Anode structure for cathodic protection of steel reinforced concrete and relevant method of use

Abstract: The present invention relates to a method for cathodic protection of steel reinforced concrete which comprises using an anode structure made of an array of valve metal strips activated by an electrocatalytic coating and having voids therein, supported by or inserted into insulating spacers, said strips being connected by connection means either provided with voids or without voids, or rods, bars, insulated cables. The anode structure is applied to the reinforcing steel cage during construction before the concrete is poured. The anode structure of the present invention exhibits a remarkable mechanical resistance and has an anode surface which may be tailored in order to provide for the necessary protection current on the basis of the density of the reinforcing bars contained in the structure to be cathodically protected.

Description: BACKGROUND OF THE INVENTION Cathodic protection of metal structures is well known. Substantially the metal structure is made the cathode in a circuit including a direct current source, an anode and an electrolyte between the anode and the cathode. The exposed surface of the anode is made of a material which is resistant to corrosion, for example platinum or mixed metal oxides, on a base structure made of a valve metal such as titanium or an organic polymer containing a dispersion of carbon black or graphite. There are many types of metal structures which need protection from corrosion, including steel reinforcing members in concrete, which are often referred to as "rebars". Concrete is sufficiently porous to allow passage of oxygen and liquid through it. Consequently, salt solutions, which remain in the concrete or which permeate the concrete from the outside, will cause corrosion of the rebars in the concrete. This is especially true when the electrolyte contains chloride ions, as for example in structures which are contacted by the sea water, and also in bridges, parking garages, etc. which are exposed to water containing salt used for deicing purposes or, finally, when calcium chloride has been added to the mortar as a hydration accelerator. The corrosion products of the rebars occupy a much larger volume than the metal consumed by the corrosion. As a result, the corrosion process not only weakens the rebars, but also, and more importantly, causes cracks and spalls in the concrete. It is only within the last ten or fifteen years that it has been appreciated that corrosion of rebars in concrete poses problems of the most serious kind, in terms not only of cost but also of safety. There are already many reinforced concrete structures which are unsafe or unusable because of deterioration of the concrete as a result of corrosion of the rebars, and unless some practical countermeasures to the problem are applied the number of such structures will increase dramatically over the next decade. Consequently, much efforts and expenses have been devoted to the development of methods for cathodic protection of rebars in concrete. As a result, cathodic protection has been recently proposed for the prevention against corrosion at the stage of the construction of concrete structures which are expected to be contaminated by chlorides during their lifetime (for example bridges in mountain areas, docks, structures operating in sea environments). Cathodic protection, applied to already built new structures, comprises several steps which are time and labor consuming. In fact, it comprises making slots in the concrete to expose the rebars, installing connection cables, sandblasting the concrete surface, positioning the anodes and covering the same by a cementious overlay. If installation is carried out during the construction phase before pouring of the concrete, there would be no need for these preparation with obvious remarkable savings. The anode for cathodic protection of new structures, which should be installed on the reinforcing steel cage before concrete pouring, needs to be kept apart with appropriate insulating means and should also exhibit outstanding mechanical characteristics to avoid possible ruptures during pouring of the cement or sagging due to the weight of the concrete. In this event the anode would come into contact with the metal of the reinforcing bars causing shortcircuiting of the system. The structures of the prior art anodes are not suitable for installation as above illustrated. For example, British patent no. 2,175,609 describes an extended area anode comprising a plurality of wires in the form of an open mesh provided with an anodically active coating which may be used for the cathodic protection of steel rebars in reinforced concrete structures. U.S. Patent no. 4,708,888 describes a cathodic protection system using anodes having a highly expanded structure with more than 90% of void areas with respect to the empty areas. The anode systems described in the cited patents cannot be utilized during construction before pouring of the concrete because the flimsiness of the highly expanded titanium meshes would easily result in mechanical damage and possible shortcircuit with the rebar cage during the pouring operation and subsequent vibration of the concrete. OBJECTS OF THE INVENTION It is an object of the present invention to overcome the shortcomings of prior art by providing