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Concrete degradation may have many different causes.Concrete is mostly damaged by thecorrosion ofreinforcement bars, thecarbonatation of hardenedcement paste or chloride attack under wet conditions. Chemical damage is caused by the formation of expansive products produced by chemical reactions (fromcarbonatation, chlorides, sulfates and distillate water), by aggressive chemical species present ingroundwater andseawater (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.[1]
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 asolvent, or a reacting medium, making transport ofsolutes 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.
The expansion of thecorrosion products (iron oxides) ofcarbon steel reinforcement structures may induce internalmechanical stress (tensile stress) that cause the formation of cracks and disrupt the concrete structure. If rebars have been improperly installed or have inadequateconcrete cover at surfaces exposed to the elements,oxide jacking andspalling 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 hardrocks, is a material very resistant to compression but which cannot withstand tension, especially internal tensions. Itstensile 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,CaCO3). In the absence of steelreinforcement 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 thePantheon building in Rome made withRoman concrete more than 2000 years ago.
When atmosphericcarbon dioxide (CO2), orcarbonate ions (HCO−3,CO2−3 dissolved in water)diffuse into concrete from its external surface, they react withcalcium hydroxide (portlandite,Ca(OH)2) and thepH of the concrete pore water progressively decreases from 13.5 – 12.5 to 8.5 (pH of water in equilibrium withcalcite). Below a pH value of about 9.5 – 10, thesolubility ofiron oxides present at the surface ofcarbon steel increases and they start to dissolve. As a consequence, they no longer protect the underlying metalliciron againstoxidation by atmosphericoxygen and the reinforcement bars are no longer passivated againstcorrosion. It is the considerableforces internally created by the expansion of the iron corrosion products (about 6 – 7 times lessdense 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 andstainless steel would be too costly a metal to replacecarbon steel.Zinc-galvanization orepoxy-coating can improve the corrosion resistance of rebar, but have also other disadvantages such as their lower surfaceadhesion to concrete (risk of slip), the possible formation ofcathodic andanodic zones conducive togalvanic corrosion if the protective coating is locally punctured or damaged, and their higher costs.
As hard rocks, concrete can withstand highcompressive stress but nottensile 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 theiron oxides produced by theoxidation of thecarbon 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.
Various types of aggregates can undergo different chemical reactions and swell inside concrete, leading to damaging expansive phenomena.
The most common are those containing reactiveamorphoussilica, that can react in the presence of water with thecementalkalis (K2O and Na2O). Among the more reactivesiliceous mineral components of some aggregates areopal,chalcedony,flint and strainedquartz. Silica (in factsilicic acid when hydrated) is easily dissolved bysodium hydroxide (NaOH) to formsodium silicate (Na
2SiO
3), a strongdesiccant with a high affinity for water. This reaction is at the core of thealkali–silica reaction (ASR):
Following this reaction, ahygroscopic and expansive viscoussilicagel 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 3 cm (1 in) in diameter), corresponding to aggregate particle size.
A quite similar reaction (alkali-silicate reaction) can occur whenclay minerals are present in some impure aggregates, and it may also lead to destructive expansion.
With some aggregates containingdolomite, adedolomitization reaction, also known asalkali-carbonate reaction (ACR), can occur where themagnesium carbonate (MgCO
3) reacts with thehydroxyl ions (OH−
) and yieldsmagnesium hydroxide (brucite,Mg(OH)
2) and acarbonate ion (CO2−
3). 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.
Far less common are degradation and pop-outs caused by the presence ofpyrite (FeS
2), aFe2+
disulfide (–S-S–) very sensitive tooxidation by atmosphericoxygen, that generates expansion by forming less dense insolubleiron oxides (Fe
2O
3),iron oxy-hydroxides (FeO(OH), orFe
2O
3·nH
2O) and mildly solublegypsum (CaSO
4·2H
2O).
