04-12-2012, 03:52 PM
INDUSTRIAL VISIT TO TNEB PRESTRESSING YARD
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INTRODUCTION
The purpose of this industrial visit is to understand the basic process of prestressing through the prestressed electric poles that are manufactured in this yard.
The wires that are used in this yard are of 4 mm dia HTS (High Tensile Strength) bars. These bars are provided with corrugations along the length. The main purpose of it is to provide grip to the bars while the concrete is placed. The concrete used in this site is of M42 grade with 1:1.4:2.3 mix proportion. Unlike RCC, here Ordinary Portland Cement is used. The factor of safety assumed is 2.5. The poles are designed for a wind load of 140 kg/cm^2 and a total load of 350 kg.
The yard has two ends- anchor end and tension end. The anchor end is the area where the bars are arranged in position. Ten wires can be inserted at one time. The form boxes are of two sizes 8 and 7.5 m. 26 poles can be made in one line. The wires are held in position using a wedge and barrel system. This is similar to the bolt and washer system. The tension end is the one where the bars are tensioned. Out of the ten wires that are positioned, eight are given the full tension while the remaining two are applied with a minimum tension so that they do not sag. The tension that is applied through the hydraulic pressure equals to 1.76 tons which is 120kg/cm^2. The aggregate used is of size 10 to 12 mm and the concrete is compacted using electrical vibrators. The water for the concrete is used only after the approval of TWAD board.
PROCEDURE
After the form boxes are arranged in place, the wires are anchored at the anchor end using the wedge and barrel system.
The bars are then tensioned at the tension end.
The concrete is then poured and compacted.
It is then allowed to develop strength for three days by curing, after which it is tested for 50% of the strength.
The testing is done with the Universal Testing Machine, and if the concrete has developed the required strength i.e. 50% value, then the rods are detensioned in a zigzag manner.
This is done in order to prevent the occurrence of cracks if the detensioning is done in a linear fashion.
If the adequate strength is not developed, the detensioning is done after a day or two after the strength has been attained.
TESTING FOR WIND LOAD
The poles are tested for wind load- 140 kg/cm^2, by the deflection cum destruction test.
In this test, the bottom 1.5 m of the pole is fixed analogous to the pole being fixed to the ground under working conditions.
The load is applied 0.6 m from the top using a chain lock and chain pulley.
A reading gauge is attached to this arrangement to monitor the application of load.
At the ultimate point, hairline cracks are developed at the bottom 1.5 m that is fixed.
The loading is stopped and the corresponding load is taken as the failure load.
ADVANTAGES AND DISADVANTAGES
The PSC poles are very economical when compared with its RCC counter parts, as the cost involved is three times lesser than that of the manufacture of RCC poles.
Thus 3 PSC poles can be made with the amount incurred in making one RCC pole.
The aggregate size is less compared to RCC which is in the range of 12-20mm.
The major disadvantage with regard to PSC poles is that it should be properly handled or it will lead to the formation of cracks.
CONCLUSION
Totally 108 poles are manufactured per day in this yard with 56 nos from the 8m form box and 52 from the 7.5 m form box. The poles that are made in this are mostly supplied for use in the rural areas. The labour involved is on a contract basis and so are the equipments. The poles are mainly man-handled as the usage of cranes requires more space.