and connected to the testing machine, while the laminated bamboo block at the right end was directly connected to the testing machine and the designed loading was applied.

Figure 6.

BFRP sheet–laminated bamboo composite specimen for double shear test.

The effects of different types of bonding materials on the ultimate bond strength, effective bond length, and distribution of the bond stress were compared. The mechanical properties of bonding materials are provided in Table 5. L-500 epoxy resin adhesive can be used as the bonding material or the reinforced resin for the BFRP sheet, but TLS-401 structural epoxy adhesive can only be used as the bonding material, as shown in Figure 6. Based on the different features of the two bonding materials, two fabrication methods were adopted. When L-500 epoxy resin adhesive is used as both the reinforced resin and bonding material, the fabrication process for the BFRP sheet–laminated bamboo composite specimen is summarized as coating the BFRP sheet with bottom glue, coating epoxy resin on the laminated bamboo surface, dipping the BFRP sheet and attaching the BFRP sheet to the laminated bamboo surface. When TLS-401 epoxy structural adhesive is adopted as bonding material, L-500 epoxy resin adhesive should be employed as the reinforced resin. The fabrication process for the BFRP sheet–laminated bamboo composite specimen has the following features: (1) the BFRP sheet is dipped in L-500 epoxy adhesive in advance and (2) the dipped BFRP sheet is then attached to the surface of the laminated bamboo using TLS-401 epoxy structural adhesive. The surfaces of the two laminated bamboo blocks should be controlled at the same level. In addition, to guarantee good bonding between the BFRP sheet and the bonding material, the BFRP sheet surface was treated to be uneven.

Table 5.

Mechanical properties of the bonding materials.

	L-500	TLS-401
Tensile stress (MPa)	48.8	32.6
Flexural stress (MPa)	77.5	48.0

The layout of the strain gages is shown in Figure 7. The four surfaces of the BFRP sheet–laminated bamboo composite specimen had strain gages attached to monitor the variation in the BFRP sheet and laminated bamboo strain, the distance of which is shown in Figure 7. As shown in Figure 8, the composite specimen was vertically installed and tested with a microcomputer-controlled electrohydraulic servo universal testing machine. The loading pattern was adopted as displacement control with a displacement rate of 1 mm/min. Test data were collected byTDS-530.

Figure 7.

The layout of strain gages.

Figure 8.

Testing setup.

Failure patterns and mechanism

As shown in Figure 9, the failure patterns of the BFRP sheet–laminated bamboo composite specimen were categorized into two types: (1) shear failure between the bonding material and the laminated bamboo and (2) shear failure between the BFRP sheet and the bonding material. In failure pattern 1, specimen failure initiated from the bonding material interface with the laminated bamboo. Bamboo chips were observed in the peeled BFRP sheets. In failure pattern 2, specimen failure started from the BFRP sheet interface with the bonding material, accompanied by fracture of BFRP fibres.

Figure 9.

Failure patterns: (a) shear failure between the bonding material and the laminated bamboo and (b) shear failure between the BFRP sheet and the bonding material.

The mechanism of the BFRP sheet–laminated bamboo composite specimen failure pattern under a double shear test was explained as follows:

Failure pattern 1 is observed in specimens JQ1, JQ2 and JQ3 in Table 6: the failure of the BFRP sheet–laminated bamboo composite specimen with L-500 epoxy adhesive as both the reinforced resin and bonding material was due to the shear-resistance stress at the bonding material–laminated bamboo interface being less than the increasing shear stress. In addition, the failure position of the specimen developed from the loaded to the free end.

Failure pattern 2 is observed in specimens JQ4, JQ5 and JQ6 in Table 6: unlike failure pattern 1, the BFRP sheet–laminated bamboo composite specimen failure with L-500 epoxy resin adhesive as reinforced resin and TLS-401 epoxy structural adhesive as the bonding material initiated from the interface of the BFRP sheet and the bonding material. Then, the BFRP fibres gradually tore and split from the bonding material, and this also developed from the loaded to the free end.

Table 6.

Test results of BFRP sheet–laminated bamboo composite specimens under double shear test.

Specimen label	Reinforced resin	Bonding material	Failure mode	P_u (kN)	P_u/F_bs (%)
JQ1	L-500	L-500	1	36.5	72
JQ2	L-500	L-500	1	38.5	76
JQ3	L-500	L-500	1	35.0	69
JQ4	L-500	TLS-401	2	43.4	86
JQ5	L-500	TLS-401	2	51.8	103
JQ6	L-500	TLS-401	2	48.9	97

BFRP: basalt fibre-reinforced polymer.