When complete (i.e., when allFe2+
ions are also oxidized into less solubleFe3+
ions), pyrite oxidation can be globally written as follows:
Thesulfuric acid released bypyrite oxidation then reacts withportlandite (Ca(OH
2)) present in the hardened cement paste to give gypsum:
When concrete iscarbonated by atmosphericcarbon dioxide (CO2), or iflimestone aggregates are used in concrete,H
2SO
4 reacts withcalcite (CaCO
3) and water to also form gypsum while releasing CO2 back to the atmosphere:
Thedihydrated gypsum is relativelysoluble in water(~ 1 – 2 g/L) atroom temperature and thus mobile. It can easily beleached by infiltration water and can formefflorescences on the concrete surface while the insolubleFe
2O
3·nH
2O remain in place around the grains of oxidized pyrite they taint in red-ocre.
Thesulfateanions 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:
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 slowreaction 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.
Carbon dioxide (CO2) from air(~ 412 ppm vol.) andbicarbonate (HCO−
3) orcarbonate (CO2−
3)anionsdissolved in water react with thecalcium hydroxide (Ca(OH)
2,portlandite) produced byPortland cementhydration in concrete to formcalcium carbonate (CaCO
3) while releasing a water molecule in the following reaction:
Exception made of the water molecule, thecarbonation reaction is essentially the reverse of the process ofcalcination oflimestone taking place in acement kiln:
Carbonation of concrete is a slow and continuous process of atmospheric CO2diffusing 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.[2]
Carbonation has two antagonist effects for (1) the concrete strength, and (2) its durability:
This explains why the carbonation reaction ofreinforced concrete is an undesirable process in concrete chemistry. Concrete carbonation can be visually revealed by applying aphenolphthalein solution over the fresh surface of a concrete samples (concrete core, prism, freshly fractured bar). Phenolphthalein is apH indicator, whose color turns from colorless at pH < 8.5 to pink-fuchsia at pH > 9.5. A violet color indicates still alkaline areas and thus non-carbonated concrete. Carbonated zones favorable for steel corrosion and concrete degradation are colorless.[3][4]
The presence of water in carbonated concrete is necessary to lower the pH of concrete pore water around rebar and to depassivate the carbon steel surface at low pH. Water is central to corrosion processes. Without water, the steel corrosion is very limited and rebar present in dry carbonated concrete structures, or components, not affected by water infiltration do not suffer from significant corrosion.
The main effect ofchloride ions onreinforced concrete is to causepitting corrosion of thesteelreinforcement bars (rebar). It is a surreptitious and dangerous form oflocalized corrosion because the rebar sections can be decreased to the point that the steel reinforcement are no longer capable of withstanding thetensile efforts they are supposed to resist by design. When the rebar sections are too small or the rebar are locally broken, the reinforcement is lost, and concrete is no longer reinforced.
Chlorides, particularlycalcium chloride, have been used to shorten the setting time of concrete.[5] However, calcium chloride and (to a lesser extent)sodium chloride have been shown to leachcalcium hydroxide and cause chemical changes inPortland cement, leading to loss of strength,[6] as well as attacking thesteel reinforcement present in most concrete. The ten-storey Queen Elizabeth hospital in Kota Kinabalu contained a high percentage of chloride causing earlyfailure.
The alkali–silica reaction (ASR) is a deleterious chemical reaction between thealkali (Na
2O andK
2O), dissolved in concrete pore water as NaOH and KOH, with reactiveamorphous (non-crystalline)siliceous aggregates in the presence of moisture. The simplest way to write the reaction in a stylized manner is the following (other representations also exist):
This reaction produces agel-like substance ofsodium silicate (Na
2SiO
3 • nH
2O), also notedNa
2H
2SiO
4 • nH
2O, or N-S-H (sodium silicate hydrate). Thishygroscopic gel swells inside the affected reactive aggregates which expand and crack. In its turn, it causes concrete expansion. If concrete is heavily reinforced, it can first cause some prestressing effect before cracking and damaging the structure. ASR affects the aggregates and is recognizable by cracked aggregates. It does not directly affect the hardened cement paste (HCP).
