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Effect of reinforcing wool fibres on fracture and energy absorption properties of an earthen material
Construction and Building Materials 27 (2012) 66–72
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of reinforcing wool fibres on fracture and energy absorption properties of an earthen material F. Aymerich a,⇑, L. Fenu b, P. Meloni c a
Department of Mechanical Engineering, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy Department of Structural Engineering, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy c Department of Chemical Engineering and Materials, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy b
a r t i c l e
i n f o
Article history: Received 5 January 2011 Received in revised form 28 July 2011 Accepted 1 August 2011 Available online 28 September 2011 Keywords: Earthen materials Wool fibres Toughness Fracture
a b s t r a c t The study investigated the improvements in strength and crack resistance induced by the introduction of wool fibres in an earthen material. Earthen samples reinforced by wool fibres of various fibre lengths at different fibre weight fractions were tested under flexural loading to examine the structural response of the material in terms of first-crack resistance, post-cracking residual strength and energy absorption capability. It was found that the fibrous reinforcement greatly improved the residual strength, the ductility and the energy absorption of the reinforced material as compared to the unreinforced soil. The results of the study also showed that fibre length had a notable influence on the post-fracture response of the material at large deformation regimes. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction In recent years there has been a revival of interest in earthen construction materials, not only for use in restoration and repair of historic and cultural heritage buildings, but also for use as low-energy and environmentally friendly materials in bio-ecological and sustainable architecture [1–9]. Earthen construction, which was widely diffused in the past, is still common today especially in regions of favourable (i.e. hot/arid and temperate) climate conditions and about one-third to one half of the world’s population are estimated to live in houses constructed of unbaked earth [4,6]. The main advantages of the use of earth as a building material are related to the significant reduction in environmental impact, owing to the use of locally available raw materials with ensuing minimal transportation costs, and to simple and energy-efficient manufacturing processes [1–5,9]. In addition, earth-based materials allow better balance and control of thermal and acoustic indoor climate, as compared to typical industrial building materials, because of their superior performances in terms of humidity absorption/desorption rates, heat storage capacities and sound transmission properties [2–5,10,11]. The main weaknesses of earth-based materials are related to shrinkage occurring during the drying phase (which results in the development of internal or surface cracking), to poor tensile ⇑ Corresponding author. Tel.: +39 070 6755727; fax: +39 070 6755717. E-mail addresses: [email protected] (F. Aymerich), [email protected] (L. Fenu), [email protected] (P. Meloni). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.08.008
and flexural strength and ductility, and to the relatively low resistance to water erosion [1–4]. Additives or stabilizers are typically used to address these issues and in general to improve particular properties of the material for specific applications. A wide range of modifiers, including mineral binders (cement, limen, bitumen), animal and vegetal stabilizers (blood, casein, animal glue, oil, latex), natural fibres (hair, wool, straw, flax, cellulose, sisal) have been in fact used in earthen construction [1–4]. In particular, various research works have shown that hygrometric shrinkage and associated cracking of earth-based materials can be greatly reduced by the introduction of fibres into the mixture [12–17]. The presence of reinforcing fibres typically also improves the tensile and, to a lesser extent, the compressive strength of earthen materials [12,18], although decreases in compressive or tensile strength were observed in a few studies as a consequence of the addition of cut straw reinforcement in the material [4,13,15]. On the other hand, only a very limited number of studies have been carried out to explore the influence of fibrous reinforcement on ductility, fracture resistance, and post-fracture behaviour of earthen materials [19–23]. Increases in material ductility with increasing jute fibre content, even though associated to decreases in compressive strength, were for example observed by Islam and Iwashita [19] in a detailed investigation of the effect of natural fibres on the compressive strength and ductility of adobe blocks. Increasing the length of the reinforcing fibres was also found to be effective for improving ductility of the material, but only up to a critical fibre length value.
