In the past decades, prestressed concrete poles have become famous and have replaced the traditional poles made of timber, steel or reinforced concrete. The earliest prestressed concrete poles were designed in 1933 by Freyssinet, a French engineer.
Different shapes of Prestressed Concrete PolesThere are different shaped cross sections of prestressed concrete poles are available but for any type of pole, base should consists maximum cross sectional area since it requires maximum moment of resistance. Rectangular or square shaped poles are suitable for smaller lengths up to 40 ft. They provide good facilitation for prestressed wires at required depth. Transportation of such poles is much easier. Vierendeel poles are also used for smaller lengths but they are made of thin elements and contains larger exposed area which makes them susceptible to corrosion failure. Circular poles are suitable for longer lengths. Circular hollow poles are less in weight and possess equal strength in all direction. In hollow type poles, concrete is consolidated by high speed spinning which makes the concrete denser and stronger. Cylindrical tapered poles are also made from spin casting method. The tapering should be uniform and it ranges from 0.15 to 0.18 inch per foot. Hexagonal, octagonal, triangular poles can also be made using spin cast method.
Design Considerations of Prestressed Concrete PolesPrestressed concrete poles are designed as members with uniform prestressing since they has to resist equal bending moments in opposite directions which is not the case for other prestressed structures. The magnitude of prestress required is half that of the normally provided for bending in one direction. Prestressed poles are casted in filed and transported to job site later. So, handling, transportation and erection stresses should also be considered while designing prestressed poles. These poles are designed as cantilever structure and one should consider both axial and bending loads acting it. Bending moment is predominant in prestressed poles when compared to axial loads and shear forces. These are resilient members.
Manufacturing Methods of Prestressed Concrete PolesThree methods are generally used to manufacture prestressed concrete poles and they are
- Centrifugal Casting Method
- Long Line Method
- Mensel’s Method
1. Centrifugal Casting MethodCentrifugal casting method is also called as spin casting method which is used to manufacture hollow and tapered prestressed concrete poles. In this method, concrete is partially filled in a steel forms and they are placed in spinning machine The concrete in the forms is consolidated by the centrifugal force created by spinning machine which will rotate for several minutes. While spinning, the concrete squeeze out water from it and this excess water is poured from the hollow cavity created in the center of the pole. Finally, the form is exposed to steam for curing for a period until the strength of concrete reaches 3500 psi. After that prestressed wire is released and allowed for air curing for a period of 28 days. Finally hollow prestressed concrete pole is obtained.
2. Long Line MethodLong line method is most commonly used method for making solid prestressed concrete poles. In this method, molding forms are positioned end to end on casting bed. These forms are placed up to a length of 400 feet. The molding forms contains bulkheads at its ends and holes are provided to these bulkheads using which prestressing wires are threaded. These wires are pretensioned against abutments at each end of line of forms. This pretensioning is done once at a time for multiple poles. Now Molding forms are filled with concrete which was vibrated externally. Using this method, many shapes of solid poles such as square, rectangular, I shaped, Y shape etc. can be manufactured. This method can be done at any precast site or yard.
3. Mensel’s MethodMensel’s method of prestressed poles making is more mechanized process. In this, poles are made on a production line which consists of horizontal molds of light weight. These molds will move from one station to other in production line. The concrete is poured in these molds and a block out is provided in the middle of mold while pouring concrete to make hollow concrete poles. The concrete in the molds is consolidated by vibration. When the concrete is begun to harden, block out in the middle is rotated and removed at fully hardened stage. These poles are heated to a temperature of 73oC for 24 hours and cooled down to room temperature.
Advantages of Prestressed Concrete PolesAdvantages of prestressed concrete poles over normal reinforced concrete poles are as follows:
- Less in weight and easy to handle.
- Installation of prestressed concrete poles in drilled holes is easier and simple.
- They are less permeable and provides good Resistance to corrosion of prestressing wire especially in hot weather regions.
- Good Resistant to erosion in desert regions.
- Good fire resistance which is useful when bush fires or grass fires occur near the ground line.
- Good resistance against freezing and thawing occurred in colder regions.
- They have more rigidity and can carry higher loads than normal RCC poles.
- They can be manufactured in number of shapes with clean and neat finishing which makes it good in appearance.
- Requires less maintenance because of good resistance properties.
Uses of Prestressed Concrete PolesBecause of their durability prestressed concrete poles are widely used all around the world now a days. They can be used as
- Lighting poles
- Railway power and signal supporting poles
- Telephone poles
- Flag poles
- Antenna masts etc.