Inspired by the studies about wooden beam applied with prestressed steel plate and bamboo beam strengthened by fiber-reinforced polymer (FRP), this paper aims to explore the applicability of the prestressed basalt fiber-reinforced polymer (BFRP) sheet to the laminated bamboo beam and the variation of the flexural performance of the laminated bamboo beam applied with prestressed BFRP sheet. Two series of tests were conducted in the present study. In the first series of tests, the prestress loss of the prestressed BFRP sheet was classified and analyzed based on measured strains and deflections, which led to a derivation of the effective prestressed force considering the prestress loss. Analyses showed that the recommended value of prestress loss compared with the initial prestressed force was 22.0% based on the existing test data in the specimen preparation stage. In the second series of tests, the static loading test was performed to investigate the flexural performance of the laminated bamboo beam applied with prestressed BFRP sheet and analyze the difference between the laminated bamboo beams applied with prestressed and non-prestressed BFRP sheets. Test results showed that the no significant variation of the ultimate load and a reduction of the ultimate deformation capacity were caused by the application of the prestressed BFRP sheet.
As a traditional structural material [
The stiffness-control design is often adopted for the modern bamboo structure [
Similar to the bamboo, the wood also encounters the difficulties in broadening its application due to the stiffness-control design. As early studied by Peterson [
Inspired by the above studies, this paper explores the application of the prestressed BFRP sheet in the laminated bamboo beam. In order to obtain the reliable performance of the laminated bamboo beam applied with the prestressed BFRP sheet, the prestress loss is firstly investigated. Until now, the standard prestress tensioning method had not been formed, and various prestress loss would be resulted from different tensioning facilities and methods. Based on the prestress tensioning method proposed in this paper, the prestress loss was classified and calculated. Referable studies included the prestress loss in the posttensioned CFRP sheet-strengthened concrete beam [
The mechanical properties of the bamboo engineering material were found to be related to the dimension of the tested specimen. In order to take the size effect into consideration, one laminated bamboo beam specimen with a dimension (
Material properties of the laminated bamboo considering size effect.
Type |
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Laminated | 3000 | 2700 | 900 | 50 | 150 | 150 | 11329.5 | 61.2 |
Material test setup.
The flexural modulus,
The BFRP sheet with density of 300 g/cm2 was provided by the Basalt Fiber Center of Southeast University. Six BFRP sheets with a nominal thickness of 0.147 mm were tested under tension per ACI 440 [
Failure photos of BFRP sheets under standard tension testing.
The above test on mechanical properties of the BFRP sheet was based on the standard testing specimen. However, the possible variation of mechanical properties of the BFRP sheet with long span and large width is not clear until now, which is of important necessity to be evaluated. One test was thus conducted to assess the tensile performance of two layers of BFRP sheets with a length of 3000 mm and width of 50 mm after impregnated with epoxy resin adhesive L-500. The control tensile force was 12.0 kN. The strain distributions of the BFRP sheet along the length direction were detected based on three groups of strain gages, and each group is composed of three strain gages, as shown in Figure
Layout of strain gages for BFRP sheets with long span and large width.
The load-strain curve of the two layers of BFRP sheets is shown in Figure
Load-average strain curve obtained from strain gages in group B.
When the initial prestress was relatively too high for the laminated bamboo beam applied with prestressed BFRP sheet, the following problems may be expected: (1) relatively large initial deflection, (2) relatively large slip between the BFRP sheet and laminated bamboo beam, and (3) the fracture or significant creep of the BFRP sheet under high prestress. When the initial deflection was large, the laminated bamboo beam applied with prestressed BFRP sheet may be in the antiarch state in the normal use state. Therefore, it is of important necessity to determine the appropriate range of the initial prestressed force of the BFRP sheet to avoid the above problems.
The recommended initial deflection caused by the initial prestressed force of the BFRP sheet was defined as
When the initial prestress level is 35% of the standard tensile stress of the BFRP sheet,
The bond slip between the BFRP sheet and the laminated bamboo should be checked to confirm the reliability of the employment of the prestressed BFRP sheet to the laminated bamboo beam when the bonding material is adopted as the epoxy resin adhesive L-500. Based on test results of a series of double shear tests on the bond behavior between the two-layer BFRP sheets and laminated bamboo (width of 50 mm and bond length of 300 mm) listed in Table
Test results of double shear test.
