Yellow leaves with green veins is known as chlorosis due to iron deficiency and the iron deficiency is probably due to the acidity of the soil. Did you recently do something like add some special fertilizer?

Its most likely the concrete that is the culprit. It makes the soil alkaline and the gardenia can't uptake iron unless the soil is on the acidic side.

Look into a soil acidifier or transplanting it away from the concrete.

This is why all my Gardenias died. They are one tough customer! I have two Vietnamese Gardenias and they started to get yellow leaves on the bottom too. So yesterday I potted them in the ground. If they die, then I give up!

There are just too many other fragrant plants out there that I don't want to waste any more time, energy and money on something that I simply cannot grow!

Good luck with yours. Hopefully you can diagnosis the problem and correct it before it's too late.