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

Resumo: This paper reports the results of an investigation on the influence of pre-heating on the mechanical behavior of PVA strain hardening cement-based composites (PVA-SHCC) by using uniaxial tensile, bending and compression tests. Tests were conducted on specimens that were previously heated to temperatures ranging from 90 °C to 250 °C and unheated specimens. The temperatures were selected based on the results of thermal analyses tests carried out to investigate the thermal degradation of the PVA fiber. The results indicated that specimens preheated to 90 °C presented a strain hardening behavior with a pronounced multiple cracking pattern. This response is nearly the same as observed for the unheated specimens. After heating to 110 °C and 145 °C, although the composite still presented a strain hardening behavior the strain capacity was reduced. At 190 °C the strain capacity of the SHCC is even smaller and at 250 °C no further typical strain hardening behavior was observed; the reinforcing fiber failed suddenly after the cracking of the matrix. A similar behavior was observed for the samples submitted to bending loads, indicating that bending test was also efficient to indicate the composite degradation under thermal loads. Compressive strength of the composite has indicated an increase up to 190 °C due to a refinement of the matrix pore structure as indicated by the microstructural studies.

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Autor(a/res): Saulo Rocha Ferreira, Flávio de Andrade Silva, Paulo Roberto Lopes Lima e Romildo Dias Toledo Filho.

Resumo: Several fiber treatments can be applied to mitigate the high water absorption of vegetal fibers. Wetting and drying cycles are usually performed in the industry of paper and cellulose to reduce the volume variation of the natural fibers. This procedure stiffens the polymeric structure of the fiber-cells (process known as hornification) resulting in a higher dimensional stability. The aim of this study is to determine the effect of the hornification on the interface of natural fibers. For this purpose, cycles of wet and drying was applied on Sisal, Curaua and Jute fibers. Fiber pull-out tests were performed in embedment lengths of 25mm. Furthermore, the influence of the hornification in the fibers mechanical (under tensile loading) and microstructural (surface modifications of the fiber and changes in the fiber-cell structure) behavior were investigated. The results indicate changes on the tensile strength and strain capacity of the studied fibers, showing that morphology and chemical composition play an important role on the efficiency rate of hornification.

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Autor(a/res): Alex Neves Junior, Romildo Dias Toledo Filho, Eduardo de Moraes Rego Fairbairn, Jo Dweck

Resumo: Mechanical and porosity related properties of high initial strength sulfate resistant Portland cement (HS SR PC) pastes subjected to early age carbonation curing were investigated. This work presents the characterization through mechanical properties, of carbonated HS SR PC pastes and of non-carbonated references. They were treated at two different times of carbonation (1 h and 24 h), at the best conditions of capturing, previously determined by the authors. The mechanical properties were characterized by compressive strength, tensile strength by diametrical compression, elastic modulus and Poisson’s coefficient. It was found that 1 h of carbonation improved the mechanical properties, while the increase of carbonation time to 24 h, decreased significantly the same properties in relation to the reference specimens. The porosity related properties were characterized by total absorption, absorption by capillarity, gas permeability, and mercury intrusion porosity. It was noticed that the increase of the carbonation time from 1 h to 24 h, increased substantially the absorption properties. Despite the mechanical properties of the 1 h carbonated paste were better than those of the reference, the absorbed water content of the carbonated specimen was slightly higher than that of the reference.

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Autor(a/res): Dweck Jo, Rosangela S. Leonardo, Frank K. Cartledge, Oscar A. Mendoza Reales & Romildo D. Toledo Filho 

Resumo: Gypsum quantitative determination in cements may be done from their thermogravimetric analysis (TG) data, which uses the mass of the water released from its gypsum dehydration during cement analysis, to determinate its content stoichiometrically. Depending on the operational conditions used to mill the gypsum with clinker, its partial dehydration may occur, which leads to unreal results when only unhydrated cement TG data are used. To solve this problem, TG analysis of the 1-h paste of respective cement must be done on respective initial cement mass basis. The method was applied to four different classes of Portland cements, showing that for all of them, the application of the method was needed to avoid erroneous determinations. As mineral composition data show that the cements have other sulfated phases, different from gypsum and its dehydration phases, the total cement SO3 content cannot be used directly to quantify gypsum. However, using the present suggested thermogravimetric method, the original gypsum content can be determined, which also estimates and includes the amount which was consumed to form ettringite during the first hour of hydration.

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