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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.
Reference: “Understanding
Wet-Mix Shotcrete: Mix Design, Specifications, and Placement” by Marc Jolin and Denis Beaupré
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