Abstract:
The invention provides a concrete structure of which at least a part has been formed by slip forming with panels inside the slip form, the panels facing the slip form.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to concrete structures. More specifically, the invention relates to slip formed concrete structures, particularly structures made of high strength concrete. 
       BACKGROUND OF THE INVENTION AND PRIOR ART 
       [0002]    Currently, many concrete structures, such as concrete shafts, are typically formed by slip forming, alternatively termed slip casting. Compared to forming with fixed forms, slip forming is very favorable, particularly from an economical point of view, since the extent of the work is greatly reduced. However, the surface of a slip formed concrete structure includes irregularities, particularly when high strength, abrasion resistant concrete qualities are used. The result is reduced erosion and abrasion resistance, reduced service life and reduced surface quality, compared to a structure having a smooth surface, all of which has technical and economical consequences. Repair of irregularities or eroded or abraded surface is often very expensive and the quality is still reduced. Forming with fixed forms for all or a part of a concrete structure is often very expensive. 
         [0003]    Structures used in the sea in areas infested by drifting surface ice have so far not been protected in the zone abraded by ice by a slip formed concrete abrasion allowance, but with a protective steel structure, due to the above mentioned technical problems. For concrete structures such steel protection is very expensive, requiring extensive scaffolding and additional work at high elevation, and may not be a good technical solution since the integrity has been questionable. Protection using concrete has so far not been possible for the desired reliable, long-lasting, affordable and simple solutions sought for by the industry. 
         [0004]    The objective of the invention is to provide a concrete structure and a method of building said structure, providing improvements with respect to the above mentioned problems and disadvantages. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention provides a concrete structure, distinctive in that at least a part of the structure has been formed by slip forming with panels inside the slip form, the panels facing the slip form. 
         [0006]    Preferably the concrete structure is for use offshore in ice-infested areas, the structure has an increased thickness as an abrasion allowance in a zone abraded by ice drifting on the sea, and the abrasion allowance has been formed by slip forming with panels inside the slip form, the panels facing the slip form. 
         [0007]    The invention also provides a method of building a concrete structure, distinctive in that at least a part of the structure is formed by slip forming with panels inside the slip form, the panels facing the slip form. In one embodiment the method is for building a concrete structure for use offshore in ice-infested areas, the structure has an increased thickness as an abrasion allowance in a zone abraded by ice drifting on the sea, the abrasion allowance is formed by slip forming with panels inside the slip form, the panels facing the slip form. 
         [0008]    The invention also provides use of panels inside a slip form, the panels facing the slip form, for building a slip formed concrete structure or a part thereof. 
         [0009]    The structure, method and use of the invention surprisingly result in a slip formed concrete structure or -part having smooth, plane and hard surface without irregularities such as small cracks, crazes or voids, which has been impossible so far in full scale production, particularly when using hard high strength, abrasion resistant air-rich concrete qualities. Without wishing to be bound by theory, it is assumed that the present invention eliminates or reduces irregularities, particularly lifting crazes or—cracks, in the surface of the ice abrasion allowance as the slip form is lifted upwards, and that such irregularities previously have been the main reason for reduced service life and high abrasion rate. The technical effect can be beneficial for any concrete structure, particularly for high strength quality concrete structures exposed for erosion or abrasion or wear for any reason, for any structures for which reduced drag or friction can be beneficial, and structures for which subsequent treatment can be facilitated. Deterioration, wear, ageing, ingress of salts and chemicals all take place in principle form the surface and inwards, for which reason the structure, the method and use according to the invention can be advantageous since better resistance is provided. Testing so far has confirmed the beneficial technical effect; however, it may take many years of service and testing in order to quantify the technical effect in all of the different aspects thereof. 
         [0010]    The term panel means in this context any in substance two dimensional structure useful for the intended purpose. Examples are plates of any feasible material such as metal, polymer material, composite material, concrete and ceramic material. Panels also include any grid, grating, mesh or honeycomb-like plate-like structures. The panels are preferably having a shape adapted for the site it is used, such as the curvature of a platform shaft with round cross section shape. The panels are arranged on the outer side of the abrasion allowance, or for fixed panels as a part of the abrasion allowance, and for all embodiments of the invention the panels are arranged nearest to the slip form, i.e. facing the slip form. 
