Most cementious adhesives rated C2 (improved cementious) will do fine for old concrete (12 months or older). Flexible (s1) or highly flexible (s2) is mostly only required for substrates which aren't entirely dimensionaly stable, very non-absorbant (or if the tiles are very non-absorbant), and relatively fresh concrete. Most c2 rated adhesives can handle 6-12 month old standard concrete, whereas s1 and s2 falls in the region between 1 and 3 months old concrete. It varies between brands and types though. Read the adhesive specs to be sure.
Tip of the day: Read the specs of the products you use or normally encounter, and commit them to memory. It simplifies things.
Here's a small guide to the letters fround in the EN 12004, EN 12002, and EN 13888 standards:
Cx = Cementious (x = 1 = normal), (x = 2 = improved)
Dx = Dispersion (x = 1 = normal), (x = 2 = improved)
Rx = Resin (Epoxy) (x = 1 = normal), (x = 2 = improved)
Sx = Flexible (x = 1 = flexible), (x = 2 = highly flexible)
F = Rapid setting
T = Anti-slippage
E = Extended open time
RGx = Resin Grout (epoxy)
CGx = Cementious Grout
x = 1 = Normal
x = 2 = Improved wear resistance and reduced water absorbation
These classifications set a base line. Conforming materials may have properties with their figurative noses just above the base line, or way way above it. It doesn't say. That is why it is also important to read the product specs and guidelines, and follow them.