for an improved anode structure having enhanced mechanical properties and comprising metal strips supported by spacers which can be applied to the reinforcing steel structure during construction, before the step where concrete is poured. It is another object of the present invention to supply for an improved anode structure having a suitable geometry to be adjusted so that the current distribution conforms to the density of rebars in the structures to be cathodically protected. It is a further object of the present invention to provide for a method for forming the anode structure of the invention onto the last layer of the reinforcing rebars or inside the reinforcing steel cage before pouring the concrete during the construction of the structure to be cathodically protected. DESCRIPTION OF THE INVENTION The anode structure of the present invention is made of an array of anode elements mechanically connected by suitable means and supported by spacers. Such connection means may have various geometries, such as metal strips with or without voids, bars, rods, insulated metal cables. Said anode elements have elongated shapes, having also various geometries, such as rods, wires, plates. However, the most preferred shape is strips of valve metals, having voids and provided with an electrocatalytic coating. The voids on the strips may be punched on the metal but most economically an expanded metal is used. These voids provide for the best contact between the anode surface and the concrete which penetrates the voids during pouring. The valve metal of the strips is titanium, tantalum, zirconium, and niobium. Titanium is best preferred in view of its mechanical resistance, corrosion resistance and availability and cost. As an alternative valve metal alloys or intermetallic compounds may be used. Activation, that is the step of providing said electrocatalytic coating, is carried out according to the procedures well known in the art, either on the punched or expanded metal before cutting into strips or alternatively on the strips after cutting from the punched or expanded metal sheet. Bending of the strips, as discussed below, may be carried out before or after activation. Preferred activation is provided by electrocatalytic coatings based on mixed oxides of valve metals and platinum group metals, such as titanium, tantalum, iridium and ruthenium or mixtures of the same. Another suitable coating is a cobalt spinel or a coating comprising an intermediate layer of platinum and iridium metals or a mixed oxide of titanium and tantalum under the electrocatalytic surface coating. Provided that certain titanium alloys contaiing small amounts of catalytic metals such as ruthenium or palladium are used, the activation step may be avoided. The strips width is over 3 mm and the thickness is in the range of 0.25 mm to 5 mm, preferably between 0.5 and 3 mm. The spacers, directed to avoid any risk of short-circuit between the anode strips and the reinforcing steel may be prefabricated elements made of plastic or cementitious material having a high mechanical resistance, to ensure easy handling and transport, as well as adequate stiffness once installed on the metal structure to be protected. Typically the spacers may have a square, rectangular, circular, elliptic or triangular cross-section. The spacers may have a diameter from 2 to 10 cm or cross-section dimensions of 2 to 10 cm. The most general practice comprises applying said spacers to the metal cage to be protected so that they are mechanically secured and firmly held in position. Thereafter the anode strips are fixed to said spacers. For example, they are inserted in a slot suitably provided in the spacers. Alternatively the strips are applied onto the spacers either by fastening by means of plastic or metallic nails, screws, clips, e.g. titanium clips, hooks or staples or by adhesion by means of glues, epoxy adhesives or the like. In an embodiment of the present invention the anode strips are first applied to said spacers as above described and then the strip-spacer assemblies are positioned on the last layer of the reinforcing metal cage before pouring the concrete. In a further embodiment of the present invention the anode strips may be curved in the widthways dimension for all the length of the strip so to obtain the maximum rigidity and mechanical resistance to the thrust of the poured concrete and to the lateral pressure exerted by the concrete which distributes inside the reinforcing cage. The direction of the curve may be either towards the inside as towards the outside with respect to the spacer surface. Other types of bending may be also resorted to as a multiple ply to offer a higher mechanical resistance or bending of the strip may be such as to bring the two edges of the strip together and fixing the same by spot-welding, thus forming a cylinder. Any angle of bending may also be used so that the strips may be bent to form a geometrically square, rectangular, triangular cross-section. In another embodiment the strips may be interposed between two spacers, forming a sandwich structure. The anode strips, which have a distance from each other higher than their width, will not cause obstruction to the concrete flow during pouring as compared to the use of the expanded meshes and relevant support, as taught by the prior art. Uniform and optimum distribution of current on the reinforcing metal structure is attained according to the present invention by suitably varying the dimensions and expansion degree of the strips, as well as the the distance with each other. The strips are connected together by means of connection elements welded thereto or simply mechanically attached by cold-heading, preferably forming 90° angles, other angles being also acceptable. As explained before, said means of connection may be manufactured by using the same material as the strips as well as different materials, such as insulated copper wires or strands. In this latter case electrical connection is preferably carried out either by means of a pull box or by plastic deformation of the cable on the strips. The cathodic protection system according to the present invention comprises applying electric current to the anode structure made of the strips spaced apart and connected by means of connection elements. Current distribution and therefore optimum cathodic protection is obtained by the arrangement of the present invention which may be specifically tailored on the density of reinforcing bars per unit area of concrete. For example in highway bridges the density of reinforcing bars is higher in the slabs areas corresponding to the piers than in the middle section to guarantee the optimum structural resistance. The corresponding ratio between square meters of reinforcing steel and square meters of concrete surface is indicatively 5 and 1. Such substantial variation of said ratio is by no means a problem with the anode structure of the present invention. In fact, as the strips are applied before pouring the concrete, their void area, number, dimensions and spacing apart may be suitably tailored depending on said density of reinforcing bars in order to obtain the best current distribution and thus the most efficient cathodic protection of the reinforcing bars avoiding an excessive protection in some areas and underprotection in others. The need of homogeneously distributing current is of the outmost importance as steel will undergo corrosion when unprotected, that is fed with a current density having a value lower than the optimum one. On the contrary, overprotection will cause hydrogen embrittlement, especially if the steel to be protected is characterized by a high fatigue limit as for that used in the case of prestressed or post-tensioned reinforced concrete structures. The invention will now be illustrated in detail by making reference to the figures, wherein: fig. 1 is a plan view of the anode assembly of the present invention figs. 2, 3, 4, and 5 are cross-section views of different embodiments of the present invention. With reference to fig. 1, the anode strips 1) are applied onto the cage 2) of reinforcing bars by means of spacers not shown in the figure. The connection elements 8) provide for the electrical continuity between the strips. The cathodic protection system is completed by a direct current source 7) and by main feed cables 4 which connect the positive pole of said source to said connection means thanks to the junction boxes 5) and main feed cables 6) which connect the negative pole of said source to the reinforcing barscage 2). The spacing among the strips is lower in area A in correspondence of the higher density of reinforcing bars and higher in area B where the density is lower. In fig. 2, the anode strips 1), after bending to increase the overall stiffness, are applied onto the reinforcing bars cage 2, in a parallel direction with respect to the plane defined by the more external layer of the cage. Said strips are insulated from the reinforcing bars by means of spacers 3). The concrete 4) is poured on the structure following the direction indicated by the arrows. Said spacers 3) are in the form of elongated flat bars, made either of plastics or cementitious material having protruding rims which increase the overall stiffness and also allow an easy positioning of said bent strips 1). Said strips are firmly held into position by means of suitable fasteners not shown in the figure, such as nails, screws, clips, made either in plastic material or metal. In this latter case a valve metal, and especially titanium, is highly preferred. Both procedures of assembling may be practiced , the first one comprising installing said spacers 3) on the reinforcing bars cage 2) and then positioning and fastening the activated strips 1) onto such spacers, the second one comprising first assembling said strips 1) onto said spacers 3) by means of said fasteners and then installing the strip-spacer assembly onto said reinforcing bar cage 2). Fig. 3 shows an alternative embodiment of the present invention, wherein activated flat strips 1) are applied onto the reinforcing bars cage 2) in a perpendicular position with respect to the plane defined by the more external layer of the cage. Spacers 3), made of plastic or cementitious material, are in the form of elongated bars or pins having a slot therein where the activated strips 1) are positioned. Fig. 4 shows a further embodiment of the present invention wherein activated flat strips 1) are just superimposed to flat spacers 3) made of plastic or cementitious material having the form of elongated bars with a rectangular section. Fig. 5 gives a better understanding of how the activated strips 1) may be fastened to spacers 3) by means of nails or pins 5, made of plastics or metal.

