Well, I do know what you have been faced with that troubled water.
If I were you (using Ver 102.0 with no patch), just try this resolution out:
(1)Apply the "Public Player" (Shift-P), and open your factory builder.
(2)Build the concrete factory on the island which accept those concrete.
(3)Link the concrete factory newly-built to its sourcing factories (Stonequarries,Cement mills) AND the the consumption shops (Materialswholesale) to make the factory works.
And it should work perfectly without the concrete ship.
Sounds a better solution than pak-ing a ship for concrete only?
PS In real world, concrete is too easy to harden without rolling it.
That's why ships can't do the job-they must be sunk when concrete start to dry and harden.
[And concrete factories always located near the end-users' site. Believe it.]
