Construction World - Indian Edition | September 2008

Special Report – Equipment

Prepared to Precast

Engineer JAGVIR GOYAL tells us about the equipment required to set up a precast concrete component workshop.

Using precast concrete components often results in quicker completion of a job or a project. Precast components can be stored and kept ready for transportation to site as and when required. During in-situ working, a lot of time is consumed in erection of scaffolds, laying of shuttering and, above all, keeping the shuttering in position till the laid concrete gains desired strength. On the other hand, precasting concrete components becomes a parallel activity and saves considerable time. Engineers have enlisted many such building components that can be precast, transported to site and erected in position.

Pre-stressed components
Pre-stressed concrete components gain preference over others as pre-stressing can be well controlled in a precast component workshop and desired results can be achieved. Precasting of pre-stressed concrete components in a workshop requires methodical working, meticulous planning and a workable layout of various yards and storage capacities. Producing pre-stressed precast concrete components and supplying them against orders often proves highly profitable and saves a great deal of time.
Pre-stressed concrete poles
The manufacture of pre-stressed concrete poles in a precast pre-stressed concrete pole workshop is a highly profitable proposition. Achieving cent-per-cent rural electrification in India is a major item on government’s agenda and many schemes are being floated to promote rural electrification. At such times, obviously, demand for pre-stressed concrete poles is very high and existing workshops are unable to meet the demand. Setting up such a workshop can prove to be a million-dollar idea for an investor.

Size of poles
The Rural Electrification Corporation of India has devised certain guidelines for manufacturing precast poles. Mostly, the length of poles required for laying electricity distribution lines is either 8 m or 9 m. There are other lengths required but these are not as common as the 8 m and 9 m long ones. A manufacturer should, therefore, manufacture poles of these lengths as these get lifted after the necessary period of curing and gaining strength.

Equipment for manufacturing
An additional advantage of setting up a pole workshop is that the equipment required for manufacture of pre-stressed precast poles is not heavy duty or costly. Light duty, cheap and indigenously built equipment can be used without facing any problems. This factor comes as a major relief to manufacturers.

The equipment required for a pole workshop are:
Concrete mixers
Concrete carrying trolleys
Shuttering vibrators
Electric pre-stressing machines
Winch machines
Electric pump sets
Welding sets
Air compressor with electric motor
Trolleys for transportation of poles
Pole testing equipment
Sprinkler system for curing

Concrete mixers
Batch-type tilting drum concrete mixers with loaders and steel wheels are suitable for mixing of concrete. Depending upon the daily production capacity of the pole workshop, the number and capacity of mixers can be decided. Preferably, 200 litre capacity 200-t mixers powered with 5 hp electric motors can be selected and used. To avoid frequent breakdowns, motors of standard manufacturers like Crompton should be preferred. For a workshop producing 200 to 250 poles per day, four such mixers should suffice.

One or two mixers should be kept diesel-operated so that work continues uninterrupted in case of power failures. As the mixers are to be continuously used throughout the year to achieve the daily production target, their drums should be provided with extra stiffeners at the bottom to avoid any wear and tear around the axis of rotation, thus avoiding leakage of concrete slurry.

Concrete carrying trolleys
Eight to 10 concrete carrying trolleys or wheelbarrows of sufficient capacity are required to transport concrete from the mixer yard to pole beds, where moulds are fixed in position for casting of poles. Depending upon the production capacity of the workshop and the number of mixers being run, the number of trolleys can be decided.

Use of ready-mix concrete (RMC)
Whenever an RMC plant is available near the pole manufacturing workshop, use of RMC instead of production of concrete in the workshop proves economical and beneficial on many counts. Better quality control of concrete can be ensured in RMC than in production of concrete through mixers at the workshop. Independent procurement of concrete ingredients such as sand, coarse aggregate and cement is avoided. The biggest advantage is that RMC proves cheaper than concrete produced at site as most of the RMC plants in India are set up by cement manufacturing companies. They allocate bulk quantities of cement to their RMC plants. In pre-stressed concrete, the use of only OPC is allowed — the use of PPC is not allowed. An owner of a pole manufacturing workshop finds it hard to buy OPC from the market as this cement is now rarely available with the main market share belonging to PPC. Thus, the pole manufacturer avoids all problems involved in procurement of OPC, aggregates and production of concrete and can simply place the order for RMC at a rate lower than concrete production in the workshop. As the quantity of concrete required per day is fixed and linked to the production capacity of the workshop, the RMC plant owner has a firm regular order in hand and is happy to supply RMC at competitive rates. Whenever RMC is used, procurement of concrete production equipment can be avoided and cost can be saved on this account too.

Shuttering vibrators
The cross-section of pre-stressed poles is normally very small while the number of high tension wires varies from eight to 12. Thus, it is better to compact the concrete with the use of external shuttering vibrators instead of the normally used needle vibrators. For a workshop producing 200 poles per day, about 10 shuttering vibrators shall be required for compaction of concrete. Low voltage high frequency vibrators equipped with 2 hp electric motors should be chosen to produce vibrations in the moulds in the range of 8,000 vibrations per minute.

While choosing shuttering vibrators, care should be kept that the copper winding and terminal box connections in them are vibration-resistant. The bearings should be capable of carrying high vibration load and full operation should be guaranteed even with full centrifugal force. All components should have high wear resistance. A good length of copper cable should be kept available for the vibrators.

