BLOCKS OF CONCRETE REINFORCED WITH NATURAL SISAL FIBRES FOR USE IN MASONRY

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1 BEFIB2012 Fibre reinforced concrete Joaquim Barros et al. (Eds) UM, Guimarães, 2012 BLOCKS OF CONCRETE REINFORCED WITH NATURAL SISAL FIBRES FOR USE IN MASONRY Indara Soto Izquierdo *, Marcio Antonio Ramalho * and Orieta Soto Izquierdo * * EESC-USP, Dep. Structural Eng., School Eng., University of São Paulo São Carlos São Paulo, Brasil indara@sc.usp.br, ramalho@sc.usp.br, orieta@sc.usp.br, web page: Keywords: Sisal fiber, blocks, structural masonry, deformation capacity. Summary: In this work sisal fibre is studied as reinforcement in the cement matrix of concrete blocks. Brazil is currently the largest producer of sisal in the world and the state of Bahia is responsible for 80% of domestic fibres. Although the use of fibres is most popular in the cordage industry, its value can be multiplied if used as reinforcement in composites. The interest in using natural sisal fibres as reinforcement for concrete block manufacturing is associated to its low cost, high availability and low power consumption for its production. In addition to the economic benefits, these fibres improve the mechanical performance of materials, increasing the tensile strength by controlling the opening and propagation of cracks and increasing the ductility, allowing relatively large deformations without loss of integrity. This paper evaluates the incorporation of sisal fibres of 20 mm and 40 mm in length and volume fraction of 0.5 and 1% for structural masonry in concrete blocks, and determines the use of these units to produce prisms and mini-walls. The laboratory tests were carried to characterize the physical properties of the fibre, blocks and mortar, as well as axial compression tests of the units, prisms, and mini-walls. The axial compression test results showed that mini-walls reinforced with fibres obtained values very close to or even superior to those obtained for the mini-walls without fibres, showing better performance than the blocks and prisms. All elements with the addition increased the deformation capacity and ductility afforded by the fibres, observed in the stress/strain curves. The rupture mode of blocks, prisms and mini-walls was characterized by an abrupt and catastrophic fracture, and the reinforced elements kept their composite parts together by the fibres, without losing its continuity, becoming a progressive rupture. 1 INTRODUCTION The Sisal (Agave sisalana, family Agavaceae) plant is native to Mexico, which spread rapidly to other regions of the world such as Africa, Europe and Asia. This plant is resistant to dry weather, intense sun and is cultivated in tropical and subtropical regions. Sisal is a common plant in northeastern Brazil, with almost one million workers directly dependant on the fibre for their livelihoods. There are numerous experiences in Brazil using cemented matrices reinforced with natural fibres for the production of building components, namely tiles, vertical fence panels, water boxes and kitchen sinks. Around the world there are already alternative programs to asbestos-cement by technology transfer, especially when it comes to inexpensive roofing systems [1]. The main feature of structural masonry is it compressive strength. This is the crucial concept to be taken into account when discussing masonry as a construction process for the elaboration of structures. The rupture of blocks in this effort occurs due to rupture by traction. Thus, the role of the existing fibres in concrete blocks would be to allow the transfer of tensile loads, reducing the

2 propagation of cracks, controlling apertures, resulting in a gain in deformation capacity and ductility. 2 EXPERIMENTAL PROGRAME The detailed characterization of materials was performed, as well as axial compression tests of the blocks, prisms and mini-walls with and without added fibre. The testing equipment used belongs to the Structures Laboratory of the School of Engineering of São Carlos (EESC), University of São Paulo (USP). 18 blocks in series were tested, totalling 90, 60 prisms (12 for each series) and 15 mini-walls of 3 by series. Each series differed from each other by the fibre content added to the volume of concrete and by the length. The Table 1 shows the nomenclature adopted for the sample species from each series. Table 1: Series of elements made Sample Species Volume fraction Nomenclature (%) Fibre length (mm) BP - - BP20 0.5% 0,5 20 BP40 0.5% 0,5 40 BP20 1% 1 20 BP40 1% 1 40 BP: Blocks, prisms or mini-walls 3 CHARACTERIZATION OF MATERIALS 3.1 Fibres The fibres are characterized as dry fibres, brushed in a good state of maturation, with brightness and normal resistance, and maximum humidity of 13.5%. The fibres were impregnated with a mineral oil-based treatment that can decrease the high water absorption capacity, protect them against alkaline aggression, reducing impurities and residual dust, see Figure 1. Figure 1: Sisal fibres Figure 2 shows water absorption measures of fibres in pre-established times relative to dry mass. 2

