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The placement of high-strength wet-mix shotcrete is
sometimes complicated by the compromise required between pumpability and
shootability requirements. At the pump,
a relatively fluid concrete that will be easy to pump is required; at the
nozzle, a stiff material is desired so it does not sag or slough on the wall.
Most of the time, the simplest solution is to add a set-accelerator at the nozzle and/or
adapt the application schedule to allow sufficient time for initial stiffening
of the in-place material before the next layer of shotcrete is applied. Due
to the stringent quality requirements of modern shotcrete, however, limiting
the amount of accelerator to what is strictly required for reaching stability
and early strength requirements is by far a better method because accelerators
can downgrade shotcrete quality.
An alternative was sought where wet-mix shotcrete could be applied
in relatively thick layers (100 to 150 mm, 4 to 6 in.) without the use of
set-accelerators. In 1994, Beaupré developed the Temporary High Initial Air Content concept. This concept is a
clever and simple system by which the workability of the fresh concrete is
increased to meet the pumpability requirement by introducing a large amount of entrained air bubbles into the mix,
instead of relying on water-reducing admixtures to increase workability.
The trick is that during pumping and
particularly during shooting, a large amount of air is lost due to the
compaction process. This reduces the slump of the in-place shotcrete, thus
increasing the shootability of the shotcrete. This air loss upon impact on
the shooting surface is often referred to as a slump killing effect.
This method of shotcrete production has been used in a number of
applications with success over the past few years. The technicalities of this
concept are simple. Instead of adjusting the plasticizer’s content (normal or
high-range water reducers and super-plasticizers) to produce a 75 to 100 mm (3
to 4 in.) slump at the pump, the plasticizer content should be reduced so as to
produce a 25 to 50 mm (1 to 2 in.) slump. The air-entraining admixture is then
incorporated to produce the slump required for pumping, typically between 75
and 150 mm (3 to 6 in.).
Remember that the slump killing effect works best if
there is a high initial air content at the pump, typically between 10 and 20%.
One should not be afraid to batch the shotcrete with this high air content as
it will be reduced to 3 to 6% in the in-place shotcrete due to the compacting
effect. Hence, it will not result in the negative effect on compressive
strength associated with high air content.
Reference: “Understanding
Wet-Mix Shotcrete: Mix Design, Specifications, and Placement” by Marc Jolin and Denis Beaupré
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