F_bs is the calculated ultimate tensile load of four layers of BFRP sheets attached to the laminated bamboo based on the ultimate stress of the BFRP sheet obtained in Table 1.

Analysis of the bond behaviour between BFRP sheet and laminated bamboo

The test results for BFRP sheet–laminated bamboo composite specimens are listed in Table 6. The average ultimate load, P_u, of composite specimens JQ1, JQ2 and JQ3 with L-500 epoxy adhesive as both the reinforcing resin and bonding material was 36.7 kN, and the average ultimate load of the composite specimens JQ4, JQ5 and JQ6 with L-500 epoxy resin adhesive as the reinforced resin and TLS-401 epoxy structural adhesive as the bonding material was 48.0 kN. It is obvious that TLS-401 epoxy structural adhesive withstands larger average ultimate load for the BFRP sheet–laminated bamboo composite specimen than the L-500 epoxy resin adhesive. The ultimate loads of composite specimens JQ1, JQ2 and JQ3 ranged from 69% to 72% compared to the calculated ultimate tensile load of four layers of BFRP sheets, F_bs. The calculation of F_bs is expressed in the following equation

F_{bs} = 4 σ_{tu} b_{f} t_{f}

(4)

where σ_tu is the ultimate tensile stress of the BFRP sheet, which is 1725.4 MPa in Table 1; b_f is the width of the BFRP sheet, which is 50 mm; and t_f is the nominal thickness of the BFRP sheet, which is 0.147 mm. However, the ultimate loads of specimens JQ4, JQ5 and JQ6 ranged from 86% to 103% of F_bs, which demonstrated that the BFRP sheet ability was almost fully developed. Therefore, TLS-401 epoxy structural adhesive is more suitable for bonding the BFRP sheet and laminated bamboo, and the failure at the BFRP sheet and bonding material interface was viewed as better for the BFRP sheet–laminated bamboo composite specimen.

The load–slip curves at the loaded ends of specimens JQ3 and JQ6 under double shear test are depicted in Figure 10. For both composite specimens, a nearly linear stage can be observed before and after the ultimate load, and the curves gradually decrease with fluctuation. The BFRP sheet–laminated bamboo composite specimen still sustains significant force for a relatively large slip after the peak point, which demonstrates good ductility for the composite specimen. As seen from Figure 10, the elastic stiffness of specimen JQ3 is larger than specimen JQ6 and the ductility is worse. In specimen JQ6, an obvious dentate line was observed because of gradual fracture of the BFRP fibres.

Figure 10.

Load–slip curves at the loaded end under double shear test.

Strain distributions of the BFRP sheet

The representative strain distribution of a BFRP sheet monitored by strain gages (Figure 7) is shown in Figure 11, where L_d is the distance from the strain gage to the loaded end. Strain distribution in two stages, including before and after the ultimate load, was compared to deeply analyse the bond behaviour. In general, the strain distribution of BFRP sheets under double shear tests was similar in all specimens. The maximum strain amplitude near the loaded end significantly increased with increasing applied load.

Figure 11.

Strain distributions of the BFRP sheet in (a) JQ1 before the peak point, (b) JQ1 after the peak point, (c) JQ2 before the peak point, (d) JQ2 after the peak point, (e) JQ4 before the peak point and (f) JQ4 after the peak point.

As shown in Figure 11(a), (c) and (e), the BFRP sheet strain variation was mainly concentrated within 100 mm from the loaded end, which demonstrated that the shear stress (see section ‘Effective bond length’) of the BFRP sheet–laminated bamboo composite specimen was mainly distributed within 100 mm away from the loaded end before the ultimate load. However, as shown in Figure 11(b), (d) and (f), the strain became relatively even and spread to the free end and after the ultimate load.

