ENCAPSULATED METAL NODULES FOR FORMING SACRIFICIAL ANODES IN REINFORCED CONCRETE OR MORTAR, AND A METHOD OF PRODUCING SUCH NODULES 5 TECHNICAL FIELD The present invention relates to the technical field of providing protection against corrosion for the metal reinforcement of structures made of reinforced concrete or the like. 10 It is known that metal reinforcement, in particular when made of steel, is subject to a phenomenon of passivation when used in hydraulic concretes or mortars given the high alkalinity thereof that results from dissolving alkaline ingredients in cement. In the more 15 common applications, this passivation provides adequate protection for the metal reinforcement of concrete. However, that technique for protecting reinforcement is ineffective in the context of works that are immersed in sea water or liable to come into contact with Cl- ions 20 which destroy the passivation of steel reinforcement. This applies in particular to structures that are subjected to salting in winter in order to combat icing. In the presence of a high concentration of chlorine ions, the pH of the concrete diminishes and passivation 25 disappears so that the metal reinforcement then becomes the seat of oxidation phenomena leading to structural damage. Expanding rust appears on the surface of the reinforcement causing the concrete of the structure to crack and burst. Similarly, oxidation of the 30 reinforcement leads to structural weakening thereof which can compromise the strength of the work in question. The presence of Cl- ions in high concentration is not the only cause of a drop in an initially-high pH that leads to more or less premature aging of concrete. This 35 drop in pH over time is also the result of the action of atmospheric carbon dioxide gas which, in the presence of moisture, becomes fixed on the lime that is released at 2 the end of cement hydration in a reaction known as carbonation. This reaction leads to the reinforcement of reinforced concrete being depassivated whereupon it becomes subject to corrosion and to the formation of 5 expanding rust. PRIOR ART In order to combat those corrosion phenomena, it is known to implement so-called "anodic protection" of the 10 reinforcement. This protection can involve either connecting the reinforcement to a DC source in order to cause a protective current to flow therein, or else providing sacrificial anodes based on one or more metals presenting an electrochemical potential lower than that 15 of iron. The present invention relates more particularly to the second method which protects metal reinforcement by implementing sacrificial anodes. In this particular field, for example, French patent 20 application No. FR 2 689 921 proposes incorporating zinc based particles within a repair concrete or mortar for applying locally to regions of a structure in need of repair, in which the metal reinforcement has been subjected to a high level of corrosion. Given the 25 difference in electrochemical potential between iron and zinc, oxidation-reduction reactions develop within the repair mortar in contact with the oxidized reinforcement, thereby contributing to reducing the rust formed on the reinforcement. In addition, it constitutes a kind of 30 battery which electrically biases the reinforcement so as to protect it. Thus, the sacrificial anodes constituted by the zinc grains are oxidized instead of the metal reinforcement which is then protected. Under certain situations, such a method can provide 35 satisfactory repair, restoration, and protection for reinforcement and the concrete containing it.
3 Unfortunately, in other situations it has been found that the electrochemical potential difference between reinforcing steel and zinc is not sufficient to provide effective protection of said reinforcement. It is then 5 necessary to consider implementing metals or alloys that present an electrochemical potential lower than that of zinc, such as aluminum, for example. Nevertheless, aluminum is highly reactive in the very alkaline aqueous medium constituted by fresh concrete or mortar. The 10 reaction then leads to a large amount of hydrogen being given off which disrupts the mortar or concrete internally and leads to it being weakened. There is therefore a need for means enabling metals or metal alloys that are highly reactive in a very 15 alkaline aqueous medium to be used as sacrificial anodes in mortars, concretes, or the like without running the risk of spoiling the mechanical properties of the concrete or the mortar by gases being given off as a result of the electrolytic reactions that develop in 20 association with metallic anodes. Furthermore, it has been found that when zinc particles are implemented in a hydraulic concrete or mortar, electrolytic reactions occur spontaneously between the zinc, the alkaline ingredients of the cement, 25 and water. These reactions thus lead to a fraction of the zinc being consumed before it is able to react with the reinforcement in need of repair or protection. There is therefore a need for means that enable electrolytic reactions between the sacrificial anodes 30 used and the ingredients of the corresponding concrete or mortar to be blocked temporarily so as to make it possible for alloys or compounds that present electrochemical potential lower or much lower than that of iron or of the steels used for making reinforcement to 35 be used as sacrificial anodes in concretes, mortar, or the like.
