ES2968550A1 - REFERENCE ELECTRODE FOR MONITORING SYSTEMS OF REINFORCED CONCRETE STRUCTURES AND ITS USE (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Reference electrode for monitoring systems of reinforced concrete structures and its use. The invention relates to the reference electrode for monitoring systems of reinforced concrete structures, which is based on a metal/metal oxide 1/metal oxide 2 redox system and which comprises a metal sheet or wire (2), where the metal sheet or wire (2) is embedded in a cement mortar (1). The invention also refers to the use of said reference electrode for measuring corrosion potentials of embedded reinforcement or to be part of the configuration of sensor systems, or multisensors, with a stable measurement potential to apply a sequence of voltammetric procedures that They allow monitoring the presence of triggering and accelerating agents of corrosion. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

REFERENCE ELECTRODE FOR MONITORING SYSTEMS OF REINFORCED CONCRETE STRUCTURES AND ITS USE

TECHNIQUE SECTOR

The present invention relates generally to the field of concrete structures, and more specifically to the field of monitoring systems for concrete structures.

BACKGROUND OF THE INVENTION

Corrosion has been identified as the most common cause of loss of durability in reinforced concrete structures, generating large repair and maintenance costs, causing overconsumption of raw materials and energy, as well as environmental pollution problems.

To reduce the incidence of this problem, its causes must be known and its effects quantified. One of the processes that has the greatest impact on the degradation of structures due to corrosion is the existence of macrocell currents. This type of process appears due to the presence of large areas of armor in a passive state, in electrical contact with areas in active corrosion (electron sources). The access of oxygen to regions of passive armor causes a considerable increase in the corrosion rate of the active corrosion zones, which causes localized deterioration that can be very damaged in a short time, which is why a large number of works have been carried out to model the effects of this type of process.

The diagnosis of these processes requires the implementation in the structure of specific and non-destructive sensor systems that allow detecting and evaluating structural corrosion and its causes. For this, electrochemical techniques are usually used. To apply most techniques, a reference electrode (RE) is needed that does not polarize and maintains a stable electrochemical potential over time.

The measurement of corrosion potential (Ecorr) is one of the most used methods due to its simplicity and speed. It is based on measuring the electric potential difference (AE) that exists between the armature and the ER. The value of Ecoobtained is associated with the probability of corrosion of the reinforcement in the evaluated area. When this technique is applied to an already executed structure, the reinforcement must be uncovered to establish electrical contact and place an external ER on the concrete surface of the area to be evaluated. To determine the corrosion rate (Vcorr) of the reinforcement, the portable devices developed also incorporate external ERs, since they must measure the intensity versus the applied potential, if the technique is voltammetric, or the variation of the potential versus time, if the technique is It is galvanostatic. The most commonly used external ER are saturated calomel (Hg/Hg<2>Ch), silver/silver chloride (Ag/AgCl) and copper/copper sulfate (Cu/CuSO4).

Real-time and long-term monitoring of reinforced concrete structures presents great attractions, since it represents a reduction in costs because it minimizes inspection and maintenance operations and allows the acquisition and analysis of data, thus being able to dispense with qualified personnel who perform these operations.

To monitor the condition of a reinforced concrete structure, the Ecorre measurement is essential parameters. Recently, some systems incorporate potentiometric sensors with which additional information is obtained about the presence of chlorides or the carbonation of the concrete. Reliable monitoring of chloride diffusion or carbonation of the material requires continuous monitoring of the electrical potential of the sensors and therefore the use of reference electrodes.

In all cases, the reference electrode is currently an essential element for any system for monitoring structures. The ideal reference electrode must:

1) be robust enough to be embedded,

2) maintain stable long-term potential,

3) allow the passage of a small amount of current without substantially polarizing,

4) present an affordable cost and

5) that the manufacturing process does not pose an environmental risk.

