It’s flow rather than volume. It’s one of the reasons that the USA is doing so terribly in this compared to other nations who focus on volume.
US tends to prefer working with fast flow of product and everything’s predicated upon that. Stop the machine even for a moment and it all falls apart very quickly.
No backup. It’s not volume based but flow based. Been that way starting around the late 1970s and increasing.
America’s got a concrete mixer economy.