Gas corrosion of concrete. Why does concrete corrosion appear and how to deal with it? Corrosion of reinforcement in concrete
What is it - corrosion of concrete and reinforced concrete? Why in iron concrete structures corrosion processes occur? In what ways can their development be prevented? In the article we will try to answer these questions.
What it is
Concrete corrosion is a process of falling and reinforced concrete structures associated with aggressive environmental influences. It seems that the reader does not need to explain how corrosion proceeds. metal structures. with concrete in in general terms the same thing happens: over time, it partially degenerates into other materials with completely different mechanical properties.
To clarify: reinforced concrete structures, of course, also suffer from ordinary rust. In most cases, reinforcement does not have high corrosion resistance.
Types and mechanisms
Remember the proverb “where it is thin, it breaks there”? It fully applies to the degradation of any structural materials.
Reinforced concrete is a composite of several types of raw materials that differ mechanical strength and resistance to various types of external influences.
| Material | Properties |
| Sand | Quartz crystals are exceptionally chemically stable, do not degrade over time |
| rubble | Gravel is usually used as filling rocks, which differs little from quartz sand in its chemical and mechanical properties. Only concentrated alkalis and acids can affect its strength. |
| fittings | The contact of steel in water and air (and concrete, as we remember, is vapor permeable) always gives a very predictable result. Even under protective layer concrete reinforcement will gradually rust. The release of reinforcement to the surface due to the destruction of the structure will greatly accelerate the process. |
| cement stone | The binder - cement - after setting turns into a relatively strong, but not chemically inert cement stone. One of its main components is slaked lime Ca(OH)2 - Easily soluble in water and reacts with other chemicals. It is with the destruction of the cement stone that the corrosion process usually begins. |
Let's look at the main types of corrosion and the mechanisms of their occurrence.
Washout
Despite its high density, concrete is a porous material. The reason is that the setting of the cement and the subsequent drying of the mortar are accompanied by a significant decrease in its volume.
Please note: porous gas and foam concrete is a separate conversation. In their case, pores are intentionally created by introducing foam or gas-forming components (usually aluminum powder) into the solution. The goal is to give concrete maximum thermal insulation qualities.
Wetting the concrete, followed by uneven evaporation of water, will cause the water to gradually move through the pores. In the process of movement, the same slaked lime Ca (OH) 2 will gradually be washed out; well, since there is less binder in the thickness of concrete, its strength decreases.
The process of leaching is most clearly demonstrated by efflorescence - white stains and growths on the surface of concrete, remaining where it often gets wet. Their presence indicates that the structure is rapidly losing strength.
Decomposition by acids
Under the influence of acids and their aqueous solutions, many destructive processes can occur in concrete.
Let's take a look at the simplest ones.
- When exposed to acids, slaked lime combines with atmospheric carbon dioxide to form an insoluble salt and water. The formula describing the reaction is Ca(OH)2 + CO2 = CaCO3 + H2O.
It would seem - why be upset if a soluble calcium compound is replaced by a more stable one? After all, the process of washing out in this case should completely stop. Not here - it was: CaCO3 crystals do not just fill the pores - they tend to expand, crack them; as a result, the concrete begins to crack.
- With an excess of water (in other words, in wet concrete), the further transformation of minerals takes the form CaCO3 + CO2 + H2O = Ca(HCO3)2. The resulting calcium bicarbonate is again soluble in water; moreover, it is too soluble: it is rapidly washed out, leaving behind pores and ... a drop in structural strength.
- In the presence of a hydrochloric acid solution, slaked lime turns into calcium chloride: Ca (OH) 2 + 2HCl \u003d CaCl2 + 2H2O. And this salt is exceptionally easy to dissolve in water; the result is quite predictable - again, the weakening of the structure.
Sulphate decomposition
In the conditions of chemical industry enterprises (in particular, those producing fertilizers), a fairly common case is the so-called sulfate corrosion of concrete.
As a result of interaction with slaked lime sulfates and aluminates present in cement, in particular, ettringite hydrosulfoaluminate (3СaO Al2O3 3CaSO4 32H2O) is formed. Crystals in the process of growth cause significant stresses, significantly exceeding the strength characteristics of cement stone.
Rebar rusting
Everything is simple and clear here: the contact of low-carbon steels with water and air leads to the formation of low-strength Fe2O3 and more complex oxides and salts. Reinforcement must take tensile loads; with a drop in the strength of the reinforcement, significant bending loads lead to the appearance of cracks and ... an accelerated drop in the strength of the surviving reinforcement due to direct contact with water and air ().
Biodegradation
The consequences of high humidity at temperatures above zero are well known: constructions made of brick, stone and concrete become habitable with moss and mold.
As a result, destruction occurs in two ways:
- The notorious lime and its compounds serve as food for the fungus.
- The accumulation of metabolic products in pores leads to an increase in internal stresses.
Frost Destruction
Imagine what happens to a section of a wet concrete structure when the temperature drops below zero.
- The water in her pores begins to crystallize.
- Ice, which has a larger volume than water, tends to expand the pores. Microcracks appear in the structure; as they expand, reinforcement corrosion is connected to the destruction of reinforced concrete.
Protection methods
So, we have studied the mechanisms of destruction. Is it possible to protect concrete and reinforced concrete structures from corrosion? Can appropriate measures be taken at home, with one's own hands?
