Advanced concrete technology9 reinforcement corrosion

Advanced concrete technology9 reinforcement corrosion Advanced concrete technology9 reinforcement corrosion Advanced concrete technology9 reinforcement corrosion Advanced concrete technology9 reinforcement corrosion Advanced concrete technology9 reinforcement corrosion Advanced concrete technology9 reinforcement corrosion Advanced concrete technology9 reinforcement corrosion

Reinforcement corrosion G.K Glass Corrosion of reinforcing steel in concrete is the most significant deterioration process affecting reinforced concrete structures It has, for example been estimated that the remediation of concrete bridges in the USA undertaken as a direct result of corrosion of the reinforcing steel cost the US state departments $5 billion in the year 2000 (Federal Highway Administration, 1999) In addition to bridges, other highway structures, buildings and marine facilities are affected An estimate of the cost incurred in the UK in 1997 as the result of all forms of corrosion damage to concrete was £750 million (BRE, 2001) The two most important causes of corrosion of the reinforcing steel are carbonation and chloride contamination of the concrete Carbonation is a problem that mainly affects buildings Chloride contamination affects structures that are exposed to de-icing salts or marine environments Other less common causes of corrosion are acidic gases such as sulphur dioxide, aggressive ions such as sulfates, fluorides and bromides and stray electrical currents Corrosion is accompanied by a loss of rebar cross-section and a build-up of corrosion products The corrosion products occupy a larger volume than the original metal from which they were derived (Figure 9.1) This generates the tensile stresses causing cracking and spalling of the concrete cover Very often the first indication of a problem is the appearance of a crack following the line of reinforcement (Figure 9.2) 9/2 Reinforcement corrosion Figure 9.1 Expansive corrosion products on steel in chloride contaminated concrete Figure 9.2 Corrosion induced by carbonation of a concrete beam which has resulted in a crack in the concrete cover following the line of the reinforcement Corrosion may be defined as the deterioration of a metal by reaction with species in the environment to form chemical compounds At room temperatures it is an electrochemical process; that is, the chemical reactions occur in conjunction with the flow of an electric current This is illustrated in Figure 9.3 The metal dissolves at one site to form positive ions leaving free electrons in the metal A reaction that results in a more positive product is termed oxidation, and the location Reinforcement corrosion Fe -~ 2e ~'~ Cathod Cathode Figure 9.3 Schematic diagram of the electrochemical processes occurring during corrosion where oxidation occurs is called an anode For example, iron dissolves (is oxidized) to form positive iron ions The residual electrons in the metal are consumed by a reaction which produces a more negative product Such a reaction is termed reduction and the location where reduction occurs is called the cathode For example, oxygen is reduced to form negative hydroxyl ions An electronic conductor on which an oxidation or reduction reaction occurs is termed an electrode Between the anode and cathode, ionic and electronic current must flow to prevent the build-up of charge The ionic conductor is termed an electrolyte and is usually an aqueous solution of ions (e.g Fe 2+, OH-, Na +, C1-) The positive ions are termed cations and the negative ions are anions The metal provides the electronic conductor If there is no external source of electrons (provided by an external power supply), the anodic oxidation reaction must generate electrons at exactly the same rate as the cathodic reduction reaction consumes them and the flow of electrons in the metal and ions in the environment between the anode and cathode must prevent any accumulation of electric charge Two important ionic species present in all aqueous environments are hydrogen ions (H +) and hydroxyl ions (OH-) When combined these ions produce water Most common cathodic reactions result in the generation of hydroxyl ions An excess of hydroxyl ions is termed alkalinity (a more alkaline environment is produced at the cathode) By contrast, the iron ions produced at the anode may react with water (hydrolysis) to produce hydrogen ions (acidity) which will lead to acidification of the local environment at the anode The processes involved in the formation of rust include the precipitation of iron ions as iron hydroxide and the oxidation of the iron hydroxide by further reaction with oxygen A potential difference (voltage) is the driving force for an electrochemical reaction A more positive potential results in a higher