Concrete degradation has many different causes. Concrete is mostly damaged by the corrosion of reinforcement bars, the carbonatation of hardened cement paste or chloride attack under wet conditions. Chemical damage is caused by the formation of expansive products produced by chemical reactions (from carbonatation, chlorides, sulfates and distillate water), by aggressive chemical species present in groundwater and seawater (chlorides, sulfates, magnesium ions), or by microorganisms (bacteria, fungi...) Other damaging processes can also involve calcium leaching by water infiltration, physical phenomena initiating cracks formation and propagation, fire or radiant heat, aggregate expansion, sea water effects, leaching, and erosion by fast-flowing water.
The most destructive agent of concrete structures and components is probably water. Indeed, water often directly participates in chemical reactions as a reagent and is always necessary as a solvent, or a reacting medium, making transport of solutes and reactions possible. Without water, many harmful reactions cannot progress, or are so slow that their harmful consequences become negligible during the planned service life of the construction. Dry concrete has a much longer lifetime than water saturated concrete in contact with circulating water. So, when possible, concrete must first be protected from water infiltration.
Corrosion of reinforcement bars
The expansion of the corrosion products (iron oxides) of carbon steel reinforcement structures may induce internal mechanical stress (tensile stress) that cause the formation of cracks and disrupt the concrete structure. If rebars have been improperly installed or have inadequate concrete cover at surfaces exposed to the elements, oxide jacking and spalling can occur during the structure's lifetime: flat fragments of concrete are detached from the concrete mass as a result of the rebar's corrosion.
Concrete, like most consolidated hard rocks, is a material very resistant to compression but which cannot withstand tension, especially internal tensions. Its tensile strength is about 10 times lower than its compressive strength. In itself carbonated concrete is a very solid material because its compressive strength increases due to its porosity decrease by the precipitation of calcium carbonate (calcite, ). In the absence of steel reinforcement bars and without the formation of expansive reaction products inducing tensile stress inside the concrete matrix, pure concrete is most often a long-lasting material. An illustration of the concrete intrinsic durability is the dome of the Pantheon building in Rome made with Roman concrete more than 2000 years ago.
When atmospheric carbon dioxide (), or carbonate ions (, dissolved in water) diffuse into concrete from its external surface, they react with calcium hydroxide (portlandite, ) and the pH of the concrete pore water progressively decreases from 13.5 – 12.5 to 8.5 (pH of water in equilibrium with calcite). Below a pH value of about 9.5 – 10, the solubility of iron oxides present at the surface of carbon steel increases and they start to dissolve. As a consequence, they no longer protect the underlying metallic iron against oxidation by atmospheric oxygen and the reinforcement bars are no longer passivated against corrosion. It is the considerable forces internally created by the expansion of the iron corrosion products (about 6 – 7 times less dense than metallic iron, so 6 – 7 times more voluminous) that cause the cracks in the concrete matrix and destroy reinforced concrete. In the absence of iron (and without some harmful chemical degradation reactions also producing expansive products) concrete would probably be one of the most durable materials. However, steel reinforcement bars are necessary to take over the tensile efforts to which concrete is submitted in most engineering structures and stainless steel would be too costly a metal to replace carbon steel. Zinc-galvanization or epoxy-coating can improve the corrosion resistance of rebar, but have also other disadvantages such as their lower surface adhesion to concrete (risk of slip), the possible formation of cathodic and anodic zones conducive to galvanic corrosion if the protective coating is locally punctured or damaged, and their higher costs.
Formation of expansive phases in concrete As hard rocks, concrete can withstand high compressive stress but not tensile stress. As a consequence, concrete is easily damaged when expansion phases are formed in its mass.
The most ubiquitous, and best known, expansive phases are probably the iron oxides produced by the oxidation of the carbon steel reinforcement bars embedded into concrete. Corrosion products are formed around rebar located in carbonated concrete (and thus no longer passivated against corrosion), or directly exposed to the atmospheric oxygen once cracks have started to form. The damages produced by rebar corrosion are clearly visible with bare eyes and their diagnostic is easy.
Other deleterious expansive chemical reactions more difficult to characterize and to identify can occur in concrete. They may be first distinguished according to the location where they occur in concrete: inside the aggregates or in the hardened cement paste.
Expansion inside the aggregates
Various types of aggregates can undergo different chemical reactions and swell inside concrete, leading to damaging expansive phenomena.
