Monitoramento da evolução da resistência à compressão do concreto por meio da técnica de impedância eletromecânica

Abstract

There is a difference of the characteristics of the material received and sampled in relation to those of the material that is effectively applied to the structure, due to the executive process that the concrete is exposed to. This difference results in different hydration processes, directly influencing the final mechanical strength of concrete in the construction. In addition, the characteristic compression strength is usually specified for 28 days of age, causing delays to the work if the strength in project is not reached. The properties of concrete in the hardened state, such as mechanical strength, are closely related to the formation of products by the chemical reaction of cement with water, so monitoring the hydration of the cement paste is a relevant way to predict the behavior of concrete in the hardened state. Thus, techniques that are enable to monitor of the structure in real time, have been widely studied and are excellent options to complement the procedures already adopted for technological control. Among the non-destructive techniques employed for this purpose, is the electro-mechanical impedance, which consists of coupling piezoelectric ceramic (PZT) sensors to the concrete that monitors the variation of impedance signals, caused by change of mechanical parameters of the material, such as stiffness. This dissertation seeks to complement existing studies, evaluating the sensitivity of the electro-mechanical impedance technique in monitoring the development of stiffness and strength of concrete and identification of hydration phases. For this, two concrete compositions were developed: the A with resistance to compression, at 28 days, of 30 MPa and the B, of 60 MPa, each of which was executed with high strength cement and repeated with blast furnace cement. Four cylindrical specimens were molded to compression testing at ages: 1, 3, 7, 28 and 91 days; as well as a void ratio test at 28 days. For each group, the electromechanical impedance technique was applied, at the referred ages, in a specimen containing two piezoelectric sensors incorporated, one positioned vertically and the other horizontally. Finally, the signs of impedance received statistical treatment using metrics proposed in the literature. The technique proved to be sensitive and efficient for both compositions with both cements, representing well the development of the compressive strength of concrete up to 7 days. In this interval, composition A could be analyzed qualitatively and the hydration stages identified. On the other hand, the composition B could only be analyzed quantitatively by the RMSD metric. In advanced ages, the two compositions performed with blast furnace cement showed reliability by correlating the RMSD and M metrics with compressive strength, in the frequency range of 180 to 300 kHz. While those produced with high strength cement, presented metrics with different behavior than expected, at 28 and 91 days, raising some hypotheses such as the incompatibility of the high initial resistance rate with the rate of evolution of the metrics, need for change in frequency range, durability of sensors and need for more tests for ages over 91 days. Regarding positioning, the PZTs positioned horizontally showed better results, this is due to the electromechanical coupling that is compromised with the difficulty of compacting the concrete around the PZT positioned vertically.