The appropriate operating range here is to work within that portion 
of the curve that is relatively flat and to avoid the steep part of the 
curve that can lead to blockage. 
Variation in the consistency of the concrete, represented for 
example by slump, also causes variation in resistance as seen in Figure 8. 
Very fluid concretes, say with slumps 6 in. and greater, have low resis- 
tance to flow but are very susceptible to bleeding and thus blockage by 
arching at which point the resistance suddenly becomes excessively large 
and flow is stopped. Concrete with slumps in the range of 2 to 4 inches 
have higher resistances. Very low slump concretes, say zero-slump, have 
high resistance yet can be pumped under high pressure without segregation 
or blockage. However, zero-slump concrete is difficult to use in a 
pumping operation because the stiff concrete does not draw into the pump 
cylinders readily on the intake stroke. Also, it may be too stiff at 
the point of placement to consolidate well. Slumps in the practical 
range of 1 to 4 inches are commonly recommended for pumping concrete. 
For the present case, slumps of 2 to 3 inches will probably be used. 
Pipeline Blockage Prevention 
Blockage of concrete flow in a pipeline is variously called stoppage, 
plugging, clogging, refusal and other terms. There are several types of 
blockage. The two most important types are: (1) blockage due to bridging 
or arching of the aggregates across the diameter of the pipeline, and 
(2) stoppage, or lack of flow, that occurs when the total force of 
friction between the concrete and the pipe wall along the overall length 
of the pipeline is greater than the driving force. Other types of 
blockage include stoppage due to premature setting of the concrete while 
still in the pipeline, and that due to gradual build-up of material on 
the inside of the pipe wall. 
A pictorial representative of pipeline blockage due to arching of 
the aggregates is shown in Figure 9. This is the most serious type of 
blockage since once it occurs it usually cannot be overcome by increasing 
the pumping pressure; this only makes the blockage worse by dewatering 
the material through bleeding. Vibration may help but cannot be depended 
on. Usually, the only remedy is to take the pipe apart and flush it 
out. 
The most common causes are too wet a mix and improper aggregate 
gradation. Other causes are improper size and shape of aggregate. An 
important contributing cause may be a localized high pressure differen- 
tial due, for example, to a leaky pipe joint, a partially closed valve, 
a sharp bend in the pipeline, or an abrupt reduction in pipe diameter. 
Air bubbles or voids in the pipeline will cause bleeding which may lead 
to blockage. The onset of arching is sudden and once initiated usually 
goes to completion. Therefore, conditions (concrete mix design and 
operating procedures) conducive to arching should be avoided by a wide 
margin. 
High resistance to flow due to friction between the moving concrete 
and the pipe wall is a serious limitation in conventional concrete 
pumping operations on land since it demands higher horsepower pumps and 
limits the distance and height to which concrete can be pumped. If the 
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