Seediscussions,stats,andauthorprofilesforthispublicationat:http://www.researchgate.net/publication/267636737

ReinforcementCorrosioninConcreteStructuresandServiceLifePredictions–AReview

CONFERENCEPAPER·NOVEMBER2014

READS

242

3AUTHORS,INCLUDING:

SureshBhalla

IndianInstituteofTechnologyDelhi

164PUBLICATIONS1,188CITATIONS

SEEPROFILE

Allin-textreferencesunderlinedinbluearelinkedtopublicationsonResearchGate,

lettingyouaccessandreadthemimmediately.

Availablefrom:SureshBhalla

Retrievedon:23December2015

9th International Symposium on Advanced Science and Technology in Experimental Mechanics, 1-6 November, 2014, New Delhi, India

Reinforcement Corrosion in Concrete Structures and Service Life Predictions – A Review

Sushil DHAWAN1, Suresh BHALLA2 and B. BHATTACHARJEE3

1, 2, 3 Department of Civil Engineering, Indian Institute of Technology Delhi (IITD), India, 110016

Abstract This article provides an overview of the mechanism of reinforcement corrosion, its initiation, progress and factors that expedite the process of reinforcement corrosion. Since the desirable requirement of high electric resistivity of concrete and high alkalinity of the pre solution is not achieved in practice, a monitoring system of the structures become essential to assess the damage of the structure over time due to reinforcement corrosion. Once the reinforcement corrosion is initiated, it shortens the service life of the structures by crack initiation, propagation and subsequently spalling of the cover concrete due to expansion of corrosion steel. Hence prediction of the remaining service life of the structure becomes essential in the assessment of the stability of structures. The prediction of the remaining service life of a corroded structure may be carried out with the help of various available prediction models on experimental techniques with the utilization of the data through monitoring. Key words Corrosion, concrete, service life, rebar & crack 1. Introduction Reinforced concrete is one of the most common materials used by the construction industry all over the world. The raw materials required for its construction are widely available and the built structures are in general durable. Owing to the wide variety of applications, reinforced concrete structures are subjected to a range of exposure conditions including marine, industrial, or other severe environments. Actually, high durability requirements is not always achieved in practice due to which corrosion of reinforcement in concrete is one of the main cause of deterioration in RC structures. Reinforcement corrosion has been widely reported and it is one of the main durability problems. Reinforced concrete is used for construction of transportation infrastructure such as bridges, tunnels, and harbour structures. It is also used for offshore platforms and a wide range of public and private buildings. Owing to the wide variety of applications, reinforced concrete are subjected to a range of exposure conditions, including marine, industrial or other severe environments. The functionality and reliability of infrastructure is crucial for a society and its economy to function. Concrete normally provides a high degree of protection to the reinforcing steel against corrosion, by virtue of the high alkalinity (pH 13.5) of the pore solution. Under high alkalinity steel remains passivized. In addition, well consolidated and purely cured concrete with low w/c ratio

has a low permeability, which minimizes the penetration of corrosion including agents such as chlorides, CO2, moisture etc. to steel surfaces. Further the high electrical resistivity of the concrete restricts the rate of corrosion by reducing the flow of electric current from the anodic to the cathodic sites. At the outset, it must be mentioned that usually in a properly, constructed and maintained structure, there should be little problem of corrosion during its service life. In general there are two major features which cause corrosion of reinforcement in concrete to an unacceptable degree; (i) Carbonation (ii) presence of chloride ions which may either have been present in the concrete constituents’ right from the beginning or are introduced into the concrete during the service life. When the rebar in the concrete is exposed to the chlorites, either contributed from the concrete ingredients or penetrated from surrounding chloride bearing environment, carbonation of concrete or penetration of acids into the concrete are the causes of reinforcement corrosion along with others related to the external environment such as moisture, oxygen, humidity, temperature and bacterial attack. 2. Mechanism of corrosion of Rebar in Concrete Corrosion of steel embedded in concrete is an electrochemical process. The surface of the corroding steel functions as a mixed electrode that is a composite of the anode and the cathode electrically connected through a body of steel itself, upon which coupled anodic and cathodic reactions take place.

Fig.1 Schematic representation of corrosion of reinforcement steel in concrete as an electrochemical process [1] The assessment of the causes and the extent of corrosion is carried out using various electrochemical techniques. In this paper a review on the mechanism of reinforcement corrosion techniques utilized to monitor

9th International Symposium on Advanced Science and Technology in Experimental Mechanics, 1-6 November, 2014, New Delhi, India

reinforcement corrosion and methodologies that are utilised for the production of remaining service life of the structures. Concrete is a complex material of construction that enables the high compressive strength of natural stone to be used in concrete structures. In tension, however, concrete can be no stronger than the bond between the cured cement and the surfaces of the aggregate. This is generally much lower than the compressive strength of concrete. Concrete is therefore frequently reinforced, usually with steel. When the system of steel bars or a steel mesh is incorporated in the concrete structures in such a way that the steel can support most of the tensile stresses and leave the immediately surrounding concrete comparatively free from the tensile stress, then the complex is known as Reinforced Concrete.

