CONCRETE MAY 2007 13

This is the first in a series of RepairGuidance Notes to BS EN 1504, Productsand systems for the protection and repairof concrete structures – Definitions,requirements, quality control andevaluation of conformity. It is plannedthat these articles will be published inCONCRETE on a regular basis and will, indue course, be compiled into a ConcreteSociety Technical Report.

The aim of these Notes is to guide consultants and con-tractors through the application of BS EN 1504(1) and

other related concrete repair Standards for the evaluation,design, specification and concrete repair process. The Noteswill help ensure that appropriate materials are correctlyspecified and applied. The development of the guidancematerial has been undertaken by members of the JointLiaison Committee of The Concrete Society, Institute ofCorrosion and Corrosion Prevention Association.

BackgroundThere are a number of causes of deterioration in concretebuildings and structures. Even when they are adequatelybuilt, properly used and well-maintained, the environmentwill affect a structure and components will degrade or wearout. The largest single cause of deterioration in reinforced-concrete structures is corrosion of the reinforcing steel.Consequently, corrosion is the subject of this first RepairGuidance Note. In addition, there are a number of deteriora-tion processes that attack the concrete directly, some fromprocesses within, such as alkali-silica reaction, and somefrom external sources, such as freeze/thaw damage. Someare related to initial construction problems while others aredue to subsequent use or lack of maintenance of the struc-ture. These will be discussed in Repair Guidance Note 2.

This Note summarises the major causes of corrosion-related defects, damage and decay in concrete buildings andstructures. Any attempt to remedy problems must start witha thorough understanding of the cause and extent of thedeterioration. It is essential that a detailed investigation iscarried out as part of the appraisal process, the results are

interpreted, and the repair options fully evaluated to ensurethat the right repair option is selected for the building and itsowner. This is discussed in Section 5 of BS EN 1504 Part 9(1).

Corrosion of steel in concrete by carbonation and chloridesThere are two major mechanisms for the acceleration ofcorrosion of steel in concrete that do not require the degra-dation of the concrete before the steel is attacked, althoughany degradation such as cracking is likely to further exacer-bate the problem. The first of these is carbonation and thesecond is chloride contamination.

CarbonationReinforcement corrosion is prevented by the alkali contentof the concrete. During the hydration process calcium,sodium and potassium hydroxides are formed, which dis-solve in the pore water of the concrete to form a very alka-line solution of approximate pH 12–13.5. At this level, steelforms a very thin, protective oxide known as a passive layer.This is a durable film that is far better than synthetic ormetallic coatings that deteriorate or are consumed. The pas-sive layer will also sustain and maintain itself indefinitely aslong as the concrete stays highly alkaline and remains freefrom contamination.

Carbonation, however, is inevitable and is caused by theingress of atmospheric carbon dioxide reacting with the porewater in the concrete cover to form carbonic acid, whichneutralises the alkalinity in the concrete. This occurs pro-gressively, and a carbonation front moves through the con-crete until it reaches the steel. The passive layer then breaksdown as the pH falls from over 12 to approximately 8.Corrosion can start in the presence of oxygen and water asthe pH falls below 11. The carbonation front moves approxi-mately according to the following parabolic relationship:

Carbonation depth = Constant × Square root of time

A typical Portland cement concrete may show a carbon-ation depth of 5–8mm after ten years, rising to 10–15mmafter 50 years(2). Therefore, structures with low concretecover over the reinforcing steel will show carbonation-induced corrosion more quickly than those with good cover.A method for determination of carbonation depth is givenin BS EN 14630(3).

The rate of decay of reinforcing steel in concrete is alsoaffected by the concrete quality. Concretes made with ahigh water:cement ratio and with a low cementitious con-tent will carbonate more quickly than well-specified con-cretes because they are more porous and have lowerreserves of alkali to resist the neutralisation process. Forconcretes with pulverised-fuel ash, ground-granulatedblast-furnace slag or other cement replacement materials,the drop of alkaline reserves is usually balanced by theincrease in concrete quality for an equivalent Portlandcement, except at high replacement levels in dry condi-tions(2). The rate of carbonation is also affected by environ-mental conditions. Carbonation is more rapid in fairly dryand wet/dry cycling environments. It may therefore occurmore rapidly in bathrooms and kitchens in blocks of flats,and in multi-storey car parks where the carbon dioxide lev-els are high due to exhaust fumes.

REPAIR GUIDANCE NOTE

This is one of aseries of notes

prepared by theJoint Liaison

Committee ofThe Concrete

Society, Instituteof Corrosion and

CorrosionPrevention

Association.

Repair Guidance Note 1:

Corrosion of steel in concrete

JOHN P BROOMFIELD, BROOMFIELD CONSULTANTS AND CHRIS

ATKINS, MOTT MACDONALD

Figure 1: The corrosionmechanism for steel in

concrete.

CONCRETE 1 16 MAY 07 19/4/07 5:49 pm Page 13

Repairs are formulated according to approaches basedon BS EN 1504 Part 9 Principle 7, Preserving or restoringpassivity, which includes electrochemical realkalisation BSEN/TS 14038-1(4). Additional approaches might bePrinciple 8, Increasing resistivity or Principle 10, Cathodicprotection (covered in BS EN 12696(5)). There will also be arequirement for Principle 3 on concrete restoration.

Chloride attackThe second major cause of reinforcement corrosion is chlo-ride contamination. This is usually due to one of the follow-ing causes:

• de-icing salt ingress from roads and vehicles• sea-salt ingress in marine environments• cast-in salt from contaminated mix components• cast-in calcium chloride as a set accelerator.

A threshold level of chloride is needed for corrosion tooccur. This can range from approximately 0.1–1.0% chlo-ride by mass of cement, but the most commonly usedthresholds are 0.3% (used by the Highways Agency) or0.4% found in much of the European literature. Once thechloride level at the reinforcement exceeds 0.4% by weightof cement (approximately 0.06% or 600ppm by weight ofsample assuming 15% cement content), there is a signifi-cant risk of corrosion, especially in the presence of mois-ture. Obviously, if cast-in chlorides exceed 0.4%, then thecorrosion risk rises(2). There are also issues of the degree ofbinding chemical of the chloride with some literature say-ing it should be taken into account and others saying it isultimately irrelevant(6). A test method for determination ofthe chloride content of hardened concrete is given in thedraft EN 14629(7).

As with carbonation, chloride-ingress rates are a func-tion of concrete quality and environment. For concrete withlow reinforcement cover, and particularly for poor-qualityconcrete, chlorides can be transported rapidly by wettingand drying absorption and by capillary action on the chlo-ride-laden water into the concrete. The water then evapo-rates and leaves the salt behind. In good-quality concretewith good cover to the reinforcement and little cracking,diffusion processes predominate.

Repairs are formulated according to approaches basedon ENV 1504 Part 9, Principle 7, Preserving or restoringpassivity, which includes electrochemical chloride extrac-tion. Additional approaches might be Principle 8,Increasing resistivity and Principle 10, Cathodic protection(covered in BS EN 12696(4)). There will also be a require-ment for Principle 3 on concrete restoration.

The corrosion processRegardless of the cause of corrosion, once the steel’s pas-sive layer protection is lost, corrosion proceeds by themechanism illustrated in Figure 1 (page 13).

Oxidation (corrosion) of steel is a natural process thatrequires the reaction at the anode and the reaction at thecathode occurring in balance. Corrosion of steel in concreteis an electrochemical reaction in which the major con-stituent of steel (iron) goes into solution as positivelycharged iron ions, releasing electrons (electrical flow). Thissite is called the anode and hosts the oxidation process.

The electrons flow through the reinforcement towardssites on the steel surface where they react with oxygen andwater from outside to produce additional hydroxyl ions.These sites are called cathodes and host the reductionprocess.