However, Ag/AgCl, Hg/Hg<2>Cl<2>and Cu/CuSO<4>electrodes cannot be used in embedded systems, since they require periodic maintenance. Furthermore, the internal dissolution of these RE could pass into the concrete, contaminating it with dissolved chlorides or sulfates, which would cause local corrosion of the reinforcement.

And although at the end of 1970 the first solid-state reference electrodes began to be developed to be embedded and to control the corrosion potential of structures subjected to cathodic protection systems, after several decades of work, it has not yet been possible to develop electrodes that are sufficiently reliable in the long term.

Consequently, the use of pseudo-references (PsR) is common, electrodes whose potential varies depending on the properties of the concrete in which they are embedded. If the conditions are reliably known, a prior laboratory calibration of the PsR can be performed, allowing its use as ER. Although graphite (Duffó, Farina, & Giordano, 2009; A. Poursaee, 2009), graphene (Jin et al., 2019) and stainless steel (Brenna, Lazzari, & Ormellese, 2015) electrodes have been used as PsR, It is the metal/metal oxide (M/MO) type electrodes that offer the greatest guarantees. The most used M/MO are Mn/MnO<2>, Mn/MnO<3> (Feliu, González, Miranda, & Feliu, 2005), NiFe<2>O<4> (Karthick Subbiah et al. , 2018) and Ti/TiO<2>(Dong, Lin, Hu, Li, & Du, 2011; Muralidharan et al., 2007). Recent studies (Duffó et al., 2010) demonstrate that the Ti/TiO<2> type is sensitive to pH variations, while the Ag/AgCl type is affected by the concentration of chlorides and presents stability problems at low temperatures (Gao et al., 2010). The Mn/MnO type is the most stable in the face of possible variations that usually occur in concrete structures (Karthick Subbiah et al., 2018; Muralidharan et al., 2007) (access of chlorides, carbonation, changes in the concentration of oxygen, etc.). Although this type of PsR has been implemented in real structures and also as ER in chloride sensors, its medium-long term stability has not yet been demonstrated.

On the other hand, a solid-state Ag/AgCl reference electrode for monitoring corrosion in reinforced concrete structures is known from document CN108169109A. This electrode is based on the measurement of the potential difference between the reference electrode described in the document and the metallic armor that is to be monitored. For its manufacture, a "lost" mold PVC cylinder is used, inside this cylinder a gel is introduced that imitates the dissolution of the pore of the concrete and to ensure the ionic exchange with the medium that surrounds it, the gel is in contact through a porous plug located at the end of the electrode. It has been designed for the measurement of corrosion potentials in the reinforcement of reinforced concrete structures.

On the other hand, a reference electrode for measuring the corrosion potential of reinforcement in reinforced concrete structures is known from document CN101144790A. The electrode is based on the Ag/AgCl/KClsat redox system. This electrode has a cylindrical shape in such a way that in the center is the Ag/AgCl electrode, surrounded by a porous layer of three components (AhO<3>, KCl and PTFE with PTFE) and surrounding the other layers a layer of slurry of cement. Like the electrode mentioned in the previous paragraph, it has been designed for measuring corrosion potentials in the reinforcement of reinforced concrete structures.

A potentiometric multisensor system is also known from document WO9630741A1. A combination of electrodes is disclosed, specifically a system of three central electrodes: a silver electrode (Ag), the second metallic lead (Pb) and the third copper (Cu), the latter being able to be replaced by carbon (C), nickel (Ni) or palladium (Pd). In addition to a reference electrode, each electrode combination may also comprise a temperature sensor for measuring the temperature in the concrete adjacent to the electrodes of the combination. However, it is not a development of a reference electrode, but rather a sensor measurement system that, with the help of a reference electrode, measures the potential differences between them and the reference.

Currently, there are reference electrodes on the market suitable for embedding in concrete, such as the following:

The ERE20 reference electrode manufactured by the company Papworths Construction Testing Equipment (PCTE) ("PCTE - Papworths Construction Testing Equipment," n.d). It is a MnO<2> electrode with a corrosion-resistant casing that allows it to be embedded in concrete, both in construction and existing structures. Before use, it must be conditioned, for 48 hours, in a saturated calcium hydroxide solution.

