Chloride-Induced Macrocell Corrosion of Steel

Construction and Building Materials, Vol. 10, No. 5, pp. 329-338, 1996 Copyright 0 1996 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 095&0618/96 $15.00+0.00

095o-M18(95)ooo1~

Chloride-induced macrocell corrosion of steel in concrete--theoretical background and practical consequences

M. Raupach

Bauingenieur Sozietst Aachen, Steppenbergalle 226,52074 Aachen, Germany

Received 16 February 1995

Due to the large extent of corrosion problems in reinforced concrete structures all over the world the durability of concrete structures exposed to aggressive environments has become a problem of major importance. New results from research on chloride-induced corrosion of steel in concrete show that the corrosion mechanisms are quite complex. Normally locally separated anodically and cathodically acting steel surface areas are formed on the steel surface. As the cathodically acting steel surface areas are not visible, the corrosion mechanisms can only be investigated indirectly using new electrochemical testing methods. In this paper the theoretical background of chloride- induced macrocell corrosion of steel in concrete is explained and additionally examples for macrocells in practice are given. In particular, the formation of macrocells in the area of cracks in concrete and the problem of macrocell corrosion after local repair measures and the consequences on the design of durable new concrete structures and repair measures are discussed. Copyright Q 1996 Elsevier Science Ltd.

Keywords: chlorides; corrosion; steel

During the last few years, corrosion of the reinforcement induced by chlorides, carbonation of the concrete or low quality of the concrete cover have caused serious damage to concrete structures all over the world. Bridges and parking structures have been damaged by the use of de-icing salts so seriously that many have had to be repaired or replaced. Offshore structures like jetties, piers, dams, docks or harbour structures are also considerably attacked by chlorides from seawater especially in the tidal, splash and spray- water zones.

The mechanisms of chloride-induced corrosion of steel in concrete structures are extremely complex. Extensive research studies have been carried out during the last few years all over the world, so that today much more is known about the corrosion process and the influencing parameters of chloride-induced corrosion than a few years ago. To a large extent this new knowl- edge is based on the development of new electrochemical monitoring techniques. One of these electrochemical techniques is the measurement of electrical currents between locally separated anodes and cathodes in concrete specimens which has been further developed within a research programme at the Institute for Building Materials Research of the Technical University of Aachen, Germany. Using these so called macrocell current measurements some new results concerning the mechanisms and the influencing factors of chloride- induced corrosion of steel in concrete have been found.

The results of these and other tests show that in the case of chloride induced corrosion normally locally separated anodically and cathodically acting steel

surface areas are formed. While the anodically acting steel surface areas normally can be detected directly by the formation of rust and a certain amount of steel removal, the cathodically acting steel surface areas are not visible and can only be detected indirectly using electrochemical testing methods.

The consequences of the formation of macrocells have to be taken into account for the design of new concrete structures in aggressive environments, the diagnosis and assessment of existing concrete structures and the design of repair measures. One effect of the formation of macrocells is that the corrosion rate of steel in the area of a crack within the concrete is influenced by the quality of the concrete between the cracks. Another interesting and important consequence is that corrosion problems are to be considered near locally patch-repaired areas due to the formation of macrocells.

In the following sections the theoretical background of chloride-induced macrocell corrosion will first be discussed and then the above-mentioned examples for macrocells in practice will be explained.

Theoretical background

Generul

Following basic thermodynamics, steel exposed to natural environments has the tendency to corrode, i.e. to return to the lower level of energy given for iron oxides. However, experience shows that steel in good quality concrete does not corrode even if sufficient moisture and oxygen are available. This is due to the

329

330 Chloride-induced macrocell corrosion: M. Raupach

alkalinity of the pore solution of the concrete which exhibits pH values in the range between pH = 12.5 and 13.5. Under these conditions a microscopically thin oxide layer (called a passive film) is formed at the steel surface suppressing the iron dissolution to negligibly low values.

Normally this passivity is stable during the whole service life of a reinforced concrete structure. But there are two different mechanisms which can destroy the passive film:

carbonation of the concrete, which is the chemical reaction of CO, with the alkaline substances of the pore solution of the concrete, causing a decrease of the pH value below about 10; or exceeding a critical limit value of the chloride content in the pore water solution.