Effective bond length

Similar to the bond between BFRP and concrete, the ultimate bond strength does not increase with increasing bond length after it reaches a certain value called the effective bond length (Nozaka et al., 2005; Ouezdou et al., 2009). Within the effective bond length, the interface shear stress is effectively transferred. Based on the following equation, shear stress, τ, at the interface of the BFRP sheet and laminated bamboo can be obtained via the strain difference

τ = t_{f} E_{f} \frac{d ε_{f}}{dx}

(5)

where E_f is the tensile elastic modulus of the BFRP sheet, which is 73.8 GPa in Table 1, and dε_f is the strain difference in the BFRP sheet at a certain distance dx. Because the strain gages attached to the composite specimen were relatively sparse with 20 mm between adjacent strain gages, the detailed shear stress distribution near the loaded and the free ends could not be achieved based on equation (5). As shown in Figure 12, the shear stress at the interfaces of specimens JQ3 and JQ6 was between 35 and 115 mm from the loaded end. The shear stress quickly decreased to zero with increasing distance from the loaded end. It is obvious that the effective bond length able to transfer the shear stress was approximately 100 mm for both specimens, which agreed well with the significant strain variation within 100 mm of the loaded end obtained from Figure 11. Therefore, the recommended effective bond length for BFRP sheet–laminated bamboo composite specimens using L-500 epoxy resin adhesive or TLS-401 epoxy structural adhesive as the bonding material is 100 mm.

Figure 12.

Distributions of shear stress: (a) specimen JQ3 and (b) specimen JQ6.

Conclusion

In this article, the bonding behaviour of BFRP bar and both laminated and constituted bamboo materials was investigated by the pull out test, and the bonding behaviour of the BFRP sheet and the laminated bamboo material was studied by the double shear test. The main conclusions are summarized as follows:

BFRP bar and laminated, reconstituted bamboo

Forming-once processing technology is proposed for the BFRP bar and bamboo engineering material composite specimen for convenient, low-priced time-saving material. The reliability of the forming-once method is proven based on the pull out test on BFRP bar–laminated and BFRP bar–reconstituted bamboo composite specimens.

Based on the test results, the average maximum bond stress of BFRP bar–laminated bamboo composite specimens decreased with increasing bond length. The average maximum bond stress of BFRP bar–reconstituted bamboo composite specimens was larger than that for the BFRP bar–laminated bamboo composite specimens. In addition, the ultimate bond strength increased almost linearly with increasing bond length.

Based on the load–relative slip curves of BFRP bar–bamboo engineering material composite specimens, the load–slip relationship is divided into five stages: (1) microslip, (2) ascending, (3) slip adjustment, (4) strengthening and (5) descending stages. Based on the aforementioned experimental observations, four types of failure modes are categorized including BFRP bar pull out, bamboo engineering material split, local bamboo engineering material compression failure and high strength non-shrinkage CGM grouting material crush.

The simplified bond–slip model is established, and the recommended bond length for the BFRP bar–laminated bamboo composite specimen is 300 mm.

BFRP sheet and laminated bamboo

Based on test results, the BFRP sheet–laminated bamboo composite specimen using TLS-401 epoxy structural adhesive as the bonding material had larger average ultimate load than the one using L-500 epoxy resin adhesive. TLS-401 epoxy structural adhesive is thus more suitable for bonding BFRP sheet and laminated bamboo.

Based on experimental observation, the BFRP sheet–laminated bamboo failure pattern is categorized as shear failure between the bonding material and laminated bamboo or the BFRP sheets. The failure of BFRP sheet–laminated bamboo with different bonding materials is ductile, and the mechanisms of the two failure patterns are discussed. In addition, failure of the BFRP sheets and bonding material interface was viewed as better for the BFRP sheet–laminated bamboo composite specimen.

Based on strain analysis, strain variation is significant within 100 mm from the loaded end. The interface shear stress is thus transferred within the 100 mm effective bond length.

Footnotes

Acknowledgements

The authors would like to acknowledge financial support from the National Natural Research and Development Fund (9Z05000049D0) and Integrated Key Precast Components and New Wood-bamboo Composite Structure (2017YFC0703502).

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship and/or publication of this article.

ORCID iD

Ye Liu

References

Banibayat

Patnaik

(2014) Variability of mechanical properties of basalt fiber reinforced polymer bars manufactured by wet-layup method. Materials & Design (1980–2015) 56: 898–906.

Barbero

Davalos

Munipalle

(1994) Bond strength of FRP-wood interface. Journal of Reinforced Plastics and Composites 13: 835–854.

Coelho

Sena-Cruz

Neves

(2015) A review on the bond behavior of FRP NSM systems in concrete. Construction and Building Materials 93: 1157–1169.

