REMEDIAL TREATMENT OF CORRODED REINFORCED CONCRETE STRUCTURES PRESENTATION REMEDIAL TREATMENT OF CORRODED REINFORCED CONCRETE STRUCTURES By: Prof. Dr Gamal Elsayed Abdelaziz Professor of Properties and Strength of Materials, Civil Engineering Department, Faculty of Engineering in Shoubra, Benha University

Background and Introduction Corrosion is the single most important problem in concrete structures The cost of corrosion (in the USA) was estimated to be approximately 400 Billion!!! It’s estimated that 25-30% of this cost is attributed to corrosion of concrete structures. It has been estimated that the annual cost of repair to UK concrete structures due to reinforcement corrosion is above £500 million. In the Middle East, 74% of reinforced concrete structures show significant corrosion damage after as little as ten to fifteen years. In Egypt, The cost of corrosion was estimated to be 4% of national income

Mechanism of Corrosion Fe  Fe ++ + 2e- (anodic reaction) (1) 4e- + O2+ 2H2O  4 (OH)- (cathodic reaction) (2) Fe ++ + 2 (OH)-  Fe (OH)2 (Ferrous hydroxide) (3) 4Fe (OH)2 + 2H2O +O2 4Fe (OH)3 (Ferric hydroxide) (4)

Causes of corrosion - Chlorides - Carbonation - Dissimilar Metals

Corrosion-induced cracking of the concrete

Deep pitting corrosion due to chloride attack

Causes of Corrosion There is still a need to understand the mechanism of corrosion due to sulphate ions with/without chloride ions. The effectiveness of alkali-aggregate reactions on the reinforcement corrosion processes is not fully understood so far. The nature of the passive film coating the reinforcing steel in concrete has not clarified yet.

Assessment of reinforcement corrosion 1. Open circuit potential (OCP) measurements 2. Surface potential (SP) measurements 3. Concrete resistivity measurement 4. Linear polarization resistance (LPR) measurement 5. Tafel extrapolation 6. Galvanostatic pulse transient method 7. Electrochemical impedance spectroscopy (EIS) 8. Harmonic analysis 9. Noise Analysis 10. Embeddable corrosion monitoring sensor 11. Visual inspection

Techniques used for monitoring corrosion of steel in concrete structures Half-cell technique Linear polarization resistance measurement Zero-meter approach Galvanostatic pulse technique

Techniques used for monitoring corrosion of steel in concrete structures Zero meter apparatus seemed to be a reliable technique for measuring the corrosion current density.

Preventive and protection measures for stopping/ delaying reinforcement corrosion Producing dense concrete through using low w/c ratio, adequate compaction, enough water curing and cement replacement materials (silica fume, fly ash, slag, metakolin, etc). Coating the reinforcement with corrosion inhibitors prior to casting. Adopting chemical corrosion inhibitors admixturs (such as calcium nitrite, calcium nitrate, sodium benzoate and organic materials). Setting-up cathodic-prevention technique.

Corrosion potential of OPC mortars made with calcium nitrite and exposed to 3% NaCl environment

Corrosion potential of OPC mortars made with calcium nitrite and exposed to 3% NaCl environment

Corrosion potential of OPC mortars made with sodium benzoate and exposed to 3% NaCl environment

Remedial Treatment of Corroded Reinforcement There are two approaches for remedying the corroded reinforced concrete structures, destructive and non-destructive. The destructive approach includes patch repair The non-destructive approach includes the following: Migrating corrosion inhibitors Cathodic protection Electro-chemical chloride removal Electro-chemical re-alkalization

Patch Repair Method Patch repair method usually can be carried out with the removal of the deteriorated zones and cleaning of the steel surface. This method is expensive and may damage the structural integrity when large areas have to be treated. Repairs are often repeated every several years, which each successive repair being increasingly greater in magnitude. The presence of high levels of chloride ions remaining in concrete will allow the corrosion process to continue. The repair material also proves to be a problem since corrosion cells are in advertently created between steel embedded in the chloride-free repair material and the steel embedded in the existing chloride contaminated concrete. This result in corrosion damage along the periphery of the patch and eventually complete failure will occur within the surrounding material and the repair itself.

