Autor(a/res): Caroline Santana Rangel, Mayara Amario, Marco Pepe, Yiming Yao, Barzin Mobasher, Romildo Dias Toledo Filho

Resumo: This study proposes a comprehensive analysis on the structural performance of reinforced Recycled Aggregate Concrete members. Particularly, it summarizes the results of an experimental investigation aimed at analyzing the tension stiffening behavior of normal and high strength class concretes produced with Recycled Concrete Aggregates (RCAs). The mixtures were proportioned in order to achieve 25 and 65 MPa of compressive strength and, moreover, several recycled-to-natural coarse aggregates replacement ratios were considered: 0%, 25% and 50%. The results derived from this type of test furnish a comprehensive analysis on both the steel-to-matrix interaction and the crack formation and propagation on concrete elements as well as distributed cracking mechanisms. Using a finite difference numerical model, the experimental results are used to back-calculate and identify the steel-to-concrete bond slip law. Also, it is an alternative mean of developing the stress-crack-width law for concrete in tension. The results showed that the use of recycled concrete aggregate does not affect the resulting concrete performance and, therefore, the RCAs can be successfully employed, up to the levels analyzed herein, for the production of structural elements made with normal and high strength class concrete mix.

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Autor(a/res): Yemcy Calcina Flores, Guilherme Chagas Cordeiro, Romildo Dias Toledo Filho, Luís Marcelo Tavares

Resumo: This work compares the influence of nano-silica with three different silica-based materials (quartz, ultrafine quartz, and silica fume) on hydration, rheology, porous structure, and compression strength in cement pastes with 2.5% cement replacement. Results of calorimetry showed that the inclusion of the silica fume and of nano-silica resulted in an acceleration of hydration of the pastes. In regard to quartz, no change in comparison to the reference was observed until a median particle size of 0.6 µm was reached. The use of nano-silica and silica fume was responsible for refinement of the porous structure and for an increase in compressive strength of the paste in comparison to the reference, caused by their role on heterogeneous nucleation and pozzolanic reactivity. On the other hand, quartz and ultrafine quartz did not have a marked effect on these properties. The rheological properties were also found to improve with addition of the different silicas.

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Autor(a/res): Marco Antônio da Silva, Marco Pepe, Rodolfo Giacomim Mendes de Andrade, Michèle Schubert Pfeil, Romildo Dias Toledo Filho

Resumo: This study reports the results of a comprehensive experimental campaign aimed at demonstrating the feasibility of using river gravels in substitution of ordinary crushed aggregates for the production of high strength Steel Fiber-River Gravel-Self Compacting Concrete (SFRGSCC). Due to geomorphological reasons, the river gravels represent the most common type of aggregates used in Amazon region for ordinary structural concrete production but only few researches focused on the use of this kind of raw material for the production of high performance cement-based composites. In fact, the river gravels present different intrinsic characteristic in comparison with crushed rocks such as, higher density and elastic modulus, rounded shape with a smoother surface and a more brittle behavior. As a consequence, when embedded in a cement-based matrix they can significantly affect the rheology and mechanical performance of both self-compacting concrete matrices (RGSCC) and Fiber Reinforced Concrete (SFRGSCC).

In this context, the present study firstly analyzes the physical and mechanical properties of the alternative aggregates and then, investigates how the complete replacement of crushed aggregate by river gravel can influence the flowability, segregation potential, yield stress and plastic viscosity of the RGSCC and SFRGSCC in the fresh state as well as the stress-strain behavior under compression, direct tension and bending in the hardened state. The results highlight as the river gravel aggregates shape and surface roughness have relevant effects on the concrete performance as they improve the concrete rheology increasing the concrete flowing and reducing the corresponding yield stress and entrapped air while, on the other hand, reduce the strain capacity of the matrix resulting in a more fragile mechanical response under compression, tension and bending. The addition of steel fiber to the RGSCC resulted being more beneficial than to the reference crushed aggregate SCC. The reinforcement significantly enhanced the RGSCC toughness being this improvement even more pronounced when the samples were submitted to direct tension and bending loads.

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Autor(a/res): Mayara Amario, Caroline Santana Rangel, Marco Pepe, Romildo Dias Toledo Filho

Resumo: This paper analyzes the possibility of applying the Compressible Packing Model (CPM) for the proportion of concrete mixtures produced with Recycled Concrete Aggregates (RCAs). As a matter of fact, the RCAs are composed of natural aggregates and attached mortar and, as a consequence, they generally present a higher porosity in comparison with ordinary natural aggregates. The higher porosity of RCAs can affect the resulting Recycled Aggregate Concretes (RACs) properties and, for this reason, the mix design procedure available in literature for ordinary concrete mixture cannot be applied as such in the case of RACs. In this context, the present work first presents a preliminary study in which the optimal mixing procedure for RACs is investigated and then, a possible extension of the CPM in the case of RACs is analyzed. Several structural RAC mixtures were designed for three strength classes (25, 45 and 65 MPa) by considering the variation of the aggregate replacement from 0 to 100%. Finally, the proposed procedure is experimentally validated by performing mechanical and durability tests on selected mixtures for the three strength classes with a RCAs content up to 60%. The results reported herein demonstrate the applicability of the CPM for recycled concrete mixtures and highlight as the rational use of RCAs lead to produce structural RAC without affecting its mechanical and the durability performance.

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