Experimental investigations on coconut-fibre rope tensile strength and pullout from coconut fibre reinforced concrete

https://doi.org/10.1016/j.conbuildmat.2012.12.052Get rights and content

Abstract

The utilisation of coconut-fibre ropes as vertical reinforcement in mortar-free interlocking structures is under consideration for use in cost-effective earthquake-resistant housing. The walls are intended to be constructed with novel interlocking blocks, and coconut fibre reinforced concrete (CFRC) is used as a construction material (presented in a separate study). The rope anchorage is achieved by embedding it in the foundation and top tie-beams. The bond between the rope and the CFRC plays an important role, and the rope tensile strength is also significant in the overall stability of the proposed structure. The rope tension generated due to earthquake loading should be less than both the pullout force and the rope tensile load to avoid the structure collapse. As a pilot study, the scope of the current work is limited to the axial pullout behaviour and tensile capacity of the rope. Therefore, the bond strength between the rope and CFRC, and the energy required to pull out ropes from CFRC are investigated experimentally using rope pullout tests. The factors considered include rope embedment length, rope diameter, pre-treatment condition, concrete mix design ratio, fibre content and knot in the material matrix. The tensile strength and elongation of coconut-fibre ropes were determined considering the parameters of rope diameter and pre-treatment. To increase the pullout energy, bond strength and tensile strength of the rope, the boiling treatment was found to be beneficial compared to chemical treatment. The pullout energy increases with an increase in embedment length, rope diameter, cement and fibre content in the matrix. With the knowledge obtained, empirical equations are proposed to determine the pullout energy, bond strength and tensile strength of the rope.

Highlights

► The rope tensile strength and rope pullout from concrete is investigated experimentally. ► Load–slippage curves are obtained from the rope pullout tests. ► The stress–strain curves of the coconut-fibre ropes are obtained. ► Empirical equations are proposed for tensile and bond strength. ► The increased tensile and bond strength is achieved with boiling treatment.

Introduction

Mortar-free construction, capable of reducing the imposed earthquake energy during a seismic event, is considered for seismic-resistant housing [1]. Coconut fibre reinforced concrete (CFRC) members with cracks produce more damping than those without cracks [2]. Coconut fibres are used because of their highest toughness compared to that of other natural fibres as reported by Munawar et al. [3] for a fibre bundle and Satyanarayana et al. [4] for a single fibre. These studies reported a tensile strain of 24% and 39%, respectively for coconut fibre specimens, while other natural fibres are in the range of 3–6%. To enhance the damping capability of the structure, the newly invented CFRC interlocking blocks, described in Ali et al. [5], are intended to be used in a mortar-free construction. The proposed wall under gravity and in-plane and out-of-plane earthquake loadings is shown in Fig. 1. Ropes, made of coconut fibres, are utilised as the vertical reinforcement of the wall to avoid its ultimate collapse by limiting the vertical displacement of the block to less than the key height to maintain an integrated stable configuration. The rope anchorage is achieved by embedment in the foundation and top tie-beams (shown in enlarged view in Fig. 1). The gravity load is taken by the compressive strength of the blocks. Because of the movability of all blocks relative to each other, the earthquake forces induced in the structure is expected to reduce. The activated lateral forces are resisted by the interlocking keys of the blocks in both the in-plane and out-of-plane directions. This resistance depends on the shear strength of the interlocking keys. The compression and shear capacities of the novel blocks are presented in Ali et al. [5]. The tension in the ropes would be generated because of the block uplift during the strong ground motion. This tension should be less than both the tensile strength of the ropes and the pullout force. As a preliminary study, the scope of the current work is limited to the axial pullout and tensile force of the ropes. The maximum force required to pull out the rope from the matrix is called the pullout force. The maximum axial force required to break the rope in tension is called the tensile force. The pullout force may govern the depth of the foundation and tie beams (i.e. the embedment of ropes into the foundation or tie beam should be selected such that there is no pullout). Therefore, the pullout force (i.e. ultimately the bond strength) as a function of different parameters as well as the tensile strength of the ropes is presented in this paper. The determination of rope tension within the proposed wall due to dynamic loading is outside the scope of this manuscript. The current study is one step towards the global goal (i.e. easy-to-build, economical and earthquake-resistant housing) and further studies are planned.