Macro-cell corrosion through concrete patching

It looks of interest to adopt alternative remedial measures which would avoid the need of an extensive concrete removal or rebar cleaning, and generally to overcome the disadvantages of patch repair method

Migrating Corrosion Inhibitors Over the past decade, the concrete repair industry has developed novel techniques that are claimed to restore the protective character of cover concrete by introducing surface-corrosion inhibitors into the carbonated and / or chloride-contaminated material. Surface-applied corrosion inhibitors include calcium nitrite, sodium nitrite and sodium mono fluro phosphate (MFP). Migrating Corrosion Inhibitors are applied to the surface of corroded RC to penetrate into the concrete cover till reaching the reinforcing steel, thus modifying the electrochemical behavior of the reinforcing steel, to stop or to delay corrosion processes. Both calcium nitrite and sodium nitrite are widely used and proved to be reliable techniques for controlling the corrosion activity of reinforcement. Whilst, MFP is not commonly used in concrete repair industry and still in infant stage,

Corrosion potential and corrosion current density of reinforcement imbedded in OPC mortar treated with various concentrations of MFP, contaminated with 0 .60% NaCl

Corrosion potential of reinforcement imbedded in OPC mortar containing either SF or FA treated with various concentrations of MFP, contaminated with 0 .60% NaCl

Electrochemical Remedial Techniques There are three electro-chemical corrosion remedial treatment methods, cathodic protection, Electro-chemical realkalisation and electro- chemical chloride extraction (ECE). The main differences between the three methods related to the magnitude of the applied current density, the duration of the treatment and the used anodes and the electrolyte.

Comparison between various electro-chemical corrosion remedial treatment methods. Electrolyte type Treatment period Applied current density Method Cementitious material All service life 3 to 20 mA/m2 Cathodic protection -Calcium hydroxide -Water - Sodium borate 4 to 8 weeks 0.5 to 5 A /m2 Electro-chemical chloride extraction -Sodium carbonate - Lithium hydroxide 2 to 8 weeks 0.8 to 5 A/m2 Electro-chemical realkalisation

Cathodic Protection The idea of cathodic protection is to artificially shift the potential of a metal so that it becomes either immune or passive. Two types of cathodic protection systems are nowadays available, namely, sacrificial anode and impressed current anode. In sacrificial anode cathodic protection, a galvanic cell is set up by connecting the steel to a more reactive metal, usually zinc. The zinc then undergoes the anodic reaction and corrodes whilst the steel is rendered entirely unreactive, the iron no longer dissolves. This may also be thought of as the anodic sites on the steel being shifted to the zinc.

Cathodic Protection The idea of cathodic protection is to artificially shift the potential of a metal so that it becomes either immune or passive. Two types of cathodic protection systems are nowadays available, namely, sacrificial anode and impressed current anode. In sacrificial anode cathodic protection, a galvanic cell is set up by connecting the steel to a more reactive metal, usually zinc. The zinc then undergoes the anodic reaction and corrodes whilst the steel is rendered entirely unreactive, the iron no longer dissolves. This may also be thought of as the anodic sites on the steel being shifted to the zinc.

Cathodic Protection The idea of cathodic protection is to artificially shift the potential of a metal so that it becomes either immune or passive. Two types of cathodic protection systems are nowadays available, namely, sacrificial anode and impressed current anode. In sacrificial anode cathodic protection, a galvanic cell is set up by connecting the steel to a more reactive metal, usually zinc. The zinc then undergoes the anodic reaction and corrodes whilst the steel is rendered entirely unreactive, the iron no longer dissolves. This may also be thought of as the anodic sites on the steel being shifted to the zinc.

Cathodic Protection With impressed current cathodic protection, the steel is connected to the negative terminal of an electrical power supply forcing it to undergo a cathodic reaction. If the potential of the steel is made negative enough to make it immune, the cathodic reaction becomes one whereby water is broken down and hydrogen is liberated as follows: 2H2O +2e- → H2 + 2OH- Principle of cathodic protection of steel in concrete

Types of anodes for cathodic protection of reinforced concrete Distributed anode CP system. Slotted anode CP system. Embedded probe anode CP system. Conductive coating anode CP system.

