Method for monitoring chloride content in porous materials

Abstract

The present invention relates to a method for monitoring chloride content in porous materials, preferably in reinforced concrete structures, in which a voltammetric sensor comprising a metal electrode is used, characterised in that the electrode metal forms poorly soluble ionic compounds with chlorides and a potentiodynamic excitation signal is applied to the electrode, causing an oxidation-reduction process to take place that favours the formation of the poorly soluble ionic compounds. The formation of the poorly soluble ionic compounds produces a response in the form of electric current intensity that is related to the chloride content in the monitored material.

Description

[0001]DESCRIPTION PROCEDURE FOR MONITORING CHLORIDE CONTENT IN POROUS MATERIALS TECHNICAL FIELD The present invention relates to the field of chloride content monitoring systems in porous materials, especially in reinforced concrete structures, and more specifically to a method for monitoring chloride content. BACKGROUND OF THE INVENTION The effective monitoring of the presence of chlorides in porous materials is important because it affects the productivity of agricultural soils and the durability of reinforced concrete structures (EHA) subjected to marine environments and de-icing salts. On the one hand, anthropogenic activities cause excessive accumulation of chloride anion (Cl-) in soils, either due to the excessive use of fertilizers or atmospheric deposition from industrial sources. An overabundance of Cl- in soils can reduce their fertility and cause toxicity in crops.From the perspective of R&D, depassivation of reinforcement due to the presence of chlorides is one of the most common causes of failure in Reinforced Concrete Structures (RCS). This phenomenon particularly affects large structures located in marine environments, such as bridges, seawalls, and offshore platforms, as well as road bridges exposed to de-icing salt. The degradation of these types of structures requires very costly repair work and can also affect the safety of both people and the environment. Furthermore, premature repair or demolition of these large concrete structures generates excessive resource consumption and waste production, which jeopardizes the sustainable development of our societies.Therefore, the detection, estimation, and prediction of chloride presence in soils and concrete structures (CSS) are current challenges in the development of smart agriculture and in the monitoring of the lifespan of structures and the sustainable development of our society. For this reason, the detection, quantification, and prediction of chloride presence in CSS has been the subject of study for many years, using both destructive (laboratory testing) and non-destructive techniques. The latter offer the most promising results, as they allow the presence of chlorides to be monitored in real time, enabling the development of estimation and prediction models that allow for optimal structural interventions, i.e., minimizing economic costs and material resources while maximizing the structure's lifespan.The most widely used techniques are destructive and based on potentiometric and Volhard methods. They consist of obtaining the amount of free or total chlorides in concrete samples extracted from the structure, which are obviously usually slow and expensive, and do not allow continuous, real-time monitoring of the presence of this anion in the EHA. Furthermore, in recent decades, different non-destructive techniques have been developed for the determination of the amount of chlorides in EHA, for example: Zhang Z, Hu J, Ma Y, Wang Y, Huang H, Zhang Z, et al. “A state-of-the-art review on Ag / AgCl ion-selective electrode used for non-destructive chloride detection in concrete.” Compos Part B Eng [Internet]. 2020;200(August):108289 o Torres-Luque M, Bastidas-Arteaga E, Schoefs F, Sánchez-Silva M, Osma JF "Non-destructive methods for measuring chloride ingress into concrete: State-of-the-art and future challenges. Constr Build Mater [Internet].2014;68:68–81. These techniques can be classified according to the nature of the measurement signal as electrochemical, optical, or other types of electromagnetic signals (radiofrequency, laser, etc.). Most of these methods are very sensitive to changes in pH, humidity, or temperature, which entail a series of negative effects that reduce the accuracy and reliability of the measurements obtained. Among the electrochemical signal systems, the most researched and widely used in EHA, due to the great simplicity of the measurement system, are the embedded potentiometric sensor systems. Among them, those manufactured using Ag stand out due to its high reactivity with chlorides, for example, those reported in the following document: Tian Y, Zhang P, Zhao K, Du Z, Zhao T. “Application of Ag / AgCl Sensor for Chloride Monitoring of Mortar under Dry-Wet Cycles.” Sensors [Internet].2020 Mar 4;20(5):1394 o Jin M, Ma Y, Zeng H, Liu J, Jiang L, Yang G, et al.“Developing a Multi-Element Sensor to Non-Destructively Monitor Several Fundamental Parameters Related to Concrete Durability.” Sensors [Internet]. 