Patent Publication Number: US-2012042585-A1

Title: Prefabricated wall element for tower construction, and tower construction

Description:
TECHNICAL FIELD 
     The invention relates to a prefabricated wall element for a tower construction according to the preamble of claim  1 . The invention also relates to a tower construction according to the preamble of claim  15 . The invention also relates to a mobile antenna system. The invention also relates to a wind power plant. 
     BACKGROUND ART 
     Today a great number of variants of wind power plants exist. A wind power plant comprises a turbine connected to blades, and a tower arranged to support the turbine. One type of wind power plant is steal towers which may be cylindrical or have a grid structure or analogous. Steal towers have a number of disadvantages though. For example they are affected by weather and thus not suitable on the sea, they require a great deal of maintenance and thus high maintenance costs, they require very thick walls in order to withstand load of powerful wind power plants and are from a technical and economical point of view not suitable for towers higher than 70 meters due to material costs and the rigidity required. The compressive strength for steel is relatively poor in relation to weight. Production of steel towers also results in transportation problems, and requires much installation work due to e.g. many bolts. 
     The development of wind power plants is towards higher and higher power, higher positioning of the turbine blades and bigger turbine blades, and thus higher towers, up to and above 100 m. Hereby the loads are great and steel towers not suitable. Further, the desire to build wind power plants at sea has increased, where steel becomes difficult to handle and maintenance-demanding. Therefore reinforced concrete is used instead which is more weather proof and more cost efficient. Hereby according to a variant prefabricated concrete rings are used, which are stacked on top of each other by means of a lifting crane and are connected by means of tension ropes or the corresponding. This production technique though has the drawback that the large concrete rings are difficult to transport and complicated to manufacture, which results in high production costs. Another variant is to cast the tower on site, wherein mould is produced on site and the concrete is added. This has the disadvantages that the quality of the concrete and thus the strength becomes impaired, the production is weather dependent, the production of the tower is time consuming, it requires a big crane and ladder scaffolding, and dismantling/destruction of mould. 
     WO 03/069099 discloses a wind turbine with a tower being built up by prefabricated wall elements essentially of concrete, the wall elements forming a number of wall portions of circumferential shell portions of one of several shell portions stacked on each other. The prefabricated wall elements are equally thick solid wall elements being even on both the outside and the inside in order to provide structural rigidity and loading capacity. The wall elements have a curved cross section. The curved cross section is according to an embodiment V-shaped with obtuse angle in order to form a facet shaped cross section. A disadvantage with such wall elements is that they are relatively complicated to cast. They are further du to the shape relatively difficult to transport and due to the weight relatively difficult to handle during assembly. Further a lot of concrete is required which makes them relatively expensive to produce. 
     EP 1876 316 A1 discloses a wind turbine with a tower which is built up by prefabricated wall elements essentially of concrete, the wall elements forming a number of wall portions of circumferential shell portions of one of several shell portions of the tower stacked on each other. The prefabricated wall elements have according to variants reduced thickness reinforced with an internal structure of horizontal and vertical stiffeners, said wall elements having an arched cross section and being stretched both horizontally and vertically by means of flexible metal cables. A disadvantage with such wall elements is that they are relatively complicated to cast. They are further due to the shape relatively difficult to transport and due to the weight relatively difficult to handle during assembly. 
     Bracing cables, e.g. in high-tensile twisted steel, are used to reduce the amount of reinforcement and also to reduce the assembly time wherein concrete constructions such as towers for wind power plants according to above are stretched after casting. The stretching force provided gives deformations being counter directed to the influence from outer loads. This improves the static properties of the construction. It used to be common to provide post-tensioned constructions with a stretching force of such a magnitude that no tensile stresses arose, but now partial pre-stressing is most common, i.e. tensile stresses are allowed and are taken by non-tensioned reinforcement. One reason is that a construction with post-tensioned reinforcement is subjected to great concentrated compressive forces from the bracing cables to certain points that may provide unwanted deformations. When the bracing cable consists of twisted steel the bracing cables need to be pulled and further fastened by means of wedges, which causes anchor slip. The bracing cable thereto tends to creep by itself, particularly during the first year. The concrete both shrinks and creeps and all together causes forced forces in concrete and connections with resulting cracks. Bracing cable of thin twined steel is also more sensitive to temperature rises during fire wherefore securing of the construction is provided by non-tensioned reinforcement. The fact that the bracing cable also needs to be stretched with a dumb craft means that it may not be to heavily constructed since the dumb craft in that case becomes unwieldy. This means that the amount of cables needed to be pulled and stretched becomes extensive and demands both heavy equipment and professional competence to be performed correctly. 
     Conventional towers for mobile antennas are today built in steel constructions. A problem with such constructions is that communication equipment arranged in the steel tower is accessible in the tower, and such communication equipment is liable to be stolen. Steel towers quickly become expensive if they are to handle large pressure loads since the thickness of the goods increases strongly. 
     One solution to this problem is known through a tower construction with circular cylindrical shape in concrete, which is cast and reinforced in a ring-shaped section, where several rings are stacked on each other. In the bottom bracing cables are anchored, by means of which the construction is stretchable according to the compressive strength that the construction can handle such that it becomes stable. The rings are arranged to be locked such that a rigid tower is achieved. According to a variant the tower construction is about 40 m high. The tower construction is configured such that the central/communication equipment may be arranged uppermost in the tower where it is accommodated. This solution prevents theft, and simplifies wire laying and cooling of the entire system. The tower however becomes relatively expensive and demands relatively thick, about 7 cm, concrete rings in order to evenly distribute and withstand the compressive loads without to big a risk for local tensions and deformations running the risk of the occurrence of cracks and at the same time providing a covering layer on the reinforcement. The thick circular cylindrical concrete rings, which may be 10 m high and 2-3 m in diameter, are difficult to manufacture, heavy and unwieldy to transport. Alternatively the rings are segmented but that does not improve the situation to any appreciable extent. Masts for mobile systems are often placed in rough terrain such as jungle and mountain ground in order not to disturb the environment. Transporting the concrete rings/concrete segments to and then build up the tower construction in such terrain is complicated. 
     OBJECT OF THE INVENTION 
     An object of the present invention is to provide a wall element for a tower construction which facilitates easy manufacturing, easy transport and easy assembly, and which is cost efficient. 
     An additional object of the present invention is to provide a tower construction which facilitates easy manufacturing, easy transport and easy assembly, and which is cost efficient. 
     An additional object of the present invention is to provide a tower construction which is suitable for wind power plants with high loads and which demands towers with a height in the magnitude of 100 m which facilitates easy and cost efficient manufacturing and transport. 
     An additional object of the present invention is to provide a tower construction which is suitable for mobile antenna systems with demands on high rigidity and which require towers with a height in the magnitude of 40 m which facilitates easy and cost efficient manufacturing, transport and assembly. 
     SUMMARY OF THE INVENTION 
     These and other objects, apparent from the following description, are achieved by means of a wall element for a tower construction, a tower construction, a wind power plant, and a mobile antenna system, which are of the type stated by way of introduction and which in addition exhibits the features recited in the characterising clause of the appended independent claims  1 ,  15 ,  16  and  18 . Preferred embodiments of the device are defined in appended dependent claims  2 - 14 ,  17  and  19 . 
     According to the invention the objects are achieved by a prefabricated wall element for a tower construction, essentially of concrete, arranged to form one of several wall portions of a building formed by circumferential shell portions of one of several shell portions stacked on each other, wherein the wall element is constituted by a substantially flat sheet portion comprising a pair of opposite sides intended to run substantially horizontally in the building and a pair of opposite sides intended to run in a direction forming a predetermined angle to the horizontal plane in the building, and along which sides the wall element includes compressive and tensile load absorbing pillar portions and is intended to be connected to adjacent wall elements. Hereby easy and cost efficient production and transport of wall elements is facilitated. The flat configuration of the wall element is easy to cast and thus easy to manufacture. Further, the flat configuration results in transportation and handling of the wall elements becoming very easy, which reduces the costs. Thanks to the compressive and tensile load absorbing pillar portions the amount of concrete may be reduced which consequently reduces the material costs. 
