CCAA Guideline Water Wash Out or Blow Back

Allowing concrete to be blown back into the agitator barrel is an inherently risky procedure unless it is carefully controlled. Air pressure can cause anything inside the pipeline to act as a high velocity projectile.
Water wash out is generally a safer alternative.
This guideline outlines the risks and suggested controls for cleaning out concrete pump lines using blow back or water wash out procedures.

Read the guidelines here.