Concrete corrosion: types and methods of elimination

Because of its strength and longevity, concrete is a basic component of modern construction. It may, however, eventually experience wear and damage, just like any other material. Corrosion is one of the biggest problems concrete structures face. Concrete’s integrity may be compromised by this process, posing a risk to public safety and necessitating expensive repairs.

Concrete corrosion comes in a variety of forms, each brought on by unique chemical reactions and environmental influences. To determine the best ways to address and prevent damage, it is essential to understand these types. The reasons behind concrete corrosion are numerous and intricate, ranging from chemical assaults by acids to the expanding forces of corroding steel reinforcement.

Thankfully, concrete corrosion can be prevented and its effects reduced with the help of efficient techniques. These include sealants and surface treatments, as well as more sophisticated methods like cathodic protection. Concrete structures can be kept safe and functional for longer periods of time by adopting preventative action.

We’ll examine the various forms of concrete corrosion in this post, along with the most effective ways to deal with it. Understanding these ideas is crucial for maintaining strong, long-lasting concrete structures, whether you’re a professional in the construction industry or a homeowner trying to safeguard your property.

General information, types and nature of destruction

The concrete’s composition and the characteristics of the environment in which a structure is used determine how long-lasting it is.

Aggressive environments are classified into three categories based on the extent of their influence:

• highly aggressive;
• medium aggressiveness;
• mildly aggressive;
• non-aggressive.

The reagents are separated into the following categories based on how they have aggregated:

• liquid;
• gaseous;
• hard.

Depending on the operating conditions of the structure, the aggressive impact on reinforced concrete structures is estimated for each environment separately.

• presence and concentration of aggressive reagents;
• the temperature at which a chemical reaction occurs;
• speed of fluid movement at the surface.

• types of gaseous substances and their concentration;
• solubility of components in water;
• air humidity and temperature.

Regarding aggressive solids (dust, salts, aerosols):

• dispersion;
• Possibility of dissolution in water;
• environmental humidity.

Concrete corrosion rate is influenced by the following variables:

• by the nature and chemical composition of the aggressive environment;
• aggregate state (solid, liquid, gaseous);
• chemical concentration and composition of aggressive substances;
• temperature and humidity values ​​​​of the environment;
• rate of influx of aggressive substances to the surface of products and elimination of corrosion products;
• features of the design composition of concrete (permeability, thermal conductivity, etc.);
• the nature of the products and the internal structure of the material (the thickness of the protective layer, the form, the presence of cracks, the density of reinforcement, etc.D.);
• type of physical or thermal effects on reinforced concrete (freezing, heating, mechanical loads, etc.).

Classification of corrosion processes in a liquid environment

Three materials are involved in destructive chemical reactions: water, cement stone, and filler. The time interval between the facility’s opening and the beginning of damage and deterioration of its performance attributes determines a structure’s durability.

The various forms of concrete corrosion and the degree of aggression exhibited by a liquid environment can be categorized based on the overall indications of their effects on particular structures.

Our research and examination of the harmed structures enable us to determine that there are three categories into which all active causes can be subdivided:

1. The components of cement stone, under the influence of liquid reagents, dissolve and are removed from the structure of concrete. These processes are especially active when liquid is filtered through the thickness of the structure.
2. Chemical exchange reactions occurring between cement stone and an aggressive environment. As a result of such reactions, decay products with weak binding properties are formed, which are easily washed out of the concrete structure under the influence of liquids.
3. The third type of destruction includes the processes of formation of crystalline poorly soluble compounds, under the influence of which the porous structure expands, which leads to the appearance of cracks and subsequent disruption of the integrity of the entire structure.

The first type of corrosion

The strength properties of concrete structures are greatly influenced by the ability to dissolve the byproducts of the cement hydration process in an aqueous environment and wash them out of the cement stone structure. Lime is the most soluble part of the cement mortar structure made with Portland cement. Thus, the process of calcium hydroxide dissolution in this instance is defined as corrosion of reinforced concrete.

By nature, cement mortar is an intricate, unstable system made up of cement hydration products in an equilibrium state and non-hydrated clinker grains. This equilibrium is upset by water action, and the system as a whole enters a new stable state with different interaction parameters.

There are two phases to the component dissolution (leaching) process:

• during the leaching of calcium hydroxide, free Ca(OH) moves into the solution₂;
• as a result of the decrease in the amount of CaO that forms a compound with the cement stone, hydrolysis (destruction) of the remaining hydrates occurs, the stable state of which is possible only in calcium hydroxide compounds of a certain concentration (see. photo).

Calcium hydroxide’s capacity to dissolve, even in purified water, inhibits the harmful mechanisms that encourage corrosion.