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The role played by straw fibres in improving toughness and crack control properties of earth-based materials under flexural loadings has been investigated in a series of papers by Lenci and co-workers [21,22]. Experimental results obtained by bending tests on pre-cracked straw-reinforced earth blocks indicate, in particular, that the resistance to crack propagation significantly improves with crack propagation, as a direct effect of the bridging action of fibres across the crack surfaces. Increases in flexural strength and ductility with the addition of wool fibres to a soil and alginate matrix have been also recently reported by Galán-Marín and co-workers [23]. There is still, however, a lack of experimental data and analyses concerning the effect of fibrous reinforcement on the fracture performance of earthen materials, with special reference to properties controlling crack initiation and propagation, and to energy absorption capabilities at large deformation regimes. This paper reports the results of an experimental study aimed at investigating the modifications in strength, crack resistance and post-fracture behaviour induced by the insertion of reinforcing fibres in an earthen material. Samples made with soil material and reinforced with sheep wool fibres were manufactured using various fibre contents and fibre lengths. The earthen samples were tested under flexural loading to compare the pre- and post-damage mechanical response of the different materials. It is worth remarking that while sheep and dairy farming has always been a significant source of agricultural income in Sardinia (Italy), there is at present only a very limited use in the textile industry of Sardinian sheep’s wool, which could thus deserve consideration as a local, renewable and sustainable resource for use in natural and environmentally-compatible building materials. 2. Materials and methods 2.1. Materials A soil from quaternary sediments alongside the Tirso River near Oristano (Sardinia, Italy), traditionally used in the past to manufacture earth bricks, was used in this study. Fig. 1 shows the granulometric curve of the soil, which was obtained by sieving and hydrometer analyses carried out in accordance with ASTM standard D422 [24]. With reference to ASTM particle size limits [24], the soil may be defined as silt (about 60%), with fine and medium sand (about 35%) and less than 7% of clay size particles. Density and Atterberg limits of the soil are reported in Table 1. The soil may be classified as CL, i.e. inorganic clay with low plasticity and low liquid limit, in the Unified Soil Classification System (USCS) [25]. X-ray diffraction (XRD), used to examine the mineralogical composition of the soil, revealed the presence of dominant quartz, together with other constituents including illite and plagioclase. Grain size and values of Atterberg limits indicate that the soil is suitable for earth construction [1,3], as also suggested by the local traditional use of this resource as building material. The strengthening and toughening effect of wool fibres on the earthen material was examined in this study. Wool fibres obtained from black and white Sardinian sheep were supplied by Edilana (Guspini, Sardinia), a manufacturer of wool products for use in thermal and acoustic building insulation. Wool fibres, which had an average diameter of 35 lm, were cut in various lengths (1, 2 and 3 cm) to investigate the effect of fibre length on the structural properties of the reinforced material. 2.2. Sample preparation Specimens 360 mm long, with a cross section of 75 mm 75 mm and a central notch 25 mm in depth (Fig. 2), were used in this study to investigate the fracture behaviour of wool reinforced soil under bending loads. Notched samples with comparable dimensions were used in previous investigations by Lenci et al. [21,22]. It should be observed, on the other hand, that none of the existing standards or recommendations on the use of earth-based construction materials (see for example [26–30]) specifically define size and geometry requirements for specimens to be tested in flexure. Samples were manufactured using various mix compositions, as summarized in Table 2. The soil, water and fibre fractions were poured in a bucket in the proportions indicated in Table 2, and thoroughly mixed by hand until a mass of
Fig. 1. Grain size distribution curve of soil.
Table 1 Physical properties of soil. Density (g/cm3)
Liquid limit (%)
Plastic limit (%)
Plasticity index (%)
2.63
28
17
11
homogeneous consistency was obtained. The water content of the various mixtures was chosen by preliminary experiments so as to guarantee similar levels of plasticity and workability across the different material systems. The specimens were obtained by casting the mixture into a plastic prismatic mould (Fig. 3a) in five successive layers. Each layer was first levelled out and afterwards compacted with the aid of a tamper to remove voids from the mixture. A single edge notch was introduced at the midspan of the sample by placing a rectangular insert of 2 mm thickness across the width of the mould, as visible in Fig. 3b, before pouring the mixture. All specimens were cured at room temperature (20 °C), first for 48 h inside the mould and then for 28 days after demoulding so as to allow the sample to reach a stable weight. Samples were finally dried up in an oven at 40 °C for two weeks before testing. A typical specimen is shown in Fig. 4. 2.3. Experimental testing Notched specimens were used for evaluation of flexural and fracture properties, with the specific aim of investigating the toughening effect, crack control capabilities and energy absorption properties induced by the presence of reinforcing fibres in the earthen material. Specimens were tested using a three-point loading configuration with a lower span length of 310 mm (Fig. 5) for assessing and comparing the fracture properties of the investigated materials. All tests were performed with the use of a closed-loop servo-electric 5 kN tension/compression machine. Tests were carried out in displacement control at a displacement rate of 1 mm/min. Displacement control was used since it provided stable crack propagation that was required for reliable measurements of energy absorbed during deflection. The applied load and the sample deflection (evaluated in the proximity of the notch on the lower side of the sample, as illustrated in Fig. 5) were recorded continuously for the duration of the test. The load and the displacement were respectively measured using the load cell of the testing machine and a linear variable differential transducer (LVDT). In order to acquire accurate and reliable force–displacement curves, particular care was taken to avoid support disturbances and external deformations (mainly associated to ‘‘seating’’ of the sample on the supports and to local indentation at the load application regions) that may affect significantly the values of the measured displacements [31,32]. To this end, as shown in Fig. 5, the LVDT was mounted on a special frame so as to obtain displacement values unaffected by local indentations of the material induced by the support and loading rollers. All tests were terminated when mid-span deflection reached the value of 16 mm. At least four specimens of each type of mixture described in Table 2 were tested during the study.
3. Results and discussion Representative sets of load–deflection curves of unreinforced and wool-reinforced samples are shown in Figs. 6 and 7, while the appearance of a fractured reinforced sample after test (with a
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Fig. 2. Geometry of samples.
Table 2 Proportion of mixtures.
a
Mixture designation
Water/soil weight ratio
Fibre weight fractiona
Fibre length (cm)
U W2_1 W2_2 W2_3 W3_1 W3_2 W3_3
0.18 0.29 0.29 0.29 0.29 0.29 0.29
– 2% 2% 2% 3% 3% 3%
– 1 2 3 1 2 3
Fig. 4. Reinforced earthen specimen after demoulding.
