For over 30 years in Canada we have been designing wet mix shotcrete for exterior exposure (rock-slope stabilization, tunnel portals, canals and beams, infrastructure rehabilitation, etc.) to have air content at the point of discharge into the pump to be in the 7 to 10% range. Pumping and the impact on shooting reduces the air content in the in-place shotcrete by about half. i.e. we find the in-place air content in the shotcrete to consistently be in about the 3.5 to 5.0% range. (Only about 1 to 2% air content is lost in pumping; the rest is lost in impacting on the receiving surface).

The air content is measured either by digging out the in-place shotcrete (or dig it out of a shot test panel) and reconsolidating it in the base of the air pressure meter in the ASTM C231 test and conducting the test. Alternatively the shotcrete can be shot directly into the air pressure meter base. It provides virtually the same value as obtained with dug-out shotcrete (as described above), provided the nozzle is held perpendicular to the air pressure meter base, and at the appropriate distance for proper consolidation of the shotcrete.

Testing on numerous projects has demonstrated that shotcrete with 3.5 to 5% in-place air content has a good air voids system ( air content, spacing factor and specific surface), when analyzed in the ASTM C457 test. Such shotcrete has been demonstrated to have good freeze/thaw durability in the ASTM C666 test and deicing salt scaling resistance in the ASTM C672 test. More importantly, feedback from the field demonstrates that such air entrained shotcrete with many thousands of cycles of freezing and thawing in the field over several decades display good durability. There are many research and case-history examples in the published shotcrete literature to support these observations. (See references 1 and 2 below)

With respect to the use of very high air contents at the pump (15-22%), this has been more of a research initiative, used on only a few projects in Quebec, and is not common practice, nor in this writer’s opinion, necessary.

There is another benefit which accrues from the use of air entraining admixtures to get 7-10% air content in the shotcrete discharged at the pump. As any concrete user knows, as the air content increases, the slump goes up. For shotcrete mixes (which have high cementitious contents and low rock contents compared to concretes) this makes the mix easier to pump and shoot. Thus it is common to shoot air entrained wet mix shotcrete at 100 to125mm (4 to 5 inch) slump. On impacting on the receiving surface, as the air content is reduced by about half, the slump of the in-place shotcrete is also instantaneously reduced by about half. (This can be demonstrated by digging the shotcrete out of the in-place material, or a test panel and conducting a slump test on it). We refer to this phenomenon as the “slump killing “process and have used it to advantage on many shotcrete projects. With a good air entrained shotcrete mix design (particularly when silica fume is used) we commonly shoot vertical sections as much as 500mm (20in) thick at 100 to 125mm (4 to 5 inch) slump in a single pass with no problems of sagging or sloughing (fall-out), without having to resort to the use of accelerators.

Finally, there are a few situations where 7 to 10% air content in the shotcrete at discharge into the pump may not work. These are situations where excess air content reduction could occur during shotcrete conveyance, such as dropping shotcrete down a pipe from the surface in an underground mine and catching it in a kettle or remixer unit. In this case, air, if needed, is best added underground in the remixer. Also, pumping shotcrete long distances (particularly pumping shotcrete downhill) may result in excessive loss of air content in the line, which could cause a slump reduction in the line and possible pumping problems. Other than for situations such as these, we always use 7-10% air content in the shotcrete at the point of discharge into the pump (even if it is not needed for frost resistance reasons) because of its enhanced pumping and “slump killer effects”.

Reference 1: Morgan, D.R., “Freeze-Thaw Durability of Shotcrete”, Concrete International, Vol. 11, No. 8, August, 1989, pp 86-93

Reference 2: Morgan, D.R., Kirkness, A.J., McAskill, N. and Duke, N., “Freeze-Thaw Durability of Wet-Mix and Dry-Mix Shotcretes with Silica Fume and Steel Fibers”, ASTM Cement, Concrete Aggregates, Vol. 10, No. 2, Winter 1988, pp 96-102.