ES2292353B2 - PROCEDURE FOR ELECTROCHEMICAL EXTRACTION OF REINFORCED CONCRETE CHLORIDES USING A DRIVING CEMENT PASTE AS AN ANODE. - Google Patents

Abstract

Electrochemical extraction procedure of reinforced concrete chlorides using a conductive cement paste as an anode. The invention consists in the application of conductive cement pastes as an anode in the technique of electrochemical extraction of chlorides (E. E. C.) used in the construction sector. The conductive character of cement pastes is conferred by the addition of properly conductive materials such as carbonates in the form of dust or fibers. The invention has the advantage that the anode, like a paste, can be projected in the form of a thin layer adapting to any type of surface with the possibility of reuse. This process is most effective in traditional methods based on the use of rigid anodes.

Description

Electrochemical extraction procedure of reinforced concrete chlorides using as a anode a paste of conductive cement

Technical sector

Building. Pathology and Rehabilitation of structures

State of the art

E. E. C. is a non-destructive technique of corrosion prevention of steel reinforcement, main Pathology of structural concrete. It began to be used to early seventies as an alternative to protection cathodic [1-3], being studied with growing interest so far with the introduction of successive improvements.

DR Lankard , JE Slater , WA Hedden , D. E :. Niesz , Neutralization of chloride in concrete, Batelle Columbus Laboratories, Federal Highway Administration (USA), Report No. FHWA-RD-76-60, ( 1975 ) 1-143.

JE Slater , DR Lankard , PJ Moreland , Electrochemical removal of chlorides from concrete bridge decks, Materials Performance, 15 ( 1976 ) 21-26.

GL Morrison , YP Virmani , FW Stratton and WJ Gilliland , Chloride removal and monomer impregnation of bridge deck concrete by electro-osmosis, Kansas Department of Transportation, Federal Highway Administration (USA), Report No. FHWA-Ks-RD-74-1 , ( 1976 ) 1-38.

Between 1986 and 1994 they got patented various procedures to apply the technique of E. E. C. [4-7].

OR. Vennesland , OA Opsahl and AP Russell-Rayner , Removal of chlorides from concrete, European Patent Application number 86302888.2, Publication number 0 200 428 ( 1986 ).

OR. Vennesland , OA Opsahl and JB Miller , Process for rehabilitating internally reinforced concrete by removal of chlorides, European Patent Application number 90108562.1, Publication number 0 398 117 ( 1990 ).

H. Roper , Method for inhibiting concrete cancer, World Intellectual Property Organization, International Patent Publication number WO 93/21130 ( 1993 ).

JE Bennett , JR Blasius , TA Mitchell , TR Turk and TJ Schue , Apparatus for the removal of chloride from reinforced concrete structures, US Patent number 5296120 ( 1994 ).

It basically consists of applying a current continues between the steel reinforcements and an electrode that is arranged externally on the concrete surface. The armor of steel are the negative pole (the cathode) and the external electrode the positive pole (the anode). How chlorides are charged ions negatively, they migrate due to the established electric field from steel trusses through concrete pores to the positively charged external electrode, drawing from this way notable percentages of them in a short period of time [8, 9].

C. Andrade , M. Castellote and C. Alonso , An overview of electrochemical realkalisation and chloride extraction, in DWS Ho, I. Godson and F. Collins Eds., Proceedings of 2nd Int. RILEM / CSIRO / ACRA Conference on Rehabilitation of Structures , Melbourne, Australia ( 1998 ) 1-12.

J. Tritthart , Electrochemical chloride removal: an overview and scientific aspects, in J. Skalny and S. Mindess Eds., Materials Science of Concrete V, published by the American Ceramic Society, Westerville, Ohio, USA ( 1998 ) 401-441.

Explanation of the invention.

Traditionally the investigation of cementitious materials used in civil works and building has been traditionally oriented to the study of its properties mechanical due to its main and unique structural function. Do not However, lately there has been a new trend in this line; the integration of other properties that allow it to have functions complementary, becoming materials "multifunctional" [10].

DDL Chung , Electrically conductive cement-based materials, Multifunctional cement-based materials, Marcel Dekker, Inc., The State University of New York, Buffalo, New York, USA, 2004 .

There are various categories within "multifunctional" cementitious materials; one of them are conductive cementitious materials. Electrical properties are achieved through conductive additions such as materials carbonaceous (for example, graphite powder or carbon fibers).