When the temperature of concrete exceeds 65 °C for too long a time at an early age, the crystallization ofettringite (AFt) does not occur because of its higher solubility at elevated temperature and the then less soluble mono-sulfate (AFm) is formed. After dissipation of the cement hydration heat, temperature goes back to ambient and the temperature curves of the solubilities of AFt and AFm phases cross over. The mono-sulfate (AFm) now more soluble at low temperature slowly dissolves to recrystallize as the less soluble ettringite (AFt). AFt crystal structure hosts more water molecules than AFm. So, AFt has a higher molar volume than AFm because of its 32 H2O molecules. During months, or years, after young concrete cooling, AFt crystallizes very slowly as small acicular needles and can exert a considerable crystallization pressure on the surrounding hardened cement paste (HCP). This leads to the expansion of concrete, to its cracking, and it can ultimately lead to the ruin of the affected structure. The characteristic feature of delayed ettringite formation (DEF) is a randomhoneycomb cracking pattern similar to this of the alkali-silica reaction (ASR). In fact, this typical crack pattern is common to all expansive internal reactions and also torestrained shrinkage where a rigid substrate or a dense rebar network prevents the movements of a superficial concrete layer. DEF is also known as internal sulfate attack (ISA). External sulfate attack (ESA) also involves ettringite (AFt) formation and deleterious expansion with the same harmful symptoms but requires an external source ofsulfate anions in the surrounding terrains or environment. To avoid DEF or ISA reactions, the best way is to use a low C3A (tri-calcium aluminate) cement precluding the formation of ettringite (AFt). Sulfate resisting (SR) cements have also a low content inAl2O3. DEF, or ISA, only affects the hardened cement paste (HCP) and leaves intact the aggregates.
DEF is exacerbated at high pH in cement with a too high content inalkalis and therefore inhydroxides. This is caused by the transformation ofettringite (AFt) into aluminoferrite monosulfate (AFm) under the action of thehydroxylanions (OH–) as schematized as follows:
The complete reaction can be derived from the molecular formulas of the reagents and products involved in the reaction. This reaction favors the dissolution of AFt and the formation of AFm. When combined, it is an aggravating factor of the harmful effect of too high temperatures. To minimize DEF, the use of low-alkali cements is also recommended. The detrimental crystallization of ettringite (AFt) preferentially occurs when concrete is exposed to water infiltrations and that the pH decreases due to the leaching of the (OH–) ions: the reaction is reversed as when temperature decreases.
Sulfates in solution in contact with concrete can cause chemical changes to the cement, which can cause significant microstructural effects leading to the weakening of the cement binder (chemical sulfate attack). Sulfate solutions can also cause damage to porous cementitious materials through crystallization and recrystallization (salt attack).[7] Sulfates and sulfites are ubiquitous in the natural environment and are present from many sources, including gypsum (calcium sulfate) often present as an additive in 'blended' cements which includefly ash and other sources of sulfate. With the notable exception of barium sulfate, most sulfates are slightly to highly soluble in water. These includeacid rain where sulfur dioxide in theairshed is dissolved in rainfall to produce sulfurous acid. In lightning storms, the dioxide is oxidized to trioxide making the residual sulfuric acid in rainfall even more highly acidic. Concrete sewage infrastructure is most commonly attacked bysulfuric acid andsulfateanions arising from theoxidation ofsulfide present in the sewage. Sulfides are formed whensulfate-reducing bacteria present in sewer mains reduce the ubiquitous sulfate ions present in water drains intohydrogen sulfide gas (H
2S).H
2S is volatile and released from water in the sewage atmosphere. It dissolves in a thin film of water condensed onto the wall of the sewer ducts where it is also accompanied by hydrogeno-sulfide (HS−
) and sulfide (S2−
) ions. WhenH
2S andHS−
anions are further exposed to atmospheric oxygen or to oxygenated stormwater, they are readily oxidized and producesulfuric acid (in fact acidichydrogen ions accompanied bysulfate andbisulfate ions) according to the respective oxidation reactions:
or,
The corrosion often present in the crown (top) of concrete sewers is directly attributable to this process – known ascrown rot corrosion.[8]
Thaumasite is a calciumsilicate mineral, containing Si atoms in unusualoctahedral configuration, with chemical formulaCa3Si(OH)6(CO3)(SO4)·12H2O, also sometimes more simply written as CaSiO3·CaCO3·CaSO4·15H2O.