This electrode has been on the market for more than 30 years and although its use is quite widespread, after tests carried out in the laboratory it has been detected that they are not as stable as claimed (Arup, Klinghoffer, & Mietz, 1997). In a stability study of these references embedded in a concrete slab without chlorides (low corrosion, passive steel) and in a slab with chlorides (high corrosion, active steel), the fairly high coefficient of variation has been observed in the period of time studied (300 days). This can cause erroneous corrosion measurements to be obtained.

The company Silvion ("Silvion Ltd," n.d.) has been marketing different Ag/AgCl reference electrodes such as WE10, WE50 or WE100 since 1987. All of them are reference electrodes that allow them to be installed embedded in reinforced concrete structures. permanent. The difference between the indicated references is the size, the amount of silver and the area of the measurement interfaces. While WE50 and WE100 are more commonly used for reinforced concrete, the WE10 electrode is often used for buildings with a metal frame. steel where aesthetics are important.

The MN15 reference electrode is marketed by the company Concrete Preservation Technologies ("Concrete Preservation Technologies Limited," n.d.). It is a Mn/MnO<2> electrode that can be used in existing steel or reinforced concrete structures. The resistivity of the mortar used in its installation must not exceed 20 kü.cm. Before application, the electrode must be immersed in water for a minimum of 2 hours and a maximum of 24 hours ("On the company's website. Concrete Preservation Technologies Limited,” n.d.) you can access different case studies where this reference electrode has been used.

The company Castle Electrodes ("Castle Electrodes," n.d.) markets different types of reference electrodes such as:

• RB15. This is a Mn/MnO<2> reference electrode. The core is wrapped in a casing with a mortar plug, which ensures good contact with the concrete and eliminates errors due to ion diffusion. The electrodes are calibrated under laboratory conditions against a saturated calomel reference electrode (CSE) and a calibration certificate is issued for each batch of electrodes.

• LD10, LD15 and LD25. They are Ag/AgCl electrodes designed to monitor steel potentials in reinforced concrete, steel frame buildings and buried steel. They are not suitable for submerged areas of seawater or splash areas. The electrodes are calibrated under laboratory conditions against a saturated calomel reference electrode (CSE) and a calibration certificate is issued for each batch of electrodes, but the stability of the electrode potential must be checked before installation. To do this, the electrode must be immersed in a 3% sodium chloride solution for a minimum of 2 hours and a maximum of 3. After this, the potential will be measured against a calomel electrode (SCE), which must be stable and oscillate. between ± 20mv with respect to the calibration values.

• Graphite Electrode. It is a pseudo-reference electrode designed to be embedded. It is not suitable for measuring absolute potentials or relative potentials over a period of weeks or months. Its potential is variable depending on the oxygen and pH levels of the concrete, and the type of graphite.

• M/MO reference electrode. This is a pseudo-reference composed of a titanium rod with mixed metal oxide coating. It is suitable for short-term potential measurements, such as in monitoring cathodic protection systems. Its potential is variable depending on the environment and pH.

• Zinc reference electrode. This is a high purity zinc rod often used as a reference in marine environments and occasionally in concrete. They are pseudo-reference electrodes that can be used for short-term potential measurements.

In view of the above, it can be concluded that most of the currently known electrodes are not sufficiently reliable in the long term. Furthermore, the few existing solid-state reference electrodes have a very high cost, which makes them unaffordable from a practical point of view.

The need to continue studying and improving reference electrodes is clearly deduced. Only in this way will it be possible to provide reliability to the monitoring systems for concrete structures.

EXPLANATION OF THE INVENTION

Today, electrochemical techniques are the most used for the study and evaluation of these types of processes in reinforced concrete structures. These monitoring techniques require a robust, reliable and long-lasting reference sensor. The electrode developed according to the present invention allows control and monitoring using non-destructive techniques to obtain real-time information on the factors that can favor corrosion processes.

