Concrete Technology 2/12 · hinder the advancement of corrosion. Concrete Technology 2. If occasional mechanical loads are so severe that there is change in the crack width or flowing

Concrete Technology 2/12

Aalto UniversitySchool of EngineeringDepartment of Civil and Structural EngineeringBuilding Materials Technology

Addition of air-entrainmentadmixture to produce

protective pores

Increase concrete strengthclass

Increase concrete strengthclass

Increase the thickness ofconcrete cover over

reinforcementApply non-corroding

reinforcement

Concrete Technology 2


Carbonation is term used for the reactions between carbondioxide originating from air and cement paste components

Air contains a small amount of carbon dioxide (0.03% at ruralareas and 0.3% in large cities) and it dissolves into pore waterof concrete producing carbonic acid. Carbonic acid reactsreadily with calcium hydroxide situated in pore water indissolved and crystalline form. Reaction products are waterand CaCO3 or its polymorphs aragonite and vaterite.Calcium hydroxide is mainly responsible for the high alkalinityof pore water in concrete (pH-value 12.4-13.5) while CaCO3 isnearly neutral (pH ~ 7). In this way, carbonation decreasesalkalinity in pore water. Carbonation reaction needs suitablewater amount in the pore system to proceed.

The highest rate of carbonation occurs at a relative humiditybetween 50 and 70% in the ambient air. When concrete isvery dry or nearly saturated, carbonation rate is minimal.

The reaction product of carbonation, CaCO3, is larger thanCa(OH)2 and, thus, the pore volume in concrete decreaseswhen calcium carbonate precipitates on the pore walls ofconcrete. This decreases permeability of concrete. However,cracking caused by carbonation shrinkage impairs thedecreased permeability and it can be assumed to remainnearly unchanged with respect to carbonation.


When most of the available Ca(OH)2 in the pore water hasbeen consumed, C-S-H gel will also begin to disintegrate.Eventually, all concrete constituents possessing CaO in theirstructure will carbonate after a long time. The compressivestrength of carbonated, normal strength concrete remainsnearly unchanged because CaCO3 and its polymorphs carryalso loads in similar manner as the virgin concrete.

Carbonation of reinforced concrete structures poses nodurability problems if the structure is situated in a dryenvironment, for example, indoors. However, if concrete iswet, carbonation of the reinforcement cover concrete canshorten the expected service life-span of the structureremarkably.


Alkalinity of pore water forms a thin passivity layer of oxideon the reinforcement steel surface. This passivity layercompletely protects the reaction of steel with oxygen andwater, and reinforcement bars in non-carbonated concretedo not rust. When carbonation has decreased the alkalinityof pore water near the surface of the steel bars toapproximately a pH-value of 9, the protective passivity layeron steel surface is broken and if oxygen and water arepresent, rusting of steel reinforcement begins.

According to test results, the depth of the carbonation layerfrom the surface of the concrete structure increases inproportion to the square root of time if the environment ofthe structure is under steady hygrometric condition.


=

in which x is carbonation depth in [mm],k is carbonation coefficient in [mm/year0.5],and t is exposure time in [years].

This equation becomes effective after couple of yearscarbonation.The higher the CaO-content is, the slower the carbonationrate. There is a big difference in CaO-content which cancarbonate in concretes produced by different blendedcements, especially if they are pozzolans such as fly-ash orsilica fume. They react with Ca(OH)2 and form C-S-H, andthe calcium hydroxide content in the concrete decreases.


Rusting of the reinforcement is a chemical reaction and,therefore, also a temperature-related phenomenon. In lowsubzero temperatures, corrosion rate is considerably slowercompared to situation when temperature is high +20…40 oC.

The passivity protective oxide layer can also be broken, if thechloride content in pore water in the vicinity of the steel barsexceeds a certain threshold value which is dependent on theOH--ion concentration of the pore water. Also, in chloride-initiated corrosion, water and oxygen must be available nearthe reinforcement surface.





After the initiation period, corrosion is an electrochemicalprocess in which electrons and OH--ions are transportedbetween anode and cathode parts of the reinforcement and anelectric circuit is formed. At the anode, positive metal ionsFe2+ are dissolved into the pore water and electrons move tothe cathode via reinforcement.At the cathode, a chemical reaction takes place betweenelectrons, oxygen, and water to form hydroxyl ions whichmove to the anode through pore water.At the anode, hydroxyl ions react with iron ions and Fe(OH)2or rust is forming. There has to exist a difference in electricalpotential between the anode and cathode as a driving force tokeep the reaction happening.