Specimen label | Bonding material | Ultimate load |
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JQ1 | L-500 | 36.5 kN |
JQ2 | L-500 | 38.5 kN |
JQ3 | L-500 | 35.0 kN |
The main process of the preparation for the laminated bamboo beam applied with prestressed BFRP sheet is summarized as follows: Impregnating and interfacial treatment: Two layers of BFRP sheets were impregnated with epoxy resin adhesive L-500. For ease of applying the initial prestress force to the BFRP sheet, a special fixture which was composed of two steel plates with two bolt holes shown in Figure Applying prestressed force: Firstly, both of the BFRP sheet and surface of the laminated bamboo beam were coated with the structural adhesive and then pressed together. Then, graded prestressed forces, including four force levels of 5 kN, 8 kN, 19 kN, and 11.49 kN (or 12.0 kN), were applied to the BFRP sheet. The load holding duration for each level of prestressed force was about 2 minutes, and the BFRP sheet was pressed to make the distribution of the structural adhesive even during the holding duration. Releasing BFRP sheet: Both ends of the laminated bamboo beam were constructed with the structural measurement shown in Figure
Specimen preparation: (a) special fixture for BFRP sheet and (b) structural measurement.
The photo of the laminated bamboo beam applied with prestressed BFRP sheet.
In the present study, the whole tensioning system at the tensioned and fixed ends is depicted in Figure
Assembly of tensioning system: (a) tensioned end, (b) tension fixture, and (c) fixed end.
Taking the release of the prestressed BFRP sheet as a dividing line, the specimen can be divided into preparation stage and storage stage. The former is defined as the preparation process of the specimen until the release of the BFRP sheet and the latter is recognized as the specimen to be stored or used after the release. Based on different stages, types of the prestress loss observed in the laminated bamboo beam applied with prestressed BFRP sheet are summarized as follows.
In specimen preparation stage, (1)
In specimen storage stage, (1)
In order to quantitatively measure the prestress loss in the laminated bamboo beam applied with prestressed BFRP sheet, three specimens including P-1, P-2, and P-3 were tested in this section, details of which are listed in Table
Specimens for measuring the prestress loss.
Label |
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Preparation stage | Storage stage |
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P-1 | 50 × 150 × 2978 | 12.02 | 51.2 | 10820 | 2-9 11 : 38 to 2-13 9 : 09 | — |
P-2 | 50 × 150 × 2978 | 12.54 | 51.9 | 11135 | 2-18 10 : 45 to 2-24 9 : 08 | 2-24 9 : 08 to 3-03 8 : 11 |
P-3 | 50 × 150 × 3000 | 12.54 | 51.1 | 11310 | 3-03 11 : 35 to 3-10 8 : 55 | 3-10 8 : 55 to 3-14 8 : 55 |
Strain gages for specimens P-1, P-2, and P-3 (the number in brackets is adopted for specimen P-3).
The measured values related to the prestress loss in the three specimens are listed in Table
Measured results related to prestress loss.
Label |
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P-1 | 11.78 | 9.90 | 10288 | 9519 | 9053 | – | 6.29 | – | – |
P-2 | 12.02 | 10.49 | 9811 | 8113 | 7449 | 5991 | 4.54 | 3.04 | 4.15 |
P-3 | 12.08 | 10.82 | 10093 | 8916 | 8324 | 8108 | 4.62 | 3.53 | 3.77 |
The calculation of different types of prestress losses was conducted based on the preparation stage and storage stage, respectively. Firstly, the prestress losses occurred in the preparation stage were analyzed. In the present test, the interface between the tensioning fixture and BFRP sheet was rough and the tensioning equipment was rigid enough, which made the prestress loss
After reaching the target initial prestressed force and before the release of the BFRP sheet, the prestress loss
Load variation at the fixed end.
Calculations for prestress loss
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P-1 | 1.88 | 15.6 | 24.6 | 52.6 | 91.3 |
P-2 | 1.53 | 12.2 | 30.1 | 73.2 | 77.3 |
P-3 | 1.26 | 10.0 | 30.9 | 68.3 | 85.0 |
When the prestressed BFRP sheet was released, both flexural and compressive deformations would result in the prestress loss
The prestress loss
Strain distribution of specimen P-1 for
The further prestress loss can be caused by the creep of the structural adhesive and laminated bamboo beam, as well as the further relaxation of the BFRP sheet after release. Among the three factors, the creep is main reason for the prestress loss. As shown in Figure
(a) Strain variation of specimens P-2 and P-3; (b) deflection variation of specimens P-2 and P-3.
In Figure
The effective prestress,
Details of all tested laminated bamboo beams applied with prestressed BFRP sheet are listed in Table
Details of tested laminated bamboo beams applied with prestressed BFRP sheet.
Specimen label |
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B-1 | 50 × 150 × 3000 | 2700 | 0 | 51.3 | 0 | 0 | 0 |
B-2 | 50 × 150 × 2978 | 2700 | 12.54 | 51.9 | 821.8 | 641.0 | 3.04 |
B-3 | 50 × 150 × 3000 | 2700 | 12.54 | 51.1 | 836.7 | 652.6 | 3.53 |
The four-point bending test was adopted in the present experimental protocol shown in Figure
Test setup and layout of strain gages and displacement transducers.