Strategy
First, let's find out which ways we have to move.
| Package of measures | clarification |
| Armature protection | Increasing the corrosion resistance of the reinforcing frame will prevent it from rusting inside the concrete and when it comes to the surface. |
| Sealing Chemical Additives | As a rule, they reduce the number of pores or make the pores closed. As a result, the permeability of the material to water and air decreases, less often unstable slaked lime is replaced by more chemically resistant compounds. |
| Pore filling | The finished concrete structure can be modified with penetrating impregnations injected through holes drilled into it or simply applied to the surface. |
| Surface protection | This includes all kinds of measures for waterproofing (roll and coating). Painting with paints and varnishes falls into the same category. |
| Biosecurity | Antiseptic impregnations nullify biological decomposition, killing the mold itself, its spores and preventing their reappearance. |
Tactics
And now let's concretize the list of possible measures a little by describing some of them.
Industrial conditions
How is the protection of reinforced concrete structures from corrosion carried out under conditions industrial enterprises, multi-apartment construction, etc. - in other words, when it is possible to use sophisticated technologies requiring special equipment?
We will mention a few frequently used solutions.
- Cementing. Through the holes drilled in the thickness of the structure, cement milk is injected under pressure, prepared in a ratio of 1:10 (cement-water), with a small (no more than 7% by weight of cement) addition of calcium chloride. Filling the pores contributes to an increase in the density of concrete and a decrease in the number of open pores in it.
- Silicization is reduced to the sequential injection of sodium liquid glass and calcium chloride. During processing, the pores are filled with a mixture of poorly soluble calcium hydrosilicate and insoluble silica.
- Bitumization– the process of filling the pores with bitumen at a temperature of 200-220C. The method is exceptionally effective, but can only be carried out with a minimum moisture content of the structure.
Useful: the main problem when drilling pits for pumping solutions is not to cause an increase in internal stresses in the thickness of the structure. From this point of view, diamond drilling of holes in concrete is optimal: it does not create shock loads and does not cause chipping of the edges of the hole.
Reinforced concrete cutting is used to open and dismantle structural elements diamond circles: they have a much greater resource compared to abrasive wheels on stone and, most importantly, they cut reinforcement perfectly.
Home conditions
Of course, the protection of concrete from corrosion is possible without the use of high-tech equipment.
- Protective painting is the simplest and most obvious solution. In particular, we can recommend the so-called rubber water-dispersion dyes: they reliably waterproof the concrete surface with minimal time and effort. The price of a kilogram of rubber paint starts at about 130 rubles.
- Treatment with liquid glass is also able to protect concrete from destruction. Instructions for its use are extremely simple: sodium liquid glass diluted with water 1:1 and applied to the concrete surface with a brush or roller in 2-3 layers without intermediate drying.
- Most effective solution- penetrating waterproofing impregnations (Penetron and its analogues). They are applied on wet concrete and penetrate to a depth of up to a meter. Penetron causes crystallization of calcium compounds that completely fill the pores.
- At the stage of concrete preparation, various reinforcing additives can be introduced into it. Here are the names of several domestic preparations: Mylonaft, SDB (sulfite-yeast brew), GKZH-94 (organosilicon liquid).
Silicone (silicone) impregnations can also be used for hydrophobization of finished structures. In the photo - silicone hydrophobic primer Tiprom D.
Conclusion
Of course, within the framework of a short article, we have touched on only a few of the long list of possible solutions (
Many building materials, including concrete, are exposed to corrosive attack. It is the destruction of metals under the influence of physicochemical or chemical environmental factors. To prevent destruction in structures made of concrete and reinforced concrete, there are various methods of protection. These can be surface coatings with a special resistant material or a variety of varnishes, impregnations.
Definition of corrosion
Corrosion is the erosion of building materials under the influence of physical, chemical and biological factors in contact with the environment. Concrete has in its composition the least durable component - it is a cement stone. It is from this part of the material that the corrosion process begins. Destruction occurs as a result of impact various kinds water, namely:
- sewage;
- water in trenches or pipes;
- marine;
- river;
- ground.
Most dangerous for concrete ground water near industrial enterprises due to the presence of chemical emissions in them. Also, when exposed to concrete and reinforced concrete, they cause significant harm. wastewater. Corrosion of concrete affects hydraulic structures, pollutes the air, however, such a concentration of gas in the environment does not harm human health, but contributes to the destruction of concrete structures.
Destruction of building materials is diverse and destructive microorganisms can be found both in direct contact and inside the structures. Accelerates corrosion in concrete high humidity environment.
Types and description
There are types of concrete corrosion:
- Radiation, which depends on the dose of ionizing radiation and the amount of cement stone. As a result, the crystal lattice of minerals is distorted, the filler expands, which leads to microcracks, macrocracks in the material, and further to complete destruction.
- Chemical, occurring as a result of precipitation and under the influence of carbon dioxide included in the air. Thus, in construction there is gas corrosion, which is especially relevant with a large amount of moisture.
- Biological. Corrosions associated with biological corrosion appear as a result of exposure to chemical substances obtained during the operation of concrete structures.
- Physico-chemical corrosion occurs as a result of freezing water. In a liquid state, water enters the pores of the material, and as a result of sub-zero temperatures, it freezes. The resulting ice expands and bursts buildings, resulting in cracks.
Chemical attacks
Formed under interaction concrete stone with environmental substances. Chemical corrosion processes fall into three categories:
- As a result of crystallization of materials, cracking occurs. Cracks are a consequence of the expansion of the volume of the material due to low temperatures.