Corrosion takes place only at the anode and if the reactioncan proceed freely, ferric hydroxide Fe(OH)3 will form as theend product. The volume of the corrosion products canincrease by a factor of over 5 which causes tensile stressesaround the reinforcement bar. Eventually, in low strengthconcretes, this can cause cracking, spalling, or evendelamination of the concrete cover over the reinforcement.

Cracks in the concrete cover have only a small influence onthe service life-span of the structure if there are no chloridespresent and the cracks are generated in perpendiculardirection to the reinforcement bars. This holds even if thecrack width is relatively large. Corrosion products andrealkalization in the crack over the reinforcement effectivelyhinder the advancement of corrosion.


If occasional mechanical loads are so severe that there ischange in the crack width or flowing water rinses thecracked surface, cracks will decrease the service life span ofthe structure.

When there is a chloride concentration exceeding a thresholdvalue in the pore water, chloride ions break the protectiveoxide layer over the steel to form an anode, while theunbroken surface forms the cathode. During the chemicalreaction, ferrous chloride is formed at the intermediate stageof the reaction, but, as ferrous hydroxide contains nochloride, Cl- is regenerated by formation of HCl.


Only free chlorides in the pore water are effective ininitiating the chloride-induced corrosion of reinforcement.Part of the chlorides have reacted with the aluminates in thebinder paste and are bound into the binder matrix.

Time

Deg

ree

of c

orro

sion

Initiation time ProgressionService life span

Rei

nfor

cem

ent b

egin

s to

rust Allowed

corrosiondegree

CO2 Cl-

TRH

O2


Estimation of service life-span of concretestructures

In the 2004 Finnish concrete code, a calculation method forestimating service life-span with regard to frost exposure andcarbonation is presented. The expected service life-span can beestimated by equation

For which tL is the estimated service life-spantLr is the reference service life-span (50 years)A…G are life-span coefficients reflectingvarious factors


Coefficients A-G in the table are different for frost resistanceand for carbonation. For example, in frost resistance,coefficient A takes into consideration air content,water/cement ratio, and maximum aggregate size. CoefficientB depends on massiveness (volume to surface area ratio) ofthe structure and possible coating of the structure.Coefficient C takes into consideration the curing measures.Coefficient E depends on the geographical direction andgeographical location of the structure and coefficient G givesthe impact of inspection and maintenance frequency.

In carbonation the effects of concrete strength class, cementtype, and air content are taken into consideration incoefficient A.


Coefficient B depends on concrete cover thickness overreinforcement and possible coatings on the concrete surface.Coefficient C depends on the curing measures and coefficientE1 takes into consideration the effects of exposure class,coefficients E2-E4 take into consideration geographicaldirection, geographical location, and frost exposure.Coefficient G again depends on inspection and maintenancefrequency.


Factors influencing frost resistance in Finnishconcrete code 2004

Coefficient Factor Design parameterA Materials, porosity Air content and water/cement ratioB Design, structural details Structure type, coatingC Performance of work Curing timeD Interior climate -E Exterior exposure to

weatherFrost exposure class, geographicaldirection

F Working load -G Maintenance measures Inspection and maintenance

frequency


Values of coefficient A in exposure class XF3. Shaded values can be usedonly in studying the effects of existing structures in the case of poor quality

Air content [%]