oxidation state and the release of more electrons (positive ions are repelled from a more positive electrode surface) Likewise, a more negative potential will drive negative ions away from an electrode An oxygen electrode has a more positive potential than an iron electrode Thus, iron dissolves as positive ions because its potential is raised by the couple to an oxygen electrode, and oxygen is reduced to negative hydroxyl ions because its potential is lowered by the same couple The potential of an electrode supporting an electrochemical reaction can be measured relative to a stable standard reference electrode The stability of compounds in a given environment and potential range is determined by thermodynamics In the case of iron, a number of possible electrochemical reactions exist The most stable products as a function of potential and pH for a fixed set of environmental conditions are given in a Pourbaix diagram illustrated in Figure 9.4 This diagram does not indicate the rate at which the most stable state will be achieved However, if the most stable products at a given electrode potential and pH are insoluble oxides, the 9/3 9/4 Reinforcement corrosion risk of high corrosion rates is lower than if the most stable products are soluble ions If the most stable product is the metal then corrosion is thermodynamically impossible Also included on this diagram is the potential - pH relationship of the oxygen reduction (line b) and hydrogen evolution (line a) reactions The oxygen reduction reaction is the most important cathodic reaction stimulating the corrosion of reinforcing steel in concrete 1.8 1.6 1.4 1.2 LU -r CO = 0-_ 0.8 Fe 3+ 0.6 0.4 I., •> 0.2 > "~ -0.2 -.~ Fe 2+ '- - o -0.6 la -0.8 -1 -1.2 -1.4 -1.6 -1.8 I I I I I I I I I I I I 10 I I 12 I I I 14 pH Figure 9.4 Simplified Pourbaix diagram for iron showing the most stable products at a given pH and potential Concrete consists of a graded mix of aggregate particles in a cement paste matrix The cement paste consists of unhydrated cement, hydration products and the residue of the water-filled space which gives rise to capillary porosity The hydration products consist mainly of calcium silicate hydrate gel and calcium hydroxide The various phases present are evident in the backscattered electron image of a polished cross-section of concrete containing a steel ribbon given in Figure 9.5 The grey scale in these images depends on the electron density of the material The phases of interest, graded in terms of their brightness, are the steel (lightest) > unhydrated cement grains > calcium hydroxide (CH) > gel (predominantly calcium silicate hydrate (C-S-H)) ~ aluminate-bearing hydrates ~ aggregate > porosity and voids (darkest) The hydration products consist mainly of calcium silicate hydrate gel, which has a porosity of around 28 per cent (the gel pores), and calcium hydroxide Capillary pores are up to lxm in diameter, whereas gel pores are around nm Concrete may also contain entrained air, entrapped air and other voids Intentionally entrained air voids are bubbles typically 0.1 mm in diameter and are distributed evenly throughout the cement paste Accidentally entrapped air usually forms very much larger voids, often up to several millimetres in diameter This will typically account for per cent of the volume of the concrete Other defects include internal cracks, voids under Reinforcement corrosion Steel Figure 9.5 Backscattered electron image of a polished cross-section of steel in concrete (Grass et al., 2001) aggregate particles created by bleed water in concrete, and 'honeycomb' voids (Glass and Buenfeld, 2000a) The pore solution is an aqueous electrolyte that may be present in the pores and larger defects It contains ions such as sodium (Na+), potassium (K+), calcium (Ca2+), hydroxyl (OH-) and sulphate (SO]-), as well as dissolved oxygen Thus, the necessary reactants are present to permit corrosion of the embedded steel However, a significant feature of cement hydration is that the aqueous phase rapidly acquires a high pH Furthermore, the material contains a substantial portion of reserve alkalinity in the form of sparingly soluble hydroxides that resist downward pH changes at values above 10 When steel is in contact with such an alkaline solution, the thermodynamically most stable products of any reaction between steel, water and oxygen are insoluble oxides (cf Figure 9.4) A film consisting of a metal-oxide layer covers the steel and presents a barrier to further metal dissolution The steel is said to be passive and the oxide film is termed a passive film Passivation is thus the primary mechanism of corrosion protection for steel in concrete No significant corrosion will occur if this environment remains intact However, concrete is exposed to a wide variety of external environments and the