The concrete layer also protects from corrosion, which should be homogeneous, for which the fixers are used.
Concreting the bottom and walls of the bowl should be phased and very accurate that the bottom is smooth and with an exact slope for draining water, and the sides are even. Concrete uses a class not lower than B15 and brands not lower than W4, which respectively provides strength and water permeability. For frost resistance, concrete should be F100-F150 (per 100-150 cycles), as well as additives for concrete Saturfix and Fluxan to increase waterproofing.