Thaumasite is formed under special conditions in the presence ofsulfateions inconcrete containing, or exposed to, a source ofcarbonateanions such aslimestone aggregates, or finely milled limestone filler (CaCO3).Bicarbonate anions (HCO−3) dissolved in groundwater may also contribute to the reaction. The detrimental reaction proceeds at the expense ofcalcium silicate hydrates (C-S-H, with dashes denoting here their non-stoichiometry) present in the hardenedcement paste (HCP). The thaumasite form of sulfate attack (TSA) is a particular type of very destructivesulfate attack. C-S-H are the "glue" in the hardened cement paste filling the interstices between theconcrete aggregates. As the TSA reaction consumes the silicates of the "cement glue", it can lead to a harmfuldecohesion and a softening ofconcrete. Expansion and cracking are more rarely observed. Unlike the commonsulfate attack, in which thecalcium hydroxide (portlandite) andcalcium aluminate hydrates react withsulfates to respectively formgypsum andettringite (an expansive phase), in the case of TSA the C-S-H ensuring the cohesion of HCP and aggregates are destroyed. As a consequence, even concrete containing low-C3A sulfate-resistingPortland cement may be affected.[9]
TSA is sometimes easily recognizable on the field when examining the altered concrete. TSA-affected concrete becomes powdery and can be dug with a scoop, or even scrapped with the fingers. Concrete decohesion is very characteristic of TSA.
TSA was first identified during the years 1990 inEngland in theUnited Kingdom in the foundation piles of bridges of the motorway M5 located in theKimmeridgianmarls. These marls are a mixture ofclay andlimestone sedimented underanoxic conditions and are rich inpyrite (FeS2, aFe2+ disulfide). Once these marls were excavated,pyrite was exposed to atmospheric oxygen or oxygen-rich infiltration water and rapidlyoxidized.Pyrite oxidation producessulfuric acid. In its turn,H2SO4 reacts with portlandite (present in the hardened cement paste, HCP) andcalcite (CaCO3 (present in limestone aggregates or in carbonated HCP). The strong acidification of the medium caused by pyrite oxidation releasesbicarbonate ions (HCO−3) orcarbon dioxide (CO2) along withcalcium (Ca2+) andsulfate ions (SO2−4).
Full pyrite oxidation can be schematized as:
Thesulfuric acid released bypyrite oxidation then reacts withportlandite (Ca(OH
2)) present in the hardened cement paste to give gypsum:
When concrete also containslimestone aggregates or a filler addition,H
2SO
4 reacts withcalcite (CaCO
3) and water to also form gypsum while releasing CO2:
Gypsum is relativelysoluble in water(~ 1 – 2 g/L), so there is plenty of calcium and sulfates ions available for TSA.
Simultaneously,carbonic acid (H2O + CO2 ⇌ H2CO3) dissolves calcite to form soluble calciumbicarbonate:
So, when all the chemical ingredients necessary to react with C-S-H from the hardened cement paste in concrete are present together the TSA reaction can occur. When grounds rich in pyrite, such as many clays or marls, are excavated for civil engineering works, the strong acidification produced by pyrite oxidation is the powerful driving force triggering TSA because it frees up and mobilizes all the ions needed to attack C-S-H and to formthaumasite (CaSiO3·CaCO3·CaSO4 · 15H2O).
TSA is favored by a low temperature, although it can be encountered at higher temperature in warm areas. The reason is to be found in theretrograde solubility of most of the ingredients needed for the TSA reaction. Indeed, the solubility of dissolvedcarbon dioxide (CO2),portlandite (Ca(OH)2),calcite (CaCO3), andgypsum (CaSO4·2H2O), increases when the temperature is lowered. This is because the dissolution reactions of these mineral species areexothermic and release heat. A lower temperature facilitates the heat release and therefore favors the exothermic reaction. Only the solubility ofsilica (from C-S-H) increases with temperature because silica dissolution is anendothermic process which requires heat to proceed.