     According to an embodiment the wall element further comprises a substantially horizontally running compressive and tensile load absorbing strut portions. Thanks to the compressive and tensile load absorbing strut portions the amount of concrete may be reduced which consequently reduces the material costs. 
     According to an embodiment of the wall element the pillar portions comprises pillar channel portions running in the longitudinal direction of the pillar portion. Hereby a simple and stable connection between circumferential shell portions for forming of the tower construction is facilitated. 
     According to an embodiment of the wall element the strut portions comprises strut channel portions running in the longitudinal direction of the strut portion. Hereby reinforcement by means of strut elements and individual post-tensioning of the strut portions of the wall element is facilitated. 
     According to an embodiment of the wall element said circumferential shell portions are connected by means of rigid bar elements running in the channel portions. This results in a stable connection. 
     According to an embodiment of the wall element said bar elements are stretchably arranged in the pillar channel portions. Hereby wall elements may be post-tensioned in a factory or after assembly of said shell portions. 
     According to an embodiment of the wall element a rigid bar element is stretchably arranged in the respective strut channel portion. Herby wall elements may be post-tensioned in a factory of after assembly of said shell portions. 
     According to an embodiment of the wall element the pillar portions of the wall element are arranged to be releasably locked to adjacent pillar portions of wall elements by means of locking elements for the formation of said building. Hereby dismantling of the wall elements of a tower construction is facilitated such that the wall elements may be reused for building up of a tower construction on e.g. a different location. This results in a construction suitable for towers of mobile antenna systems. 
     According to an embodiment the wall elements are arranged to be connected by means of cast concrete in the channel portions. Hereby a very stable and rigid connection with high structural strength is obtained in order to withstand great loads and which is suitable for supporting of a turbine of a wind power plant. 
     According to an embodiment of the wall element the concrete of the wall element is high performance concrete composed of cement and ballast with a weight ratio between amount of water and amount of cement, vct, being lower than 0,39. Hereby the sheet portion may be made water, salt and acid proof. 
     According to an embodiment of the wall element the composition of the high performance concrete comprises a mixture of 10-20% sharp sand, and/or 1-5 percentage by volume of aerogel and/or slag in glass phase and/or mineral fibres such as carbon, silicate and/or basalt fibre. With such an admixture a concrete with such properties is achieved that the tensile strength increases, almost doubled, which surprisingly results in the high performance concrete being fire proof. 
     With a vct lower than 0,39 and admixture of ballast in the cement according to above it is facilitated to provide a long-term constructive sheet with a thickness down to only about 20 mm, i.e. way below the norm for covering layers, served to protect the reinforcement steel from corroding through water, salt and acid penetration or quickly lose its strength during fire. Hereby the amount of concrete may be reduced considerably which results lighter and consequently more easily handled wall elements, and reduces the manufacturing costs. This consequently facilitates providing a thickness of the sheet portion being thinner than the norm for covering layers, i.e. thinner covering layers on the respective side of the reinforcement net than 30 mm, the reinforcement net being according to an embodiment about 10 mm, may be provided with maintained fire protection avoiding capsizing and maintained water resistance avoiding corrosion. 
     According to an embodiment of the wall element said high performance concrete has a flexural strength greater than 10 MPa. According to an embodiment of the wall element said high performance concrete has compressive strength greater than 90 MPa. The Pillar and strut portions may thanks to the good compressive and tensile strength be dimensioned to take all occurring vertical and horizontal compressive and tensile forces of the tower construction while the relative to the pillar and strut portion thin sheet portions may be made so thin that they only answer for bracing. 
     According to an embodiment of the wall element the pillar portions have an extension in the range of 5-15 metre, preferably in the range of 8-13 m. This is a suitable range for managing easy transport and handling and keeping the manufacturing time down. 
     According to the invention the objects are achieved with a tower construction according to any of the embodiments above. 
     According to the invention the objects are achieved with a mobile antenna system comprising a tower construction according to embodiments above, and communication equipment arranged in the upper part of the tower construction. By simple and cost efficient manufacturing and transport of wall elements the wall elements may be easily transported to rough terrain such as jungle and mountain ground in order not to disturb environment, wherein the mobile antenna system then, thanks to the wall elements being easy to handle, easily may be built up in the rough terrain. 
     According to an embodiment of the mobile antenna system the tower construction has a height in the range of 25-50 m. This is a suitable height of a tower construction for mobile antenna systems. 
     According to an embodiment the objects are achieved with a wind power plant comprising a turbine, turbine blades connected to the turbine, and a tower construction according to embodiments above, which tower construction is arranged to support said turbine. By easy and cost efficient manufacturing and transport of wall elements the wall element may be easily transported to a suitable location such as out at the sea by means of a boat, the tower construction advantageously being arrangeable there due to the fact that it is not sensitive to weather. The wind power plant may then, thanks to the wall elements being easy to handle, easily be built up in the rough terrain. 
     According to an embodiment of the wind power plant the tower construction has a height in the range of 60-140 m. this is today considered as being a suitable height of a tower construction for a wind power plant. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will be had upon the reference to the following detailed description when read in conjunction with the accompanying drawings, wherein like reference characters refer to like parts throughout the several views, and in which: 
         FIG. 1  schematically illustrates a side view of a portion of a wall element according to a first embodiment of the present invention; 
         FIG. 1   a - c  schematically illustrate different cross sections of the wall element in  FIG. 1 ; 
         FIG. 2  schematically illustrates a plan view of respectively one portion of two interconnected wall elements according to  FIG. 1 ; 
         FIG. 3   a - b  schematically illustrate side cross sections of portions of two wall elements according to  FIG. 1  stacked on each other; 
         FIG. 4  schematically illustrates a plan view of wall elements according to  FIG. 1  interconnected to a tower section; 
         FIG. 5   a  schematically illustrates a tower construction according to an embodiment of the present invention during assembly; 
         FIG. 5   b  schematically illustrates a tower construction according to  FIG. 5  an interconnected; 
         FIG. 5   c  schematically illustrates a part of a bar element for assembly of tower sections according to an embodiment of the present invention; 
         FIG. 6  schematically illustrates a side view of a portion of a wall element according to a second embodiment of the present invention; 
         FIG. 6   a - c  schematically illustrate different sections of the wall element in  FIG. 6 ; 
         FIG. 7  schematically illustrates a plan view of respectively one portion of two interconnected wall elements according to  FIG. 6 ; 
         FIG. 8   a  schematically illustrates a side cross section of a portion of the wall element according to  FIG. 6 ; 
         FIG. 8   b  schematically illustrates side cross sections portion of wall elements according to  FIG. 6 ; 
         FIG. 9  schematically illustrates a tower section composed of wall elements according to  FIG. 6 ; and 
         FIG. 10   a - d  show different measured data of high performance concrete according to the present invention compared to conventional concrete. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  schematically illustrates a side view of a portion of a flat wall element  20  for a tower construction according to a first embodiment of the present invention, and  FIG. 1   a - c  schematically illustrate different cross sections A-A, B-B, C-C of the wall element in  FIG. 1 . 
     The flat wall element  20  is prefabricated. The flat wall element  20  is obtained by casting in a mould which mould has recesses for pillars and struts. The flat wall element  20  is consequently easy to produce since it may be cast in one piece with a simple mould. 