- Leaching with soft waters, followed by the formation of a white coating.
- Cement bacillus, which is a consequence of moisture, destroys concrete structures. They form cracks and cracks.
Physico-chemical
Scheme of the corrosion process. In this case, the cement stone disperses in water. As a result, calcium hydroxide is washed out or dissolved. The dissolution of reinforced concrete due to the action of water happens at different speeds. So, for example, dense massive structures are subject to corrosion only after many decades. In buildings with thin shells, calcium leaching occurs after 2-3 years. When water passes through concrete, the decomposition process is accelerated many times over, and the strength characteristics of the material decrease.
biological destruction
Corrosion with the formation of large volumes of biological compounds in the stone is the result of the influence of various substances penetrating concrete. This contributes to the appearance of internal stress and cracks in the concrete structure. Biological corrosion is determined by the presence of bacteria, mosses, fungi or lichens on the cement stone.
Biological damage develops due to direct contact of microorganisms with the material. As well as bio-organisms that can harm the material, being at a distance. Biological corrosion develops under conditions industrial environment with a high moisture content in the atmosphere.
radiation
Corrosion of concrete is radiation, which occurs as a result of radiation. It contributes to the removal of crystallized liquid from the concrete structure and thereby leads to a violation of the strength of the structure. Prolonged exposure to radiation exposure leads to a liquid state of crystalline substances. Stress appears in the concrete solution, and cracks appear.
Influencing factors
Corrosion of concrete occurs under the influence of the following circumstances, on which the rate of destruction of buildings and structures depends:
- surface skill concrete mortar resist substances;
- material porosity;
- substances found in atmospheric precipitation;
- capillarity.
The main component of concrete is its porosity, which determines the number of pores and the presence of density in the structure of the material. The possibility of moisture absorption of the structure during the melting of snow masses or other atmospheric precipitation depends on the porosity of concrete. Material with a significant number of pores is subject to greater opportunity destruction as a result of physical and chemical corrosion. Therefore, the protection of concrete from corrosion should begin at the initial stage of construction of buildings and structures, because all types of concrete corrosion lead to the destruction of buildings.
Anti-corrosion protection
Types of corrosion destruction of concrete are different and diverse. Many builders are interested in the issue of protecting concrete structures from the influence of negative external factors environment.
Often the upper layers of concrete are destroyed, then protection consists in the use of concrete with a small amount capillaries in its structure. Using the drug from the occurrence of cracks at the initial stage of construction, this will help protect structures from leaching and washing out.
Protection against destruction in the form of rust is divided into:
- methods that change the composition of concrete, while making it more durable and resistant to negative impacts environment;
- measures related to coating the surface of the material with hydraulic preparations;
- combined measures, which include coating the concrete with an anti-corrosion preparation with its further penetration deep into the material.
The use of belite cement in the concrete composition will reduce the amount of calcium hydroxide released, which contributes to the evaporation of the liquid. Such a component will allow to compact the material and thereby stop the penetration of liquid through the concrete solution.
Another type of destruction of a concrete structure from rust is sulfate corrosion of concrete. It appears as a result of the relationship of sulfates with the stone in the mortar cement. Destruction is observed in the form of structural distortions and bursting of structural elements.
The metal parts of the structure are covered with special protective materials. Corrosion of concrete due to exposure to water is prevented in various ways. A variety of additives and preparations are used at the initial stage of preparing a concrete solution: drainage systems or waterproofing.
Protection of concrete against corrosion is divided into: primary and secondary. They are also susceptible to corrosive corrosion of reinforced concrete structures. Inhibitors are used to save them. metal corrosion during the preparation of concrete. Thus, a film is formed on the reinforced concrete components, which stops the contact of metal with concrete.
First of all, the state of building materials is negatively affected by an aggressive environment.
Water, carbon dioxide, salts, temperature changes very often cause corrosion. In this regard, the most important problem and task number one in the construction and subsequent operation of any facilities is protection from concrete corrosion.
Causes of corrosion
The structure of concrete produced on a mineral basis is capillary-porous. Therefore, it is very susceptible to negative influences.
Atmospheric phenomena in the porous structure of concrete form crystals. Then they increase and cause cracks.
Chlorides, sulfates and carbonates, dissolved in the air in large quantities, also have a destructive effect on building structures.
Concrete corrosion and its types
Concrete corrosion there are three types. The main criterion for classification is the degree of deterioration of its properties and characteristics.
Corrosion of the 1st degree - the constituent parts of concrete are washed out;
Corrosion of the 2nd degree - corrosion products without binding properties are formed;
Corrosion of the 3rd degree - poorly soluble crystallizing salts accumulate, which increase the volume.
Concrete Protection Methods
For guard concrete from corrosion, as well as increasing its durability, it is necessary to apply its primary and secondary protection.
Primary protection involves the introduction of a variety of modifying additives. These can be stabilizing (prevent delamination), plasticizing (increase), water-retaining and regulating the setting process of the concrete mix, its porosity, density, etc.
Methods of secondary protection against concrete corrosion involves the application of protective coatings:
Lacquer coatings. They are used when exposed to liquid media, and direct contact of concrete with an aggressive solid environment.
Lacquer and acrylic coatings. These products form a strong weather-resistant and durable protection. For example, acrylic creates a polymer film, thereby preventing concrete corrosion. Moreover, it protects the surface from microorganisms and fungi.
