approximate strength of poured reinforced concrete slab
The slab you’re driving on is a slab on grade that does not rely on reinforcing to span voids.
If, as you say, the ground is compacted to 95%, (which is about the most aggregates can be compacted,) then the load will be transferred to the ground without the slab bending.
With 6 1/2 sack concrete, it’s compressive strength is probably between 4,000 psi and 5,000 psi depending on the amount of water, aggregate, etc.  Therefore, one square foot of concrete will support between 576,000 lbs. (4,000 x 12 x 12) and 720,000 lbs. (5,000 x 12” x 12”).
Therefore, one square foot of concrete is substantially less than the area of 8 or 10 tires from the concrete truck.
Note: Don’t let the truck “bump up” onto the edge of the slab...it’ll crack the edge. If possible let it bump up on timbers and have timbers along the edge of the slab and on the slab for the first 12” or so. This will allow the weight to distribute out into the slab.