Construction and Building Materials 40 (2013) 991–997
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Fracture energy of natural ﬁbre reinforced concrete I. Merta ⇑, E.K. Tschegg Institute for Building Construction and Technology, Building Construction and Maintenance, Faculty of Civil Engineering, University of Technology Vienna, Karlsplatz 13/206-4, 1040 Vienna, Austria
h i g h l i g h t s " Hemp ﬁbres enhance the fracture energy of concrete for 70%. " Elephant grass ﬁbres increase the fracture energy of concrete for up to 5%. " Straw ﬁbres increased the fracture energy of concrete solely up to 2%.
a r t i c l e
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Article history: Received 13 December 2011 Received in revised form 29 October 2012 Accepted 22 November 2012 Available online 28 December 2012 Keywords: Fibre reinforced concrete Natural ﬁbre Fracture energy Wedge splitting test
a b s t r a c t This paper reports on an experimental study of the fracture energy of concrete reinforced with natural ﬁbres of hemp, elephant grass, and wheat straw. Concrete specimens containing 0.19% of ﬁbres by weight and of 40 mm of length were uniaxially tested with the wedge splitting test (WST) method. The addition of ﬁbres was found to improve the fracture toughness of plane concrete. The most distinctive increase in the fracture energy has been observed by hemp reinforced concrete, up to 70%, when comparing with non reinforced concrete, whereas for straw and elephant grass reinforced concrete this increase was moderate, up to 2% and 5%, respectively. The beneﬁcial effect of hemp ﬁbres is believed to be the result of the ﬁbre’s high tensile strength and the ﬁbre’s ﬁneness, resulting in a better bonding between ﬁbres and concrete matrix. The presence of ﬁbres in concrete decreased minimally the tensile strength of concrete, for 4%, 7%, and 8% for hemp, straw and elephant grass reinforced specimens, respectively. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Concrete is one of the most widely and commonly used building material in civil engineering around the world. Concrete is strong in compression, however, as a very brittle material, has low strain capacity in tension and consequently low toughness. As a result, cracks develop whenever loads give rise to tensile stresses exceeding the tensile strength of concrete. Adding ﬁbres to concrete matrix has been long recognised as a way to enhance the energy absorption capacity and crack resistance of the plane concrete [1–3]. In ﬁbre reinforced concrete (FRC) by bridging ﬁbres across the cracks a post-cracking ductility is provided, and consequently, the toughness of concrete is considerably enhanced. Consideration of toughness and the fracture energy is important since it determines the ductility and crack resistance of the structure assuring the safety and integrity of the structural element prior to its complete failure . Concrete is typically reinforced with steel or synthetic ﬁbres like carbon, glass, or aramid. Despite of their advantages the high ⇑ Corresponding author. Tel.: +43 1 58801 21512; fax: +43 1 58801 21599. E-mail address: [email protected]
(I. Merta). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.11.060
material costs, the high energy-consuming process by the production, and their adverse environmental impact has initiated the search of new environmental friendly and sustainable alternatives. In the framework of international research, a considerable effort is going on in the exploitation of fast growing, annually renewable, cheap agricultural crops and crop residues as possible ﬁbre reinforcement in concrete. The basic advantage of natural ﬁbres is that they are a low cost and widely available resource in many agricultural areas. They are biodegradable, non-abrasive and there is no concern with health and safety during handling. Natural ﬁbre reinforced materials are environmental friendly materials producing less green-house gas emissions and pollutants. The use of natural ﬁbres as reinforcement is a way to recycle these ﬁbres and to produce a high performance material. One of the ﬁrst motivations to use natural ﬁbres in building materials was the effort to ﬁnd a replacement for asbestos in ﬁbre cement products. Australian research  was focused on this subject and ultimately wood pulp ﬁbre was responsible for a great replacement of asbestos in the Australian cement industry. Recently, the use of different types of natural ﬁbres as reinforcement mainly in cementitious matrices has been researched. Savastano et al. [6–9] in Brazil considered pulp from eucalyptus waste,
I. Merta, E.K. Tschegg / Construction and Building Materials 40 (2013) 991–997
Fig. 1. Concrete specimen for wedge splitting test (WST) under uniaxial load according to Tschegg [25,26].
