Development of kenaf fibre reinforced polymer laminate for shear strengthening of reinforced concrete beam

Abstract

Fabrication of green fibre composite laminate for strengthening of reinforced concrete structure is one of the current interests in the field of construction industry. The aim of this research was to develop kenaf fibre reinforced polymer (KFRP) laminate for shear strengthening of reinforced concrete beam. Comprehensive design and theoretical models were also proposed for KFRP laminate shear strengthened beam. In the experimental programme, KFRP laminate had been fabricated with various fibre content to obtain optimal mix ratio. Physical and mechanical properties of KFRP laminates were experimentally investigated. Three reinforced concrete beam specimens were prepared for structural investigations. Results showed that KFRP laminate with maximum fibre content had the highest tensile strength and the laminate was found to be elastic isotropic in nature. The KFRP laminate strengthened beam had 100 % higher shear crack load and 33 % ultimate failure load as compared to un-strengthened control beam. It reduced the numbers and width of cracks and had shown strain compatibility behavior with shear reinforcement. The failure load, ductility, crack patterns and strain characteristics of KFRP laminate strengthened beam were found to be closely comparable with CFRP laminate strengthened beam. The experimental results satisfactorily verified the proposed design and theoretical models.

Appendix

A 150 mm × 300 mm × 2,300 mm full scale reinforced concrete beam was fabricated with 2–16 mm flexural reinforcement and 6 mm shear reinforcement of 110 mm spacing. The beam was supported using roller with the span of two meters. It was tested for two point load with the shear span of 650 mm. The properties of materials are shown in Table 7. The beams need to be strengthened for shear using KFRP laminate for it’s maximum capacities.

1.1 Effective strain of CFRP laminate based on ACI 440.2R-02

Active bond length,

$$L_{\text{e}} = \frac{23,300}{{\left( {n t_{\text{frp }} E_{\text{frp}} } \right)^{0.58} }} = \frac{23,300}{{\left( {1 \times 1.2 \times 165 \times 10^{3} } \right)^{0.58} }} = 19.74 {\text{mm}},$$

$$k_{1} = \left( {\frac{{ f_{\text{c}}^{'} }}{27} } \right)^{2/3} = \left( {\frac{ 22.4}{27} } \right)^{2/3} = 0.88 ,$$

$${\text{k}}_{2} = \left( {\frac{{d_{\text{f}} - 2L_{\text{e}} }}{{d_{\text{f}} }}} \right) = \left( {\frac{261 - 2 \times 19.74 }{261}} \right) = 0.85,\quad {\text{for two sides laminate,}}$$

$$k_{v} = \left( {\frac{{ k_{1} k_{2} L_{\text{e}} }}{{11,900\varepsilon_{\text{fu}} }}} \right) = \left( {\frac{0.88 \times 0.85 \times 19.74}{11,900 \times 0.01}} \right) = 0.104,$$

The effective strain ε _fe in FRP is assumed to be smaller than the ultimate strain ε _fu = 0.012

This can be computed putting value of k _v and ε _fu.

Effective strain of CFRP laminate,

$$\varepsilon_{fe} = k_{v} \varepsilon_{fu} = 0.104 \times 0.012 = 0.0013 < 0.004$$

1.2 Debonding strain of CFRP laminate based on proposed model of Alam et al. (2014)

The debonding strain of CFRP laminate for beams without connectors could be predicted based on the proposed model of Alam et al. (2014),

$$\varepsilon_{\text{frp, debonding}} = \frac{{F_{\text{bu}} \left( {d - d^{'} } \right)}}{{t_{\text{frp}} E_{\text{frp}} }} = \frac{{1.58\left( {261 - 39} \right)}}{{\left( {1.2} \right)\left( {165,000} \right)}} = 0.00177$$

1.3 Flexural capacities of beam

As per Eq. 2, the flexural capacities of beams,

$$\begin{gathered} M = Tz = A_{s} f_{tk} = \left( {402} \right)\left( {654} \right)\left[ {261 - \frac{{0.588\left( {402} \right)\left( {654} \right)}}{{\left( {25.6} \right)\left( {150} \right)}}} \right] \hfill \\ \;\;\; = 58.02 \, {\rm kN - m} , \hfill \\ \end{gathered}$$

Total failure load,

$$P = 2V_{\text{d}} = \frac{2M}{{L_{\text{s}} }} = \frac{2(58.02)}{0.65} = 178.5 {\text\,{kN}} .$$

Design shear force

$$V_{d} = \frac{178.5}{2} = 89.25 {\text\,{kN}} .$$

1.4 Design strain of KFRP laminate

Yield strain of shear reinforcement,

$$\varepsilon_{\text{y, link}} = \frac{{f_{\text{y, link}} }}{{E_{\text{s}} }} = \frac{520}{200,000} = 0.0026.$$