2020 Sep 30;20(19):5607. In potentiometric systems used in EHA, the chloride ion-selective electrode (ISE) is usually made of Ag coated with AgCl (Ag / AgCl), because these sensors have a Nernstian-type response to variations in chloride and silver activity. Studies indicate that good results are obtained with this type of sensor even 4 years after being embedded in concrete. The measuring system manufactured with this type of sensor consists of Ag / AgCl ion-selective sensors (ISE) and a reference electrode (Ref), connected to the ends of a high-impedance voltmeter. The ISE is embedded in the concrete and the Ref may or may not be embedded.The standard equilibrium potential difference is measured over time between the ISE and the Ref with the voltmeter, so that it is related to the activity ratio of Cl- and Ag. +, allows the quantification of chloride concentrations in the pore solution of concrete. However, the functionality of these sensors for quantifying Cl- has limitations, such as their response is greatly affected by both temperature and the adsorption of other analytes on the sensor surface, they require frequent calibration, and they are not good at detecting very low concentrations of the analyte of interest due to interference from the OH- anion, which is usually found at very high concentrations (generally around 0.1 moles / liter). Furthermore, the reference electrodes used in the system tend to have a shorter lifespan than the structure being monitored. More recently, electrochemical sensors based on the application of the Impedance Spectroscopy technique have begun to be developed, such as the one defined in the patent by Torres Luque MM et al.ES2683618T3, and the sensor developed by Huang, B. et al. “Sensors and Actuators^: B . Chemical Hybrid cement composite-based sensor for in-situ chloride monitoring in concrete structures.” Sensors Actuators B Chem [Internet]. 2023;385(March):133638. This type of system is configured by two electrodes between which an alternating current signal is applied whose frequency changes over time. The results found in the bibliography show that this type of sensor measures changes in chloride concentration with good sensitivity. However, they still show interference with pH and temperature variations. Furthermore, the type of excitation signal is complex from the operational point of view of electronic equipment, so its implementation in real structures can be very difficult.Therefore, considering the shortcomings of current technologies, the development of a system for detecting, estimating, and predicting the presence of chlorides in AHA remains a challenge. Voltammetric sensors are widely used in various fields related to the characterization of dissolved systems and quality control, such as food technology or for monitoring wastewater treatment processes, but this technology has hardly been exploited in non-aqueous media, such as concrete.The application of voltammetric sensors in a porous medium such as concrete presents certain difficulties, but is promising because it can allow the detection and quantification of agents that affect the durability of EHA, as well as the development of control and prediction models related to their deterioration, resulting in these models being more precise than those developed with existing monitoring systems. EXPLANATION OF THE INVENTION The present invention is based on the application of voltammetric techniques in systems for monitoring reinforced concrete structures. An electrochemical sensor has been developed, applicable for use in porous materials such as concrete and mortar. In particular, a method capable of monitoring the chloride content in these media has been developed, when the electrochemical sensor is embedded in them.The sensor allows measuring the chloride content of concrete, but can be used in other media such as soil, chemical reagent storage tanks, wastewater, biological media, and the like. The present invention relates to a method for monitoring chloride content in porous materials, preferably in reinforced concrete structures. This method uses a voltammetric sensor comprising a metal electrode, with the particularity that the metal of the electrode forms sparingly soluble ionic compounds with chlorides. A potentiodynamic excitation signal is applied to the electrode, producing an oxidation-reduction process that favors the formation of sparingly soluble ionic compounds. The formation of said sparingly soluble ionic compounds produces a response in electric current intensity that is related to the chloride content in the monitored material.Preferably, the applied potentiodynamic excitation signal is rectangular or triangular in shape. Preferred embodiments of the present invention are described in the dependent claims. Throughout the description and claims, the word "comprise" and its variants are not intended to exclude other technical features, 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 be apparent 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 In order to complement the description being provided and in order to assist in a better understanding of the features of the invention, a set of drawings is attached as an integral part of said description, in which the following is represented for illustrative and non-limiting purposes: Figure 1A - Triangular potential excitation signal. Figure 1B - Graph showing the normalized electrical intensity response by the surface of the working electrode, versus the potential difference applied to the working electrode (voltage wave). Figure 2 - Rectangular wave signal. Figure 3A - Single-sensor configuration. Figure 3B - Multi-sensor configuration. Figure 4 - Response of the sensor embedded in concrete. Accumulated load: Continuous line - concrete without the presence of chlorides. Dashed line - concrete with the presence of chlorides. Figure 5 - Response of the sensor embedded in concrete.Electric current density. Continuous line - concrete without the presence of chlorides. Dashed line - concrete with the presence of chlorides. Figure 6 - Response of the Ag sensor embedded in concrete: Continuous line - concrete without the presence of chlorides. Dashed line - concrete with the presence of chlorides. Figure 7A - Correlation αCl vs. %ClvsCC. Figure 7B - Correlation of βCl vs. %ClvsCC. Figure 8A - Correlation of humidity variation vs. αCl. Figure 8B - Correlation of humidity variation vs. βCl. Figure 9 - Correlation σCl vs. %ClvsCC. Figure 10 - Correlation of cement quantity vs. m1. PREFERRED EMBODIMENT OF THE INVENTION The present invention relates to a method for monitoring the chloride content in porous materials. This method uses a voltammetric sensor, and its application in a medium such as concrete is a very novel aspect.According to a preferred embodiment of the present invention, a rectangular potentiodynamic excitation signal, as shown in Figure 2, is applied to the sensor in the potential range where the reaction of interest occurs, such that, after subjecting the sensor to an oxidative excitation pulse, a reductive relaxation pulse is applied in order to alter the surface of the sensor as little as possible (recovering its original metallic state), thereby increasing and ensuring its durability. According to another preferred embodiment of the present invention, a triangular-shaped signal, which may be of the type shown in Figure 1A, can be applied in the potential range where the reaction of interest occurs, in this case being the potential zone where the formation / nucleation reaction (oxidation) and reduction of the chloride compounds with the metal from which the electrode is formed takes place, as shown in Figure 1B.The voltammetric type Chloride sensor can be prepared from metals such as silver, gold, copper or lead, in which their ionic forms Ag. + , Cu + , Au + and Pb 2+ They form poorly soluble compounds with the species to be detected (solubility Kps<10 -6). The current intensity of the peaks related to the formation / reduction of the ionic compound formed with the Chloride anion varies with the concentration in the medium and due to the high reactivity shown by the cations generated during the electrochemically induced oxidation process of the metallic electrode in the presence of chlorides. From the processing and analysis of the electric current obtained as a response, the chloride content of the electrolyte in which the sensor has been embedded is estimated. Preparation of the Chloride Sensor As an example, the manufacture of a chloride sensor made using 99% purity metallic Ag is described (the purity limit is 90% allowed for any of the metals used). The procedure for preparing the sensor with any of the other metals mentioned (silver, gold, copper and lead) is similar.Electrodes can be manufactured using sheets or wires, which, for example, are sized as follows: • Sheets: 0.1 to 0.5 mm thick; 5 mm to 20 mm long; • Wires: diameters between 0.1 mm and 2 mm; lengths between 5 mm and 20 mm. The effective electrode length, understood as the one in contact with the concrete, will depend on the porosity and electrical resistivity of the material in which the sensor is embedded. In concretes with lower porosity and higher electrical resistivity, larger surface areas will be used. Table 1 specifies some recommendations according to the water-accessible porosity of the concrete obtained by the standardized test UNE 83980. Table 1 Recommended wire electrode dimensions according to water-accessible porosity of the concrete: Sensor System Operation The distribution of sensors in the reinforced concrete structure makes it possible to detect and quantify the presence of chlorides and their accumulation in the concrete mass over time. It allows the depth and velocity of the chloride advance front to be estimated at different points in the material mass. Depending on the requirements for system monitoring, two different configurations are proposed. Single-sensor Configuration A single-sensor configuration is shown in Figure 3A. An electrode is embedded in a cylinder of electrically conductive material (such as graphite or stainless steel), which acts as an auxiliary measurement electrode. Inside the cylinder, the electrical connection to the electrode is made using a multi-core cable covered with Teflon or any other polymer equivalent in terms of its air and water porosity values.Next, to protect the cable-electrode connection and electrically isolate the electrode from the cylinder of electrically conductive material, the cylinder is filled with epoxy resin. The cylinder of electrically conductive material that acts as the auxiliary measuring electrode must have an exterior surface area at least 40 times the useful surface area of ​​the sensor electrode. The cylinder of electrically conductive material is the counter electrode (CE) of the Cl sensor system. The electrical connection to the CE is made by means of a multi-core cable covered with Teflon; the junction between the cable and the auxiliary conductive cylinder is protected by epoxy resin. Multi-sensor configuration This configuration allows a single area to be monitored by several sensors, thereby increasing the reliability of the estimation result. A compact multi-sensor system is shown in Figure 3B.N sensors of the specified metals are embedded in the electrically conductive cylinder, in the same way as with a single electrode. The different wire sensors are located at known and fixed distances from each other. Measuring the concentration detected over time makes it possible to determine the velocity of the chloride front and estimate its depth over time. If the depth of the reinforcement is known, estimating the initiation time from which depassivation of the structure's reinforcements occurs and