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Study of the compressive and tensile strenghts of self-compacting concrete with sugarcane bagasse ash

Estudo das resistências à compressão e à tração de concreto autoadensável com cinza do bagaço da cana-de-açúcar

Abstract

In this work, the axial compressive strength at 3, 7 and 28 days of age and the tensile strength at 28 days of age of specimens of two self-compacting concrete (SCC) compositions were evaluated. The difference between them was the presence of the sugarcane bagasse ash (SCBA) in the substitution rate of 10% of the sand mass in the second trace. The traces were developed based on the rheological parameters of the NBR 15823 Brazilian standard. The strengths at 28 days were evaluated by the confidence interval of the Student’s t-test with α of 5%. The results in the fresh state have proved that the rheological performances between the SCC’S are similar. Moreover, the SCC with SCBA obtained equivalent performances in the compressive and tensile strengths at all ages when compared to the SCC without SCBA, with an effective reduction of 87 Kg of sand per cubic meter of concrete.

Keywords:
self-compacting mortar; rheology of concrete; mechanical strength of concrete; sustainability

Resumo

Neste trabalho foram avaliadas as resistências à compressão axial aos 3, 7 e 28 dias de idade e a resistência à tração aos 28 dias de idade, de corpos de prova de duas composições de concreto autoadensável (CAA). A diferença entre ambas, era a presença da cinza do bagaço da cana-de-açúcar (CBC), na taxa de substituição de 10% da massa da areia no segundo traço. Os traços foram desenvolvidos com base nos parâmetros reológicos da norma NBR 15823 da Associação Brasileira de Normas Técnicas. As resistências aos 28 dias foram avaliadas pelo intervalo de confiança do teste t de Student com α de 5%. Os resultados no estado fresco comprovaram que os desempenhos reológicos entre os CAA’s são similares. Destaca-se também que o CAA com CBC obteve desempenhos equivalentes nas resistências à compressão e à tração em todas as idades, quando comparado ao CAA sem CBC, com a redução efetiva de 87 Kg de areia por metro cúbico de concreto.

Palavras-chave:
argamassa autoadensável; reologia do concreto; resistência mecânica do concreto; sustentabilidade

1. Introduction

The self-compacting concrete (SCC) is a category of concrete that is capable of flowing, self-compacting by its own weight, filling molds and passing through obstructions (reinforcements, curves, inserts) with success, quality and reliability. During these events, the mixture must remain homogeneous, in the stages of mixing, transportation, placement and finishing, in order to characterize efficient results in the control of its segregation [1[1] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto auto-adensável. Parte 1: Classificação, controle e aceitação no estado fresco (Self Compacting concrete. Part 1: Classification, control and acceptance in the fresh state). - NBR 15823-1, Rio de Janeiro, 2017., 2[2] GOMES, P. C. C.; BARROS, A. R. de. Métodos de dosagem de concreto autoadensável, São Paulo: Pini, 1ed, 1999, 165 p.].

The methodologies and techniques to obtain SCC are associated with challenges inherent to the rheological study in its fresh state, with a special focus on the control of the fluidity, cohesion and segregation resistance aspects. To this end, these methodologies usually perform preliminary studies of the paste and/or the mortar, and of the granular structure of the aggregates and fine materials [3[3] EUROPEAN FEDERATION FOR SPECIALIST CONSTRUCTION CHEMICALS AND CONCRETE SYSTEMS. EFNARC. The Europe Guidelines for Self-Compacting Concrete - Specfication, Production and Use. In: EFNARC Maio, 2005. Disponível em: <http://www.EFNARC.org/pdf/SCCGuidelinesMay2005.pdf> Acesso em: 25 de set. 2017.
http://www.EFNARC.org/pdf/SCCGuidelinesM...
, 4[4] VANDERLEI, R. D.; PEINADO, H. S.; NAGANO, M. F.; MOLIN FILHO, R. G. D. Uso da cinza do bagaço de cana-de-açúcar como agregado em concretos e argamassas. REEC - Revista eletrônica de Engenharia Civil, v. 8, n. 1, 2014, p. 21-31., 5[5] OKAMURA, H.; OUCHI, M. Self-compacting concrete. Jornal of advance concrete technology, v. 1, n. 1, 2003, p. 5-15., 6[6] GOMES, P. C. C. Optimization and characterization of high-strength self-compacting concrete, Barcelona, 2002, Thesis (doctorate) - School of D’Enginyeria of Construction of Camins, Universitat Politecnica of Catalunya, 139 p.]. However, even though this concrete has special focus of development in its fresh state, with an intense program of rheological analysis, it must also present characteristics appropriate in its hardened state, as all the other concrete classes with structural purposes.