     The wall element has an outer side  20   a  and an inner side  20   b . The wall element  20  is essentially constituted by a flat sheet portion  22 , a pair of opposite sides of which one  20   c  is shown, intended to run substantially horizontally in the tower construction and a pair of opposite sides  20   e ,  20   f  intended to run in a direction forming a predetermined angle to the horizontal plane in the tower construction. Said angle to the horizontal plane is according to an embodiment in the range of 90 degrees +/−30 degrees. According to an embodiment the opposite sides  20   e ,  20   f  of the wall elements are intended to run substantially vertically in the tower construction, i.e. perpendicular relative to the horizontal plane or with a certain inclination relative to the vertical plane. 
     The wall element  20  includes compressive and tensile load absorbing pillar portions  23   a ,  23   b  running along the sides  20   e ,  20   f . The wall element is intended to be connected to adjacent wall elements. The wall element includes compressive and tensile load absorbing strut portions  24   a  of which one is shown, and preferably at least one intermediate strut portion  24   b  of which one is shown running between the pillar portions parallel to and at a distance from the strut portions running along the sides  20   c . Said strut portions  24   a ,  24   b  is referenced to below as the strut portions. 
     The wall element  20  consequently constitutes a flat tetragonal module or cassette with dimensions adapted to the purpose. According to this embodiment the tetragonal wall element is rectangular. According to another embodiment the tetragonal wall element is trapezoidal, preferably with equal angle on the respective inclined side, such that it receives the shape of a truncated equally sided triangle. The tetragonal wall element  20  has according to this embodiment a height being approximately three times its width. According to an embodiment the height of the wall element is in the range of 5-15 m, preferably 8-13 m. Other extensions of height of the wall element  20  are conceivable and depend among others on the application. 
     The wall element  20  comprises according to an embodiment a reinforcement configuration, not shown, which according to a variant comprises a reinforcement net or the corresponding which preferably has an extension or surface essentially corresponding to the surface of the mould, the reinforcement net according to an embodiment constituting the reinforcement in the flat sheet portion  22 . The reinforcement net thus preferably has a substantially flat configuration. According to an alternative embodiment the sheet portion has no reinforcement/no reinforcement net, which is made possible by the pillar portions  23   a ,  23   b  and the strut portions being arranged to absorb the vertical and horizontal loads, wherein the bracing to be handled by the sheet portion  22  is highly insignificant. Preferably the sheet portion according to this embodiment without reinforcement comprises fibres. 
     The pillar portions  23   a ,  23   b  of the wall element  20  are internally cast in the wall element  20  and consequently arranged to run in a direction forming a predetermined angle to the horizontal plane in a tower construction, preferably arranged to run substantially vertically in a tower construction. The strut portions  24   a ,  24   b  are internally cast in the wall element  20  and consequently arranged to run in a direction substantially horizontally in a tower construction. 
     The wall element  20  comprising pillar portions  23   a ,  23   b  and strut portions  24   a ,  24   b  is cast according to the configuration of pillars and struts in the mould. The wall element  20  further comprises the casted flat sheet portion  22  reinforced with the reinforcement net. According to this embodiment the external side  20   a  of the wall element is essentially even and the internal side has enhancements formed by the pillar and strut portions. 
     The pillar portions  23   a ,  23   b  have through channel portions  26  running in their longitudinal direction. According to an embodiment the pillar channel portions  26  are constituted by tubular bars. According to another embodiment the pillar channel portions are constituted by tubular channels formed during casting of pipes which are removed after casting. 
     The strut portions  24   a ,  24   b  have through channel portions  27  running in their longitudinal direction. According to an embodiment the channel portions  27  are constituted by tubular bars. According to another embodiment the pillar channel portions are constituted by tubular channels formed during casting of pipes which are removed after casting. 
     Removal of a tubular element from the formed pillar channel portion  26  and strut channel portion  27  is facilitated e.g. by the tubular element being waxed or lubricated prior to being cast. 
     Bar elements  44  are arranged to be inserted into the strut channel portions  27  for post-tensioning of the wall element  20 . According to yet another embodiment the channel portion  27  is formed by means of a bar element  44  which is arranged to be embedded such that it may be post-tensioned, wherein the channel portion then already has a bar element introduced therein. According to an embodiment the post-tension of strut portions  24   a ,  24   b  of the wall element  20  is provided in factory, i.e. the post-tension is prefabricated. According to another embodiment the post-tension of strut portions  24   a ,  24   b  of wall elements  20  is arranged to be provided after assembly. The bar element  44  in the respective channel portion is preferably rigid. The bar element  44  is preferably of steel. The bar element is preferably straight, each strut channel portion consequently being straight. 
     According to this embodiment the bar element  44  is arranged in the respective strut channel portion  27 . According to an alternative embodiment two or more bar elements are arranged in the respective strut channel portion  27 , the bar elements being dimensioned for a certain load, wherein, according to a variant with several bar elements, the bar elements are thinner than if one bar element is used per strut channel portion. The bar elements may be arranged as an interconnected group or separately arranged in the respective channel portion. 
     According to an embodiment fasteners  46   a ,  46   b  for bar elements  44  are arranged at the respective pillar portion  23   a ,  23   b , which fastener  46   a ,  46   b  according to a variant is an embedded sheet metal portion to which a bar element  44  is arranged to be fixed, wherein the bar element  44  according to an embodiment may be post-tensioned by means of e.g. a nut  44   a . This is more apparent in  FIG. 2 . 
     According to this embodiment the pillar portions  23   a ,  23   b  are bevelled externally, i.e. having a grade along its respective external side, which sides constitute the pair of opposite sides  20   e ,  20   f  of the wall element. Hereby each pillar portion increases gradually in width from its inside to its outside. The external grade or angle along the respective pillar portion  23   a ,  23   b  is adapted for the number of wall elements  20  to be interconnected side to side in order to form a ring-shaped section such as described in connection to  FIGS. 2 and 3 . The grade of each pillar portion is provided during casting in that the mould having a corresponding shape. 
     The casted pillar portions  23   a ,  23   b  and the strut portions  24   a ,  24   b  constitute reinforcements of the wall element  20  arranged to withstand compressive and tensile forces. The sheet portion  22  of the wall element  20  is according to this embodiment arranged to only handle smaller bracing forces and may therefore be made very thin such that the amount of concrete may be reduced considerably. 
     By means of the flat form of the wall element  20  easy transport is facilitated in that theses wall elements  20  easily may be stacked in a pile on top of each other and transported and be transported on e.g. a truck, boat or the like. They take up little space and are not unwieldy. Due to the fact that the amount of concrete is reduced thanks to the relative to the pillar and strut portions thin sheet portions  22  they become relatively light weight and thus easy to handle. 
     According to a preferred embodiment the wall element  20  is made of high performance concrete with such properties that the wall element  20  with a sheet portion  22  having a thickness thinner than the norm of covering layers, i.e. thinner than covering layers on the respective side of the reinforcement net  18  than 30 mm, the reinforcement net  18  according to an embodiment being about 10 mm, may be provided. According to an embodiment the sheet portion of the wall element  20  is thus thinner in thickness than 70 mm whereby a thickness of the sheet portion  22  of the wall element  20  down to 20 mm by be provided with maintained fire protection avoiding capsizing and maintained water resistance avoiding corrosion. 
     By using high performance concrete with the above mentioned properties a considerably lighter construction with maintained compressive and tensile strength properties which further simplifies transport and assembly in rough terrain and for manufacturing of towers for mobile masts. 
       FIG. 2  schematically illustrates a plan view of respectively one portion of two interconnected wall elements according to  FIG. 1 ,  FIG. 3   a - b  schematically illustrate side cross sections of portions of two wall elements according to  FIG. 1  stacked on each other, and  FIG. 4  schematically illustrates a plan view of the wall element according to  FIG. 1  interconnected to a ring-shaped tower section. 