Within this paper only chloride-induced corrosion will be considered. As mentioned above, chlorides can penetrate into the concrete together with de-icing salts or seawater. In exceptional cases critical amounts of chlorides may also be present in the fresh concrete mix or penetrate into the concrete by PVC fires.

Tuutti’ has presented a principal scheme for damage caused by corrosion of steel in concrete (Figure 1) which clearly shows that the initiation period - carbonation or penetration of chlorides or both - has to be observed separately from the propagation (deterioration) period, because both processes are subject to different rate determining factors. The initiation period is not the subject of this paper (see, for example, the State of the Art Report2).

Electrochemical corrosion process

Chloride-induced corrosion of steel in concrete is an electrochemical process comparable to the operation of a battery. The poles of the battery are different surface areas of the reinforcement acting anodically or cathodically. Unlike the process in a rechargeable battery, the

Deterioration of the reinforcement

I critical deterioration

initiation period deterioration period 4 we

carbonation, corrosion of the ingress of chlorides reinforcement

Figure 1 Deterioration scheme for corrosion of steel in concrete’

corrosion process is not, however, reversible. The voltage of a corrosion cell is set up through differences in potential on the steel surface.

The differences in potential needed for corrosion of the reinforcement may be caused by overlapping local differences in the composition of the concrete, differing aeration conditions, inhomogeneities in the steel surface or uneven coating of the steel surface with corrosion products. In the case of steel in concrete, there are always differences in potential. If such a so-called corrosion cell consisting of anodically and cathodically acting steel surface areas is active, an electrical current flows between anodes and cathodes which is proportional to the rate of iron removal by galvanic action according to Faraday’s law. The basic mechanisms are described in detail by Schiep12.

The corrosion process takes place in two sub- processes, as outlined in Figure 2.

l At the anode, iron ions pass into solution, separat- ing from the electrons. They are converted into rust products in further reactions.

l At the cathode, electrons, water and oxygen are converted into hydroxyl ions. The cathodic process does not cause any deterioration of the steel, but on the contrary, it protects the steel. This effect is known as cathodic protection.

l These hydroxile ions transport the negatively charged ions in the electrolyte through the electrical field created between anode and cathode, towards the direction of the anode. Near the anode, they react with the steel ions in solution. Depending on the moisture and aeration conditions, this inter- mediate product may continue to react, producing the final corrosion products (Figure 2). The individ- ual processes are in fact much more complicated2.

Corrosion process

Cl- 02

Reacttons

Anode : Fe - Fe”+ 2e-

Cathode : $02 + Hz0 + 2e- -+ Z(OH)-

Fe(0f-Q + X ‘0, + Y.HPO - rust

Figure 2 Schematic representation of the corrosion process of steel in concrete and the electrochemical reactions

Chloride-induced macrocell corrosion: M. Raupach 331

Types of corrosion cells

Corrosion cells consisting of metallically and electrolyt- ically connected anodes and cathodes can occur as microcells leading to uniform iron removal and macrocells causing local iron removal or pitting corrosion:

l Uniform corrosion is generally caused by carbonation of the concrete over a wide area or uniformly very high chloride contents in the vicinity of the steel. This leads to the formation of so-called microcells, consisting of pairs of immediately adjacent anodes and cathodes. These are microscopic in size, so that externally they appear to produce uniform removal of the steel.

l Macrocells normally occur in the case of chloride- induced corrosion. They consist of anodically acting areas, normally where the critical chloride content has been reached, and large cathodes being next to the anodes or sometimes also quite far away from the anodes up to a distance of a few metres.

Microcells and macrocells are schematically shown in Figure 3. Normally the loss of steel in the anodically

Microcells (uniform iron removal)

Parallel anodes and cathodes preferably occur if one side of a concrete structure is exposed to chlorides and the opposite side is exposed to air (Figure 4). If the reinforcement at the side where chloride penetrates into the concrete is depassivated, it acts as anode while the opposite reinforcement can act as cathode. Such corrosion macrocells normally are generated by high driving voltages, because the potential of the anodes is very low due to the high chloride content, the depassivation of the steel and the high water content and on the other side the potential of the cathode is very positive due to the high oxygen content and the passivity of the steel surface. In these cases the driving forces are in the range of several 100 mV up to more than 0.5 V.