therefore the start of reinforcement corrosion processes becomes a simple calculation. Applied electrochemical techniques In the sensor system, the electrochemical technique used can be cyclic voltammetry in the form of a triangular wave or potential step voltammetry, where the excitation signal is of the rectangular wave type.A dimensionless parameter is obtained, independent of temperature variations, which is directly correlated with the amount of chlorides present in the concrete pore solution. Rectangular wave signal For the rectangular wave signal: the applied rectangular signal is characterized by the potential being ΔE during half a period. t=n≠0 (excitation pulse), and during the other half period it equals 0 (relaxation pulse). In this case, to determine the presence of the chloride anion, quantify it and minimize errors caused by variations in electrical resistivity due to variations in the humidity content of the material, two types of signals can be applied depending on the nature of the phenomenon to be evaluated. Two types of signals can be applied depending on their frequency. Excitation signal for non-faradic processes, related to the accumulation of charges at the sensor / material interface; it is applied at high frequencies (between 25 Hz and 100 Hz). Excitation signal for faradic processes, related to the oxidation and reduction phenomena that occur on the sensor surface; it is applied at low frequencies (between 1 Hz and 5 Hz). Pulses for the induction of non-faradic processes: The signal frequency can be between 25 Hz and 100 Hz (high frequency pulses).The maximum signal amplitude is 0.8 V and the minimum is -1 V. The signal amplitude (V) changes, in absolute value, at a rate of 50 mV / T, where T is the signal period. With this type of pulse, the electric current response depends on the ionic mobility in the concrete and the accumulation of charges in the double layer. Pulses to induce faradic processes: The signal frequency can be between 1 Hz and 5 Hz (low frequency pulses). The maximum signal amplitude is 0.8 V and the minimum is -1 V. The signal amplitude changes, in absolute value, at a rate of 50 mV / T, where T is the signal period. With this type of pulse, the electric current response depends mainly on the REDOX reactions that occur on the surface of the WE.In order to optimize the investment costs required for monitoring large reinforced structures, the two excitation and measurement techniques can be applied using two different electronic and connection configurations: Traditional 3-electrode configuration, where the working electrode (WE) is the sensor and the counter electrode (CE) is the cylinder of electrically conductive material shown in Figure 3 or the structure's own reinforcement. A reference electrode (REF), also embedded in the concrete, is used. 2-electrode configuration, where the working electrode (WE) is the sensor and the counter-pseudo-reference electrode (CER) is used either the cylinder of electrically conductive material shown in Figure 3 or the structure's own reinforcement. An example of the morphology of the applied excitation signals is shown in Figure 2.The applied potential range covers the area where the AgCl formation and nucleation (oxidation) reaction and the reduction of this same ionic compound occur. The design and duration of the applied potential step sequence make it possible to distinguish the metal oxidation processes that give rise to different types of oxides, hydroxides, or carbonates of the oxidized metals, and to distinguish these species from the metal chlorides formed during the process. The purpose of the cathodic potential step sequence is to ensure complete regeneration of the sensing electrode surface and, therefore, that the response obtained in another measurement sweep is not affected by the previous working history of the sensing electrode. Figure 4 shows the accumulated charge curves obtained from the current density curves in Figure 5.It can be easily identified that the maximum charge is correlated with the chloride concentration in the cementing matrix, since there is an increase in this as the chloride concentration increases. Triangular wave signal For cyclic voltammetry (CV) in triangular form: The sweep speed, in absolute value, can be between 20 mV / s and 70 mV / s. CV is applied in the potential range between 0.8 V and -1 V vs. SCE (saturated calomel electrode). An example of the morphology of the applied excitation signals is shown in Figure 1A. The applied potential range includes the area where the formation and nucleation (oxidation) reaction of AgCl and the reduction of this same ionic compound occur. An example of the sensor response to the type of excitation described is shown in Figure 6.A change in the morphology of the peaks associated with the reaction of the metal, in this case Ag, with the presence of chlorides is observed. The reduction peaks show a more stable behavior during tests in porous media (Figure 6, peaks P1, P2 and P3). Without the presence of chlorides, during the cathodic scan, peak P1 is associated with the reduction of the compounds formed by the metal with O2 and OH-. In the presence of chlorides, a new peak appears (P3, Figure 6) and peak P1 decreases. Since the formation of AgCl is favored over the formation of AgO and AgOH during oxidation, the reduction also favors the reduction of the most abundant product, giving rise to the predominant peak P3. These peaks P1 and P3 will be those used to determine the chloride concentration using the sensor.In order to optimize the investment costs required for monitoring large reinforced structures, the excitation and measurement