     Each prefabricated wall element  20  is arranged to form one of several wall portions  20  of circumferential shell portions  30  of a tower construction formed of several shell portions  30  stacked on each other, as is shown in  FIG. 5 . The circumferential shell portions  30  form the ring-shaped tower section  30 . 
     The wall elements  20  are arranged to be interconnected by arranging the external side of a pillar portion  23   a ,  23   b  of a wall element  20  to an external side of a pillar portion  23   b ,  23   a  of another wall element  20  such that they abut against each other according to  FIG. 2  such that the internal sides of each wall element  20  are angled inwardly towards each other. 
     Additional wall elements  20  are interconnected according to above such that a ring-shaped section  30  or a circumferential shell portion is achieved. The prefabricated flat wall elements  20  are thus placed along each other such that a ring-shaped section  30  or a circumferential shell portion  30  is formed. The ring-shaped section is here constituted by identical flat wall elements  20  wherein a facet-shaped ring  30  is achieved. According to this embodiment the number of wall elements  20  in one section is six, wherein the ring-shaped section  30  has a hexagonal cross section in the horizontal plane. Hereby the external bevel or grade of the pillar portions is 15 degrees. 
     The number of wall elements  20  according to alternative embodiments may be more or fewer, more resulting in a more circular shaped section and thus more stable from a strength point of view and lighter wall elements  20 , and fewer results in quicker assembly and fewer wall elements  20  to handle. 
     The wall elements  20  are arranged to be fixedly locked by means of releasable fasteners or locking elements  40   a  such that said ring shaped section  30  or circumferential shell portion is achieved, see  FIG. 2 . The releasable fasteners  40   a  are according to an embodiment constituted by fittings  40   a . The fittings  40   a  is according to a variant arranged at the strut portions  24   a ,  24   b  in connection to the adjacent pillar portions  23   a ,  23   b  to releasably lock the wall elements  20 . According to this embodiment the locking element is arranged to releasably lock the wall elements to each other by fixing the locking element  40   a  to fasteners  46   b ,  46   a  of each wall element  20 , here the embedded sheet metal portion  46   b ,  46   a , illustrated in  FIG. 2 , by means of or corresponding. The locking element is arranged to extend substantially horizontally internally  20   b  between two adjacent wall elements  20  for said locking. 
     The tower sections are arranged to be formed by stacking tower sections on each other, wherein wall elements in  FIG. 3   a - b  are stacked on each other, wherein a lower end of the respective pillar portion  23   a ,  23   b  of the respective wall element  20  of the upper section  30  rests on an upper end of the respective pillar portion  23   a ,  23   b  of the lower tower section  30  wherein upper and lower end of the respective pillar portion  23   a ,  23   b  according to a variant has a step such that they engage for preventing lateral sliding of the tower section, se  FIG. 3   a . The pillar portions  23   a ,  23   b  of the lower tower section  30  are consequently arranged to support upper tower section  30 . Further during stacking of wall elements according to above a lower strut portion  24   a  of the wall element  20  of the upper tower section  30  is arranged to rest on an upper strut portion  24   c  of the wall element  20  of the lower tower section  30  wherein upper and lower strut portions according to a variant have a step such that they engage for preventing lateral sliding of the tower sections, se  FIG. 3   b.    
     The wall elements  20  of the upper tower section are arranged to be fixedly locked with the wall element of an underlying tower section by mends of releasable fasteners or locking elements  40   b . The releasable fasteners  40   b  are according to an embodiment constituted of fittings  40   b . The fittings  40   a  are according to a variant arranged at the strut portions  24   a ,  24   c  in connection to the adjacent pillar portions  23   a ,  23   b  to releasably lock the wall elements  20 . According to this embodiment the locking element is arranged to releasably lock the wall element to each other by fixing the locking element  40   a  to fasteners  46   a ,  46   b  of the respective wall element  20 , here the embedded sheet metal portion  46   b ,  46   a , illustrated in  FIG. 2 , by means of nuts or the corresponding. The locking element is arranged to extend substantially vertically internally  20   b  between to wall elements  20  stacked on each other for said locking. 
       FIG. 5   a  schematically illustrates a tower construction according to an embodiment of the present invention composed of wall elements  20  according to  FIG. 1 , and consequently tower section  30  according to  FIG. 4 , during assembly comprising tower sections according to  FIG. 4 ,  FIG. 5   b  schematically illustrates the tower construction according to  FIG. 5   a  interconnected, and  FIG. 5   c  schematically illustrates a part of a bar element for interconnection of tower sections stacked on each other according to an embodiment of the present invention. 
     The tower section  50  is built up of ring-shaped tower sections  30  or circumferential shell portions as described in connection to  FIG. 4 , wherein tower sections  30  are arranged to be stacked on each other as described in connection to  FIG. 3   a - b . The sections  30  are arranged to be connected to each other for forming of the tower construction  50  by aligning the respective pillar portion  23   a ,  23   b  of the respective tower section to each other such that a tower pillar with a through channel running in its longitudinal direction ay be formed. 
     The tower construction  50  comprises the bar elements  43 . According to an embodiment of the tower construction said circumferential shell portions  30  stacked on each other are connected by means of bar elements  43 , suitably of stainless steel, running in the pillar channels  26 . The bar elements are according to this embodiment anchored at the top and bottom of the tower construction by means of anchorages  45   a ,  45   b , or in portions thereof. The bar elements are preferably single rigid bar elements which have the advantage that they may be dimensioned and post-tensioned with greater forces and with a simpler method than a flexible bracing cable. 
     In the construction according to the invention a high performance concrete which among others has such properties that it does not shrink wherefore a rigid straight bar element  43 ,  44  is preferred in the channel portions  26 ,  27  of pillar portions  23   a ,  23   b  and strut portions  24   a ,  24   b  in the wall element  20  since fastening by means of a rigid bar element  43 ,  44  does not creep, which results in easy post-tensioning in pillar portion and strut portion. Such as mentioned above in connection to  FIG. 1  the strut portion  24   a ,  24   b  is suitably post-tensioned prior to final assembly, either in factory or on the site. 
     The pillar portions  23   a ,  23   b  may according to an embodiment also be post-tensioned in each single wall element  20  in a factory combined with strong joints between the floor levels of the tower construction, the pillar portions  23   a ,  23   b  of the wall elements being releasably locked by means of the locking element  40   a . According to an embodiment bar elements  43  are therefore connectable in single positions or as a joint series of bars and finally lockable by threading devices, the threading device according to a variant being constituted by threads  43   a  in the bar element  43  and nuts  43   b  with threading adapted for the threads such as is apparent from  FIG. 5   c .  FIG. 8   a - b  illustrate a variant for connecting bar elements in pillar channel portions which is applicable also in this embodiment. Preferably the pillar portions are post-tensioned on the site, preferably also from the bottom to the top, by bar elements  43  connected in series. Thereby work is made easier but above all it is facilitated to provide a final strain to the whole tower in an easy way, e.g. by means of a light weight and simple hydraulic tool, to a desired strain. No undesired lock creeping occurs, and for maximum rigidness after possible initial creeping of the bar element of steel during e.g. the first year, final strain may easily be achieved. This results in a considerably more stable connection and rigidness. 
     Hereby also dismantling of the wall elements of a tower construction  50  is facilitated such that the wall elements may be reused for building of a tower construction on e.g. a different location. This results in a suitable construction for towers of mobile antenna systems. 
