Fracture energy of natural fibre reinforced concrete

Fracture energy of natural fibre reinforced concrete

Construction and Building Materials 40 (2013) 991–997 Contents lists available at SciVerse ScienceDirect Construction and Building Materials journal...

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Construction and Building Materials 40 (2013) 991–997

Contents lists available at SciVerse ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

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

i n f o

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 [4]. 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 [5] 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,

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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 [10] 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 [18] 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. [19], 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. [23] 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. [24] 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.

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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.

Fibre type

Tensile strength (N/mm2) measured

Tensile strength (N/mm2) from the literature [28]

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 [27]. 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 [28]. 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 [32] 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

Specimens

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)

42.2

34.6

31.7

30.5

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]

4,0

3,5

3,0

2,5

Concrete Straw

Hemp Elephantgrass

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.

(a) 2,0

Specimens Fig. 4. Notch tensile strength of the specimens.

(b)

50μm

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.

(a)

Concrete Hemp Straw Elephantgrass

250 200

150

(b)

100 50

0

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.

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(a)

(b)

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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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