technique can be applied using two different electronic and connection configurations: Traditional 3-electrode configuration, where the working electrode (WE) is the sensor and the cylinder of electrically conductive material shown in Figure 3 or the structure's own reinforcement is used as the counter electrode (CE). A reference electrode (REF) also embedded in the concrete is used. 2-electrode configuration, where the working electrode (WE) is the sensor and the cylinder of electrically conductive material shown in Figure 3 or the structure's own reinforcement is used as the counter-pseudo-reference electrode (CER).Data processing and chloride estimation for the non-Faradic rectangular wave signal The average value of the ionic circulation resistance (Rs) of the excitation pulses for the non-Faradic signal is obtained. To do this, the value of the electric current at time 0 of each of the high-frequency excitation pulses is determined by extrapolation, and applying Ohm's law, the value of Rs is calculated for each of them, making an analogy to the simple Randles circuit for electrochemical systems. With all the values, the arithmetic average value of this parameter is obtained. To obtain a value of Rs related to the chloride content and changes in humidity, the value of Rs is normalized with respect to the day of commissioning when the chlorides have not yet penetrated the concrete (Rs at exposure=0, Rs0). In accordance with the previous proposal, the correlation parameter, called αCl, is defined according to Eq.1. where: t: is the time at which the test is performed, α Cl : is the dimensionless parameter defined from the sensor response with the pulse signal for the induction of non-faradic processes, Rs t : is the value of the resistance to ionic circulation of the system for the t at which the test is carried out (Ω), Rs0: is the value of the resistance to ionic circulation of the system for the time of start of exposure (Ω). The coefficient α Cl It is directly related to the chloride concentration, as demonstrated below. The resistance that the electrolyte (concrete pore solution) presents between WE and CE to the passage of ions is known as Rs. This resistance is the sum of the resistances equal to the number of analytes in the electrolyte. In the pore dissolution of concrete, under "normal" conditions the predominant anion is OH-, so the rest of the ionic conduction branches can be ignored (Eq.3) Eq.3 In the case where chlorides are present, the ionic conduction branch associated with this anion (Eq. 4) must be considered. 1 1 1^^^^^^^ + ^^^^= + = ^ ^^^^ ^^ ^^^^ ^^^^^^^ ^^^^^^^^ · ^^^ ^^^^Eq.4 ^^^^ ^^^^ ^ ^ Substituting Eq.3 and Eq.4 in Eq.1 and simplifying, we obtain Conductance, the inverse of resistance, is directly related to ion concentration. Therefore, using this relationship in Eq. 5, we arrive at the expression that relates the defined parameter α Cl with the chloride concentration. [^^^^−] = ^^^^^^ · ^^ · [^^^^ −] Eq.6 where K is a constant and [OH-], if there have been no pH variations, it will also remain constant. Data processing and chloride estimation for the faradic rectangular wave signal The system’s response to the low-frequency faradic pulse rectangular wave signal is processed through the accumulated charge density. The charge is obtained from the integral of the intensity function with respect to time (Eq.7). This integral can be solved geometrically taking into account that the integral is equal to the area under the curve (Eq.8). The accumulated charge curve is defined at each time step according to Eq. 9. Dividing this value by the sensor surface area gives the accumulated charge density. ^^^^^^^^^^^^^^^^^^^ = ^^^^ + ^^^^−1 Eq.9where: Q: is the accumulated charge (C), i: is the electric current intensity (A), dt: is the time differential, qn: is the charge for a time instant dt (C), Qaccumulated: is the accumulated charge for a time (C). To obtain a maximum accumulated charge value due only to the reactions in which the Chloride anions are involved, the accumulated Qmax is normalized with respect to the value obtained when the penetration of chlorides has not yet occurred in the concrete (maximum accumulated charge at exposure=0, Qaccumulated MAX 0). On the other hand, taking into account that the auxiliary electrode and the reference will not always be at the same distance from the working electrode in all tests and for all electrodes, the cell constant will vary from one case to another, so to compensate for this variation the subtraction defined above is divided by the value of the maximum accumulated charge at tposition=0.According to the above, the correlation parameter β. cl is defined according to Eq.19. ^^^^^^^^^^^^^^^ − ^^^^^^^^^^ ^^Á^^ ^^^^^^^^^^^^^^^^^^ ^^Á^^ 0 ^^^^ = ^^ ^^^^^^^^^^^^^^^^^^ ^^Á^^ 0where: Q acumulada MÁX : is the value of maximum accumulated load for a period of exposure to an environment with chlorides t, Q acumulada MÁX 0 : is the value of maximum accumulated load when chloride penetration has not yet occurred. The coefficient β ClIt is directly related to the concentration of chlorides in the pore solution of the concrete as demonstrated below. The charge is defined according to Eq. 7, where i is the sum of all the faradic currents produced by the reaction of the electrode with different ions contained in the electrolyte and that can be expressed according to Eq. 11, Cottrell's equation, in this equation, the variation of the intensity with respect to time is expressed for a certain ion, as a function of its diffusion coefficient (D) and its concentration (C). 