Ingresso de íons cloreto no concreto : estudo da influência dos agregados.

Abstract

Technology advances, especially in concrete and structural softwares for analysis and design, allowed reinforced concrete structures to become slimmer and work closer to their ultimate loads. Therefore, the theme of durability of reinforced concrete structures is fundamental, and should be taken into account already in the design phase, to obtain structures that meet the requirements of service life prescribed in the standards, preventing accidents and avoiding human and material losses. The structures in coastal regions tend to suffer from corrosive processes of the reinforcement, caused by the entry of chloride ions from the sea, through the concrete cover. Therefore, the concrete that protects the reinforcement must be the less porous and less permeable as possible. To guarantee this, the concrete must attend some requirements, such as a low chloride diffusion coefficient. As the entry of chloride ions is a very complex and relatively time-consuming process, simplifications and accelerated tests are necessary. In this study we analyzed the influence that aggregates of different mineralogical and granulometric compositions could have on the diffusion coefficient of concrete, since the interfacial transition zone formed at the paste-aggregate interface is a porous and, so, a preferred path of ions chloride. Coarse aggregates of basaltic and limestone rock, and fine aggregates of natural river sand, crushed granite and crushed basalt were used. The method for obtaining the diffusion coefficients was the proposed by Andrade (1993), being the chloride migration test. After carrying out the characterization steps of the materials used, the tests of resistance to axial compression, water absorption and voids index were carried out, and finally, the migration test. The results obtained show that the fine aggregates exert more influence than the coarse aggregates in the diffusion of chlorides. The mineralogical composition affected the results considerably, since the concretes that used crushed granitic rock as fine agregate, presented higher diffusion coefficients than the others. The coarse aggregates of basalt and limestone achieved similar performances, without considerable differences. Specimens with two different water-cement ratios (w/c) were analyzed, and it was noticed that this parameter is not always the most suitable to predict the durability of a structure, considering that a concrete with a strength of 29.8 MPa and w/c factor of 0.60 obtained a lower diffusion coefficient than another specimen with 44.2 MPa and w/c factor of 0.50, due to changes in the fine aggregate. Finally, correlations were found between the electric current measured in the test and the diffusion coefficient of the samples, contributing to the development of new techniques for analyzing durability-related properties of concrete.