     According to this embodiment three tower sections  30  are stacked on each other, wherein the respective tower section  30  is tapering upwardly such that the formed tower construction  50  is upwardly tapering. An advantage with an upwardly tapering tower construction is that it reduces moment and thereby dimensioned loads. Each wall element  20  of the tapering sections  30  has a trapezoidal shape with equal angle on the respective inclined side such that they get the shape of a truncated equally sided triangle. Alternatively the tower construction could be arranged to run vertically wherein the tower has straight section of which each wall element is rectangular. Alternatively the tower section could be formed by a mixture of tapered and straight tower sections, where a tapering tower section may be either upwardly or downwardly tapering. E.g. the lowermost tower section could be upwardly tapering and the uppermost tower section downwardly tapering and intermediate tower sections straight. The number of tower sections could be more of fewer than three. The height of the respective tower section may be the same or different. 
     By using releasable fasteners  40   a  such as fittings the tower construction  50  may also be dismantled. According to an embodiment the tower construction has a height in the range of 25-50 m, e.g. about 40 m. Such a tower is suitable for mobile antenna systems. The tower construction may be constructed with any suitable height, i.e. also higher than 50 m if so desired. The tower construction has according to a variant a bottom diameter in the range of 3-7 m, preferably 4-6 m. 
     The tower construction is according to an embodiment configured such that the central/communication equipment of a mobile antenna system may be arranged uppermost in the tower construction, which prevents theft of the communication equipment, and simplifies wire laying and cooling of the entire system. 
     According to this embodiment such a bar element  43  is arranged in the respective pillar channel portion  26  or a bar element  43  arranged to run through two or more pillar channel portions  26 . According to an alternative embodiment two or more bar elements are arranged in the respective strut channel portion  26 , the bar elements being dimensioned for a certain load, wherein, according to a variant with several bar elements, the bar elements are thinner than if one bar elements would be used per pillar channel portion. The bar element may be arranged interconnected as a group or separately arranged in the respective channel portion. 
       FIG. 6  schematically illustrates a side view of a portion of a flat wall element  70  for a tower construction according to a second embodiment of the present invention, and  FIG. 6   a - c  schematically illustrate different cross sections A-A, B-B and C-C of the wall element in  FIG. 6 . 
     The flat wall element  70  is prefabricated. The wall element  70  is arranged to be cast in a mould which according to a variant is a reinforcement configuration. The mould has longitudinal recesses along the sides which in profile have a curved shape or loop-shape. By casting the reinforcement configuration in the mould a wall element  70  is obtained. 
     The wall element has an external side  70   a  and an internal side  70   b . The wall element  70  is essentially constituted by a flat sheet portion  72 , a pair of opposite sides of which one  70   c  is shown, intended to run substantially horizontally in the tower construction and a pair of opposite sides  70   e ,  70   f  intended to run in a direction forming a predetermined angle to the horizontal plane in the tower construction. Said angle to the horizontal plane is according to an embodiment 90 degrees +/−30 degrees. According to an embodiment the opposite sides  70   e ,  70   f  of the wall elements are intended to run substantially vertical in the tower construction, i.e. perpendicular relative to the horizontal plane or with a certain inclination relative to the vertical plane. 
     The wall element  70  comprises compressive and tensile load absorbing pillar portions  73   a ,  73   b  running along the sides  70   e ,  70   f . The wall element  70  is intended to be connected to adjacent wall elements. The wall element comprises compressive and tensile load absorbing strut portions  74   a  running substantially horizontally along the sides  70   c  of which one is shown, and preferably at least one intermediate strut portion  74   b , of which one is shown, running between the pillar portions and running substantially parallel to and at a distance from the strut portions  74   a  running along the sides  70   c . Said strut portions  74   a ,  74   b  is referred to as the strut portions below. 
     The wall element  70  consequently constitutes a flat tetragonal module or cassette with dimensions adapted for the purpose. According to this embodiment the tetragonal wall element is rectangular. According to another embodiment the tetragonal wall element is trapezoidal-shaped, preferably with equal angle on the respective inclining side such that it gets the shape of a truncated equally sided triangle. The tetragonal wall element  70  has according to this embodiment a height being about three times its width. According to an embodiment the height of the wall element is in the range of 5-15 m, preferably 8-13 m. 
     The wall element  70  comprises a not shown reinforcement configuration which according to a variant comprises reinforcement net or the corresponding preferably having an extension or surface substantially corresponding to the shape of the mould, the reinforcement net constituting the reinforcement in the flat sheet portion  72 . The reinforcement net thus has a substantially flat configuration. 
     The pillar portions  73   a ,  73   b  of the wall element  70  are internally cast in the wall element  70  and consequently arranged to run in a direction forming a predetermined angle to the horizontal plane in a tower construction, preferably arranged to run substantially vertically in a tower construction. The Strut portions  74   a ,  74   b  are internally casted in the wall element  70  and consequently arranged to run in a direction substantially horizontally in a tower construction. 
     The wall element  70  comprising pillar portions  73   a ,  73   b  and strut portions  74   a ,  74   b  is cast in accordance with the configuration of pillars and struts of the mould. The wall element  70  in addition comprises the casted flat sheet portion  72  reinforced with the reinforcement net. According to this embodiment the external side  70   a  of the wall element  70  is essentially even and the internal side has enhancements formed by the pillar and strut portions. 
     The pillar portions  73   a ,  73   b  have through channel portions  76   a ,  76   b  running in its longitudinal direction. Said channel portions  76   a ,  76   b  are formed by the channel portions  73   a ,  73   b  having a loop-shaped cross section projecting from the sheet portion and having a curved portion outwardly from the long side of the sheet portion  72 . Hereby the channel portion  76   a ,  76   b  running along the respective side of the wall element  70  is formed. 
     The reinforcement configuration comprises embedded reinforcement bars  78  running in the longitudinal direction of the pillar portions. 
     The reinforcement configuration comprises ring-shaped reinforcements  79  partly embedded in the pillar portions  73   a ,  73   b  transversally arranged along the respective pillar portion, a number of ring-shaped reinforcements  79  being arranged at a distance from each other along the pillar portion. The ring-shaped reinforcements  79  are arranged along the pillar portions in such a way that a pillar portion  79   a  of the ring-shaped reinforcement projects over the channel portion wherein a projecting loop  79   a  is formed which loop forms a circumferential opening with the channel portion  77 . The embedded portion of the partly embedded ring-shaped reinforcement  79  is arranged to run around the reinforcement bar  78  running in the longitudinal direction of the pillar portion. The ring-shaped reinforcements  79  are vertically arranged in the pillar channel portions such that they form a series of loops with a c/c-measure varying depending on dimensioned loads. 
     The strut portions  74   a ,  74   b  have channel portions  77  running in the longitudinal direction. According to an embodiment the cannel portions  77  are constituted by tubular bars. According to another embodiment the channel portions are constituted by tubular channels formed during casting of tubes which after casting have been removed. This is explained in more detail below in connection to  FIG. 7 . 
     According to this embodiment the pillar portions  73   a ,  73   b  are bevelled, i.e. having grade along its respective external side, which sides constitute the pair of opposite sides  70   e ,  70   f  of the wall element. Hereby each pillar portion  73   a ,  73   b  increases gradually in width from its inner side to its outer side. The external grade or angle along the respective pillar portion  73   a ,  73   b  is adapted to the number of wall elements  70  being interconnected side to side in order to form a ring-shaped section as described in connection to  FIGS. 7 and 9 . The grade of the respective pillar portion is provided during casting by the mould having a corresponding shape. 
     The casted pillar portions  73   a ,  73   b  and strut portions  74   a ,  74   b  constitute reinforcements of the wall element  70  arranged to withstand compressive and tensile forces. The sheet portion  72  of the wall element  70  is according to this embodiment arranged to only handle smaller bracing forces and may therefore be made very thin such that the amount of concrete may be reduced considerably. 
     Easy transportation is facilitated by the flat shape of the wall element  70  in that these wall elements  70  easily may be stacked on each other and transported on e.g. a truck, a boat or the like. They take up little space and are not unwieldy. By the fact that the amount of concrete is reduced thanks to the thin sheet portions  72  they become relatively light weight and thus easy to handle. 