Sealing impregnations. These substances give concrete hydrophobic properties. They greatly increase the water resistance and also reduce the water absorption of materials. They are used in conditions of high humidity and in places requiring special sanitary and hygienic measures.
Adhesive coatings. They are used when exposed to liquid media (for example, if a concrete pile is flooded groundwater). In addition, they are used as an impermeable underlayer for facing coatings. For example, polyisobutylene sheets, polyethylene film, oil bitumen rolls, etc.
biocidal materials. They are designed to destroy and suppress fungal formations on concrete structures. Chemically active elements penetrate into the concrete structure and fill microcracks and pores.
Anti-corrosion coatings for concrete are used everywhere: in the walls and floors of residential premises, in garage complexes, foundations, collectors, sewage treatment plants, greenhouses, greenhouses.
Ticket number 19
1) 100 g of water was removed from 400 g of a 50% (by mass) solution of H2SO4 by evaporation. what is equal to mass fraction H2SO4 in the remaining solution??
Mass of sulfuric acid in solution
m (H2SO4) \u003d m1 (H2SO4 solution) * W1 / 100 \u003d 400 * 50 / 100 \u003d 200 g.
The mass of the resulting solution
m2(H2SO4 solution) = m1(H2SO4 solution) - m(H2O) = 400 - 100 = 300 g.
The concentration of sulfuric acid in the resulting solution:
W2 \u003d m (H2SO4) * 100 / m2 (solution H2SO4) \u003d 200 * 100 / 300 \u003d 66.67%
2) Elements that exhibit metallic and non-metallic properties in compounds are called amphoteric, these include elements of the A-groups of the Periodic system - Be, Al, Ga, Ge, Sn, Pb, Sb, Bi, Po, etc., as well as most elements B-groups - Cr, Mn, Fe, Zn, Cd, Au, etc. Amphoteric oxides are called the same as the main ones, for example:
BeO - beryllium oxide
FeO - iron(II) oxide
Al2O3 - aluminum oxide
Fe2O3 - iron(III) oxide
SnO - tin(II) oxide
MnO2 - manganese(IV) oxide
SnO2 - tin(IV) dioxide
ZnO - zinc(II) oxide
Amphoteric hydroxides (if the oxidation state of the element exceeds + II) can be in ortho - or (and) meta - form. Here are examples of amphoteric hydroxides:
Be(OH)2
- beryllium hydroxide
Al(OH)3
- aluminum hydroxide
AlO(OH)
- aluminum metahydroxide
TiO(OH)2
- titanium dihydroxide
Fe(OH)2
- iron(II) hydroxide
FeO(OH)
- iron metahydroxide
Amphoteric oxides do not always correspond to amphoteric hydroxides, since when trying to obtain the latter, hydrated oxides are formed, for example:
SnO2 . nH2O
- tin(IV) oxide polyhydrate
Au2O3 . nH2O
- gold(I) oxide polyhydrate
Au2O3 . nH2O
- gold(III) oxide polyhydrate
If several oxidation states correspond to an amphoteric element in compounds, then the amphotericity of the corresponding oxides and hydroxides (and, consequently, the amphotericity of the element itself) will be expressed differently. For low oxidation states, hydroxides and oxides have a predominance of basic properties, and the element itself has metallic properties, so it is almost always a part of cations. For high oxidation states, on the contrary, hydroxides and oxides have a predominance of acidic properties, and the element itself has non-metallic properties, so it is almost always included in the composition of anions. Thus, manganese(II) oxide and hydroxide are dominated by basic properties, and manganese itself is part of the 2+ type cations, while acidic properties dominate in manganese(VII) oxide and hydroxide, and manganese itself is part of the anion of the MnO4- type. Amphoteric hydroxides with a large predominance of acidic properties are assigned formulas and names based on the model of acid hydroxides, for example HMnVIIO4 - manganese acid.
Thus, the division of elements into metals and non-metals is conditional; between elements (Na, K, Ca, Ba, etc.) with purely metallic properties and elements (F, O, N, Cl, S, C, etc.) with purely non-metallic properties, there is a large group of elements with amphoteric properties
3) Write an expression for the equilibrium constant of the CO2+C↔ 2CO heterogeneous system. How will the rate of the direct reaction-formation of CO change if the concentration of CO2 is 4 TIMES REDUCED?
K = 2 / - expression for the equilibrium constant.
Let there be x mol / l CO 2, then after a decrease in concentration by 4 times it will be x / 4 mol / l.
Direct reaction rate (up to):
v = k* = k*[x]
Forward reaction rate (after):
v" = k*" = k*
n \u003d v "/ v \u003d (k *) / (k * [x]) \u003d 1/4 - the speed will decrease by 4 times.
With increasing pressure, the equilibrium shifts in the direction in which the total number of moles of gases decreases, i.e. to the left.
4)Standard hydrogen electrode- an electrode used as a reference electrode in various electrochemical measurements and in galvanic cells. A hydrogen electrode (HE) is a plate or wire made of a metal that absorbs well hydrogen gas(usually platinum or palladium is used) saturated with hydrogen (at atmospheric pressure) and immersed in an aqueous solution containing hydrogen ions. The plate potential depends on [ specify] on the concentration of H + ions in the solution. The electrode is a standard against which the electrode potential of the determined chemical reaction is measured. At a hydrogen pressure of 1 atm, a proton concentration in the solution of 1 mol/l, and a temperature of 298 K, the SE potential is assumed to be 0 V. When assembling a galvanic cell from the SE and the electrode to be determined, the reaction proceeds reversibly on the platinum surface:
2Н + + 2e − = H 2
that is, either hydrogen reduction or its oxidation occurs - this depends on the potential of the reaction taking place on the electrode being determined. By measuring the EMF of a galvanic electrode at standard conditions(see above) determine the standard electrode potential of the chemical reaction being determined.
