Concrete degradation in an over 100-year-old drinking water treatment plant, a case of lime leaching from the water retaining tanks

The problem of the concrete durability counted in tens of years is not a frequent subject of research, despite the fact that the oldest structures made with the use of Portland cement are about 140 years old, and the large size engineered constructions (like dams,breakwaters, bridges, chambers, halls) are 110 years old. The considered tank belongs to the last group of objects.In the years 1908–1909, a drinking water treatment station was built for the city of Poznan, which was then 196. 000, taking it from theWarta River. The plan dimensions are 65.5 ×21.5 m, and the height is 9.0 m above and 5.0 underground. It is a pearl of industrial architecture of the end of the 19th and the beginning of the 20th century with the most modern technological equipment in the world at that time. In the underground part, a large-size reinforced concrete tank with a ribbed ceiling supported in the middle with reinforced concrete columns was constructed throughout the projection.Those were the times when the first bold constructions from the cement concrete were built in the world, very few of which have been preserved till the present day. In order to estimate the durability of the concrete, after several decades of influence of very high relative air humidity, it is very significant to check the changes in its microstructure, which influence its strength parameters. This year the author of the project conducted a material and structural analysis of the technical condition of the over hundred ten year old tank structure, which proved that the compressive strength of the concrete is lowered, and there are numerous cracks of the walls and the ceiling, and also some accumulations of white lines and icicles outside. The SEM, EDS and XRD analyses proved that the phase composition of the concrete in the barrier thickness is variable. In the lower layers of the concrete a calcium hydroxide concentration is observed, which outside the barrier changes into the calcium carbonate. The migration of the Portlandite in the direction of the inside of the tank with very high relative air humidity (>95%) results from the mass diffusion caused by different concentration of the porous liquid in the outer and inner layers of the barriers. Those are very slow processes, that last tens of years during the exploitation of the tank. They are beneficial in terms of the gradual alkalinity increase, which protects the steel against corrosion, but they can be harmful due to a local increase of the porosity and the decrease of the concrete strength.This phenomenon has been described in the literature as elution of calcium ions from the concrete structure. Numerous articles in this field focus on creating a sufficiently durable concrete structure by dosing appropriate mineral additives, rather than on several dozen years of observations of the objects in use. Therefore, the intention of the author of this article is not to model the Ca2+ leaching process in a laboratory, but to analyze the gradual degradation of an industrial facility subjected to direct action of water over a period of one hundred years.