Table 1 Mix design proportion of the concrete matrix. Materials
Mass unit (kg/m3)
Aggregate 0–4 mm Aggregate 4–8 mm Aggregate 8–16 mm Cement (CEM I) Fly ash Water Superplasticizer
1143 288 545 265 40 180 1.37
residual sisal (Agave sisalana) coir, and banana pulp as a replacement for asbestos in cementitious matrices, whereas Coutts and Ni  in Australia extensively studied bamboo ﬁbres in cement matrix. The applications of coconut coir and sugar cane bagasse as well as ﬂax have been also studied as possible reinforcement of cementitious composites [11–14]. Recently extensive research work is going on in Brasil concerning application of sisal ﬁbres as reinforcement in cement based composites [15–17]. The cracking mechanism as well as the tensile-, impact- and fatigue behaviour of the composite has been in detail studied. With application of wood ﬁbres in cementitious matrix Sierra Beltran  developed a promising ductile cement-based composite. However, concerning the toughness or energy absorption capacity of concrete reinforced with natural ﬁbres of straw, elephant grass and hemp very limited work exist. The only work found in the literature is that of Li et al. , where the mechanical and physical properties of hemp ﬁbre reinforced concrete have been experimentally studied. They stated that ﬂexural strength and ﬂexural toughness all increase with increasing ﬁbre content. Despite all the aforementioned advantages there are some serious concerns for successful application of natural ﬁbres as concrete reinforcement. These are the high variation of ﬁbre properties, high moisture absorption capacity, and the main concern is the durability of ﬁbres in the alkaline environment of cement matrix. The degradation of ﬁbres in cement matrix occurs as a consequence of dissolve of the lignin and the hemicellulose in the middle lamellae of the ﬁbres through the alkaline pore water. As a consequence with aging the composite may undergo a reduction in strength and toughness. Much experimental research has been conducted
related to the durability of natural FRCs [20–22]. It is generally accepted that there are two ways to improve the durability of the composite. One is to modify the surface of the ﬁbres with physical or chemical agents. The second is to modify the composition of the matrix with addition of pozzolanic materials (ﬂy ash, silica fume or metacaolin) which reduce the alkalinity of the matrix. Recently the previous horniﬁcation of ﬁbres has been proposed by Claramunt et al.  and it was reported that the previous treatment of softwood kraft pulp and cotton linters had beneﬁcial effect on the mechanical performance and durability of the composite reinforced with these ﬁbres. Merta et al.  experimentally investigated the durability of hemp ﬁbres in the alkaline environment of cement matrix in terms of tension strength loss of the ﬁbres after accelerated aging test by elevate temperature. The objective was to ﬁnd the most effective protective agent which will reduce water absorption of ﬁbres and, in a long-term preserves the tension strength loss of ﬁbres. The protective substances used to saturate the ﬁbres and provide the water resistance protection were: linseed oil, linseed oil with catalyst, parafﬁn, and bees wax. It was observed that linseed oil with catalyst seemed to offer the best protection against the alkaline environment with lowest tensile strength loss of the ﬁbres. Despite of all advantages of natural ﬁbres as reinforcing materials, their employment as reinforcement in concrete is still limited investigated area and a challenging idea. In order to investigate the inﬂuence of the ﬁbres on the energy absorption capacity of concrete, in this research ﬁbres of plants that are widely available and cheap in European countries has been selected. Fibres of hemp, elephant grass, and wheat straw were added to the concrete matrix and with the wedge splitting test (WST) method according to Tschegg [25,26] the uniaxial fracture toughness of the obtained composites has been studied. 2. Experimental program 2.1. Specimens and materials For experimental work concrete specimens of dimensions 150 150 120 mm3 with a 30 mm long and 3 mm wide starter notch cut at the top have been employed. In order to obtain a concrete prism with a height of 120 mm on the bottom of the cube molds a 30 mm thick piece of plastic was placed. The rectangular groove on the upper side of the specimen, needed for the load transmission pieces, was achieved by gluing two stone pieces thereon (Fig. 1). The concrete matrix was prepared according to the mix design proportion listed in Table 1. The coarse aggregate in the matrix used was river gravel with maximum particle size of 16 mm. The water/cement ratio was 0.67. For fracture mechanics tests a series of ﬁve specimens, whereas for compression tests three specimens were produced. Chopped ﬁbres of hemp, wheat straw, and elephant grass with 40 mm length (Fig. 2) were added to the concrete matrix as ﬁbre reinforcement. The ﬁbres content was 4.5 kg/m3 which resulted in a ﬁbre percentage of 0.19% by weight. The ﬁbres were used as they come from nature without any kind of preparation ensuring in such a way a low cost building material. Hemp (Cannabis sativa L.) is categorised as a bast ﬁbre crop. Its stem consists of a woody core with a hollow open sponge like structure, surrounded by an outer skin containing long and strong ﬁbres. After processing the stems two materials are produced: hemp hurd (or shiv) and hemp ﬁbres. Hemp hurd is so far extensively used in hemp lime products. It is a composite building material formed from the mixture of hemp hurd as aggregate and lime based binders. It is usually used as material for insulating walls or insulation layers for ﬂoors and roofs.