The present invention refers to the reference electrode, as described in claim 1, which can be implemented in continuous monitoring systems of parameters related to the durability of reinforced concrete structures, such as corrosion speed and potential, pH, Cl- penetration or oxygen diffusion. Unlike the systems known in the state of the art, the present reference electrode has the following advantages:

- High durability and reproducibility.

- Stable electrochemical potential over time

- Low coefficient of variation of potential with temperature.

- Low coefficient of variation of potential versus pH.

- Low coefficient of variation of the potential in the presence of chlorides.

- Zero maintenance cost.

- Low manufacturing cost.

Specifically, the present invention refers to the reference electrode for monitoring systems of reinforced concrete structures, based on the metal/metal oxide 1/metal oxide 2 redox system (M/MO1/MO2), which comprises a sheet or a metal thread, said sheet or thread embedded in a cement mortar.

The present invention also refers to the use of the reference electrode for measuring corrosion potentials of embedded reinforcement or to be part of the configuration of sensor or multisensor systems.

Preferred embodiments of the present invention are described in the dependent claims.

Throughout the description and claims the word "comprises" and its variants are not intended to exclude other technical characteristics, additives, components or steps. Furthermore, the word "comprises" includes the case "consists of." For those skilled in the art, other objects, advantages and features of the invention will emerge partly from the description and partly from the practice of the invention. The following examples are provided by way of illustration, and are not intended to be limiting of the present invention. Furthermore, the present invention covers all possible combinations of embodiments indicated herein.

BRIEF DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, a set of drawings is attached as an integral part of said description, where, with an illustrative and non-limiting nature, the following has been represented. following:

Figure 1.- Shows the M/MO1/MO2 redox reaction, taking as an example the one that occurs on the surface of the Ni electrode.

Figure 2.- Shows the diagram of the reference electrode.

PREFERRED EMBODIMENT OF THE INVENTION

The present invention discloses a new type of solid state reference electrode of the metal/metal oxide 1/metal oxide 2 (M/MO1/MO2) type, based on the stability of the OCP of the REDOX system, embedded in a cement mortar (1), for application in the measurement and/or control of the electrical potential in the reinforcement of reinforced concrete structures. The developed electrode is characterized by a very stable electrical potential over time and very low or no maintenance needs.

The reference electrode comprises a metal sheet or wire (2), preferably it is Ni, Cu or Co and the redox system corresponding to M/MO1/MO2 is Ni/Ni(N)/Ni(IN), Cu/Cu (I)/Cu(II) or Co/Co(N)/Co(NI). So the reference electrode can be generated from the following metals: Ni, Co and Cu, taking into account the high stability of the Ni(M)/Ni(MI), Cu(I)/Cu(II) reactions, Co(N)/Co(NI), in alkaline media. The different electrodes can be previously subjected to thermal treatments to generate an oxide layer of adequate thickness, in order to promote the stability of the reactions.

The electrodes can be manufactured with the specified metals using sheets or threads, according to the following dimensions:

Sheets: 0.1 to 0.5 mm thick

Threads: 1mm to 5mm thick

The reference electrode according to the present invention can be used to measure corrosion potentials of embedded reinforcements, but it has also been designed to be part of the configuration of sensor systems (or multisensors) with a stable measurement potential to apply a sequence of voltammetric procedures that allow monitoring the presence of corrosion triggering and accelerating agents.

The electrochemical reaction associated with the potential acquired by the system occurs at the metal/mortar interface; this interfacial region has the appropriate chemical conditions (humidity, ionic conductivity and pH) that guarantee its electrical response over time.

These conditions are defined according to the following parameters:

- pH between 12 and 13.

- Internal relative humidity in the porous system of the material very close to the saturation value.

- Air permeability coefficient (k) of the functional material according to UNE 83981 between 413 x 10-18 m2 to 1.5 x 10-19 m2

According to the invention, medium (fc<28>days between 25 and 50 MPa) or high strength cement mortar (fc<28>days >50 MPa) is used.