IA’ Ic Anodes and cathodes parallel

ACAC.

Macrocells (pitting.local iron removal)

IA >> Ic Ic >> IA

A anode t Anodic current

C cathode 1

Cathodic current

Figure 3 Schematic representation of microcells and macrocells

acting surface areas is much higher in the case of macrocell corrosion, because large cathodically acting areas support the local iron removal.

According to Raupach3 anodes and cathodes can occur in parallel or side by side. Figure 4 shows typical examples for macrocells with different geometries. As mentioned above, the distance between anodes and cathodes can be a few metres from one another. In Laase and Stiche14 a macro corrosion cell on a support- ing wall is reported in which anodes and cathodes were several metres apart.

C tubes, containers

Retaining wails. offshore structures, piers

C Bridges, parking structures

Anodes and cathodes side by side

Bridge decks and Supports. parking decks walls

Figure 4 Examples for different types of macrocells (schematic)


Especially in the case of local chloride attack, e.g. in the area of cracks or at the bottom of supports or walls (Figure 4) anodes and cathodes can also be formed side by side. In these cases the ratio between cathodically and anodically acting steel surface areas can be quite high, resulting in high corrosion rates and consequently in high losses in cross-section.

SimpliJied electrical circuit model

Based on a simplified electrical circuit model the relationship between driving voltage, the resistances of the corroding system and the electrical macrocell current which is proportional to the corrosion rate, can be calculated (Figure 5). This model consists of the driving voltage U,, the resistances of steel, anode, cathode and electrolyte (concrete) and the electrical current Z, flowing between anode and cathode. The galvanic current can be calculated using the following basic equation:

4 ‘A/ k

where:

z, =

u R,c =

U R,a =

ra =

A, = =

2 =

;1 = k 1

electrical current between anode and cathode (corrosion rate) rest potential at the cathode rest potential at the anode specific anodic polarization resistance anodically acting steel surface area specific cathodic polarization resistance cathodically acting steel surface area specific resistance of the electrolyte (concrete) cell constant geometry

The resistance of the steel related to the transport of electrons is negligibly small compared to the other active resistances. The resistances of anode, cathode and electrolyte can again be sub-divided into different partial resistances3. It can be concluded from this equation that the corrosion rate is negligibly small, if only

Figure 5 Simplified electrical circuit model for the corrosion of steel in concrete (schematic

one of the three resistances (r,, rC or pel) is infinite, for example, under the following conditions:

if the steel surface is not depassivated the anodic resistance is infinite; if nearly no oxygen is available, e.g. underwater, the cathodic resistance is infinite; and if the concrete is dried out totally, the electrolytic resistance is infinite

As is well known, for all these cases no corrosion problems are to be expected.

Besides the anodic, cathodic and electrolytic processes the geometry of the corrosion cell also influ- ences the corrosion rate directly. All influencing parameters are discussed in the following section.

Basic influencing parameters

The parameters influencing the corrosion rate of macrocells in concrete structures are schematically shown in Figure 6.

It is evident that a prediction of the macrocell current is quite difficult due to the large number of influencing factors. Additionally the relationships between the different parameters and the three resistances are not the same: water saturation of the concrete leads, e.g. to a very low electrolytic resistance by a high cathodic resistance.

To investigate which factors predominantly influence the corrosion rate a research programme has been carried out using more than 200 so-called concrete corrosion cells, i.e. concrete specimens with locally separated anodes and cathodes and constant geometry. The results of these tests have already been published and presented at various conferences3T5-8. Therefore the

macrocell current le m

----

Figure 6 Influencing factors for the corrosion rate of chloride- induced macrocells

Chloride-induced macrocell corrosion: M. Raupach

ammety /

salt -water reservoir

1 1

Figure 7 Test set up for the investigations on the corrosion mechanism in the area of cracks (schematic)

results will not be discussed within this paper in detail. It has been found that especially the chloride content, the water content and the type of cement and admix- tures influence the corrosion rate considerably.

