Laboratory and field tests (Ref 13-18) have shown that for a saturated 
concrete flowing through a straight pipe of constant cross section in 
plug flow the total resistance to flow is (approximately) directly 
proportional to the length of pipe and to the velocity of flow, and 
inversely proportional to the pipe diameter. Resistance is essentially 
independent of radial pressure and total pressure, for the range of 
pressures tested to date (up to about 1,000 psi), provided that the 
pressure does not change the characteristics of the concrete mix, for 
example, by decreasing the interparticle void volume by compressing 
entrapped air or forcing water into porous aggregates. 
It is assumed that the above stated relationships between resistance, 
length, diameter and pressure can be extended to the pressure environments 
of interest in the deep ocean (8,900 psi at 20,000 ft). However, this 
must be verified by laboratory tests. 
Different concrete mixes will have different characteristics of 
flow resistance; that is, the above linear relationships are relative to 
a specific mix. If the mix changes, then the resistance properties may 
also change. The effect of mix characteristics on friction is discussed 
in a later section. 
When the concrete is not saturated the water does not completely 
fill the interparticle void volume, the particles come in contact with 
each other, an intergranular pressure develops, the ratio between the 
axial and radial pressures changes, and the contact pressure between the 
solid particles and the pipe wall increases dramatically. The resistance 
to flow is much greater for unsaturated than for saturated concrete. 
Additionally, unsaturated concrete usually exhibits dilatancy; any 
movement of solid particles reiative to one another tends to cause an 
increase in the total interparticle void volume, and thus, if the volume 
increase is constrained, an even greater intergranular stress and wall 
friction are developed. 
The resistance to flow of concrete in the unsaturated state is not 
linearly proportional to pipe length, but is a function of the radial 
pressure; also the relationship between the radial pressure and axial 
pressure changes with change in axial pressure. 
The difference in flow resistance between a given mix in a saturated 
state and the same mix in an unsaturated state is great. The unsaturated 
mix can develop the same amount of total resistance in a few feet of 
pipe length as that developed in several hundred feet of pipe length in 
saturated flow. Additionally, unsaturated concrete may be subject to 
bridging and arching of the aggregates across the diameter of the pipe 
as discussed below. 
In practice, then, the concrete mix must be saturated. If it 
becomes unsaturated, blockage may occur almost immediately. A common 
cause of change from saturated to unsaturated is bleeding of the water. 
Bleeding is prevented by providing a mix that is "plastic" and "cohesive," 
by avoiding high pressure differentials (such as an abrupt change in 
pipe diameter), and by avoiding large discontinuities (such as air 
bubbles and voids) in the water phase. 
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