     According to a preferred embodiment each wall element  70  is made of high performance concrete with such properties that wall elements  70  with a sheet portion  72  with a thickness being thinner than the norm for covering layer, i.e. thinner covering layers on the respective side of the reinforcement net than 30 mm, the reinforcement net according to an embodiment being about 10 mm may be obtained. According to an embodiment the sheet portion of the wall element  70  is thus thinner in thickness than 70 mm. 
     The properties of the high performance concrete according to the present invention, the concrete of the wall element and tower construction  50 ,  100  according to the first and second embodiments of the present invention preferably being constituted thereof, are described in more detail in connection to  FIG. 10   a - d  below. 
     By using high performance concrete with the above mentioned properties a considerably lighter construction with maintained compressive and tensile strength properties, which further simplifies transport and assembly in rough terrain for manufacturing of towers for wind power plants is facilitated. 
       FIG. 7  schematically illustrates a plan view of a portion of two interconnected wall elements according to  FIG. 6 . 
     Each prefabricated wall element  70  is arranged to form one of several wall portions of circumferential shell portions  80  of one of several shell portions stacked on each other according to  FIG. 9 . The circumferential shell portions  80  form the ring-shaped tower section  80 . 
     The wall elements  70  are arranged to be interconnected by arranging the external side of a pillar portion  73   a ,  73   b  of a wall element  70  to the external side of a pillar portion  73   b ,  73   a  of another wall element  70  such that they abut against each other such that the internal sides of the respective wall element  70  are angled towards each other. 
     The pillar portions  73   a ,  73   b  have a cross section such that when two long sides  70   a ,  70   b  of the wall element  70  abut against each other a through channel portion  76  is formed by the channel portions  76   a ,  76   b  facing each other by the thus interconnected pillar portions  73   a ,  73   b.    
     The portion  79   a  of the respective pillar portion projecting over the channel portion of the partly embedded ring-shaped reinforcements  79  transversely arranged along the respective pillar portion is arranged to overlap a corresponding projection portion  79   a  of an abutting wall element  70  such that the respective loop  79   a  overlaps the other loop  79   a , wherein a loop from the ring-shaped reinforcement  79  of a first wall element  70  extend inwardly towards the channel portion  76   a  of the pillar portion  23   a  of an adjacent second wall element  70  and the loop from the ring-shaped reinforcement  79  of the adjacent second wall element  70  extend inwardly towards the channel portion  76   b  of the channel portion  73   b  of the first wall element. Consequently ring-shaped reinforcements are transversely arranged along the respective wall element such that when two long sides  70   a ,  70   b  of the wall element  70  abut against each other several ring-shaped loops are formed of projecting portions of opposite reinforcements. 
     The pillar portions  73   a ,  73   b  is according to an embodiment dimensioned to withstand forces arising in tower constructions for wind power plants. Depending on size of aggregate and wings of wind power plants answering to dimensioned load (not weight of aggregate) the pillar dimensions varies with suitable measures normally between 200×200 mm to 300×300 mm. For mobile antenna towers they may naturally be dimensioned considerably slimmer as the most important thing for such towers is that they are rigid. 
     Removal of tubular element from the formed strut channel portion  77  is facilitated as in the first embodiment e.g. by the tubular element being waxed or lubricated prior to being embedded. Alternatively removal of the tubular element is facilitated by having the tubular element covered in plastic prior to being embedded. 
     Bar elements  94  are arranged to be inserted in the strut channel portions  77  for post-tensioning of wall elements  70 . According to yet another embodiment the channel portion  77  is formed by means of a bar element  94  which is arranged to be embedded such that is post-tensioned, the channel portion already having a bar element inserted therein. According to an embodiment post-tensioning of strut portions  74   a ,  74   b  of wall elements  70  is obtained in a factory, i.e. the post-tensioning is prefabricated. According to another embodiment the post-tensioning of strut portions  74   a ,  74   b  of wall elements  70  is arranged to be provided after assembly. According to this embodiment the bar element is arranged to be tensioned/tightened by means of a nut  94   a . According to a variant the post-tensioning is provided by screwing by means of a hydraulic tool, which post-tensioning by the threading may be performed with less power than if corresponding struts are to be tightened. According to a variant the edge of the channel portion is arranged to resist the nut. 
     According to this embodiment a bar element  94  is arranged in the respective strut portion  77 . According to an alternative embodiment two or more bar elements are arranged in the respective strut channel portion  77 , the bar elements being dimensioned for a certain load, wherein, according to a variant with several bar elements, the bar elements are thinner than if one bar element is used per strut channel portion. The bar element may be arranged interconnected in a group or separately arranged in the respective channel portion. 
       FIG. 8   a  schematically illustrates a side cross section of portions of wall elements according to  FIG. 6 , and  FIG. 8   b  schematically illustrates a side cross section of portion of two wall elements stacked on each other according to  FIG. 6 .  FIG. 9  schematically illustrates a tower section  80  interconnected by wall elements according to  FIG. 6 . 
     The tower construction  100  is built up of tower sections  80  stacked on each other. Tower sections are obtained by interconnecting wall elements  70  according to above such that a ring-shaped section  80  is obtained. When two long sides  72   a ,  72   b  of the wall element  70  abut against each other a through channel portion is formed through the thus interconnected pillar portions as described above, i.e. that the respective channel portion  76   a ,  76   b  of the respective pillar portion  73   a ,  73   b  are facing each other such that said channel portion  76  is formed. Two pillar portions  73   a ,  73   b  arranged towards each other in that way form a pillar  73  with a through channel portion running in the longitudinal direction of the pillar. The prefabricated flat wall elements  70  are placed along each other such that a ring-shaped tower section  80  is formed. The ring-shaped tower section  80  is here constituted by identical flat wall elements  70  wherein a facet-shaped ring is obtained. According to this embodiment the number of wall elements  70  in a tower section is twelve, wherein the ring-shaped section has a dodecagonal cross section in the horizontal plane. Hereby the external bevel or grade of the sheet portion is 7,5 degrees. 
     Due to the fact that the respective wall element  70  is trapezoidal-shaped with equal angle on the respective inclining side such that they get the shape of a truncated equally sided triangle an upwardly tapering tower section is obtained, which reduces the moment and thereby dimensioned loads. 
     The number of wall elements  70  may according to alternative embodiments be more of fewer where more results in a more circular tower section and thus more stable from a strength point of view and lighter wall elements  70 , and fewer results in fewer wall elements  70  which results in quicker assembly and fewer wall elements  70  to handle. 
     Due to the fact that the respective section  80  is tapering, the wall elements  70  of one section  80  to be stacked on another section is smaller in width than the wall element  70  of the section  80  below such that an upwardly tapering tower construction  100  is obtained. 
     The tower sections are arranged to be formed by stacking tower sections on each other, wherein wall elements according to  FIG. 8   b  are stacked on each other, wherein a lower end of the respective pillar portion  73   a ,  73   b  of the respective wall element  70  of the upper tower section  80  rests on an upper end of the respective pillar portion  73   a ,  73   b  of the lower tower section  70  wherein the upper and lower end of the respective pillar portion  73   a ,  73   b  according to a variant has a step such that they engage into each other for prevention of lateral sliding of the tower section, see  FIG. 8   b . The pillar portions  73   a ,  73   b  of the lower tower section  80  are consequently arranged to support the upper tower section  80 . 
     The sections  80  are thus arranged to be stacked on each other for forming of a tower construction  100  by aligning the respective pillar of the respective section with each other. Hereby each pillar of the respective section  80  forms a tower pillar such that the tower has a corresponding number of tower pillars, here twelve tower pillars, as the respective section. A through channel is thus formed running in the longitudinal direction of the respective tower pillar by means of the channel portion  76  formed in the pillars of the respective section by alignment of the pillars during stacking of the sections on each other. 
