It has been two years since I installed hardwood floor over concrete slab. I did not see any distress so far. The most time consuming task was to secure the plywood to the slab---I screwed the subfloor to slab every square foot.

This winter, I plan to install 2 1/4" strips over the existing 3/4" plywood in my living room---I have basement below the plywood---do I need to leave an extra gap in anticipation of wood expanding during the summer season? There will be 1/2" expansion gaps allaround the walls but I would like to know if an extra gap would be needed, say after every 6 rows or so? My plan is to slip a paint scraper between the strips after laydown 6 rows or so.

Thank you in advance.

Provided that you maintain proper humidity levels, there is no reason to have internal expansion gaps in normal room sized installations.

The moisture meter is going to tell you what is needed.