4 SUMMARY OF THE INVENTION In order to achieve this object, the invention provides metal nodules for constituting sacrificial anodes in reinforced concrete, mortar, or the like, 5 the nodules being characterized in that each comprises: - a metal core containing one or more metals or metal alloys selected from those presenting electrochemical potential lower than that of iron; and 10 - a protective coating surrounding the core and made in such a manner that when the nodules are used in the preparation and implementation of concrete, mortar, or the like: - firstly it presents sufficient mechanical 15 strength to withstand mixing with the aggregate constituting the concrete, mortar, or the like; and - secondly it constitutes a temporary barrier to water, at least until the concrete, mortar, or the like has set partially. 20 When implemented in a concrete or mortar while it is being prepared and used, nodules of the invention present the advantage of not reacting immediately. Thus, the sacrificial anodes constituted by the metallic cores are not consumed prematurely and no gas is given off in 25 untimely manner even though the cores are suitable for reacting strongly with the highly alkaline aqueous medium of the concrete or mortar. This advantageous characteristic of the nodules is mainly the result of the mechanical and physico-chemical 30 qualities of the protective coating on the cores. The coating has sufficient strength to withstand the impact and abrasion stresses to which the cores are subjected during the preparation and mixing of the concrete, mortar, or the like in a mixer or truck mixer without 35 breaking. This strength associated with the water repellent nature of the coatings prevents any premature contact between the cores and the alkaline aqueous 5 medium. Furthermore, the temporary nature of the water repellent properties of the protective coating provide a kind of delayed action effect allowing oxidation reduction reactions to occur on the cores, but only after 5 the concrete or mortar has set, at least partially. Thus, these reactions develop only after the mortar or concrete has been used and already presents sufficient strength to be able to withstand any gas being given off, in particular hydrogen gas, without any risk of plastic 10 deformation and/or structural damage such as microcracking, for example. Implementing nodules of the invention thus makes it possible to obtain concretes, mortars, or the like which present both mechanical strength characteristics that are 15 compatible with use in structures, for example, and chemical properties adapted to preparing and repairing metal reinforcement. The cores of the nodules are preferably constituted by or contain mainly a metal selected from aluminum, 20 magnesium, zinc, and alloys thereof. According to an advantageous characteristic of the invention, the cores are made from aluminum casting waste which constitutes a relatively low cost material. The invention thus provides a route for recycling such waste which is 25 difficult to reuse elsewhere. Preferably but not exclusively, the cores of the nodules present a mean grain size lying in the range about 1 millimeter (mm) to about 10 mm, while preferably but not exclusively, the nodules present grain size lying 30 in the range about 2 mm to about 11 mm. This grain size can vary depending on the use to which the nodules are put. Thus, this range of grain sizes can be used for incorporating in concrete. However, when the nodules are to be incorporated in mortar it is preferable to use 35 nodules presenting a mean grain size of about 2 mm to about 6 mm, or nodules possessing cores having a mean grain size of about 1 mm to about 4 mm.
6 The protective coating of the nodules is made out of any suitable material or matter that presents mechanical strength characteristics and physico-chemical properties that are suitable for the function of nodules of the 5 invention. Thus, the protective coating can be made out of a polymer or a resin that is hard enough after polymerization to form a waterproof shell around each core that can withstand the impacts and the abrasion that 10 result from being mixed with the aggregate of the concrete or mortar used. Furthermore, this polymer or resin must be suitable for being degraded by the alkaline nature ofthe concrete or mortar so that after a certain length of time, corresponding at least to the time 15 required for the concrete or mortar to set partially, oxidation-reduction reactions involving the core can begin. According to a preferred characteristic of the invention, the protective coating is constituted by a 20 water-porous shell which presents suitable mechanical strength and which is impregnated with or covered in a water-repellent substance that is to be degraded over time by the alkaline nature of the medium in which the nodules are used, thereby constituting a temporary 25 barrier against water. The porous shell is preferably made of a compound based on calcium. By way of example, the shell can be based on chalk or limestone, or more preferably it can be based on cement. 30 The water-repellent substance is selected so as to be degradable by the alkaline ingredient of the mortar thus allowing water and ions responsible for oxidation reduction reactions to pass through once it has become degraded. In preferred but not absolutely necessary 35 manner, the water-repellent substance is selected from silicones, silanes, siloxanes, stearates, fatty acids, paraffins, and mixtures thereof.
