Concrete in vertical slipforms : A process for determining a suitable concrete composition

Abstract: In the construction industry of today, many projects are getting bigger, higher and more complicated than ever before. The most important material in these projects is concrete and the process of casting has developed a lot during the last century. One specific casting procedure, the slipforming technique, is a well established method throughout the world and it is profitable when there are high demands on the aesthetics, the given time period is short, the structures are high and big or when a large number of standardized structures are to be cast. Slipforming is performed around the clock and due to the generally high work phase of today, slipforming is suitable because it can keep a steady phase without stopping. The material concrete is a subject that is constantly evolving and often new prescriptions, grades and types appear on the market. Experiments about for example alternative binders and different recipes have resulted in concrete being more flexible now than ever before. In general, one cannot find much information about how the concrete mix specifically should be when slipforming. Therefore, there is a need for investigating this further. This master thesis examines important properties and features that a concrete mix should possess to be suitable for slipforming. The choice of material composition and other affecting parameters are put forward to get a clear picture of how to accomplish an eminent slipform process. The study includes thorough investigations regarding both concrete and slipforms which formed an adequate foundation for the further research. Also, an examination on literature of how the two components work together, i.e. how the concrete behaves in a slipform, was carried out to the greatest possible extent. As expected, there was not much information written down about this subject which supported the alleged gap in the current literature. Subsequently, to obtain a deeper understanding of the matter, fifteen interviews were performed. The interviews formed a qualitative research process and the interviewees were consultants specialized in slipforming, project contractors and employees at concrete manufacturers. From the results, it can be concluded that there is no specific concrete mix advocated for slipforming. Important parameters such as stickiness, workability and hardening procedure are key aspects that need to be considered to create an adequate concrete mix. The basic things to keep in mind are that the amount of fine material should be small to avoid a sticky concrete, if crushed gravel is used it should be washed, the alternative binder should be adapted to the CEM I and the recommended alternative binders are PFA and GGBS. Further, the concrete mix is dependent on various other parameters that can be difficult to predict, such as weather and temperature at site, transport distance to site from the concrete factory and how well the site can keep a constant workflow around the clock. Accordingly, there is no standard recipe that is guaranteed to work in every project, but the most appropriate method is to perform tests on the possible mixes to be able to foresee how they will behave in the slipform.