ingress of aggressive species may render it corrosive A schematic diagram of the process of corrosion-induced deterioration is given in Figure 9.6 Deterioration starts with the loss of protection provided by the concrete cover This is followed by corrosion initiation and then propagation The loss of protection provided by the concrete cover occurs as the result of the ingress of aggressive species such as chloride ions and carbon dioxide (see aggressive species listed in section 9.1) These species are transported through the pore system in the cement paste At low porosities, entrained and entrapped air voids are distinct cavities isolated from one another This will restrict their influence on the transport of aggressive species When they are not filled with solution, they will act to block the solution transport of ions while promoting transport in the gaseous phase Cracks and other large voids may be oriented to produce a continuous network through the concrete thereby significantly 9/5 9/6 Reinforcement corrosion Damage 'ira'; -" I Physical ~ i steel/ damage / ete ".~ "~ o Corrosion | initiation-~ f:> Cover protection / Chloridecontamination CI-free _~| L J high pH ~ carbonation y 11 .&iy~/ to tl Time Figure 9.6 Schematic diagram of corrosion-induced deterioration of reinforced concrete reducing resistance to transport If such large defects are absent, capillary pores will have the dominant effect on the transport of aggressive species into concrete The rate of any transport process will depend on the volume fraction, tortuosity and connectivity of the pores This is determined by factors such as the water/cement ratio (w/c), cement content, cement fineness, cement type, use of cement replacement materials (for example ground granulated blast furnace slag (ggbs), pulverized-fuel ash (pfa) and silica fume (sf)), concrete compaction, and degree of hydration (Atkinson and Nickerson, 1984) Reducing w/c (which controls the original spacing of the cement grains) and prolonged hydration may, for example, result in the capillary pores becoming blocked by gel so that they are interconnected solely by gel pores It may be noted that no physical damage occurs while aggressive species contaminate the concrete cover Indeed a gain in concrete strength may occur Nevertheless the loss of protection provided by the concrete cover is a deterioration process that reduces the remaining maintenance-free service life The period of this first stage normally forms a substantial proportion of the service life before the first maintenance is necessary As indicated above, this period is strongly dependent on the cover depth and the properties of the cover concrete affecting the rate of transport of the aggressive species The second stage of the deterioration process involves corrosion propagation The transition between the first and second stages is precipitated by the breakdown of the passive film resulting in corrosion initiation During the second stage a loss of steel section occurs Furthermore, because the high volume corrosion products exert tensile forces, spalling of the concrete cover may occur (cf Figure 9.1) This may affect the integrity of the structure by reducing the tensile and bond strength of the steel In addition a safety hazard may result from the falling debris More detail on the stages in the deterioration process is given in the sections below 9.5.1 Concrete carbonation :.: : :~: When atmospheric carbon dioxide (CO2) dissolves in the cement pore solution, carbonic acid (H2CO2) is formed A reduction in the pH of the concrete pore solution then occurs Reinforcement corrosion and some of the alkaline solid phases are neutralized For example, calcium hydroxide (Ca(OH)2) will be converted into calcium carbonate (CaCO3) As the reserve levels of the alkaline solid phases are depleted, a zone of low pH (the carbonated zone) extends from the surface into concrete This process is termed carbonation A fall in pH to values below 10 at the steel may render the steel passive film thermodynamically unstable Figure 9.4 Factors associated with both the concrete and the external environment affect the rate of carbonation The nature of the porosity and the alkaline reserves of the cement hydration products are the main factors associated with the concrete that affect carbonation Thus, for example, a high water/cement ratio will increase the capillary porosity and the rate of carbonation (Figure 9.7) Cracks resulting from tensile stresses in the concrete will also increase the carbonation depth 40 ~" vE 30 325 kg/m 30PC days > 95% RH curing 20 years UK \¢,~.~,¢I" , 20 1~ J 0.2 0.4 1t m - T = ~.