Abstract:
A protective coating for application to steel reinforced concrete structures, comprising a liquid composition of elastomeric polymeric material having electrically conductive particulate matter evenly dispersed therein. A cathodic protection system embodying the coating comprises a moisture impervious membrane of elastomeric polymeric material having electrically conductive particulate material evenly dispersed therein, an electrically conductive grid embedded in the membrane or contiguous thereto, and direct current means connecting the grid electrically with the reinforcing bars.

Description:
RELATED APPLICATION 
     This application is a continuation in part of U.S. application Ser. No. 061,473 filed June 15, 1987 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a protective coating for steel reinforced concrete. 
     BACKGROUND OF THE INVENTION 
     All suspended concrete decks in parking facilities contain reinforcing steel. When steel encounters both oxygen and water, rusting can occur. The process may be referred to as galvanic action, electrochemical corrosion, or ionic activity. If sodium chloride (de-icing salt) is used on roadways, it is carried into the garage on the underside of vehicles, drops to the deck surface, and eventually permeates through the concrete to the reinforcing steel (rebars), acting as an electrolyte in the presence of moisture and oxygen. This process greatly accelerates the rusting, which in turn reduces the steel to scale. This transformation of the steel increases its volume and produces extreme stress within the concrete slab, initially creating cracks and eventually spalled areas or potholes. The pressure created by corroding rebars can reach several thousand pounds per square inch. 
     Since no present system can eliminate oxygen from the concrete slab, it is customary to employ a waterproofing system using an impervious membrane and/or coating with a sealer, which stops the penetration of both moisture and chlorides into the slab. Another method of inhibiting corrosion is to apply a constant negative electric voltage to the reinforcing steel to reverse its anodic property. 
     An example of such a coating and voltage application presently in use is disclosed in U.S. Pat. No. 4,506,485 issued Mar. 26, 1985 to J. Apostolos which shows a coating of molten metal and a direct current circuit joining the coating and the embedded rebar. The Apostolos system suffers from the disadvantage that the coating provides a sacrificial anode and ablates over a period of time. Also the coating of Apostolos is relatively rigid and would be ineffective to bridge larger cracks often occurring in concrete. 
     The present invention provides a moisture impervious membrane or coating of improved conductivity which when given an electropositive charge reverses the anodic property of the reinforcing steel and thus all the reinforcing steel network becomes cathodic whereby no corrosion occurs. 
     Known waterproofing coating systems also suffer from the disadvantage that they are relatively inflexible and inelastic. Since hairline cracks in concrete can develop which are one-eighth of an inch or more in width, such coatings will fracture when stressed at the site of the crack. Shrinkage cracking of the coating may also occur. 
     The present invention provides a moisture impervious membrane or coating with inherent flexibility to bridge cracks without itself being fractured under normal extension under stress. 
     SUMMARY OF THE INVENTION 
     Essentially the invention consists of a protective coating for steel reinforced concrete structures, comprising a liquid composition of elastomeric polymeric material having electrically conductive particulate matter evenly dispersed therein and curable at ambient temperature to form a moisture impervious conductive membrane. 
     In another aspect the invention consists of a concrete structure containing steel reinforcing bars, a cathodic protection system comprising: a moisture impervious membrane of elastomeric polymeric material applied to the structure, the membrane having electrically conductive particulate matter evenly dispersed therein; an electrically conductive grid embedded in the membrane or contiguous thereto; and direct current means connecting the grid electrically with the reinforcing bars. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example embodiment of the invention is shown in the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of a reinforced concrete slab having a protective coating system. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     As seen in FIG. 1 of the drawings an impermeable membrane or coating 10 of the invention is applied to a concrete structure 12 containing reinforcing bars 14. Coating 10 has embedded in it, or contiguous to it, a primary anode of conductive wire material which is formed in a grid 16 connected electrically by a conductor 18 to a source 20 of direct current the source also being connected by a conductor 22 to reinforcing bars 14. A low electrical current flows between grid 16 and rebars 14 substantially equal and opposite to the current which would result from an electrical connection between the rebars and the grid, which inhibits electrolytic action on the rebars. A wear course 24 may be laid over coating 10. 
     The composition of coating 10 allows the current to be uniformly distributed throughout the coating. Examples of suitable formulations for forming the coatings of the invention are set forth below. 
     
