Title of Invention

A DEVICE FOR DETECTING CORROSION OF STEEL REINFORCEMENT IN CONCRETE

Abstract A device for detecting corrosion of steel reinforcement in concrete characterized by a probe essentially consisting of a reference electrode placed at the centre of a U-shaped stainless steel counter electrode, the said probe being embedded in a wetable sponge, the said electrodes being comiected to electronic circuitry such as herein described, for impressing anodic galvanostatic pulse and monitoring the resultant potential change of the steel reinforcement in concrete with respect to the reference electrode
Full Text This invention relates to a device for detecting corrosion of steel reinforcement in concrete.
Detecting the corroded areas of reinforcement rods in concrete structures is very important as it helps in evaluating the condition of the structures and in evolving strategic repair and rehabilitation procedures. The device of the present invention is useful in detecting the corroded areas of reinforcing steel in concrete. In this device, the DC Galvanostatic Pulse Technique is used, with which the polarisation ability of the reinforcing steel and hence the corroded areas in the concrete can be detected. Thnefore the detectioa of actual corroded areas of reinforcing steel in concrete becomes possible with the help of this device. -This device is useful for field measurements for the following reasons : (i) It works on battery (ii) It is of light weight and hence portable (iii) It is rugged (iv) Simple operations are involved in the measurements
Hitherto for detecting and locating the corroded areas of reinforced rods in concrete, the only technique available as on date is potential mapping. This method is based on measuring the open circuit potentials (OCP) of reinforcing steel at various spots and mapping iso-potentials. The potential values can be interpreted in a way that more negative potentials than -350 mV vs Copper Sulphate Electrode (-280 mV vs Saturated Calomel Electrode) suggest 90 % probability for occurrence of corrosion (ASTM (876-1977).
This method however suffers serious setbacks due to the following reasons :
1. The cover depth directly influences tlw potential field measured at the sur&ce, as with increasing distance fix>m the reinforcing steel, i.e. with larger concrete covo-, the differences between anodic and cathodic areas decrease. Therefore, the sensitivity of this method concerning localization of small corrosion spots decreases.
2. The effect of a high resistance paint coating on concrete could be to enhance the effect of cover depth, as the macro-cell current paths tend to avoid the highly resistive concrete. Thus corroding areas could be masked and not detected.
3. The concrete humidity and the presence of ions in the pore solution affect the electrical resistivity of the concrete. Though this does not significantly affect the potential contour pattern, the numerical potential values may fluctuate by 50 mV.
4. The conditions of aeration, i.e. oxygen access, strongly determine the open circuit potential values of passive steel in concrete. Low oxygen content leads to a pronoimced shift of open circuit potential to more negative values. In particular, in wet concrete due to very low oxygen difiEiisivity conditions may arise resulting in a shift of the open circuit potential of reinforcing steel to comparably negative values. Consequently, non-corroding steel reinforcement may show negative potentials similar to those of corroding steel reinforcement. This leads to the high risk of mis-classification of non-coiToding areas as corroding areas under low aerations.
The main object of the present invention is to provide a device for detecting corrosion of steel reinforcement in concrete which obviates the drawbacks as detailed above.
Another object of the present invention is to provide a portable device to detect and locate the corrosion of steel in reinforced concrete based on the anodic galvanostatic pulse technique.
Accordingly, the present invention provides a device for detecting corrosion of steel reinforcement in concrete characterized by a probe essentially consisting of a reference electrode placed at the centre of a U-shaped stainless steel counter electrode, the said probe being embedded in a wetable sponge, the said electrodes being coimected to electronic circuitry such as herein described, for impressing anodic galvanostatic pulse and monitoring the resultant potential change of the steel reinforcement in concrete with respect to the reference electrode.
In an embodiment of the present invention the reference electrode used is such as a saturated calomel electrode (SCE).
In another embodiment of the present invention the electronic circuitry used is such as a constant current generator the output of which being cormected to the counter electrode and the reinforcing steel in concrete, the input to the said constant current generator being connected to the output of a triggerable monostable multivibrator, the output of the said multivibrator being also connected in parallel to a first sample and hold amplifier (SHI) through a positive slope detector and timer and a second sample & hold amplifier (SH2) through a negative slope detector and timer, the SHI input being coimected to the reference electrode, the outputs of SH1 & SH2 being connected through a switch to a panel meter.
In the drawings accompanying this specification figure 1 represents the block diagram of an embodinent of the device of the present invention with the probe.
The present invention provides a device for detecting corrosion of steel reinforcement in concrete which consists of a special probe design and a circuitry for impressing anodic galvanostatic pulse and monitoring the potential change of the reinforcing steel in concrete.
In an embodiment of the present invention, the special probe (of size 40 X 120 mm), made of PVC (6) comprises a stainless steel counter electrode (1) ('U' sh^Ted with arm width 5mm and length 80 mm) and a saturated calomel reference electrode (2) placed at the center of 'U" shaped counter electrode with a wetted sponge (5). Electrical contacts are made with these electrodes with a simple BNC connector. This hand held probe can be placed over reinforced concrete (4) containing steel rod (3) and the wetted sponge ensures good flow of signal.
In another embodiment of the present invention the device consists of three timers Tl, T2, T3 and two sample and hold amplifiers (S/H). The first timer (Tl) gives a square pulse of 60 seconds duration. This pulse is fed to a V-to-I converter (constant current generator) which is the main current source for the cell system consisting of a reinforcing rod inside a concrete, and a probe with a reference electrode and a counter electrode embedded in a wet sponge. The current source injects a set current (amplitude either 100 or 300 fiA) for 60 seconds into the cell system. At the end of 50 seconds of the pulse duration, timer T2 switches on the first sample and hold amplifier (S/Hl) to measure the potential of the reinforcement steel in the concrete. The potential read is held by the S'Hl (ON potential). Alter the expin. of the total pulse duration (60
