By Eng.Abel de la Cruz
The combined use of steel and concrete materials has allowed the individual contribution of their intrinsic properties, to obtain as a result a material with great resistance to tensile and compression. In addition to a protective action of concrete against corrosion of concrete.
Concrete protects the low-alloy steel armature through two mechanisms:
a.- Due to the level of alkalinity (pH: approx. 12/ 13) of the concrete in contact with the steel, it reaches a state of passivation when a layer of submicroscopic, impermeable and adherent oxide is superficially formed, preventing the anodic reaction of oxidation.
b.- The concrete acts as a barrier layer against the presence of moisture, oxygen, and other corrosive contaminants that want to reach the steel surface.
Consequently, a homogeneous concrete structure, properly designed, balanced, continuous, without defects and imperfections, can achieve long service life times, otherwise corrosion problems will occur in the armor. The rate of deterioration will increase if the environmental corrosive aggressiveness is high, also losing the qualities of structural strength. This can be constituted not only in a contingency of high cost of repair, but in a construction of high risk to human life because the corrosive phenomenon acts many times, silently, inside the structure and its collapse is usually unpredictable.
When these reinforced concrete structures are exposed to environments of high corrosive aggressiveness and in contact with contaminants of the chloride ion type, the phenomenon of carbonation occurs with a decrease in pH, the passivated state of the steel reinforcements in the concrete can be lost, generating corrosion of the same.
Chloride Ion Corrosion
Chloride ions are the main cause of localized, point or pitting corrosion problems in armor. Although sulfate and sulfide ions are also despasivant, they are less frequent and dangerous agents than chlorides.
The source of chlorides can not only be atmospheric, by splashing or immersion in seawater that penetrate the concrete over time, it can also come from the water used in the preparation of the mixture, from the impurities of the concrete due to the quarry of origin or due to the additives used in the concrete as setting accelerants.
The effect of chloride ions has the particularity of locally destroying the passivation layer that had formed superficially on the steel rods, initiating the corrosive process in the form of pitting and acting as anodes in a typical electrochemical corrosion pile, leading to the deepening of the bite.
The severity of the attack, due to the chlorides that penetrate by diffusion into the concrete, not only depend on the amount of these ions that reach the metal surface, but also on other factors such as oxygen availability and the number and size of empty spaces adjacent to the reinforcements.
Fortunately, in submerged structures, the water that fills the pores of the concrete makes it very difficult to diffuse oxygen to the metal, limiting the rate of corrosion. The speed, with which chloride ions present in free form in solutions or in combined form as part of the raw materials used in concrete, reach the armatures and have an effect on the speed and extent of localized or pitting corrosion and this, in turn, will depend on:
The level of chloride concentration in the external medium.
Nature of the cations that accompany chloride ions.
Quality of raw materials and aggregates, formulation and execution of concrete, mainly in terms of water/cement ratios, types of additives, water quality, etc.
Porosity of the concrete and degree of cracking of the same, etc.
The critical content of total chlorides, generally accepted in concrete, is established between 0.05 to 0.1 % in relation to the weight of the concrete, being able to use for its determination the ASTM D 1411-93 Standard Test Method for Water-Soluble Chlorides Present as Admixes in Graded Aggregate Road Mixes. Annual Book of ASTM Standards Vol. 04.08.
Concrete Carbonation Corrosion
The corrosion caused by the carbonation of concrete, according to its morphology, is of the uniform type, that is, the loss of the material of the steel reinforcements is generalized, unlike the corrosion located by pitting, which tends to deepen more quickly than uniform corrosion, and consequently constitutes greater danger.
This phenomenon consists of the action of atmospheric agents that reduce alkalinity at the steel/concrete interface. The causes of this effect are the acidic constituents of the atmosphere or the environment where the reinforced concrete structure is in service, particularly CO2 and SO2.
Between them, the one that has the greatest presence in the atmosphere and plays the most activity in the process is CO2, hence the phenomenon is called carbonation. The characteristic of the process is the appearance of separate gaps of different pH, usually an area with pH of high alkalinity pH> 12 and another with pH < 9.
Carbonation is actually a slow process that becomes apparent after several years. The chemical process consists of the reaction of CO2 (slightly acidic in solution) with calcium hydroxide Ca(OH)2 (which is formed during the combination of cement silicates and alkalis in sulfates in Clincker) which is found in solution contained in the pores of the concrete and/or with the hydrated components of the same, the product of the reaction is the formation of carbonates whose pH is less than 9.
The speed of the carbonation reaction also depends on the rate of diffusion of CO2 through the already carbonated concrete and the diffusion of the water formed in the first stage of the process.
The carbonation of the concrete when it reaches the steel surface of the armor destroys the layer that passes it because it is unstable at pH In general, it can be defined that the progress of carbonation is determined more by the porosity of the concrete than by its own composition. In the water/concrete ratios between 0.4 to 0.7, we will always have more water in concrete than can be chemically combined with cement and it is known that this determines the porosity of the set concrete. So if we resort to lower water/concrete ratios, we could obtain a less porous concrete with greater resistance to carbonation. Apart from these internal factors, the carbonation and consequently the corrosion process of steel reinforcements will also depend on external factors, such as atmospheric humidity. Therefore, values between 50 and 70% of H.R. will be optimal, since the pores will not become saturated with water, allowing at these levels, the diffusion and penetration of CO2, reaching more easily the concrete adjacent to the armor and with the possibility of carrying out the chemical reaction of carbonation in the aqueous medium. A combined effect of chlorides and CO2 (Carbon dioxide) on concrete can cause serious corrosive consequences, but also a side effect caused by CO2 on the chlorinated aluminates of the concrete, which can be dissociated and the combined chlorides free. Protective Coatings It is important as in the protection of steel to follow the procedures established for the application of organic coatings: a.Inspect and evaluate the conditions of the concrete to be protected: humidity, contaminants, state of conservation, integrity and resistance of the same, defects, alkalinity or pH, etc. *General Manager- AmericanConsult Peru. [email protected]
At humidity below 50%, the pores will be dry and will not allow the reaction of CO2 in the liquid medium. On the contrary, with humidities greater than 70% of H.R. the pores are saturated with water and the carbonic gas CO2, penetrates with greater difficulty towards the armor.
There is a wide variety of coatings that protect reinforced concrete surfaces and act through the barrier layer mechanism: Epoxy-Polyamide Paints, Epoxy-amines, Epoxy-Novolacas, Polyurethane Paints etc. which get in the way and delay the entry of external contaminants such as chlorides, carbon dioxide, sulfur dioxide, moisture, oxygen, etc. extending the durability of reinforced concrete structures.
b.Design and selection of the painting system with coatings according to the operating conditions.
c.Preparation of the surface of the concrete, according to technical standards.
d.Application of the different coating layers.
e.Quality inspection of the integral concrete surface treatment process.
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