The innovation of this patent is the viability of application of a conductive cement paste as an anode in the E. E. C. technique for its potential advantages over others most usual anodes, such as the possibility of being projected in Thin layer shape, adapt to any type of surface or the possibility of reuse; In short, its versatility.

Materials and preparation of concrete specimens contaminated with chloride ions

Portland cement was used with filler (filler) limestone CEM II / B-L 32,5N, sand and gravel limestones crushing with maximum size 20 mm and distilled water with chloride sodium dissolved to introduce 2% chlorides with respect to mass of  cement. Two different dosages were followed that were They differ fundamentally by the ratio a / c (water / cement), and consequently, in the porosity and compressive strength, denominated HM (bad concrete) and HB (good concrete) (Table 1).

100

The type test tube consisted of a concrete cube 18x18x12 cm with a 16x16 cm metal mesh embedded in horizontal position and 6 cm from the concrete surface. He metal mesh was made up of 6 steel bars in each one of the two perpendicular directions, 16 cm long and 2 mm in diameter, welded by tapping with a wheelbase 3 cm and with two copper wires welded with tin to it. It has an average electrical resistance of 0.54. A hollow cube without PVC bottom was embedded 0.5 cm in the concrete surface to Pool mode to accommodate the conductive cement paste and the electrolyte.

Concrete kneading was done following standardized procedure in two tons, among which placed the metal mesh on some supports of the mold and sewed by electric needle vibrator. Flush the mold was placed the pool with the help of a mace. Then it was covered with a layer of polyethylene and was introduced into a humidity chamber (RH ≥90% and T = 20ºC) for 28 days, taking them out only at 24 hours for demoulding and the application of a resin on the electrodes and a concrete waterproofing mortar to avoid future electrolyte losses (Figures 1 and 2).

Materials and preparation of conductive cement paste

Portland cement was used with filler (filler) CEM II / B-L 32.5N limestone, distilled water and addition of graphite powder by 50%, in relation to / c = 0.5. Owns a average electrical resistance and porosity of 4 \ Omega-50.4%. Of these high values of conductivity and porosity depends largely on the success of your application. These dosages are the result of a study previously carried out by the authors on resistance electronic and electrolytic conductive cement pastes with addition of carbonaceous materials.

After 28 days of curing the concrete, it they practiced four holes in the midpoints of the four faces of the pool and at a height that depended on the thickness of the paste, and four other electrodes were passed through them graphite so that there is a hole in the center of 3x3 cm. He kneading the conductive cement paste was done following a standardized procedure, applying it superficially with a spatula inside the pool in two tons the fixed thickness (2 or 0.7 cm) as if it were a plaster. He turned to enter the test tube in a humidity chamber for 7 days. Past the paste curing time, the water used was placed as electrolyte and the E. E. C. could be started (Figure 1).

Electrochemical extraction treatment

The negative pole of a power supply analog direct current connected to the metal mesh (cathode) using the two copper wires and the positive pole connected to the conductive cement paste (anode) by means of graphite electrodes Between the cathode and the negative terminal of the power supply is permanently connected in series a coulometer to control the past load. Between the anode and the positive terminal of the power supply connects daily in series a multimeter that will work as an ammeter to keep the intensity constant. This same multimeter is will also use as a voltmeter in parallel between one of the graphite electrodes and one of the welded wires of the test tube to record the increasing variation of potential. It was also used A clock to control the treatment time. The level of Electrolyte was kept constant daily. Within this assembly can be introduced so many specimens and sources of serial feeding as desired; in this investigation it they got to connect three test tubes and two power supplies in Serie.

The EEC was performed galvanostatically at a constant current intensity of 2A / m2, reaching voltages per test tube of up to 18V and using charge densities of 10 6 C / m2 for a first treatment and of 4 · 10 6 C / m2 for one second (in total, 5 · 10 6 C / m2). The rest time elapsed in case of two treatments was seven days in a humidity chamber without electrolyte. Both the load density and the current intensity used were calculated based on the exposed concrete surface (which coincides with that of the cathode and that of the anode) and is equal to
16 x 16 cm 2 = 0.0256 m2, resulting in 51.2 mA, 25600C and 102400C respectively. The duration of the treatments was approximately 139 and 556 hours.