When water flows throughcracks present in concrete, water may dissolve variousminerals present in the hardenedcement paste or in theaggregates, if the solution is unsaturated with respect to them. Dissolved ions, such as calcium (Ca2+), are leached out and transported in solution some distance. If the physico-chemical conditions prevailing in the seeping water evolve with distance along the water path and water becomes supersaturated with respect to certain minerals, they can further precipitate, makingcalthemitedeposits (predominatelycalcium carbonate) inside the cracks, or at the concrete outer surface. This process can cause theself-healing of fractures in particular conditions.
Fagerlund[10] (2000) determined that, “About 15% of the lime has to be dissolved before strength is affected. This corresponds to about 10% of the cement weight, or almost all of the initially formed Ca(OH)2.” Therefore, a large amount of "calcium hydroxide" (Ca(OH)2) must be leached from the concrete before structural integrity is affected. The other issue however is that leaching away Ca(OH)2 may allow the corrosion of reinforcing steel to affect structural integrity.
Within set concrete there remains some free "calcium hydroxide" (Ca(OH)2),[2] which can further dissociate to form Ca2+ and hydroxide (OH−) ions".[11] Any water which finds a seepage path through micro cracks and air voids present in concrete, will readily carry the (Ca(OH)2) and Ca2+ (depending on solution pH and chemical reaction at the time) to the underside of the structure where leachate solution contacts the atmosphere.[12]Carbon dioxide (CO2) from the atmosphere readily diffuses into the leachate and causes a chemical reaction, which precipitates (deposits)calcium carbonate (CaCO3) on the outside of the concrete structure. Consisting primarily of CaCO3 this secondary deposit derived from concrete is known as "calthemite"[12] and can mimic the shapes and forms ofcave "speleothems", such asstalactites,stalagmites,flowstone etc.[13] Other trace elements such as iron from rusting reinforcing steel bars may be transported and deposited by the leachate at the same time as the CaCO3. This may colour the calthemites orange or red.[14][15]
The chemistry involving the leaching ofcalcium hydroxide from concrete can facilitate the growth of calthemites up to ≈200 times faster than cave speleothems due to the different chemical reactions involved.[16] The sight of calthemite is a visual sign that calcium is being leached from the concrete structure and the concrete is gradually degrading.[12][17]
In very old concrete where the calcium hydroxide has been leached from the leachate seepage path, the chemistry may revert to that similar to "speleothem" chemistry in limestone cave.[12][13] This is where carbon dioxide enriched rain or seepage water forms a weakcarbonic acid, which leaches calcium carbonate (CaCO3) from within the concrete structure and carries it to the underside of the structure.[18] When it contacts the atmosphere, carbon dioxide degasses and calcium carbonate is precipitated to create calthemite deposits,[12] which mimic the shapes and forms of speleothems.[13] This degassing chemistry is not common in concrete structures as the leachate can often find new paths through the concrete to access free calcium hydroxide and this reverts the chemistry to that previously mentioned where CO2 is the reactant.[12]
Concrete exposed toseawater is susceptible to itscorrosive effects. The effects are more pronounced above thetidal zone than where the concrete is permanently submerged. In the submerged zone,magnesium andhydrogen carbonate ions precipitate a layer ofbrucite (magnesium hydroxide: Mg(OH)2), about 30micrometers thick, on which a slower deposition ofcalcium carbonate asaragonite occurs. These mineral layers somewhat protect the concrete from other processes, which includes attack bymagnesium,chloride andsulfate ions andcarbonatation. Above the water surface, mechanical damage may occur byerosion bywaves themselves orsand andgravel they carry, and bycrystallization of salts from water soaking into the concretepores and then drying up.Pozzolanic cements and cements using more than60 wt.% ofblast furnace slags as cementitious material are more resistant toseawater than purePortland cement. Seawater attack presents aspects of both chloride and sulfate attacks.
