Assuming the initial test was at the back (or front) of the mixer and that the test from the middle of the load was not at the end of the pump line, other causes could be as simple as the slump decreased and made the air content lower. The mixer could have been stopped for a while and the...
As long as the contractor doesn't spread deicers on the non-air entrained concrete it should be fine. Use sand for traction on localized pathways across the slab.