The medium or high compact mortar that surrounds the metal electrode acts as a porous partition that ensures the balance of the electronic and ionic current involved in the operating principle of the system. The dosage and composition of the mortar has been specifically designed to confer stability properties to the pseudo reference. Its high compactness is achieved by using CEM I 52.5 and by sieving the aggregate, achieving a tmax ^ 1mm and a high percentage of fines.

CEM I 52.5, according to Royal Decree 256/2016, of June 10, which approves the Instruction for the reception of cements (RC-16), is a Portland cement with a Clinker content between 95-100 % and between 0-5% of minority components, its nominal resistance at 28 days being equal to or greater than 52.5 MPa.

On the other hand, in the mortar the amount of chlorides and other soluble impurities, such as sulfates, Mg2+, bicarbonates or sulfides, has been minimized by washing the aggregate with distilled water and using distilled water instead of tap water to carry out the kneading the mortar. In this way, the quantity of these species is below the limits established in the standard (Spanish Structural Code), understanding that the materials used in the manufacture of the mortar comply with European Regulations and therefore the content of these compounds is subject to the same. . The limit for sulfates and sulfides will be 0.8% and for chlorides 0.05% of the weight of the aggregate sample.

The base dosage of the mortar is that established in Table 1, this may be modified depending on the requirements of the functional material according to the conditions of the structure (requirements of the concrete used):

- by adding silica fume or aluminum silicate in a proportion between 0.22% and 21% of the weight of the cement

- aggregate humidity correction

- quantity of cement can vary by ± 200 kg depending on the additions

- water/cement ratio between 0.3 and 0.5.

Table 1. Base dosage of the functional material

Superplasticizer is an additive that is used to reduce the amount of water in concrete, its percentage is limited by the supplier.

The reference electrode according to the present invention in a preferred embodiment has a cylindrical shape with a conical tip finish (Fig. 2), in order to facilitate its embedding in fresh reinforced concrete structures. Having this shape will offer less resistance when must be inserted in the places of the structure that you want to monitor.

This form also facilitates its implementation in already forged structures. To do this, a cylindrical drilling of the appropriate diameter and depth must be made, and inserted into the existing or already set work with a small amount of repair mortar, avoiding the formation of air bubbles that could worsen its electrical connectivity.

Preparation of the reference electrode

According to Fig. 2, for the manufacture of the reference electrode, as an example, conical bottom tubes made of polyethylene (PE) or polypropylene (PP), or any other polymer with similar properties, are used as manufacturing molds. 29.5 mm external diameter, 27.2 mm external diameter and 115 mm high, where the mortar (1) (functional material) is poured in a fresh state.

Sheet or wire (2) of Ni, Co or Cu is used with a purity of no less than 90%, preferably higher. Prior to manufacturing, the metal is mechanically polished using sandpaper of different granulometry, alumina and diamond powder. The electrode is placed in the central axis of the cylinder.

The electrical connection is made using a multi-wire cable (3) or a shielded multi-wire coated with a Teflon polymer or polymers derived from it, with low porosity and permeability to oxygen and water vapor. The connection between cable and metal is protected by a coating (4) of hydrophobic nature and low oxygen permeability, such as an epoxy-type resin or photocurable resins.

As the active region of ionic circulation, a cement mortar of medium or high compactness is used, composed of a Portland cement type CEM I 52.5 and limestone and/or siliceous aggregate (tmax 1mm), with a water/cement ratio between 0.3 and 0.5. With this cementing paste, the mold is filled, and the electrode (Ni, Co or Cu) is then embedded. Finally, the system is immersed in an ultrasonic bath to homogenize the material and eliminate any air (dissolved or in the form of small bubbles) that the mixture may contain.

This mixture is allowed to set at room temperature for 12 hours, then, without removing the polymer mold tube, it is filled from the top with epoxy resin until the height of the mold is complete. The resin is allowed to solidify in ambient conditions for 12 hours and then the sensors are placed in a curing chamber for 28 days (T=23 ± 2°C, HR«99%).

