Why concrete decays
Concrete goes through decay phenomena due to congenital causes such as lack of casting and tamping mix, and pathological causes connected with the atmospheric environment and operating conditions which affect the work itself. In both cases corrosion mechanisms are triggered from gases in the atmosphere and dissolved in water which, as rain and damp, penetrates in the concrete. Using an expression taken from the medical field, we can say that water is not directly responsible for the decay, but it’s the carrier of the viruses which cause the illness, i.e. corrosion of metal armor in concrete and dissolution of some of its components (concrete carbonation).
Like the human body, concrete has a natural protection: its “immune defenses” come from the basicity of the cement (approx.. pH 13) which preserves the metal armor from corrosion. The industrial process has been altering the composition of the atmosphere since the 1950s: the concentration of gases like carbon monoxide has increased and other gasses have appeared on the scene, like carbon monoxide, sulphur and sulphuric dioxide, nitrogenous monoxides, etc. These gasses, when dissolved in the water in clouds, cause acid rains which are the main causes of corrosion and decay of stone and concrete works.
The decay process
Decay shows after a triggering stage which may last a few years and during which the surface of concrete is apparently healthy. The decrease in pH of the cement paste has nonetheless started oxidation of metal armors for some time. The rust formed has a higher volume and adds pressure between the surface and the armors; at first you can notice micro-crazes, then plates of material detach (concrete cover) revealing the rusty armor and the corroded concrete.
Solubilization of the components of cement paste increases surface porosity of the concrete and consequently how easily the “contaminated” water can penetrate. Decay speed is exponential, as is the increase of maintenance costs if there is no timely intervention, which would be more effective during the triggering stage of the decay. Decay of concrete, in addition to the extremely evident damage, involves a decrease of mechanical resistance.
Areas of use
Restoration of concrete is going to become more and more important in the construction sector: anything built in concrete is going to be subject to restoration in the upcoming years.
CIVILIAN BUILDING: Restoration of beams, pillars, balconies and risers of house balconies, blocks of flats, etc.
INDUSTRIAL BUILDING: Restoration of warehouses, industrial tanks and effluent treatment plants.
PUBLIC WORKS: Restoration of stadiums, schools, hospitals, bridged, viaducts, galleries, etc.