Concrete is vastly used in construction due to its high compressive strength, and its ease of shaping. Formworks are used to render the desired shape to the material during its change from a liquid to a solid state, until the concrete develops the required strength. Removing the formwork early can cause excessive deformations or even failure; however, postponing removal also has a delaying effect in construction with economic consequences. In practice, the removal of the formworks is based on the development of the compressive strength vs. the self-weight and temporary loads related to the construction. This study aims to monitor the hardening of concrete columns by means of the reflection of ultrasonic waves. As concrete hardens, its mechanical impedance increases influencing the reflection that can be received by one-sided non-invasive measurements. By suitably selecting the stiffness of the ultrasonic buffer material, the moment that the concrete obtains a specific stiffness can be safely determined for the first time. Results demonstrate sensitivity of the technique on real scale formworks, as it can pinpoint the moment in which the formwork can be removed, while numerical simulations help in the understanding of the complicated wave field and validate the methodology.
In the present study, a combination of experimental techniques is studied to monitor the state of fresh concrete inside composite formworks. The aim is to acquire information about the stiffness of the concrete for which safe removal of the formworks can be done. Determining early age properties of concrete is important not only for ensuring safety on construction sites but also for presenting economic advantages. Removal of formworks in the appropriate time avoids delays in the construction works and allows the reuse of formworks in an efficient manner and thus reduce the associated cost. Experimental monitoring was complemented by numerical simulations to clearly investigate the ultrasonic propagation in fresh concrete and how the hydration process affects this propagation. The experiments showed promising results in monitoring the increase of stiffness in the concrete inside the formwork through an one-sided measurement. Strain measurements carried out at different heights of a cylindrical formwork gave information about the pressure distribution on the formwork to ensure limit stress values are not reached. In addition, monitoring the strain variation confirmed the trends of increase in the stiffness by giving indications about the shrinkage and thermal expansion of concrete.
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