         ______________________________________I CHLOROPRENE MEMBRANES______________________________________EXAMPLE IINGREDIENT       PERCENT BY WEIGHTchloroprene latex solids            28.8conductive carbon black            8.7clay             1.35thickening agent (polyacrylate)            0.45water            60.3anti-oxidant     0.4PROPERTIESelectrical resistivity            20-100 ohm-cmtensile strength 870 psielongation at 72° F.            500%elongation at 0° F.            150%EXAMPLE IIINGREDIENT       PERCENT BY WEIGHTchloroprene latex solids            31.0nickel powder (particulate)            33.5water            33.39thickening agent 0.6anti-oxidant     0.51wetting agent    0.2PROPERTIESresistivity      1-15 ohm-cmtensile strength 800 psielongation at 72° F.            500%elongation at 0° F.            300%EXAMPLE IIIINGREDIENT       PERCENT BY WEIGHTchloroprene latex solids            38.03conductive carbon black            3.7conductive carbon fibers            8.1water            49.55anti-oxidant     0.42wetting agent    0.2PROPERTIESresistivity      10-20 ohm-cmtensile strength 600 psielongation at 72° F.            450%EXAMPLE IVINGREDIENT       PERCENT BY WEIGHTchloroprene latex solids            39.0nickel coated graphite fibers            0.36water            45.74clay             14.9PROPERTIESresistivity      20-80 ohm-cmtensile strength 840 psielongation at 72° F.            550%EXAMPLE VINGREDIENT       PERCENT BY WEIGHTchloroprene latex solids            30clay             8.7conductive carbon black            5.45nickel coated graphite            0.3water            55wetting agent    0.05PROPERTIESresistivity      40-50 ohm-cmtensile strength 800 psielongation at 72° F.            450%EXAMPLE VIINGREDIENT       PERCENT BY WEIGHTchloroprene latex solids            25.44clay             0.17nickel powder    4.23other inorganic filler            1.36conductive carbon black            10.6water            58.0wetting agent    0.2PROPERTIESresistivity      16-30 ohm-cmtensile strength 500 psielongation at 72° F.            400%______________________________________II URETHANE MEMBRANES______________________________________EXAMPLE IINGREDIENT       PERCENT BY WEIGHTblocked urethane prepolymer            46.91conductive nickel powder            24.27nickel coated graphite fibers            0.21xylene           21.86curing agent     3.88anti-oxidant     1.00other            1.87PROPERTIESresistivity      0.5 ohm-cmtensile strength 420 psielongation at 72° F.            240%EXAMPLE IIINGREDIENT       PERCENT BY WEIGHTblocked urethane prepolymer            58.91conductive nickel powder            6.09nickel coated graphite fibers            0.15conductive carbon fibers            5.69conductivity enhancer            0.10xylene           19.95curing agent     4.87anti-oxidant     1.00other            2.36PROPERTIESresistivity      0.5-4 ohm-cmtensile strength 505 psielongation at 72° F.            835%EXAMPLE IIIINGREDIENT       PERCENT BY WEIGHTblocked urethane prepolymer            60.21conductive nickel powder            7.79nickel coated graphite fibers            0.11conductive carbon fibers            3.11xylene           20.39curing agent     4.98anti-oxidant     1.00other            2.41PROPERTIESresistivity      2-10 ohm-cmtensile strength 355 psielongation at 72° F.            1000+%EXAMPLE IVINGREDIENT       PERCENT BY WEIGHTblocked urethane prepolymer            56.78conductive carbon black            7.84nickel coated graphite fibers            0.44wetting agent    0.29xylene           26.67curing agent     4.7anti-oxidant     1.00other            2.28PROPERTIESresistivity      2-5 ohm-cmtensile strength 245 psielongation at 72° F.            195%______________________________________ 
    
     A suitable chloroprene polymer is sold by Dupont de Nemours Inc. under the trade mark NEOPRENE. A suitable blocked urethane prepolymer is sold by Bayer AG under the trade mark DESMOCAP. 
     The composition for the coatings of the invention are liquid when formulated and are curable (that is, dryable) at ambient temperatures (that is, at temperatures in the range about 40°-120° F.) in order to form a moisture impervious conductive membrane. 
     The thickness of membrane 10 depends upon the size of cracks in concrete 12 to be bridged. Generally membrane thickness of 0.020 inch to 0.030 inch would be required. In case of excessive roughness a parge coating on the concrete may be required to prepare the surface to receive membrane 10. The wire of grid 16 may be encapsulated in (i.e. embedded within) membrane 10 or the membrane may be laid as a secondary anode on top of the grid which would then be contiguous with the membrane. A suitable material for grid 16 is platinum (over niobium copper wire) which may be laid in strands about twenty feet apart, the spacing and gauge of the wire being dependent on the specific requirements of the system. The composition of coating 10 allows the current to be distributed evenly throughout the coating and reduces its resistivity which may be as low as 10 ohm-cm. 
     By providing uniform distribution of electric current localized disintegration, and the consequent destruction of the adjacent concrete, is avoided where chloride ions already exist in the concrete from salt penetration before the membrane is applied. Consequently the membrane performs the dual function of keeping water and salt from penetrating the concrete and inhibits the corrosive effects of chloride ions already present in the concrete. 
     A separate wear course such as a coal tar epoxy (sand) may be applied over membrane 10 in known manner. 
     If desired, membrane 10 may be prefabricated in sheets or sections and applied to the concrete substrate using an auxiliary binding agent such as a conductive primer adhesive. 
     The invention provides a moisture impervious membrane which is resistant to chloride ion penetration and neutralizes chloride ions already present in the concrete, has a high flexibility and elongation characteristics, and has low electrical resistivity. Additionally, the membrane has the ability to bond to concrete substrates and to have wear courses bond to it. 
     The term &#34;impervious&#34; means highly resistant to moisture transmission.