Possible side effects of cathodic protection Reduction of bond between steel and concrete. Hydrogen evolution and embitterment of the reinforcing steel. Increase the risk of alkali-silica reaction in concrete made with reactive silica aggregate. However, these limitations have not experimentally been confirmed yet.

Electro-chemical chloride removal Electrochemical chloride extraction (ECE), desalination, is a temporary technique which has been shown to clearly remove a lot of chloride from the cover of concrete. In this method, the anode, and the cathode are connected such as CP method but the typical current density is 0.5 to 2 A/m2 of concrete surface, the duration of this treatment is 2 to 8 weeks and the electrolyte is water, sodium borate or calcium Hydroxide. Schematic arrangement of ECE

Electro-chemical chloride removal Due to the electric field, the chloride ions migrate to the anode and away from the reinforcement. When the anode put in an electrolyte, the chloride is taken away together with the electrolyte after the treatment; where a proportion of the chlorides can be completely removed from the concrete with very significant removal immediately around the steel, and a high level of repassivation of the steel is obtained. Chloride profile of untreated and treated OPC mortar with ECE

Electrochemical chloride extraction (ECE) has been confirmed to be a successful temporary remedial treatment of reinforcement corrosion. It can leads to a reasonable decrease in the chloride profile of the cover zone and a substantial reduction in the corrosion rate of reinforcement, thus transforming the state of reinforcement corrosion from active to passive. The long-term efficiency of ECE on the corrosion rate of reinforcement after halting ECE treatment is reasonably improved with increasing the amount of impressed current charges and using of water as an electrolyte. However, the source and content of chloride ions and type of cement has insignificant effect on the long-term efficiency of ECE. On the other hand, all the above-mentioned parameters had a slight effect on improving short-term efficiency of ECE, compared to their effects on the long-term efficiency of ECE

Effect of impressed current density, treatment period, electrolyte type and cement type on the corrosion rate (Icorr) of reinforcement

Electrochemical Realkalization Electro-chemical realkalisation (ECR) is used for restoring the passive film of reinforcement and increase the alkalinity of cover concrete subjected to carbonation. The operating mechanism of this technique is similar to that of electro-chemical chloride removal. The surface of the structure is usually covered with a layer of gelatinous composition containing some alkaline compounds, normally a mixture of sodium carbonate and sodium hydroxide. On the top of this layer, a conducting net of titanium mesh is placed and the net is connected to the positive terminal of an electric power source. The negative terminal of the power source is connected to the reinforcing bars. The voltage difference is adjusted so that electrolysis starts around the steel which increases the pH of the interior of the concrete, as a result of generation of OH- ions. The alkaline material from the electrolyte move into the concrete cover by capillary absorption, diffusion and electro-osmosis.

Schematic arrangement of ECR

Effect of current density on the realkalised depths Effect of ECR treatment period on the realkalised depths

Electrochemical Realkalization The rate of the electrolyte penetration into carbonated hardened cement pastes is significantly influenced by the moisture content of the pores (%RH). The drier condition allows greater migration of the electrolyte. It is not however, directly related to the increasing level of polarization during the ECR treatment. Capillary absorption is considered to be the major controlling mechanism of the ECR process at the near surface zone compared to other mechanisms such as diffusion and current-induced (electro-osmosis). Electro-osmosis in particular, seems to play no significant role on the realkalisation rate of the surface zone. Hydrolysis and electro-migration are the main mechanisms controlling the processes of ECR at the cathode zone and are dependent on the level of polarization and the period of treatment but was not significant to the internal RH of the cementitious matrix.

The use of sodium phosphate as an electrolyte resulted in a substantial increase (300%) in the realkalisation rate, measured as a penetration depth, compared to that observed when sodium carbonate was used. Lithium hydroxide also increased the realkalisation rate by about 50%. The beneficial effect of using sodium phosphate as an electrolyte has not been understood yet and the role of this substance on any possible enhancement of properties of the cementitious material should be subject to further investigation. Effect of electrolyte type on the realkalised depth measured from concrete surface

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