The failure pictures of the laminated bamboo beam applied with non-prestressed or prestressed BFRP sheet are shown in Figure
Failure photos of (a) overall and (b) local views for specimen B-1, (c) overall and (d) local views for specimen B-2, and (e) overall and (f) local views for specimen B-3.
The load-deflection curves of specimens B-1, B-2, and B-3 are shown in Figure
Load-deflection curves at the midspan.
The ultimate loads of the three specimens were 29.5 kN, 29.0 kN, and 29.5 kN, respectively, which meant that the application of the prestressed force in the BFRP sheet did not affect the ultimate load. Until the failure point, the ultimate midspan deflections of specimens B-1, B-2, and B-3 were 131.41 mm, 96.41, and 96.41 mm, respectively. It is demonstrated that the ultimate deformation capacity of the laminated bamboo beam applied with prestressed BFRP sheet was reduced compared with that applied with non-prestressed BFRP sheet.
As shown in Figure
Midspan deflection under levels of loads.
The load-strain curves of the three specimens are shown in Figure
Load-strain curves of the three specimens: (a) B-1, (b) B-2, and (c) B-3.
Until the failure point, the maximum compressive and tensile strains of the specimen B-1 applied with non-prestressed BFRP sheet were −12345
The strain distribution along the beam height (captured from strain gages Y-3, Y-4, Y-5, and Y-6) under different loads shown in Figure
Strain distribution along beam height under different loads: (a) B-1, (b) B-2, and (c) B-3.
Based on the detected strain values in Figure
Positions of neutral axis under different loads.
Load (kN) | B-1 | B-2 | B-3 | |||
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5 | 72.97 | 0.49 | 73.90 | 0.49 | 75.26 | 0.50 |
10 | 72.15 | 0.48 | 73.82 | 0.49 | 75.37 | 0.50 |
15 | 69.99 | 0.47 | 73.67 | 0.49 | 75.02 | 0.50 |
20 | 65.82 | 0.44 | 72.05 | 0.48 | 73.62 | 0.49 |
25 | 61.89 | 0.41 | 68.82 | 0.46 | 69.86 | 0.47 |
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57.93 | 0.39 | 65.49 | 0.44 | 63.86 | 0.43 |
The laminated bamboo beam applied with prestressed BFRP sheet was investigated in this paper. Both prestress loss and flexural performance of the laminated bamboo beam applied with prestressed BFRP sheet were analyzed. Main conclusions are summarized as follows: A preliminary design of the laminated bamboo beam applied with prestressed BFRP sheet was discussed in this paper, including the reasonable controlled initial prestressed force and prevention of the bond slip. To overcome the deflection resulted from the dead load, the controlled initial prestress is recommended as 55% of the standard tensile stress of the BFRP sheet, including a 5% super tension. The bond slip between the BFRP sheet and the laminated bamboo is small and limited with the 100 mm range away from the loaded end. Considering the release of the prestressed BFRP sheet, the prestress loss was analyzed according to the specimen preparation stage and specimen storage stage. Based on analyses of prestress loss, the total prestress loss in the preparation stage is recommended as 22.0%, but more calibrated data are required to obtain an acceptable value for the prestress loss in the storage stage due to the diversity of the experimental data. The test results of four-point bending showed that the failure of the laminated bamboo beam applied with non-prestressed or prestressed BFRP sheet initiated from the loading point and propagated to a certain distance of the laminated bamboo beam, which can be explained as the tensile strain of the specimen reached the maximum tensile strain. The ultimate load of the laminated bamboo beam was not affected by the application of the prestressed BFRP sheet, but the ultimate deformation capacity of the laminated bamboo beam applied with prestressed BFRP sheet was reduced compared with that applied with non-prestressed BFRP sheet. The stiffness of the laminated bamboo beam applied with prestressed or non-prestressed BFRP sheet was almost the same in the linear stage, and then after the linear stage, the stiffness of the laminated bamboo beam applied with prestressed BFRP sheet became relatively larger. The maximum compressive strain was larger than the maximum tensile strain in all specimens. Based on the strain distribution along the beam height, the position of the neutral axis was found to be near the middle height of the beam at the initial loading stage and gradually went down with the increase of the applied load. The neutral axis of the laminated bamboo beam applied with prestressed BFRP sheet finally became higher than that of the laminated bamboo beam applied with non-prestressed BFRP sheet.
All data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors would like to acknowledge financial support from the National Natural Research and Development Fund (9Z05000049D0) and Research and Demonstration Application of Green Bamboo and Wood Structure System (2017YFC0703502).