Building Materials, Third Edition

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Vibration: No vibration is required during placement. SCC flows around rebar and other
inclusions in the form under its own weight.

Flow spread: Flow spreads of 45 cm diameter or grater are obtainable.

Set time: The initial set time in many SCC mixes increase upwards of 90 minutes, depending
on the admixtures used and water content of the mix.

Workability: Workability of Self-Compacting concrete is equilibrium of its fluidity, deformability
and resistance to segregation and filling ability. This equilibrium has to be maintained for a
sufficient time period to allow for its transportation, placing and finishing. For specific uses, for
example Ready Mixed Concrete this time could be 90 minutes or more, if required, while for
the precast concrete production, 30 minutes could be sufficient.

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Self-Compacting concrete has been used as a “special concrete” only by large construction
companies. In order for Self-Compacting concrete to be used as a standard concrete rather than
a special one, new systems for its design, manufacturing and construction need to be established
and standardized.

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 The main reason for the employment of Self-Compacting concrete are the
shortened construction period, assured compaction in the structural elements; especially in
confined zones where vibrating compaction is difficult and, to eliminate noise due to vibration;
effective especially at products plants. Other applications of Self-Compacting concrete are
bridge (anchorage, arch, beam, girder, tower, pier, joints between beam and girder); box
culvert; building; concrete filled steel column; tunnel (lining, immersed tunnel, fill of tunnel);
dam (concrete around structure); concrete products (block, culvert wall, water tank, slab, and
segments); diaphragm wall; tank (side wall, joint between side wall and slab).

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Shotcrete is the concrete conveyed through a hose and pneumatically projected at a high
velocity on a surface. It is similar to gunite (mortar) but with coarse aggregates. The normal
specifications with respect to cement, aggregate and water also apply to shotcrete but the
coarse aggregate used should be harder to account for attrition and of small size. The w/c ratio
is kept quite low. The admixtures such as accelerators are used to permit quick setting of
shotcrete. Consolidation of shotcrete is achieved by the impact of the high-velocity jet impinging
on the surface.
Shotcrete can be made by dry-mix or wet-mix process, the former being more successful and
generally used. In the dry-mix method the cement and aggregates are mixed dry, conveyed
through a hose into a special nozzle by compressed air. Water is sprayed, inside the nozzle,
under pressure and intimately mixed. Shotcrete is projected at high velocity on the surface to
be gunited. In the wet process the mixing is done as for normal concrete. The maximum rate of
deposition of shotcrete is 15m^3 /hr for the dry process but it can be more with the wet process.
The high cost of shotcrete and the wastage due to rebound has to be weighed with other
techniques before recommending it. It is also to be remembered that it is not eco friendly as lot
of dusting problem and waste due to rebound is there.