Q: Is there any admixture that will keep cement and sand from being blown out of the jet stream of nozzle-mixed shotcrete when work is being done in windy weather?

On one bad day we couldn't keep the ingredients together so we had spots where we were practically blasting the wall with water at the same time that we were accumulating a lot of dry sand and cement on the ground.

This seems like too much to ask of an admixture. Good shotcrete really can't be produced if it is windy. When the wind isn't too bad it sometimes helps to fit a metal cone over the end of the nozzle to protect sand and cement from being blown out of the jet stream. Another bad effect of wind is that it can cause cracking from drying shrinkage as well as from evaporative cooling. The shotcreting area should be protected against wind or operations should be suspended.

A: The published suggestion about a metal cone on the nozzle is impractical and also presupposes that the material coming out of the nozzle tip is still unmixed. The problem seems to be one of a faulty nozzle if, as the inquirer states, the water and sand and cement are being separated by the wind. The questioner should check out one of the more efficient nozzles on the market which produce a stream of fully mixed materials. At the least he should insert a 24- to 30-inch hose extension between his water injection body and the nozzle tip he is presently using. An efficient nozzle that is mixing properly will not only prevent wind separation of the ingredients but will also help the nozzleman eliminate laminations in the material in place.

E.R. "Woody" Rogers
Contractors Warehouse Inc.
Alexandria, Virginia