Prediction of the static modulus of elasticity in concrete cylinders using resonant tests
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Concrete is widely used in construction nowadays, and it is crucial that its behaviour conforms to the original design. In every construction site, standard concrete cylinders are cast for quality control. Concrete cylinders are routinely tested to validate their design requirements. Concrete cylinders are traditionally tested for their compressive strength, disregarding the modulus of elasticity. Mostly because the evaluation of the modulus of elasticity is more time consuming and costly than the evaluation of the compressive strength Non-destructive test (NDT) are often used to estimate the properties of concrete. The use of NDT methods allows the continuous monitoring of specimens; the required equipment is portable. Hence NDT methods are affordable and fast than traditional destructive tests. Nevertheless, the elastic properties obtained from non-destructive tests (e.g. dynamic tests) differ from traditional tests (e.g. static tests); Mostly because the strain levels used in the dynamic tests are more than three orders of magnitude lower than in the static tests. This work has two main objectives: 1) To develop a simpler, more affordable and faster methodology for testing the concrete cylinders using a cellphone. 2) To develop a reliable forecasting methodology for the static modulus using the methodology from the first point. To achieve the main objectives, a comprehensive experimental program was undertaken. Over 100 cylinders from four different concrete mixes were tested. Their resonant frequencies were evaluated using the standard microphone of a smartphone. To validate the methodology, the results obtained using the cell microphone were compared with the ones obtained with an accelerometer. Different supports, impact locations, impulse hammers and sensor positions were tested to find the optimal test setup. The signal processing algorithm for extracting the resonant frequencies from the microphone signal is presented. Using this algorithm along with the optimal test setup, it was possible to find the right frequencies in all cases. The methodology developed for forecasting the static modulus is based on the linearity between static and dynamic moduli, and the linear development of concrete stiffness with the logarithm of time. An example of forecasting the static modulus is presented. The example given yielded a 95% confidence interval within the ±15% of the expected value, although the actual estimation error was less than 2%.
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Cristobal Lara (2019). Prediction of the static modulus of elasticity in concrete cylinders using resonant tests. UWSpace. http://hdl.handle.net/10012/15202