Max. aggregate

Coefficient A

size [mm] Effective water cement ratio

8 12 >16 0,3 0,35 0,4 0,45 0,5 0,55 0,6 0,65 0,7

2,5 2,0 1,5 1,04 0,69 0,52 0,43 0,36 0,32 0,29 0,26 0,24

3,0 2,5 2,0 3,12 1,30 0,84 0,63 0,52 0,44 0,38 0,34 0,31

3,5 3,0 2,5 4,00 2,51 1,26 0,86 0,66 0,55 0,47 0,41 0,37

4,0 3,5 3,0 4,00 4,00 1,91 1,14 0,83 0,66 0,55 0,47 0,42

4,5 4,0 3,5 4,00 4,00 3,08 1,50 1,01 0,77 0,63 0,54 0,47

5,0 4,5 4,0 4,00 4,00 4,00 2,00 1,23 0,90 0,72 0,60 0,52

5,5 5,0 4,5 4,00 4,00 4,00 2,77 1,50 1,04 0,81 0,67 0,57

6,0 5,5 5,0 4,00 4,00 4,00 4,00 1,84 1,21 0,91 0,74 0,62

6,5 6,0 5,5 4,00 4,00 4,00 4,00 2,28 1,39 1,02 0,81 0,67

7,0 6,5 6,0 4,00 4,00 4,00 4,00 2,91 1,61 1,13 0,88 0,73

7,5 7,0 6,5 4,00 4,00 4,00 4,00 3,85 1,88 1,26 0,96 0,78

8,0 7,5 7,0 4,00 4,00 4,00 4,00 4,00 2,21 1,41 1,05 0,84

8,5 8,0 7,5 4,00 4,00 4,00 4,00 4,00 2,62 1,58 1,14 0,90

9,0 8,5 8,0 4,00 4,00 4,00 4,00 4,00 3,16 1,77 1,24 0,97

9,5 9,0 8,5 4,00 4,00 4,00 4,00 4,00 3,91 1,99 1,35 1,03

10,0 9,5 9,0 4,00 4,00 4,00 4,00 4,00 4,00 2,25 1,47 1,11


As an example, the estimated service life-spans of concretestructures situated in environment class XF3 (frost action,horizontal structure, no salt exposure) can be assessedaccording to the table. Then only the material-relatedparameters are taken into consideration and all othercoefficients have a value of 1.

The estimated life-span can be calculated by multiplyingcoefficient A by 50 years according to the previouslypresented equation. The deterioration formulae by which thecoefficient values have been calculated are also presented inthe concrete code.


Other durability features of concreteSulfate attackSulfate attack is initiated when water soluble sulfates (SO4

2-),originating from ground or from sea water, penetrate intoconcrete pore water and react with aluminates or calciumhydroxide in cement paste. Reaction products expandremarkably which causes crack propagation and decreasesthe strength properties of concrete.Four reaction mechanisms are responsible for sulfatedamage in concrete. Sulfate ions can react with calciumhydroxide and gypsum (CaSO4·H2O) is formed. Aluminatesfrom cement or sometimes from aggregates can react withsulfates and trisulfate (ettringite 3CaO·Al2O3·3CaSO4·31H2O) is formed. The increase in volume of the solid phasesin these reactions is 124 and 227 percent, respectively.


The third sulfate deterioration mechanism is attributed tosulfate absorption into silicates or to a reaction with C-S-H.In these instances thaumasite (CaSiO3·CaCO3·CaSO4·15H2O) is produced. This reaction takes place in lowtemperatures.

The fourth mechanism does not need outside source ofsulfates to cause expansion and cracking into concrete. Thedeterioration mechanism can be named inner sulfate attackcaused by excessive heat treatment in concretes produced byPortland cement. When temperature rises to 70-100 oCduring hydration, ettringite transforms into monosulfate(3CaO·Al2O3·CaSO4·12H2O) and sulfate.


At lower temperatures monosulfate becomes metastable and,if in hardened concrete there is sufficiently water available orlater water content in concrete increases, ettringite can beformed again. This reaction is accompanied by expansion inthe concrete structure and subsequent cracking.This reaction can happen after a couple years time span and,therefore, it is sometimes called delayed ettringite formation.This deterioration mechanism has been observed in façadeprecast units, concrete railway sleepers, and basement slabs.The severity of sulfate corrosion expansion caused by outsidesulfate attack is different depending on the salt composition.The severity increases in the order calcium sulfate, sodiumsulfate, and magnesium sulfate. The severity of the attackincreases also when the moisture content in concreteincreases.


Remedy for sulfate attack

Sulfate attack can be mitigated by minimizing the C3A-content of the cement by applying sulfate resisting cements.Sulfate resisting cements have a C3A-content below 3% orblast furnace slag content in the binder exceeds 70%.The other mitigation method is to reduce the Ca(OH)2–content in concrete by applying blended cements in which thepozzolanic reaction decreases the calcium hydroxide amount.


