Assessment of Reinforcement Corrosion

8/12/2019 Assessment of Reinforcement Corrosion

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50thASEP Anniversary International Convention & Exposition, Safer, Smarter & GreenerTechnical Proceedings ISSN 1656-7757, September 28-30, 2011 Makati City, Philippines

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ASSESSMENT OF REINFORCEMENT CORROSION IN FULLY

CARBONATED CONCRETE THROUGH HALF-CELL POTENTIAL

TECHNIQUE: INVESTIGATION OF THE INFLUENCE OF

CONCRETE SURFACE PRE-WETTING

Ronaldo S. Gallardo and Perlie F. Bohol

ABSTRACT: The corrosion of reinforcement is one of the major causes of pre-mature

deterioration of reinforced concrete structures. In a tropical country like the Philippines, the

cyclic wetting and drying climatic condition favors corrosion induced by carbonation. The half-

cell potential (HCP) technique is a simple, cost-effective and rapid tool for locating corroding

rebars and assessing the corrosion risk of concrete structures. This study reports the results of

half-cell potential measurements obtained on the service life of reinforced concrete columns,

which are fully carbonated and contaminated with chloride ions. Corrosion predictions from

HCP data obtained at four different levels of wetness of concrete surface: (1) dry; (2) minimally

wet; (3) thoroughly wet; and (4) after 2 hours drying of thoroughly wet concrete, werecompared with the actual corrosion revealed from the direct visual inspection of reinforcements.

Results showed that HCP measurement is most effective when performed on minimally wetted

concrete surface. At minimally wetted concrete surface condition, 7 out of 9 severely corroded

rebars were detected and two of the most corroded columns have been identified through the

potential difference technique and statistical evaluation of HCP data. The HCP technique is

fairly reliable in assessing the corrosion of reinforcements in fully carbonated concrete. But a

more reliable corrosion prediction can be attained if HCP data is complemented with

carbonation depth measurements.

KEYWORDS: corrosion; carbonation, half-cell potential

1. INTRODUCTION

1.1 Background of the Study

Steel reinforced concrete is a durable and widely used material in civil engineering structures.

Although known to be durable, when exposed in aggressive environment will undergo prematuredeterioration, which eventually results to shortened service life. One of the major causes of

deterioration in concrete structures is the corrosion of steel reinforcement. This problem is

recognized worldwide and has been a topic of major importance since the past decades. The

problem with corrosion of steel in concrete structures is the damage cannot be recognized untilvisible signs of cracking, spalling or rust staining are evident. By the time these corrosion signs

manifested on the concrete surface, the damage is already significant and it may be too late toprevent the damage growth. The dissolution of steel bars poses serious hazard to public safety. In

addition, the repair or rehabilitation seems to be uneconomical as it exceeds the originalconstruction cost. And not to forget the traffic and work disruptions the repair works could

cause. This suggests that corrosion in reinforced concrete (RC) structures should be detected and

monitored before any corrosion signs become apparent to be able to carry out applicable repair

strategies, cut high repair costs, and therefore extend its service life. It is then highly desirable tocarry out corrosion monitoring procedures, especially in environments of high aggressivity like


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in the Philippines, where the wet and dry cycles seems to favor the corrosion process. One of the

non-destructive techniques in monitoring the corrosion in RCS is the half-cell potential or opencircuit potential method. The half-cell potential (HCP) test is universally adopted in the condition

surveys of RC structures due to its simplicity, cost effectiveness and quick diagnosis of

reinforcement corrosion. In this method, the measurement of the potential difference between a

reference electrode and the reinforcing bar allows the estimation of the probability ofreinforcement corrosion. HCP method is carried out to locate corroding zones where further

destructive tests will be performed. However, the method cannot provide the degree or the

quantitative rate of corrosion. Half-cell potential technique has been widely used and a large

number of field experiences have proven its reliability, specifically in concrete structuressubjected to chloride attack. This has led to its standardization as ASTM C876: Standard Test

Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete. However, the

reliability of the half-cell potential technique has been questioned in cases of corrosion induced

by carbonation. According to The International Union of Laboratories and Experts inConstruction Materials, Systems and Structures (RILEM, from the name in French), detection of

corroding zones in carbonated concrete is difficult because of its high resistivity. The high

resistivity of carbonated concrete is masking the corroding areas with less negative potentialvalues, and in some cases, positive values resulting to the same or close potential readingsbetween the corroding and non-corroding zones which eventually lead to incorrect corrosion

prediction and to inadequate conclusions. Practical applications of HCP on carbonated concrete

as reported in the literature have shown that adopting the ASTM C876 fixed potential criteria can

lead to incorrect corrosion prediction. The ASTM C876-91 itself restricts the use of its fixedpotential criteria in evaluating the corrosion condition of reinforcing steel in concrete where

concrete cover is fully carbonated, unless either experience or destructive examination of some

areas, or both, imply its applicability. Some studies have shown that wetting of concrete surfacecan reduce the resistivity of concrete and may help reveal the corroding areas, but this has been

studied in chloride-contaminated concrete. ASTM, RILEM and other authors do recommend the

pre-wetting of concrete surface but do not explain how it will influence the HCP measurementsand how it will help achieve reliable corrosion predictions. To the knowledge of the author, thereis no study conducted in the past that investigated if pre-wetting of concrete surface can help

unmask the corroding areas in fully carbonated concrete. This study reports a practical

application of half-cell potential technique on existing RC columns of a fence located in Quezon

City. This study presents the experience with half-cell potential technique in identifying the mostcorroded column member and in locating the corroding areas on RC columns, which are fully

carbonated and contaminated with chloride ions. Concurrently, an investigation of the influence

of concrete surface pre-wetting on the fluctuation of half-cell potentials as well as on the

alteration of HCP result interpretation is presented. Moreover, the reliability of HCP techniquewas verified and its limitations were discussed.

1.2Significance of the Study

Resistivity is sensitive to the moisture content of concrete. Some studies have shown that wetting

of concrete surface can reduce the resistivity of thick concrete cover and can help obtain

effective HCP readings on concrete contaminated with chloride ions. Wetting of concrete surface

can also reduce the high resistivity of carbonated concrete. Better understanding of the effect ofconcrete surface pre-wetting is essential in identifying the ideal concrete surface condition for


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effective HCP measurements. Effective HCP measurements would lead to corrosion prediction

that is close to the actual corrosion of the structure being investigated, hence, a correct andaccurate corrosion prediction. The additional experience and evidence of this study will provide

baseline information to engineers towards improved application of half-cell potential technique

and more reliable evaluation of reinforcement corrosion on concrete structure, which are fully

carbonated and contaminated with chloride ions. Verifying the applicability and reliability ofhalf-cell potential technique in fully carbonated concrete structures will pave the way to its

extensive application in the country and will stimulate further studies for the development of

HCP evaluation criteria suitable for fully carbonated concrete.

1.3 Objectives of the Study

The main goals of this study are to investigate on the influence of concrete surface pre-wetting

on the fluctuation of half-cell potential values and on the alteration of HCP result interpretation;and to verify the reliability of half-cell potential method in detecting the corroding zones in

concrete structure which are fully carbonated and contaminated with chloride ions. The specific

objectives are:

a. To investigate how the half-cell potential values of passive and actively corroding zones will

fluctuate at minimal and thorough pre-wetting of concrete surface

b. To examine how the shift in potentials caused by concrete surface pre-wetting will change theresults interpretation deduced from statistical analysis and potential difference technique

c. To compare the corrosion prediction formulated from HCP data and the actual state ofreinforcement corrosion

d. To verify the applicability of the ASTM C876 fixed potential criteria in evaluating thecorrosion risk of steel reinforcements and locating the corroding zones on carbonated concretestructure

e. To explore the relationship between half-cell potential and carbonation depth Section

headings, subsection headings and subheadings should be flushed left.