7 According to another characteristic of the invention, each nodule includes one or more conductive strands associated with the core. These strands in direct contact with the cores of the nodules and covered 5 together with the nodules in the protective coatings facilitate the flow of electric current within the concrete or mortar that incorporates nodules of the invention. The electrically conductive strands can be made of any suitable material that is inert relative to 10 the galvanic couples formed by the cores and the reinforcement. The conductive strands are preferably made of carbon fibers. The invention also provides a method of producing nodules to constitute sacrificial anodes in reinforced 15 concrete, mortar, or the like, characterized in that it consists in coating metallic nodules containing one or more metals or metal alloys selected from those presenting an electrochemical potential lower than that of iron, in a protective coating which, when the nodules 20 are used in the preparation and use of concrete, mortar, or the like: - firstly presents sufficient mechanical strength to withstand mixing with the aggregate constituting the concrete, mortar, or the like; and 25 - secondly constitutes a temporary barrier to water, at least until the concrete, mortar, or the like has set partially. Thus, the coating can be made by coating the cores with any material presenting the required properties 30 matching the function of the nodules. This coating can be performed by any method adapted to the nature of the cores and of the material(s) constituting the protective coating such as, for example, and in non-limiting manner: mixing, dipping, spraying, deposition in a fluidized bed. 35 According to a preferred characteristic of the invention, the production method consists, when making the protective coatings on the cores, in: 8 - optionally screening the cores; - agglomerating powder around the cores to form porous shells about said cores; - crystallizing the shells to confer an abrasion 5 resistant and protective nature thereon; and - applying a water-repellent substance to the crystallized shells, said substance being suitable for being degraded by the alkaline nature of the medium in which the nodules are used. 10 By way of example, the powder used is a powder based on calcium or on a compound thereof such as a powder based on chalk or limestone. Preferably, the powder used is a cement or cement-based powder. When dry cement is used for making the shells, the 15 quantity of cement employed represents about 50% to 70% of the total weight of the nodules. After the powder has agglomerated to form a kind of shell around the cores, the shells are crystallized so as to impart sufficient mechanical strength thereto. For a 20 powder based on chalk, limestone, or cement, an aqueous silicate solution is applied to the shells as a crystallizing agent. The crystallizing agent preferably uses an aqueous solution of water glass. Thus, in order to ensure that the shells formed by cement powder 25 agglomerating around the cores becomes crystallized, an aqueous solution of sodium silicate at a concentration of about 20% is used and the quantity thereof represents no more than 10% of the weight of cement used for constituting the shells. 30 The shells made in this way are porous so the invention recommends applying a water-repellent substance to cover and/or impregnate the shells. This water repellent substance is selected in such a manner as to be suitable for being degraded by the alkaline nature of the 35 concrete, mortar, or the like in which the nodules are used. The water-repellent substance serves firstly to prevent any diffusion of water and ions through the 9 shells of the nodules for a length of time that is not less than the time required by the concrete or mortar to set or acquire sufficient stiffness to ensure that it is no longer sensitive to any gas that might be given off 5 and give rise to the cores being oxidized. Thereafter, degradation of the water-repellent substance serves to destroy the temporary barrier it once formed, thereby allowing oxidation-reduction reactions to develop that involve the cores. 10 It should also be observed that the nature of the porous shell advantageously contributes to providing cohesion between the nodules and the concrete or mortar in which they are used. As mentioned above, the water-repellent substance is 15 preferably selected from silanes, silicones, siloxanes, stearates, fatty acids, paraffin, and mixtures thereof, with a preference for silanes. The water-repellent substance is applied to the crystallized shells so that its concentration preferably, but not exclusively, 20 represents 1% to 5% of the total weight of the nodules. The nodules made in this way in accordance with the invention can be used in concretes, mortars, or the like for making new structures or for repairing damaged structures. 25 When used for preparing hydraulic concretes or mortars, the nodules preferably, but not necessarily, constitute no more than 10% of the total weight of the aggregate plus cement. Insofar as the nodules are used in concretes, 30 mortars, or the like which are not naturally highly alkaline, for example concretes or mortars that make use of an organic binder, it is possible to envisage correcting alkalinity by adding alkaline active elements such as soda, for example. 35 The following examples enable the invention and its various aspects relating to the nodules, to producing them, and to using them to be better understood.
