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Non-destructive corrosion rate monitoring for reinforced concrete structures Dubravka Bjegovic, Dunja Mikulic, Dalibor Sekulic Contact

ABSTRACT

The corrosion of reinforcements has resulted to be one of the most frequent causes of their premature failures. Monitoring the corrosion rate, assuming the uniform corrosion and the loss in diameter decreases linear with the corrosion rate, allows calculating the remaining load carrying and the safety of the structure. There are several methods of measuring true, instantaneous rate of corrosion, based on electrochemical methods.
This paper describes electropotential mapping method and measuring techniques with the Gecor 6 device, their application, advantages and disadvantages based on our own experiences, and the interpretation of the measurement results.

1. INTRODUCTION

A serious problem of concrete infrastructure deterioration, as a result of reinforced steel corrosion, goes along with great economical consequences. New-built concrete is alkali and reinforcement corrosion is obstructed. Decrease of pore water pH value causes depasivation of metal surface and initiation of corrosion. Penetration of chloride ions from environment in concrete and reaction with atmospheric carbon dioxide are the main source of corrosion. Corrosion products have bigger volume than steel, which causes tensile strains in concrete. If the tensile strains are bigger than tensile strength of concrete, the result is cracking of concrete. Penetration of chlorides and diffusion of carbon dioxide are increased at the places of cracks, which further increases corrosion. Another consequence of crack formation is the development of galvanic cells with anodic and cathodic areas with corrosion at unprotected (anodic) areas. Pitting corrosion and galvanic macrocell formation generate small losses of steel, but create areas with concentrations of strains.
The main investigation of corrosion researchers is detection and measuring of defects in the initial stage of corrosion process. This paper presents actual methods for corrosion characterisation in reinforced concrete (Table 1).

ELECTROCHEMICAL METHODS	NONELECTROCHEMICAL METHODS
STATIC MEASUREMENTS Half-cell potential measurements    Hand held equipment    Embedded reference electrodes Corrosion macrocell current measuring Electrochemical noise	Visual inspection Seismic method Infrared thermography Acoustic emission Radiography and radiometry Radar Electrical resistivity of concrete Electrical resistance method Optical fibber sensors Magnetic technique Microwave based Thermoreflectometry
POLARISATION MEASUREMENTS Linear polarisation method    Hand held equipment    Embedded linear polarisation sensors Electrochemical impedance spectroscopy    Localised electrochemical impedance spectroscopy Galvanostatic pulse method Scanning reference electrode method
Table 1: Actual methods for corrosion characterisation in reinforced concrete

2. ELECTROCHEMICAL METHODS

2.1 STATIC MEASUREMENTS


2.1.1 HALF-CELL POTENTIAL MEASUREMENTS


Fig 1: Principle of the half-cell method

The principle involved in this method is appearance of an electrical potential between the reinforcing steel and a reference electrode named half-cell. The half-cell consists of a metal rod immersed in a solution of its own ions (Fig. 1).

The role of the half-cell is to insure constant reference potential. The metal rod is connected with reinforcement steel by a voltmeter, and the ion solution is connected to the pore water via moist porous plug [1]. Measuring method is based on many measurements of potential and correlation of measured potentials with observed corrosion rate at reinforcement. Table 2 presents criteria according to ASTM C-876 standard for cooper-cooper sulphate electrode, and also for calomel and silver-silver chloride. The main application of this method is in situ.

Cu/CuSO4	Calomel (SCE)	Ag/AgCl	Interpretation
E>-200mV	E>-126mV	E>-119mV	Greater than 90% probability that no corrosion is occurring
-200mV < E < -350mV	-126mV < E < -276mV	-119mV < E < -269mV	Corrosion activity is uncertain
E<-350mV	E<-276mV	E<-269mV	Greater than 90% probability that no corrosion is occurring
Table 2: Interpretation of corrosion potential measurements

Hand held equipment
The half-cell is moved across the concrete surface to be investigated, and the electrode potentials are measured at many points. The measured potential is drawn as equipotential lines to identify the corrosion areas [2]. Extra devices are constructed to accelerate measuring [3].

