|Title of Invention||
"AN EMBEDDABLE REFERENCE ELECTRODE FOR USE IN CONCRETE STRUCTURES"
|Abstract||An embeddable reference electrode for use in concrete structures which comprises, an open ended tubular casing (8) of inert material, the bottom end of the said casing being plugged (7) with cement paste, characterised in that above the said cement plug (7) is provided with layers of saturated calcium hydroxide (6), mercuric oxide (5) and mercury (4) respectively, a stainless steel wire (3) is fixed onto the top end of the said casing (8) by teflon gasket (2) in such a manner that the said wire dips in the mercury (4) & mercuric oxide (5), the top end of the said wire (3) is provided with copper wire (1) for electrical connections.|
|Full Text||This invention relates to an embeddable referenceselec-trode for use in concrete structures.
The present invention relates to the improvements in or relating to the development of a stable and rugged reference electrode for use in reinforced and pre-stressed concrete bridges and structures.
It is now fairly well recognised that reinforced and prestressed concrete bridges and structures exposed to marine or industrial environment need continuous monitoring of their condition because of their susceptibility to corrosion and environmental degradation. Electrochemical techniques such as half cell potential measurements, linear polarization measurements and impedance measurements have been suggested as monitoring tools. Use of a reliable reference electrode is imperative to these monitoring techniques.
Cathodic protection of reinforced concrete structures has emerged as a fool-proof rehabilitation technique for corrosion damaged reinforced concrete structures. Potential criterion is widely adopted for optimizing the cathodic protection design. Reference electrode is essentially needed for continuous optimization of cathodic protection.
Hitherto various reference electrodes such as copper/copper sulphate, silver/silver chloride, mercury/mercurous oxide, titanium substrate insoluble
electrode and graphite electrode have been suggested and utilised in concrete structures. ASTM C 876 specifies
the use of Cu/CuSO as reference electrode.
Hitherto known electrodes have the following drawbacks:-
(a) Leakage of copper sulphate solution has been found to
stain the concrete. The seepage of CuSO solution would
contaminate the concrete and these copper ions would
influence the steel behavior.
(b) Graphite electrodes are not sufficiently rugged.
(c) Surface-mounted reference electrodes are subjected to ultra violet radiation effect which considerably influences the measured values and lead to erroneous data [F.J.Ansuini and J.R.Dimond, "Long term stability testing of reference electrodes for reinforced concrete", Corrosion/94, Paper No.295 (Houston) TX NACE, 1994; Frank J.Ansuini and J.R.Dimond, "Factors affecting the accuracy of reference electrodes", Materials Performance, Vol.13, 1994, p.14]
(d) Ag/AgCl electrodes are photosensitive [P.J.Ansuini and
J.R.Dimond "Long-term stability testing of reference
electrodes for reinforced concrete", Corrosion/94, Paper
No.295 (Houston), TX, NACE, 1994; Frank J.Ansuini and J.R.Dimond, "Factors affecting the accuracy of reference electrodes", Materials Performance, Vol.13, 1994, p. 14; M.H.Peterson and R.E.Groover, "Material performance", May 1972, p.14.
(e) PbO -MnO coated graphite electrodes are found to be
unstable in alkaline concrete environment [unpublished results, CECRI].
(f) Hg/HgO electrode as it is, is unstable when embedded in
concrete [C.Dehaghanian, C.R.Root and C.E.Locke; Review
of embeddable reference electrode for Portland cement concrete corrosion 81, NACE conference, April 1981, Ontario, Canada, Paper 45.]
(g) Mo/MoO and AgCl electrodes were found unstable during
o winter when the temperature dropped below 0 C.
[H.C.Schell,D.G Manning : evaluation of performance of
cathodic protection system on reinforced concrete
bridge super structure, corrosion 85, Massachusetts,
March, 1985, paper 263].
(h) Zn / ZnSO electrode show erratic variations in
potentials. [D.G.Manning and H.C.Schell, Early performance of eight environmental cathodic protection systems at the Burlington Bay Skyway Test site", Transportation Research Record 1041, Transportation Research Board, National Research Council, Washington D.C. 1985].
The main object of the present invention is to provide an embeddable reference electrode for use in concrete structures which is rugged, stable and can give reliable
data. Another object of the present invention is to provide an embeddable electrode which is unaffected by temperature changes and chloride ion concentration changes in the surrounding medium. Still another object of the present invention is to develop an electrode to be sued in soil. An embodiment of the embeddable reference electrode of the present invention is shown in Figure 1 of the drawings accompanying this specification. The system consists of a glass tube (8) in which 316 stainless steel wire (3) of approximately 1.5 mm dia is dipped in mercury (4). The mercury is in contact with mercuric oxide (5) which in turn is in contact with saturated calcium hydroxide (6). The plug (7) is made of a cement paste to act as a bridge with the bulk concrete. The stainless steel wire is fixed in a Teflon gasket (2) and electrical connections are made using coaxial copper wire (l).This reference electrode is not going to be used in cocreting relating to atomic power station and atomic energy.
