Flow of Concrete in a Pipeline 
The flow of concrete through a pipeline is usually described as 
"plug flow" in which the main mass of the concrete slides along on a 
thin lubricating film of water, cement and very fine sand on the inside 
wall of the pipe. The following discussion of concrete flow behavior 
and resistance in a pipe is based on work reported in References 13 
through 16 and summarized in References 17 and 18. 
The plug of concrete consists of the particles of cement, sand and 
coarse aggregate, and a water phase which occupies the continuous inter- 
particle void space. As shown in Figure 5 the flow velocity is essen- 
tially constant across the plug and drops rapidly across the lubricating 
film to zero at the pipe wall. Thus in a straight pipe of constant 
diameter there are no internal shear stresses within the plug. The 
resistance to flow is considered to be due to the friction between the 
peripheral surface of the plug and the inside surface of the pipe wall, 
and to the internal shear force within the lubricating layer. At a 
change in pipe direction or diameter, resistance to flow changes since 
the solid particles within the plug move relative to each other and 
develop internal shearing forces similar to those in a fluid in laminar 
or turbulent flow. For mild bends and tapers whose total length is 
small compared to the overall length of the pipe, the contribution to 
resistance due to internal shear in bends and tapers is a small percentage 
of the total resistance. 
If flow velocity increases above about 7 to 10 ft/sec (depending on 
the pipe diameter and on the individual mix) the central core of the 
concrete tends to move faster than the concrete closer to the pipe wall. 
The flow thus assumes a laminar or quasi-laminar flow profile with 
internal shear stresses. The behavior of concrete in this flow regime 
is not well known, but a tendency for the coarse aggregate to accumulate 
in the central core can be postulated. Such a moderate segregation by 
aggregate size could be aggravated by the cumulative effect of long 
distances to the point of concentrating the coarse aggregate into rock 
pockets which could then cause sudden blockages by arching. Since this 
is an unknown regime, velocities higher than about 10 ft/sec should be 
avoided until this problem can be studied. 
Effect of Saturation on Flow Resistance 
The resistance due to friction of the solid particles in contact 
with the pipe wall is strongly affected by whether or not the concrete 
is saturated. In the saturated state the water completely fills the 
interparticle void volume. In fact, it is necessary to have sufficient 
water to overfill the voids in the dry material. Thus, the water separates 
the solid particles and the intergranular pressure between particles is 
negligibly small, as is the normal force of the particles against the 
pipe wall. In this saturated state, the concrete is forced through the 
pipe by the hydraulic pressure in the water. The radial pressure against 
the pipe wall is nearly equal to the longitudinal pressure since it is 
due almost entirely to the hydraulic pressure of the water and little to 
the solid particle pressure. 
16 