~te,' 0.6 0.8 Water/cement ratio 1.0 Figure 9.7 The effect of free water/cement ratio and external exposure environment on the depth of carbonation in concrete (Hassanein, 1997) Carbon dioxide reacts with all the major constituents of hydrated cement This includes both calcium hydroxide and the calcium silicate hydrate gel Many solid phases will release hydroxyl ions as the pH is reduced These alkaline reserves in hydrated cement depend on the cement type (Sergi and Glass, 2000; Glass et al., 2000a) Table 9.1 shows the acid required to reduce the pH of aqueous suspensions of ground cement paste and concrete to a pH of approximately 10 It is apparent that blending ordinary Portland cement (opc) with pfa or ggbs may substantially reduce the alkaline reserves in the hydrated paste and therefore reduce the resistance to carbonation Table 9.1 Acid (moles H ÷ / kg binder) required to reduce the pH to the phenolphthalein endpoint together with values extrapolated from the titration data at pH 10 Acid neutralization capacity (moles/kg) Cementitious binder OPC SRPC 30%PFA 65%GGBS Cement paste (Sergi and Glass, 2000) (Phenolphthalein) (pH 10) 19.8 19.1 13.1 13.0 19.8 18.6 13.0 Concrete (Glass et al., 2000a) (pH 10) 18.9 17.5 15.4 14.5 9/7 9/8 Reinforcement corrosion A very important factor affecting the rate of carbonation is the moisture content of the concrete This is determined by the environment to which the concrete is exposed Water is necessary for the carbonation reaction, but if the pores are filled, the ingress of carbon dioxide is severely hindered (Hassanein, 1997) Thus concrete exposed in dryer environments carbonates more quickly (Figure 9.7) Models of the carbonation process may be formulated by assuming that it is controlled by diffusion of carbon dioxide through concrete and reaction with the constituents of hydrated cement However, the diffusion coefficient for carbon dioxide is difficult to measure as the carbonation process is slow and results in a change in the pore structure Simple models of the rate of carbonation of concrete often assume that the carbonation depth is proportional to the square root of time (t) depth = k ~r/ (9.1) The coefficient of proportionality (k) has to be determined for each concrete/environment combination Examples of measured relationships are given in Figure 9.8 25 • 65% GGBS, internal [] 65% GGBS, external • OPC, internal zx OPC, external 20 E E ,- 15 i , v o "o to [] A • to r~ I'11-i [] o • • [] • A A I • A A A • A i • ' ' 4' ' I ', Time (qyears) Figure 9.8 Measured natural carbonation depths over a 20-year period in 65 per cent GGBS and OPC concretes plotted as a function of the root of time (Hassanein, 1997) 9.5.2 Corrosion initiation and propagation After the depth of carbonation has reached the reinforcing steel, the passive film is no longer stable However, significant rates of corrosion are not automatic As indicated in Figure 9.3, corrosion requires the presence of an electrolyte to conduct ions between the anodic and cathodic sites This is normally provided by moisture In dry concrete the resistance of the environment to the flow of ionic current may be high Increasing the moisture content increases the risk of significant corrosion occurring (Figure 9.9) but it may reduce the carbonation depth (Glass et al., 1991) The binding capacity for aggressive ions such as chloride is also affected by carbonation Bound chloride ions are released as the pH of the concrete is reduced This reduces the concrete resistivity and helps to retain moisture Thus the presence of low levels of chlorides exacerbates the carbonation problem Severe corrosion problems are encountered Reinforcement corrosion t lO E A v 0% Chloride • 0.4% Chloride A 1% Chloride 0.1 40 ' ' ' ', ' ' ' ' I 50 60 I 70 I 80 ', ' ' ' , 90 100 Relative humidity (%) Figure 9.9 Steel m o r t a r (Glass corrosion rate as a f u n c t i o n of chloride c o n t e n t and relative h u m i d i t y in c a r b o n a t e d OPC et al., 1991) when carbonation is accompanied by high levels of chloride contamination However, this is unusual as the dry conditions, which promote high carbonation rates, are not compatible with wet conditions which promote high rates of chloride contamination An example of carbonation-induced corrosion is given in Figure 9.10 which shows a beam removed from a building The depth of carbonation is shown by the indicator phenolphthalein This changes colour (from pink to colourless - dark to light grey in Figure 9.10) when the pH decreases below Figure 9.10 A c a r b o n a t e d concrete beam s h o w i n g areas of corrosion 9/9 9/10 Reinforcementcorrosion ~'~ii~ ~ ~ ~'~ ~ ~ ~ ~ ~i~i~iiii~i~iiiii~i~i!ii~i!!i!iiiiiiiiiiiiii! !i!i!i!!!!!!!!!iiiiii!ii~iii!!!iiiiiiiii~~ ~ii~i~i!iiiiii~ii S~!O~!i!~i~iiii~i~i~i~ii~i~i'~ili~ii~i~i~!~i~i ~i~i~!~i~ii~i~!~ii~!~!~i~i~i~!~i~i~i!~!~ii~i~i~i~!~i~i~i~i~i~i~i~i~i~:i~!~ii~!~!~!~i~i i~ ~ii!