Fig. 2. Chopped ﬁbres of hemp, wheat straw, and elephant grass.
I. Merta, E.K. Tschegg / Construction and Building Materials 40 (2013) 991–997 Table 2 Tensile strength of the ﬁbres.
Table 3 Test results of the concrete specimens.
Tensile strength (N/mm2) measured
Tensile strength (N/mm2) from the literature 
Hemp Straw Elephant grass
600–700 40 40–60
310–1110 30 180–260
Hemp ﬁbres are the most valuable part of the plant. Usually hemp ﬁbres are used as insulation products. They have about 70% cellulose and contain low levels of lignin (around 8–10%). The ﬁbre diameter ranges from 16–50 lm . Hemp ﬁbres have an extremely high tensile strength of between 310 and 1110 N/mm2 (Table 2). Tensile strength tests of hemp ﬁbre or bundles are not standardised, and in the literature very different test methods are used . Elephant grass (Miscanthus) is a tall perennial grass that has been planted extensively in Europe during the past 5–10 years, as a new bioenergy crop. Its stem has a similar hollow woody core like hemp, surrounded by a thinner outer core. Miscanthus concrete is made from chopped stem of the elephant grass and used for insulating walls. In this research solely the outer core of the elephant grass stem was used as ﬁbre reinforcement for concrete. The tensile strength of these ﬁbres is between 180 and 260 N/mm2 (Table 2). Wheat straw, as a widely available agricultural resource in European countries, has been also selected as ﬁbre reinforcement for concrete, although its tensile strength is considerable lower than that of hemp and elephant grass (Table 2).
2.2. Test method 2.2.1. Fracture mechanical test For fracture mechanical characterisation and determination of the softening properties of concrete and other quasi-brittle materials usually the uniaxial tensile test (UTT) and the three-point bending test (TPBT) [29,30] have been recommended. However, there are some drawbacks observed by these test methods. The UTT method is difﬁcult to carry out, while the TPBT method requires relatively large specimens and, due to the inﬂuence of the self-weight, a special care in fracture analysis is required. An additional inconvenience of the loadpoint deﬂection measurements is that they are strongly affected by the support conditions. Recently a novel procedure for measuring the fracture properties of concrete and other quasi-brittle materials, the wedge splitting test method (WST), has been widely adopted. The WST method was originally developed by Tschegg [25,26]. It is a very stable fracture mechanics test capable to determine accurately the load displacement diagram of the test specimens beyond the maximum load. The major advantages of the WST are that the specimens are small and compact, the method does not require any sophisticated test equipment; it stores little elastic energy during testing and is well suited for inverse analysis. The WST method was comprehensively investigated by many scientists and it has been proved reliable for fracture testing of ordinary concrete at early age and later, for lightweight concrete and for concrete reinforced with steel and synthetic ﬁbres [31–34]. Löfgren  made a comparison of the experimental results obtained from UTT and TPBT with the results of the WST. The results demonstrated the applicability of the WST showing that the scatter of the test results is lower than for the other tests. Fig. 3 shows the fundamental setup of the WST method for uniaxial loading of a cubic specimen. The specimen has been provided with a rectangular groove and a
Plane concrete C1 C2 C3 C4 C5 Mean Standard deviation COV [%] Hemp FRC H1 H2 H3 H4 H5 Mean Standard deviation COV (%) Straw FRC S1 S2 S3 S4 S5 Mean Standard deviation COV (%) Elephant grass FRC E1 E2 E3 E4 E5 Mean Standard deviation COV (%)
Compressive strength (N/mm2)
Notch tensile strength sNTS (N/mm2)
Speciﬁc fracture energy Gf (N/mm2)
3.88 3.76 3.84 3.51 3.73 3.74 0.14 3.85
136.55 121.46 130.56 114.07 95.37 119.6 16.04 13.41
3.78 3.66 3.33 3.55 3.64 3.59 0.17 4.67
185.16 256.53 169.05 159.32 251.79 204.37 46.41 22.71
3.56 3.53 3.43 3.66 3.3 3.5 0.14 3.91
126.86 126.22 110.83 133.33 115.03 122.45 9.25 7.55
3.49 3.48 3.41 3.32 3.56 3.45 0.09 2.63
153.23 131.98 119.09 148.58 136.01 137.78 13.61 9.88
starter notch at the top (Fig. 1). The specimen is than positioned on a narrow linear support and the two load transmission pieces and a slender wedge are inserted in the groove (Fig. 3). The load FM produced by the testing machine is transferred by the load transmission pieces from the wedge into the specimen, which leads to the splitting of the specimen. The friction between the wedge and load transmission pieces (equipped with ball bearings) is negligibly small (<1%) and the splitting force FH can be determined by means of a simple calculation. The vertical force FM, of the testing machine is converted to a large horizontal force FH and into small vertical force FV that does not disturb the propagation of the crack. The splitting force FH breaks the specimen in mode I.