Electric pre-stressing machines
Each length of pole beds shall require pre-stressing machines for the high-tension wires laid inside the moulds. In general, a pole manufacturing bed may be 170 to 180 m long for 8-m-long poles and about 200 m long for 9-m-long poles. This is done to fix moulds for at least 20 poles in one line by leaving small gaps between every two adjacent moulds and to pre-stress the continuously running high-tension wires with a single pre-stressing machine. In a pole workshop producing 100 poles of 8 m length and 100 poles of 9 m length every day, 20 beds each of 180 m length should be prepared for 8-m-long poles and another 20 beds of 200 m length for 9-m-long poles should be laid. On each bed, 20 poles shall be cast every day while the wires shall run continuously through all the moulds on one bed. Every fourth day, poles precast 72 hours earlier will be de-moulded and beds shall be again available for next concreting. Pre-stressing machines equi-pped with 5 hp motors and able to pre-stress two wires at a time are suitable for pre-stressed poles. Wire rope of at least 20 mm diameter should be used in pre-stressing machines. Solen-oid electromagnetic braking devices should be available on all these machines for auto-braking during power failures. Anchor grips for high-tension wires with wedges duly hardened and tempered should be fail safe.

Winch machines
The use of wire rope winch machines is necessary to lift the poles from the inside of moulds. In general, poles are de-moulded and shifted from the beds after 72 to 96 hours of laying of concrete. These poles are taken to the curing yard and beds are prepared to lay the next batch of poles; 10 or 12 mm diameter wire rope is sufficient for the winches used to lift the poles. The winches should be manually operated — no motor-operated winches are necessary. Single drum winches with cast iron gears for rotation of drum and lifting of poles serve the purpose.

Gantries of sufficient height of about
7 ft are required for lifting poles from the beds. They are provided with base channels having 12 to 14 inch size cast iron wheels on each end for moving them to required positions above the pole beds. Angle supports should be provided at all corners and angles to provide strength and stiffness to angle and channel framework of gantries. Different heights of gantries are required for de-moulding poles, loading poles to trolleys, and stacking poles in the yard. Low height of about 4 ft is required for de-moulding poles while maximum height, say 12 ft, is required during loading of poles. A 7 ft height of gantries is suitable for stacking poles.

Electric pump sets
Sufficient quantity of water must be available in the workshop for mixing and curing concrete. For satisfactory perform-ance, it is very important to properly cure pre-stressed poles. Thus, submersible pumps should be used for extraction of water from the ground; three to four such pumps should be installed at site. Keeping in view the required discharge and head, the choice of pump capacity can be made. Pumps should not suffer from any suction or priming problems, should be noiseless, able to work without any necessity of pump house, have a single shaft for motor and pump to have permanent correct alignment, and have high operating efficiency. Also, they should be able to work under wide fluctuations in ground water level.

Welding sets
Small but frequent welding jobs are required to be carried out during the manufacturing of poles and availability of welding transformers makes the job easier and quicker. A minimum of two arc welding transformers should form part of the pole workshop equipment — 400 ampere, oil-cooled, copper-wound, three-phase, 50 Hz. frequency welding sets with A class insulation and mounted on wheels for easy shifting should be selected.
A few step-down transformers, each of about 4 kva capacity should be procured and kept ready for the pole manufacturing workshop. Air-cooled, copper-wound, three-phase, 50 Hz. transformers should serve the purpose.

Air compressors
Availability of an air compressor in the pole manufacturing workshop is also important. A 120 litre air compressor of 1.5 BHP with a single phase motor of this capacity can serve the purpose. The air compressor should be equipped with a long flexible hose pipe of 15 to 20 m length. The pressure gauge should be accu-rate and calibrated by a reputed test house.

Special trolleys should be provided in the pole workshop to shift concrete poles from the pole beds to the curing yard. If 100 poles each of 8 m length and 9 m length are being precast in the workshop, two trolleys each for 8 m long poles and 9 m long poles shall be sufficient. These trolleys should be fabricated in a steel works shop according to site requirements as ready-made trolleys are generally not available.

Pole-testing equipment
A pole manufacturing workshop should have the unique feature of testing the final product itself instead of relying upon the cube strength of concrete. A pole workshop should therefore be equipped with pole-testing equipment. The guidelines given by Rural Electrification Cell of India prescribe testing of about 0.5 per cent poles for required strength. A workshop producing about 200 poles per day would produce about 70,000 poles a year leaving aside non-working days — that means testing about 350 poles a year. As and when about 2,000 poles are ready, 10 poles from the lot are sampled out for testing. Pole-testing equipment should therefore be fabricated and installed in the testing yard of the workshop.

Sprinkler system
For the curing of poles, a sprinkler system set up at site proves useful. A network of perforated water pipes should be run above the poles stacked in the curing yard. Water should be continuously sprinkled over the stacked poles for 28 days to ensure full curing of concrete.

Quick Bytes

The manufacture of pre-stressed concrete poles in a precast pre-stressed concrete pole workshop is a highly profitable proposition.

Precasting of pre-stressed concrete components in a workshop requires methodical working, meticulous planning and a workable layout of various yards and storage capacities.

The equipment required for manufacture of pre-stressed precast poles is not heavy duty or costly.

Use of RMC instead of production of concrete in the workshop proves economical and beneficial.

A pole workshop should be equipped with pole-testing equipment.



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