~i~i!:~i~il 9.6.1 Chloride contamination Chloride contamination of concrete may arise from both internal and external sources Internal sources of chloride include contamination of the mix materials and the use of calcium chloride as a set accelerator in construction Limitations are placed by current codes of practice on the acceptable levels of chloride contamination resulting from the use of contaminated mix materials, while the use of chloride containing admixtures for reinforced concrete is generally not permitted (BSI, 1988) Thus internal chlorides tend to only affect older existing structures External sources of chlorides include de-icing salts and sea salt in marine environments (Bamforth et al., 1997) Chloride ingress into concrete can occur by a number of mechanisms This includes diffusion due to a concentration gradient, migration in an electric field, and water flow (Bamforth et al., 1997) The rate of diffusion is described by the diffusion coefficient This parameter gives the flux of a species (quantity passing through a unit area per unit of time) per unit of concentration gradient Migration is determined by the ionic mobility This gives the average velocity per unit of electric field Water flow may result from a pressure gradient, absorption into partially dry concrete, wick action (absorption at one location with drying at another location) or electro-osmosis (the movement of water under the influence of an electric field) each of which may have its own transport coefficients (Glass and Buenfeld, 2000a) As indicated above, chloride transport is affected by the pore structure The physical effect of the pore structure is generally incorporated into the measured values of the transport coefficients (e.g the diffusion coefficient or ionic mobility) defining the various transport processes In addition, chemical interactions between ions in the pore solution and with the pore walls affect the movement of chloride ions through concrete The two main effects on chloride transport arising from interactions between ions in the pore solution, are the effective concentration of a species may be altered and, the separation of charge is constrained (the condition of electroneutrality) The effective concentration of an ion resulting from its non-ideal behaviour is termed its activity The theoretical description of the transport of a species may, to a good approximation, be expressed in terms of its concentration only when the concentration is low The condition of electroneutrality implies that, when no ions are produced or consumed, the net quantity of positive charge transported by cations must be equal to the negative charge transported by anions If this were not the case a local charge would continue to build up The transport of an excess of ions of one particular type (e.g C1-) in a given direction may therefore only occur if other ions of the same sign (e.g OH-) are transported in the opposite direction (Yu et al., 1993) Thus, when sodium chloride diffuses into concrete, chloride diffusion into the concrete might be retarded by its association with a slower charge balancing sodium ion if no significant counter diffusion of hydroxyl or sulphate ions out of the concrete occurs The two main interactions between ions and the pore walls are chloride binding and membrane effects (Glass and Buenfeld, 2000b; Zhang and Buenfeld, 2000) Chloride binding in concrete may be defined as the interaction between the porous matrix and chloride ions which results in their effective removal from the pore solution All cements bind a proportion of the chloride present This will remove chloride from the transport Reinforcement corrosion initiation in concrete follows the model of pitting corrosion It is a two-stage process in which pit nucleation is followed by pit growth (Glass et al., 2000b) The causes of pit nucleation are still subject to much debate However, it is widely recognized that pit nucleation is usually followed by repassivation If the pits are to grow, pit nucleation must be accompanied by a local fall in pH and increase in chloride content at the pit nucleation site The local fall in pH occurs as the result of the hydrolysis of dissolved iron ions An example of such a reaction is given by: Fe 2÷ + 2H20 ~ Fe(OH)2 + 2H + (9.5) The presence of an excess of chloride ions provides the charge balancing anion to stabilize the local reduction in pH Hydrochloric acid (HC1) is effectively formed (Figure 9.12) The presence of chloride ions promotes the continued dissolution of iron Salt Concrete Passive film Steel Figure 9.12 Schematic representation of the process of corrosion initiation arising from chloride contamination The effect of this mechanism is evident at voids at the steel-concrete interface It was noted that corrosion tends to initiate at the location of such defects (Figure 9.13) This has been attributed to the absence of solid phases