Durability of natural fibres has remained a topic of interest for many researchers. In a study by Saravanan and Sivaraja [6], the effect of an acidic and alkaline environment on the curing of concrete was investigated for slabs (750 mm long, 500 mm wide and 40 mm thick). The considered parameters were two different grades of concrete (M20 and M30, showing different strengths of concrete i.e. 20 and 30 MPa, respectively), three contents of coir rope reinforcement (0.95%, 1.43% and 1.91%) and rope pre-treatment (untreated and coated with epoxy resin). The mass loss and strength deterioration against HCl and Na2SO4 attacks over 2 years was determined. The reduction in flexural strength was approximately 2% and 3–4% for acid and sulphate environments, respectively. It was concluded that the slabs were not affected by the acid or sulphate environment and the durability of the proposed coir rope reinforced bio-composite concrete panels was good. In another study by Ramakrishna and Sundararajan [7], the variation in chemical composition and tensile strength of four natural fibres, i.e. coconut, sisal, jute and hibiscus cannabinus fibres, were investigated when subjected to alternate wetting and drying, continuous immersion for 60 days in water, saturated lime and sodium hydroxide. The chemical composition of all the fibres changed because of immersion in the solutions described above. Continuous immersion was found to be critical due to the loss of their tensile strength. However, coconut fibres were reported as the best option for retaining a good percentage of their original tensile strength in all tests. In most practical situations, raw natural fibres were not able to provide adequate interfacial bond strength. A considerable amount of work has been done to improve the fibre surfaces for increasing the bond strength with the surrounding matrix. The pre-treatment can be achieved by physical and chemical modifications of the fibre surface. The main constituents of the surface of a coconut fibre are lignin and cellulose. Boiled and washed fibres are stiffer and tougher compared to raw fibres because they have higher lignin and cellulose contents due to the washout of extractives [8]. The process could also remove a part of the extraneous fibre surface components which may resist the formation of the bond between fibres and cement paste. It was shown in Asasutjarit et al. [8] that compared to fibres without pre-treatment, the internal bond between washed and boiled fibres and the cement paste was doubled. Thus, washed and boiled fibres will effectively increase the fibre–matrix bond strength. Mani and Satyanarayana [9] concluded that the combined use of sodium alginate and calcium chloride effectively improved the ultimate tensile strength of fibres by 18%. Sodium alginate is a weak acidic food gel while calcium chloride is almost neutral, both of them are safe to use and are environmental friendly. In the current study, pre-treatment with boiling water and calcium alginate is adopted for coconut-fibre ropes. Only water soaked fibres are used in concrete to prepare CFRC.

