Autor(a/res): BR Moura; GE Oliveira; RD Toledo Filho; FGS Junior.

Resumo: As well known, the fibers can be used as mechanical reinforcement fillers in the composites. In the specific case of vegetable fibers in cementitious matrices, there are some concerns about low adhesion on the matrix, high water absorption by the fiber and low durability in alkaline media, which can be improved by some chemical treatments on the fiber surface. This present work aim to evaluate a single coating of a magnetite layer and a double coating of polyaniline layer and magnetite layer on the mango seed fiber surface. These coating aimed to get better the fiber/matrix adhesion as well as to promote a chemical protection of the fiber. The natural and coated mango seed fibers were characterized by FTIR and TGA, to evaluate the coating process result. The morphology of the composites presents no change using natural or coated fibers. The mechanical properties of the composites containing the fibers, natural and coated, were determined by compression tests. The fibers presence, independent on the surface treatment, got better the all composites mechanical properties. On the other hand, the composite with 0.5% of double coated fibers had similar young’s modulus than natural fiber, associated the chemical protection on the fiber.

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Autor(a/res): G. C. Cordeiro, T. R. Barroso e R. D. Toledo Filho

Resumo: In this study, the feasibility and effectiveness of a controlled laboratory re-calcination process was evaluated in order to mitigate the negative effects of Sugar Cane Bagasse Ash (SCBA) with high carbon content on hydration and fresh properties of concrete. Measurements of particle size distribution, chemical composition, BET specifc surface area, and pozzolanic activity were realized to characterize the as-received and re-processed SCBA. Moreover, the distinct SCBAs were evaluated based on results of isothermal calorimetry and time of setting by Vicat method in cement-SCBA pastes and compressive strength, Young’s modulus, and water absorption in a 35-MPa concrete. The results showed that the re-calcination process decreased the loss on ignition from 20.9% to 2.1% at laboratory calcination thus increasing the silica content of the ash. Re-burnt SCBA provided the control of setting times and the evolution of the compressive strength of concrete changed with the nature of the used ash with a superior behavior being observed for lab-conditioned re-calcination SCBA.

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Autor(a/res): B Rodríguez, J H Quintero, Y P Arias, O A Mendoza-Reales, J C Ochoa-Botero and R D Toledo-Filho

Resumo: This work studies the reinforcing effect of Multi Walled Carbon Nanotubes (MWCNT) on cement pastes. A 0.35% solid concentration of MWCNT in powder was dispersed in deionized water with sodium dodecyl sulfate (cationic surfactant), cetylpyridinium chloride (anionic surfactant) and triton X-100 (amphoteric surfactant) using an ultrasonic tip processor. Three concentrations of each surfactant (1mM, 10mM and 100mM) were tested, and all samples were sonicated until an adequate dispersion degree was obtained. Cement pastes with additions of carbon nanotubes of 0.15% by mass of cement were produced in two steps; first the dispersions of MWCNT were combined with the mixing water using an ultrasonic tip processor to guarantee homogeneity, and then cement was added and mixed until a homogeneous paste was obtained. Direct tensile strength, apparent density and open porosity of the pastes were measured after 7 days of curing. It was found that the MWCNT/surfactants dispersions decrease the mechanical properties of the cement based matrix due to an increased porosity caused by the presence of surfactants.

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Autor(a/res): Cristina Frazão, Joaquim Barros, Romildo Toledo Filho, Saulo Ferreira e Delfina Gonçalves.

Resumo: This research proposes the development of an innovative structural panels based on the use of thin outer layers of Sisal Fiber-Cement Composites (SiFCC) together with a core layer of Polypropylene Fiber-Reinforced Lightweight Concrete (PFRLC).
The influence of sisal fibers was studied in two different ways, short sisal fibers (50 mm) randomly distributed in the matrix, and long unidirectional aligned sisal fibers (700 mm) applied by a cast hand layup technique. Lightweight aggregates and polypropylene fibers were used in the concrete layer forming the panel's core in order to reduce its density and improve its post-cracking tensile strength and energy absorption capacity.
The behavior of the sandwich panels in four-point bending test is described, and the various failure mechanisms are reported. Mechanical properties of both SiFCC and PFRLC were obtained, which were also used in the numerical simulations. Pull-off tests were performed to evaluate the bond strength between the outer SiFCC layers and the core PFRLC. The results revealed that the long sisal fibers were more effective in terms of providing to the panel higher flexural capacity than when using short sisal fibers, long fibers ensured the development of a deflection hardening behavior followed by the formation of multiple cracks, while short sisal fibers promoted a softening response after cracking.

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