Fibre weight fraction is evaluated on the total weight of the mixture.
Fig. 5. Schematics of test setup for flexural tests of samples.
Fig. 3. Mould used for preparation of samples (a) and detail of central insert for notch introduction (b).
typical macroscopic crack propagating from the tip of the initial notch) is visible in Fig. 8. The plots of Figs. 6 and 7 show that unreinforced and reinforced samples have a similar initial response, characterized by an approximately linear behaviour between origin and first-crack load, which may be defined as the load at which the force–deflection curve starts exhibiting significant nonlinearity [31,32]. The first-crack point was experimentally observed to correspond with the onset of continuous and visible macrocracking propagating from the notch tip (Fig. 9).
Fig. 6. Typical load–deflection curves for unreinforced and reinforced (2% fibre weight fraction) samples.
Load–deflection curves of unreinforced samples show an abrupt load drop with a drastic loss of residual strength after first cracking, while complete separation of the sample into two pieces, with ensuing total loss of load carrying capability, usually occurs for deflections smaller than approximately 5–6 mm. Reinforced samples, in contrast, exhibit a characteristic strain hardening behaviour after first cracking, with continuously raising load–deflection curves, only followed by a falling load segment for large deflection values.
F. Aymerich et al. / Construction and Building Materials 27 (2012) 66–72
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Fig. 7. Typical load–deflection curves for unreinforced and reinforced (3% fibre weight fraction) samples.
Fig. 10. Detail of fibre bridging across a macrocrack growing from the notch (W3_3 sample).
Fig. 8. Reinforced sample after test.
Fig. 9. Onset of macrocrack at the notch in unreinforced (left) and reinforced (right) samples.
The plots of Figs. 6 and 7 show that the presence of fibres does not substantially modifies the elastic response and the first-crack flexural strength of the material. A similar behaviour was observed in numerous studies on concretes or cementitious composites reinforced with relatively small fibre fractions [33–35] and may be attributed to the fact that fibres are too widely spaced to affect the pre-crack response of the matrix [33]. It is seen however that fibres enormously improve the post-cracking behaviour of the material in terms of ductility, energy absorption and residual strength over large deformation regimes. The experimental results also suggest that the beneficial action of fibres may only develop when macrocracks are wide enough to activate the typical crack-control mechanisms associated to fibrous reinforcement, such as bridging (which arrest or delay opening of macrocracks, Fig. 10) or debonding and frictional pull-out (which impart toughness and ductility by energy dissipation through fracture and friction at the fibre-matrix interface) [35,36]. Increases in
fibre weight fraction and fibre lengths are thus expected to improve the post-crack performances of earthen samples, especially for large deflections. Longer fibres, in particular, should correspond, for a given fibre weight fraction, to better toughness and residual strength properties at large deflections, as a direct consequence of the higher loads required for fibre debonding and frictional pullout after complete debonding. To confirm this hypothesis, first-crack loads and peak loads were evaluated from the load–deflection curves of the whole set of samples subjected to test (2% or 3% fibre weight fraction, with 1, 2 or 3 cm fibre length). The first-crack load was estimated by considering an offset method with a 0.05 mm offset deflection [31], while the peak load was defined as the maximum load attained during the test up to the maximum deflection of 16 mm. Average values of first-crack loads and peak loads are reported in the graphs of Figs. 11 and 12. Error bars in graphs of Figs. 11 and 12 (and in the graphs of the following Figs. 13–16) indicate plus and minus one standard deviation from the mean. A comparison of the first-crack and peak load values show that while the onset of first cracking is basically unaffected by fibre length, peak loads are significantly increased when the length of fibres is increased from 2 cm to 3 cm. It is worth noting that no evident peak load improvement is on the other hand observed when fibre length is increased from 1 cm to 2 cm, thus suggesting that a minimum critical fibre length should be used to efficiently introduce and exploit potential toughening mechanisms typical of this class of materials. In order to better characterize and compare the post cracking response and the load-carrying capabilities of the investigated materials at various deformation stages, the values of two different performance indicators (namely, the energy absorbed and the residual strength retained by the sample) were evaluated at increasing mid-span displacements. Figs. 13 and 14 show the energy absorbed by W2 (2% fibre weight fraction) and W3 (3% fibre weight fraction) samples at various deflection levels between 1 and 15 mm. The energy absorbed by the samples was calculated as the area lying under the force– displacement curve up to the selected displacement. We may observe that while for small deflections the energy absorbed by samples is roughly independent of fibre length, for large deflections the energy absorption tends to be higher in specimens reinforced with longer fibres. As an example, as visible in Fig. 14 with reference to samples with 3% fibre fraction, the energy
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Fig. 11. Average values of first-crack loads and peak loads for samples reinforced with 2% wool fibres. Error bars indicate plus and minus one standard deviation.
Fig. 12. Average values of first-crack loads and peak loads for samples reinforced with 3% wool fibres. Error bars indicate plus and minus one standard deviation.
Fig. 13. Average values of absorbed energy at various deflection values for samples reinforced with 2% wool fibres.
Fig. 14. Average values of absorbed energy at various deflection values for samples reinforced with 3% wool fibres.
