Concrete is extremely vulnerable to SUGAR. I don't know the chemistry, but even during WWII the French underground was encouraged to add sugar to German concrete fortifications -- the Atlantic Wall. It turns out that a tiny amount of sugar makes concrete stronger -- anything beyond tiny makes concrete weaker -- to the point of crumbling if hit by a hammer.
It takes quite a bit of sugar to hurt concrete and I doubt the French resistance had that much sugar in a war ravaged Europe. The biggest problem for the Germans was salt, on the Atlantic wall. They had to wash the beach sand before they could use it.
Both of these cause the problem because they are water soluble and after the concrete cures, the salt or sugar washes away in the rain, leaving small voids throughout the matrix.
I doubt pouring a liter or two of soda pop in a concrete entombed drain will do much. My wife had this problem building houses, just from the tile guys washing out their grout tools in the sink and not chasing it with enough water. Usually they were chipping concrete and replacing pipe. You might have better luck with a gallon of muriatic acid, as long as all of the pipe is PVC. Just don't pour it in through the porcelain fixtures. Pull the crapper and pour it in the hole. It is heavier than water so it will go to the bottom.