1 ^^ · ^^ · ^^ · ^ 2^^(^^) = ^ · ^^ ^ Eq.11 √ ^ · ^^ where: n is the number of electrons in the reduction or oxidation reaction of the analyte, F is the Faraday constant (96485 C / mol), S is the area of ​​the flat electrode in cm 2 , t the time in seconds, in the test. Therefore, solving the integral: In equation Eq. 12, the subscript a refers to all ionic species contained in the electrolyte and reacting with the electrode during the test, while m refers to the test duration during which the integral is performed. The diffusion coefficient and the analyte concentration are fixed for a given exposure time to the environment. The maximum accumulated charge will occur for a given t of the test (tQMAX). Thus, the maximum accumulated charge can be expressed as: Eq. 13 Furthermore, in the case of chlorides, the sum n a ·D a 1 / 2 ·C a , can be broken down by separating the term referring to the Cl- contained in the concrete pore solution. Eq.14 is replaced in Eq.19 and is obtained as a common factor, both in the numerator as in denominator, [2 · ^^1 / 2] ^^ ^^^^^^^^ 0 , so the terms cancel out. Eq.15 The chloride concentration at time 0 is equal to 0 (^^^^^^−0 = 0), and the concentration of the remaining analytes is assumed to be invariant over time. Eq. 16 To put it simply: Considering that the diffusion coefficients will remain practically constant with the exposure time, as well as the concentration of the rest of the analytes other than the anion 1 ^ ^^^ 2 chloride, then ^^ − ·^^ ^^^^− 1 will be equal to a constant that we will call K2. And Eq.17 ∑^^≠^^^^− (^^^^·^^^^ 2·^^ ^^ ) is as follows: ^^^^^^ = ^^2^^^^^^− Eq.18where K2 is a constant for a given concrete, since it depends on the rest of the analytes contained in the concrete pore solution and C Clthe Cl- concentration at a given exposure time. Data processing and chloride estimation for the triangular signal The system's response to the triangular signal is processed through the current density (j) of the main reduction peak, understanding as main the one with the highest absolute value of j, which in the example in Figure 6, would be peak P1 for the case of the condition without chlorides, and peak P3 for the case of the condition with chlorides. The electric current density (j) is defined as the electric current (i) normalized by the effective surface area of ​​the sensor, where i is the sum of all the faradic currents produced by the reaction of the electrode with different ions contained in the electrolyte. In the case where chlorides are present, Figure 6 shows how there is an overlap, with peaks due to other reactions (P2).To obtain a peak current density value due only to reactions involving chloride anions, j is normalized with respect to the value obtained when chloride penetration into the concrete has not yet occurred (main reduction peak current density, or highest absolute value at texposition=0, j0). Taking into account that the auxiliary and reference electrodes will not always be at the same distance from the working electrode in all tests and for all electrodes, the cell constant will vary from case to case, so to compensate for this variation, the previously defined subtraction is divided by the maximum accumulated charge value at texposition=0. In accordance with the above, the correlation parameter ^^ Cl is defined according to Eq.19. where: j t : is the electric current density of the maximum peak in reduction for a time from the beginning of the exposure t (A / cm 2), j0: is the electric current density of the maximum peak in reduction for a time from the beginning of exposure 0 (A / cm 2 ). The coefficient σ Cl It is directly related to the chloride concentration in the concrete pore solution as demonstrated below. The peak current in cyclic voltammetry can be expressed according to Eq. 20, Randles-Sevcik equation, in this equation, the peak electric current is expressed with respect to the number of electrons in the redox reaction (n), area of ​​the working electrode (S), diffusion coefficient for electroactive species (D), the sweep velocity (v), and the concentration of electroactive species (C). Thus, jt and j0 are defined as follows: In equations Eq. 22 and Eq. 23, the subscript a refers to all ionic species contained in the electrolyte and reacting with the electrode during the test, t refers to the exposure time. The diffusion coefficient and the analyte concentration are fixed for a given exposure time to an environment. Eq. and Eq. 23 are substituted in Eq. 19, canceling the terms that remain constant (vy 2.69 10 5 ). The chloride concentration at time 0 is considered to be 0 (^^^^^^−0 = 0), and the concentration of the remaining analytes is assumed to be invariable over time. To put it simply: Considering that the diffusion coefficients will remain practically constant with the exposure time, as well as the concentration of the rest of the analytes other than the anion 3 1 ^^ ^^^^ − ^^ 2 ·^^ ^^^^ − ^^2 chloride, then 3 1 will be equal to a constant that we will call K2. And the Eq. ∑^^≠^^^^− (^^^^0 2·^^ ^^0 2·^^ ^^0 ) 17 is as follows: ^^^^^^ = ^^3^^^^^^− ^^ Eq.27 where K3 is a constant for a given concrete, since it depends on the rest of the analytes contained in the concrete pore solution and C Cl the concentration of Cl- at a given exposure time. Example 1 Model for estimating and predicting the amount of chlorides in concrete (rectangular signal) As an example, Figure 7 shows, through empirical results obtained with an Ag sensor, a single-sensor type with a 2 cm long and 1 mm diameter thread, that the two defined parameters have a direct linear relationship with the percentage of chlorides, referred to the amount of cement per m 3 of concrete, contained in the concrete, showing