Examples for macrocells in practice

General

Some examples of chloride-induced macrocells in concrete structures have already been shown in Figure 4. In the following section two typical macrocells are presented in more detail:

l macrocells in the area of cracks; l macrocells before and after local patch repair.

Corrosion mechanisms in the urea of cracks within the concrete

Generul. Whereas in untracked concrete a good quality cover will generally prevent depassivation of the reinforcement, prevention is practically impossible in the crack zone, since both the carbonation front and the chlorides penetrate much more rapidly into the crack zone than elsewhereg. The capillary suction capacity of the cracks plays an important role in the penetration of chlorides.

The corrosion mechanism in the crack zone is of decisive significance for the corrosion rate of the reinforcement. Generally different fundamental mechanisms for corrosion phenomena in crack zones are conceivable. Cell current and voltage measurements on cracked reinforced steel beams exposed to chloride attack have clearly demonstrated KM that large macrocells are formed under certain conditions. The reinforcement in the crack zone acts as an anode, the reinforcement between the cracks set up to a distance of several decimetres as a cathode, often leading to extremely high measured corrosion rates in the crack zone.

Cuthodic reuction of steel between trucks in concrete. As already mentioned the cathodic reaction is not

directly visible. To show the intensity of the cathodic reaction between the cracks and the influence of the distance from the cracks laboratory tests have been carried out using electrical current measurements between different locally separated steel bar@.

In order to be able to generate cracks with defined crack widths, reinforced concrete beams (70 X 15 X 10 cm3) were clamped against steel girders (see Figure 7). The potentially cathodic reinforcement between the cracks was simulated by adding three reinforcing steel sections on each side of the crack, allowing cathodic action to be determined as a function of crack distance (see Figure 8). The rod diameter was 14 mm in each case. The side faces and underside of the beams were coated with an epoxy-resin-based coating to prevent most of the effects of rapid drying of the beams via these areas; this simulates the situation in a large component more closely.

On the basis of results from studies in the splash zone of a motorway near Dusseldorf during the harsh winter of 1986/87i7, a 1% chloride solution was poured into the wetting frame above the crack once weekly for a period of 24 h. Twelve wetting periods were followed by two periods in which tapwater without added chlorides was

300kg/m3 OPC 35 F

w/c =0.60

Concrete cover * 15mm

Crack width w=Obmm

Storage: 20-C. 00% r.h.

Concrete-age: 29d

20 IO 0 I

IO 20 30

Distance from the crack (cm)

Figure 8 Anodic and cathodic currents measured at the specimen shown in Figure 7


Mass loss after 24 weeks (mg)

Figure 9 Calculated mass losses of the and w/c-ratio

introduced. The specimens were then stored without further wetting at 80% relative atmospheric humidity in a climatic chamber; after one year the cycle of wetting periods was repeated in order to simulate real conditions as closely as possible.

By measuring the electric current between the reinforcing steel electrodes (see Figure 7) it was possible to determine for each region of the reinforcing steel whether more electrons had been received than sacri- ficed (cathode) or vice versu (anode). Figure 8 shows the current balance of each of the reinforcing steel sections for the basic mix on the first day after chloride wetting, with a crack width of 0.5 mm. In order to show the effect of the distance of the reinforcement to the crack, the widths of the bars correspond to the lengths of the reinforcing steel sections in Figure 8. It is apparent that regions of the reinforcement outside the crack behave cathodically up to a crack distance of more than 20 cm. The current density of the cathodes falls with increasing distance from the crack, because electrolytic resistance rises with crack distance. The evaluation of the current

balances for the specimens aged up to four months and for the other concrete mixes and covers investigated in the tests showed that the current distribution deviates only slightly from that depicted in Figure 8.

InjIuences of’ truck width, concrete cover und concrete quality. To investigate which parameters influence the corrosion rates of the steel in the cracked concrete areas specimens with different crack widths, concrete covers and w/c-ratios have been produced and tested.

