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TECHNICAL PAPER

Table 5: Design of SCC with optimized paste and aggregates such as hematite, magnetite, ilmenite etc. One
aggregate combinations for different w/b problem that is encountered with high density concrete is the
segregation of the heavy aggregates, which requires a careful
W/P POWDER PASTE AGGREGATE SLUMP T 500 (s)
RATIO CONTENT CONTENT CONTENT FLOW mixture design of the paste fraction. Excess paste is not really
(Vol.) (kg/m ) (lit. /m ) (lit. /m ) (mm) an option since it would lower the density. Hence, the design
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3
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0.8 550 388 612 555 6.0 includes a careful control of the packing of the heavy aggregate
0.9 521 388 612 600 5.0 by choosing optimal particle distributions for the coarse and
1.0 495 388 612 600 4.0 fine fractions [12] . Further, the viscosity of the cement paste
1.1 472 388 612 610 1.9 can be controlled by the judicious use of SP and VMA. While
1.2 450 388 612 635 1.0 segregation control can be achieved by these optimal design
methods, there are several applications where it is desirable to
with the optimal paste composition and aggregate gradation, pump the high density concrete, in order to facilitate the rapid
the flowability as well as the compressive strength improves with construction of such structures. This necessitates the adoption of
packing density. Further, the design of SCC with different w/b the rheology control approach for the design of HDC.
(from 0.8-1.2) with optimized dosages of SP and VMA, keeping
the paste content constant at 388 kg/m , results in concrete The slump test can sometimes lead to an erroneous judgment
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with controlled self-compactability, as presented in Table 5. As about the pumpability characteristics of HDC. While it gives
expected, the slump flow (which is indicative of the yield stress) an assessment of the consistency of the concrete, it is not able
increases and the T 500 (which is indicative of plastic viscosity) to simulate the effect of the lubrication layer caused by the
excess paste in the concrete that leads to improved pumping
decreases when the w/b is increased.
characteristics. A simple approach to the determination of
Comparing the results in Tables 3 and 5, the plastic viscosity rheological characteristics for HDC to assess the pumpability
[13]
of the paste fraction is plotted against the slump flow and the was presented by Venkatachalapathy et al. . This involved the
T 500 for the corresponding concrete mixtures in Figure 9. It is use of a coaxial cylinder set up in which the inner cylinder was
clear that there is a close correlation between the rheology of suspended from a buoyancy balance into an outer cylinder
the paste and concrete, when the aggregate fraction is kept with the freshly-mixed HDC. The outer cylinder was kept on a
constant. platform that was moved downward at a steady rate, resulting
in a shear stress on the walls of the inner cylinder, as depicted
4. PUMPABILITY OF HIGH DENSITY CONCRETE in Figure 10. The shear resistance exerted by the moving
concrete around the inner cylinder created a change in the mass
High density concretes (HDC) with unit weights exceeding measured on the weighing balance. From the maximum mass
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3000 kg/m are used for radiation shielding purposes in nuclear recorded, the shear stress value was computed for the concrete.
facilities. These are primarily designed with the use of heavy A validation of the approach was performed by comparing the
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results obtained for normal concrete (of density 2400 kg/m ) with

Weighing balance

Flexible rope

Inner cylinder
Outer container
Moving platform

Figure 10: Coaxial cylinder set up to determine the yield stress of fresh
[13]
HDC – the moving platform moves the outer container and the
Figure 9: Relation between paste and concrete rheology for the SCC resultant drag felt by the inner cylinder is indicated by the mass change
studies reported in Tables 3 and 5 on the balance

THE INDIAN CONCRETE JOURNAL | JANUARY 2026 55