• The most favorable temperature for the development of the process of dissolution of calcium hydroxide is 20 ° C. With a further increase in temperature, solubility decreases.
• With prolonged exposure of water to cement stone, this component can be completely removed from the structure of concrete with the complete decomposition of other hydrate components – alumina, iron oxide and silica to an amorphous loose state.
• The intensity of the leaching process is directly proportional to the density of the material and the amount of mineral fillers containing calcium hydroxide.
• Signs of damage of the first type are visible on sections of structures in the form of drying spots after exposure to water.

Note: Calcium hydroxide dissolved in water is carbonized and released on the surface of the structures as a white coating of calcium carbonate as a result of chemical reactions taking place in the material’s structure.

The first kind of damage is more common in subterranean and hydraulic structures that are subject to the transient or ongoing influence of fresh water. Pozzolanic additives, such as flasks, trassa, tripoli, and others, aid in lowering the first type’s rate of destruction, binding Ca(OH)2 compounds, and decreasing the permeability of concrete.

Additionally, the following tasks must be completed in order to strengthen materials’ resistance to the first type of corrosion’s effects:

• use high-density concrete for the manufacture of corrosion-resistant structures;
• conduct artificial carbonization of bases;
• use pozzolanic or other special cements;
• apply waterproofing of surfaces;
• treat finished structures with special impregnations.

The ability of products to withstand exchange reactions that take place in the structure of structures during the second type of corrosive action process is positively impacted by the resistance of materials to leaching.

Second type of corrosion

Interactions between the components of the solution and cement stone through chemical exchange represent the second class of destructive processes that take place in a liquid medium. The products of the reaction either precipitate as amorphous compounds lacking binding properties or readily dissolve in water and are carried out of the concrete structure by the filtration flow.

When coatings are exposed to strong chemical compounds found in some salts and acid solutions, corrosion of this kind may occur. The process of cement stone destruction is both deeper and shorter the more intense the substitution reaction is and the faster the products that result dissolve.

The primary forms of the second kind of corrosion are:

The pattern of adverse effects of the second type is slightly different from the destruction of the first type, occurring in the upper layers of concrete that are in direct contact with the aggressive environment, and it involves a gradual dissolution of substances obtained as a result of the hydrolysis of cement.

Water washes away new formations that were created on the surface as a result of exchange reactions but lacked adequate density and astringent qualities. This exposed the next layer of concrete, which is when destruction reactions started.

This layer disappears and dissolves as well. The subsequent stages of the second type of corrosion happen in this order and according to this scheme until the structure is completely destroyed.

Destruction of structures under the influence of carbon dioxide compounds

What is carbon dioxide corrosion, and this chapter will examine the mechanism of action of this kind of destruction.

Defects in concrete caused by carbon dioxide waters are among the most frequent examples of the second kind of destruction. All naturally occurring liquids contain some carbon dioxide in some amount.

The biochemical processes that take place in the liquid itself as well as in the soil that the water is constantly in contact with are the cause of the presence of CO2 in natural waters.

The microbiological processes that take place during the decay of plant residues at varying depths are linked to the release of carbon dioxide. additionally, the potential emission of CO2 resulting from the reaction of groundwater flowing with carbonate sedimentary rocks.

In this instance, the concentration of carbon dioxide in the solution controls the rate of destruction. The solution’s acidic properties and the rate at which carbon dioxide corrodes increase with the concentration of H2CO3.

The nature of acidic destruction of structures

Aggressive impact on organic or inorganic acid structures also initiates the second type of corrosion processes in the material, which eventually can change into the first type of corrosion and result in the total destruction of the cement stone in the product’s structure.

In addition to carbon dioxide, the most frequent reactions among the inorganic acids that corrode concrete are:

Furthermore, from organic ones (lactic, acetic, etc.).

Acid causes the cement stone to be nearly entirely destroyed. Furthermore, at the site of reactions, the chemical products of destruction are partially dissolved and partially preserved.

The concentration of hydrogen ions and the potency of the active acid dictate the level of acid corrosion activity. The surface of the cement stone forms a loose amorphous mass and calcium salts as a result of the acid reaction.

Water-soluble calcium salts are removed from the structure, leaving behind the loose mass. All of these procedures weaken the structure until eventually they destroy it entirely.

The development of the process of acidic aggressive effects is significantly influenced by the rate of exchange reactions at the surface of the affected structure.

Alkaline reactions of destruction

This kind of corrosion can happen when binders and fillers used to prepare concrete have a high alkalis content.

• The most common type of alkaline effects is the chemical reaction of potassium and sodium compounds with silica fillers. In modified concretes, the increased content of alkalis is explained by the presence of potassium and sodium oxides in the cements used.
• The increased content of these elements is caused by the high purification of gases coming out of the chimneys of cement kilns and the subsequent return of cement dust from dust collectors to the structure of the technological process. In addition, the alkali content can be increased by using mineral, chemical or organomineral modifying additives.
• Potassium and sodium chlorides found in sea water, saline soils or anti-icing reagents can also react with silica.
• Not only silica, but also certain types of quartz fillers can interact with alkalis.