Figure 9 shows the total mass losses of steel removed during the 24-week test period, as calculated from the measured cell current curves, against the tested concrete covers, w/c-ratios and crack widths. Figure 10 shows the total mass losses after a period of two years. As expected, it is evident that mass losses increase substan- tially: whereas mass losses from specimens depassivated during the first wetting period are roughly twice as high after two years as after 24 weeks, particularly high growth rates for the mass losses are observed in specimens not depassivated until the second year. Only one

Mass loss after 2 years (mg)

c,=l5mm; w/c--O.6

c,=35mm; w/c=O.6

Crock width (mm c,=35mm; w/c -0.5

0.5

Figure 10 Calculated mass losses of the anodes in the cracked areas after 2 years depending on crack width, concrete cover and w/c-ratio


specimen with a concrete cover of 35 mm, w/c = 0.5 and a crack width of 0.1 mm suffered virtually no corrosion throughout the whole test period.

In terms of concrete cover, it may be inferred from Figures 9 and 10 that increasing concrete cover from 15 to 35 mm leads to much reduced removal rates. This relationship is already known from other research workis.

A reduction in the w/c-ratio from 0.6 to 0.5 yielded a further reduction in steel mass loss in the crack zone. This influence is especially pronounced after 24 weeks, while the influence of the w/c-ratio becomes much smaller after one year. This relationship may be explained by the fact that the period up to depassivation is prolonged by a reduction of the w/c-ratio, but that after the onset of corrosion the w/c-ratio has only a negligible influence. This result applies, however, only to the tested w/c-ratios between 0.5 and 0.6. With larger differences (e.g. between 0.4 and 0.7) a significant influence is to be anticipated even after the onset of corrosion; this factor was not investigated in the present study.

The results in Figures 9 and IO indicate that corrosion currents usually increase with growing crack width under the selected test conditions and observation periods, but it is also evident that concrete cover and composition have a much greater influence than crack width.

It is obvious that the quality of the concrete cover is the dominant influencing variable for the corrosion rate in the crack zone. This is, however, only true for crack widths up to about 0.4 mm. With larger cracks, which do not generally occur if the relevant regulations for the dimensioning of reinforced concrete components are observed, crack width is also of decisive importance, since a different corrosion mechanism is dominant for crack widths of this orderi9.

InJIuence of u crack width limit&ion by reduction of the steel diameters. Generally crack widths can be reduced by limiting the rod diameter. For the following calculations it is assumed that the number of reinforcement rods is selected to ensure that the overall steel cross-section remains constant. If it is likewise assumed that the length of depassivated steel in the crack zone

increases in direct proportion to the steel diameter, the depassivated surface of the steel rod in the crack zone will be four times the rod cross-section. Given a constant overall steel cross-section, it follows that the total depassivated area will remain the same irrespective of rod diameter.

In terms of the cathodically-acting regions of the steel surface, a reduction in rod diameter has two main effects:

l The total surface area of a single steel bar between the cracks increases in inverse proportion to the ratio of the diameteP; and

l the crack distance decreases less than proportion- ately to the ratio of the diameters, in accordance with Equation (5) in Schie131zo.

Whereas increasing the total surface area of the steel fundamentally tends to accelerate the reinforcement corrosion rate in the crack zone, reduction of the crack distance has the opposite effect. Since the steel diameter has a greater influence on the total surface of the steel than on the crack distance, however, a reduction in rod diameter will generally lead to a higher corrosion rate in the crack zone, as shown below on the basis of the comparative calculation given in Table I: a reduction in rod diameter from 20 mm to 8 mm results in an increase in annual losses in cross-section by a factor of 1.8 X

20/8 = 4.5 for the assumptions given in SchieSl and Raupachi6.

Conclusions. Cell current measurements on cracked reinforced concrete specimens showed that chloride induced steel corrosion in the crack zone involves the formation of macro-corrosion cells. The steel in the crack zone acts as the anode and the steel between the cracks, up to a distance of several decimetres from the cracks, as the cathode. The steel corrosion rate in the crack zone is therefore influenced by the conditions between the cracks.