Bacteria themselves do not have noticeable effect on concrete. However,sulfate-reducing bacteria (SRB) in untreatedsewage water tend to producehydrogen sulfide (H2S), which is thenoxidized insulfuric acid (H2SO4) by atmosphericoxygen (abiotic reaction) and byaerobic bacteria present inbiofilm (biotic reaction) on the concrete surface above the water level. The sulfuric acid dissolves thecarbonates in the hardened cement paste (HCP), and alsocalcium hydroxide (portlandite: Ca(OH)2) andcalcium silicate hydrate (CaO·SiO2·nH2O), and causesstrength loss, as well as producingsulfates which are harmful to concrete.[19]
In each case the soft expansive and water-soluble corrosion product ofgypsum (CaSO4) is formed. Gypsum is easily washed away inwastewater causing a loss ofconcrete aggregate and exposing fresh material to acid attack.
Concrete floors lying on ground that containspyrite (iron(II)disulfide) are also at risk. As a preventive measure sewage may be pretreated to increase pH or oxidize or precipitate the sulfides in order to minimize the activity of sulfide-reducing bacteria.[citation needed]
As bacteria often prefer to adhere to the surfaces of solids than to remain into suspension in water (planktonic bacteria), thebiofilms formed by sessile (i.e., fixed) bacteria are often the place where they are the most active. Biofilms made of multiple layers (like an onion) of dead and living bacteria protect the living ones from the harsh conditions often prevailing in water outside biofilm. Biofilms developing on the already exposed surface of metallic elements encased in concrete can also contribute to accelerate their corrosion (differential aeration and formation ofanodic zones at the surface of the metal).Sulfides produced by the SRB bacteria can also inducestress corrosion cracking in steel and other metals.
Damage can occur during the casting and de-shuttering processes. For instance, the corners of beams can be damaged during the removal of shuttering because they are less effectively compacted by means of vibration (improved by using form-vibrators). Other physical damage can be caused by the use of steel shuttering without base plates. The steel shuttering pinches the top surface of a concrete slab due to the weight of the next slab being constructed.
Concrete slabs, block walls and pipelines are susceptible to cracking during ground settlement, seismic tremors or other sources of vibration, and also from expansion and contraction during adverse temperature changes.
Thecement hydration process consumes water molecules. The sum of the volumes of the hydration products present in the hardened cement paste is smaller than the sum of the volumes of the reacting mineral phases present in thecement clinker. Therefore, the volume of the fresh and very young concrete undergoes a contraction due to the hydration reaction: it is what is called "chemical shrinkage" or "self-desiccation". It is not a problem as long as the very fresh concrete is still in a liquid, or a sufficiently plastic, state and can easily accommodate volume changes (contraction).
Later in the setting phase, when the fresh concrete becomes more viscous and starts to harden, water loss due to unwanted evaporation can cause "plastic shrinkage". This occurs when concrete is placed under hot conditions, e.g. in the summer, and not sufficiently protected against evaporation. Cracks often develop above reinforcement bars because the contraction of concrete is locally restrained at this level and the still setting and weakly resistant concrete cannot freely shrink.
The curing of concrete when it continues to harden after its initial setting and progressively develops its mechanical strength is a critical phase to avoid unwanted cracks in concrete. Depending on the temperature (summer or winter conditions) and thus on the cement hydration kinetics controlling the setting and hardening rate of concrete, curing time can require a few days only (summer) or up to two weeks (winter). It is then critical to avoid losses of water by evaporation because water is necessary for continuing the slow cement hydration. Water loss at this stage aggravates concrete shrinkage and can cause unacceptable cracks to develop. Cracks form in case of a too short, or too poor, curing when young concrete has not yet developed a sufficient early strength to withstand tensile stress caused by undesirable and premature drying. Crack development occurs when early-age concrete is insufficiently protected against desiccation and too much water evaporates with heat because of unfavorable meteorological conditions: e.g, high temperature, direct solar insolation, dry air, lowrelative humidity, and high wind speed during summer, or in hot conditions. Curing is intended to maintain moist conditions at the surface of concrete. It can be done by letting the formworks in place for a longer time, or by applying ahydrophobic thin film of an oily product (curing compound) at the concrete surface (e.g., for large slabs or rafts) to minimize water evaporation.
After sufficient setting and hardening of concrete, the progressive loss of capillary water is also responsible for the "drying shrinkage". It is a continuous and long-term process occurring later during the concrete's life when under dry conditions the larger pores of concrete are no longer completely saturated by water.