The high consumption of fine aggregates and fine materials, which promote the cohesion with the desired controlled viscosity among other factors, is a determining factor in the SCC composition. In this study, the incorporation of fines was performed adding calcitic limestone filler and sugarcane bagasse ash (SCBA). The SCBA particles contributed to the expected rheological control, where about 73% of the particles had a size between 150 and 250 micrometers, typical of fine sand phases. According to EFNARC [3[3] EUROPEAN FEDERATION FOR SPECIALIST CONSTRUCTION CHEMICALS AND CONCRETE SYSTEMS. EFNARC. The Europe Guidelines for Self-Compacting Concrete - Specfication, Production and Use. In: EFNARC Maio, 2005. Disponível em: <http://www.EFNARC.org/pdf/SCCGuidelinesMay2005.pdf> Acesso em: 25 de set. 2017.
http://www.EFNARC.org/pdf/SCCGuidelinesM...
], the remaining 27%, with dimensions smaller than 150 micrometers, added mass as fine materials.

The SCBA is a by-product from de burning of the sugarcane bagasse in the process of energy cogeneration. In general, it is predominantly composed of silica (SiO2), as well as the sand, and depending on the burning conditions, it may be in the amorphous or crystalline state [7[7] CORDEIRO, G. C.; TOLEDO FILHO, R. D.; FAIRBAIRN, E. M. R. Use of Ultra-Fine Sugar Cane Bagasse Ash as Mineral Admixture for Concrete. ACI Materials Journal, v. 105, n. 5, 2008, p. 487-493., 8[8] CRESPI, M.; MARTINS, Q.; ALMEIDA, S.; BARUD, H.; KOBELNIK, M.; RIBEIRO, C. Characterization and thermal behavior of residues from industrial sugarcane processing. Journal of Thermal Analysis and Calorimetry, v. 106, n. 3, p. 753-757, 2011., 9[9] BAHURUDEEN, A.; SANTHANAM, M. Influence of different processing methods on the pozzolanic performance of sugarcane bagasse ash. Cement and Concrete Composites, v. 56, Feb. 2015, p. 32-45.].

According to CONAB [10[10] COMPANHIA NACIONAL DE ABASTECIMENTO. CONAB. Acompanhamento de safra brasileira: cana-de-açúcar. Segundo levantamento da safra de cana-de-açúcar, abril. Brasília: CONAB 2017. Disponível em: <http://www.conab.gov.br/conteudos.php?a=1253&>. Acesso em: 19 set. 2017.
http://www.conab.gov.br/conteudos.php?a=...
], about 38.7 million tons of sugarcane are expected in the current harvest (2017/2018) only in the state of Paraná. This volume can generate more than 240,000 tons of SCBA, based on the rate of ash generation by burning the bagasse reported in FIESP/CIESP [11[11] FEDERAÇÃO DA INDUSTRIAS DO ESTADO DE SÃO PAULO/ CENTRO DAS INDÚSTRIAS DO ESTADO DE SÃO PAULO. FIESP/CIESP. Ampliação da oferta de energia através da biomassa. São Paulo: 2001. Disponível em: <http://www.fiesp.com.br/indices-pesquisas-e-publicacoes/ampliacao-da-oferta-de-energia-atraves-de-biomassa/>. Acesso em: 19 set. 2017.
http://www.fiesp.com.br/indices-pesquisa...
]. With the same data base and considering the same logic of calculation, it is estimated that the expected volume of 645,000,000 tons of sugarcane in the national harvest in 2017/2018 would be able to generate more than 4,000,000 tons of SCBA, if all the bagasse was used in processes of energy cogeneration. It is emphasized that such volume generated would be able to decelerate the same sand consumption of natural extraction.