Claims: A method for assembling an anode structure for cathodic protection of steel reinforced concrete which anode comprises a plurality of strips (1) of valve metal or alloys thereof and connection means (8), characterized in that it comprises positioning said strips (1) onto the steel reinforcing structure (2) before pouring the concrete, electrically insulating said strips from said structure by means of spacers (3) ensuring stiffness, electrically connecting said strips by means of said connection means (8) and pouring concrete onto said structure provided with said strips, spacers and connection means. The method of claim 1 characterized in that the strips are positioned onto the spacers before or after installing the spacers onto the steel reinforcing structure. The method of claim 1 or 2 characterized in that the strips and the spacers are positioned on the external surface of the steel reinforcing cage or inside the steel reinforcing structure. The method of claim 1 characterized in that the spacers are made of concrete or of plastics. The method of claim 1 characterized in that the spacers have a rectangular, polyhedral or circular cross-section. The method of claim 2 characterized in that the strips are positioned onto the spacers and covered by other spacers in order to obtain sandwich structures. The method of claim 1 characterized in that the strips are fastened to the spacers by means of metallic or plastic nails, screws, clips, staples or hooks, preferably the nails, clips, screws, staples or hooks are made of titanium. The method of claim 1 wherein the strips are fixed to the spacers by means of adhesives. The method of claim 1 characterized in that the strips are connected together by means of connection means attached thereto by welding or mechanically by plastic deformation. The method of claim 1 characterized in that the strips are connected to the connection means forming 90 degrees angles. The method of claim 1 characterized in that the connection elements are valve metal strips without or with voids. The method of claim 1 characterized in that the connection elements are insulated copper cables. The method of claim 1 characterized in that the strips are made of expanded valve metal sheet. The method of claim 6 characterized in that the strips have a width higher than 3 mm. The method of anyone of claims 1 to 14 characterized in that the strips have one or more bendings in the longitudinal direction, or the strips are bent to form a cylinder, preferably the strips are provided with an electrocatalytic and corrosion resistant coating. The method of anyone of claims 1 to 15 characterized in that the distance between one strip and the adjacent one, the dimensions of the strips and the ratio of void areas in the same is varied in order to comform to the density of the steel in the concrete structure. An anode structure for cathodic protection of a steel reinforced concrete structure (2) comprising a plurality of valve metal or valve metal alloys strips (1) connected together by means of connection means (8), wherein said strips are inserted in or superimposed to insulating spacers (3) ensuring stiffness. The structure of claim 17 characterized in that the spacers, the strips and the connection means are as defined in anyone of claims 1 to 16. A steel reinforced concrete structure characterized in that it is provided with the anode structure of claim 17 or 18. A method for cathodically protecting steel reinforced concrete by means of the anode structure of claim 17 or 18 characterized in that a direct current is applied to said anode structure by means of power supply source, the positive pole of which is connected to said anode structure and the negative pole to steel reinforcing cage.

Inventors: BIAGIOLI MARCELLO, TETTAMANTI MICHELE, BIAGIOLI, MARCELLO, TETTAMANTI, MICHELE
Applicants: ORONZIO DE NORA SA, ORONZIO DE NORA S.A.
Application year: 1992
Main classification: C23F13
IPC codes: C23F13