2.0 THEORETICAL CONSIDERATIONS

2.1 Considerations and Framework

Corroding areas on RC structures are difficult to detect due to the existence of highly resistive

layers [RILEM TC154, Gu & Beaudoin, 1998] resulting from the densification of concrete porestructure caused by carbonation [Chang et al., 2004; Chi et al., 2002 and Yeih & Chang, 2005].

The high resistivity of carbonated concrete can mask the corroding areas [Elsener & Bohni,

1990] by showing less negative potentials of actively corroding zones that is similar or close tothe potentials of passive zones [RILEM TC154]. The presence of high resistance surface layer

can enhance the effect of concrete cover depth [Elsener & Bohni,1990]. As the concrete cover

depth increases the potential difference between the passive and actively corroding zones

decreases [Elsener & Bohni, 1990 and Ouglova et al., 2005]; and this can make the identification


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of corroding zones difficult. The dense pore structure may restrict the rate of oxygen diffusion

into concrete [Baccay et al., 2006; and Song & Kwon,2007]. Oxygen depletion in the concretecover caused by fully saturated concrete pores can shift the potentials to more negative values

[Qian et al., 2003] but this very negative potential reading may not be necessarily associated with

high probability of reinforcement corrosion [Gu & Beaudoin, 1998]. Hence, it is possible to

obtain a very negative potential reading on anon-corroding area due to oxygen starvation. Theinfluence of concrete cover depth, concrete electrical resistance and oxygen availability in the

case of carbonated concrete is significant to cause the fluctuation of potential values away from

its true corrosion potential. Thus, corroding areas in fully carbonated concrete may be masked

and therefore would be difficult to detect.

Concrete having potential range from +100 to -600 mV CSE [RILEM TC154].ASTMs numeric

magnitude technique cannot be used to evaluate the corrosion risk of carbonated concrete not

only for the reason that it (fixed potential criteria) was empirically derived from chloride-inducedcorrosion cases but also on the fact that potentials obtained on concrete surface may be

significantly influenced by various factors as highlighted by different authors (Gu & Beaudoin,

1998; Qian et al., 2003; Elsener & Bohni,1990; Ouglova et al., 2005; Assouli et al., 2008; andNakamura et al., 2008). Moreover, ASTM C876-91 itself does restrict the use of its numericmagnitude technique in evaluating the corrosion condition of steel reinforcements in fully

carbonated concrete, unless experience and/or destructive examination of some areas have

implied its applicability. Hence, it is obvious that a fixed potential value for the evaluation of

half-cell potential data is not always applicable and may lead to incorrect corrosion prediction.

Theoretical considerations and practical experience on a large number of structures have shown

that the results of HCP measurements on existing structures need careful interpretation [RILEMTC 154; Elsener, 2001; and Qian et al., 2003]. Different recommendations for sound HCP data

analysis are available in the literature. Some (Poupard,2006; Nakamura et al., 2008, ASTM C87-

91 and RILEM TC 154) suggest the use of potential difference technique i.e. gradient in thepotential field, which is more reliable than the numeric magnitude technique in locating thecorroding areas [Carino, 1999]. While others (RILEM TC 154 and Gulikers, 2008) recommend a

statistical evaluation of HCP data which allows the comparison of corrosion risk among the same

structural members and permits the derivation of fixed potential criteria for corrosion risk

assessment of a particular structure at a given environment.

REFERENCES

ABOUT THE AUTHORS

Author One is a full time faculty and the chairman of the Civil Engineering Department of De La Salle

University . He may be contacted at DLSU, 2401 Taft Avenue, Manila. Tel. 524-4611 local 203 and faxno. 521-1609. E-mail: [email protected].

Author Two obtained his Master of Science in Civil Engineering from the De La Salle University ,

Philippines in 2011 E-mail [email protected]


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ACKNOWLEDGEMENTS

The equipment and methodology used in the measurement of carbonation were through the guidance and

sponsorship of Prof. Nobuaki Otsuki, a professor and concrete technology and materials expert from

Tokyo Institute of Technology, Japan.

Documents

Assessment of Reinforcement Corrosion