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Autor(a/res): Thiago Melo Grabois, Guilherme Chagas Cordeiro, Romildo Dias Toledo Filho
Resumo: This paper presents the results of a comprehensive experimental characterization on the fresh and hardened state of self-compacting lightweight concrete (SCLC) reinforced with steel fibers. Two classes of SCLC were produced containing either coarse or coarse and fine lightweight aggregates. Steel fibers were used as reinforcement in fiber volume fraction of 0.5%. Slump flow, inverted slump flow and “V” funnel tests were performed to characterize the self-compacting behavior of the concretes. The mechanical behavior was evaluated by means of compression, tensile and flexural tests. Thermal characterization was performed by specific heat, thermal diffusivity and conductivity tests. Autogenous and drying shrinkage tests were also carried out in the study. The results showed slump flow within 600–700 mm without segregation even for fiber reinforced SCLC mixes. All mixes have shown 28-day compressive strength above 30 MPa and density within 1700–1900 kg/m3. The fiber reinforcement has increased the mechanical properties under direct tensile and bending tests. Adequate thermal insulation properties were verified when compared to normal-weight concrete. Moreover, autogenous shrinkages were around 150 microstrain. Finally, concretes containing coarse and fine lightweight aggregates presented higher drying shrinkage than those only coarse lightweight aggregate.
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Autor(a/res): Joaquim A.O. Barros, Flávio de A. Silva, Romildo D. Toledo Filho
Resumo: This work is dedicated to the assessment of the flexural capacity of thin mortar panels (about 12 mm thick) reinforced with unidirectional continuous layers of sisal fibers. By adopting five stacked layers, each one involved in a mortar layer of about 1 mm thickness, which constitute a fiber volume of 10%, a maximum flexural strength of 30 MPa was obtained at a deflection of around 1/10 of the specimen’s span length. At this deflection the toughness is 46 times higher the elastic energy at crack initiation. Fiber pullout tests were carried out to assess the relevant properties of the fiber–matrix bond behavior. By adopting a multidirectional smeared fixed crack model to simulate the crack initiation and propagation in the cement matrix, considering the fibers like truss elements characterized by their elastic properties, and applying interface finite elements with a local bond-slip equation for modeling the sliding mode of the fiber–matrix interface, the two series of panels tested experimentally were simulated accurately. By adopting this numerical strategy, a parametric study was carried out to demonstrate that the load carrying capacity at serviceability limit state conditions can be significantly increased with the elasticity modulus of the fibers, which can be assured by applying a certain prestress level to the fibers, while the ultimate flexural capacity of these layered panels increases with the fiber–matrix bond strength.
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Autor(a/res): Ocar Aurelio Mendoza Reales, William C. Pearl Jr., Maria D. M. Paiva, Cristiane R. Miranda e Romildo Dias Toledo Filho.
Resumo: This work studies the influence of a commercial dispersion of multi wall carbon nanotubes (MWCNT) on the hydration of a class G cement paste, at room and elevated down-hole temperatures. The MWCNT dispersion was produced with a solids concentration of 3.0 % by mass and an anionic surfactant as dispersing agent. Cement pastes with water-to-cement ratio of 0.45 and additions of solid MWCNT by mass of cement up to 0.50 % were studied. Isothermal calorimetry results showed a clear retardation of the hydration of cement caused by the surfactant contained in the MWCNT dispersion. Nevertheless, thermogravimetric evaluations showed that once the hydration reaction resumed, the retardation effect of the surfactant did not have a negative impact on the amount of hydration products precipitated. It was concluded that the commercial MWCNT dispersion presents a good potential to be applied in oil well cement pastes.
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Autor(a/res): Ocar Aurelio Mendoza Reales, William C. Pearl Jr., Maria D. M. Paiva, Cristiane R. Miranda e Romildo Dias Toledo Filho.
Resumo: This work studies the influence of a commercial dispersion of multi wall carbon nanotubes (MWCNT) on the hydration of a class G cement paste, at room and elevated down-hole temperatures. The MWCNT dispersion was produced with a solids concentration of 3.0 % by mass and an anionic surfactant as dispersing agent. Cement pastes with water-to-cement ratio of 0.45 and additions of solid MWCNT by mass of cement up to 0.50 % were studied. Isothermal calorimetry results showed a clear retardation of the hydration of cement caused by the surfactant contained in the MWCNT dispersion. Nevertheless, thermogravimetric evaluations showed that once the hydration reaction resumed, the retardation effect of the surfactant did not have a negative impact on the amount of hydration products precipitated. It was concluded that the commercial MWCNT dispersion presents a good potential to be applied in oil well cement pastes.
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Autor(a/res): Jo Dweck, Maura Berger Maltez Melchert, Frank K. Cartledge, Rosangela Silva Leonardo e Romildo Dias Toledo Filho.
Resumo: The capture of CO2 and SO2 from industrial gas effluents has been done usually by lime-containing products. For this purpose, cement pastes also can be used, due mainly to their calcium hydroxide content formed during hydration. To select the best cement for this purpose, TG and DTG curves of different Portland cement pastes (types I, II, III and G), prepared with a water-to-cement ratio (W/C) equal to 0.5, were analyzed at different ages, at same operating conditions. The curves were transformed into respective cement calcined and initial mass basis, to have a common and same composition reference basis, for a correct quantitative hydration data comparison. This procedure also shows that there is an unavoidable partial drying effect of the pastes before starting their analysis, which randomly decreases the W/C ratio at which were prepared, which indicates that, when results are compared on respective paste initial mass basis, assuming that the ratio W/C has not changed, possible calculation errors may be done. Type I, II and G analyzed cements have shown similar hydration characteristics as a function of time, while the analyzed type III cement has shown a different hydration behavior, mainly due to its highest Al2O3 and lowest SO3 contents, promoting the formation of hydrated calcium aluminates, by the pozzolanic action of the excess of alumina, consuming Ca(OH)2, which final content at 28 days was the lowest one, among the hydrated cements.
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