When pumping concrete there is normally a change from hopper to hose end. I understand what causes air content to drop. The question I have is, what causes air content increase from hopper to hose end?

really low slump concrete and an empty hopper are couple of causes

Bad concrete is the only time it ever increased on the test

bad cups can increase air. do you have grout in your waterbox when cleaning out

 Had these problems 1000 times and most of the time its the batch plants issue. When you have these issues make sure not to add water because it will often increase the air. Severe wheather changes Or a new product line of add mixtures not used by the conrete compant before can be another issue. When I see these problems I just step back and let the QC's, Inspecter's and the General Contractor's fight it out.

http://www.concretepumpers.com/content/air-entrainment-report

Telebelt=no air loss.  Pump can be minimal loss if free fall is reduced....I always liked to lay hose on ground and pump up into testers wheelbarrow.. instead of just dropping it in..  like one project engineer is rumored to have said...."put a piece of plywood over the mixer chute to keep that air out". Air was too high to suit him I guess.  Caused a few laughs at his expense.

Does anyone know if higher pumping pressures contributes to air loss?