Determination of Concrete Formwork Removal Time Based on Ultrasound Reflection

Abstract

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.