It was also established that the steel removal rates in

the crack zone fall significantly with increasing concrete cover and are slightly lower for a concrete with a w/c- ratio of 0.5 than for one with a w/c-ratio of 0.6. The influence of crack width declined significantly with test duration, until after a period of two years no clear rela-

Table 1 Calculations on the influence of a crack width limitation by reducing the steel diameters on the theoretically expected corrosion rates according to SchieBl and Raupachr6

Parameters Units Case 1 Case 2 Case 2

J= Case 1

Reinforcement Cross-section of steel Depassivated steel surface area Total perimeter of the steel bars Distance from crack to crack Total macrocell current Mean anodic current density Mean rate of steel removal Mean annual loss in cross-section of steel bars

mm 2 0 20 12 0 8 cm2 6.28 6.03 0:96 cm2 25.1 24. I 0.96 cm 12.6 30.1 2.4 cm 16.2 9.5 0.6 pA 18 135 1.7

uA cmm2 3.11 5.60 1.8 urn a-r 36.1 65.0 1.8

% a-r 0.36 I .63 4.5


tionship between crack width and steel removal rates in the crack zone was observable.

Finally, it was shown that limiting rod diameters in order to restrict the crack widths results in higher losses in cross-section despite the reduced crack distance and crack width. Calculations indicated that the increase is in the range of about one to three times the reciprocal of the ratio between the rod diameters.

This calculation and the results of the laboratory tests clearly indicates that the problem of reinforcement corrosion in crack zones cannot be solved only by crack width limitation. Corrosion protection must be assured primarily through adequate concrete quality and cover.

Corrosion risk uj’ter locul patch repair

General. Due to inhomogeneities of the quality of the concrete cover, damage caused by corrosion of the reinforcement (e.g. cracking and spalling) normally appears in locally limited areas. If only the damaged areas are repaired by replacing the spalled concrete using repair mortar it can sometimes be observed that the concrete in the vicinity of the local repair zone is damaged by reinforcement corrosion only a few months or years after the local repair.

Corrosion mechanisms before and ufter local patch repair. The corrosion mechanisms leading to corrosion problems after local patch repair near the repaired area are schematically shown in Figure II. If the critical chloride content has been reached at the steel surface, the reinforcement starts to corrode locally while the steel surface areas adjacent to the corroding areas act cathodically. As the cathodically acting steel surface areas are protected against corrosion by the cathodic current flow, the critical corrosion initiating chloride content increases in these areas. Therefore the corroding pits tend to grow more in the direction of the inner part of the steel and not in the direction along the steel surface to the cathodically protected areas. Due to the expansion of the rust products, cracks and spalling can occur near the corroding steel surface areas.

Figure II also shows the situation after repair of the damaged surface areas by local concrete removal and refilling with mortar. If the concrete is only removed in the area where the concrete was cracked and the steel surface showed corrosion products, besides the patch often areas with high chloride contents remain. Due to the repair of the anodically acting zone the cathodic protection effect for the areas adjacent to the anodes is also removed. Therefore these areas may start to corrode, because the critical corrosion initiating chloride content is reduced by the missing cathodic protection effect. The corrosion rate can be extremely high, because the passive steel surface areas besides these critical zones often show very positive potentials resulting in potentially high corrosion rates (see Figure ZZ).

This corrosion mechanism normally cannot be prevented just by coating the steel in the repaired area

Local corrosion before repair

Rust Crack

Depth of carbonation or critical chloride content

eel

! &Concrete I ~.__~__._____________ $ C A c ’

A = anode C = cathode

Situation after patch repair

Repalr mqrtar I

High corrosion risk!

Steel

I \ I

-IConcrete I I __-.-.-_-_-.-.-__.-__ ‘C A C A C’

Influence of a steel coating

,Repair mqrtar Steel coating

High corrosion risk!

Steel

&Concrete I I

T_-~-__.-‘_‘-‘-‘-~-‘-

C A $: A C’

Figure 11 Corrosion mechanism due to macrocell action before and after patch repair (schematic)

(see Figure IZ). Using adequate coating systems, the cathodic reaction rate can be reduced significantly in the coated areas, but in most cases other passive steel surface areas remain which support the anodic action of the area next to the critical zone. As a consequence the steel coating normally will only reduce the corrosion rate slightly, but not prevent the corrosion process near the patched areas.

To prevent corrosion problems near the patched areas, all concrete has to be removed which contains a chloride content higher than the critical corrosion initiating chloride content for the situation after repair. This means that it is normally not sufficient to remove only the concrete in cracked and spalled areas and in areas where the steel surface shows rust products. However, it is also possible to ensure corrosion protection by addi- tional methods, e.g. by reduction of the water content of the concrete using adequate coating systems on the concrete surface.