R values 2greater than 0.92 (Figure 7). The free chloride content in the concrete was obtained according to the test collected in the RILEM TC 178-TMC standard. To obtain the parameter αCl, in the results presented in Figure 7, the signal frequency used was 25 Hz (period (T) 40 ms). In the case of obtaining the parameter βCl - the signal frequency used is 5 Hz (period (T) 200 ms). The tests were carried out with the sensor embedded in two types of concrete: CC1, concrete water cement ratio 0.8, without additions, with a quantity of cement per m 3 of 255 kg concrete. CC2, concrete water cement ratio 0.6, without additions, with a quantity of cement per m 3 of 340 kg concrete. As the percentage of chlorides refers to the quantity of cement per m 3concrete (CC), the slope of the fitting line will vary with CC, as shown in the graphs in Figure 7. In addition, the parameters αCl and βCl also vary with the moisture content of the concrete (Figure ). Therefore, the estimation model is based on two linear functions, one for each parameter, Eq.28 and Eq.29, where the estimated value is the percentage of chlorides as a function of the amount of cement per m 3of concrete. The parameters m1, m2, n1 and n2 will be constant for a specific quantity of cement and humidity conditions. These parameters must be corrected for the humidity and the quantity of cement through empirical equations. %^^^^^^^^ = ^^1 ^^^^^^ + ^^1 Eq.28%^^^^^^^^ = ^^2 ^^^^^^ + ^^2 Eq.29Furthermore, a prediction model is also defined for estimating the time it will take for the chlorides to reach the sensor height in a given percentage. The inputs for this model are the parameters obtained from the sensor response and the time it takes for the chlorides to reach the sensor height. The governing equations have been obtained empirically and through dimensional analysis.Example 2 Model for estimating and predicting the quantity of chlorides in concrete (triangular signal) As an example, Figure 9 shows, through empirical results obtained with a single-sensor Ag sensor with a 2cm long and 1mm diameter thread, that the defined parameter has a direct linear relationship with the percentage of chlorides, referred to the quantity of cement per m. 3 of concrete, contained in the concrete, showing R values 2greater than 0.92 (Figure 9). The free chloride content in the concrete was obtained according to the test collected in the RILEM TC 178-TMC standard. To obtain the parameter σCl, triangular cyclic voltammetry was used as the excitation signal (Figure 1A), with a sweep speed of 20 mV / s. Triangular wave voltammetry was applied in the potential range between 0.8 V and -1 V vs. SCE. The tests were carried out with the sensor embedded in five types of concrete: CC1, concrete with a water-cement ratio of 0.9, without additions, with a quantity of cement per m 3 of 225 kg concrete. CC2, concrete water cement ratio 0.8, without additions, with a quantity of cement per m 3 of 250 kg concrete. CC3, concrete water cement ratio 0.6, without additions, with a quantity of cement per m 3 of 315 kg. CC4 concrete, water cement ratio 0.5, without additions, with a quantity of cement per m 3of 385 kg concrete. CC5, concrete water cement ratio 0.4, without additions, with a quantity of cement per m 3 of 490 kg concrete. As the percentage of chlorides refers to the quantity of cement per m 3 of concrete (CC), the slope of the fitting line will vary with CC, as shown in the graphs in Figure 10. Therefore, the estimation model is based on a linear function Eq. , where the estimated value is the percentage of chlorides as a function of the amount of cement per m 3 of concrete. The parameter m1 will be constant for a specific quantity of cement and humidity conditions. This parameter must be corrected for humidity and cement quantity through empirical equations. %^^^^^^^^ = ^^3 · ^^^^^^ Eq.30 The results of the study demonstrate that the relationship of m1 with CC follows a function of the type ^^ =^^^^+^^ where a, b, c, and d are constants, and their values ​​can be obtained by fitting the real data to the theoretical function (Figure 10). In the case at hand, the equation for m1 as a function of CC is shown in Eq. 31 ^^ 0.67 · ^^^^ − 88.17 3 =2.17 ^^^^ − 460.69Eq.31 Furthermore, a prediction model is also defined for estimating the time it will take for chlorides to reach the sensor height in a given percentage. The inputs for this model are the parameters obtained from the sensor response and the time it takes for the chlorides to reach the sensor height. The governing equations have been obtained empirically and by dimensional analysis. Conclusions The method according to the present invention has the following advantages: - The sensor is capable of detecting the presence of chlorides in concrete exposed to a NaCl concentration of 0.1 m or higher. - The sensor has a high sensitivity, being able to differentiate the presence of chlorides when their percentage in the concrete with respect to the amount of cement is equal to or greater than 0.01%.- The parameter defined from the sensor response used to estimate the presence of chlorides has been verified to be independent of temperature. - The parameter defined from the sensor response used to estimate the presence of chlorides has been verified to be dependent on concrete moisture content, which is taken into account in the model. - The fit of estimated versus calculated values ​​gives a slope of 0.931 and an intercept of 0.039, with an acceptable R2 value of 0.90. - The sensor is capable of estimating the time at which a given percentage of chlorides will occur with an error typically less than five percent.