Debonding strain of KFRP laminate,

$$\varepsilon_{\text{kfrp, debonding}} = \frac{{F_{\text{bu}} \left( {d - d^{'} } \right)}}{{t_{\text{kfrp}} E_{\text{kfrp}} }} = \frac{{1.58\left( {261 - 39} \right)}}{{\left( 6 \right)\left( {11400} \right)}} = 0.00512 > \varepsilon_{\text{y, link}} .$$

Thus, the design strain of KFRP laminate is,

$$\varepsilon_{\text{kfrp(design) = }} \varepsilon_{\text{y, link}} = 0.0026.$$

1.5 Required cross sectional area of KFRP laminate for shear strengthening of RC beam

As per proposed guideline, the cross sectional area of CFRP laminate can be calculated based on Eq. 12. For conservative design, the shear crack inclination of beam could be considered as 45°.

$$\begin{gathered} V_{{{\text{y, link}}\left( { 4 5} \right)}} = A_{\text{s, link}} f_{\text{y, link}} \left[ {\frac{{\left( {d - d^{'} } \right)\cot 45}}{s}} \right] = \left( {47.5} \right)\left( {520} \right)\left[ {\frac{{\left( {261 - 39} \right)\cot 45}}{110}} \right] \hfill \\ = 49840 {\rm{N}} = 49.84 {\text\,{kN}} \hfill \\ A_{\text{kfrp}} = \frac{{E_{s} s_{\text{kfrp}} \left( {V_{d} - V_{\text{link}} } \right)}}{{2E_{\text{kfrp}} f_{\text{y, link}} \left( {d - d^{'} } \right)\cot \theta }} = \frac{{\left( {200000} \right)\left( {110} \right)\left( {89250 - 49840} \right)}}{{2\left( {11000} \right)\left( {520} \right)\left( {261 - 39} \right)\cot 45}} = 341.39 \, {\text{mm}}^{2} \hfill \\ = 6 {\text{mm}} \times 54 {\text{mm }} \approx 6 {\text{mm}} \times 50 {\text{mm}} .\hfill \\ \end{gathered}$$

Thus, the provided KFRP laminate was 6 mm × 50 mm × 300 mm with 110 mm spacing for shear strengthening of beam.

1.6 Theoretical shear capacity of KFRP laminate strengthened beam

$$V_{\text{t, link}} = A_{\text{s, link}} f_{\text{t, link}} \left[ {\frac{{\left( {d - d^{'} } \right)\cot 45}}{\text{s}}} \right] = \left( {47.5} \right)\left( {556} \right)\left[ {\frac{{\left( {261 - 39} \right)\cot 45}}{110}} \right] = 53.3 {\text\,{kN}}$$

$$\begin{gathered} V_{\text{kfrp}} = 2A_{\text{kfrp}} E_{\text{kfrp}} \varepsilon_{\text{kfrp, design}} \left[ {\frac{{\left( {d - d^{'} } \right)\cot 45}}{{s_{\text{kfrp}} }}} \right] \hfill \\ \quad \quad = 2\left( {300} \right)\left( {11400} \right)\left( {0.0026} \right)\left[ {\frac{{\left( {261 - 39} \right)\cot 45}}{110}} \right] = 35.89 {\text\,{kN}} .\hfill \\ \end{gathered}$$

Total shear force of KFRP laminate shear strengthened beam is,

$$V_{\text{SB}} = V_{\text{y, link}} + V_{\text{kfrp}} = 53.3 + 35.89 = 89.2 {\text\,{kN}} .$$

Shear failure load is 178.4 kN

1.7 Theoretical shear capacity of CFRP laminate strengthened beam

$$\begin{gathered} V_{\text{debonding, link}} = A_{\text{s, link}} f_{\text{s}} \left[ {\frac{{\left( {d - d^{'} } \right)\cot 45}}{s}} \right] = \left( {47.5} \right)\left( {200000} \right)(0.00177)\left[ {\frac{{\left( {261 - 39} \right)\cot 45}}{110}} \right] = 33.93 {\text\,{kN}} \hfill \\ V_{\text{cfrp}} = 2A_{\text{cfrp}} E_{\text{cfrp}} \varepsilon_{\text{cfrp, design}} \left[ {\frac{{\left( {d - d^{'} } \right)\cot 45}}{{s_{\text{cfrp}} }}} \right] \hfill \\ = 2\left( {1.2} \right)(50)\left( {165000} \right)\left( {0.00177} \right)\left[ {\frac{{\left( {261 - 39} \right)\cot 45}}{110}} \right] = 70.72 {\text\,{kN}} .\hfill \\ \end{gathered}$$

Total shear force of CFRP laminate shear strengthened beam is,

$$V_{SB,CFRP} = V_{y,link} + V_{kfrp} = 33.93 + 70.72 = 104.6 \, {\rm{kN}}$$

Shear failure load due to debonding of CFRP laminate is 209.2 kN

See appendix Table 7

Table 7 Design parameters