Can I use 2"x6" as roof rafters for porch addition over concrete patio?
It looks like the answer is "probably not." If this calculator from the American Wood Council is to be believed (and if my assumptions about your materials and load are correct), the max span is about 10ft for a pine 2x6.

This table agrees that 10-12ft is the max span for typical grade lumber, at typical spacings, for a 5psf dead load.

While your particular circumstances might come up with slightly different numbers, I'm going to bet that you're in the vicinity of the max span for that material. How closely you want to shave that risk is up to you.