As the critical chloride content causing depassivation is not a fixed value, but depends on many different factor$, different research projects have been carried out to investigate the value of the critical chloride content causing the above mentioned corrosion problems and the influencing parameters21-23.


Results from research on the durability of local repuir measures. Different laboratory tests have been carried out on reinforced concrete specimens to determine the corrosion behaviour of steel in concrete after local repair of damage caused by corrosion of the reinforcement in carbonated or chloride contaminated areas of the concrete21-23. In the following the preliminary results of the investigations are summarized:

l Where carbonated concrete was not removed completely in the vicinity of the reinforcement, macrocell corrosion occurred after repair in all specimens when water was applied. This means that local realkalization according to RILEM TC 12424 is applicable only after all carbonated concrete at the level of the reinforcement has been removed or if a sufficiently low water content of the concrete can be assured.

l In the case of chloride-induced macrocell corrosion, all areas in which the critical chloride content is exceeded must be removed, irrespective of whether damage is visible in such areas or not. Under the test conditions, a critical chloride content of 0.5-1.0% chloride was found for concrete containing ordinary Portland cement and a critical chloride content of 0.5-2% chloride for blast furnace slag cement and fly ash cement concretes depending on the concrete cover, w/c-ratio and the water content of the concrete.

l Cathodic reaction of the reinforcement within the repaired areas was reduced to negligibly low values by epoxy-coating the steel. However, under real conditions, there are usually large steel surfaces outside the repaired area of reinforced concrete structures which act additionally as cathodes, as demonstrated by macrocell current measurements.

Studies in this field have not yet been completed.

Conclusions

Chloride-induced corrosion of steel in concrete is an electrochemical process. The steel surface areas can be divided into anodically and cathodically acting areas. At the anodes steel is removed by corrosion supported by the cathodic reaction occurring in areas adjacent to the anodes or in special cases up to some metres apart. Macrocells normally occur, when the chloride content of the concrete is not constantly high, but unevenly distributed along the steel surface.

The effect of macrocell corrosion has to be taken into account for the design of durable new structures, diagnosis and assessment of existing structures and the design of repair measures. As practical examples macrocell corrosion in the area of cracks and adjacent to local patch repaired areas is presented within this paper. The main conclusions for these cases are summarized in earlier sections of this paper.

The consequences of the potential corrosion risk of

macrocell corrosion after local repair measures are already integrated in the technical recommendation on ‘Repair of concrete structures damaged by steel corrosion’ prepared by RILEM TC 124. A final draft of this recommendation has already been published24.