F. Aymerich et al. / Construction and Building Materials 27 (2012) 66–72
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Fig. 15. Average values of load-carrying capability at various deflection values for samples reinforced with 2% wool fibres.
Fig. 16. Average values of load-carrying capability at various deflection values for samples reinforced with 3% wool fibres.
absorbed by W3_3 samples (3 cm fibres) is, at 15 mm deflection, more than 20% larger than that of W3_1 samples (1 cm fibres) and about 14% larger than that of W3_2 samples (2 cm fibres). These results further highlights how the role exerted by fibre length on the efficiency of the toughening and strengthening mechanisms increases in importance with increasing deflection and macrocrack opening. The progressively greater role of fibre length with increasing deflections is confirmed by the analysis of the strength data illustrated in the graphs of Figs. 15 (specimens with 2% fibre weight ratio) and 16 (specimens with 3% fibre weight ratio). The diagrams, which report the load carrying capability retained by the samples at various mid-span displacements, clearly show that the largest influence of fibre length is again exerted at the largest deflections, with longer fibres always corresponding to higher residual strengths.
2. The beneficial action of fibres is effectively exploited only when macrocracks are sufficiently open to trigger typical strengthening mechanisms such as bridging and frictional fibre pull-out. 3. Fibre length was found to have a significant influence on the postfracture response of the material only for large deflection levels. 4. Further studies are needed to assess validity and significance of the above findings for more realistic full-scale earthen building units.
Acknowledgement The authors wish to thank Ms. Daniela Ducato of Edilana (www.edilana.com) for providing the wool fibres used in this study. References
4. Conclusions The study was aimed at investigating the enhancements in strength and post-fracture performances achievable by the introduction of fibre reinforcement in an earthen material. Notched samples made with a soil material reinforced by wool fibres were prepared with two fibre weight fractions (2% and 3%) and using different fibre lengths (1, 2 and 3 cm). The samples were tested under flexural loading to compare the mechanical response of the various materials in terms of first-crack resistance, post-cracking performance and energy absorption capability over the deflection range investigated. The main conclusions of the study may be summarized as follows: 1. The presence of wool fibres does not substantially modify the initial elastic response and the first-crack strength of the unreinforced earthen material; in contrast, the fibrous reinforcement greatly improves the residual strength, the ductility and the energy absorption of samples after first cracking.
[1] Houben H, Guillaud H. Traité de construction en terre. Marseille: Parenthèses; 1989. [2] Minke G. Earth construction handbook: the building material earth in modern architecture. Southampton: Wit Press; 2000. [3] Minke G. Building with earth. Design and technology of a sustainable architecture. Basel: Birkhäuser; 2006. [4] Avrami E, Guillaud H, Hardy M, editors. Terra literature review. An overview of research in earthen architecture conservation. Los Angeles: The Getty Conservation Institute; 2008. [5] Berge B. The ecology of building materials. Oxford: Architectural press; 2000. [6] Jiménez Delgado MC, Cañas Guerrero I. Earth building in Spain. Constr Build Mater 2006;20:679–90. [7] Walker P, Keable R, Martin J, Maniatidis V. Rammed earth: design and construction guidelines. Bracknell (UK): BRE Bookshop; 2005. [8] Jiménez Delgado MC, Cañas Guerrero I. The selection of soils for unstabilised earth building: a normative review. Constr Build Mater 2007;21:237–51. [9] Achenza M, Fenu L. On earth stabilization with natural polymers for earth masonry construction. Mater Struct 2006;39:21–7. [10] Binici H, Aksogan O, Bakbak D, Kaplan H, Isik B. Sound insulation of fibre reinforced mud brick walls. Constr Build Mater 2009;23:1035–41. [11] Binici H, Aksogan O, Nuri Bodur M, Akca E, Kapur S. Thermal isolation and mechanical properties of fibre reinforced mud bricks as wall materials. Constr Build Mater 2007;21:901–6. [12] Ghavami K, Toledo Filho RD, Barbosa NP. Behaviour of composite soil reinforced with natural fibres. Cem Concr Compos 1999;21:39–48.
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[13] Bouhicha M, Aouissi F, Kenai S. Performance of composite soil reinforced with barley straw. Cem Concr Compos 2005;27:617–21. [14] Quagliarini E, Lenci S. The influence of natural stabilizers and natural fibres on the mechanical properties of ancient Roman adobe bricks. J Cultural Heritage 2010;11:309–14. [15] Piattoni Q, Quagliarini E, Lenci S. Experimental analysis and modelling of the mechanical behaviour of earthen bricks. Constr Build Mater 2011;25(4): 2067–75. [16] Yetgin S, Çavdar Ö, Cavdar A. The effects of the fiber contents on the mechanic properties of the adobes. Constr Build Mater 2008;22(3):222–7. [17] Galán-Marín C, Rivera-Gómez C, Petric J. Clay-based composite stabilized with natural polymer and fibre. Constr Build Mater 2010;24:1462–8. [18] Binici H, Aksogan O, Shah T. Investigation of fibre reinforced mud brick as a building material. Constr Build Mater 2005;19:313–8. [19] Islam MS, Iwashita K. Seismic response of fiber-reinforced and stabilized adobe structures. In: Proceedings of the Getty Seismic Adobe Project 2006 Colloquium, 11–13 April 2006, Los Angeles, USA; 2006. [20] Segetin M, Jayaraman K. Harakeke reinforcement of soil–cement building materials: manufacturability and properties. Build Env 2007;42:3066–79. [21] Clementi F, Lenci S, Sadowski T. Fracture characteristics of unfired earth. Int J Fract 2008;149:193–8. [22] Lenci S, Piattoni Q, Clementi F, Sadowski T. A mechanical characterization of unfired dry earth: ultimate strength, damage and fracture parameters. In: Proceedings of XIX Aimeta Conference, 14–17 September, Ancona (Italy); 2009. [23] Galán-Marín C, Rivera-Gómez C, Petric J. Effect of animal fibres reinforcement on stabilized earth mechanical properties. J Biobased Mater Bioenergy 2010;4(2):121–8. [24] ASTM Standard D422-63. 2007 Standard test method for particle-size analysis of soils. West Conshohocken (PA): ASTM International; 2007. doi:10.1520/ D0422-63R07..
