Building Materials, Third Edition

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The method for achieving self-compactability
involves not only high deformability of paste
or mortar, but also resistance to segregation
between coarse aggregate and mortar when
the concrete flows through the confined zone
of reinforcing bars. To achieve self-
compactability, the aggregate content are
limited, the water-powder ratio is kept low,
and super plasticizers are used. The methods
to achieve self-compactability are shown in
Fig. 20. 8, and the influence of all these factors
are as to follow.
The frequency of collision and contact between aggregate particles can increase as the
relative distance between the particles decreases and then internal stress can increase when
concrete is deformed, particularly near obstacles. The energy required for flowing is consumed
by the increased internal stress, resulting in blockage of aggregate particles. Limiting the coarse
aggregate content, whose energy consumption is particularly intense, to level lower than
normal will be most effective in avoiding this kind of blockage.
Highly viscous paste is also required to avoid the blockage of coarse aggregate when
concrete flows through obstacles (Fig. 20. 9). When concrete is deformed, paste with a high
viscosity also prevents localized increase in internal stress due to the approach of coarse
aggregate particles. High deformability can be achieved only by employment of super-plasticizer,
keeping the water-powder ratio to a very low value.

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Non-Segregating: The aggregate stay in suspension in the mix as it flows into the form.

Non-Bleeding: Water does not rise to the top of the mix or is observed on the outer edges of
a flow test.