Autor(a/res): Paulo Roberto Lopes Lima, Romildo Dias Toledo Filho, José Mário Feitosa Lima

Resumo: Embora a utilização de fibras vegetais como reforço de matrizes a base de cimento remonte à década de 70, poucos estudos têm sido realizados para investigação experimental e analítica desses compósitos sob tração direta. Nesse trabalho, foi avaliado o comportamento de argamassas compósitas reforçadas com fibras curtas de sisal sob ensaio de tração. O diagrama carga-deslocamento foi expresso em termos de tensão-deformação até a abertura da fissura e, após esse ponto, em termos de tensão-abertura de fissura. Comparado com a matriz, que apresentou ruptura frágil, os compósitos reforçados com 1%, 2% e 3% de fibras de sisal apresentaram propagação da fissura a grandes deslocamentos com manutenção de uma resistência residual. Modelos analíticos baseados na Mecânica da Fratura foram utilizados para determinação da tenacidade de fratura e comprimento característico do material e demonstraram que a adição de fibra aumenta a capacidade de absorção de energia do material.

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Autor(a/res): Viktor Mechtcherine, Michaela Gorges, Christof Schroefl, Alexander Assmann, Wolfgang Brameshuber, António Bettencourt Ribeiro, Daniel Cusson, João Custódio, Eugênia Fonseca da Silva, Kazuo Ichimiya, Shin-ichi Igarashi, Agnieszka Klemm, Konstantin Kovler, Anne Neiry de Mendonça Lopes, Pietro Lura, Van Tuan Nguyen, Hans-Wolf Reinhardt, Romildo Dias Toledo Filho, Jason Weiss, Mateusz Wyrzykowski, Guang Ye e Semion Zhutovsky. 

Resumo: The article presents the results of a round-robin test performed by 13 international research groups (representing fifteen institutions) in the framework of the activities of the RILEM Technical Committee 225-SAP “Applications of Superabsorbent Polymers in Concrete Construction”. Two commercially available SAP materials were used for internal curing of a high-performance, fine-grained concrete in combination with the addition of extra water. The concrete had the same mix composition in all laboratories involved but was composed of local materials. All found a considerable decrease in autogenous shrinkage attributable to internal curing. Also, with regard to the shrinkage-mitigating effect of both particular SAP materials, the results were consistent. This demonstrates that internal curing using SAP is a robust approach, working independently of some variations in the concretes’ raw materials, production process, or measuring technique. Furthermore, the effects of internal curing on other properties of concrete in its fresh and hardened states were investigated. These are consistent as well and expand considerably the existing data basis on properties of concrete materials containing SAP.

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Autor(a/res): C. A. A. Rocha, G. C. Cordeiro, R. D. Toledo Filho

Resumo: The present work aims to study the replacement of Portland cement (PC) by stone cutting waste (SW) and ground waste clay brick (BW) in binary and ternary pastes. Thermogravimetry and differential thermal analysis tests were carried out at various ages in order to investigate the development of the cement hydration reactions in the presence of those wastes. The packing density was calculated in accordance with the Compressible Packing Model to understand the physical effect of those wastes. Compressive strength tests were also performed and the results were related to hydration and packing. Considering the substitution levels studied, the results indicated that the use of SW in the binary mixture accelerated the hydration reactions, and the particles packing density and compressive strength were maintained. The use of BW in the binary mixture caused a small acceleration in the hydration reactions and there was an indication of pozzolanic activity, although the compressive strength was reduced in comparison with the reference paste. In the ternary mixture, the combined effect of both wastes resulted in the maintenance of compressive strength for cement replacement content of 30%.

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Autor(a/res): Margareth Silva Magalhães, Romildo Dias Toledo Filho e Eduardo Moraes Rego Fairbairn.

Resumo: In the present paper, the thermal durability of polyvinyl alcohol fibers (PVA) was studied after fiber samples had been subjected to temperatures ranging from 90°C to 250°C. Residual mechanical properties, such as tensile strength, elastic modulus and elongation at break, and physical properties, such as density were determined. Weibull statistics were used to quantify the degree of variability in fiber strength, at the different temperature. In addition, thermal analysis of PVA fibers were carried out employing thermogravimetry and differential scanning calorimetry up to the temperature of 800°C. SEM analysis of heated and unheated samples had been carried out in order to allow the identification of the changes in the microstructure of the fibers. The degradation process of PVA fibers manifests itself in a significant loss of mass, stiffness and strength of the fibers, which is progressive with increasing temperature. Thermal analysis has shown that the melting point of PVA fibers begins at approximately 200°C and thermal degradation initiates at about 239ºC. However, progressive loss in tensile strength and elastic modulus was observed starting at a temperature as low as 90°C, due to glass transition temperature of PVA fibers at approximately 66°C. At 220°C, the elastic modulus and strength were reduced at about 45% and 52%, respectively, when compared with respective values of unheated samples. With regards to Weibull modulus, the statistical parameter did not exhibit significant influence on temperature for samples heated up to 145°C, which ranged from 23.4 to 28.8. However, samples heated to 220°C showed a sudden reduction in Weibull modulus to 8.6, indicating that a significant change occurred in the populations of fracture inducing flaws at this temperature level, which clearly affect the tensile strength and Weibull modulus.

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