Fig.2 Progress of corrosion in concrete and eventual spalling [2] Anodic and cathodic reactions are broadly referred to as “Half- Cell reactions”. The anodic reaction is the oxidation process, which results in the loss of metal, while the ‘cathodic reaction’ is the reduction process which results in reduction of dissolved oxygen forming Hydroxyl Ions [3]. De-passivation around rebar would bring down the pH value and would result to corrosion in rebar. Corrosion rate is affected by the following factors (i) The pH of the electrolyte in concrete which is affected mainly by the carbonation, (ii) The availability of oxygen and capillary water and (iii) The concentration of Fe2+

in concrete near the reinforcement [4]. 3. Factors affecting Corrosion of Steel in Concrete Structures [5] • Availability of oxygen and moisture at the rebar level • Relative humidity and temperature • Carbonation and entry of acidic gaseous pollutants to

rebar level • Chloride ions reaching to the rebar level either through

the concrete ingredients or from the external environment.

• Aggregate Size and grading • Construction Practice • Cover over reinforcing steel • Cement composition • Impurities in aggregate • Impurities in mixing and curing water

• Permeability of concrete which is a function of w/c ration affects the corrosion of the rebar

The detrimental effects on the durability of RC structures are that it causes volume expansion developing tensile stress in concrete, which ultimately results in cracking and spalling of concrete. Due to loss of concrete cover there may be significant reduction in the load bearing capacity of the structure. 4. Durability of Concrete (Construction Practice)

• Aggregate washing of deleterious materials • Recommended w/c ratio, minimum cement

content, cover thickness etc • Proper mixing, placing, compaction of freshly

placed concreteProperties of fresh concrete- workability, uniformity, segregation, bleeding.

• Segregation and bleeding should be avoided • Properties of hardened concrete – Strength and

durability • Proper curing of concrete • Engineered concrete [cement, graded coarse

aggregates, graded fine aggregates, mineral admixtures , chemical admixtures (use of water reducing admixtures or super plasticisers), water]

5. Time Dependent States of Reinforcement Corrosion Corrosion process has three distinct stages: • De-passivation (process of de-passivation takes an

initiation period) • Propagation (propagation phase starts from the time of

de-passivation to the final state) • Final State

Fig.3 Stages of Rebar Corrosion

Critical tcr is the time of unacceptable corrosion damage and this critical time can be the service life. For the reinforced concrete it is assumed to equate the unacceptable corrosion damage to the onset of spalling of concrete. Corrosion-induced deterioration of reinforced concrete can be modelled in terms of these components: 1. Time for corrosion initiation (Ti) 2. Time, subsequent to corrosion initiation, for

appearance of a crack on the external concrete surface (crack propogation, Tp); and

9th International Symposium on Advanced Science and Technology in Experimental Mechanics, 1-6 November, 2014, New Delhi, India

3. Time for surface cracks to progress into further damage and develop into spalls (Td) to the point where the functional service life(Tf) is reached.

Figure 3 shown illustrates this schematically as a plot of cumulative damage versus time. 6. Reasons of Corrosion The two most common causes of reinforcement corrosion are: 1. Localised breakdown of the passive film on steel by

chloride ions and 2. General breakdown of passivity by neutralization of

the concrete, predominantly by reaction with atmospheric CO2.

Durable concrete (alkaline with pH~13.5) is an ideal environment for steel but the increased use of salts and the increase concentration of Carbon dioxide in modern environment due to industrial pollution has resulted in corrosion of the rebar becoming the primary cause of failure of this material. Loss of alkalinity due to carbonation – due to depassivation alkalinity is lost as a result of 1. Reaction with acidic gases (such as CO2) in the

atmosphere 2. Leaching by water from the surface

Ca(OH)2 + CO2 --------à CaCO3 + H2O

It consumes alkalinity and reduces pore water pH to 8.9 range, where steel is no longer passive. 6.1 Loss of alkalinity due to chlorides [6] The chloride ion locally de-passivates the metal and promotes active metal dissolution. Chloride reacts with the calcium aluminate and calcium alumino ferrate in the concrete to form insoluble calcium chloro aluminates and calcium chloro ferrates in which the chlorite is bound in non active form; it is the chlorite in solution the is free to promote corrosion of steel. 6.2 Cracks due to mechanical loading If the cracks penetrate to the steel, protection can be lost. This is especially under tensile loading, for de-bonding of steel and concrete occurs, thus removing the alkaline environment and so destroying the protection in the vicinity of the de-bonding. Corrosion of steel reinforcement also can be due to atmospheric pollution. 7. Prevention [7] 1. Keep concrete always dry, so that there is no moisture

to form rust. If concrete is always wet, then there is no oxygen to form rust.

2. Epoxy coating to rebar to protect them from moisture and aggressive agents. The embedded epoxy coating on steel bars provide a certain degree of protection to the steel bars and thereby, delay the initiation of corrosion. These coatings prevent movement of

moisture to the steel surface but restrict oxygen penetration.