Accordingly, the present invention provides an embeddable reference electrode for use in concrete structures which comprises, an open ended tubular casing (8) of inert material, the bottom end of the said casing being plugged (7) with cement paste, characterised in that above the said cement plug (7) is provided with layers of saturated calcium hydroxide (6), mercuric oxide (5) and mercury (4) respectively, a stainless steel wire (3) is fixed onto the top end of the said casing (8) by teflon gasket (2) in such a manner that the said wire dips in the mercury (4) & mercuric oxide (5), the top end of the said wire (3) is provided with copper wire (1) for electrical connections.
The present invention provides an embeddable, rugged stable reference electrode for use in concrete structures. This reference electrode exhibits better micro reversibility and a minimum temperature coefficient, unaffected by the influence of chloride ions present in the system. This reference electrode offers minimum IR drop in the potential measurements. This reference electrode is used for corrosion potential measurements, corrosion rate determinations and to follow cathodic protection potentials of steel reinforcements. The electrode is usually placed close to the steel reinforcement. Prior to the casting of concrete, electrical connections are made to the electrode through extended shield copper wires. The said reference electrode doesn't have any harmful effect in chemical reaction of cement hydration process
The following examples are given by way of illustrations of the present invention and should not be construed to limit the scope of the present invention. Example 1 Micro-reversibility performance:-
This test is carried out on a reference electrode to check the reversibility of the electrode and reproduci-
bility of the electrode potentials. It is carried out on a three electrode electrochemical cell. The developed embeddable electrode is used as a working electrode. A large platinum foil (2.5 x 2.5 cm area) as a counter electrode and saturated calomel electrode commercially available as reference electrode respectively. Bio Analytical System (BAS 100A) is used for small amplitude cyclic voltammetry. The amplitude of potentials is + 15 mV and the sweep rate is 1 mV/sec. The resultant current-potential curve (voltammogram) is recorded. From the open circuit potential, the cyclic polarisations are done. The zero current crossing potentials (ZCCP) for the forward and backward scans are noted. The difference in potentials values ( E) is the measure of irreversibility. For a perfect reference
electrode system, E → 0 as the sweep rate tends to
Figure 2 presents the cyclic current-potential (cyclic voltammogram) of the saturated calomel electrode supplied by Elico P Ltd, Hyderabad.
Figure 3 presents the cyclic current-potential (cyclic voltammogram) of the saturated calomel electrode supplied by Toshniwal Brothers P Ltd, Mumbai.
Figure 4 presents the cyclic current-potential (cyclic voltammogram) of the developed embeddable reference electrode.
Figure 5 presents the cyclic current-potential (cyclic voltammogram) of the laboratory-made Hg/HgO reference electrode of the present invention.
Table 1 presents the results of the micro-reversibility tests.
MICRO-REVERSIBILITY TESTS (Table Removed)
It may be seen from the above table that the newly-developed embeddable reference electrode has the desirable reversibility characteristics when compared to the commercially available systems.
The developed reference electrode with glass tubing is
o o stable in the temperature range of 30 C to 50 C; Figure
6 presents the variation of the developed electrode potential versus saturated calomel electrode with temperatures. A temperature coefficient of 4.5 mV per degree centigrade is obtained. By proper thermal insulation, this can be minimised and the performance of the
electrode at higher temperatures (65 C) can be studied.
Influence of chloride ion concentration present in the
To study the stability and reproductibility of the embed-
dable reference electrode in presence of chloride ions
in the surrounding medium, the following tests were
carried out. Saturated calcium hydroxide solution and
saturated calcium hydroxide containing chloride ion
solutions were used. The potentials of three embeddable
electrodes were used for study. The potentials of these
electrodes were monitored in saturated Ca(OH) solutions
using saturated calomel electrode for 8 days. Figure 7
presents the variation of the electrode potentials for 8
days in saturated Ca(OH) solution. A value of - 56 + 2
mV Vs SCE was obtained.
The potentials of another three electrodes were monitored in saturated Ca(OH) containing 500 ppm chloride
ions. Figure 8 presents the variation of the electrode
potentials for 8 days in saturated Ca(OH) solution
containing 500 ppm of chloride ions. A constant value
of - 53 mV Vs SCE was obtained.
It may be seen from the above example that the performance of the newly-developed embeddable electrode is not influenced even in the presence of 500 ppm of chloride
ion in the medium.
Long-term stability :
10 x 10 x 10 cm concrete cubes of strength M were
used. The reference electrodes were embedded 1 cm away
from the 5 mm steel rod. After curing for 28 days in
distilled water, the potentials of steel rods were
followed up to 28 days. Three types of concrete cubes
were used: (i) Plain concrete; (ii) Concrete containing
50 ppm chloride ions and (iii) concrete containing
30,000 ppm chloride ions.