3 Figure 2: Water absorption of sisal fibres over time The maximum absorption values in 24 hours were lower than those obtained by [2] and [1], 193% and 151%, respectively. This fact confirms that the oil treatment of the fibres contributed to a decreased high absorption and porosity that characterizes this type of material. In spite of this, the fibres continue to present significant absorption values, which may affect the degree of adhesion with the cementitious matrix and compromise the workability of the mixture. For this reason it was necessary to double the feeding time and vibrancy of the vibro-press for the production of blocks with fibres. 3.2 Blocks The blocks presented actual dimensions of 140 mm x 190 mm x 390 mm (width, height and length). In manufacturing the blocks with and without addition of fibres, materials, except for the cement and the fibres, were measured by volume. Table 2 shows a summary of the physical properties of five types of concrete blocks. Table 2: Physical characteristics of the blocks Block Type Water Absorption (%) Liquid area (mm 2 ) Dry air density (g/cm 3 ) BE 7, ,11 2,18 BE20-0,5% 10, ,92 2,12 BE40-0,5% 10, ,37 2,08 BE20-1% 10, ,81 2,11 BE40-1% 8, ,12 2,18 According to [3] the water absorption of concrete structural blocks must be less than or equal to 10%. The presence of fibres in the concrete caused a greater absorption of the units, indicating a higher incidence of permeable pores. The fibres also caused the dry air density values to be smaller for the reinforced blocks in comparison to the reference blocks, for the apertures introduced into them, 3

4 acting as air incorporators. All units had very similar liquid area values, since they were produced using the same machine with the same mould. 3.3 Mortar bedding The prisms and mini-walls were made with a single type of (ii) medium-resistance mortar, as specified by [4]. Table 3 shows the compressive strength of mortar. Type of block Table 3: Compressive strength of mortar Number of sample specimens Average resistance (f a ) (MPa) f a /f bm BE 6 6,08 0,54 BE20-0,5% 6 5,93 0,83 BE40-0,5% 6 6,15 1,03 BE20-1% 6 7,02 0,98 BE40-1% 6 6,23 0,97 Overall average resistance 6,28 According to Table 3, the average mortar compressive strength was similar to that specified by [4] for this type of trace (6.5 MPa). The relation between the compressive strength of the mortar and the block without fibres was slightly lower than the limit recommended by many authors, including [5], which establishes values between 0.7 and 1. 4 COMPRESSIVE STRENGTH OF BLOCKS, PRISMS, AND MINI-WALLS. Table 4 presents the compressive strength of the blocks and tested elements at work. The age of testing was 28 days after made the blocks and elements. Type of block Table 4: Compressive strength of blocks, prisms, and mini-walls Average Resist. of Blocks (f bm ) (MPa) Charact. strength (f bk ) (MPa) Average Resist. of Prisms (f p ) (MPa) Average Resist. of mini-walls (f mp ) (MPa) CP 11,26 9,43 5,19 3,08 CP20-0,5% 7,11 6,25 3,49 2,95 CP40-0,5% 6,00 4,81 3,25 2,96 CP20-1% 7,16 6,10 4,37 3,30 BE40-1% 6,43 5,22 3,82 3,07 In the compression test, the blocks with fibres showed a decrease in resistance by an average of 4