Embedded reference electrodes
Results obtained by means of the hand held equipment are not accurate, because there is a concrete layer between half-cell and steel with variations in resistance and thickness. To avoid negative effects of the concrete layer, half-cells can be embedded in concrete close to the reinforcing steel. Different reference electrodes are commercially available. Pseudoreference mixed metal oxide electrodes (Fig.2) consists of mixed metal oxide activated titanium rod, cast in specially developed cementuos body, which has long term stability of the electrochemical potential [4]. The ERE 20-Embeddable reference electrode, developed and manufactured by the FORCE Institute using a manganese dioxide electrode in a steel housing with an alkaline, chloride free gel (Fig.3) [5]. The relevant sensor is manufactured by the Austrian Ingenierbüro Wietek (Fig.4) [6]. The PVC covered sensor in the form of a wire is wrapped around the steel to be monitored. A potential between the steel and electrode can be measured by using the half-cell. The advantage of the method is great sensitivity, which makes the method suitable for measurements of pitting corrosion at the large concrete structures.

Fig 2: MMO Ti probe

Fig 3: ERE 20 probe

Fig 4: Wire sensor

2.1.2 CORROSION MACROCELL CURRENT MEASURING
During the corrosion process, corrosion macrocells are formed with a distribution of anodic and cathodic areas. Voltage in a macrocell element equal to potential difference between active and passive steel gives the corrosion current:

I=DU/(RE+RA+RC)	(1)

Where,I - electrical current (mA)

D U -voltage in the macrocell element (mV)
R_E - concrete electrical resistance (W)
R_A - anode reaction electrical resistance (W)
R_C - cathode reaction electrical resistance (W)

Mass of the steel loss can be directly calculated from the Faraday law:

(2)

Where, W_m - molecular mass (g/mol)

t -time (s)
V -valence
F -Faraday constant (96500 C)

Different measuring configurations for in-situ testing are developed. Figure 5 shows a measuring system developed by Schiessel and Rupach [7, 8]. The system consists of the steel electrodes and insulting supports. The sensor can be built in a new construction or during the repair. Steel electrodes are placed at different depths which makes depasivation front monitoring possible. Another configuration is a Corrowatch Multiprobe manufactured by Germann Instruments (Fig.6) [5]. A multi-probe test unit (Fig.7) developed by the Swedish FORCE Institute consists of 20 embedded steel electrodes, which are potentiostatically held at a fixed potential. The test unit is exposed to chloride ions diffusing from one side. Initiation of corrosion can be detected by a sudden rise in the anodic current [7]. These test methods have an advantage in providing direct indication of electrochemical activity in the system.

Fig 5: Schiessel probe

Fig 6: Corrowatch probe

Fig 7: FORCE probe

2.1.3 ELECTROCHEMICAL NOISE
Fluctuations of potential and current, generated spontaneously by the corrosion process, make electrochemical noise. Analysis of fluctuations after spectral decomposition gives not only finding of corrosion, but characterisation of the corrosion process. Advantage of the electrochemical noise method is absence of external current or voltages supply which perturbate system. Measured signals can be analysed by mathematical analysis. In the case of complicated kinds of corrosion, like metastabile pitting corrosion or corrosion inhibitor induced by unstable passivation, mathematical analysis becomes unsuccessful, and some researchers suggest application of chaos theory at corrosion electrochemistry [9].

2.2 POLARISATION MEASUREMENTS


2.2.1 LINEAR POLARISATION METHOD
In the linear polarisation method, a potential scan is applied at the specimen in a range E_corr±25mV. The resulting current has linear dependence versus the potential, which can be evaluated from the equation:

(3)

Rate between the applied current and the potential response DE gives polarisation resistance Rp:

(4)

Where,

Di - applied current (mA)
DE -potential response (mV)
i_corr -corrosion intensity (mA/cm²)
R_p - polarisation resistance (kW)
B - value of 13 to 52 mV in the most metal / media systems
b_a - anodic Taffel constant
b_c-cathodic Taffel constant

This is Stern-Geary relation, which can be used for corrosion current calculation. The linear polarisation technique has been used widely in laboratory work for corrosion rate determination, but some modifications are needed for its application to structures in the field [10].

Hand held equipment
A difficulty with the linear polarisation technique is requirement for determination of area of steel being polarised without which accurate corrosion determination can't be achieved. This problem is avoided by use of an extra ring electrode placed around the central electrode. In this way, signal application is limited at the known rebar area. Based on this principle, in situ devices are developed. "Gecor 6" (Fig.8) consists of the rate meter that automatically controls the system and two sensors. Sensor A is for the corrosion rate and half-cell measurements and sensor B is for the concrete resistivity, temperature and relative humidity measurements [11]. Another device is MS 4500 Polarisation Resistance Monitor for accurate determination of polarisation resistance even in high resistance concrete environments (Fig.9) [12].