     The tower section further comprises bar elements  93  arranged to be inserted into the channel portions  76  for connecting the circumferential shell portions, i.e. the tower section  80  by means of the bar elements running in the pillar channel portions  76 . 
     According to an embodiment bar elements  93  are of steel or other suitable material composition arranged to be lead through the respective through channel portion  76  extending in the longitudinal direction of the tower pillar. Thereafter concrete y 1  is arranged to be filled in the respective channel portion such that a permanent locking of the wall element  70  and the sections  80  is provided with bar elements  93  and said ring-shaped reinforcements through which overlapping ring-shaped loops bar elements  93  are arranged to be introduced. In such a way the tower construction  100  is permanently locked and a very stable construction is obtained. Hereby no welding is required. Such a construction with tower pillars with through channels running in its longitudinal direction results in a simple solution which may be controlled from factory and where the tower construction then easily may be built on the site. 
     According to an embodiment each bar element is rotatably arranged in the respective channel portion. Hereby according to an embodiment a hollow tubular element is arranged to be embedded in the respective channel portion  76 , forming a channel dimensioned such that bar elements may be introduced and rotated in the channel. 
     According to an embodiment the tubular element is removably embedded in the channel portion. According to an embodiment the tubular element is waxed, lubricated, or treated with another suitable agent which does not stick to or is locked by concrete prior to being embedded, which facilitates removal of the tubular element such that it may be removed such that a channel formed by the concrete cast in the pillar portion is formed, dimensioned such that the bar element may be introduced and rotated in the channel. Alternatively removal of the ring-shaped element is facilitated by having the tubular element covered in plastic prior to it being embedded. Any suitable means for providing. According to these embodiments the tubular element does not need to be hollow. 
     According to an embodiment an upper axially extending end portion and a lower axially extending end portion of the tubular element a bigger circumference, i.e. diameter, than the remaining portion of the tubular element running there between. Hereby a channel  77 ′ of the pillar portion is obtained when removing the tubular element which has a greater circumference along an upper portion  77 ′ a  and along a lower portion  77 ′ b  of the channel. Through this solution easy post-tensioning of introduced bar elements is facilitated. 
     Such a solution is advantageously used also during connection of tower construction by means of the bar elements according to the first embodiment of the present invention. 
     The pillar portions  73   a ,  73   b  are consequently according to an embodiment also arranged to be post-tensioned in each single wall element  70  in factory combined with strong joints between the different floor levels of the tower construction. According to an embodiment bar elements  93  are therefore in single positions or as an interconnected series of bar elements  93  connectable and finally lockable by threading devices, the threading devices according to a variant being constituted by threads  93   a  in bar elements  93  and nuts  93   b  with threading adapted for the threads such as is apparent from  FIG. 8   a - b.    
     The threads of the respective bar element is preferably adapted such that when the bar element is arranged through a pillar portion  73   a ,  73   b  of a wall element, alternatively through two or several pillar portions of wall elements  70  of tower sections  80  stacked on each other, the thread has an extension corresponding to the wider part of the channel  77 ′ surrounded by concrete y 1 . A bar element may thus have a length corresponding to a one pillar portion, two pillar portions or several pillar portions, the respective end of the bar element having threads corresponding to the extension of the wider portion  77 ′ a ,  77 ′ b  of the channel  77 . 
     The respective nut  93   b  preferably has an extension corresponding to the double extension of the wider portion  77 ′ a ,  77 ′ b  in the channel  77 . When a bar element is arranged in the channel portion, i.e. the channel such that the threads of the bar element are present at the level of the wider portion of the channel and the nut is screwed thereon, the nut  93   b  will project corresponding to the length of a wider portion of the channel in a pillar portion. Hereby the projection portion of the nut  93   b  is during stacking of an additional tower section arranged to be introduced in a wider portion of the channel of a pillar portion of the tower section stacked thereon, wherein a bar element may be introduced through it and threaded to the nut for post-tensioning. 
     Preferably the pillar portions are post-tensioned on the location, preferably also from the bottom to the top, by bar elements  93  connected in series. Thereby the work is made easier but above all it is facilitated to provide a final tensioning to the entire tower construction  100  in an easy way, e.g. by means of a light weight and simple hydraulic tool, to a desired tension. No lock creeping which is hard to handle occurs, and for maximum rigidity after possible initial creeping of the bar element of steel during e.g. the first year final strain may easily be obtained. This results in a considerably more stable connection and rigidity. 
     According to a variant some bar elements at the time are connectably strained, and by the fact that the forces decrease higher up in the tower construction the bar elements are adapted to the actual forces decreasing with height since the moment becomes less. According to an embodiment the bar elements, preferably comprising steel, are consequently dimensioned after the forces they are arranged to absorb such that the dimension of bar elements and/or the dimension on the pillars and struts higher up in the tower construction are dimensioned to absorb less force than bar elements in lower tower sections, which reduces the material consumption. 
     Hereby tower sections for forming of the tower construction may be connected by means of post-tensioned bar elements and/or by means of concrete added to the channel portions for fixing tower sections by casting. 
     According to an embodiment of the tower construction  100  according to the second embodiment, ten sections  80  are stacked on each other, wherein the respective section  80  is upwardly tapering. The respective wall element  70 /section  80  is according to this embodiment 10 m high such that the tower construction  100  becomes 100 m high. The tower construction may of course be built to desired height. 
     The tower construction  100  is according to an embodiment configured such that it is arranged to support the turbine of a wind power plant and thus constitutes a tower for a wind power plant. According to an embodiment the height of the tower construction is in the range of 60-140 m, but higher towers are possible to obtain. The tower construction has according to a variant a bottom diameter in the range of 4-8 m, preferably 5-7 m. 
     According to this embodiment a bar element  93  is arranged in the respective pillar channel portion  76  or a bar element  93  is arranged to run through two or more pillar channel portions  76 . According to an alternative embodiment two or more bar elements are arranged in the respective strut channel portion  76 , the bar elements being dimensioned for a certain load, wherein, according to a variant with several bar element, the bar elements are thinner than if one bar element is used per pillar channel portion. The bar elements maybe arranged interconnected in a group or separately arranged in the respective channel portion. 
     Above different variants for tensioning strut portions  24   a ,  24   b ;  74   a ,  74   b  and pillar portions  23   a ,  23   b ;  73   a ,  73   b  of a tower section  50 ,  100  by means of bar elements  43 ;  93 ,  44 ;  94 , and connecting tower section  30 ;  80  of tower constructions  50 ,  100  by means of bar elements, have been described. Post-tensioning of strut portions and pillar portions may, as mentioned, be provided in different ways. One way is that it is made in factory, both in pillar portion and strut portion. Another way is to leave post-tensioning in pillar portions to after assembly of a tower section  30 ;  80  and thus connect several tower sections stacked on each other. According to a variant some wall elements are stretched at a time and by the fact that the forces are reduced upwardly the bar elements are adapted to the actual forces reducing with height since the moment becomes smaller. According to another variant the bar elements have the same dimension and are stretched from the bottom to the top, which has the advantage that if then, e.g. after one year a need exists for additional stretching due to the steel of the bar element creeping and slacken. The bar element  44 ;  90  in the strut portions are straight and stretchably arranged in strut portions of the respective wall element  20 ;  70 . The advantage with a straight bar element is that it reduces the number of tensions and stretches, it is possible to stretch easy and precise with nuts without creeping, and they may be post-tensioned by screwing. 
     Above wall elements  27 ,  70  for forming of tower constructions  50 ,  100  for mobile antenna systems and wind power plants have been described. By means of a wall element  20 ,  70  according to the present invention a waste silo or manure well could be constructed. 
