I'll try to be brief, but one could write a book on this subject. The following is based on my experience in upstate NY, which has similar climate to Chicago, where I assume the garage in question is located.
There are many potential reasons why the spalling has occurred. Freeze-thaw is a definite possibility. The ground level normally carries the heaviest chloride load and is the wettest, with cars bringing road salt and water in from the entrance roadway. This combined with the winter climate in Chicago result in challenging conditions for any exposed concrete. Other possibilities include inadequate concrete curing, low air entrainment, poorly finished concrete (water added or brought up to surface due to overworking) and alkali-silica reaction (ASR) due to poor reactive coarse aggregate in the concrete, although this should be evident throughout the structure.
You can core the distressed concrete areas (and also the good concrete areas for comparison) and have them tested for chlorides, freeze-thaw resistance, ASR susceptibility, and petrographic analysis performed for air entrainment. Prevention of further damage is normally addressed by repairing the spalled areas, cleaning and sealing the entire surface of the ground floor with a penetrating type sealer. Chloride extraction is sometimes employed, depending on the level of intrusion, but this is typically not warranted.