Claims

1. A method for monitoring the chloride content in porous materials, characterized in that: - a voltammetric sensor comprising a metal electrode is used, with the particularity that the metal of the electrode forms sparingly soluble ionic compounds with chlorides, - a potentiodynamic excitation signal is applied to the electrode which produces an oxidation-reduction process that favors the formation of sparingly soluble ionic compounds, - the formation of sparingly soluble ionic compounds produces a response in electric current intensity that is related to the chloride content in the monitored material.

2. A method according to claim 1, characterized in that the porous materials are made of concrete.

3. A method according to any of the preceding claims, characterized in that the metal of the electrode forms ionic compounds with chlorides with a solubility Kps ≤ 10 -64. Method according to any of the preceding claims, characterized in that the electrode metal is selected from the group comprising Ag, Cu, Au or Pb.

5. Method according to claim 4, characterized in that the electrode metal is Ag.

6. Method according to any of the preceding claims, characterized in that the electrode metal has a purity of ≥ 90%, preferably 99%.

7. Method according to any of the preceding claims, characterized in that the metal electrode is in the form of a sheet or a wire.

8. Method according to claim 7, characterized in that the sheet has a thickness of between 0.1 and 0.5 mm and a length of between 5 and 20 mm; and the wire has a diameter of between 0.1 and 5 mm and a length of between 5 and 20 mm.

9. Method according to any of the preceding claims, characterized in that the potentiodynamic excitation signal is applied to the electrode in a range of potentials that produces the oxidation reaction of the metal / Cl- ionic compound and its reduction.

10. Method according to claim 9, characterized in that the potential range has a maximum signal amplitude of 0.8 V and a minimum amplitude of -1 V.

11. Method according to any of the preceding claims, characterized in that the potentiodynamic excitation signal that is applied is rectangular or triangular in shape.

12. Method according to claim 11, characterized in that for the rectangular shaped signal, pulses are applied to induce non-faradic processes with a signal frequency of between 25 and 100 Hz. 13.

14. Method according to any of claims 12-13, characterized in that the amplitude of the signal changes at a rate of 50 mV / T, T being the period of the signal.

15. Method according to claim 11, characterized in that for the triangular-shaped signal, a sweep speed of between 20 mV / s and 70 mV / s is applied.

16. Method according to any of the preceding claims, characterized in that the voltammetric sensor is used in a single-sensor configuration comprising an electrode embedded in a cylinder of electrically conductive material. 17.Method according to any of claims 1-15, characterized in that the voltammetric sensor is used in a multi-sensor configuration comprising n electrodes embedded in a cylinder of electrically conductive material.

18. Method according to any of claims 1-12 or 14, characterized in that - excitation pulses for the non-faradic signal are applied to the electrode and. - the chloride content is calculated based on a correlation parameter αCl according to the following formula: 1 − 1 ^^^^ ^^ ^^ ^^^^0 ^^^^ = Eq.1 1 ^^^^0where: t: is the time at which the test is performed, Rs t: is the value of the resistance to ionic circulation of the system at the time t at which the test is carried out (Ω). Rs0: is the value of the resistance to ionic circulation of the system at the start of exposure (Ω).

19. Method according to any of claims 1-11 or 13-14, characterized in that - excitation pulses for the faradic signal are applied to the electrode and - the chloride content is calculated based on a correlation parameter β Cl according to the following formula: where: Q accumulated MAX: is the maximum accumulated charge value for a period of exposure to an environment with chlorides t. Q accumulated MAX 0: is the maximum accumulated charge value when chloride penetration has not yet occurred.

20. Method according to any of claims 1-11 or 15, characterized in that - triangular excitation pulses are applied to the electrode and - the chloride content is calculated based on a correlation parameter ^^ Cl according to the following formula: ^ ^ ^^ ^^ − ^^0 ^^^^ = Eq.209 ^^0where: jt: is the electric current density of the maximum peak in reduction for a time from the beginning of exposure t (A / cm 2 ) j0: is the electric current density of the maximum peak in reduction for a time from the beginning of exposure 0 (A / cm 2 ).

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