In a passive (high alkaline) environment this reduction-oxidation (redox) reaction is desired in order to sustain andmaintain the passive layer. However, if depassivationoccurs rapid corrosion can begin, accelerated by the pres-

ence of carbon dioxide or chloride ions.As can be seen in the diagram, the cathodic reaction

requires water and oxygen. The initial anodic reaction doesnot require any reactants until the iron has become solubleferrous ions that can then react with hydroxyl ions (the alka-linity in the concrete), and then with oxygen and water tocreate the solid rust whose volume increase will crack andspall the concrete.

The fact that oxygen is not required at the anode isimportant because the exclusion of oxygen from anodicareas without stifling the cathodic reaction will lead to dis-solution of the reinforcement rather than cracking andspalling of the concrete, i.e. the structure is weakened with-out there being evidence of deterioration. This can happenin local saturation conditions where the concrete is very wetand therefore conductive enough to allow good separationbetween anodes and cathodes. This process is known as dif-ferential aeration where the electrons are used to form H+

ions at the cathode that are free to react with chloride toform hydrochloric acid within, for example pits andcrevices on the steel surface. Further oxygen starvationwithin the pit or crevice leads to enhanced degradation andrapid failure.

The ingredients for corrosion are therefore:• carbonation or sufficient chloride at reinforcement

depth to depassivate the steel• oxygen to fuel the cathodic reaction and to create the

expansive oxide• water to fuel the cathodic reaction and to create the

expansive oxide • concrete of low enough resistivity to allow the electro-

chemical anode and cathode reactions to proceed.

These ingredients, along with the electrical/electro-chemical nature of the reactions, can therefore be used toassess the corrosion condition. Methods for corrosionassessment of reinforced concrete are given in ConcreteSociety Technical Report 60(8). �

MAY 2007 CONCRETE

REPAIR GUIDANCE NOTE

14

References:

1. BRITISH STANDARDS INSTITUTION, BS EN 1504. Productsand systems for the repair and protection of concretestructures – Definitions, requirements, quality control andevaluation of conformity, Part 1: Definitions, Part 2: Surfaceprotection systems for concrete, Part 3: Structural and non-structural repair, Part 4: Structural bonding, Part 5: Concreteinjection, Part 6: Grouting to anchor reinforcement or to fillexternal voids, Part 7: Reinforcement corrosion protection,Part 8: Quality control and evaluation of conformity, Part 9:General principles for the use of products and systems. Draft.

2. BUILDING RESEARCH ESTABLISHMENT. Digest 444.Corrosion of steel in concrete, Part 1: Durability of reinforcedconcrete structures, Part 2: Investigation and assessment,Part 3: Protection and remediation, 2000.

3. BRITISH STANDARDS INSTITUTION. BS EN 14630. Productsand systems for the protection and repair of concretestructures. Test methods. Determination of carbonation depthin hardened concrete by the phenolphthalein method, 2006

4. BRITISH STANDARDS INSTITUTION. BS EN TS 14038.Electrochemical realkalization and chloride extractiontreatments for reinforced concrete, Part 1: Realkalization.Draft.

5. BRITISH STANDARDS INSTITUTION. BS EN 12696. Cathodicprotection of steel in concrete. 2000.

6. BROOMFIELD, J.P. Corrosion of steel in concrete:understanding, investigation and repair. 2nd Edition, Taylorand Francis, London, 2007.

7. BRITISH STANDARDS INSTITUTION. BS EN 14629. Productsand systems for the protection and repair of concretestructures – Test Methods – Determination of chloride contentin hardened concrete. Draft.

8. THE CONCRETE SOCIETY. Electrochemical tests forreinforcement corrosion, Technical Report 60, Camberley,2004.

“The aim of theseNotes is to guideconsultants and

contractorsthrough the

application of BSEN 1504 andother related

concrete repairStandards for the

evaluation,design,

specification andconcrete repair

process. The Noteswill help ensurethat appropriate

materials arecorrectly specified

and applied.”

CONCRETE 1 16 MAY 07 19/4/07 5:49 pm Page 14