During the pumping of shotcrete, what type of air is lost if the concrete free falls in the pump system?

When shotcrete is pumped, the process involves pushing the mixture through a pump system, which can sometimes lead to free fall within the system. In this context, when the concrete mixture free falls, trapped air is displaced and can escape. This "trapped air" refers to the air pockets that may have been introduced or mixed with the concrete during the mixing process or while it is being transported.

As the mixture moves through the pump, especially during a free fall, these air pockets can be released, which affects the quality and density of the concrete. In comparison, natural air refers to the surrounding air, ambient air is the air present in the environment, and trained air is not a standard term associated with concrete pumping or shotcrete. Hence, in this scenario, the focus is correctly on trapped air, as it aligns with the behavior of air during free fall in the pumping system.