This work is dedicated to the assessment of the structural capacity of a new lightweight block for one-way precast concrete slabs made of short sisal fiber reinforced concrete (SSFRC) containing natural and recycled aggregate. Flexural tests were carried out on SSFRC block samples, as well as on ceramic and EPS blocks used commercially. Slab panels including SSFRC, ceramic and EPS blocks were tested under four point bending configuration to assess the benefits of the new SSFRC block in quasi-real slab conditions. The results of the tests indicated the influence of the sisal fibers and recycled aggregate on the physical and mechanical properties of the concrete and the best structural performance of the SSFRC blocks in relation to commercial solutions. The flexural behavior and cracking of slabs were presented and discussed. Numerical models were also developed to predict the nonlinear behavior of the SSFRC and to simulate the flexural behavior, crack initiation and propagation in the blocks. By adopting this numerical strategy, a parametric study was carried out to simulated new design of blocks and to demonstrate that the load carrying capacity at serviceability limit state conditions can be significantly increased with the thickness of SSFRC block.

Concrete with recycled aggregate is a fragile material under tensile stresses. However, like conventional concrete, it is possible that its contribution is relevant in the design of reinforced concrete elements under tension or bending, even after cracking. The objective of this work is to evaluate the application of the analytical models used to predict the effect of tension stiffening on recycled reinforced concrete. Tests of reinforced concrete under tensile were performed using conventional concrete and concrete containing 25% and 50% replacement of the natural aggregate with recycled aggregate. From the experimental results of reinforced concrete, the contribution of the concrete was isolated and a parametric study was carried out to identify which analytical model in the literature may be more appropriate. The models proposed by Carreira and Chu (1986), Vecchio and Collins (1986) and Hsu and Mo (2010) were evaluated. A numerical analysis, based on the finite element method, was implemented to model the mechanical behavior of the reinforced concrete under tensile using the analytical models already adjusted to concrete with recycled aggregate. The stress distribution in steel and concrete and the cracking mode were evaluated numerically. The results indicate that the parameters used in the analytical models for conventional concrete cannot predict the behavior of concrete with recycled aggregate and need to be modified to obtain a more accurate answer.

Resumo: Embora a utilização de fibras vegetais como reforço de matrizes a base de cimento remonte à década de 70, poucos estudos têm sido realizados para investigação experimental e analítica desses compósitos sob tração direta. Nesse trabalho, foi avaliado o comportamento de argamassas reforçadas com fibras curtas de sisal sob ensaio de tração. O diagrama carga-deslocamento foi expresso em termos de tensão-deformação até a abertura da fissura e, após esse ponto, em termos de tensão-abertura de fissura. Comparados com a matriz, que apresentou ruptura frágil, os compósitos reforçados com 1%, 2% e 3% de fibras de sisal apresentaram propagação da fissura a grandes deslocamentos com manutenção de uma resistência residual. Modelos analíticos baseados na Mecânica da Fratura foram utilizados para determinação da tenacidade de fratura e comprimento característico do material e demonstraram que a adição de fibra aumenta a capacidade de absorção de energia do material. Palavras-Chave:mecânica da fratura;argamassa; fibras vegetaisAbstract:Although the use of vegetables fibers as reinforcement in cement based matrices goes back to the 70s, few studies have been performed for analytical and experimental investigation of these composites in direct tensile. In this study, It was evaluated the tensile behavior of composite mortars reinforced with 1%, 2% and 3% of short sisal fibers. The softening curve was represented by stress-strain diagram until first crack and, after this point, it was represented by stress-crack opening diagram. Compared with the matrix, which showed brittle fracture, the composites showed crack propagation to large displacements with maintenance of residual stress. Analytical models based Fracture Mechanics were used to determine fracture energy and characteristic length of material and the results showed that presence of fibers increment the toughness of cement based mortar under tensile.

RESUMOO concreto é um material muito versátil e de larga utilização na construção civil. A avaliação do seu comportamento não linear físico, porém, ainda é um dos grandes desafios da engenharia estrutural. Neste trabalho foram investigados três métodos de fissuração distribuída do concreto utilizados para modelagem numérica de vigas de concreto armado: fixo, rotacional e multidirecional. Para validação dos métodos foram modeladas numericamente vigas ensaiadas por Bresler e Scordelis (1963) e Álvares (1993) utilizando o programa computacional DIANA. Foi verificado, que para vigas com falhas por flexão, todos os três modelos apresentaram bons resultados com capacidade para prever o caminho de equilíbrio. Para vigas que rompem por cisalhamento, no entanto, não foi possível encontrar soluções aceitáveis com o modelo fixo de fissuração, enquanto que, com o modelo rotacional de fissuração foi obtida grande instabilidade numérica para determinar o caminho de equilíbrio logo após o surgimento da fissura diagonal. Palavras-Chave: viga de concreto armado; modelos de fissuração; método dos elementos finitos. ABSTRACTThe concrete is an outstanding material with vast utilization in the civil construction area. The determination of nonlinear behavior of concrete is still a major challenge in structural engineering. In this study there were investigated three smeared cracking models used for numerical modeling of reinforced concrete beam: fixed, rotational and multidirectional. In order to validate the models, three experimental samples studied by Bresler & Scordelis (1963) and Álvares (1993) were selected and modeled using the software DIANA. The results indicated that the three adopted smeared crack models performed well and where capable of predicting the equilibrium path. However, to beams with shear rupture it was impossible to find acceptable solutions for the fixed model, whereas with the rotational model it was obtained major numerical instability in order to determine the equilibrium path after materializing the diagonal cracking.

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