Alkali–silica reaction The most common are those containing reactive amorphous silica, that can react in the presence of water with the cement alkalis (K2O and Na2O). Among the more reactive siliceous mineral components of some aggregates are opal, chalcedony, flint and strained quartz. Silica (in fact silicic acid when hydrated) is easily dissolved by sodium hydroxide (NaOH) to form sodium silicate (), a strong desiccant with a high affinity for water. This reaction is at the core of the alkali–silica reaction (ASR):
Following this reaction, a hygroscopic and expansive viscous silicagel phase forms inside the affected aggregates which swell and crack from inside. In its turn, the volumetric expansion of the swollen aggregates damages the concrete matrix and extensive cracks propagate causing structural damages in the concrete structure. On the surface of concrete pavements, the ASR can also cause pop-outs, i.e. the expulsion of small cones (up to in diameter), corresponding to aggregate particle size.
A quite similar reaction (alkali-silicate reaction) can occur when clay minerals are present in some impure aggregates, and it may also lead to destructive expansion.
Alkali–carbonate reaction With some aggregates containing dolomite, a dedolomitization reaction, also known as alkali-carbonate reaction (ACR), can occur where the magnesium carbonate () reacts with the hydroxyl ions () and yields magnesium hydroxide (brucite, ) and a carbonate ion (). The resulting expansion caused by the swelling of brucite can cause destruction of the material:
Often the alkali–silicate reaction and the dedolomitization reaction are masked by a much more severe alkali–silica reaction dominating the deleterious effects. Because the alkali-carbonate reaction (ACR) is often thwarted by a coexisting ASR reaction, it explains why ACR is no longer considered to be a major detrimental reaction.
Pyrite oxidation Far less common are degradation and pop-outs caused by the presence of pyrite (), a disulfide (S-S) very sensitive to oxidation by atmospheric oxygen, that generates expansion by forming less dense insoluble iron oxides (), iron oxy-hydroxides (FeO(OH), or ·n) and mildly soluble gypsum (·2).
When complete (i.e., when all ions are also oxidized into less soluble ions), pyrite oxidation can be globally written as follows:
The sulfuric acid released by pyrite oxidation then reacts with portlandite ()) present in the hardened cement paste to give gypsum:
When concrete is carbonated by atmospheric carbon dioxide (), or if limestone aggregates are used in concrete, reacts with calcite () and water to also form gypsum while releasing back to the atmosphere:
The dihydrated gypsum is relatively soluble in water at room temperature and thus mobile. It can easily be leached by infiltration water and can form efflorescences on the concrete surface while the insoluble ·n remain in place around the grains of oxidized pyrite they taint in red-ocre.
Expansive chemical reactions inside the hardened cement paste The sulfate anions reacting with different phases of the hardened cement paste (HCP) to form more voluminous reaction products can cause 3 types of expansive reactions called sulfate attack inside HCP: The delayed ettringite formation (DEF) also known as internal sulfate attack (ISA); The external sulfate attack (ESA), and; The thaumasite form of sulfate attack (TSA).
These three types of sulfate attack reactions are described into more details in specific sections latter in the text. When the hardened cement paste (HCP) is affected, the detrimental consequences for the structural stability of concrete structures are generally more severe than when aggregates are affected: DEF, ESA and TSA are much more damaging for concrete than ASR and ACR reactions.
A common points to all these various chemical expansive reaction is that they all require water as a reactant and as a reaction medium. The presence of water is always an aggravating factor. Concrete structures immersed in water as dams and bridge piles are therefore particularly sensitive. These reactions are also characterized by slow reaction kinetics, depending on environmental conditions such as temperature and relative humidity. They develop at a slow rate and may take several years before damages become apparent. Often a decade is needed to observe their harmful consequences. Protecting concrete structures from water contact may help to slow down the progression of the damages.
Chemical damages
Carbonation
Carbon dioxide () from air and bicarbonate () or carbonate () anions dissolved in water react with the calcium hydroxide (, portlandite) produced by Portland cement hydration in concrete to form calcium carbonate () while releasing a water molecule in the following reaction:
Exception made of the water molecule, the carbonation reaction is essentially the reverse of the process of calcination of limestone taking place in a cement kiln:
Carbonation of concrete is a slow and continuous process of atmospheric diffusing from the outer surface of concrete exposed to air into its mass and chemically reacting with the mineral phases of the hydrated cement paste. Carbonation slows down with increasing diffusion depth.
Carbonation has two antagonist effects for (1) the concrete strength, and (2) its durability: The precipitation of calcite filling the microscopic voids in the concrete pore space decreases the concrete matrix porosity: so, it increases the mechanical strength of concrete;