[25] ASTM Standard D2487-11. Standard practice for classification of soils for engineering purposes (Unified Soil Classification System), ASTM International: West Conshohocken (PA); 2011. doi:10.1520/D2487-11.. [26] 14.7.4 New Mexico Earthen building materials code. Santa Fe, New Mexico (USA): Construction Industries Division of the Regulation and Licensing Department; 2009. [27] Standards New Zealand. NZS 4297:1998. Engineering design of earth buildings. Wellington: Standards New Zealand; 1998. [28] Standards New Zealand. NZS 4298:1998. Materials and workmanship for earth buildings. Wellington: Standards New Zealand; 1998. [29] ASTM Standard E2392/E2392M – 10e1. Standard guide for design of earthen wall building systems. West Conshohocken (PA): ASTM International; 2010. doi:10.1520/E2392_E2392M-10E01. . [30] Compressed earth blocks: materials identification tests and mechanical tests. ENTPE/CRATerre-EAG, Lyon, France; 1998. [31] Gopalaratnam VS, Gettu R. On the characterization of flexural toughness in fiber reinforced concretes. Cem Concr Compos 1995;17:239–54. [32] ASTM C1018-97, 1997 Standard test method for flexural toughness and firstcrack strength of fiber-reinforced concrete (using beam with third-point loading). West Conshohocken (PA): ASTM International; 1997. doi:10.1520/ C1018-97. . [33] Lange DA, Ouyang C, Shah SP. Behavior of cement based matrices reinforced by randomly dispersed microfibers. Adv Cem Based Mater 1996;3(1):20–30. [34] Johnston CD. Fiber-reinforced cements and concretes. Amsterdam: Gordon and Breach Science Publishers; 2001. [35] Tatnall PC. Fiber-reinforced concrete. In: Lamond JF, Pielert JH, editors. Significance of tests and properties of concrete and concrete-making materials STP 169D. Philadelphia: ASTM International; 2006. p. 578–90. [36] Bentur A, Mindess S. Fibre reinforced cementitious composites. London: Taylor and Francis; 2007.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of reinforcing wool fibres on fracture and energy absorption properties of an earthen material F. Aymerich a,⇑, L. Fenu b, P. Meloni c a
Department of Mechanical Engineering, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy Department of Structural Engineering, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy c Department of Chemical Engineering and Materials, University of Cagliari, Piazza d’Armi, 09123 Cagliari, Italy b
a r t i c l e
i n f o
Article history: Received 5 January 2011 Received in revised form 28 July 2011 Accepted 1 August 2011 Available online 28 September 2011 Keywords: Earthen materials Wool fibres Toughness Fracture
a b s t r a c t The study investigated the improvements in strength and crack resistance induced by the introduction of wool fibres in an earthen material. Earthen samples reinforced by wool fibres of various fibre lengths at different fibre weight fractions were tested under flexural loading to examine the structural response of the material in terms of first-crack resistance, post-cracking residual strength and energy absorption capability. It was found that the fibrous reinforcement greatly improved the residual strength, the ductility and the energy absorption of the reinforced material as compared to the unreinforced soil. The results of the study also showed that fibre length had a notable influence on the post-fracture response of the material at large deformation regimes. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction In recent years there has been a revival of interest in earthen construction materials, not only for use in restoration and repair of historic and cultural heritage buildings, but also for use as low-energy and environmentally friendly materials in bio-ecological and sustainable architecture [1–9]. Earthen construction, which was widely diffused in the past, is still common today especially in regions of favourable (i.e. hot/arid and temperate) climate conditions and about one-third to one half of the world’s population are estimated to live in houses constructed of unbaked earth [4,6]. The main advantages of the use of earth as a building material are related to the significant reduction in environmental impact, owing to the use of locally available raw materials with ensuing minimal transportation costs, and to simple and energy-efficient manufacturing processes [1–5,9]. In addition, earth-based materials allow better balance and control of thermal and acoustic indoor climate, as compared to typical industrial building materials, because of their superior performances in terms of humidity absorption/desorption rates, heat storage capacities and sound transmission properties [2–5,10,11]. The main weaknesses of earth-based materials are related to shrinkage occurring during the drying phase (which results in the development of internal or surface cracking), to poor tensile ⇑ Corresponding author. Tel.: +39 070 6755727; fax: +39 070 6755717. E-mail addresses: [email protected] (F. Aymerich), [email protected] (L. Fenu), [email protected] (P. Meloni). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.08.008
