Avaliação do estado de fissuração de lajes convencionais utilizando testes modais

Abstract

The study of the dynamic behavior of structures is an alternative to investigate its structural behavior. This study aims to investigate whether modal tests are effective in assessing the state of cracking associated with the load capacity of conventional reinforced concrete slabs (solid and ribbed). Four conventional slabs (two solid and two ribbed) were tested using static and dynamic tests, and had their results studied. All slabs have width and length of 3,00m x 1,35m . The thickness of solid slabs is 0.08 m and the ribbed slabs of 0,26m. To support the slabs were used metal profiles set in concrete blocks, such support simulated isostatic situation, being one pin and one roller support. All four slabs were subjected to charge and discharge cycles with their deflections measured and modal tests were made at the end of each cycle. Firstly, static tests were in solid slabs, it could be measured deflections experienced by the slabs in each load stage. It was observed that during the application of 16 kN load the slabs left their elastic regime and its deflection curves abruptly changed slope. After application of each load stage, it was proceeded with modal tests. The results of the modal parameters for the solid slabs were obtained by decay method for free vibration and through StarModal software. These results were in accordance with the gradual loss of rigidity of the slabs during implementation of charge and discharge cycles, because the natural frequencies were falling. A damage index based on natural frequencies variation was applied. The results showed that only after 8kN stage the slab suffers damage that can be repaired, which was also shown by cracks found during testing. For solid slabs, it was made a slope based analysis of the natural frequency along the decay curves and it was noted that the slope of the fitted straight curves increases as decreases the stiffness of the tested case. Static tests were also made for the ribbed slabs, however, they showed very small linear displacement when subjected to applied loads because of their high stiffness. Only after 28kN stage some deflection curves showed the abrupt change in slope, representing the output of the linear regime. The modal tests were performed in ribbed slabs similarly to that performed for solid slabs. There was no high frequency loss and damage detection rate did not show any damage that could not be repaired, all due to the high rigidity of the slab. Finally, it was concluded that the modal tests have shown a promising alternative to be studied for detection of structural damage, mainly due to the results obtained for solid slabs.