SE is used to measure the standard electrode potential of an electrochemical reaction, to measure the concentration (activity) of hydrogen ions, as well as any other ions. VE is also used to determine the solubility product, to determine the constants
Device
Scheme of a standard hydrogen electrode:
1. Platinum electrode.
2. Hydrogen gas supplied.
3. Acid solution (usually HCl), in which the concentration of H + = 1 mol/l.
4. A water seal that prevents the ingress of oxygen from the air.
5. Electrolytic bridge (consisting of concentrated solution KCl), allowing you to connect the second half of the galvanic cell.
Concrete is an artificial stone material consisting of cement, sand, water and crushed stone. When the compacted mixture of binder (cement) with aggregate hardens, concrete is formed. Crushed stone, sand, gravel can be used as a filler.
- the process of destruction of its structure, embrittlement under the influence of the environment. concrete can be of three types.
Types of concrete corrosion:
1. Dissolution of the constituent parts of the cement stone.
This is the most common type of concrete corrosion failure. Concrete products are operated mainly in the open air. At the same time, they are exposed to atmospheric precipitation and other liquid media. Integral part concrete is the resulting calcium oxide hydrate (Ca (OH) 2) - slaked lime. This is the most easily soluble component, so over time it dissolves and is gradually removed, while violating the structure of concrete.
2. Corrosion of concrete during the interaction of cement stone with acids contained in water.
Under the influence of acids, the corrosion of concrete proceeds either with an increase in its volume, or with the washing out of easily soluble lime compounds.
The increase in volume occurs according to the reaction:
Ca(OH) 2 + CO 2 = CaCO 3 + H 2 O
CaCO 3 is insoluble in water. Gradually, its deposition occurs in the pores of the cement stone, due to which there is an increase in the volume of concrete, and further its cracking and destruction.
When concrete comes into contact with aqueous solutions of acids, easily soluble calcium bicarbonate is formed, which is aggressive for concrete, and in the presence of water it dissolves in it and is gradually washed out of the structure of the concrete stone. The formation of calcium bicarbonate is described by the reaction:
CaCO 3 + CO 2 + H 2 O \u003d Ca (HCO 3) 2.
In addition to dissolution, the chemical corrosion of concrete is also observed:
Ca (OH) 2 + 2HCl \u003d CaCl 2 + 2H 2 O,
in this case, salts of calcium chloride are washed out.
If the destruction of concrete occurs under the influence of water sulfates, pozzolanic Portland cement is used, as well as sulfate-resistant Portland cement.
3. Corrosion of concrete due to the formation and crystallization of sparingly soluble substances in the pores.
In addition to the above-described corrosion destruction of concrete, in the presence of microorganisms, biocorrosion may occur. Fungi, bacteria and some algae can penetrate into the pores of the concrete stone and develop there. The products of their metabolism are deposited in the pores and gradually destroy the structure of the concrete stone.
During corrosion of concrete, several types of destruction usually occur simultaneously.
Corrosion of concrete (reinforced concrete structures) under extreme operating conditions
Extreme conditions can be called the impact on the concrete stone of very low temperatures and various substances with increased aggressiveness.
A fairly common case of concrete corrosion under extreme conditions is the destruction of the material under the influence of sulfates (chemical corrosion of concrete). First of all, aluminate components of concrete stone and calcium hydroxide interact with sulfates. The interaction of aluminate minerals and sulfates is highly undesirable. As a result, several modifications of hydrosulfoaluminate are formed, the most dangerous of which is ettringite(3СaO Al 2 O 3 3CaSO 4 32H 2 O). This salt, as it grows (crystals increase), forms very high stresses inside the concrete, which significantly exceed the strength characteristics of cement stone. As a result, under the influence of solutions containing sulfates, the corrosion destruction of concrete proceeds very intensively.
When calcium hydroxide interacts with sulfates, CaSO 4 2H 2 O is formed. Over time, the substance accumulates in the pore space of concrete, gradually destroying it.
The resistance to sulfate-containing environments depends very much on the mineralogical composition of the concrete. If the content of minerals based on aluminum and tricalcium silicate is limited in cement, then it is more stable in this environment.
If iron reinforcement poured with concrete is used in the structures, i.e. reinforced concrete, another type of destruction is possible - corrosion of reinforcement in concrete. Under the influence of environmental waters or in the presence of hydrogen sulfide, chlorine, sulfur dioxide in the air, the reinforcement in the middle of the concrete rusts and corrosion products of iron are formed. In terms of volume, they exceed the initial volume of the reinforcement, which leads to the emergence and growth of internal stresses, and subsequently to cracking of concrete.
Through the pores in the cement stone, air and moisture penetrate to the reinforcement. Their supply to the metal surface is not carried out uniformly, which is why different potentials are observed in different parts of the surface - electrochemical corrosion proceeds. The rate of electrochemical corrosion of reinforcement depends on the moisture permeability, porosity of the concrete stone and the presence of cracks in it.