on the steel at these locations that would otherwise release hydroxyl ions to prevent a local fall in pH thereby inhibiting corrosion initiation (Page, 1975) Titrametric methods have been developed to determine the resistance to pH reduction that gives rise to this inhibitive property of the solid phases In one method, termed differential acid neutralization analysis, the resistance to pH reduction is given by the acid added per unit of pH reduction of an aqueous suspension of ground Figure 9.13 Corrosion initiation evident from the corrosion product on the concrete surface at the location of a void at the steel-concrete interface 9/13 9/14 Reinforcement corrosion solid An example of the data obtained on an aqueous suspension of ground OPC concrete that had previously been exposed to a neutral chloride solution is given in Figure 9.14 The dissolution of the solid phases gives rise to peaks in the data at various pH values On the basis of such data, inhibitive effects have been attributed to many hydration products of cement that release hydroxyl ions into the pore solution at high pH values (Glass et al., 2000a, 2001 a) -~ [] = 100 o E -~ Soluble - ~ 90 "5 ~:~ "5 -o.~ 80 o~ 70 "= o 60 o ~ -r" o o •~- rr I r "~ ~ ~1_ R e s i s t a n c e to uction = 13 12.5 12 11.5 11 10.5 10 50 9.5 pH Figure 9.14 The resistance to a reduction in pH and soluble chloride content determined on chloride contaminated 0PC concrete (Glass et aL, 2000) Chloride ions bound in the solid phase may act to promote corrosion via the same mechanism that operates when hydroxyl ions bound in the solid phase act to inhibit corrosion (Glass et al., 2000b; Glass and Buenfeld, 1997a) The pH-dependent dissolution characteristics of chloride are included in Figure 9.14 It was noted that, for this sample, virtually all the chloride that was bound in the solid phases was released before the pH reduced to 11 As the passive film is still thermodynamically stable at this pH, it might be assumed that all the chloride released will be available to sustain pit growth Evidence of the participation of bound chloride in corrosion initiation comes from the absence of any significant correlation between chloride binding and chloride threshold level (Glass et al., 1999) However, there is substantially more bound hydroxide than bound chloride which gives the solid phases a net inhibitive effect The chloride threshold level may be defined as the quantity of chloride at the steel that is necessary to sustain local passive film breakdown, and hence initiate the corrosion process It is usually presented as a ratio of the chloride to cement content of concrete (expressed as a weight percentage) Typical values range between 0.2 per cent and 2.5 per cent by weight of cement (Glass and Buenfeld, 1997a) While the chloride content is relatively easy to determine, it should be noted that the cement content is often only estimated as laboratory verification of this is more difficult The acid neutralization capacity of a suspension of ground concrete can give an indication of the cement content or it may be directly used in an alternative representation of the chloride threshold level that has been proposed (Sergi and Glass, 2000) Low chloride threshold levels are often observed under site conditions It is believed that these are the result of the defects such as voids, millscale and rust on the steel The presence of such defects combined with moisture (the electrolyte) and oxygen, which Reinforcement corrosion raises the steel potential to positive values will limit the amount of chloride that is necessary to cause passive film breakdown In technical terms such defects are said to lower the pitting potential of steel This is the potential at which local breakdown of the passive film occurs It generally shifts to more negative values as the chloride content increases Because the condition of the steel-concrete interface is seldom characterized, the associated variations in the chloride threshold level appear to be random Thus the chloride threshold level may be regarded as representing a risk of corrosion initiation The risk of corrosion initiation occurring on UK bridges as a function of the chloride content may be estimated from the data in Figure 9.15 (Vassie, 1984) 80% 70% 60% = 50% ~ 40% ~ _~ 30% rr 20% 0 10% 0% 0.2 > "~ -0 .2 -. ~ Fe 2+ '- - o -0 .6 la -0 .8 -1 -1 .2 -1 .4 -1 .6 -1 .8 I I I I I I I I I I I I 10... 8.61E-13 m2/s I ~:~ • • :=: ~ E ,:.: ,~ 1.5 t'• O t~ '9_ !.! i-i i=i i-i • !! t- - :.:i :;: !'i ',.~:i !r! : !-' ! !'! 1. '-, ~ Measured -' - Fitted _% _- O !! 0.5 iii :" ! i:.', • -% :... spontaneous corrosion process _ _,oo [] Decrease in anodic polarization/ film breakdown LU o -2 00co / = I1) -3 00 ,~ r,/~ - [] [] [] / "~ 4o0 .-' :-~ ~/ ," • g 400 o o n ;'7 -5 00 el ~V ~-. - ' " [] -6 00 -7 00