I made the mistake of using asphalt in my garage instead of concrete. I would like to cover the asphalt with concrete to a height of approx 2.5 in. As I said, this is inside so there won't be as fast a temperature change in sunlight as there would be on a driveway so the expansion and contraction will probably not be as drastic as a driveway. Does anyone think pouring 2.5 in will be OK if I put scribe lines at 4 ft spacing?

More detail is needed, but two and one half inches of normal concrete is insufficient for vehicle loads. It may crush on a good base or deflect and break on a bad base. Are you planning to use a special high strength mix with additives? What is the condition of the asphalt? Is it rutted or otherwise distorted? What problems do you have with the asphalt now?

Agreed that more info is needed but as a general reference: What you propose is/was called whitetopping, more commonly used on airport runways, and other high traffic areas where the asphalt is not cracked, alligatored, or has potholes, etc as concrete will not bond well in these areas leading to (most likely) failure of the concrete. If none of these conditions exist, you have to mill the asphalt to grade while maintianing about 3" of support base. joints are usually (in all directions) 12" per inch of topping. Toppings range from 2 - 8" thick and (as smearcase states) you can admix the topping with Tuffstrand or fibrestrand (or similar product) which will make the concrete stronger but decreases the initial workability.