Fig 8: Gecor 6

Fig 9: MS 4500 polarization device

Embeddable linear polarisation sensors
Based on the linear polarisation principle, embeddable minisensors are developed. Different types of minisensors are commercially available. The C-probe CP100 (Fig.10) is a combination of Silver/silver chloride reference half cell and graphite counter electrode [7]. CORROATER 800/800T (Fig.11) is manufactured using carbon steel measures corrosion rate of reinforcing steel in concrete [12]. General Building Research Corporation of Japan developed sensor, as shown in the Figure 12. Three electrochemical characteristics of natural potential, polarization resistance and electrolyte resistance can be measured [13]. All this probes have possibility of automated measurements using computer-controlled equipment.

Fig 10: C-probe type CP 100

Fig 11: CORROATER

Fig 12: Minisensor

2.2.2 ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS)
Electrochemical impedance spectroscopy (EIS) uses polarisation with alternating current. Instrumentation for measurements is more sophisticated than for other polarisation measurements, consisting of a potentiostat and spectrum analyser. Reinforcement is maintained at its corrosion potential E_corr by the potentiostat, with application of a sinusoidal potential (10 to 20 mV) in a wide frequency range. The response at input signal is also sinusoidal with phase shift relative to the input signal. The EIS method in its basic formulation is very attractive because it can determine polarisation resistance and add extra information about the corrosion process. High frequency range can give information about dielectric properties of concrete, and low frequency range information about dielectric properties of passivity film on the steel. In spite of these possibilities, the method hasn't had wide application to reinforced concrete, because diagrams become complex and difficult for interpretation [14].

Localised electrochemical impedance spectroscopy (LEIS)
Data obtained by the conventional EIS technique are averaged across the entire area of the sample, and this technique isn't suitable for application for chloride induced pitting corrosion. To avoid problems, localised electrochemical impedance spectroscopy (LEIS) is developed 15 . The principles of LEIS are similar to those in conventional EIS, but LEIS combines both established direct current scanning probe methods with alternating current impedance techniques. The probe consists of two separate platinised electrodes. The first electrode has a tip, which is electrochemically sharpened to 5 mm in diameter. The second ring electrode is positioned at a fixed distance of 2-3 mm away from the tip electrode. Method is suitable for the corrosion inhibitor effectiveness investigation.

2.2.3 GALVANOSTATIC PULSE METHOD
A short time anodic pulse (typically 8 s) is applied galvanostaticially on the reinforcement and the resulted change in potential is monitored. Potentials are measured with a reference electrode and the high impedance voltmeter. When a current impulse I_app is applied to a corrosion system, the potential V as a function of time can be expressed as [16]:

(5)

Where, R_p -polarisation resistance (W)

C_dl -capacity of double layer (mF)
R_W -ohmic resistance (W)

Fig 13: The GalvaPulse device

The Galvanostatic pulse method allows rapid measurements of polarisation resistance, ohmic resistance and open circuit potential. An example of an instrument used in this method is GalvaPulse by the Gemann Instruments (Fig 13.) This is a rapid non-destructive device for determining the corrosion rate of reinforcement in concrete. The device is equipped with software, which enables displaying the corrosion rate, electrical resistance and half-cell potential, together with the graphs of the galvanostatic pulse [5].

2.2.4 SCANNING REFERENCE ELECTRODE METHOD (SRET)
In localised corrosion, anodic and cathodic reactions usually occur at separate sites. These reactions produce small but measurable ionic transport in the electrolyte local to the anodic and cathodic sites. The Scanning reference electrode technique (SRET) measures micro galvanic potentials existing locally on the surface of the specimen using uniquely designed scanning electrode. Specimens rotate in a solution by means of a stepper electromotor. Measurement is made by means of the scanning electrode and differential amplifier, which gives two-dimensional picture of any region of interest. Method allows dynamical information of corrosion activity, which has been done by the variations of ionic flow at the microscopic scale. The SRET method can be used for pasivation research of corrosion at grain boundaries, and cracking under corrosion induced strain [17].

3 NONELECTHROCHEMICAL METHODS

Many nonelectrochemical methods, are suitable for determining corrosion of reinforced steel in concrete 18, 19, 20, and 21 . The list of the nonelectrochemical methods is given in Table 1. These methods will be described in the next paper.

4 CONCLUSION

An overview of most frequent nondestructive corrosion determination methods as applied for the reinforced concrete is presented. Nondestructive methods are advantageous when compared to destructive methods. Continuos monitoring of reinforcement condition is enabled, measurements can be done at the level of the entire structure, and nondestructive methods have proven to be fast and inexpensive. On the other hand, determination of reinforcement steel with nondestructive methods is complex and may lead to wrong interpretation of results. To avoid misinterpretation it is recommended to combine several nondestructive testing methods, before making any conclusion about reinforcement steel corrosion.

REFERENCES

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