         [0011]    The panels are left in the structure after the slip forming or the panels are removed from the structure after the slip forming. A smooth inner surface is preferred for panels that are removed. An irregular inner surface, such as for feasible grid, grating, mesh or honeycomb-like plate-like structures, is preferred for panels that are left as part of the structure. 
         [0012]    The abrasion allowance is formed by concrete, preferably without steel armour reinforcement except of possible reinforcing fibres that optionally may be steel fibres. Any steel reinforcement armouring of the abrasion allowance is preferably without electrical or mechanical contact with the main steel reinforcement armouring. 
         [0013]    The length of increased thickness, that is the elevation range of the ice abrasion allowance, preferably encompass the range abraded by drifting ice, which is from the lowest ice draught level at lowest water tide level to the highest expected ice top at the highest water tide level for a gravity base structure. For a floating concrete structure, the tidal range is replaced by the ballast range for the specification of required elevation range having abrasion allowance. 
         [0014]    The transition from the ordinary structure to the structure of increased thickness is preferably gradual and preferably formed by an insert form onto which the panels are arranged. Preferably both the panels and the insert form have means for being arranged or connected together, such as by a wedge system, bolts or male-female means. 
     
    
     
       FIGURES  
         [0015]    The invention is illustrated with figures, of which: 
           [0016]      FIGS. 1 to 5  illustrate sections through a structure according to the invention having an abrasion allowance. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Reference is made to  FIG. 1 , illustrating a section through a wall of a gravity base shaft structure  1  that is slip formed and which shall be provided with an abrasion allowance according to the invention. A slip form yoke  2  and working platforms  3  are lifted upwards concurrently as the structure is slip formed upwards, in a conventional way. 
         [0018]    In  FIG. 2 , a bolted support  4  and an insert form  5  have been arranged on the outer wall of the structure. In  FIG. 3  the first panel  6  has been arranged on the insert form  5 . The volume inside the panel is filled with concrete of a feasible quality for abrasion resistance and behaviour in the forming operation, which quality may be determined by testing. The armouring is not illustrated. Preferably the steel armouring is not extended into the abrasion allowance, but the abrasion allowance may preferably comprise reinforcing fibres such as steel fibres, carbon fibres, boron fibres or other fibres or ceramics or other material for increased abrasion resistance and/or strength. The main structure armouring will thereby not be exposed as the erosion allowance is eroded. 
         [0019]      FIGS. 4 and 5  illustrate how the structure is slip casted further, arranging panels successively upwards in order to cover the structure with abrasion allowance over the intended distance or elevation range. The figures also illustrate how the abrasion allowance is terminated at the upper end in a corresponding way as it was started in the lower end, i.e with an insert form and a bolted support. The supports, insert forms and panels are preferably provided with means for being connected together, preferably in a releasable way. During slip forming the slip form slip or slide on the panels, not on the concrete of the abrasion allowance. Accordingly, the concrete of the abrasion allowance is not subjected to shear forces by the slip form. 
         [0020]    Preferably the panels, any insert forms and any bolted supports are all removed after the forming operation, leaving a smooth, plane, hard and abrasion resistant regular surface of the abrasion allowance on the structure. Extensive testing has revealed that a concrete quality such as B70 (CEN: C70/85, ref. ISO 19906) is feasible for abrasion allowance. Testing and modelling has revealed that an abrasion allowance thickness of 105-122 mm, over an elevation range of typical 6.6 m encompassing the ice drift abraded zone, for a service life of 40 years on shafts of a gravity base structure in ice infested areas, is convenient. 
         [0021]    Testing has revealed that the contents of small cracks or crevices on a concrete surface is dramatically reduced, and the surface become far smoother, with far less irregularities, by slip forming with panels with smooth panel inner surface and removing said panels after forming, according to a preferred embodiment of the invention. The result is inter alia an improved ice abrasion resistance, a reduction in ice formation on the structure per se, increased resistance to repeated cycles of freezing and melting, reduced friction and prolonged service life of the structure.