When concrete is subject to an excessive temperature increase during its setting and hardening as in massive concrete structures from where cement hydration heat cannot easily escape (semi-adiabatic conditions), thetemperature gradients and the differential volume changes can also cause the formation of thermal cracks and fissures. To minimize them a slowly-setting cement (CEM III, withblast furnace slags) is preferred to a quickly setting cement (CEM I:Portland cement). Pouring concrete under colder conditions (e.g., during the night, or in the winter), or using cold water and ice mixed with cooled aggregates to prepare concrete, may also contribute to minimizing thermal cracks.
When a concrete structure is heavily reinforced, the very dense rebar network can block the contraction movement of the protectingconcrete cover located above the external layer ofreinforcement bars due to the natural drying shrinkage process. As a consequence, a network of fissures with the characteristic honeycomb pattern also typical for the cracks resulting from the expansive chemical reactions (ASR,DEF,ESA) forms.
The formation of fissures in the concrete cover above the reinforcement bars represents a preferential pathway for the ingress of water and aggressive agents such asCO2 (lowering ofpH around the rebar) andchloride anions (pitting corrosion) into concrete. The physical formation of cracks therefore favors the chemical degradation of concrete and aggravates steel corrosion. Physical and chemical degradation processes are intimately coupled, and the presence of water infiltrations also accelerates the formation of expansive products of harmful swelling chemical reactions (iron corrosion products, ASR, DEF, ISA, ESA).
Different approaches and methods have been developed to attempt to quantitatively estimate the influence of cracks in concrete structures on carbonation and chloride penetration.[20] Their aim is to avoid underestimating the penetration depth of these harmful chemical agents and to calculate a sufficient thickness for theconcrete cover to protect the rebar against corrosion during the whole service life of the concrete structure.
In winter conditions, or in cold climates, when the temperature falls below0 °Celsius, thecrystallization ofice in thepores of concrete is also a physical mechanism (change of state) responsible for the volumetric expansion of a substance exerting a hightensile strength inside the concrete matrix. When thetensile strength of concrete is exceeded, cracks appear. Adding anair entrainment agent during the mixing of fresh concrete induces the formation of tiny airbubbles in the fresh concreteslurry. This creates numerous small air-filled micro-cavities in the hardened concrete serving as empty volume reserve to accommodate the volumetric expansion of ice and delays the moment tensile stress will develop. Air entrainment makes concrete more workable during placement, and increases its durability when hardened, particularly in climates subject tofreeze-thaw cycles.
Overload, shocks and vibrations (bridges, roads submitted to intense truck traffic...) can inducemechanical stress and deformations in concrete structures and be responsible for the mechanical degradation of concrete. Beside the long-term drying shrinkage of concrete, pre-stressed and post-tensioned civil engineering structures (bridges, primary containment domes ofnuclear power plants can also undergo slow concrete creep and deformation.
Due to its lowthermal conductivity, a layer of concrete is frequently used forfireproofing of steel structures. However, concrete itself may be damaged by fire, with one notable example being the1996 Channel Tunnel fire where fire damage extended along several hundred meters of the tunnel's length. For this reason, common fire testing standards, such as ASTM E119,[21] do not permit fire testing of cementitious products unless the relative humidity inside the cementitious product is at or below 75%. Otherwise, concrete can be subject to significant spalling.
Up to about 300 °C, the concrete undergoes normalthermal expansion. Above that temperature, shrinkage occurs due to water loss; however, the aggregate continues expanding, which causes internal stresses. Up to about 500 °C, the major structural changes are carbonatation and coarsening of pores. At 573 °C,quartz undergoes rapid expansion due tophase transition, and at 900 °Ccalcite starts shrinking due to decomposition. At 450-550 °C the cement hydrate decomposes, yielding calcium oxide.Calcium carbonate decomposes at about 600 °C. Rehydration of the calcium oxide on cooling of the structure causes expansion, which can cause damage to material which withstood fire without falling apart. Concrete in buildings that experienced a fire and were left standing for several years shows extensive degree of carbonatation from carbon dioxide which is reabsorbed.