Cosenza

Manfredi

Realfonzo

(1997) Behavior and modeling of bond of FRP rebars to concrete. Journal of Composites for Construction 1: 40–51.

Davalos

Qiao

Trimble

(2000) Fiber-reinforced composite and wood bonded interfaces: part 1. Durability and shear strength. Journal of Composites, Technology & Research 22: 224–231.

De Lorenzis

Scialpi

La Tegola

(2005) Analytical and experimental study on bonded-in CFRP bars in glulam timber. Composites Part B: Engineering 36: 279–289.

Eligehausen

Popov

Bertero

(1982) Local bond stress-slip relationships of deformed bars under generalized excitations. In: Earthquake Engineering Research Center Report. 82/23. Berkeley: University of California.

GB/T1938-2009. (2009) Method of Testing in Compressive Strength Parallel to Grain of Wood. China, Beijing: State Administration of Quality Supervision, Inspection and Quarantine.

Gentile

Svecova

Rizkalla

(2002) Timber beams strengthened with GFRP bars: development and applications. Journal of Composites for Construction 6: 11–20.

10.

Kaur

Satya

Pant

, et al. (2016) Eco-friendly preservation of bamboo species: traditional to modern techniques. Bioresources 11: 10604–10624.

11.

Yang

Zhou

, et al. (2019) Bending performance of glubam beams made with different processes. Advances in Structural Engineering 22: 535–546.

12.

Madhoushi

Ansell

(2004) Experimental study of static and fatigue strengths of pultruded GFRP rods bonded into LVL and glulam. International Journal of Adhesion & Adhesives 24: 319–325.

13.

Mahdavi

Clouston

Arwade

(2012) A low-technology approach toward fabrication of laminated bamboo lumber. Construction and Building Materials 29: 257–262.

14.

Nozaka

Shield

Hajjar

(2005) Effective bond length of carbon-fiber-reinforced polymer strips bonded to fatigued steel bridge I-girders. Journal of Bridge Engineering 10: 195–205.

15.

Nugroho

Ando

(2001) Development of structural composite products made from bamboo II: fundamental properties of laminated bamboo lumber. Journal of Wood Science 47: 237.

16.

Ouezdou

Belarbi

Bae

S-W

(2009) Effective bond length of FRP sheets externally bonded to concrete. International Journal of Concrete Structures and Materials 3: 127–131.

17.

Qin

(2009) Research on surface color, properties of thermo-treated reconstituted bamboo lumber after artificial weathering test. Advanced Materials Research 79–82: 1395–1398.

18.

R-02 A (2003) Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. Farmington Hills, MI: American Concrete Institute.

19.

Renjie

Yunshui

(2002) Bamboo Based Panel Technology. Beijing, China: China Forestry Publishing House.

20.

Sena-Cruz

Branco

Jorge

, et al. (2012) Bond behavior between glulam and GFRP’s by pullout tests. Composites Part B: Engineering 43: 1045–1055.

21.

Shan

Gao

, et al. (2012) Research and applicant of solar energy-prefabricated bamboo pole house. In: The 12th international symposium on structural engineering, Beijing, China, 17–19 November.

22.

Sharma

Gatóo

Bock

, et al. (2015) Engineered bamboo for structural applications. Construction and Building Materials 81: 66–73.

23.

Spaun

(1981) Reinforcement of wood with fiberglass. Forest Products Journal 31: 26–33.

24.

Wan

Smith

Qiao

, et al. (2013) Experimental investigation on FRP-to-timber bonded interfaces. Journal of Composites for Construction 18: A4013006.

25.

Wei

Duan

, et al. (2017) Flexural performance of bamboo scrimber beams strengthened with fiber-reinforced polymer. Construction and Building Materials 142: 66–82.

26.

Yang

Shenxue

Qingfang

, et al. (2010) Experimental study on flexural performance of bamboo beams. Building Structure 40: 88–91.

27.

Yang-lun

YW-jY

Ding-hua

ZYR

(2006) Studies on factors influencing properties of reconstituted engineering timber made from small-sized bamboo. China Forest Products Industry 6: 7.

28.

Yuan

Teng

Seracino

, et al. (2004) Full-range behavior of FRP-to-concrete bonded joints. Engineering Structures 26: 553–565.

29.

Zhao

Zhou

Long

, et al. (2018) Compression performance of thin-walled square steel tube/bamboo plywood composite hollow columns with binding bars. Advances in Structural Engineering 21: 347–364.