Effect of Concrete Mixtures on Flow Resistance 
The characteristics of the materials and their proportions in the 
concrete mixture affect the flow resistance in several ways. The two 
most important characteristics of the mixture are the inherent resistance 
of the mix to bleeding and the magnitude of the flow resistance when the 
mix is saturated. 
To provide a cohesive mix that will not bleed easily the mix is 
proportioned by the method of minimizing the total interparticle void 
volume between the solid particles of cement and aggregates. The void 
space is continuous but the individual channels that remain for water 
passage are extremely small and intricately interconnected. Thus when 
the mix is saturated with water, hydrostatic pressure can be transmitted 
via the water phase yet the small diameter, high specific-surface-area 
interstices limit the flow of water through the mix. 
Mix design is by careful gradation of the successively smaller 
sizes of aggregate and cement so that the voids between larger solid 
particles are filled with smaller particles and the voids between them 
with smaller still. This provides small tortuous paths for the water to 
migrate through the aggregates. In this condition, hydraulic pressure 
moves the mass of concrete down the pipe rather than just the water 
through the interstices of the solids. 
Three ways in which the mix proportions affect resistance to flow 
of saturated concrete are shown qualitatively in Figures 6, 7 and 8. 
Quantitative information depends on the specific mix and size of pipe. 
The information presented is based on laboratory tests in which the 
pressure required to pump concrete was measured while the following 
individual factors were varied: (1) cement content, (2) size, shape and 
percentage of coarse aggregate, (3) amount and gradation of fine aggregate, 
(4) consistency of mix, and (5) pipe diameter and length (Ref 16). 
The saturated state resistance to flow is strongly affected by the 
amount and fineness of the fine material, particularly the total amount 
of very fine material; i.e., the cement and fine aggregate passing the 
number 200 sieve. 
In Figure 6 it is seen that the resistance is a minimum at a certain 
amount of total fines per cubic yard indicated by point "A." As the 
amount of fines is decreased, the resistance increases gradually until 
at a certain minimum amount of fines indicated by point "B" blockage of 
the pipe occurs abruptly due to arching where the resistance suddenly 
increases to a very large value. On the other hand, if the amount of 
fines is increased past point "A," the resistance increases smoothly to 
a higher value than at "A" but without blocking the pipeline. 
Thus the control of terminal velocity of concrete traveling down 
the pipeline is best achieved by adjusting the total amount of "fine 
fines" in the mix so that the system is operating in a resistance range 
in the vicinity of point "'C." The velocity of flow can then be further 
controlled by varying the pump pressure. 
The influence of the amount and size of coarse aggregates on flow 
resistance is shown in Figure 7. Increasing the amount of coarse aggregate 
as a percentage of total aggregate has a moderately increasing effect on 
resistance until values in the range of 50% are exceeded at which point 
resistance rises rapidly and can lead to blockage by arching. 
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