Técnicas não destrutivas para detecção da reação álcali-agregado em prismas de concreto

Abstract

In recent decades there has been an increasing interest in studying the alkali-aggregate reaction (RAA), as numerous manifestations of this phenomenon have occurred not only in dams and hydraulic structures, but also in foundations of buildings, foundations of transmission towers energy, and even concrete sleepers. Such phenomenon begins to occur without external signs that indicate its presence. In this work we intend to verify if, through two non-destructive techniques, such as ultrasonic wave propagation velocity and resonance frequency, we could anticipate internal damage in the material before any external manifestation. The Brazilian standard, in order to evaluate whether an aggregate is reactive or not, uses prismatic mortar specimens, in the so-called accelerated test that lasts only 30 days. Of course, the kinetics of forced reactions, occurring in a short time, is very different from what occurs in the concrete that is applied in the works. To further approximate the phenomenon of reality, there is also in ABNT NBR 1557-6 the test on concrete prisms with dimensions 7.5 cm x 7.5 cm x 28.5 cm, now in a one-year test. In this research, in addition to prisms with these dimensions, 10 cm x 10 cm x 114 cm concrete bars were used to verify if the dimensions of the prisms have any influence on the expansion due to RAA. By using a large, recognizably reactive aggregate, prisms were molded with the two dimensions cited. Two cement consumptions of 420 kg / m3 and 500 kg / m3 were used. To activate the RAA, sodium hydroxide was incorporated into the blends. Some prisms were confined by the application of a compression stress at their ends. The behavior of the prisms was monitored over time, following the expansion, the ultrasonic wave velocity and the resonance frequency. The results indicate that the cement consumption adopted did not significantly influence the expansion or the resonance frequency, but at the ultrasonic wave velocity, higher consumption led to higher velocity. It has also been found that, after some time, the resonant frequency begins to decrease, indicating that it is capable of accusing internal damage in the concrete due to RAA before external manifestations occur. The ultrasonic wave velocity was not able to detect these damages.