and flexural strength and ductility, and to the relatively low resistance to water erosion [1–4]. Additives or stabilizers are typically used to address these issues and in general to improve particular properties of the material for specific applications. A wide range of modifiers, including mineral binders (cement, limen, bitumen), animal and vegetal stabilizers (blood, casein, animal glue, oil, latex), natural fibres (hair, wool, straw, flax, cellulose, sisal) have been in fact used in earthen construction [1–4]. In particular, various research works have shown that hygrometric shrinkage and associated cracking of earth-based materials can be greatly reduced by the introduction of fibres into the mixture [12–17]. The presence of reinforcing fibres typically also improves the tensile and, to a lesser extent, the compressive strength of earthen materials [12,18], although decreases in compressive or tensile strength were observed in a few studies as a consequence of the addition of cut straw reinforcement in the material [4,13,15]. On the other hand, only a very limited number of studies have been carried out to explore the influence of fibrous reinforcement on ductility, fracture resistance, and post-fracture behaviour of earthen materials [19–23]. Increases in material ductility with increasing jute fibre content, even though associated to decreases in compressive strength, were for example observed by Islam and Iwashita [19] in a detailed investigation of the effect of natural fibres on the compressive strength and ductility of adobe blocks. Increasing the length of the reinforcing fibres was also found to be effective for improving ductility of the material, but only up to a critical fibre length value.
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The role played by straw fibres in improving toughness and crack control properties of earth-based materials under flexural loadings has been investigated in a series of papers by Lenci and co-workers [21,22]. Experimental results obtained by bending tests on pre-cracked straw-reinforced earth blocks indicate, in particular, that the resistance to crack propagation significantly improves with crack propagation, as a direct effect of the bridging action of fibres across the crack surfaces. Increases in flexural strength and ductility with the addition of wool fibres to a soil and alginate matrix have been also recently reported by Galán-Marín and co-workers [23]. There is still, however, a lack of experimental data and analyses concerning the effect of fibrous reinforcement on the fracture performance of earthen materials, with special reference to properties controlling crack initiation and propagation, and to energy absorption capabilities at large deformation regimes. This paper reports the results of an experimental study aimed at investigating the modifications in strength, crack resistance and post-fracture behaviour induced by the insertion of reinforcing fibres in an earthen material. Samples made with soil material and reinforced with sheep wool fibres were manufactured using various fibre contents and fibre lengths. The earthen samples were tested under flexural loading to compare the pre- and post-damage mechanical response of the different materials. It is worth remarking that while sheep and dairy farming has always been a significant source of agricultural income in Sardinia (Italy), there is at present only a very limited use in the textile industry of Sardinian sheep’s wool, which could thus deserve consideration as a local, renewable and sustainable resource for use in natural and environmentally-compatible building materials. 2. Materials and methods 2.1. Materials A soil from quaternary sediments alongside the Tirso River near Oristano (Sardinia, Italy), traditionally used in the past to manufacture earth bricks, was used in this study. Fig. 1 shows the granulometric curve of the soil, which was obtained by sieving and hydrometer analyses carried out in accordance with ASTM standard D422 [24]. With reference to ASTM particle size limits [24], the soil may be defined as silt (about 60%), with fine and medium sand (about 35%) and less than 7% of clay size particles. Density and Atterberg limits of the soil are reported in Table 1. The soil may be classified as CL, i.e. inorganic clay with low plasticity and low liquid limit, in the Unified Soil Classification System (USCS) [25]. X-ray diffraction (XRD), used to examine the mineralogical composition of the soil, revealed the presence of dominant quartz, together with other constituents including illite and plagioclase. Grain size and values of Atterberg limits indicate that the soil is suitable for earth construction [1,3], as also suggested by the local traditional use of this resource as building material. The strengthening and toughening effect of wool fibres on the earthen material was examined in this study. Wool fibres obtained from black and white Sardinian sheep were supplied by Edilana (Guspini, Sardinia), a manufacturer of wool products for use in thermal and acoustic building insulation. Wool fibres, which had an average diameter of 35 lm, were cut in various lengths (1, 2 and 3 cm) to investigate the effect of fibre length on the structural properties of the reinforced material. 2.2. Sample preparation Specimens 360 mm long, with a cross section of 75 mm 75 mm and a central notch 25 mm in depth (Fig. 2), were used in this study to investigate the fracture behaviour of wool reinforced soil under bending loads. Notched samples with comparable dimensions were used in previous investigations by Lenci et al. [21,22]. It should be observed, on the other hand, that none of the existing standards or recommendations on the use of earth-based construction materials (see for example [26–30]) specifically define size and geometry requirements for specimens to be tested in flexure. Samples were manufactured using various mix compositions, as summarized in Table 2. The soil, water and fibre fractions were poured in a bucket in the proportions indicated in Table 2, and thoroughly mixed by hand until a mass of
Fig. 1. Grain size distribution curve of soil.
