Melange wrote:

It would be wise to include an admixture in the concrete that would improve its
watertightness.

I was considering doing that, many years ago i built a shorter
aquaduct style channal, except it was just on the ground and tiled, it
never leaked to my knowledge. We added a product to the cement to make
it waterproof, cant remember what it was called though, whats yours
called?


There are loads of them.

Just Google on "waterproof concrete admixture"