Concrete exposed to up to 100 °C is normally considered as healthy. The parts of a concrete structure that is exposed to temperatures above approximately 300 °C (dependent of water/cement ratio) will most likely get a pink color. Over approximately 600 °C the concrete will turn light grey, and over approximately 1000 °C it turns yellow-brown.[22]One rule of thumb is to consider all pink colored concrete as damaged that should be removed.
Fire will expose the concrete to gases and liquids that can be harmful to the concrete, among other salts and acids that occur when gases produced by a fire come into contact with water.
If concrete is exposed to very high temperatures very rapidly, explosive spalling of the concrete can result. In a very hot, very quick fire the water inside the concrete will boil before it evaporates. The steam inside the concrete exerts expansive pressure and can initiate and forcibly expel a spall.[23]
Exposure of concrete structures toneutrons andgamma radiation innuclear power plants, and high-flux material testing reactors, can induceradiation damage to their concrete structural components.Paramagnetic defects andoptical centers are easily formed, but very high fluxes are necessary to displace a sufficiently high number of atoms in the crystal lattice of the minerals present in concrete before significant mechanical damage is observed.
However,neutron irradiation with a very highneutron fluence (number of neutrons per unit of cross-section area: neutron/cm2) is known to renderamorphous a fraction of thequartz present in some concrete aggregates. This amorphization process is also calledmetamictization. Metamict quartz with its disordered lattice structure is prone toalkali–silica reaction and can thus be responsible of harmful chemical expansion in the concrete of nuclear containment structures.
It may be necessary to repair a concrete structure following damage (e.g. due to age, chemical attack, fire,[24] impact, movement or reinforcement corrosion). Strengthening may be necessary if the structure is weakened (e.g. due to design or construction errors, excessive loading, or because of a change of use).
The first step should always be an investigation to determine the cause of the deterioration. The general principles of repair include arresting and preventing further degradation; treating exposed steel reinforcement; and filling fissures or holes caused by cracking or left after the loss of spalled or damaged concrete.
Various techniques are available for the repair, protection and rehabilitation of concrete structures,[25] and specifications for repair principals have been defined systematically.[26] The selection of the appropriate approach will depend on the cause of the initial damage (e.g. impact, excessive loading, movement, corrosion of the reinforcement, chemical attack, or fire) and whether the repair is to be fully load bearing or simply cosmetic.
Concrete stitching employs metal staples or stitches to restore structural integrity to cracked concrete surfaces. This method applies torque across the crack, effectively transferring load and tension to stabilize and strengthen the affected area. Recognized for its simplicity and minimal disruption, concrete stitching is widely utilized in essential infrastructures such as bridges and buildings, significantly prolonging the lifespan of concrete structures by preventing crack propagation.
Repair principles which do not improve the strength or performance of concrete beyond its original (undamaged) condition include replacement and restoration of concrete after spalling and delamination; strengthening to restore structural load-bearing capacity; and increasing resistance to physical or mechanical attack.
Repair principles for arresting and preventing further degradation include control of anodic areas;cathodic protection, cathodic control; increasing resistivity; preserving or restoring passivity; increasing resistance to chemical attack; protection against ingress of adverse agents; and moisture control.
Techniques for filling holes left by the removal of spalled or damaged concrete include mortar repairs; flowing concrete repairs and sprayed concrete repairs. The filling of cracks, fissures or voids in concrete for structural purposes (restoration of strength and load-bearing capability), or non-structural reasons (flexible repairs where further movement is expected, or alternately to resist water and gas permeation) typically involves the injection of low viscosity resins or grouts based on epoxy, PU or acrylic resins, or micronised cement slurries.[27]
One novel proposal for the repair of cracks is to use bacteria.BacillaFilla is a genetically engineered bacterium designed to repair damaged concrete, filling in the cracks, and making them whole again.
Various techniques are available for strengthening concrete structures, to increase the load-carrying capacity or else to improve the in-service performance. These include increasing the concrete cross-section and adding material such as steel plate or fiber composites[28][29] to enhance the tensile capacity or increase the confinement of the concrete for improved compression capacity.
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