Please allow me introduce this post with a short declaration: "In the following post we are going to talk ONLY about standard types of concrete (named as "traditional concrete") where the mix is made out of cement, sands and coarse aggregates with water and superplasticizers".
Traditional concrete is a porous material such as (of course with differences in terms of dimensions and shape) wood, paper, textiles and so on, which means that when it gets in contact with water it gets wet.
Therefore, when we talk about traditional concrete, we should identify its degree of permeability and not its impermeability (actually, there is no completely waterproof or impermeable traditional concrete). Traditional concrete is NOT "impermeable or waterproof", it will get ALWAYS wet.
Why so? Because of its porousness, or to be more precise, because of its capillary porousness, small voids micron-sized, which form an interconnected channeling inside the hardened cement paste (details about the effect of capillary porousness on my post “Concrete shrinkage part 2”). This network of channels strongly influences the durability both of concrete and structures (read my post “Concrete durability”).
Microscopic holes let water and air enter the concrete, these superficial holes are than linked together digging channels toward the concrete core. Air brings aggressive chemical elements, water brings even more of them. They attack the hardened cement paste and the reinforcement bars eroding the vital part of concrete and oxidizing and corroding reinforcement.
It is a slow but irreversible decay of both concrete and structure. We are not only talking of mold onto the concrete surface and/or cosmetic damages, the main problem, when it comes about permeability, is durability, which means having structures that can withstand the test of time, keeping themselves safe while maintaining their desired performances for all the structure's forecasted life.
How to have lesser porous and thus more durable concrete? There is a threefold solution to this problem.
First, low w/c ratio. The lower the value, the lower the permeability. Of course, this means having higher strength values and higher costs, but this is the price to pay if you need a lesser permeable concrete…we will come back to that in a moment.
Let me open here a very brief parenthesis. As the w/c ratio decreases, various intrinsic characteristics of the concrete will also change. In this post we do not want to encourage the use of low w/c ratio concrete with the sole purpose of reducing permeability, this is a delicate choice that must be made wisely by evaluating its feasibility case by case, but rather underline that the permeability of concretes is a function of their capillarity which in turn is directly related to the w/c ratio.
Second, be sure of having the concrete set in place properly (read my post “Concrete compaction”). The lesser compaction, the more voids especially macro-voids.
Third, never miss curing the concrete properly (check my post “Concrete curing”), proper curing in terms of methods and timing means significant reduction in capillary porousness.
Back to the higher cost; true, higher strength concrete is more expensive, but, if this choice is made already at the design phase, it is sometimes possible to reduce the structure´s section saving concrete quantity (and so money), insulation material can be reduced in terms of expected performance hence saving money and, on top of all of that, having a lesser permeable structure brings to a longer life of the structure, minimizing maintenance and restoration costs. This is invaluable.
Knowing that concrete is not impermeable or waterproof, however, does not mean that theoretically a tank made with concrete can be made, without waterproofing, perfectly watertight. Again, the choice of concrete is the first step, the joints choice is equally important, and, as said before, casting and curing methodologies are essential.
About concrete choice….did you know that there are other types of concrete that although having the same w/c ratio are lesser permeable? If you are curious please read my post “Low permeability concrete”