In the last years, several researches have advanced significantly in the use of the SCBA in civil construction products. Cordeiro et al. [7[7] CORDEIRO, G. C.; TOLEDO FILHO, R. D.; FAIRBAIRN, E. M. R. Use of Ultra-Fine Sugar Cane Bagasse Ash as Mineral Admixture for Concrete. ACI Materials Journal, v. 105, n. 5, 2008, p. 487-493.] obtained excellent performances in the rheological tests on concretes produced by using the SCBA as a mineral mixture (replacement rate of up to 20%). In this work, a very low change of the plastic viscosity in these materials was observed. In this same line, Crespi et al. [8[8] CRESPI, M.; MARTINS, Q.; ALMEIDA, S.; BARUD, H.; KOBELNIK, M.; RIBEIRO, C. Characterization and thermal behavior of residues from industrial sugarcane processing. Journal of Thermal Analysis and Calorimetry, v. 106, n. 3, p. 753-757, 2011.], Bahurudeen and Santhanam [9[9] BAHURUDEEN, A.; SANTHANAM, M. Influence of different processing methods on the pozzolanic performance of sugarcane bagasse ash. Cement and Concrete Composites, v. 56, Feb. 2015, p. 32-45.] and Castaldelli et al. [12[12] CASTALDELLI, V. N.; MORAES, J. C. B.; AKASAKI, J. L.; MELGES, J. L. P.; MONZÓ, J.; BORRACHERO, M. V.; SORIANO, L.; PAYÁ, J.; TASHIMA, M. M. Study of the binary system fly ash/sugarcane bagasse ash (FA/SCBA) in SiO2/K2O alkali-activated binders. Fuel, v. 174, 15 June 2016, p. 307-316.] verified the existence of pozzolanic properties in the sugarcane bagasse ash, besides the bond properties of the concrete mortars.

Câmara et al. [13[13] CÂMARA, E.; PINTO, R. C. A.; ROCHA, J. C. Setting process on mortars containing sugarcane bagasse ash. Revista IBRACON de Estruturas e Materiais, v. 9, n. 4, aug. 2016, p. 617-642.] showed that it is viable to use the SCBA in mixtures with cement when the purpose is to accelerate the hydration of the first ages with projections of better results of the compressive strengths of mortars over time. Lima et al. [14[14] LIMA, S. A.; SALES, A.; ALMEIDA, F. do C. R.; MORETTI, J. P.; PORTELLA, K. F. Concretos com cinza do bagaço da cana-de-açúcar: avaliação da durabilidade por meio de ensaios de carbonatação e abrasão. Ambiente Construído, v. 11, n. 2, abr./jun. 2011, p. 201-212.] had also emphasized this information for concretes made with the SCBA and cement CP II E 32, which presented better results of compressive strength than the ones without SCBA.

Other authors have evaluated the performance of the mechanical strength of the SCBA in concretes, such as Moretti et al. [15[15] MORETTI, J. P.; SALES, A.; ALMEIDA, F. C. R.; REZENDE, M. A. M.; GROMBONI, P. P. Joint use of construction waste (CW) and sugarcane bagasseash sand (SBAS) in concrete. Construction and Building Materials, v. 113, 15 June 2016, pp.317-323.], who have proved that concretes with SCBA can achieve values of compressive strength very close to concretes without the ash. Sampaio et al. [16[16] SAMPAIO, Z. L. M.; SOUZA, P. A. B. F.; GOUVEIA, B. G. Analysis of the influence of the sugar cane bagasse ashes on mechanical behavior of concrete, v. 7, n. 4, aug. 2014, p. 626-647.] and Bahurudeen et al. [17[17] BAHURUDEEN, A.; KANRAJ, D.; DEV, V.; GOKUL; SANTHANAM, M. Performance evaluation of sugarcane bagasse ash blended cement in concrete. Cement and Concrete Composites, v. 59, may. 2015, p.77-88.] emphasized that the initiative in using the SCBA in concretes is a good destination for the agro-industrial residues, since it promotes improvement of the performance of the mechanical properties of the compound.

The need for high levels of fine materials in the SCC composition aroused the interest in studying the effect of the SCBA as an aggregate. This fact is due to the great availability of material and the targeting of this by-product (residue) in a sustainable way. Therefore, in this work, the SCBA was used as a partial replacement for the sand. Thus, it is projected that the characteristic results of the self-compacting parameters in the fresh state, and subsequently, of the strength properties in the hardened state, are not changed in relation to the concrete without the SCBA. The comparative mechanical evaluations were conducted on the performance results of the axial compressive strength at 3, 7 and 28 days and the tensile strength (diametral compression) at 28 days.