Total Chloride Determination

Sampling was done before starting E. E. C. treatments and after finishing both having Let the specimens dry for 24 hours to obtain samples totally dry A sample was used to extract the samples. drill with a drill bit 18 mm in diameter [11].

MA CLIMENT , G. DE VERA AND E. VIQUEIRA , "Bit shape geometric considerations when sampling by dry drilling for obtaining chloride profiles in concrete", Materials and Structures, 34 ( 2001 ) 150-154.

This was always done between the surface of the anode and cathode of the specimen in the geometric center of these in the case of the first treatment and in the geometric center of one of the four quadrants delimited by the four electrodes for a second treatment It was drilled at 5 mm depth intervals thick except the first and last, usually 7 mm, trying to get the centers of gravity of the samples taken before and after the E. E. C. match so that the profiles of distribution be comparable. Once the samples are removed, the hole with a plastic bag and filled with mortar PCC 20 repair of Bettor MBT. The plastic coating serves to prevent chlorides from accessing the repair mortar and a consequent sensitive dilution.

Water sampling was also performed used as an electrolyte recording the final volume.

The determination of total chlorides in the case Solid sample analysis was performed by titration potentiometric suspension of the same attacked with acid diluted nitric while in the case of liquid samples it was not necessary [11].

MA Climent , E. Viqueira , G. de Vera and MM López-Atalaya , Analysis of acid-soluble chloride in cement, mortar and concrete by potentiometric titration without filtration steps, Cement and Concrete Research, 29 ( 1999 ) 893-898.

The percentage of chlorides has been expressed referred to total mass and not cement mass to make comparable the results of the analysis of cement paste samples and of concrete, with different proportion of cement. Represented graphically the percentage of total chlorides and the depth of center of gravity of its corresponding drilling volume, it Obtain chloride distribution profiles. The difference between distribution profiles before and after each treatment allows us to calculate specific efficiencies of the treatments, expressed as percentages of chlorides removed at each point analyzed.

From the analysis of solid samples and liquid mass balances were performed to see what percentage of chlorides remained in the concrete, in the paste of conductive cement, in the electrolyte or had been removed by electrolysis.

Results

The origin of the investigation was to verify the Efficiency of E. E. C. using a conductive cement paste as an anode and compare it with another type of more traditional anode to pattern mode This anode was the titanium dioxide mesh of Ruthenium (TiRuO2) for commercial use, used with results satisfactory in the preparation of the Doctoral Thesis on E. E. C. of Mrs. M ^ a José Sánchez de Rojas Noguera in it laboratory [13,14]. Cement paste dosing was followed conductive commented above, in relation to / c = 0.5 and replacement of graphite powder by 50% (PC PG 50% (Sust.)). This it was applied in a thickness of 2 cm and a first treatment was performed of 10 6 C / m2 load density.

M ^ a José Sánchez de Rojas Noguera , Doctoral Thesis: Electrochemical extraction of chlorides from reinforced concrete: study of different variables that influence treatment efficiency, Department of Construction Engineering, Public Works and Urban Infrastructure, University of Alicante , Spain, 2004 .

MA Climent et al . Effect of type of anodic arrangements on efficiency of electrochemical chloride removal from reinforced concrete, ACI Materials Journal, 103, No.4, ( 2006 ) 243-250.

If the results obtained are compared (Figures 2 and 3), it is verified that in both cases they are actually extracted considerable amounts, 50.60% and 40.84%, being the efficiency of the TiRuO2 mesh 10% smaller than that of the paste. If concrete of the latter, highlights the high percentage of chlorides that accumulate in it (32.08%), accumulation that logically does not It is produced in the mesh, eliminating chlorides by electrolysis by 38.64% or remaining in the electrolyte in a small percentage of 2.2%. This indicates that the pasta has acted as filter being removed by electrolysis in the form of chlorine gas only a 18.52% being the chloride content present in the electrolyte negligible.

Therefore, it was positively confirmed in a first approach the possibility of applying a cement paste conductive as an anode in the E. E. C.

An important point given the significant accumulation of chlorides in pasta, was the study of how to avoid a more than likely redistribution of them again towards the concrete after the treatment of E. E. C. The redistribution of Chlorides in concrete consist of equalizing the concentrations at different depths by diffusion, due to a concentration gradient, a phenomenon observed by others researchers. This produces a regression effect towards the initial situation.

