Building Concrete Roundabouts in Switzerland

Since 2003, roundabouts in Switzerland have been built with concrete carriageways. Because space is at a premium due to the population density, the diameter of most roundabouts is less than 30 m. Roundabout carriageways with small diameters are subject to particularly high shear stress levels, which is why the choice of surface depends not only on the road traffic itself but also in-creasingly on geometrical considerations and/or the roundabout diameter. Consequently, rounda-bouts in the canton of Zurich with a diameter of less than 28 metres can now only be built with concrete carriageways, irrespective of traffic levels. In principle, the concrete surfaces of frequently used carriageways are laid on a layer of asphalt in order to avoid subsequent pumping of the flags. Structurally speaking, the circular carriageway is separated from the approach/exit areas as they demonstrate different movement behaviour. To avoid vertical misalignment in the transition zone, the flags lie on a concrete bedding (bed plate). The thickness of the concrete surface varies between 26 and 28 cm. Furthermore, it is dependent on the width of the circular carriageway. If the circular carriageway is more than 25 times wider than the thickness, it must be reinforced or a smaller flag size must be favoured by implementing a central longitudinal joint. To ensure the long-term adherence of concrete carriageways, concrete recipes are increasingly adopted in Switzerland in which the aggregate consists of 60 - 70% crushed stones. To improve visibility and contribute to road safety in Switzerland, white markings on concrete sur-faces are either underlaid in black or the concrete surface itself is coloured black. An increasing number of traffic islands and inner rings are now made in one piece from concrete with or without pigmentation, as these are the best means of handling high traffic volumes while providing a low-maintenance solution. Concrete constructions are also gaining ground for design-related reasons, with the variable shaping possibilities of concrete and its suitability for pigmenta-tion. Thanks to the very good results displayed by concrete roundabouts, 20 to 25 roundabouts are now built from concrete in Switzerland every year.