The
SCC mixes are designed and tested to meet the demands of the projects.
For
example, the mix for mass concrete is designed for pumping and
depositing at a
fairly high rate. SCC was used in the construction of the anchorages of
the Akashi-Kaikyo Suspension
Bridge in Japan.
The SCC was mixed at a batch plant at the job site and pumped through a
piping
system to the location of the anchorages 200 m away. The SCC was
dropped from a
height of as much as 5 m without aggregate segregation. For mass
concrete, the
maximum size of coarse aggregates may be as large as 50 mm. Similarly,
SCC
mixes can be designed and placed successfully for concrete members with
normal
and congested reinforcement.
The
three
important aspects in mix design are:
¶
First,
a high-range water
reducing admixture (super plasticizer) is used to provide
the high flowability of the mix, much like a high slump concrete.
¶
Second,
the aggregate content is
proportioned. The size and shape of the coarse
aggregate are very important to the successful manufacture of SCC.
Rounded
aggregate is desirable over angular aggregate because angular aggregate
will
have a tendency to lock together. The coarse
aggregate content will
usually
drop in an SCC mix, resulting in a sand and aggregate
ratio of 0.50 or greater.
The
combined
aggregate grading is the most important aspect of creating an SCC mix.
A poorly graded aggregate source may make it impossible to develop an
SCC mix.
Most admixture companies and large ready-mixed suppliers have computer
programs
to determine optimum blending of aggregate from available sources.
As
with any concrete mix, aggregate size must be limited to that which
will pass
through rebar openings. In SCC, the top size is often 1/2- to 3/8-inch.
¶
Third,
the fluid properties are
altered to provide a cohesive mix that will keep the
aggregate and paste together.
The
viscous
properties of an SCC mix are achieved
through one of three
ways:
- Higher
fines content (upwards of 1,100 cubic yards). The fines can be: cement,
fly
ash, limestone screenings, finely ground glass and granulated ground
blast
furnace slag.
- The
addition of viscosity modifying admixtures (VMA) with lower fines
content.
- The
addition of a combination of both.
