Corrosion of Steel Reinforcement in Concrete

Overview Introduction Mechanisms of Steel Corrosion Control of Corrosion

Introduction One of the principal causes of concrete deterioration in KSA. The damage is especially large in the structures exposed to marine environment , contaminated ground water, or deicing chemicals. 1991 report FHWA in U. S. reported that 134,00 (23% of the total) bridges required immediate repair and 226,000 (39% of the total) were also deficient. The total repair cost was estimated at $ 90 billion dollars.

CRACKING OF CONCRETE Heat of hydration Alkali-aggregate reactivity Carbonation Sulfate attack Acid and chemicals Reinforcement corrosion

REINFORCEMENT CORROSION Passivity High pH leading to formation of passive layer Chemical binding of chlorides Dense and impermeable structure of concrete Depassivation Chloride ingress Carbonation

MECHANISMS OF REINFORCEMENT CORROSION

FACTORS AFFECTING REINFORCEMENT CORROSION Depassivation of steel Potential variation Availability of the reaction products, namely oxygen and moisture Electrical resistivity of concrete Moisture Chloride and sulfate contamination

FACTORS INFLUENCING REINFORCEMENT CORROSION Carbonation Chlorides Moisture Oxygen diffusion Concrete mix variables Construction variables Temperature Humidity

Chloride-induced Reinforcement Corrosion Due to the external chlorides in substructures Due to chloride contamination from the mix constituents in the superstructures

Chloride Limits ACI 318 (0.1 – 0.15%; water soluble) ACI 224 (0.2%; acid soluble) BS 8110 (0.4%; total)

Damage to Concrete

Mechanisms of Steel Corrosion Corrosion of steel in concrete is an electrochemical process. The electrochemical potentials to form the corrosion cells may be generated in two ways: Two dissimilar metals are embedded in concrete, such as steel rebars and aluminum conduit pipes, or when significant variations exist in surface characteristics of the steel. In the vicinity of reinforcing steel concentration cells may be formed due to differences in the concentration of dissolved ions, such as alkalies and chlorides.

Mechanisms of Steel Corrosion As a result, one of the two metals (or some parts of the metal when only one type of metal is present) becomes anodic and the other cathodic. The fundamental chemical changes occurring at the anodic and cathodic areas are as follows:

Electrochemical Process of Steel Corrosion

Anodic and Cathodic Reactions Anode: Fe 2e- + Fe2+ (metallic iron) FeO (H2O)x rust Cathode: (½) O2 + H2O + 2e- 2(OH)- air water

Oxidation State vs. Increase of Volume

Corrosion Process

Corrosion Cells Anodic reaction (involving ionization of metallic iron) will not progress far unless the electron flow to the cathode is maintained by the consumption of electrons. For the cathode process, therefore the presence of both air and water at the surface of the cathode is absolutely necessary.

Steel Passivity Ordinary iron and steel products are normally covered by a thin iron oxide film that becomes impermeable and strongly adherent to the steel surface in an alkaline environment, thus making the steel passive to corrosion. This means that metallic iron is not available for the anodic reaction until the passivity of steel has been destroyed.

Destroying Passive Layer In absence of chloride ions in the solution Protective film on steel is stable as long as the pH of the solution stays above 11.5. When concrete has high permeability and when alkalies and most of the calcium hydroxide have either been carbonated or leached away), the pH of concrete in the vicinity of steel may have been reduced to less than 11.5. This would destroy the passivity of steel.

Destroying Passive Layer In presence of chloride ions Depending on the Cl- /OH- ratio, the protective film is destroyed even at pH values considerably above 11.5. When Cl- /OH- molar ratio is higher than 0.6, steel is no longer protected, probably because the iron-oxide film becomes either permeable or unstable under these conditions.

Destroying Passive Layer In presence of chloride ions The threshold chloride content to initiate corrosion is reported to be in the range 0.6 to 0.9 kg Cl- per cubic meter of concrete. When large amounts of chloride are present, concrete tends to hold more moisture, which also increases the risk of steel corrosion by lowering the electrical resistivity of concrete.

After the Destroy of Passivity Rate of corrosion will be controlled by: The electrical resistivity. [significant corrosion is not observed as long as the electrical resistivity of concrete is above 50 to 3 70 10 Ω.cm]. The availability of oxygen.

Sources of Chloride in Concrete admixtures, salt-contaminated aggregate, Penetration of seawater, groundwater, or deicing salt solutions.

Corrosion of the Steel Reinforced Concrete Structures MARINE STRUCTURES BURIED UTILITIES FOUNDATIONS BRIDGES & CULVERTS

Corrosion of the Reinforcing Steel in a Spandrel Beams (17 years of service)

CARBONATION Ca(OH)2 + CO2  CaCO3 + H2O Reduction in pH (up to 8.5)

Carbonation in uncontaminated cement mortar

Carbonation in OPC mortar specimens contaminated with chloride plus sulfate

Carbonation in fly ash cement mortar contaminated with chloride plus sulfate

Control of Corrosion Permeability of concrete is the key to control the various processes involved in the phenomena. Concrete mixture parameters to ensure low permeability, e.g., low water-cement ratio, adequate cement content, control of aggregate size and grading, and use of mineral admixtures.

Control of Corrosion Maximum permissible chloride content of concrete mixtures is also specified by ACI Building Code 318. Maximum water-soluble Cl- ion concentration in hardened concrete, at an age of 28 days, from all ingredients (including aggregates, cementitious materials, and admixtures) should not exceed 0.06 % by weight of cement for prestressed concrete, 0.15 % by weight of cement for reinforced concrete exposed to chloride in service,, and 0.30 % by mass of cement for other reinforced concretes, respectively.

Control of Corrosion ACI Building Code 318 specifies minimum concrete cover of 50 mm for walls and slabs, and 63 mm for other members is recommended. Current practice for coastal structures in the North Sea requires a minimum 50 mm of cover on conventional reinforcement, and 70 mm on prestressing steel. RCJY and other agencies requires 75 mm minimum concrete cover.

Control of Corrosion ACI 224R specifies 0.15 mm as the maximum permissible crack width at the tensile face of reinforced concrete structures subject to wetting-drying or seawater spray. The CEB Model Code recommends limiting the crack widths to 0.1mm at the steel surface for concrete members exposed to frequent flexural loads, and 0.2 mm to others. By increasing the permeability of concrete and exposing it to numerous physical-chemical processes of deterioration, the presence of a network of interconnected cracks and microcracks would have a deleterious effect.

Control of Corrosion Waterproof membranes: are used when they are protected from physical damage by asphaltic concrete wearing surfaces; therefore, their surface life is limited to the life of the asphaltic concrete, which is about 15 years. Overlay of watertight concrete: 37.5 to 63 mm thick, provides a more durable protection to the penetration of aggressive fluids into reinforced or prestressed concrete members.

Control of Corrosion Protective coatings for reinforcing steel are of two types: anodic coatings (e.g., zinc-coated steel) very limited use due to concern regarding the long-term durability. and barrier coatings (e.g., epoxy-coated steel), long-time performance of epoxy-coated rebars is still under investigation in many countries.

Epoxy-coated Steel

Control of Corrosion Cathodic protection techniques involve suppression of current flow in the corrosion cell, either by: Supplying externally a current flow in the opposite direction or by using sacrificial anodes. Due to its complex and high cost the system is finding limited applications.