Figure 9 presents the variations of the potential of
rebar in M concrete cubes and concrete cubes contain-20
ing various amounts of chlorides. In 30,000 ppm chloride ion containing concrete cubes the rebar exhibited very active potentials indicating the corrosion of steel. This indicates that the potentials of the embed-dable electrode may be used as a criteria to monitor the corrosion behaviour of steel rebar up to 28 days in chloride containing environments.
Application of buried pipeline :
A pipeline of 6.3 metres (21 feet) was buried below 1 metre (3 feet) under the surface of the ground. The soil charactaeristics were uniform along the length of
the pipe. The potentials of the pipe at various locations were monitored by two types of reference electrodes. The conventionally used copper/copper sulphate electrode, a surface mountable electrode, and the embeddable reference electrode were used for investigation. The embeddable reference electrode was also buried very close to the pipe and electrical connections were made. The soil surface was made wet with tap water. At the end of the 5th minute, the potentials of the buried pipe was followed with the distance from one end. The potentials were also measured using the Cu/CuSO electrode at
the end of 30th minute of wetting using a high impedance
voltmeter along its length from one side. It may be
seen from figure 10 that the potentials measured using
the embeddable electrode were nearly steady along the
length of the pipe. The Cu/CuSO reference electrode
potentials were varying along the length of the pipe.
The embeddable electrode exhibited a constant potential
along the length of the pipe indicating the uniformity
of the soil characteristics.
Application to Cathodic Protection :
This example is given to illustrate the use of the newly- developed embeddable reference electrode to follow the cathodic protection potentials of the buried pipeline.
A pipeline of 6.3 metres (21 feet) was buried below 1 metre (3 feet) under the surface of the ground. A titanium substrate insoluble anode mesh (2 feet x 0.5 feet) was buried on one side of the pipe. The embedda-ble reference electrode was buried very close to the pipe. Electrical connections were made using shielded copper wires.' A rectifier was used and different ca-thodic currents were passed.
A laboratory made Cu/CuSO electrode was used to measure
cathodic potentials of the pipeline. The soil surface was made wet by pouring tap water and the potentials of the pipe along the length at the end of 5th and 3 0th minutes after.wetting was measured using a high impedance digital voltmeter.
Figure 11 presents the variation of the potential of the buried pipe along its length at different applied cathodic currents (2 mA and 5 mA) measured using Cu/CuSO
reference electrode. The influence of applied cathodic currents are not seen.
Figure 12 presents the variation of the potential of the buried pipeline along its length at different applied cathodic currents (2 mA and 5 mA) measured using embed-dable reference electrode. The influence of applied cathodic currents are clearly seen.
The present invention of an embeddable reference electrode has the following advantages :-
1. The conventionally used platinum wire has been replaced
by 316 stainless steel wire, thereby contributing to the
ruggedness of the system. As the 316 stainless steel
does not dissolve in mercury, it offers an unique advan
tage as a rugged electrode.
2. The compatibility with the concrete medium has been enhanced by the cement plug provided at the bottom.
3. The stability of the electrode has been enhanced by encasing the mercury and the mercuric oxide in the fashion in which the new design has been proposed.
4. The new design helps in improving the stability and
electrochemical reversibility of the system by prevent
ing the chloride ion entering and contaminating the
saturated calcium hydroxide solution.
5. The new design has got the adaptability for embedding in concrete structures at any orientation.
6. The new design enhances the performance of the
mercury/mercuric oxide reference electrode up to a
o maximum temperature of 6 5 C.
7. Provision of a cement plug helps in minimising the IR
drop in the measurement of electrode potentials and
thereby increases the accuracy of measurements.
An embeddable reference electrode for use in concrete structures which comprises, an open ended tubular casing (8) of inert material, the bottom end of the said casing being plugged (7) with cement paste, characterised in that above the said cement plug (7) is provided with layers of saturated calcium hydroxide (6), mercuric oxide (5) and mercury (4) respectively, a stainless steel wire (3) is fixed onto the top end of the said casing (8) by teflon gasket (2) in such a manner that the said wire dips in the mercury (4) & mercuric oxide (5), the top end of the said wire (3) is provided with copper wire (1) for electrical connections.
An embeddable reference electrode for use in concrete structures substantially as herein described with reference to the examples and Fig. 1 of drawings accompanying this specification.
|Indian Patent Application Number||681/DEL/1996|
|PG Journal Number||43/2004|
|Date of Filing||29-Mar-1996|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG NEW DELHI-110001,INDIA.|
|PCT International Classification Number||E04C 5/07|
|PCT International Application Number||N/A|
|PCT International Filing date|