Resistance of concrete is the ability of a material to maintain its properties for a long time: fire and heat resistance, cold resistance, resistance of concrete to chemically aggressive environment, water and gas, maintaining its performance when working in adverse environmental conditions without significant damage and destruction. The article is prepared in collaboration with Telling Company experts, a successful UK-based engineering enterprise, massively utilising hydraulic lime, lime mortar and concrete solutions in its projects.
Resistance factors in a nutshell
A particularly high expansion of concrete hardening occurs during the formation of calcium sulfoaluminate. Concrete corrosion can occur if there is moisture in the air, and various acid gases. For example, sulfur dioxide discharged from furnace boilers, locomotives or sets of certain chemicals combines with moisture in the air and water vapor to form sulfuric acid, which destroys the concrete as well as the free acid in aqueous medium. Processes of chemical corrosion of concrete can not be considered independently of the physical and physico-chemical processes occurring in the concrete under the influence of an aqueous or gaseous medium. A crucial impact, in particular, has a three-dimensional deformation, resulting moisture exchange (absorption of water and its evaporation), the process of freezing and thawing, seepage and water filtration, diffusion processes, moving moisture in concrete, and so on.
An increase in resistance of the concrete regardless of the type of corrosion is achieved by providing the necessary density and homogeneity of the concrete structure. The presence of shells and different kinds of leaks in the form of open or interconnected crevices and cracks resulting from temperature and shrinkage deformation, the most favorable for the emergence and development of corrosion processes.
To increase the resistance of concrete in relation to a purely chemical process of corrosion is necessary not only to ensure a sufficient density of the concrete, but also make the selection of binders and fillers, the most stable in terms of this type of corrosion.
The question of safety valves in the concrete is inextricably linked with the question of durability of concrete, so it is appropriate to consider here.
Preservation of reinforcement in concrete
Typically, steel reinforcement, encased in concrete, is not destroyed (but rusts) and can be kept in good condition for decades. The safety of reinforcement is provided by the presence of an alkaline environment in the concrete. This is true only for a sufficiently dense concrete, which excludes a possibility of air flowing directly to the steel reinforcement rods. Therefore, the construction of reinforcement should be covered with a protective layer of concrete, the minimum thickness of which varies from 10 (for thin walls and hollow core slabs, decks) to 120 mm (for large hydraulic structures). In hostile environments (high humidity, harmful gases and so on) the thickness of the protective layer should be increased. The protective layer must be dense, with no cracks or defects, otherwise its purpose is not justified. Cracks in the protective layer provide access directly to the air valve, which causes the formation of rust film, accompanied by an increase in its volume. The latter causes tensile forces in the concrete cracking and failure of the protective layer, with all the negative consequences for the durability of concrete structure.