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Tuutti, K. Corrosion of steel in concrete. CBI forskning/research, April, 1982 Schiel31, P. Corrosion of Steel in Concrete, RILEM; Technical Committee 60-CSC, Chapman and Hall, New York, 1988 Raupach, M. Zur chloridinduzierten Makroelementkorrosion von Stahl in Beton. Dissertations. In Schrifrenreihe des Deutschen Ausschusses fur Stahlbeton Beuth, Berlin, 1992, Nr. 433 Lasse, H. and Stichel, W. Sttitzwandsanierung in Berlin: Spezielle Aspekte der Korrosion an der Riickwand. In Eautechnik 60, 1983, No. 4, pp 124129 Schiepl, P. and Raupach, M. Macrocell steel corrosion in concrete caused by chlorides. in Second CANMET/ACI Internatioanl Conference on Durability of Concrete, ed. Malhotra, V.M., Canmet, Montreal, 1991, pp 565-583 Schiel31, P. and Raupach, M. Influence on concrete composition and microclimate on the critical chloride content in concrete. In Corrosion of Reinforcement in Concrete, International Symposium, Wishaw, Warwickshire, UK, 21-24 May 1990, Elsevier, London 1990, pp 49-58 SchieBl, P. and Raupach, M. Influence on of the type of cement on the corrosion behaviour of steel in concrete. In 9th International Congress on the Chemistry of Cement, New Delhi, 23.-28 November 1992, National Council for Cement and Building Materials, New Delhi 1992, Vol. 5, pp 296233 SchieDl, P. and Raupach, M. Chloride induced corrosion of steel in concrete - investigations with a concrete corrosion cell. In The Ltfe of Structures: The Role of Physical Testing, International Seminar, 24-26 April 1989,. Brighton, UK,- Butterworths, London: 1989 Paner 21. DD 226-233 Hart], G. and Lukas, W: ‘Untersuchung zur Chlorideindringung in Beton und zum EinfluB von Rissen auf die chloridinduzierte Korrosion der Bewehrung. Betonwerk + Fertigteiltechnik 1987, 53( 7) 497-506 Okada, K. and Miyagawa, T. Chloride corrosion of reinforcing steel in cracked concrete. (AC1 SP-65). In Performance of Concrete in Marine Environment, American Concrete Institute, Detroit, Michigan, 1980, pp 237-254 Miyagawa, T. Early chloride corrosion of reinforcing steel in concrete. Dissertation, Kyoto University, Japan, 1985 Makita, M., Mori, Y. and Katawaki, K. Marine corrosion behaviour of reinforced concrete exposed at Rokyo Bay (AC1 SP-65). In Performance of Concrete in Marine Environment American Concrete Institute, Detroit, Michigan, 1980, pp 271-290 Wilkins, N. .I. M. and Stillwell, J. A. The corrosion of steel reinforcement in cracked concrete exposed to seawater immersion. In International Conference on Concrete in the Marine Environment, London, 22-24 September 1986 Grimaldi, G., Brevet, P., Pannier, G. and Raharinaivo, A. Factors influencing electrode potential of steel in concrete. &it. Corros. J. 1986, 21(l), 55-62 SchieBl, P. and Raupach, M. Untersuchungen zum Mechanismus der Bewehrungkorrosion im Bereich von Rissen. In &us- tojftechnische Einflrisse auf Konstrucktionen. Zum 60. Geburtstag von Hubert K. Hildorf Ernst & Sohn, Berlin: 1990, pp 583-599 SchieRl, P. and Raupach, M. Laboruntersuchungen und Berechnungen zum Einflul3 der RiBreite des Betons auf die chloridinduzierte Korrosion von Stahl in Beton. Bauingenieur 1994, 69, 439-445 Weber, J. Penetration of chlorides into concrete on the normal exposure conditions. In Dauerhaftigkeit nichtmetalhscher anor- gonischer Baustoffe: AbschluJkolloquium des Forschungsschwer- punktprogrammes der DFG, Karlsruhe 4-5 October 1988, Institut fur Massivbau und Bautechnologie, Karlsruhe, 1988, pp 55-60 Atimtay, E. and Ferguson, M. Early chloride corrosion of reinforced concrete - A test report. Muter. Perform. 1974, 13(12) 1882 1 SchieBl, P. Einflut3 von Rissen auf die Dauerhaftigkeit von Stahlbeton - und Spannbetonbauteilen. In: Schrtftenreihe des


Deutschen Ausschusses fGr Stahlbeton Ernst & Sohn, Berlin 1986, No. 370, pp IO-52

20 SchieBl, P. Gundlagen der Neuregelung zur Beschrankung der RiBbreite. In Heft 400 des Deutschen Ausschussesftir Stahlbeton

21 Schiegl, P., Breit, W. and Raupach, M. Durability of local repair measures on concrete structures damaged by reinforcement corrosion. In Third CANMET/ACI International Conference on Durability of Concrete, Nice, France, May 22-27, 1994

22 Raupach, M. and Breit, W. Korrosionsverhalten von Stahl in

Beton nach ortlichen Instandsetzungen. In Werkstoj’ivissens- chaften und Bausanierung, Teil 1, Expert Verlag, pp 726-744

23 Raupach, M. Untersuchungen zur Dauerhaftigkeit von Instandsetzungen bei Betonen mit hohem Chloridgehalt. In Forschung, StraJenbau und Straflenverkehrstechnik, Heft 658, 1993. Herausgegeben vom Bundesminister fur Verkehr, Abteilung StraBenbau, Bonn-Bad Godesberg

24 Draft recommendation for repair strategies for concrete structures damaged by reinforcement corrosion. RILEM TC 124 Mater. Struct. 1994, 41.5436

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Chloride-Induced Macrocell Corrosion of Steel