Nowadays in many situations, concrete is placed through pipes. Thus the Engineer in-charge must be acquainted with the pumpable concrete. Pumpable concrete is not a special concrete, but an ordinary concrete which can be pushed through a pipe line is called a pumpable concrete. This concrete should be proportioned in such a way that its friction on the inner walls of the pipe line does not increase to such a limit to prevent its movement at the pressure applied by the pump. This concrete should contain certain amount of fines (cement + fine aggregate) more than the ordinary concrete to offer lubrication on the inner walls of pipe line.
Requirements of Mix Design for Pumpable Concrete:
The pumpable concrete should have:
(a) It should have minimum fine particles smaller than 0.25 mm in size (cement + fine aggregate), approximately 400 kg/m3 of concrete for the max size of 32 mm of coarse aggregate. In case of very angular and flaky aggregate, the quantity of fine aggregate may be increased by 10%.
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(b) Amount of cement. This concrete must contain a minimum cement content of 240 kg/m3 of concrete for max. size of 32 mm of coarse aggregate. In case the max. size of coarse aggregate is 16 mm, then quantity of cement should be increased by 10%.
(c) Water/cement ratio. The water/cement ratio of pumpable concrete may vary from 0.42 to 0.65.
(d) Workability. The slump may be between 75 mm to 150 mm.
(e) The grading curve of aggregate as shown in Fig.20.13.
For designing a pumpable concrete, it is essential to understand the action on the concrete when it is pumped through the pipe line.
The first requirement of the concrete is that under the pressure from the pump, the mix must neither bleed nor segregate. It must be able to bind all constituents of the mix together. The mix should be able to deform while flowing through the pipe line at tapered sections and bends.
This property can be achieved by adding more fine particles. Thus the proportion of fines is of prime importance. For pumpable concrete the contents of fines (cement + fine aggregate) 350 kg/m3 to 400 kg/m3 are considered necessary. This amount of fines is also necessary for maintaining lubricating film around the concrete plug to reduce wear and tear of the whole system.
Blockage of Pump System:
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Assuming the pump mechanically sound, there are following two reasons for the development of blockage in the pump:
1. Water is forced out of the mix. resulting bleeding and blockage by jamming.
2. The frictional resistance is too much due to the nature of ingredients of the mix.
To produce good quality pumpable concrete, a good grading of aggregate is necessary. Flaky and elongated aggregate will produce harsh concrete for the given cement content and water/cement ratio. The grading curve of the aggregates to be used in the proposed mix should be parallel to the curves of Fig.20.13, but not coarser than curve no. 2. The curve may be brought parallel by making adjustments in the proportions of F.A. and C.A.
It has been recommended that 10-20% of fine aggregate should pass through 300 micron sieve. Sometimes to safeguard against the under sanding 3-4% extra sand is added. The total content of fine aggregate in kg per cubic metre of concrete for various maximum size of coarse aggregate is shown in Table 20.50.
Step by step design calculations for a pumpable concrete mix.
Step 1:
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Draw the grading curve of aggregate Fig. 20.15 shows a more acceptable grading curve for pumpable concrete using 20 mm max. size aggregate. The grading curve drawn should fall within the curve of Fig. 20.15. If necessary re-adjustments in the proportions of coarse and fine aggregate may be made.
Step 2:
The limits of fines contents should conform as given in Table 20.51.
Step 3:
The mix proportions should be adopted on saturated surface dry condition as shown in table 20.52 for a slump of 75 to 100 mm.
Step 4:
The mix proportion of table 20.52 is based on the grading of aggregate shown in table 20.54.
Step 5:
Absolute density of materials is taken as given in table 20.53.
Step 6:
The calculations of combined aggregate grading based on the proportion given in table 20.52 are given in table 20.55. These combined grading limits should fall within the limits given in table 20.53.
Step 7:
Find out the fires contents.
Example 4:
Design a mix for pumpable concrete from the following data:
1. The max. size of coarse aggregate = 20 mm.
2. The fine aggregate passes through the 300 micron or 0.3 mm = 19%
3. Slump of concrete = 75 to 100 mm
Procedure:
The proportions for 75-100 mm slump will be taken as given in table 20.52.
From the table 20.52,
Water = 190 kg or litres
Cement content =310
Fine Aggregate = 880
We know that total fines less than 0.25 mm for pumpable concrete should be 350 to 400 kg per cubic metre of concrete.
As the fine aggregate passes 0.3 mm sieve is 19%, the sand particles passing through 0.25 mm sieve
= 19 x 25/30 = 16% app.
∴ Fine particles in sand = [(16 x 880)/100] = 140.8 = 141 kg.
Cement content = 310
Total fines = 310 + 141 = 451 kg.
From Table 20.51 for 190 kg water solid fines per m3 of concrete should be 330 to 465 kg/m3.
Hence total fines of 451 kg/m3 as calculated are suitable.
From table 20.52, coarse aggregate = 930 kg.
Note:
The crushed aggregate usually requires higher sand contents to achieve the required void content. Crushed fine aggregate contains a lot of dust passing through 0.150 mm sieve. The excess of dust particles should be avoided which may cause high pipe line friction and blockage.