Does usage of accelerators in Shotcrete
affect its strength adversely?
Reference:
“Understanding Wet-Mix
Shotcrete: Mix Design, Specifications, and Placement” by
Marc Jolin and Denis Beaupré
The
dosage of the accelerator is a very important aspect of shotcrete
technology.
The claim is sometimes made that some accelerators do not affect the
long-term
strength or other properties of shotcrete. In fact, it is not only the
type of
accelerator that is important with respect to long-term properties, but
also
its dosage.
Normally
all accelerating admixtures increase the early-age compressive strength
when
compared with the reference shotcrete. However,
compressive strengths at later
stage (say 28 and 56 days) show marked reductions for some mixtures
when
compared with the reference mixture.
It
is therefore recommended to minimize
the amount of set-accelerator added to any
wet-mix shotcrete mixture to limit these long-term reductions in
compressive
strength. Similar reductions in quality can be measured for
absorption and
drying shrinkage, where the values can be increased (while you want to
keep
them as low as possible) as much as 100 and 50%, respectively.
It
is important to not only select the
right type of accelerator (compatible with
the cement and admixtures, early strength development, and shelf life) but also
to limit the accelerator dosage to the amount required to ensure the
stability
of the fresh shotcrete layer and to meet the early and later age
strength and
other performance requirements.