seconds) the timer (T3) switches on the S/H2 amplifier and after a preset delay of 100 µS. When the current is cut off, the potmtial of the reinforoemoit bar drops down suddenly and reaches a potential decay region. The sudden drop in voltage is the in-phase voltage whose magnitude is iproportional to the iR drop. The potential reading held by the S/H2 amplifer is the remanance voltage (OFF potential). A digital panel meter (DPM) is used to measure the OCP, S/Hl and S/H2 through a selection switch S2. Once the first pulse of 60 seconds duration is set on, the entire processing of sampling by S/Hl and S/H2 proceeds automatically, resulting in ease of measurement
Anodic Galvanostatic Pulse Technique comprises of impressing a short-time anodic current pulse galvanostatically between a small counter electrode by using a probe and the steel reinforcement and monitoring the resultant potential change with respect to a reference electrode (SCE). The potential rises because of the current application to a maximimi value which is taken as "ON" potential. After the pulse application is cut off, the potential drops suddenly to a value called "OFF" potential in a fraction of a second and then starts decaying slowly reaching the OCP finally. As passive steel can be essily polarized and hence non-corroding areas show much larger potential shift than corroding areas. This could be identified by a large positive value of the ON arid OFF potentials. In the case of corroding reinforcement steel, the ON and OFF potential values are nearly the same as that of open circuit potential due to less polarisation.
The detailed instructions for the use of the device of the present invention in detecting the corroded areas of steel reinforcement in concrete are as follows : 1. Wet the sponge of the special probe containing the reference electrode (saturated calomel electrode) and the auxillan electrode (stainless steel) with 3 % NaCl soiuiion
2. Place the special probe over the conovte where the condition of the reinforcement rod is to be assessed
3. Connect the meter with the appropriate electrodes
4. Connect the three cables of the instrument on the rear side to the battery power supply
5. Switch on the instrument
6. Set the Current knob to 100 fiA position on the rear side of the instrument
7. Measure the open circuit potential by selecting the OCP position of the selection knob
8. Press the pulse push button so as to impress the 100µA DC current pulse
9. By turning the selection knob to ON position, measure the ON potential
10. After the LED glow completes, change the selection knob to OFF position and measure the OFF potential
The following examples are given by way of illustration and should not be construed to limit the scope of the present invention.
(Table Removed)
In the above experiment, mild steel specimen of 10 mm diameter and 50 mm length was embedded in M30 cement concrete without any chloride addition. After curing for 28 days, the concrete cube was subjected to alternate wetting using tap water and drying. This cycle of alternate wetting and dr>'ing was continued for several months. At the end of the exposure period, the OCP, ON and OFF potentials were measured by the developed device. The concrete specimen was broken open for visual observation. The ON potential has shocted up to a large positive value and the difference in OFF potential and OCP is very high indicating a corrosion free, passive state of the reinforcement rod which is confirmed by the visual observation.
(Table Removed)
In this experiment, the concrete cube was made very similar to the example I but
with the addition of 3 % NaCl by weight of cement in the concrete. The cube was subjected to the alternate wetting and drying procedure verj similar to example I and the
measurements were made with the developed instrument and the results are given in the table. After the test period, the cube was broken open for visual observation. On impressing the current pulse, the ON potential shooted up by 400 mV approximately and the OFF potential by 100 mV with respect to OCP. This indicated a moderate polarisation of the reinforcing rod. The visual observation confirmed the moderate corrosion of the reinforcing rod.
(Table Removed)
In this experiment, the concrete cube was made very similar to the exzunples I & II but with the addition of 5 % NaCl by weight in the concrete. The cube was subjected to the alternate wettii^ and drying procedure very similar to the previous examples and the measurements were made with the developed instrument and the results are given in the table. After the test period, the cube was broken open for visual observation. On impressing the current pulse, the ON potential shooted by 175 mV jq>pFoxiinately from the OCP and the OFF potential is almost equal to OCP. This indicated lack of sufficient polarisation due to corrosion of the reinforcing rod. The visual observation also confirmed tlie complete corrosion of the reinforcing rod.
(Table Removed)
In this example, the concrete cube was prq>ared as in earlier examples with M30 mix without any chloride inclusion. The cube after the curing period of 28 days, was kept immersed totally in 3 % NaCl solution for 18 months. At the end of the period, the readings were taken with the developed instrument and the results arc presented in the above table. On impressing the galvanostatic pulse, the ON potential shooted by nearly 1.2 V and the OFF potential by nearly 1 V from the OCP. This high polarisation and the large difference in the OFF and OCP projects a totally corrosion free condition. On the other hand, a mere measurement of OCP would have indicated highly active condition because of a highly negative value as observed in this example. Afler the measurement, the cube was broaken open for visual observation. Observation showed a totally corrosion free condition of the reinforcing rod. Thus, this example clearly showed the misleading nature of OCP measurements, particularly in wet concrete surfaces where the measurements from newly developed instrument based on Anodic Galvanostatic Pulse Technique predicted the actual condition of the reinforcing rod.
Thus the above examples clearly point out that if the ON and OFF potentials of a measurement is above +400 mV, the condition of the reinforcing rod is clearly corrosion free.
The main advantages of the present invention are :
1. The device is portable and easy to transport and hence suitable for field operations
2. The device is rugged and of light weight for field use
3. Measurement of OCP is possible
4. Measurement of ON and OFF potentials clearly indicate the condition of reinforcing steel rod inside the concrete
5. It is possible to assess the condition of reinforcement steel rod in both dry and wet concretes.