Fig. 3. Setup of the uniaxial wedge splitting test (WST) according to Tschegg [25,26].
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The crack mouth opening displacement (CMOD) is determined at the height of the load application line on both sides by electronic displacement transducers. The two electronic displacement transducers are used on the one hand to obtain the average of the load displacement and on the other hand serve as crack behaviour detectors. If the crack runs obliquely to the notch, the specimen is eliminated. All tests were carried out at an average ambient temperature of 22 °C and an average relative humidity of 50%. The loading process was displacement controlled with constant cross-head speed rate of 1 mm/min.
2.2.2. Surface roughness of the ﬁbres Scanning electron microscopy (SEM) was applied for the characterisation of the microstructure of the ﬁbres’ surface. Micrographs of the ﬁbres’ cross sections with 500 magniﬁcation and of the ﬁbres’ surfaces with 2000 magniﬁcation are presented.
3. Results and discussion 3.1. Notch tensile strength of the specimens
Notch tensile strength σNTS [N/mm 2]
The notch tensile strength, rNTS (N/mm2), of the specimens are recorded in the Table 3. The mean values, standard deviation and coefﬁcient of variation (COV) are also given for each series of the test specimens. The mean value of the notch tensile strength of the ﬁbre reinforced specimens was up to 4%, 7%, and 8% lower for hemp, straw, and elephant grass reinforced specimens, respectively, compared to unreinforced concrete specimens (Fig. 4). As expected, the presence of the ﬁbres does not have much inﬂuence on the notch tensile strength of the concrete.
Specimens Fig. 4. Notch tensile strength of the specimens.
Fig. 7. Scanning electron micrographs of the hemp ﬁbre: (a) 500 magniﬁcation of the ﬁbre’s cross section and (b) 2000 magniﬁcation of the ﬁbre’s surface.
Specific fracture energy G f [N/m]
Fig. 5. Splitting force–displacement curve of the specimens.
Concrete Hemp Straw Elephantgrass
Specimens Fig. 6. Speciﬁc fracture energy of the specimens.
Fig. 8. Scanning electron micrographs of the straw ﬁbre: (a) 500 magniﬁcation of the ﬁbre’s cross section and (b) 2000 magniﬁcation of the ﬁbre’s surface.
I. Merta, E.K. Tschegg / Construction and Building Materials 40 (2013) 991–997
A typical load–displacement curve of a concrete specimen without ﬁbres and specimens reinforced with hemp, elephant grass, and straw is presented in Fig. 5. The results showed that the presence of the ﬁbres in concrete enhances the fracture energy of the plane concrete. This is because of the fact that, when ﬁbres are present in concrete, the cracks could not extend without stretching and debonding the ﬁbres. The most distinctive increase in fracture energy of ﬁbres specimens compared to unreinforced concrete specimens was observed by hemp ﬁbre specimens, i.e., up to 70%. Reinforcing concrete with straw and elephant grass ﬁbres resulted in minimal increase of the fracture energy, i.e., 2% and 5%, respectively (Fig. 6). 3.3. Surface roughness of the ﬁbres
Fig. 9. Scanning electron micrographs of the elephant grass ﬁbre: (a) 500 magniﬁcation of the ﬁbre’s cross section and (b) 2000 magniﬁcation of the ﬁbre’s surface.
3.2. Fracture energy of the specimens The fracture energy is deﬁned as the post-crack energy absorption ability of the material and it represents the energy that the structure will absorb during failure. The speciﬁc fracture energy, Gf (N/m), was calculated as the area under the splitting force– displacement curve up to a deﬁned displacement of 1.5 mm divided by the area of the fracture plain. The speciﬁc fracture energy of the specimens is recorded in the Table 3. The mean values, standard deviation, and coefﬁcient of variation (COV) are also given for each series of the test specimens.