The mortar pore solution replaces the solution used in electrodes of this type for in situ measurements. Its high alkalinity (pH between 12 and 13) favors the stability of the zero current potential (OCP) for the REDOX M/MO1 System. /MO2. The high compactness of the mortar helps to keep the chemical composition of the system constant (salt concentration, pH of the solution, etc.) and the high thermal inertia of the material means that temperature variations do not have a high impact on the potential. Therefore, the mortar used gives the system zero need for maintenance and high potential stability over time.

Once the concrete curing time has elapsed (28 days), the polymer mold tube is removed.

The reference electrode according to the present invention does not require any type of maintenance and is especially characterized by the stability of its electrical potential and its low coefficient of variation of the potential with respect to temperature in addition to its durability over time. This behavior is associated with the OCP value of the REDOX reactions shown in Fig. 1, when the Ni is embedded in a mortar matrix of high compactness and practically zero chloride content. The mortar jacket acts as an ionically conductive membrane. And the mortar of medium or high compactness will ensure that the conditions in the environment of the metal electrode remain constant and that the liquid junction potential is minimized, thus ensuring that the potential of the pseudo reference remains stable over time.

It has been verified that the reference electrode according to the present invention shows high stability both submerged in aqueous medium and embedded in hardened concrete. The measured variations are (-0.092 ± 0.02) V (37 days of monitoring) in aqueous medium and (-0.095 ± 0.012) V (21 days) embedded in hardened concrete.

The variation of the potential of the reference electrode according to the present invention with temperature is practically negligible, being (-0.86 ± 0.18) mV/K.

The reference electrode according to the present invention has been developed as an auxiliary component for use in voltammetric type sensor systems, or in multisensor systems (for example, electronic tongues type) that can operate autonomously, without any type of maintenance, during several decades. The system will allow periodic monitoring of reinforced concrete structures, in order to evaluate their condition and evolution over time.

This type of electrodes have special characteristics that make them useful for implementation in autonomous and remote systems for continuous monitoring of physicochemical parameters.

Claims (13)

1. Reference electrode for monitoring systems of reinforced concrete structures, characterized in that it is based on a metal/metal oxide 1/metal oxide 2 redox system and comprising a metal sheet or wire (2), where The metal sheet or wire (2) is embedded in a cement mortar (1). 2. Reference electrode according to claim 1, characterized in that the metal is Ni, Cu or Co and the corresponding redox system is Ni/Ni(N)/Ni(IN), Cu/Cu(I)/Cu(II) or Co/Co(N)/Co(IN). 3. Reference electrode according to claim 1, characterized in that the metal is Ni and the corresponding redox system is Ni/Ni(N)/Ni(IN). 4. Reference electrode according to any of the previous claims, characterized in that the sheet has a thickness of between 0.1 and 0.5 mm. 5. Reference electrode according to any of claims 1-3, characterized in that the wire has a thickness of between 1 mm and 5 mm. 6. Reference electrode according to claim 1, characterized in that the cement mortar is of medium (fc, <28> days between 25 and 50 MPa) or high resistance (fc, <28> days > 50 MPa). 7. Reference electrode according to claim 6, characterized in that the amount of chlorides has been minimized below 0.05% and the amount of sulfates and sulfides below 0.8% of the weight of the aggregate sample. 8. Reference electrode according to claim 6, characterized in that the cement is of the Portland CEM I 52.5 type. 9. Reference electrode according to any of claims 6-8, characterized in that the mortar is made of limestone and/or siliceous aggregate of tmax 1mm. 10. Reference electrode according to any of claims 6-9, characterized in that the mortar has a water/cement ratio between 0.3 and 0.5. 11. Reference electrode according to any of the previous claims, characterized in that it has a cylindrical shape with a conical tip finish. 12. Use of the reference electrode according to any of claims 1-11, for measuring corrosion potentials of embedded reinforcement. 13. Use of the reference electrode according to any of claims 1-11, to form part of the configuration of sensor or multi-sensor systems.