Condições de mistura e a permeabilidade de concretos refratários contendo fibras poliméricas

Abstract

Fibras poliméricas têm sido adicionadas a concretos refratários para aumentar sua permeabilidade e diminuir riscos de explosão durante a secagem. Entretanto, a incorporação das fibras pode dificultar a homogeneização do concreto e reduzir sua trabalhabilidade. Trabalhos anteriores dos autores indicam que a magnitude do aumento de permeabilidade gerado pelas fibras está relacionada, em parte, ao seu comprimento médio final, determinado pelas condições de processamento. O objetivo deste trabalho foi estabelecer relações entre aumento de permeabilidade e condições de processamento para concretos contendo fibras com diferentes comprimentos iniciais. Os aspectos reológicos das misturas foram avaliados em um reômetro para concretos desenvolvido pelo Grupo de Engenharia de Microestrutura de Materiais (GEMM). Duas formas de processamento foram investigadas: adição das fibras ao material seco e adição após mistura e homogeneização do concreto. Os valores de permeabilidade foram calculados utilizando-se a equação de Forchheimer. Os resultados mostraram que os métodos de adição das fibras geraram diferentes níveis de cisalhamento e de redução dos comprimentos iniciais. Observou-se, também, que fibras mais longas (12 e 24 mm) consumiram mais energia durante a mistura e apresentaram menores níveis de permeabilidade após tratamento térmico que aquelas mais curtas (3 e 6 mm).

Influence of the addition sequence of PVA-fibers and water on mixing and rheological behavior of mortars

“…The study [4] concluded that depending on the addition method different shear levels during mixing and damage to the fibers were obtained. Damage was lower for shorter fibers and when they were added after the concrete was already mixed [4]. Another investigation also showed better results when the fibers were mixed in the final stage of the processing, after all particulate materials had already been mixed with water [18].…”

“…The agglomerates are destroyed by impact and shear energy provided by the mixing equipment, thus, promoting fines and aggregates dispersion, as well as, the homogenization of the solid particles between them and with water [1][2][3][4][5][6]. The efforts related to the mixing process can be assessed by special rheometers capable to withstand and measure the high levels of torque generated during mixing of highly concentrated suspensions (like mortars and concretes) [1][2][3][4], or by mixers equipped adequately to quantify the energy consumption [5][6][7] during the process, also allowing for the identification of the moment that the material starts to behave like a fluid [2,8], the fluidity point. This point can be determined in the mixing curve as the moment in which the torque levels start to stabilize and the first derivative of the curve tends to zero asymptotically, Figure 1 [2].…”

“…An initial mixing of the fibers with the dry particulate materials also reported a better dispersion of sand and fibers [23]. In refractory castables technology, some authors investigating alumina-based concretes with PP fiber addition studied two types of processing: (i) mixing the fibers with the dry materials and (ii) addition of the fibers into the fluid concrete already mixed [4]. The study [4] concluded that depending on the addition method different shear levels during mixing and damage to the fibers were obtained.…”

“…In refractory castables technology, some authors investigating alumina-based concretes with PP fiber addition studied two types of processing: (i) mixing the fibers with the dry materials and (ii) addition of the fibers into the fluid concrete already mixed [4]. The study [4] concluded that depending on the addition method different shear levels during mixing and damage to the fibers were obtained. Damage was lower for shorter fibers and when they were added after the concrete was already mixed [4].…”