The presence of dissolved substances in water increases the corrosion of reinforcement with an increase in electrolyte concentration.
With prolonged exposure to air, a very thin (5-10 microns) protective film is formed on the surface, which does not dissolve in water and does not interact with sulfates. The process of formation of a protective film under the influence of atmospheric carbon dioxide is called carbonization. Carbonization protects concrete from corrosion, but promotes corrosion of reinforcement in concrete.
It is impossible to reinforce concrete, which includes calcium chloride (more than 2% by weight of cement). Calcium chloride accelerates the corrosion of reinforcement both in air and in water.
Corrosion protection of concrete reinforcement
There are several ways to protect steel reinforcement in concrete from corrosion: ennoble the environment surrounding the metal (i.e. use quality concrete special composition, the introduction of inhibitors); additional protection of concrete reinforcement against corrosion (films, etc.); improve the characteristics of the metal itself.
Concrete itself is located around the reinforcement, therefore it is concrete that is the medium surrounding the metal. To extend the service life of the reinforcement, it is necessary to improve the effect of concrete stone on steel. First of all, it is necessary to exclude or, if this is not possible, to minimize the substances that make up the concrete, which contribute to the intensification of the process of corrosion of reinforcement in concrete. These substances include thiocyanates, chlorides.
If a reinforced concrete product is operated under conditions of periodic wetting, it is necessary to impregnate the concrete with special impregnations (bitumen, petrolatum, etc.). This will greatly reduce the permeability of the concrete. With constant saturation of the concrete stone, the corrosion of reinforcement in concrete is practically reduced to a minimum. This is explained by the fact that the penetration of oxygen to the metal surface is very difficult, and the cathode process is significantly retarded.
To extend service life metal base reinforced concrete - concrete is ennobled. During the formation of the concrete mixture, corrosion inhibitors are introduced into the composition.
To protect reinforcement against corrosion in structural and heat-insulating concretes, the method of ohmic limitation is widely used. The bottom line is that the humidity of the concrete itself should not exceed the equilibrium value at a relative humidity of 60%. Then the processes of reinforcement corrosion almost stop, because. there is a high ohmic resistance of moisture films near the surface of the reinforcement. This method is not so simple and not effective in areas with high humidity and frequent rainfall.
Good concrete should have an initial passivating effect on the reinforcement. Concrete products completely dry out in about 2-3 years. If the climate is dry, then a little faster. It is at this time that the strongest corrosive destruction of the reinforcement occurs, because. it is in a damp concrete environment.
A good way to protect the reinforcement of concrete from corrosion is the preliminary passivation of the surface of the reinforcement, as well as the formation of oxide protective films under the influence of the aqueous alkaline environment of the concrete stone. Enhance the protective properties of the film by introducing concrete mix passivators. Often sodium nitrite is used in an amount of 2 - 3% of the initial cement weight.
Corrosion protection of concrete
To protect concrete from corrosion and extend its service life, it is not enough to use only one type of protection. So that concrete does not succumb to the harmful effects of the environment, already at the design stage, preventive actions for his protection.
Operational and preventive measures include the neutralization of aggressive environments, sealing, intensive ventilation during the operation of cement stone in the room (for air drying).
Rational design plays an important role in preventing concrete from further failure. At the same time, it is necessary to give concrete surface structural form, which will exclude the accumulation of water and various organic substances in the recesses. In addition, it is important to ensure the free flow of liquid from the surface. This can be achieved by using drainage or by forming a concrete surface with a slope.
Protection of concrete against corrosion can be divided into primary and secondary.
Primary protection of concrete against corrosion provides for the introduction of special additives into the composition of concrete during its manufacture and formation, while changing its mineralogical composition. This method is considered the most effective.
Various water-retaining, plasticizing, stabilizing, chemical modifiers, amorphous silica, etc. can serve as additives.
In addition, focusing on the operating conditions of the cement stone, during its formation, the composition that is optimal for these conditions is selected. For example, for cements operating in sulfate-containing waters, the C 3 S content is reduced.
Pozzolanization is often used. Acid hydraulic additives containing active silica are added to Portland cement.
Ca (OH) 2 + SiO 2 nH 2 O \u003d CaO SiO2 (n + 1) H 2 O,
The resulting calcium hydrosilicate is more stable than Ca(OH) 2 .
Chemical additives can greatly improve the performance properties of concrete. Increase its density, as a result of which aggressive agents in the pores slow down the speed of their movement. Reinforcement, being in dense concrete, is less susceptible to corrosion damage.
Also, with the help of chemical additives, it is possible to significantly increase the number of conditionally closed pores. As a result, the frost resistance of cement stone increases significantly.
The most common chemical additives that are used to protect concrete from destruction are: plasticizing, antifreeze, sealing, water-repellent, air-entraining, setting retarders, gas-forming, reinforcement corrosion inhibitors.
Some additives have a double effect, i.e. improve several indicators at once. Others may improve one and downgrade the other.
The most promising and common are the following additives.
Mylonaft. It is a plasticizing additive consisting of a mixture of sodium salts of water-insoluble organic acids. It helps to increase the homogeneity of the concrete mix, while reducing the friction between its individual grains. Also involves air. Produced and supplied in the form of pastes. It is necessary to introduce into the concrete mixture from 0.05 to 0.15% by weight of cement (in terms of dry matter). If the specified dosage is exceeded, the compressive strength of concrete decreases.