     Above wall elements  20 ;  70  have been described arranged to form one of several wall portions of circumferential shell portions  30 ;  80  of a tower construction  50 ;  100  formed by several shell portions stacked on each other. However, any suitable building construction may be provided by means of the wall elements according to the present invention, such as the shell of a multi-story building, where the strut portions according to a variant may constitute beams for floor levels. The wall elements need not be of the same size. Any suitable tower shape or other building shape may be provided with the device, wherein the ring-shaped section may be a regular or irregular polygon. The ring-shape may have any suitable cross section such as triangular, square, rectangular, pentagonal, hexagonal, etc. or an irregular cross section. 
       FIG. 10   a - d  show different measured data of high performance concrete y 1  according to the present invention compared to conventional concrete y 2 . 
     The high performance concrete according to the present invention is composed of cement and ballast with a water-cement-number, vct, i.e. weight ratio between amount of water and amount of cement, being lower than 0.39, wherein all added water has been chemically bound during hardening into concrete and where all capillary pores vanished into the cement paste. A low vct-number results in the cement matrix becoming stronger and denser. By these improvements in properties the sheet portion may be made water, salt and acid proof. 
     According to a variant the cement constitutes 20-30% of the high performance concrete and ballast 55-75% of the high performance concrete. The high performance concrete is composed of 5-15% water, with vct&lt;39. 
     Ballast comprises slag and/or stone and/or sand. According to a preferred embodiment the ballast comprises sharp sand which according to an embodiment constitutes 10-20% of the high performance concrete. The cement comprises according to an embodiment fine material such as micro silica, aerogel and similar materials. According to an embodiment the fine material constitutes 1-5% of the high performance concrete. 
     The high performance concrete according to an embodiment of the present invention is consequently composed of smaller admixtures of material with good grip zones, i.e. material having a rough configuration/surface, are uneven, e.g. with craters or the corresponding, such as aerosol and/or sharp sand and/or mineral fibre such as carbon fibre, silicate fibre, or basalt fibre, mixed in cement to a certain composition. According to an embodiment the high performance concrete y 1  is according to the present invention composed with an admixture of 10-20% sharp sand, and/or 1-5 percentages by volume of aerogel and/or slag in glass phase and/or mineral fibres such as carbon fibre, silicate fibre, or basalt fibre. Hereby a high performance concrete is obtained with such properties that the tensile strength increases, at least doubled, which totally surprisingly means that the high performance concrete becomes fire proof. 
     All in all means that a long-term constructive sheet with a thickness down to only about 20 mm, i.e. far below the norm for covering layers may be created, serving to protect the reinforcement steel from corroding by water, salt and acid penetration or quickly lose its strength during a fire. 
     Fire tests have been performed on the high performance concrete y 1  according to the present invention. The test was performed on the national testing laboratory in Borås, Sweden. Two pillars of the high performance concrete  71  were tested against fire according to SIS 02 48 20 during 122.5 minutes. Bothe pillars kept the load-bearing capacity during the entire test. 
     The properties of the concrete are improved by increasing the density of the cement paste and cooperation with ballast material. Thereby is obtained an increased compressive and tensile strength, good water-tightness but at the same time good diffusion-openness, higher aging durability, high carbonation and chloride resistance, high adhesiveness and that the concrete is shrink free during hardening. 
     The high performance concrete according to the present invention results in increased flexural strength from in the best case today 5-7 MPa to 10-15 MPa with possibility to doubling of compressive strength of normal concrete. By the fact the water cement number, vct, at the same time may be made low, the small amount of released steam is all of a sudden not able to split the material during fire. 
     Compressive and flexural strength tests were made on high performance concrete y 1  according to the present invention and as a comparison normal concrete  72 , the following results being obtained after 28 days. 
     High Performance Concrete y 1  According to the Present Invention: 
                                                Compressive strength after 28 days     95 MPa           Flexural strength after 28 days   12.5 MPa                        
Normal Concrete y 2 :
 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Compressive strength after 28 days 
                   45 MPa 
               
               
                   
                 Flexural strength after 28 days 
                 12.5 MPa 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 10   a - d  show tests of high performance concrete according to the present invention, denoted y 1 , and conventional concrete, denoted y 2  below. 
       FIG. 10   a  shows tests of shrinking on 40×40×160 mm samples with dowels on both sides of high performance concrete according to the present invention and normal concrete y 2 . The length was measured with a Graf-Kaufman apparatus. After 6 months no notable shrinkage of the high performance concrete y 1  according to the present invention was noted unlike normal concrete y 2 . 
       FIG. 10   b  shows water absorption through capillary suction, where the test was performed according to Swedish standard on high performance concrete y 1  according to the present invention and normal concrete y 2 . Hereby is apparent that the water-proofness of the high performance concrete y 1  according to the present invention is substantially higher than normal concrete y 2 . 
       FIG. 10   c  shows freezing and thawing in acid and chloride based solution where the test was performed according to ASTM 666, which is a standard test method for concrete resistance, on high performance concrete y 1  according to the present invention and normal concrete y 2 . The test shows that the chloride resistance of the high performance concrete y 1  according to the present invention is considerably higher than normal concrete y 2 . 
       FIG. 10   d  shows a special test of freezing and thawing in a mixture of equal parts of formic acid, lactic acid and acetic acid with pH 3, on high performance concrete y 1  according to the present invention and normal concrete y 2 . The test shows that the acid resistance of the high performance concrete y 1  according to the present invention is considerably higher than normal concrete y 2 . 
     The degree of carbonation was measured by changing the test bodies in the tests above, wet them with water and spray a phenolphthalein solution over the surfaces. Carbonated surfaces do not become pink-coloured. The carbonation depth for y 1  after 6 months was measured to 1-1.5 mm. This shows that the concrete has a very low permeability which explains the low water absorption and high ability to resist salt and acids. 
     By using high performance concrete in the wall element  20 ,  70  according to the present invention it is facilitated to obtain a thickness of the sheet portion  22 ,  72  of the wall element  20 ,  70  down to 20 mm with maintained fire protection avoiding capsizing and maintained waterproofness avoiding corrosion. 
     Due to the fact that the concrete of the wall elements  20 ,  70  according to the present invention are constituted by high performance concrete y 1  according to above, and the compressive and tensile load absorbing pillar portions  23   a ,  23   b ;  73   a ,  73   b  thereby being produced in high performance concrete the pillar portions may absorb a considerably higher compressive load than conventional concrete, more than 70 MPa in pressure load. The pillar and strut portions may therefore be dimensioned to take all existing vertical and horizontal compressive and tensile forces of the tower construction  50 ,  100  while the relative to the pillar and strut portions thin sheet portions  22 ,  72  only answers for bracing. 
     The tower construction according to the present invention with high performance concrete according to the present invention thus gets a considerably better strength than with conventional concrete. In e.g. a wind power tower the total capacity needs to be calculated according to the ability of the tower construction to withstand tensile forces on one side and equal size of compressive forces on the opposite side. 
     A pillar portion  73   a ,  73   b  with a compressive strength of e.g. 80 MPa is post-tensioned by 40 MPa. The side of the tower construction  100  being subjected to tensile load is to be withstood by the tensile strength existing in the bar elements  93 , preferably of steel, arranged in the pillar portion, while the compressively loaded opposite side of the tower construction  100  is to be withstood by the compressive forces remaining in the high performance concrete y 1 , i.e. 40 MPa. Consequently, if the high performance concrete y 1  can withstand a compressive load of 140 MPa, 70 MPa remains when post-tensioning of the bar element  93  is performed. 
     All pillar portions in a tower construction withstands the same loads since tensile loaded and compressive loaded pillars respectively varies with the wind direction. The same naturally holds also for the strut portions. 
     The foregoing description of the preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated.