Placement of RCC

Placement of RCCCooling of aggregates

In places, where the ambient temperature is more than 25 C degrees, it becomes necessary to chill the aggregates or add ice flakes to contain the temperature of concrete discharged from the batching plant to 20 C degrees and a placement temperature of <22 C degrees. The temperatures could be adjusted after going through the thermal studies.

Transportation and placement of RCC

There are various types of transportation and placement plants used in RCC dams. They are listed below: Picking up concrete from the batch plant by a belt conveyor and placing it with a jack posts system with atripper conveyor, line conveyor and crawler placer/ creter crane. This the system used for construction of RCC Burnett Dam in Australia. This placement system was supplied and operated by Rotec Industries Inc., Chicago, USA. The same system is being used in India now for concreting of Baglihar dam Jammu & Kashmir, after the cableways became non-functional due to landslides and damages to tail track.

Surface cleaning before concrete placement

There is always some time-gap in concrete placements between every two successive lifts and it is usual that the horizontal surface gets contaminated during this system-bound operational process. Contamination is injurious to the surface of the lift and must not be allowed. It is essential before concrete placement that horizontal construction surface is made perfectly clean, and any laitance, loose material or free water on the surface is removed by vacuum pickup car. High pressure air or water jets should not be used because these will damage RCC surfaces that are less than 72 hours old and prevent bond development. During cleaning process, if some loose material comes out and holes appear, these should be properly refilled, leveled and thoroughly compacted.

The contact surface aggregate should be perfectly clean and in condition to bond with the mortar layer, which is generally laid before starting the next lift. Placement rates of less than one lift in 24 hours usually require some surface preparation. Material loosened by equipment travel should also be removed. A vacuum truck driven over the old lift immediately prior to placement of the next lift works well. Water cleaning or sandblasting of green concrete tends to remove more material than necessary and tends to leave a washed sand coating on the surface, reducing bond. However, lift surfaces that are older than approximately 72 hours may be washed with low-pressure water, if necessary. Placement rates of three lifts or more per 24 hours generally do not require any surface preparation. The cleanliness of horizontal lift surface is quite critical for an RCC dam and should not be neglected; otherwise it will be permanent seepage problem in the structure affecting its long-term durability.

Mortar spreading

This has relation to maturity factor of concrete. This becomes necessary for proper fusion of two layers of concrete when the time element between two successive layers of concrete is more than permissible.

Joint cutting

Vertical contraction joints are provided in a dam body to avoid the occurrence of random cracks due to temperature stress. Construction of joints should be done with every lift after spreading and proper finishing but before compacting the lift by vibratory roller. In case of any problem, vibratory cutter can also cut the contraction joints immediately after completion of compaction. The task is completed by the use of a vibratory joint cutter to which is attached a vertical heavy-duty jack. The jack forces its joint cutting blade while applying vibration to make a narrow groove reaching down to a specified depth and installing a joint filler board in this groove. If any difficulty is experienced in inserting the joint filler board, a 3 mm thick galvanized iron sheet can be placed inside the groove in place of filler board. Care should however be taken to ensure that groove is cut vertical and the joint filler is also placed in a perfectly vertical position. Joint cutting, though simple and easy, requires proper supervision. Only trained and experienced operator should carry out this job.

Compaction

Best results have been obtained by use of 15 T double drum vibratory rollers. Concrete discharged by belt conveyor or dumper-scrappers is dozed to the required line and level,  and thereafter compacted by use of vibratory rollers. Trials are normally done to decide the number of passes of vibratory and non-vibratory rollers to get proper density. Placement of correct thickness of concrete is achieved by automatic laser guided system connected to the dozer blades. Normally 10 passes of vibratory compactors have been found to be adequate.

Curing

Under no circumstances RCC must be allowed to dry for first 21 days of placing. Curing of concrete on slopes is achieved by use of wetted hesian cloth of appropriate specifications. Curing the lift area is done by a water spray operated intermittently depending upon ambient temperature prevailing at site.

Water slopes

These are placed as close to the upstream face as possible. These are fixed to the correct locations by use of mechanical fixtures since there is no radial joint shuttering provided in RCC dams. In conventional concrete dam, water-stops are fixed by the help of radial shuttering.

Conclusion

From the text of what is stated above, it will be seen that the future of expedient construction of RCC dams to save costs and earn revenue lies in adopting the present world technology and improving it further. This will also help in a big way the utilization of fly ash, which is becoming increasingly necessary.


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