The micrographs of the ﬁbres’ cross sections and of the ﬁbres’ surfaces are presented in Figs. 7–9. The surface of hemp and elephant grass ﬁbres is relatively smooth, while the surface of straw ﬁbres has a much higher roughness. The representative failure planes of the unreinforced specimens and the ﬁbre reinforced specimens are presented in the Fig. 10. By hemp reinforced concrete specimens, due to the ﬁbres’ extreme ﬁneness and thinness (diameter 16–50 lm, belonging in the category of microﬁbres), the total ﬁbres’ surface area is high, which results in a higher chemical adhesion and friction between ﬁbre and matrix. As a consequence, the ﬁbres tensile strength could have been utilised in greater extend contributing to the increase in fracture energy of the specimens. As a result of the ﬁbres high tension strength, no rupture of the ﬁbres was observed, rather a pull-out of the ﬁbres along the fracture plane (Fig. 10b). The high surface roughness of the straw ﬁbres ensures a good bond with the concrete matrix (Fig. 10c). However, as a result of the ﬁbres’ rather low tensile strength and strong bond, rupture of the ﬁbres along the failure plane has been observed. By elephant grass reinforced specimens (Fig. 10d) as a result of a low surface
Fig. 10. Failure planes of specimens (a) unreinforced concrete, (b) concrete reinforced with hemp ﬁbres, (c) concrete reinforced with straw ﬁbres, and (d) concrete reinforced with elephant grass ﬁbres.
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the concrete matrix and ﬁbre pull-out failure without any stress transfer. The employment of these ﬁbres as concrete reinforcement is believed to be limited. 3. The splitting tensile strength of the ﬁbre reinforced specimens was up to 4%, 7%, and 8% lower compared to unreinforced concrete specimens for hemp, straw, and elephant grass ﬁbres respectively.
Acknowledgement Authors would like to thank Dr. Franz Denk from the Wopﬁnger Transportbeton Company for the fabrication of the concrete specimens, providing in such a way a generous supporting fund for this research. Fig. 11. Bundle of hemp ﬁbres in the concrete matrix.
roughness of the ﬁbres, pure bond with the concrete matrix and ﬁbre pull-out failure with almost no stress transfer was observed. The employment of both straw and elephant grass ﬁbres as concrete reinforcement is believed to be rather limited. However, the drawback of the hemp ﬁbre’s ﬁne structure is that it is extremely hard to obtain a uniform concrete mixture. During the production of the composite some ﬁbres do not disperse into individual ﬁlaments surrounded by the matrix, but rather tend to clamp together forming a bundle of ﬁbres in the matrix (Fig. 11). For the larger cement grains it is difﬁcult to penetrate within these spaces. The resulting microstructure is characterised by vacant spaces between the ﬁbres of the bundles. In such bundles the bonding with the concrete matrix is not uniform and only the external ﬁbres are more tightly bonded with the surrounded matrix. Consequently, it results in variation of the number of ﬁbres across the fracture plane. The major contributing factor to the high scatter in the test results of the hemp ﬁbre specimens (COV = 22%, see Table 3) is believed to be related to this phenomenon. Improving the mixing technique would probably lead to much more uniform matrix and lower scatter in results. 4. Conclusion Reinforcing concrete with natural ﬁbres could provide an environmental friendly and low cost building material. In this paper the fracture energy of concrete reinforced with chopped ﬁbres of hemp, wheat straw, and elephant grass was experimentally investigated with employment of the wedge splitting test (WST) method. The conclusions obtained from this study are as follows: 1. With hemp ﬁbres as reinforcement an enhancement of the concrete fracture energy, up to 70% compared to unreinforced concrete, has been achieved. It is believed to be a result of the ﬁbre’s high tensile strength and of the ﬁbre’s ﬁneness. As a result of a high total ﬁbre’s surface area good bonding between ﬁbres and matrix could be achieved enabling an efﬁcient stress transfer. 2. Straw and elephant grass ﬁbres increased the fracture energy of concrete solely up to 2% and 5% respectively. By straw FRC the reason for that is believed to be the combination of high surface roughness of the straw ﬁbres (resulting in good bond with the concrete matrix) and of low tensile strength of the ﬁbres. This results in failure by ﬁbres rupture. By elephant grass FRC the low surface roughness of the ﬁbres results in pure bond with
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