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.