Slipforming of Vertical Concrete Structures. Friction between Concrete and Slipform Panel
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Slipforming is a construction method that has been used in several decades for production of concrete structures. It is a wide range of different structures that are slipformed, but typical are vertical structures such as towers, bridge columns and offshore platforms. Slipforming are not only used for straight vertical concrete structures, but also on structures where the geometry of the structure and the wall thickness is changed. Slipforming is normally a continuous working operation (24 hours a day), which require a well-planned supply of materials. Problems that occur during this process needs to be solved instantly. Slipforming is a rather complicated operation compared to other construction techniques. The requirements to the materials, personnel and the execution of the work are therefore accordingly higher. Slipforming of concrete structures has in most cases been carried out successfully with no or only minor supplementary work. However, in some cases, surface damages have occurred during slipforming. Typical surface damages are lifting cracks and vertical lined damages caused by lumps formed on the slipform panel. These problems have during recent years caused discussion and partly also scepticism to slipforming as a reliable construction technique. The Norwegian Public Roads Administration has recommended in Publication 77 that some concrete structures should not be slipformed depending on the environmental impact at the location, geometric degree of difficulties of the concrete structure and the type of concrete. Also in other countries there are scepticism to slipforming as a construction technique. The prime objective of the research program is to improve the understanding of the slipform technique as a construction method in order to ensure high quality concrete structures. The objective is to identify the parameters affecting the net lifting stress (friction) that occur during lifting of the slipform panel. Focus is given to the importance of the concrete properties that will influence the forces that occur between the slipform panel and the concrete. Also any connection between the friction level and the surface damages is investigated. Based on the result it should be possible to define requirements for materials, mix composition and method of execution to ensure that the specified quality in the structure is obtained. The lifting stress can be divided in static lifting stress and sliding lifting stress, where the static lifting stress represents the friction that has to be overcome in order to start sliding and the sliding lifting stress is the minimum friction that occurs during sliding. The difference between the static and sliding lifting stress is caused by the decreasing effective pressure during lifting at the sliding zone and the adhesion that occurs because of no movement of the slipform panel between two lifts. Both static and sliding lifting stress are closely related, but the static lifting stress can be extremely large compared to the sliding lifting stress. The friction law can be used to describe the correlation between the net lifting stress and the effective pressure. This correlation is almost linear and applicable for both the net static and sliding lifting stress. The effective pressure, which represents the pressure between the solid particles and the slipform panel, is the difference between the normal pressure (concrete pressure against the slipform panel) and the pore water pressure. It is primarily the pressure in the pore water that is responsible for most of the variation in the effective pressure during the plastic phase and the transition period, which means that it is mainly the variation in the pore water pressure that controls the level of the lifting stress. The pore water pressure is decreasing slightly in early phase because of the settlement in the concrete. During the elastic phase, the pore water pressure start to decrease faster as an effect of the chemical shrinkage that occurs because of the cement reaction. The pore water pressure development can be characterised by the decrease rate of the pore water pressure and the minimum pore water pressure. The minimum pore water pressure is defined as the pore water pressure at the time of maximum lifting stress. The minimum pore water pressure occurs just before the pressure is increasing at the sliding zone close to the slipform panel. It is primarily the level of the minimum pore water pressure that will decide the maximum level of the static and sliding lifting stress. The pore water pressure decrease rate and the minimum pore water pressure depends on the particle concentration and particle size distribution for the finer particles and also the air content in the concrete. Higher particle concentration and finer particle size distribution will both result in a faster pore water pressure decrease rate and a lower minimum pore water pressure. A higher air content will reduce the effect from the chemical shrinkage because the existing air volume will act as a pressure release volume, resulting in a lower pore water pressure decrease rate and a higher minimum pore water pressure. Also the compaction method will have an impact on the decrease rate of the pore water pressure and the minimum pore water pressure, because the air content will be reduced with prolonged vibration time. Prolonged vibration will in general result in a higher lifting stress, depending on the response on the concrete during vibration. When lightweight aggregate is used in the concrete, the entrapped air in the lightweight aggregate will increase the pore water pressure and result in a lower lifting stress. Porous lightweight aggregate will have larger impact on the pore water pressure than denser lightweight aggregate. Pressure gradients that occur between two concrete layers will affect the decrease rate of the pore water pressure. Water will “flow” from layers with younger concrete without any negative pressure to concrete layers with lower pore water pressure. This will reduce the decrease rate in the concrete layer that receives the water. In later stage the same concrete that supplied the concrete layer below with water will receive water from the concrete layer above. The pressure gradient at the joint (between two concrete layers) will be more even as a result of the water communications between the concrete layers. Evaporation of water from a fresh concrete surface will result in a faster decrease rate and a lower minimum pore water pressure because of the drying process will form menisci near the surface. The water communication is in general good in the concrete in this phase. The time at which the minimum pore water pressure occurs will also have an impact on the minimum pressure level. A shorter period of time from the minimum pore water pressure occur to the time of initial set will result in a relatively higher minimum pore water pressure and a lower lifting stress. The minimum pore water pressure has occurred earlier when water has evaporated from an exposed concrete surface. Also when very rough slipform panel is used, the incipient vacuum between the slipform panel and the concrete is punctured early (collapse of the capillary system at the sliding zone) because of the rough panel surface and will result in a relative low lifting stress. Both the lifting frequency and the lifting height has a considerable effect on the static lifting stress. Lower lifting height or decreased lifting frequency will both result in a lower pore water pressure and a higher static lifting stress. This is probably because the interface zone is disturbed each time the slipform panel is lifted. Less disturbance of the interface will result in a lower minimum pore water pressure. The lifting stress is decreasing during lifting as an effect of the decreasing effective pressure at the sliding zone and the reduced adhesion. The effective pressure at the sliding zone is probably at minimum and the adhesion is completely broken when the lifting stress is stabilized on a minimum level. The sliding lifting stress is also affected of the lifting frequency and the lifting height if not the minimum level is reached during the lift. Surface damages caused by high lifting stress are not demonstrated in the vertical slipform rig. However, similar concrete mix design that has been used in a field project, where surface damages occurred, has been tested in the vertical slipform rig. The concrete mix in this field project was replaced with a new concrete mix, where no or only minor surface damages occurred after the replacement. Both concrete mixes is tested in the vertical slipform rig and the result show a considerable higher static and sliding lifting stress for the concrete mix that was used when surface damages occurred. This indicates that there are a connection between high lifting stress and risk for surface damages. This means also that concrete mixes that obtains high lifting stress in the vertical slipform rig is more exposed to surface damages than concrete mixes that has obtained lower lifting stress.