We claim :
1. A device for detecting corrosion of steel reinforcement in concrete characterized by a probe essentially consisting of a reference electrode placed at the centre of a U-shaped stainless steel counter electrode, the said probe being embedded in a wetable sponge, the said electrodes being connected to electronic circuitry such as herein described, for impressing anodic galvanostatic pulse and monitoring the resultant potential change of the steel reinforcement in concrete with respect to the reference electrode.
2. A device as claimed in claim 1 wherein the reference electrode used is such as a saturated calomel electrode (SCE).
3. A device as claimed in claims 1 & 2 wherein the electronic circuitry used is a constant current generator the output of which being connected to the counter electrode and reinforcing steel of concrete, the input to the said constant current generator being connected to the output of a triggerable monostable multivibrator, the output of the said multivibrator being also connected in parallel to a first sample & hold amplifier (S/Hl) through a positive slope detector and timer and a second sample & hold amplifier (S/H2) through a negative slope detector and timer, the S/Hl input being connected to the reference electrode, the outputs of said S/Hl & S/H2 being connected through a switch to a panel meter.
4. A device for detecting corrosion of steel reinforcement in concrete substantially as herein described with reference to the drawing accompanying this specification and the examples.

Documents:


Patent Number 244862
Indian Patent Application Number 639/DEL/1999
PG Journal Number 52/2010
Publication Date 24-Dec-2010
Grant Date 22-Dec-2010
Date of Filing 23-Apr-1999
Name of Patentee COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH
Applicant Address RAFI MARG NEW DELHI-110001,INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 GOPALACHARI VENKATACHARI CENTRAL ELECTROCHEMICAL RESEARCH INSTITUTE,KARAIKUDI-630006, INDIA.
2 SESHADHRI SRINIVASAN CENTRAL ELECTROCHEMICAL RESEARCH INSTITUTE,KARAIKUDI-630006, INDIA.
3 NERUR SANKARANARAYANAN RENGASWAMY CENTRAL ELECTROCHEMICAL RESEARCH INSTITUTE,KARAIKUDI-630006, INDIA.
4 KANNAMANGALAM RAMASWAMY RAMAKRISHNAN CENTRAL ELECTROCHEMICAL RESEARCH INSTITUTE,KARAIKUDI-630006, INDIA.
5 PANJALI NATARAJAN CENTRAL ELECTROCHEMICAL RESEARCH INSTITUTE,KARAIKUDI-630006, INDIA.
6 SADAGOPAN SATHIYANARAYANAN CENTRAL ELECTROCHEMICAL RESEARCH INSTITUTE,KARAIKUDI-630006, INDIA.
PCT International Classification Number G01N 17/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA