I have attached PDF Index Results of the 2015 National Championships using a .975 Concrete Factor for classes using street tires.
I would recommend using the multiplier for all "grippy" concrete sites, but that may be hard to determine. That's the major reason I don't offer that "officially". Pungo Field is concrete, but not very grippy and is small enough that using the .975 factor would be completely inappropriate, whereas sites like Lincoln Airpark, Wilmington Airport, Toledo Express Airport and Grissom Aeroplex, using this factor would help equalize the Categories.
H's & K's,