5 41% in comparison to the reference blocks, although they reached the required characteristic strength it was 4 MPa. The reference blocks had a characteristic strength higher than laid down. The prism fibres showed better performance than the blocks with fibre, with reduced average resistance of 28% compared to the prisms without fibres. The mini-walls with fibres practically did not show a decrease in compressive strength when compared to the elements without fibres. The average decrease was of 3%, and in at least one case of a wall with fibre, the resistance resulted slightly higher than the elements without fibres. Overall, taking into account the length and content by volume, the composite fibre of 20 mm and 1% addition showed the best performance for blocks and prisms as well as for the mini-walls. During the compression test without reinforcement on the blocks an instant collapse was observed, as shown in Figure 3a. The abrupt break is a consequence of higher compressive strength observed for these blocks, making them more fragile. The blocks reinforced with fibres showed the absence of brittle fracture, as shown in Figure 3b. a b Figure 3: Fragile break of the blocks without the addition of fibres (a); ductile break for the blocks with the addition of fibres (b) In most cases the rupture occurred by the development of vertical cracks along the lateral septa. They started close to the mortar joints, which then spread throughout the height of the element, with a characteristic break of traction in the region near the joint, as shown in Figure 4. Figure 4: Typical Break of the prisms under compression According to [6], due to the fact that the mortar is less rigid than the blocks, it tends to deform more 5

6 than the units. This deformation is eventually impeded by the adhesion between the components, which causes the appearance of tensile stresses in the units transversing along the vertical axis of compression. During the compression tests of the prisms with fibre, a ductile rupture of the elements was also observed, keeping the parts together because of the adhesion between fibres and matrix. However, in the elements without fibres there was an abrupt breaking, characteristic of a brittle behaviour. The major resistance to compression of the blocks that form the reference prisms can also contribute to this type of break. This fact can be seen in Figure 5. a b Figure 5: Fragile break of the prisms without fibre addition (a); ductile break of the prisms with fibre addition (b) As for the cracking state of the mini-walls, vertical cracks of traction that began in the vertical joints were observed, progressing to cutting the blocks and causing them to break. The appearance of this type of cracks can be explained by the presence of vertical joints in the mini-walls. Figure 6 shows this behaviour. Figure 6: Propagation of cracks along the vertical traction 6

7 5 MODULE DEFORMATION OF PRISMS AND MINI-WALLS The modulus of deformation was calculated for the prisms and mini-walls in agreement with the requirements of [7]. According to this text, the module is given by the inclination of the secant line in the stress/strain diagram between 5% and 33% of rupture stress. 5.1 Prisms The modulus of deformation results of these elements are displayed in Table 5. The column Def. 33% has information related to 33% of the rupture stress, and the column Def. Last is the deformation at the time of rupture. Table 5: Module deformation of the prisms with and without added fibre Type of Def. Module Def. Last Def. 33% ( ) prisms (E p ) (MPa) ( ) PR 5283,86 0,28 1,18 PR20-0,5% 3528,03 0,31 2,10 PR40-0,5% 3084,64 0,55 3,93 PR20-1% 4484,49 0,31 2,22 PR40-1% 3916,51 0,32 2,20 Analyzing the results, a reduction can be noted in the rigidity of the prisms with the addition of fibres in comparison with the prisms without fibres. This result was somewhat expected because higher strengths tend to produce a more rigid structure. According to the studies by [8], the same tendency was observed in the composites with the addition of fibres. For a better view of the results, the stress/strain curves were traced for each prism studied. They are organized by type of prism in Figure 7. 7

8 Figure 7: Stress-strain curve of each type of prisms Although there is a reduction in the compressive strength of prisms with the addition of fibre, a gain in the ability to absorb deflections can be seen because most of them had the post-peak portions of the curves. This shows that the role of the fibre improves the composite in terms of ductility and residual strength capacity after the matrix cracks. 5.2 Mini-walls The modulus of deformation results of these elements are presented in Table 6. Table 6: Modulus of deformation of mini-walls with and without added fibre. Type of miniwalls Def. Module (E mp ) (MPa) Def. 33% ( ) Def. Last ( ) MN ,19 1,30 MN 20-0,5% ,23 1,68 MN 40-0,5% ,25 1,71 MN 20-1% ,20 1,47 MN 40-1% ,22 2,01 8

9 The result analysis showed a small decrease in rigidity of the mini-wall with the addition of fibres, in comparison to the mini-walls without fibres. This reduction is equal to approximately 12%, considering the overall average. The stress/strain curves for all tested mini-walls were also mapped. They are organized for each type sample species and are presented in Figure 8. Figure 8: Curve stress/strain of each type of mini-walls The compressive strength of the walls showed no marked difference, with very close values. The deformation capacity gain is seen for elements of blocks with the addition of sisal. The fibres 9

10 maintained the faces of the cracks together, preventing the loss of continuity of the material. According to [9], the presence of fibres affects the start of cracks in the composite failure, limiting their growth. From the stage in which these cracks start to come together, the sum of its effects influences the toughness due to the large deformations experienced by the material. 6 CONCLUSIONS This work carried out a comparative study of the mechanical behaviour of blocks, wedges and miniwalls with and without the addition of sisal fibres by the axial compression tests. As for the physical properties of the fibres, they showed low apparent density and high water absorption. These are in fact common characteristics of natural fibres due to a high incidence of permeable pores. The mini-wall with the addition of fibres showed better performance because the compressive strength values were similar to the reference element, when compared to the blocks and prisms. It is probable that the sisal fibre performed better because higher tensile stresses developed in the units. This situation is explained by two main reasons. The first is that the sample specimen has larger dimensions, submitted to a lower confinement produced by the test apparatus. Furthermore, a higher traction in the blocks can also be explained by the fact that there are lagged vertical joints, which does not occur in the prisms. The fibre length with the best performance was of 20 mm. This length conferred more resistance to the blocks, prisms and mini-walls, when compared to the elements reinforced with fibres of 40 mm. As for the relationship of the fibre content in volume, the blocks with 1% addition of sisal showed better efficiency for the same length, improving the resistance capacity of the composite. Both the mini-walls and prisms showed gains in the deformation capacity for blocks with the addition of sisal, shown in the stress/strain curves. In fact, the failure mode was well characterized in all the sample species tested in this research. As for the units and elements with the addition of fibres, a ductile rupture was obtained. However, there was always an abrupt breaking in the blocks and elements without fibres, characteristic of a brittle behaviour. The fibres carried out a fundamental role after the matrix cracked, due to its low modulus of deformation. This results in greater energy absorption and gives the material a certain capacity to absorb loads after cracking, contributing to the increase in toughness and ductility. The greater social value is the possibility of increased sisal sales in poor communities that depend on production this for their livelihood. This enables a new commercial perspective for the fibres by extending its use to a broader market, namely construction. The natural sisal fibre, besides providing an increased income to those who produce it, is biodegradable and natural, thereby producing little environmental impacts. REFERENCES [1] H. J. SAVASTANO, Materiais à base de cimento reforçado com fibra vegetal: reciclagem de resíduos para a construção de baixo custo. 2000, 7p, 22 p. Tese (Livre-Docência em Engenharia Civil) - Escola Politécnica da Universidade de São Paulo, São Paulo, [2] R. D. TOLEDO FILHO, Materiais compósitos reforçados com fibras naturais: caracterização experimental. 1997, 93p. Tese (Doutorado em Engenharia Civil) - Pontifícia Universidade Católica do Rio de Janeiro, Rio de Janeiro, [3] Associação Brasileira de Normas Técnicas, NBR 6136: blocos vazados de concreto simples para alvenaria requisitos. Rio de Janeiro, [4] BRITISH STANDARDS INSTITUTION. BS 5628: Part 1. Code of practice for use of masonry Structural use of unreinforced masonry. London,

11 [5] G. MOHAMAD, Comportamento mecânico na ruptura de prismas de blocos de concreto p. Dissertação (Mestrado em Engenharia Civil) Universidade Federal de Santa Catarina, Florianópolis, [6] LA ROVERE, Alvenaria Estrutural Notas de aula, curso de pós-graduação em Engenharia Civil., Universidade Federal de Santa Catarina. [7] AMERICAN CONCRETE INSTITUTE. ACI : building code requirements for masonry. structures and specifications for masonry structures. Detroit [8] M. S. PICANÇO, Compósitos cimentícios reforçados com fibras de curauá p. Dissertação (Mestrado em Engenharia Civil) - Pontifícia Universidade Católica do Rio de Janeiro, Rio de Janeiro, [9] H. J. SAVASTANO, Materiais à base de cimento reforçado com fibra vegetal: reciclagem de resíduos para a construção de baixo custo. 2000, 7p, 22 p. Tese (Livre-Docência em Engenharia Civil) - Escola Politécnica da Universidade de São Paulo, São Paulo,