10 BEST METHOD OF PERFORMING THE INVENTION Examples EXAMPLE 1 - PRODUCING NODULES OF THE INVENTION WITH CARBON FIBER 5 CONDUCTIVE STRANDS Aluminum casting waste is used for constituting the nodules. The following method is then used to produce nodules of the invention from such waste. 1) The metal waste is stirred for about 5 minutes in 10 order to break up any clumping of the waste. 2) The waste is screened to select cores presenting a grain size cutoff in the range 1 mm to 4 mm. 3) The selected cores are washed under running water for about 5 minutes while being stirred. 15 4) The washed cores are drained and then dried in hot air. 5) 10 kilograms (kg) of cores prepared in this way are mixed with 10 grams (g) of carbon fibers for dispersing in a concrete mixer, mill, or other mixer. 20 6) Mixing is continued for 15 minutes (min) to 20 min with 5 kg of CPA CEM I or CMJ CEM II type cement being added progressively until a shell of cement agglomerates around each core. Where necessary, 1 kg to 2 kg of cement is added to complete coating. 25 7) Mixing is continued for about 30 min while spraying an aqueous solution of water glass up to a maximum of 10% of the weight of added cement so as to cause the agglomerated cement shells to crystallize. An exothermal reaction occurs and that can make it necessary 30 to cool the mixing vessel either by an external jet of water or by a flow of cold air inside it. It should be observed that the exothermal nature of the reaction is advantageously made use of to evaporate the water from the silicate solution before oxidation-reduction 35 reactions develop on the cores. 8) The cores coated in their crystallized shells are then cooled, e.g. by spreading the nodules out on a 11 ventilated plane surface. Complete cooling of the nodules can require 2 hours (h) to 4 h. 9) After complete cooling, a water-repellent solution is applied on the crystallized shells of the 5 nodules by mixing. Naturally, a sufficient quantity of this solution is applied, amounting to about 1% to 5% of the total weight, so as to ensure that the shells are well covered and/or well impregnated. 10) Finally, the nodules are dried, either by being 10 spread out for a period of 24 h in a plane and ventilated area, or else by being mixed under a flow of dry air. Nodules produced in this way are nodules in accordance with the invention and they are ready for use in preparing a concrete, mortar, or the like. 15 It should be observed that the combined coating of the carbon fibers forming the conductive strands and the cores in the common protective coating ensures that there is intimate and electrically effective contact between the carbon fibers and the sacrificial anodes constituted 20 by the cores. When the nodules are put into use, this electrical contact is subsequently encouraged by the aluminate crystals developing in the vicinity of the cores. 25 EXAMPLE 2 - PRODUCING NODULES OF THE INVENTION WITHOUT CONDUCTIVE STRANDS The method is substantially the same as in Example 1 except that step 5 of mixing the cores with carbon fibers is omitted. 30 EXAMPLE 3 - USING NODULES IN A MORTAR Nodules of the invention can be used to prepare a mortar which remains workable for about 30 min. The composition of such a mortar can be as follows: 35 . CPA CEM 1 or CPJ CEM II type cement: 300 kg * 0 to 4 mm lime, silica, or silico-lime sand: 900 kg . nodules of Example 1 or Example 2 having 12 grain size of 2 mm to 6 mm: 100 kg . for a total aggregate of: 1300 kg . mixing water about 150 liters depending on the required consistency. 5 Such a mortar can then be used for repair operations or for operations to prevent corrosion, in particular as a rendering. EXAMPLE 4 - USING NODULES IN CONCRETE 10 It is possible to use nodules of the invention when preparing concrete that remains workable for about 30 minutes. The composition of such a concrete could be as follows: 15 . CPA CEM 1 or CPJ CEM II type cement: 350 kg 0 to 25 mm lime, silica, or silico-lime sand: 1650 kg 2 mm to 6 mm nodules of the invention: 200 kg for a total aggregate of: 2200 kg mixing water about 175 liters depending on the 20 required consistency. When using nodules of the kind produced in Example 1 in a mortar of Example 3 or in a concrete of Example 4, the spontaneous electrolytic reaction on the cores begins about 2 hours after initial contact with the mixing 25 water. The hydrogen which is then given off is of no effect given the stiffness of the concrete or mortar which has already set partially. The appearance of a potential difference between the slurry as prepared in this way and applied to a structure 30 in need of repair and the steel reinforcement of said structure can be monitored by means of a "Canin" type wet electrode potential-measuring instrument. A potential difference lying in the range 300 millivolts (mV) to 1500 mV can be observed. 35 In the above examples, the nodules are used directly in preparing concretes or mortars that are for use immediately or in any event very quickly.
13 Nevertheless, nodules of the invention can also be added to ready-for-use wet or dry concretes or mortars as are available in the trade. Under such circumstances, the nodules should be added immediately before the 5 material is used. Naturally, steps should previously be taken to ensure that the ready-for-use concrete or mortar that is supplied has workability of a duration that is compatible with the reaction time of the nodules.