Sulfate expansions of test mortars produced by different binders. The watercement ratio of the mortars is 0.6, mortars have been immersed in sodiumsulfate solution in which SO4

-2 content is 30 g/l.


Acid attackDue to the high alkalinity of pore water (pH 12.5-14) inconcrete, all binder paste constituents are stable in thisenvironment. All strong acids (pH < 4.5) and many weakacids (5.5 < pH < 6.5) effectively decrease pore wateralkalinity and attack Ca(OH)2 and C-S-H gel of the binderpaste. Most aggregates endure acid attack remarkably bettercompared to the binder paste.Acid attack is a surface phenomenon similar to carbonationand, therefore, the penetration depth is a function of squareroot of time. The decomposition of hydration products formnew compounds which may be leached out from thestructure, if they are soluble. Some compounds can bedisruptive as such, for example, H2SO4 is a combination ofacid attack and sulfate attack.


Alkali-aggregate reactionsThere are two reaction types causing deleterious swelling ofconcrete in moist environment due to reactions betweenalkalis (Na2O and K2O) and certain aggregates. In alkali-silica reaction, the reactive forms of silica are opal,chalcedony, and tridymite which occur in opaline orchalcedonic cherts, siliceous limestones, and some volcanicrocks as rhyolites. Alkali-carbonate reaction is causedbetween some dolomitic limestone aggregates and the alkalisof the cement.In both deleterious aggregate reactions, all aspects of theinvolved mechanisms are not known. Reactive siliceousminerals in the aggregate react with alkaline hydroxidesoriginating usually from cement.


Alkali-silicate gel is formed in the voids and cracks of theaggregate or on the surface of the aggregate. The gel absorbswater and swells in large volume (5-20 ‰) if water isavailable in concrete and the environment. Internalpressures are generated into concrete and eventuallycracking can destroy the concrete structure totally. On thesurface, cracks form a map-like pattern and sometimes pop-outs can be noticed.A combination of mix design features and moisture conditionhave to be fulfilled for the deleterious swelling of the gel tooccur. The severity of the swelling of the gel depends on theamount of reactive material and its particle size, alkalicontent in the pore water, and the moisture content inconcrete.


For different reactive aggregates, particle size fraction, andcement type, a different pessimum combination can befound. If the reactive aggregate material amount in concreteis very small or very large and the moisture content is belowa threshold value, the expansion caused by the swelling gelcan be insignificant.To hinder the alkali-silica reaction, the maximum relativehumidity in the interior of concrete should not exceed 80-85%. The cement type should have as low alkali oxidecontent as possible. The equivalent Na2O-content in thecement should not exceed 0.60 % (eqv. Na2O = Na2O +0.659·K2O by weight). Also, application of pozzolanicsecondary cementitious binders has been shown to diminishthe deleterious expansion caused by alkali-silicate reaction.


The imperfectly understood damage mechanism in alkali-carbonate aggregate reaction involves the de-dolomitizationof the dolomite structure. When dolomite CaMg(CO3)2structure is changed into CaCO3 and Mg(OH)2, it becomesmore open and other minerals such as clay in the dolomiteaggregate begin to expand due to moisture. Pozzolanicsecondary binders are not effective in controlling the alkali-carbonate expansion which is contrary to alkali-silicatereaction.

Fortunately, alkali-carbonate reaction is quite rare.


This presentation can give the impression that a large volumeof concrete structures are subject to deterioration. However,most indoor concrete structures have usually no corrosionproblems and their service life-span can be measured inhundreds of years.

Outdoor concrete structures constitute about one third of thetotal volume of concrete structures and the majordeterioration mechanisms that affect them are reinforcementcorrosion and freeze-thaw deterioration. A large majority ofthe outdoor structures are not exposed to salts and it is notdifficult to design and build such concrete structures thatpossess estimated service life span of 200 years.


The technology and design knowledge already exists toextend the estimated life-span of outdoor structures exposedto chlorides to 100 years. All other deterioration mechanismsapply to concrete structures that comprise below 5 per centof the total concrete volume. Even in these structures,durability properties of concrete usually exceed those of thecompeting materials.


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Concrete Technology 2/12 · hinder the advancement of corrosion. Concrete Technology 2. If occasional mechanical loads are so severe that there is change in the crack width or flowing