# Determination of Concrete Formwork Removal Time Based on Ultrasound Reflection
## 1 Introduction  
In construction sites, fresh concrete is cast into formworks where it progressively hardens and gains considerable load-bearing capacity. Failures in the formworks during the construction process as well as failures originating from early removal of the formwork when the concrete has not developed sufficient strength to bear its own weight and to allow for the continuation of the construction can be catastrophic [1, 2]. Furthermore, formworks are expensive with their cost being on many occasions of the same order as the concrete mix [3].  
Since concrete should gain a minimum of strength and stiffness, it is obvious that formworks should not be removed before that point. On the other hand, unmolding should not take place unnecessarily late to avoid construction delays and eventually economic consequences. Therefore, innovation in construction points towards time management, safety and reusability of formworks which entails also strong economic benefits. It becomes clear that a methodology to confirm a minimum degree of concrete hardening necessary for the unmolding would benefit construction. The use of a robust and accurate system to monitor concrete in the formworks on construction sites can lead to more economic and safe construction practices. To date, most of the rules and recommendations for the time required for the demolding of formworks are empirical and may not always be accurate [4]. Eurocode 2 [5] mentions that the element should be designed with a partial safety factor of 1.5, which means that at the moment of demolding the structural element should have a loading capacity at least 50% higher than the load that it is subjected to. According to [PERSON] and [PERSON] [6] the formwork should not be removed until the concrete is strong enough to carry its own weight and the imposed construction load. In addition, concreteshould be sufficiently hard so that the surface is not injured during the formwork removal. In the American recommendations [7] concerning the removal of forms and supports, it is mentioned that the minimum time for formwork removal is normally a function of strength gained over the strength required. In case a strength requirement is not applied, the minimum time for column formwork removal is 12 h, while it can be modified if a retarder or accelerator is used [7,8]. It is understood that the removal of formworks is most times empirical while information on the strength may possibly come from material cast in parallel in small samples. However, a deterministic method to investigate the development of the mechanical properties of the cast concrete itself and enable safe and timely removal of formwork is not in place. In addition, the rate of hardening of concrete strongly depends on the environmental factors where the concrete is cast. Therefore, the removal time can vary between construction sites and even in the same construction site at different periods of the year. Monitoring concrete in the formworks gives the advantage of having real-time data about the state of the concrete and the formwork, consequently allowing engineering decisions about the removal time of each specific formwork.  
The objective of the study is the experimental and numerical investigation of an ultrasonic monitoring technique to determine the time for safe removal of a site cylindrical formwork. This technique can be later connected to an acquisition and internet-of-things system to transmit real-time data about the state of the formworks measured by different sensors concerning not only the stiffness of concrete, but in general additional information about the health status of the formwork and its position. This study is based on experimental results on real-size composite construction formworks filled with fresh concrete where the ultrasound reflection as measured from the external surface supplies crucial information about the hardening state, and, thus, about potential demolding of the column.