Mylonaft increases the water resistance of concrete stone by two grades, frost resistance by two times, resistance to mineral salt solutions, and crack resistance.
Sulfite-yeast brew SDB. This is a chemical plasticizing additive. It is obtained by processing calcium salts of lignosulfonic acids. The substance helps to increase the mobility of the concrete mix, entrain air in it and reduce the sticking of cement grains. Manufacturers may supply SDBs as solid or liquid concentrates. To achieve the protective effect of this additive, you need a little more than soap naphtha. In terms of dry matter of cement, it is necessary to introduce 0.15 - 0.3% of sulfite-yeast mash. It increases frost resistance by 1.5 - 2 times, by 5 - 10% strength, by one grade - water resistance, resistance to mineral salt solutions and crack resistance.
Sulfite-yeast mash has the best effect when it is introduced into a concrete stone based on high-aluminum and fast-hardening Portland cements.
Silicone liquid GKZH-94. This is a hydrophobic and gas-forming additive, which is formed during the hydrolysis of ethylhydrosiloxane. As a result of the interaction of cement and this additive, hydrogen is released and a large number of closed pores evenly distributed in concrete. It has an active water-repellent effect on the capillaries and pore walls of concrete. It almost does not affect the rheological properties of the mixture, but it greatly slows down the process of concrete hardening ( initial stages). Supplied as a 50% aqueous emulsion or 100% liquid. The second is introduced into the concrete mixture in the amount of 0.03 - 0.08%.
It helps to increase the water resistance of concrete by two grades, frost resistance - by three to four times. In addition, it increases resistance to variable moistening and drying, the effects of mineral salt solutions (under conditions of capillary suction), stretching.
Secondary protection of concrete against corrosion provides for the application of various paintwork materials, protective mixtures, coatings and cladding various plates. Those. concrete waterproofing.
Secondary protection can also include carbonization (exposure of concrete in air).
Corrosion protection of concrete with paints and varnishes acrylic coatings used when exposed to solid and gaseous media. The resulting protective film effectively protects the concrete surface not only from air and moisture, but also from the effects of various microorganisms.
Protecting concrete from corrosion with mastics it is used when exposed to moisture, contact with solid media. Mastics based on various resins (resinization) are often used.
Protection of concrete against corrosion by sealing impregnations used in almost all environments (liquid, gaseous), especially at high humidity, in addition, they are used before applying coatings. Sealing impregnations fill the outer layer of concrete, giving it good hydrophobic properties, and reduce water absorption.
Biocidal materials are used to protect concrete from the effects of various types of fungi, mold, bacteria, microorganisms. Chemically active substances of biocidal additives fill the pores of concrete and destroy bacteria.
Corrosion protection of concrete with adhesive coatings it is used in the operation of concrete stone in liquid media, soils with high humidity and places of frequent wetting with electrolyte. For example, lower part concrete breakwater is pasted over with polyisobutylene plates.
As pasting coatings, polyethylene film, polyisobutylene plates, oil bitumen rolls can be used. They can also act as an impermeable sublayer in claddings.
Most effective comprehensive protection concrete from corrosion, i.e. both primary and secondary.
Concrete, thanks to its technical characteristics and design possibilities, has won a leading position in the building materials market. However, it, being exposed to aggressive external influences, is gradually destroyed with a deterioration in consumer qualities. This process is called concrete corrosion. According to modern concepts, corrosion is a whole series of chemical, physicochemical reactions and biological processes provoked by the influence of the external environment and leading to the destruction of the material.
Types of concrete corrosion
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There are three main types of corrosion of this building material:
- The first type of corrosion includes all processes that occur in concrete under the influence of soft water. In this case, the components of the cement stone dissolve in water and are carried away by it. This process can take place with different speed. In dense concretes of massive hydraulic structures, the corrosion process proceeds slowly and can last for several decades. In thin-walled concrete structures, the components of the cement stone decompose quickly, and after several years of operation, it may be necessary to repair work. If the water filtration process begins through the concrete, then the decomposition of the concrete components accelerates, a large amount of calcium hydroxide is removed from the material and the concrete becomes highly porous, which means it is fragile.
The leaching of calcium hydroxide slows down if concrete element is in the air. Under the influence of carbon dioxide in the air, calcium hydroxide is converted into calcium carbonate. So concrete blocks, intended for the construction of hydraulic engineering facilities, are kept in the air for several months before lowering to the installation site. This measure allows time for the calcium hydroxide to carbonate on the surface of the concrete.
- Corrosion of the second type - chemical corrosion - includes those processes that occur in concrete during the interaction of chemicals contained in water or the environment with the components of the cement stone. As a result of these reactions, easily soluble products and amorphous masses are formed in the body of concrete, which do not have an astringent ability. Because of this, concrete can gradually turn into a spongy mass with extremely low strength. For example, this type includes sulfate corrosion, which occurs due to the interaction of concrete with water containing a large amount of sulfates.
From corrosion processes of the second type highest value have magnesia and carbon dioxide corrosion.
- Corrosion of the third type includes processes in which sparingly soluble salts accumulate in the capillaries and pores of concrete. The crystallization of these salts causes stresses in the capillaries and pores, which leads to the destruction of the concrete structure. Sulfate corrosion has the greatest practical significance in the processes of this category.
