The most serious cause of pipeline blockage is segregation of the 
concrete constituents, particularly "bleeding" of the water. To prevent 
segregation as the concrete travels through the pipe, it is important 
that there be no air pockets in the line and that there be no voids from 
the material separating. Also, it is important to maintain plug flow. 
At the beginning of a pour, air in the pipeline system should be 
vented at the concreting head or other high point in the line. Voids in 
the concrete can be prevented by keeping the pipe completely full of 
concrete and under positive pressure throughout its entire length. Once 
concrete flow has begun, it should continue uninterruptedly. Thus, 
there is a need for a backlog of material and backup mixing and pumping 
capability. 
The means for preventing runaway velocity of the flow is to use a 
concrete mixture that has friction head loss characteristics which cause 
terminal velocity of flow, for a given pipe diameter, within a reasonable 
range, probably about six to ten ft/sec. Higher velocities lead to 
laminar or turbulent flow which could cause segregation. A valve or 
throttle will not be used at the seafloor to control the flow velocity. 
The use of a valve or throttle is specifically avoided since the high 
pressure differential across a throttle or a partially closed valve may 
lead to blockage and thus seriously diminish the reliability of the 
system. 
Once the concrete discharge has started at the seafloor, the lower 
end of the pipe must be kept buried in the already emplaced concrete to 
prevent mixing with seawater and the cement washing out of the concrete. 
For most applications, where concrete is emplaced without confinement by 
forms or other means, it will be desirable for the concrete to build up 
into a mound of well consolidated material with fairly steep side slopes 
(say about one vertical for each two horizontal). Therefore the concrete 
will need a fairly stiff consistency, for example a slump of about two 
or three inches. 
As the concrete descends down the long pipeline, pressure can 
increase to thousands of pounds per square inch in deep water. This 
change in pressure may affect the concrete characteristics, for example, 
cause a reduction of slump and thus increased friction. 
State-of-the-art knowledge is available for placing concrete under- 
water to depths of several hundred feet, and for pumping concrete through 
a pipe at pressure heads up to about 1,000 psi and velocities of flow of 
two or three feet per second. For the proposed application concrete 
will be flowing through a pipe at much higher pressures (several thousand 
psi) and higher velocities (up to about 10 ft/sec). The concrete will 
have a much stiffer consistency (about 2 to 3 in. slump) than that 
usually placed underwater (about 6 to 7 in. slump). 
A major engineering assumption has been made: that available 
state-of-the-art knowledge on concrete mix designs can be adapted and 
extended for use in the new environment. Such an assumption needs to be 
verified by laboratory tests on concrete mixtures that will simultaneously 
meet the requirements of: (1) the flow of concrete through pipelines at 
the higher pressures and velocities and (2) the behavior of the stiff 
concrete discharged underwater. 
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