Pylons made of High-Strength Spun Concrete and prestressed with CFRP for high power transmission lines

摘要

Steel structures exposed to weather have been showing increasing corrosion damages over the last 20 years, causing serious and expensive maintenance problems. Lattice steel pylons for transmitting voltages typically need to be retreated with expensive corrosion protection coatings after 15 - 20 years, giving rise to considerable costs and producing substantial environmental pollution. Reinforced concrete elements (e.g. spun concrete pylons) are corrosion-resistant. However, it is precisely this which also makes them relatively heavy: the corrosion protection can be guaranteed only by covering the steel reinforcement with at least 3 cm of concrete. This covering is largely unnecessary, when a reinforcement consisting of carbon fibre reinforced plastic (CFRP) - which is not susceptible to corrosion - is used. Along with the simultaneous use of a high-strength concrete, a marked weight reduction can be achieved, and concrete poles exposed to weather conditions are far superior to steel towers with respect to corrosion resistance. The aim of the project presented is the production of a 27 m high CFRP-prestressed spun concrete pylon as a support for an electric power line. This Pylon will be used as a support mast in a section of the 110 kV line of the Nordostschweizerische Kraftwerke (NOK, power stations of North East Switzerland) Beznau-Baden. The high corrosion resistance of the CFRP prestressing and shear reinforcement allows minimization of the concrete cover so that a minimum cross-sectional wall thickness of only 4 cm can be aimed for (at present this thickness is about 10 cm). The low weight of the CFRP reinforcement (the density of CFRP is only 1.6 g/cm~3, which is a fifth of the density of steel) and its high tensile strength (CFRP prestressing rods have a guaranteed tensile strength of 3'000 N/mm~2, which is twice that of a prestressing steel) are also noteworthy. These two factors consent a weight reduction on the reinforcement side of 90% compared with conventional prestressed concrete constructions. On the matrix side, high-strength spun concrete of strength class B110 is used. Owing to its high strength, this achieves the stated minimization of the cross-sectional dimensions. The envisaged pylon weight of about 6 t means a 40% weight saving compared to the traditional steel reinforced spun concrete pylon. The transport and installation costs are thus lower; the expected life without maintenance is 50 years. In addition to the production of the pylon, a static test was carried out on the mast to 70% of its design bending resistance, in order to validate the static calculation methods. After the pilot mast has been installed, long-term monitoring in the field will be implemented by means of electrical resistance measurements of the CFRP pretensioning rods (parallel to conventional strain-gauge measurements). This paper describes the different stages of development and the problems encountered while designing and manufacturing the prototype pylon. The presented project results from a close co-operation of the spun concrete element production plant SACAC with the Swiss Federal Laboratories for Material Testing and Research EMPA and the power stations of North East Switzerland NOK.