2. Materials and experimental program

The SCC without the SCBA and the SCC with the SCBA, named as SCCSCBA10%, were produced and validated by the experimental methods, specifications and parameters of NBR 15823-1 [1[1] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto auto-adensável. Parte 1: Classificação, controle e aceitação no estado fresco (Self Compacting concrete. Part 1: Classification, control and acceptance in the fresh state). - NBR 15823-1, Rio de Janeiro, 2017.], Gomes and Barros [2[2] GOMES, P. C. C.; BARROS, A. R. de. Métodos de dosagem de concreto autoadensável, São Paulo: Pini, 1ed, 1999, 165 p.] and Okamura and Ouchi [5[5] OKAMURA, H.; OUCHI, M. Self-compacting concrete. Jornal of advance concrete technology, v. 1, n. 1, 2003, p. 5-15.]. For the study of the compressive and tensile strengths at 3, 7 and 28 days age, 8 cylindrical specimens of (Ø 10 cm x 20 cm) were produced for each of the investigations carried out, that is, 64 specimens in total.

2.1 Materials

The first unitary trace was composed only of sand as fine aggregate and the second one had 10% of SCBA in replacement for the sand, in mass. The materials and their respective unitary compositions are shown in Table 1.

Table 1
Unitary traces of the concretes

Both unitary traces (table 1) presented a mortar volume (Vscm) at 70% (0.7) in relation to the mass. This proportion of mortar in the concrete was able to promote an amount of fines from 380 kg/m3 to 600 kg/m3 as foreseen by the EFNARC [3[3] EUROPEAN FEDERATION FOR SPECIALIST CONSTRUCTION CHEMICALS AND CONCRETE SYSTEMS. EFNARC. The Europe Guidelines for Self-Compacting Concrete - Specfication, Production and Use. In: EFNARC Maio, 2005. Disponível em: <http://www.EFNARC.org/pdf/SCCGuidelinesMay2005.pdf> Acesso em: 25 de set. 2017.
http://www.EFNARC.org/pdf/SCCGuidelinesM...
] recommendations.

All the materials used in the production of the SCC are described below:

Cement: Portland CPII E-32, characterized by NBR 13069 [19[19] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto projetado - Determinação dos tempos de pega em pasta de cimento Portland, com ou sem a utilização de aditivo acelerador de pega. - NBR 13069, Rio de Janeiro, 2012.] and NBR 7215 [20[20] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Cimento Portland - Determinação da resistência à compressão (Portland cement - Determination of compressive strength). - NBR 7215, Rio de Janeiro, 1996.]. It had initial setting time at 67 minutes and final setting time at 212 minutes. Its characteristic strengths at 3, 7 and 28 days were of 18 MPa, 21 MPa and 40 MPa, respectively.

Fine aggregate and coarse aggregate: quartz sand and basalt gravel, respectively. Both were from the region of Maringá in the state of Paraná. Their main characteristics, evaluated by the standards NBR NM 30 [21[21] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Agregado miúdo - Determinação da absorção de água. (Fine aggregate - Test method for water absorption). - NBR NM 30, Rio de Janeiro, 2001.], NBR NM 45 [22[22] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Agregados - Determinação da massa unitária e do volume de vazios. (Aggregates - Determination of the unit weight and air-void contents). - NBR NM 45, Rio de Janeiro, 2006.], NBR NM 52 [23[23] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Agregado miúdo - Determinação da massa específica e massa específica aparente. (Fine aggregate - Determination of the bulk specific gravity and apparent specific gravity). - NBR NM 52, Rio de Janeiro, 2009.], NBR NM 53 [24[24] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Agregado graúdo - determinação de massa específica, massa específica aparente e absorção de água. (Coarse aggregate - Determination of the bulk specific gravity, apparent specific gravity and water absorption). - NBR NM 53, Rio de Janeiro, 2009.]; NBR NM 248 [25[25] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Agregados - Determinação da composição granulométrica. (Aggregates - Sieve analysis of fine and coarse aggregates). - NBR NM 248, Rio de Janeiro, 2003.] and NBR 7211 [26[26] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Agregados para concreto - Especificação (Aggregates for concrete - Specification). - NBR 7211, Rio de Janeiro, 2009.] are presented in Table 2.

Table 2
Sand and gravel characterizations

Superplasticizer additive (sp): Glenium 51 from the manufacturer Basf, which was liquid, white in color and cloudy in appearance. It is classified as a third generation additive for concrete, has the polycarboxylic ether as reference and has also total water solubility. The sp had a solid content of 28.5% to 31.5% of the total mass and a pH between 5 and 7, for a viscosity around 1067 g/cm3.

Limestone filler: calcitic origin and produced by the manufacturer Cazanga. It is composed with the presence at least 51.8 % of CaO, 37% of Ca, 1% of MgO and 0.63% of Mg. More than 94% of its particles are smaller than 0.045 mm (45 µm). The water used was from the water supply system of the city of Maringá, which was in accordance with the requirements of NBR 15900 [27[27] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Água para amassamento do concreto - requisitos. (Water for kneading the concrete - requirements). - NBR 15900, Rio de Janeiro, 2009.].

SCBA: from Iguatemi Sugarcane Mill (Maringá, Paraná, Brazil) of Usaçúcar Group. It had a specific mass of 2640 Kg/m³ and specific area of 5356 m²/Kg [18[18] MOLIN FILHO, R. G. D. Concreto auto-adensável com cinza do bagaço da cana-de-açúcar, Maringá, 2012, Dissertação (Mestrado), Departamento de Engenharia Civil, Universidade Estadual de Maringá, 164 p.]. The highest proportion of its particles, around 73%, had a size between 150 and 250 micrometers [28[28] MOISÉS, M. P.; SILVA, C. T. P. DA; MENEGUIN, J. G.; GIROTTO, E. M.; RADOVANOVIC, E. Synthesis of zeolite NaA from sugarcane bagasse ash. Materials Letters, v. 108, 2013, p. 243-246.]. It is important to emphasize that the SCBA was used in natura, being only sieved in the mesh 0.595mm (#30), in order to remove the organic impurities (Figure 1).

Figure 1
Sugarcane bagasse ash (SCBA) sample

Table 3 shows the SCBA composition studied by X-Ray fluorescence (XRF).

Table 3
X-Ray fluorescence (XRF) results of SCBA composition

In Table 3, it is possible to observe the predominant presence of silicon dioxide (SiO2)25. Figure 2 presents the X-Ray Diffraction (XRD) patterns.

Figure 2
X-Ray diffraction

The diffraction signals are identified as quartz crystalline phases (ICDS 07-346 standard). Finally, it is highlighted that the solid materials were dry and/or with moisture controlled.

2.2 Method and self-compacting parameters

The preparation of the SCC’s was performed as described by Molin Filho [18[18] MOLIN FILHO, R. G. D. Concreto auto-adensável com cinza do bagaço da cana-de-açúcar, Maringá, 2012, Dissertação (Mestrado), Departamento de Engenharia Civil, Universidade Estadual de Maringá, 164 p.]. The self-compacting parameters were proved by tests of self-compactibility recommended by NBR 15823-1 [1[1] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto auto-adensável. Parte 1: Classificação, controle e aceitação no estado fresco (Self Compacting concrete. Part 1: Classification, control and acceptance in the fresh state). - NBR 15823-1, Rio de Janeiro, 2017.] (Slump-flow, J-Ring, V-Funnel and L-Box Tests) and by Gomes and Barros [2[2] GOMES, P. C. C.; BARROS, A. R. de. Métodos de dosagem de concreto autoadensável, São Paulo: Pini, 1ed, 1999, 165 p.] the segregation resistance test by U-Pipe. It is important to emphasize that the method described by NBR 15823-6 [29[29] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 15823-6: Concreto auto-adensável. Parte 6: Determinação da resistência à segregação - Métodos da coluna de segregação e da peneira. Rio de Janeiro, 2017.] for determination of the segregation resistance by the segregation column and the sieve methods was replaced by the segregation analysis of Gomes U-Pipe method [6[6] GOMES, P. C. C. Optimization and characterization of high-strength self-compacting concrete, Barcelona, 2002, Thesis (doctorate) - School of D’Enginyeria of Construction of Camins, Universitat Politecnica of Catalunya, 139 p.], which presents the same demand and precision level. The classification parameters are shown in Table 4.

Table 4
Range of self-compacting parameters

It is highlighted that the main acceptance criterion of the respective parameters was the recommendation of NBR 15823-1 [1[1] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto auto-adensável. Parte 1: Classificação, controle e aceitação no estado fresco (Self Compacting concrete. Part 1: Classification, control and acceptance in the fresh state). - NBR 15823-1, Rio de Janeiro, 2017.]. In the cases there were specifications of the standard, the criteria of Gomes and Barros [2[2] GOMES, P. C. C.; BARROS, A. R. de. Métodos de dosagem de concreto autoadensável, São Paulo: Pini, 1ed, 1999, 165 p.] were only considered as acceptance criteria if there was disapproval by the standard.

2.3 Scheme for evaluations of the compressive and tensile strengths

For the study of the compressive strength at 3, 7 and 28 days age, 8 cylindrical specimens (Ø 10 cm x 20 cm) were produced for each of the investigations carried out, that is, a total of 48 specimens for the two traces. For the tensile tests, 8 specimens of each SCC type were also produced, totaling 16 specimens in this type of test. The failures schemes in the 100-ton hydraulic press can be seen in Figure 3.

Figure 3
Specimens in failure position

Panel “a” of Figure 3 presents a schematic image of the rupture by axial compression, for a compressive strength analysis, while panel “b” presents the rupture scheme by diametral compression for a tensile strength analysis. The moldings of the specimens did not use any energy of self-compactibility and followed the other specifications of the NBR 5738 [34[34] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto - Procedimento para moldagem e cura de corpos-de-prova (Concrete - Proceedings for molding and curing of specimens). - NBR 5738, Rio de Janeiro; 2016.]. The test recommended by NBR 5739 [35[35] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto - Ensaio de compressão de corpos-de-prova cilíndricos (Concrete - Compression tests in cylindrical specimens). - NBR 5739, Rio de Janeiro, 2007.] was performed specifically for the analysis of the axial compressive strength. For the analysis of the tensile strength by diametral compression, the test recommended by NBR 7222 [36[36] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Argamassa e concreto. Determinação da resistência à tração por compressão diametral de corpos de prova cilíndricos (Mortar and concrete - Determination of the tension stregth of cylindrical specimens submitted to diametrical compression). - NBR 7222, Rio de Janeiro, 2011.] was carried out, which was later evaluated by NBR 6118 [37[37] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Projeto de estruturas de concreto - Procedimento (Design of structural concrete - Procedure). - NBR 6118, Rio de Janeiro, 2014.]. At the end, both concretes were classified by NBR 8953 [38[38] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto para fins estruturais- Classificação por grupos de resistência (Concrete for structural use - Strength classification - Classification Descriptors: Concrete. Classification). - NBR 8953, Rio de Janeiro, 2015.].

3. Results and discussions

This topic consists of two stages. At first, a synthesis of the self-compacting tests is presented, and after, the results of the mechanical tests of the compressive and tensile strengths.

3.1 Obtaining self-compacting traces

The self-compacting properties of the concretes studied were evaluated by verifying the rheological characteristics of fluidity, cohesion and segregation resistance in their fresh state. The results achieved are shown in Table 5.

Table 5
Results of the self-compactibilty tests

It was verified, by evaluating the results presented in Table 5, that both concretes received technical approvals in all the tests of self-compactibility of NBR 15823-1 [1[1] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto auto-adensável. Parte 1: Classificação, controle e aceitação no estado fresco (Self Compacting concrete. Part 1: Classification, control and acceptance in the fresh state). - NBR 15823-1, Rio de Janeiro, 2017.]. Although the concrete without SCBA has not fulfilled the J-Ring test according to the parameters of NBR 15823-1 [1[1] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto auto-adensável. Parte 1: Classificação, controle e aceitação no estado fresco (Self Compacting concrete. Part 1: Classification, control and acceptance in the fresh state). - NBR 15823-1, Rio de Janeiro, 2017.], it had partial approval by the auxiliary parameter of J-Ring, the “Blocking step”, recommended by Gomes and Barros [2[2] GOMES, P. C. C.; BARROS, A. R. de. Métodos de dosagem de concreto autoadensável, São Paulo: Pini, 1ed, 1999, 165 p.]. Figure 4 shows illustrations of the Slump-flow test of the two concretes.

Figure 4
Illustrations of the slump-flow tests

It was considered at this moment that both concretes studied fulfilled the requirements of self-compactibility demanded for the category of SCC. Therefore, under these conditions, the specimens were produced for the study of the mechanical properties. The materials consumption for both concretes are demonstrated in Table 6.

Table 6
Materials consumption for 1 m³ of concrete

In Table 6, it is highlighted that the consumption ranges of both concretes components meet the specifications of EFNARC [3[3] EUROPEAN FEDERATION FOR SPECIALIST CONSTRUCTION CHEMICALS AND CONCRETE SYSTEMS. EFNARC. The Europe Guidelines for Self-Compacting Concrete - Specfication, Production and Use. In: EFNARC Maio, 2005. Disponível em: <http://www.EFNARC.org/pdf/SCCGuidelinesMay2005.pdf> Acesso em: 25 de set. 2017.
http://www.EFNARC.org/pdf/SCCGuidelinesM...
], including the cement consumption very close of the expected average ranges of 350 Kg to 450 Kg per cubic meter of SCC. It is also observed, in the same Table, the evident effective reduction of 87.63 kg/m3 of sand with the direct use of 79.30 Kg of SCBA in the concrete trace SCCSCBA10%. Considering also the analysis of the fresh state, it was possible to affirm, by the parameters of fluidity and flow, apparent plastic viscosity, passing ability and segregation resistance, that both the SCCSCBA10% and the SCC without SCBA present the same technical recommendations of application in accordance with the criteria of NBR 15823-1 [1[1] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto auto-adensável. Parte 1: Classificação, controle e aceitação no estado fresco (Self Compacting concrete. Part 1: Classification, control and acceptance in the fresh state). - NBR 15823-1, Rio de Janeiro, 2017.]. By the annexes of this standard, it is possible to affirm that both are appropriate to be applied in most constructions works, such as walls, beams, pillars and others.

3.2 Evaluations of the compressive and tensile strength

The results of the compressive strengths are presented in Table 7. These are the average values of the specimens of each concrete trace, as well as the respective calculations of the characteristic compressive strength values (fck).

Table 7
Axial compressive strengths at 3, 7 and 28 days

At the age of 3 and 7 days, a greater difference, about 8%, in the results of the characteristic compressive strength (fck) was observed, considering the hardening stages. However, only a small difference of 3 % in the characteristic axial compressive strength at 28 days was observed, which is the classificatory stage of NBR 8953 [39[38] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto para fins estruturais- Classificação por grupos de resistência (Concrete for structural use - Strength classification - Classification Descriptors: Concrete. Classification). - NBR 8953, Rio de Janeiro, 2015.].

The tests results of the axial compressive strength for both concretes placed them in the same level of mechanical resistance by the classification of NBR 8953 [39[38] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto para fins estruturais- Classificação por grupos de resistência (Concrete for structural use - Strength classification - Classification Descriptors: Concrete. Classification). - NBR 8953, Rio de Janeiro, 2015.]. Therefore, both of them belong to group I and to class C35 of strength. It is also worth emphasizing, by the Student’s t-test with significance level α of 5%, that the average values of the compression failures have confidence intervals with a lower and upper limit of 37.47 MPa and 39.73 MPa (error of ± 1.13 MPa), respectively, for the SCC, and of 40.38 MPa to 39.22 MPa (error of ± 0.58) for the SCCSCBA10%. Consequently, it is possible to reproduce again the two compositions of SCC with a 95% confidence level with values capable of providing resistance results within group I for the class C35.

The results obtained in the characteristic axial compressive strength at 28 days, of 39.90 MPa for the SCCSCBA10%, are similar to the 3 traces obtained by the modification of the SCBA preparation performed by Sampaio et al. [16[16] SAMPAIO, Z. L. M.; SOUZA, P. A. B. F.; GOUVEIA, B. G. Analysis of the influence of the sugar cane bagasse ashes on mechanical behavior of concrete, v. 7, n. 4, aug. 2014, p. 626-647.], which were of 38.6 MPa, 39.19 MPa and 38.64 MPa for the same replacement rate. Already, Lima et al. [14[14] LIMA, S. A.; SALES, A.; ALMEIDA, F. do C. R.; MORETTI, J. P.; PORTELLA, K. F. Concretos com cinza do bagaço da cana-de-açúcar: avaliação da durabilidade por meio de ensaios de carbonatação e abrasão. Ambiente Construído, v. 11, n. 2, abr./jun. 2011, p. 201-212.] obtained 27.98 MPa and 28.72 MPa for the concrete with 30% and 50%, respectively, of replacement of sand by SCBA.

The results of the tensile strength at 28 days are shown in Table 8, where the average values of the specimens of each concrete trace and their respective characteristic values are presented.

Table 8
Tensile strength by diametral compression at 28 days

In order to analyze the tensile strength, Table 9 was elaborated by the determination of the characteristic compressive strength (fck) and the support of NBR 6118 [37[37] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Projeto de estruturas de concreto - Procedimento (Design of structural concrete - Procedure). - NBR 6118, Rio de Janeiro, 2014.].

Table 9
Tensile strength analysis

In general, the characteristic tensile strenght values (fctk,sp), presented in Table 9, are adjusted between the characteristic average and minimum values proposed by the calculation of the characteristic compressive strength (fck) by NBR 6118 [37[37] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Projeto de estruturas de concreto - Procedimento (Design of structural concrete - Procedure). - NBR 6118, Rio de Janeiro, 2014.], with a minimum variation of 2% between the SCC’s developed. It is important to highlight, by the Student’s t-test with significance level α of 5%, that the average values of the tensile failures have confidence intervals with a lower and upper limits of 2.54 MPa and 3.56 Mpa (error of ± 0.51 MPa), respectively, for the SCC, and of 2.47 MPa to 3.37 MPa (error of ± 0.45 MPa) for the SCCSCBA10%. Therefore, it is possible to reproduce the two compositions of SCC with a 95% confidence level with values capable of providing desirable results between fct,m and fctk,inf. Figure 5 shows two SCC specimens ruptured by the diametral compression test.

Figure 5
Specimens ruptured by diametral compression

It is observed in Figure 5 that there are good coverings of the mortars in relation to the gravels in both specimens. Moreover, there is a homogeneous distribution of the coarse aggregates along the vertical and radial extension, which is a typical and necessary characteristic for SCC’s.

4. Conclusions

This work obtained two SCC traces, with and without SCBA, for the study of the mechanical performances of compressive and tensile strengths. In particular, the highlights were:

  1. The rheological parameters of fluidity and flow, apparent plastic viscosity, passing ability and segregation resistance, obtained by the SCC without SCBA and the SCCSCBA10, showed that both of them achieved the previous recommendation to be used in several structural applications following the standards of NBR 15823-1 annexes.[1[1] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto auto-adensável. Parte 1: Classificação, controle e aceitação no estado fresco (Self Compacting concrete. Part 1: Classification, control and acceptance in the fresh state). - NBR 15823-1, Rio de Janeiro, 2017.];

  2. In general, it was possible to classify, by NBR 8953 [38[38] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. Concreto para fins estruturais- Classificação por grupos de resistência (Concrete for structural use - Strength classification - Classification Descriptors: Concrete. Classification). - NBR 8953, Rio de Janeiro, 2015.], both SCC’s as belonging to group I and to class C35 of strength, with a 95% confidence level of reproducibility;

  3. It is emphasized that the SCCSCBA10% maintained the same levels of materials consumption, which are in accordance with the values ranges of this concrete technology and also contributed to the reduction of 87 kg/m³ in the effective consumption of sand;

  4. In a direct evaluation on the SCC performance in the fresh state in relation to the SCCSCBA10%, it is noted that the 10% replacement rate of sand by SCBA, in mass, do not change the rheological properties for the SCC’s in the aspects of fluidity, cohesion and consistency;

  5. Still performing a direct comparison, the results indicated that it is possible to use the SCBA in the SCC production with no significant variations in the mechanical properties of compressive strength at 3, 7 and 28 days of age and of tensile strength at 28 days.

Finally, it is worth noting that this paper considered as important the advances in the development of new materials for the civil construction industry, which directly reduce the extraction of natural resources in favor of the respect for the social and environmental values.

5. Acknowledgements

To the Special Concretes Laboratory, the Construction Materials Laboratory and the Soil Mechanics Laboratory of the Civil Engineering Department of the State University of Maringá (UEM) in the state of Paraná, for the incentive and support. To Basf do Brasil for the donation of the superplasticizer. To CAPES for the financial support given to researchers. To Usaçúcar Group for all the support and donation of the SCBA.

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Publication Dates

  • Publication in this collection
    23 Sept 2019
  • Date of issue
    Jul-Aug 2019

History

  • Received
    19 Dec 2017
  • Accepted
    23 Oct 2018
  • Published
    08 Aug 2019
IBRACON - Instituto Brasileiro do Concreto Instituto Brasileiro do Concreto (IBRACON), Av. Queiroz Filho, nº 1700 sala 407/408 Torre D, Villa Lobos Office Park, CEP 05319-000, São Paulo, SP - Brasil, Tel. (55 11) 3735-0202, Fax: (55 11) 3733-2190 - São Paulo - SP - Brazil
E-mail: arlene@ibracon.org.br