• As a result of the interaction of fillers with cement stone, hydrated compounds are formed on its surface, expanding in a humid environment.
• At the moment of swelling, stresses arise in the structure of the products, causing deformation of structures and destruction of concrete.
• As a result of such reactions, small cracks are formed on the surface, from which sodium silicate leaks in some cases.

Note: Adding 10–20% of active, finely ground mineral additives to cement is the most effective way to prevent alkaline destruction because they slow down hydration reactions and remove the stressors that lead to destruction.

Calcium silicate hydrous protective microfilms form on the filler particles as a result of the autoclave treatment’s beneficial effect on halting the alkaline impact process.

Third type of corrosion

The most well-known examples of third-type liquid aggressive environments are industrial and subterranean waters that contain sulfate compounds. Liquid sulfate solutions’ effects on structures lead to sulfate corrosion of concrete.

• Its mechanism of action is based on the ability to form crystalline sulfate formations, causing an increase in the volume of cement stone. The stresses arising as a result of such reactions can significantly exceed the strength value of concrete. The result of such plastic deformations is the destruction of erected structures.
• The processes occurring in the structure of products during sulfate corrosion are a rather complex physical and chemical scheme, which can only be explained in a simplified manner – noting only the main features.
• During the interaction of sulfate solutions, calcium hydrosulfoaluminate and gypsum are formed in the pores of the structures.
• With further development of the process, gypsum reacts with cement stone and forms several sulfate compounds. The most dangerous of them is tricalcium hydrosulfoaluminate – it, when crystallizing, is capable of increasing in volume by 2.5 times.

Bicarbonates, which form poorly soluble components, stop sulfates from penetrating deeply into structures, and the use of chlorides as additives can slow down the development of sulfate corrosion.

Biocorrosion of concrete

Concrete biological corrosion is the result of bacteria and other microorganisms having a direct or indirect impact on the technical properties of materials. This class of organisms includes mold, seaweed, lichens, and different forms of fungal formations.

When different acids of microbial origin act on concrete, the density of the concrete is violated, resulting in biodamage to concrete structures.

As microorganisms interact with their surroundings and pollutants throughout their existence, they produce acids, ammonia, and other harmful substances that are released onto the surface of structures. By reacting with cement stone, they exacerbate the breakdown of structural integrity and compromise the structural integrity of buildings.

Gaseous aggressive environments

The gas environment in which the products are used has a significant impact on the development of chemical corrosion reactions in concrete.

The air also contains trace levels of argon, hydrogen, carbon dioxide, and other elements in addition to nitrogen and oxygen. Also, emissions from motor vehicles, industrial companies, and other sources may be present in the atmosphere, depending on the region’s location and level of industrialization.

Excellent conditions are created for gas corrosion when this air environment combines with a suitable temperature and humidity level.

The aforementioned gases are readily soluble in water and can react to form acidic compounds with cement stone. Particularly hazardous is carbon dioxide, which reacts with calcium hydroxide to produce calcium bicarbonate that is readily soluble in water and readily washes out of the material’s structure when precipitation or groundwater flow through it.

One major problem in construction is concrete corrosion, which weakens structures and raises safety issues. Comprehending the distinct forms of corrosion, including carbonation, chloride assault, and sulfate attack, is imperative for efficacious mitigation and restoration. Concrete corrosion can be prevented by using the right materials, applying protective coatings, and performing routine maintenance. We can guarantee the longevity and safety of concrete structures by taking care of these factors.

Type of Corrosion	Methods of Elimination
Carbonation	Apply sealants or coatings to protect the concrete surface.
Chloride Attack	Use corrosion-resistant steel and apply protective coatings.
Sulfate Attack	Replace damaged concrete and use sulfate-resistant cement.
Alkali-Silica Reaction	Use low-alkali cement and add pozzolanic materials.

The longevity and strength of concrete structures can be considerably impacted by the serious problem of concrete corrosion. Effective corrosion prevention and repair depend on having a thorough understanding of the various forms of corrosion and their causes. Concrete can deteriorate due to a variety of causes, including chemical reactions with sulfates and acids and physical damage from freeze-thaw cycles.

Preventive and corrective measures are needed to address concrete corrosion. To reduce exposure to corrosive elements, preventive measures include the use of high-quality materials, the application of protective coatings, and the maintenance of proper drainage. In order to restore and protect the concrete when corrosion is already present, techniques like applying sealants, using corrosion inhibitors, and fixing damaged areas must be used.

Concrete structures can last longer and retain their integrity if builders and property owners take preventative measures and pay attention to corrosion indicators. A regular inspection schedule and prompt maintenance are essential for keeping small problems from growing into larger ones. In the end, spending money on the appropriate supplies and methods will ensure that concrete structures last for many years while also saving time and money.

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