Table 1 Physical properties of soil. Density (g/cm3)
Liquid limit (%)
Plastic limit (%)
Plasticity index (%)
2.63
28
17
11
homogeneous consistency was obtained. The water content of the various mixtures was chosen by preliminary experiments so as to guarantee similar levels of plasticity and workability across the different material systems. The specimens were obtained by casting the mixture into a plastic prismatic mould (Fig. 3a) in five successive layers. Each layer was first levelled out and afterwards compacted with the aid of a tamper to remove voids from the mixture. A single edge notch was introduced at the midspan of the sample by placing a rectangular insert of 2 mm thickness across the width of the mould, as visible in Fig. 3b, before pouring the mixture. All specimens were cured at room temperature (20 °C), first for 48 h inside the mould and then for 28 days after demoulding so as to allow the sample to reach a stable weight. Samples were finally dried up in an oven at 40 °C for two weeks before testing. A typical specimen is shown in Fig. 4. 2.3. Experimental testing Notched specimens were used for evaluation of flexural and fracture properties, with the specific aim of investigating the toughening effect, crack control capabilities and energy absorption properties induced by the presence of reinforcing fibres in the earthen material. Specimens were tested using a three-point loading configuration with a lower span length of 310 mm (Fig. 5) for assessing and comparing the fracture properties of the investigated materials. All tests were performed with the use of a closed-loop servo-electric 5 kN tension/compression machine. Tests were carried out in displacement control at a displacement rate of 1 mm/min. Displacement control was used since it provided stable crack propagation that was required for reliable measurements of energy absorbed during deflection. The applied load and the sample deflection (evaluated in the proximity of the notch on the lower side of the sample, as illustrated in Fig. 5) were recorded continuously for the duration of the test. The load and the displacement were respectively measured using the load cell of the testing machine and a linear variable differential transducer (LVDT). In order to acquire accurate and reliable force–displacement curves, particular care was taken to avoid support disturbances and external deformations (mainly associated to ‘‘seating’’ of the sample on the supports and to local indentation at the load application regions) that may affect significantly the values of the measured displacements [31,32]. To this end, as shown in Fig. 5, the LVDT was mounted on a special frame so as to obtain displacement values unaffected by local indentations of the material induced by the support and loading rollers. All tests were terminated when mid-span deflection reached the value of 16 mm. At least four specimens of each type of mixture described in Table 2 were tested during the study.
3. Results and discussion Representative sets of load–deflection curves of unreinforced and wool-reinforced samples are shown in Figs. 6 and 7, while the appearance of a fractured reinforced sample after test (with a
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Fig. 2. Geometry of samples.
Table 2 Proportion of mixtures.
a
Mixture designation
Water/soil weight ratio
Fibre weight fractiona
Fibre length (cm)
U W2_1 W2_2 W2_3 W3_1 W3_2 W3_3
0.18 0.29 0.29 0.29 0.29 0.29 0.29
– 2% 2% 2% 3% 3% 3%
– 1 2 3 1 2 3
Fig. 4. Reinforced earthen specimen after demoulding.
Fibre weight fraction is evaluated on the total weight of the mixture.
Fig. 5. Schematics of test setup for flexural tests of samples.
Fig. 3. Mould used for preparation of samples (a) and detail of central insert for notch introduction (b).
typical macroscopic crack propagating from the tip of the initial notch) is visible in Fig. 8. The plots of Figs. 6 and 7 show that unreinforced and reinforced samples have a similar initial response, characterized by an approximately linear behaviour between origin and first-crack load, which may be defined as the load at which the force–deflection curve starts exhibiting significant nonlinearity [31,32]. The first-crack point was experimentally observed to correspond with the onset of continuous and visible macrocracking propagating from the notch tip (Fig. 9).
Fig. 6. Typical load–deflection curves for unreinforced and reinforced (2% fibre weight fraction) samples.
Load–deflection curves of unreinforced samples show an abrupt load drop with a drastic loss of residual strength after first cracking, while complete separation of the sample into two pieces, with ensuing total loss of load carrying capability, usually occurs for deflections smaller than approximately 5–6 mm. Reinforced samples, in contrast, exhibit a characteristic strain hardening behaviour after first cracking, with continuously raising load–deflection curves, only followed by a falling load segment for large deflection values.
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Fig. 7. Typical load–deflection curves for unreinforced and reinforced (3% fibre weight fraction) samples.
Fig. 10. Detail of fibre bridging across a macrocrack growing from the notch (W3_3 sample).
Fig. 8. Reinforced sample after test.
Fig. 9. Onset of macrocrack at the notch in unreinforced (left) and reinforced (right) samples.
