A Solid gravity concrete dam construction involves placement of 5,60,000 m3 of concrete in a calendar period of 4 years. The maximum quantity of placement during a year is 2,10,000 m3 to be placed in the second year of work. It is proposed to work 2 shifts/day and 200 days in a year to complete the project.
Suitable granite rock for the aggregates is available in a hill at a distance of 2 Km from dam site. There is no overburden. The product of the quarry has to be processed to produce the aggregates in size ranges of 150 mm- 75 mm, 75 mm- 40 mm, 40 mm- 20 mm and 20 mm- 6 mm. Good quality sand is not available around the site and a blending of natural sand with the processed fines of the crusher will be necessary.
For this Project:
1) Prepare schematic arrangements of the aggregate and concrete producing plants. Give the sizes of various stockpiles of aggregates (Assume suitable mix proportion).
2) Work out the type, size and number of machines required for producing requisite quantitities of aggregates and concrete.
Solution:
Average rate of placement of concrete:
Total quantity to be placed in 4 years = 5,60,000 m3
Average quantity to be placed in a year = 1,40,000 m3
Maximum quantity to be placed in a year = 2,10,000 m3
Working hours in a year @ 200 days, 2 shifts per day of 6 hrs each = 2400
Average rate of placement during the peak year = 2,10,000/2400
= 87.50 m3/hr
Average rate of production of concrete:
There will be some loss of concrete during transportation between the RMC plant and dam site due to spillage, leakage etc. Sometimes, the mixed batch may have to be discarded due to delay in placement or due to unsatisfactory mixing. To account for these losses an allowance of 5 % may be made for concrete mixed at RMC. Therefore,
Average production of concrete required to be done = 0.05 X 87.50 + 87.50
= 91.875 m3/hr
Plant for Concrete production: – As the peak requirement of production of concrete is of 91.875 m3/hr. Hence use two batching plants with pan type mixer of capacity 3 m3 with 3.5 cycle time. This mixer will produce almost 51.42 m3 of concrete per hour.
The batcher size will depend upon the size of mixers. Assume the proportion of mix as 1:1:2 and water cement ratio 0.45. W eight of concrete is taken as 2320 Kg/m3. Weight of concrete mixed in a batch= 2320 X 3 = 6960 Kg. Taking density of aggregate as 1785 Kg/m3 and density of sand as 1545 Kg/m3.
Weight of coarse aggregate = (2 X 6960) / 1 + 1 + 2 = 3480 Kg
Volume of coarse aggregates = 3480 / 1785 = 1.949 m3
Weight of Fine aggregate = (1 X 6960) / 1 + 1 + 2 = 1740 Kg
Volume of Fine aggregates = 1740 / 1545 = 1.126 m3
Weight of Cement = (1 X 6960) / 1 + 1 + 2 = 1740 Kg
Weight of water required per batch = 1740 X 0.45 = 783 Kg = 783 litres
Adopt the proportion of different grades of material in coarse aggregates as 35:30:25:10 for 150-75 mm, 75-40 mm, 40-20 mm, 20-6 mm.
Weight of each grade in one batch of 3 m3 will be calculated as below. This gives the capacity of each batcher required in the plant.
Weight of 150-75 mm size = (35/100) X 3480 = 1218 Kg = 0.682 m3
Weight of 75-40 mm size = (30/100) X 3480 = 1044 Kg = 0.584 m3
Weight of 40-20 mm size = (25/100) X 3480 = 870 Kg = 0.487 m3
Weight of 20-6 mm size = (10/100) X 3480 = 348 Kg = 0.194 m3
The batcher may be sized for additional 33 % capacity. So use two 1600 kg and two 1200 kg batchers for coarse aggregates. For sand and cement use 2200 kg batcher. The size of the storage bins at the batching plant will depend upon the dependability of supply arrangements for concrete ingredients at the batching and mixing plant. Normally, a provision for storing about 3 hours requirement for concrete production is adequate. The suggested bin capacities for each ingredient are shown below:
| Ingredient | Requirement in tonnes for | ||||
| One batch of 3 m3 | One m3 | One Hour 51.42 m3 | Three Hours | Suggested Bin capacity In Tonnes | |
| 150-75 | 1.218 | 0.406 | 20.87 | 62.61 | 65 |
| 75-40 | 1.044 | 0.348 | 17.89 | 53.67 | 55 |
| 40-20 | 0.870 | 0.290 | 14.91 | 44.73 | 45 |
| 20-6 | 0.348 | 0.116 | 5.96 | 17.88 | 20 |
| Sand | 1.740 | 0.580 | 29.82 | 89.46 | 90 |
| Cement | 1.740 | 0.580 | 29.82 | 89.46 | 90 |
Transportation of aggregates from processing plant to batching and mixing plant for each plant: The quantity of coarse and fine aggregates required to be supplied to the bins of batching and mixing plant every hour comes to 91.6 tonnes as calculated in the above table. Allowing for transportation losses of about 3 %, the transportation system should carry 94.35 tonnes per hour. Belt conveyors may be used of capacity 100 tonnes per hour or dumpers may be employed for this work. Cement transportation may be designed for a capacity of 30 tonnes per hour.
Aggregate processing plant: - Processed coarse and fine aggregates will be required to be drawn from the processing plant at an average hourly rate of 61.6 tonnes and 30 tonnes. That is finished product capacity of about 91.6 tonnes per hour for the plant will be required. The raw material handling capacity of the plant would be greater. Assuming 10 % product reject from the plant, the capacity of processing plant to be handle raw materials working at the rate of 2 shifts per day would be 91.6/0.9 = 101.77 tonnes. The stockpile of finished aggregate at the processing plant should be adequate to supply 10 days requirement of concrete plant. The capacities of various stockpiles would be as follows:
Coarse aggregates:
150-75 mm 20.87 X 12 X 10 = 2504 tonnes
75-40 mm 17.89X 12 X 10 = 2146 tonnes
40-20 mm 14.91X 12 X 10 = 1789 tonnes
20-6 mm 5.96X 12 X 10 = 715 tonnes
Sand 29.82X 12 X 10 = 3578 tonnes
Cement 29.82X 12 X 10 = 3578 tonnes
Production of raw material at the quarry: The project requires 61.6 tonnes of processed coarse aggregate at the processing plant. With 10 % process reject the quantity of raw material necessary at the processing plant to produce this quantity of finished aggregate will be 67.76 tonnes. In addition 50 % of the sand will be produced from the quarry supplied raw material. The quantity of raw material needed for this will be 14.91/0.9 = 16.56 tonnes. Thus a total quantity of 67.76 + 16.56 = 84.32 tonnes, say 85 tonnes of raw material is needed at every processing plant every hour for 2 shifts working from quarry. If the transportation losses are taken as 5 %, the transportation system for raw material from the quarry to the processing plant should be designed for 90 tonnes per hour for every plant. This is based on 12 hour/day working.
Use of loader and dumpers can be used for transportation of raw material. The material produced at the quarry will be more than the material supplied to the processing plant as some of the blasted rock will be too large to be loaded and transport to the processing plant. Providing for a 20 % rejection of quarried material, the capacity of quarry should be 180/0.80 = 225 tonnes, with 2 shift a day working.
With a swell factor of 0.6, the quantity of rock drilled and blasted in the hill would be 225 X 0.6 = 135 tonnes per hour with 2- shift operation.
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