The bond strength between a reinforcing material (steel or fibre reinforced polymer rebar) and concrete plays an important role in the overall behaviour of the composite [10]. An investigation of a rope/matrix interface (rope pullout test) is significant in this context, because the mechanical properties of the composites depend strongly not only on the properties of the rope and the matrix but also on the rope/matrix interfacial ones. To the best of author’s knowledge to date, pullout tests on rope cast in CFRC have never been carried out. For this reason, the standard bond strength test method [11] used for steel or fibre reinforced polymer rebars cast in plain concrete is taken as a guide to investigate the bond strength between the rope and CFRC. Uni-directional load is used to measure the load–slip curve. The ASTM standard [11] specifies that the loading device must be able to measure forces that are within 2% of the applied load. The slippage at the loaded and free ends of the rebar must be measured relative to the loaded and free ends of the matrix specimen using linear variable differential transformers (LVDT’s). The loading rate should be adjusted such that the specimen does not fail before 3 min after the specimen has been loaded, and at least ten load and displacement readings should be taken before failure of the specimen. The width, height and length of the matrix specimen should also be determined according to the standard. The width of each specimen should be greater than the sum of the bar diameter and two times the required concrete cover. The minimum length of each specimen should be five times this value. The height of each specimen should be equal to the embedment length of rope. Each specimen should be cured in forms using a curing compound or a plastic membrane to avoid rapid evaporation of water from the specimen. A minimum of two cylinders is required from each batch of concrete. These test cylinders should be cured in the same manner as the specimens have been cured. In a study by Guadagnini et al. [12], the bond strength between fibre reinforced polymer (FRP) bars and concrete was determined, using two testing methods: a pullout test and a splitting test (eccentric pullout). The pullout test was used to determine the bond strength of bars in a confined condition, whereas the splitting test was used to find the bond strength of FRP bars near the surface of the concrete. The research presented in Guadagnini et al. [12] also showed that pullout tests generally produced far more consistent results than splitting tests, as the natural inconsistencies in the concrete had a significant effect on the splitting behaviour. Therefore, pullout tests using uni-directional loading are relatively easy, quick and more reliable for having first hand information about the bond strength between the reinforcing bar and the surrounding matrix. Many researchers [13], [14], [15], [16], [17], [18], [19], [20] also used direct pullout tests to determine the behaviour of bond between a reinforcing rebar and the surrounding matrix. The placement of rebars in the matrix was varied to suite the different design scenarios. The factors considered were different types of rebars (steel, FRP), rebar size, embedment length, presence of ribs, pre-treatment conditions, matrix strength and fibre content in the matrix. The effects of these parameters on the bond strength are discussed in Section 4 in order to compare the known trends with that of the current findings (i.e. in case of ropes embedded in coconut fibre reinforced concrete).

Section snippets

Experimental work

The goals of this study are to

  • i.

    investigate the tensile strength of coconut-fibre ropes for variable rope diameter and pre-treatments, and

  • ii.

    determine how different parameters affect the bond between rope and coconut fibre reinforced concrete (CFRC). The variables considered are the embedment length of rope (100, 150 and 200 mm), approximate rope diameter (18, 27 and 36 mm), pre-treatment condition (soaking, boiling and chemical), the mix design ratio i.e. cement:sand:aggregates (1:2:2, 1:3:3 and

Rope tensile behaviour

The typical stress–strain relation for coconut-fibre rope is shown in Fig. 4. The relationship relates to a soaked rope of approximately 36 mm diameter. The maximum load (shown as ‘C’ in Fig. 4) is taken for the calculation of the ultimate strength of the rope and its corresponding strain is noted from the curve. The elongation up to peak load and total elongation are measured. It can be observed from the figure that the stress–strain curve has three approximately linear segments (OA, AB, BC)

Rope pullout behaviour

Different stages of rope pullout, marked as (a–e), are elaborated in Fig. 6. Since the rope is made of coconut fibres, which have the ability of taking high strains, the upward load of 0.5 kN was applied in order to tighten the rope between the loaded end and jaw of the Avery machine (stage a). When the load was applied to pull out the rope from the specimen, the elongation was initially observed (stage b). Then slippage of the rope started at the loaded end (stage c). When slippage at the

Conclusions

The experiments on rope tensile strength and rope-CFRC bond strength revealed that:

  • i.

    Thin (18 mm diameter) ropes had higher tensile strength than that obtained with medium (27 mm diameter) and thick (36 mm diameter) ropes. However, higher tensile load was required for thick ropes compared to thin and medium ropes.

  • ii.

    Thick ropes had greater elongation than that obtained with the thin and medium ropes.

  • iii.

    Compared to the soaked treatment, the tensile strength and elongation of the ropes increased by boiling

Acknowledgements

The authors would like to thank all who helped throughout the research, particularly Golden Bay Cement and Winstone Aggregates for support of this research. The authors are also grateful to Dr. Tam Larkin for his valuable time and suggestions in improving the manuscript. The careful review and constructive suggestions by the anonymous reviewers are gratefully acknowledged. The authors also wish to thank Pakistan Higher Education Commission for supporting the PhD study of the first author at the

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