3. Stainless steel can be used in lieu of conventional reinforcements.

4. Use of fly ash concrete with low permeability which would delay the arrival of carbonation and chlorites at the level of the rebar. They form a calcium silica hydrate (CSH) compound that over time effectively reduces concrete diffusivity to oxygen, carbon dioxide, water and chloride ions.

5. Electro chemical injection of organic base corrosion inhibitors into carbonated concrete.

6. Physical properties of durable concrete would improve the extent of carbonation decline.

7. Installation of physical barrier system such as coatings, sealers, membrane.

8. The zinc surface layer applied either hot dipped or electrode deposition would result on a low corrosion rate for zinc, thereby providing galvanic cathodic protection.

9. Concrete mix design modification involves such factors as reduced water cement ratio including use of water reducing admixtures or super plasticisers, type of cement, permeability reducing admixtures such as fly ash, silica fumes, blast furnace slag, and corrosion inhibiting admixtures.

10. Remedies for corrosion-damaged concrete include removal of all delaminated concrete, cleaning of the reinforcement by abrasive blast cleaning.

8. Strategies for Investigation of a Corroded RC Structure A visual survey, whether the corrosion of rebar is really a cause of distress. The survey consists of a careful investigation of the structure for any sign of distress such as cracking, spalling and rust staining. 8.1 Reinforcement Corrosion Monitoring Techniques • Half – Cell potential, E corrosion • Concrete resistivity (P) • Corrosion current density ( I corrosion)

9. Service Life Prediction Models [8] 9.1Bazant’s Model This model predicts the corrosion damage based on volume expansion due to the formation of hydrated red rust over the residual rebar core. This rust is expansive in nature and occupies four times the volume of parent steel that creates a pressure on the concrete to produce cracks on its surface. 9.2Morinaga’s Model An empirical model to predict the service life of corroded rebars based on the instance of concrete cover crack by means of rust formation on rebar surface.

9th International Symposium on Advanced Science and Technology in Experimental Mechanics, 1-6 November, 2014, New Delhi, India

9.3Wang and Zhao’s Model The analysis of a large number of rebar corrosion data collected from laboratory and their comparision with finite element analysis, the authors (Wang and Zhao) have established an empirical expression to determine the ratio of thickness of corrosion product,∆ , to the depth of rebar penetration, H, corresponding to the cracks in cover concrete. 9.4 Dagher and Kulendran’s Model In the context of service life prediction of RC structures subjected to rebar corrosion, this model can be used more reliably to determine the radial bar expansion at which cracks in cover concrete occurs. 10. Experimental Method of Service Life Prediction In this method, Ahmad et al. predicted service life based on the cumulative damage theory. The fraction of damage due to the externally induced current is subtracted from 1 to obtain the fraction of damage due to corrosion. It is assumed that there are no alternative sources of corrosion in the rebar. The methodology is based on the cumulative damage theory.The final failure is assumed to be the effects of two mode failure.The first mode considered is the natural corrosion of rebar from the time of depassivation followed by the second mode,which is the accelerated corrosion of rebar by anodic electrolysis under the impressed anodic current for a short period.The actual cracking of the specimen after applying an optimal anodic current for a given period has been carried out by splitting the specimens under physical load.The reduction in failure tensile stress of concrete is co-related with intensity and duration of the impressed current.From this relationship,the time required for cracking under impressed current is determined corresponding to the zero residual tensile stress at failure of concrete.To demonstrate the utility of the suggested experimental methodology,the service life of a number of R.C. core specimens having different corrosion rates and cover thicknesses were determined and compared with the results obtained.

11. CONCLUSIONS The most common causes of reinforcement corrosion are chlorite ions and carbonation by atmospheric carbon dioxide. In wet and cold climates, reinforced concrete for roads, bridges, parking structures and other structures that may be exposed to deicing salt may benefit from the use of epoxy coating, hot dip galvanized or stainless steel rebar. A good structural design, detailing and a well-chosen cement mix that makes durable concrete may provide sufficient protection for many applications. Use of fly ash also delays the effect of chlorite and carbon dioxide. References [1] Shamsad Ahmad: Reinforcement corrosion in concrete

structures, its monitoring and service life prediction––a Review, J. Cement & Concrete Composites, 25 (2003), 459–471.

[2] Hansson C. M.: Comments on electrochemical measurements of the rate of corrosion of steel in concrete, J. Civil Engg. Materials, ASCE 14 (1984), 574–584.

[3] Berkely K. G. C., Pathmanaban S.: Cathodic protection of reinforcement steel in concrete, Butterworths & Co. Ltd., London, (1990).

[4] Mozer J.D.: Corrosion of reinforcing bars in concrete, J. American Concrete Institute, (1965), 909–931.

[5] Cahyadi J.H., Uomoto T.: Influence of environmental relative humidity on carbonation of concretre (mathematical modeling), Durability of building materials and components (1993), 1142–1151.

[6] ASTM C: 1152 - Standard test method for acid-soluble chloride in mortar and concrete (1990), 609–610.

[7] Uhlig H. H.: Corrosion and corrosion control, John Wiley and Sons, (1983).

[8] Bazant Z. P.: Physical model for steel corrosion in concrete sea structures-theory, J. Struct Engg., ASCE 15 (1979); 1137–1153.