Pumping of concrete is economical if the quantity of concrete required is more than about 1150 m3 or uninterrupted supply of concrete is needed over a long period as at the beginning of each period of pumping the pipes have to be lubricated by mortar (at the rate of about 0.25 in3 per 1000 m length of 150 mm diameter pipe) and also at the end of the operation considerable effort is needed for cleaning the pipes.
Pumping System:
The pumping system consists of the following components:
1. Hopper:
Concrete mixed in the mixer is discharged into this hopper, from where it is discharged at the site by the pump through pipe.
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2. Pump:
Nowadays several types of pumps such as direct acting, horizontal piston type with semi- rotary valves set etc. are available.
The pump is consisted of the following component parts:
(a) Inlet valve
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(b) Outlet valve
(c) Piston
(d) Rubber cup
(e) Pipe.
Working:
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After mixing, the concrete is discharged into the hopper, and pump started. In the suction stroke, the inlet valve is open as shown in Fig.10.5 (a) and delivery valve remains closed. During delivery stroke inlet valve closes and delivery valve opens as shown in Fig.10.5 (b). As shown in the Fig., the diameter of the piston is somewhat less than the diameter of the cylinder.
At the working end of the piston, rubber cups are attached, which check the wastage in the cylinder. At the other end, this piston works in bushes. These bushes are lubricated by the water flowing through empty space between the cylinder and the piston. This water also dissolves the cement paste leaking through rubber cups. Hence the danger of setting the cement paste is reduced. In this way the pump is kept safe from damage. The whole assembly is set in such a way that every part of it can be opened and inspected as per need and the damaged part replaced.
Capacity:
Usually concrete can be pumped along a straight and level pipe line for a distance of about 300 to 450 m and upto 40 m vertical height using a single pump. Any rise of 30 cms is equivalent to 2.4 m on the level ground and a right angled bend is equivalent to about 12 m. Hence for a bend of 90° a horizontal distance of 12 m and for 22 ½° bend 3 m horizontal distance should be reduced. No sharp bends should be attached in a concrete pipe line. In between bends straight pipe should be attached.
Pipe Line:
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Pipe lines for concrete are usually made of mild steel tubes in 3 m sections with special joints for quick assembly and release. Bends and short lengths are also available. The size of pipes varies from 11.25 cms to 20 cms inside diameter. The diameter of pipe should be at least three times the maximum size of aggregate used.
Types of Concrete Pumps:
Squeeze Type Concrete Pump:
In recent times much improvements and refinements have been incorporated in the modem concrete pumps. During 1963 squeeze type concrete pump was developed in U.S.A. In this pump concrete placed in a collecting hopper is fed into the flexible pipe by rotating blades. The flexible pipe is connected to the pumping chamber. In the pumping chamber a vacuum of about 600 mm of mercury is created. This vacuum ensures the shape of the pipe cylindrical except when squeezed by the roller.
This ensures the continuous flow of concrete. Two rotating rollers progressively squeeze the flexible pipe and move the concrete into the delivery pipe Fig.10.6 shows the action of a squeeze pump. Often squeeze pumps are lorry mounted and concrete may be delivered through a folding boom.
Most widely used modern pump is the hydraulic piston pump. Though the specifications of modern and original mechanically driven pumps differ, but the concept of working is the same.
A concrete pump consists of three parts namely:
(a) Concrete receiving hopper,
(b) Value system, and
(c) Power transmitting system.
Types of Modern Concrete Pumps:
There are three main types of concrete pumps.
They are:
(1) Mobile,
(2) Static or trailer,
(3) Mortar or screed pump.
Type of Valves:
The valve system is the most important part of any concrete pump.
Following are the main type of valves:
(a) Squeeze type valves
(b) Rotating or sliding gate valves
(c) Flapper valves
(d) Hollow transfer tube valves
(e) Rock valves
Hollow transfer tube valves are most commonly used. Rock valve are also widely used valves.
Problems in Pumping of Concrete:
The most common problem of pumping concrete is blockage. If the pump is mechanically sound, the failure of concrete to emerge at the end of the pipe line is called blockage. The increase in the pressure shown by the pressure gauge of the pump is the indication of the blockage of the pipe line. Most blockages take place or occur at the tapered sections of the pump end.
Properties of the Concrete to be Pumped:
Concrete to be pumped should meet the following specifications:
1. The Concrete should be neither too wet nor too dry. Usually the slump should vary between 5.7 cms to 10 cms. With lower water content, the solid particles instead of moving longitudinally in a coherent mass in suspension exert pressure on the walls of the pipe. When the water content is in excess, segregation will take place. When the water content is at the correct or critical value, friction develops only at the surface of the pipe and in a thin 1 to 2.5 mm layer of the lubricating mortar.
Thus all the concrete moves at the same velocity. For concrete to be pumped, slightly higher cement content has been found useful as it allows the formation of lubricating film in the pipe. The magnitude of friction developed depends upon the consistency of the mix.
2. Cement/aggregate ratio should not be more than 1:6.
3. Water/cement ratio may range from 0.5 to 0.65.
4. A mix proportion of 1:2:4 has been found ideal for this concrete.
5. Well graded aggregate should be used for pumped concrete.
6. For pumped concrete light weight aggregate should not be used.
7. Usually 20 mm size aggregate should be used. For concrete with maximum size of coarse aggregate of 20 mm, the optimum amount of fine aggregate should be between 35 to 40%; and the material finer than 300 micron should be between 15 to 20% and 3% passing through 150 micron sieve of weight of fine aggregate.
8. The quantity of fine particles i.e. cements and sand below 0.25 mm size i.e. particles below 300 micron should be between 350 to 400 kg/m3.
9. The slump of pumpable concrete may be kept 7.5 cms to collapse range and diameter of pipe line should be 3 to 4 times the maximum size of aggregate. However concrete slump may vary from 4 cm to 10 cm and compacting factor 0.9 to 0.95.