In addition to the listed types of corrosion damage caused by exposure of concrete to a liquid, biological corrosion is distinguished. It affects mainly buildings. Food Industry. The cause of its occurrence are fungi, bacteria, algae. The destruction of concrete is caused by the products of their metabolism. Especially this process is activated in conditions of high humidity.
Protection of concrete against corrosion by increasing the resistance of the material itself
One of the ways to prevent corrosion is. Read our article on how to properly increase the density of concrete.
Preparation of cinder concrete - all about how to choose the right slag and manually prepare cinder concrete.
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Many corrosion control measures are difficult or not very effective. In practice, they try to use the simplest and most inexpensive methods and, above all, increase the stability of the concrete itself by using corrosion-resistant cement or giving the material high density and waterproof.
- Use of corrosion resistant cements. In some cases, the occurrence of sulfate corrosion of concrete can be avoided by using sulfate-resistant cements instead of Portland cement or Portland slag cement. These special cements contain active ingredients that make it possible to increase the resistance of concrete not only to sulphate, but also to fresh water.
- Increasing the density of concrete. This kind of corrosion control is an effective way to protect the material from all kinds of corrosive processes. Increasing the density of concrete reduces its water resistance. This makes it difficult for aggressive media to penetrate into the pores of the material. For the manufacture of high-density concrete, cements with low water demand are used, the water-cement ratio is reduced, and the mixture is compacted with special care in the manufacture of a concrete element.
If these measures did not give a result, then they resort to the optimal method of waterproofing in a particular case.
Types of waterproofing
One of the most common waterproofing methods for concrete and reinforced concrete products - piles, pipes, columns, slabs - is impregnating waterproofing.
To effectively protect the material from the destructive effect of corrosion, it is sufficient to impregnate it to a depth of 10-15 mm. The surface waterproof layer creates protection against water penetration for the rest of the volume of the structural element.
Impregnation methods are distinguished by temperature and pressure. According to the impregnation temperature, there are hot and cold.
- For hot impregnation, petroleum bitumens, paraffins, petrolatum, and synthetic compounds are used. The impregnation operation is carried out, as a rule, in baths at temperatures of 80-180°C. When heated, the impregnating composition transforms into liquid state, its viscosity decreases, it easily penetrates into the pores of concrete, clogging them tightly when solidified.
- As cold impregnations, compositions are used, which are based on mineral binders - cement, sodium silicate, or organic low- and high-molecular substances - styrene, methyl methacrylate, polyurethane.
Impregnation waterproofing can be carried out at different pressures:
- The simplest operation is impregnation under atmospheric pressure. In this process, the penetration of the composition into the pores occurs only due to the capillary effect.
- Impregnation in autoclaves is carried out at a pressure of 0.6-1.2 MPa, but despite high pressure, the rate of the process increases by no more than two times. This is due to the presence of air in the pores, which occupies a part of the volume and counteracts the impregnating composition.
- Vacuuming increases the efficiency of concrete processing by 3-4 times. Impregnating compositions easily penetrate into the pores, from which the air is pumped out, without encountering opposition.
Surface impregnation is carried out directly on the object with highly penetrating compounds. Processing is usually carried out twice.
Other types of waterproofing: injection, hydrophobization, mastic and roll pasting waterproofing.
Corrosion destruction of reinforcement in concrete
Life time building structures reduces not only the corrosion of concrete, but also the corrosion of metal reinforcement. The process of destruction of the metal is carried out for some time, but it is theoretically impossible to determine the exact service life of metal elements. Corrosion of reinforcement in heavily loaded structures is especially dangerous.
Impregnating waterproofing with application is a very effective way to protect against corrosion if the impregnation is chosen correctly.
Do you prefer hydrophobic concrete additives? Read about how to choose and use them correctly.
If you are interested in renting an abs (concrete pump truck), with our prices and conditions.
To prevent corrosion, care must be taken that the composition of the concrete does not contain substances that are aggressive to the metal. But in practice, this task is not feasible, since it is impossible to check the chemical composition of all concrete aggregates.
Corrosion of reinforcement is initiated by elements contained in the air and moisture penetrating through the pores of the concrete. Due to the unevenness of this process, different potentials arise in different parts of the reinforcement, which causes electrochemical corrosion. The rate of this corrosion process increases with an increase in the porosity and moisture permeability of the material, and also due to an increase in the electrolyte concentration, which is increased by substances dissolved in water.
Big damage metal fittings causes electrocorrosion, which occurs due to leakage currents and stray currents that appear at the locations of electrical poles.
Reinforced concrete supports of contact networks are the most vulnerable components on electrified sections of railways.
Ways to combat corrosion of reinforcement
AT modern construction water-repellent lubricants are used and protective coatings for fittings. One of the ways to protect metal elements is to provide a concrete cushion of the required size with the help of clamps.
One of the main difficulties in combating reinforcement corrosion is the impossibility of metal re-treatment, which can be carried out for open metal structures.
The most promising direction is the use of polymer mixtures in the composition of concrete. Polymers added to concrete in combination with cement create additional protection fittings. In some cases, cement is completely replaced by polymers, obtaining polymer concrete.
For thin-walled structures, it is possible to use fundamentally new materials:
- steel fiber concrete is a concrete mix to which trimmings are added steel wire, occupying up to 6% of the total volume of the material;
- In addition to traditional components, alkali-resistant fiberglass is added to fiberglass concrete.
So far no universal and effective ways to combat metal corrosion in reinforced concrete, builders are forced to lay reinforcement in more than it should be in accordance with the technical calculations.
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