Autor(a/res): Fabrício de C. Vitorino, Romildo D. Toledo Filho & Jo Dweck

Resumo: Oil well cementing is a vital operation to assure casing stability and zonal isolation for oil and gas exploration. However, some scenarios demand the cemented region to withstand high thermal gradients and imposed deformations, as occurs in the case of oil wells subjected to cyclic steam injection at temperatures up to 250 °C, to reduce oil viscosity and to increase well pressure to facilitate heavy oil recovery. In this paper, the hydration of ductile special cement systems using styrene-butadiene latex (SBR) and carboxylated styrene-butadiene latex (XSBR) addition was studied by conduction calorimetry. The resulting heat flow curves, presented in log–log plots, were used to analyze the influence of those copolymers on the hydration stages of three families of cement pastes of different complexity. The simpler cement systems (SCCS) contained water, oil well Portland cement class G and SBR or XSBR in its composition. In medium complexity systems silica fume was added and in the higher complexity ones (HCCS), superplasticizer as well. The primary objective of adding those copolymers into the Portland cement paste is to obtain higher ductility properties after setting, silica fume to have good thermal stability up to 300 °C, while superplasticizer was added to guarantee good workability. Rheological tests were carried out to evaluate the effect of the copolymers on the composite viscosity. Thermogravimetric analysis of selected SCCS and HCCS samples was performed to quantify the main formed phases up to 24 h of cement hydration. From the obtained results, it was noticed that SBR and XSBR addition substantially affects hydration kinetics at all early age stages. Starting from pre-induction and induction periods, the main observed effect during these stages, was related to the increased viscosity of the pastes, which was higher in XSBR containing pastes, retarding the hydration reactions of respective following stages, when compared to pastes with the same cementitious matrix without copolymer addition.

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Autor(a/res): Joaquim A.O. Barros, Daniele Justo Santos,Cintia Maria Fontes, Jose Mario F. Lima & Romildo Toledo Filho

Resumo: The proper use of renewable or recycled source materials can contribute significantly to reducing the environmental impact of construction industry. In this work, cement-based composites reinforced with natural fibres were developed and their mechanical behaviour was characterised. To ensure the composite sustainability and durability, the ordinary Portland cement matrix was modified by adding metakaolin and the natural aggregate was substituted by 10 and 20% of recycled concrete aggregate. Compression and splitting tensile tests indicated that mechanical strength did not seem to be affected by recycled content. Flat sheets were cast in a self-compacted cement matrix and bending tests were performed to determine the first crack, postpeak strength and cracking behaviour of the composites. The use of short sisal fibre as reinforcement of recycled cement matrices results in a composite with multiple cracking and increment in strength after first crack. The modelling of composites using the finite-element method allowed to determine the tensile stress−strain behaviour of material and to design possible applications of this new sustainable material.

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Autor(a/res): Dimas Alan Strauss Rambo, Yiming Yao, Flávio de Andrade Silva, Romildo Dias Toledo Filho, Barzin Mobasher

Resumo: The work in hand presents results of an experimental and numerical research on the post-heating residual strength of a basalt textile refractory composite submitted to tensile loading. The tensile tests were performed after a preheating process at temperatures ranging from 25 to 1000 °C. The mechanical performance and cracking mechanisms were discussed and compared to that obtained at room temperature. Image analysis by means of digital image correlation method was used to obtain the evolution of crack width which was subsequently correlated with the stress response for all target temperatures. Scanning electron microscopy was used to investigate the damage processes in the fiber–matrix interfaces after exposure to high temperatures. A finite difference model was used to simulate the tension stiffening behavior of TRC (Textile Reinforced Concrete) systems predicting their crack spacing and stress vs. strain responses. The obtained results indicated that due to the coating decomposition the reliability of basalt TRC can only be guaranteed from room temperature to 150 °C.

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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 are used to mitigate the high water absorption of vegetable fibers. Wetting and drying cycles are usually performed in the industry of paper and cellulose to reduce the volume variation of these fibers. This procedure stiffens the polymeric structure of the fiber-cells (this process is known as hornification) resulting in a higher dimensional stability. The aim of this study is to determine the effect of the hornification on the chemical and mechanical behavior of natural fibers and how these properties influence the fiber matrix bond. For this purpose, 5 and 10 cycles of wet and drying were applied to curauá, jute and sisal fibers. Fiber pull-out tests were performed in the embedment length of 25 mm. Direct tensile tests were performed in natural and hornified fibers. Furthermore, X-ray diffraction, thermogravimetry analysis, infra red spectroscopy and nuclear magnetic resonance were used to investigate the influence of the hornification on the chemical properties of the studied fibers. Modifications on the fiber morphology were observed with a scanning electron microscope. The results indicate changes on the tensile strength and strain capacity of the studied fibers, showing that morphology, physical aspects and chemical composition play an important role on the efficiency rate of hornification. Significant improvement in the fiber-matrix interface was observed through the pullout tests. It was concluded that 5 cycles promotes a better performance to curauá and sisal fibers. Only the sisal fibers show improvement on its bond mechanisms after 10 cycles.

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