The plots of Figs. 6 and 7 show that the presence of fibres does not substantially modifies the elastic response and the first-crack flexural strength of the material. A similar behaviour was observed in numerous studies on concretes or cementitious composites reinforced with relatively small fibre fractions [33–35] and may be attributed to the fact that fibres are too widely spaced to affect the pre-crack response of the matrix [33]. It is seen however that fibres enormously improve the post-cracking behaviour of the material in terms of ductility, energy absorption and residual strength over large deformation regimes. The experimental results also suggest that the beneficial action of fibres may only develop when macrocracks are wide enough to activate the typical crack-control mechanisms associated to fibrous reinforcement, such as bridging (which arrest or delay opening of macrocracks, Fig. 10) or debonding and frictional pull-out (which impart toughness and ductility by energy dissipation through fracture and friction at the fibre-matrix interface) [35,36]. Increases in
fibre weight fraction and fibre lengths are thus expected to improve the post-crack performances of earthen samples, especially for large deflections. Longer fibres, in particular, should correspond, for a given fibre weight fraction, to better toughness and residual strength properties at large deflections, as a direct consequence of the higher loads required for fibre debonding and frictional pullout after complete debonding. To confirm this hypothesis, first-crack loads and peak loads were evaluated from the load–deflection curves of the whole set of samples subjected to test (2% or 3% fibre weight fraction, with 1, 2 or 3 cm fibre length). The first-crack load was estimated by considering an offset method with a 0.05 mm offset deflection [31], while the peak load was defined as the maximum load attained during the test up to the maximum deflection of 16 mm. Average values of first-crack loads and peak loads are reported in the graphs of Figs. 11 and 12. Error bars in graphs of Figs. 11 and 12 (and in the graphs of the following Figs. 13–16) indicate plus and minus one standard deviation from the mean. A comparison of the first-crack and peak load values show that while the onset of first cracking is basically unaffected by fibre length, peak loads are significantly increased when the length of fibres is increased from 2 cm to 3 cm. It is worth noting that no evident peak load improvement is on the other hand observed when fibre length is increased from 1 cm to 2 cm, thus suggesting that a minimum critical fibre length should be used to efficiently introduce and exploit potential toughening mechanisms typical of this class of materials. In order to better characterize and compare the post cracking response and the load-carrying capabilities of the investigated materials at various deformation stages, the values of two different performance indicators (namely, the energy absorbed and the residual strength retained by the sample) were evaluated at increasing mid-span displacements. Figs. 13 and 14 show the energy absorbed by W2 (2% fibre weight fraction) and W3 (3% fibre weight fraction) samples at various deflection levels between 1 and 15 mm. The energy absorbed by the samples was calculated as the area lying under the force– displacement curve up to the selected displacement. We may observe that while for small deflections the energy absorbed by samples is roughly independent of fibre length, for large deflections the energy absorption tends to be higher in specimens reinforced with longer fibres. As an example, as visible in Fig. 14 with reference to samples with 3% fibre fraction, the energy
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Fig. 11. Average values of first-crack loads and peak loads for samples reinforced with 2% wool fibres. Error bars indicate plus and minus one standard deviation.
Fig. 12. Average values of first-crack loads and peak loads for samples reinforced with 3% wool fibres. Error bars indicate plus and minus one standard deviation.
Fig. 13. Average values of absorbed energy at various deflection values for samples reinforced with 2% wool fibres.
Fig. 14. Average values of absorbed energy at various deflection values for samples reinforced with 3% wool fibres.
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Fig. 15. Average values of load-carrying capability at various deflection values for samples reinforced with 2% wool fibres.
Fig. 16. Average values of load-carrying capability at various deflection values for samples reinforced with 3% wool fibres.
absorbed by W3_3 samples (3 cm fibres) is, at 15 mm deflection, more than 20% larger than that of W3_1 samples (1 cm fibres) and about 14% larger than that of W3_2 samples (2 cm fibres). These results further highlights how the role exerted by fibre length on the efficiency of the toughening and strengthening mechanisms increases in importance with increasing deflection and macrocrack opening. The progressively greater role of fibre length with increasing deflections is confirmed by the analysis of the strength data illustrated in the graphs of Figs. 15 (specimens with 2% fibre weight ratio) and 16 (specimens with 3% fibre weight ratio). The diagrams, which report the load carrying capability retained by the samples at various mid-span displacements, clearly show that the largest influence of fibre length is again exerted at the largest deflections, with longer fibres always corresponding to higher residual strengths.
2. The beneficial action of fibres is effectively exploited only when macrocracks are sufficiently open to trigger typical strengthening mechanisms such as bridging and frictional fibre pull-out. 3. Fibre length was found to have a significant influence on the postfracture response of the material only for large deflection levels. 4. Further studies are needed to assess validity and significance of the above findings for more realistic full-scale earthen building units.
Acknowledgement The authors wish to thank Ms. Daniela Ducato of Edilana (www.edilana.com) for providing the wool fibres used in this study. References
4. Conclusions The study was aimed at investigating the enhancements in strength and post-fracture performances achievable by the introduction of fibre reinforcement in an earthen material. Notched samples made with a soil material reinforced by wool fibres were prepared with two fibre weight fractions (2% and 3%) and using different fibre lengths (1, 2 and 3 cm). The samples were tested under flexural loading to compare the mechanical response of the various materials in terms of first-crack resistance, post-cracking performance and energy absorption capability over the deflection range investigated. The main conclusions of the study may be summarized as follows: 1. The presence of wool fibres does not substantially modify the initial elastic response and the first-crack strength of the unreinforced earthen material; in contrast, the fibrous reinforcement greatly improves the residual strength, the ductility and the energy absorption of samples after first cracking.
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