To avoid such a negative effect there were two options:

-

Remove the paste once the EEC is finished. This is facilitated given its high porosity. This option is not desirable because it means giving up a priori the possibility of reusing the same paste in future and necessary actions of application of the EEC. O well

Modify some of the parameters of the experiment or paste to try to prevent chlorides from accumulate in it, such as the past load density or its thickness. Both variables were developed below. On the one hand it reduced the thickness of the paste up to 7 mm and on the other the load passed to a total of 5 · 10 6 C / m2.

Comparing the results obtained with it 2 cm and 7 mm thick paste and identical load density 5 · 10 6 C / m2 (Figures 6 and 7), an increase is observed of efficiency goes from 50.62% to 64.61% and what is more importantly, the accumulation of chlorides dramatically decreases 60.93% to 5.42%, allowing 33.13% to be eliminated and a 26.06% remain in the electrolyte. This could be because decreasing the thickness of the paste reduced its ability to accumulation of chlorides while it was easier for the chlorides can transfer it corresponding that residual percentage 5% to chlorides in full motion.

Until now the application of the technique with this new type of anode it had been carried out using a high-ratio concrete a / c to simulate an old concrete and porous. We wanted to redo the experience but on a concrete of better quality, low ratio a / c simulating a concrete of recent commissioning and compact.

A first treatment of 10 6 C / m2 using the PG 50% PC (Sub.) e = 7 mm it was clearly manifested insufficient because the efficiency was very low. Comparing the results obtained between the two concrete dosages for a second treatment of 5 · 10 6 C / m2 (Figures 8 and 9), clearly increases efficiency in the case of concrete compact compared to a first treatment but does not reach the level of the case of porous concrete. Efficiency decreases almost 25% (from 64.61% to 40.89%). But it should be noted that, while a higher concrete quality makes E. E. C. difficult, it also makes it difficult Chloride contamination. Note that the corroboration of the conclusion of the previous experience, as the loss is maintained accumulation of chlorides in pasta with similar percentages.

Description of the figures

Figure 1: Scheme of the application of the technique electrochemical extraction of chlorides (EEC) using as anode a conductive cement paste (1) pool, (2) paste conductive cement, (3) metal mesh, (4) concrete.

Fig. 2 and 3: Chloride content (?: Initial porous concrete,:: Conductive cement paste, ratio a / c = 0.5, replacement of graphite powder by 50%, 2 cm of thickness; X: TiRuO_2 mesh. Past load for both types of anode: 1 · 10 6 C / m2).

Figures 4 and 5: Chloride content (?: Initial HP, \ sqbullet: HP PC PG 50% (Sub.) E = 2 cm 5 · 10 6 C / m2, x: HM PC PG 50% (Sust.) E = 7 mm 5 · 10 6 C / m2).

Figures 6 and 7: Chloride content (?: Initial HC, \ sqbullet: HC PC PG 50% (Sub.) E = 7 mm 5 · 10 6 C / m2, x: HB PC PG 50% (Sust.) E = 7 mm Irrigation by drip 5 · 10 6 C / m2).

Claims (5)

1. Procedure for electrochemical extraction of reinforced concrete chlorides using the concrete reinforcement itself as an cathode and an external anode characterized in that said anode comprises: cement paste and other materials that make it conductive, applying a layer of wet cement paste resulting on one of the external faces of the reinforced concrete. 2. Procedure for electrochemical extraction of reinforced concrete chlorides according to claim 1 characterized in that the conductive cement paste acting as an anode is composed, at least by:

to)

cement

b)

Water

C)

conductive carbonaceous material.

3. Procedure. for electrochemical extraction of reinforced concrete chlorides according to claim 2, characterized in that the conductive carbonaceous material is selected, although not limited, from:

to)

powder graphite

b)

fiber carbon

C)

any other carbonaceous material driver

d)

mix of all the above in different proportions

4. Procedure for electrochemical extraction of reinforced concrete chlorides according to claims 1 and 3 characterized in that an adequate current intensity is applied between the anode and the cathode for a variable time. 5. Procedure for electrochemical extraction of reinforced concrete chlorides according to claims 1-4 characterized in that the intensity applied between anode and cathode is constant and is applied in one or different successive stages.