Abstract:
A method of resisting corrosion in concrete containing metal elements is provided. It includes introducing into fresh concrete, containing metal elements, at least one compound capable of sequestering chloride ions. The method may also involve employing a compound which is capable of establishing a corrosion resistant oxide layer on the metal reinforcing elements. The invention also includes certain compounds which may be employed in the method as well as concrete structures containing the compounds. In another embodiment of the invention, concrete structures may be rehabilitated by providing an overlay containing a compound of the type which will contribute to corrosion resistance either through chloride ion sequestering or creating barriers around metal structural elements with the overlay being provided in situ or as a preformed member and with possible use of a slurry in combination with an overlay segment.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    The present application is a divisional application based on U.S. Ser. No. 10/044,660, which is a continuation-in-part of Ser. No. 10/010,581, filed Nov. 13, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method of introducing into fresh concrete, as herein defined, compounds capable of sequestering chloride ions to establish resistance to corrosion of metal reinforcing elements contained within or contacting the concrete and provide a corrosion resistant oxide layer on the metal reinforcing elements, as well as related compositions and structures. The invention is also directed toward corrosion protection of concrete articles wherein the concrete has already set and hardened.  
           [0004]    2. Description of the Prior Art  
           [0005]    The advantageous use of metal reinforcing members, such as steel reinforcing members, in concrete for structural uses has been known for many years. Concrete is known to provide desired compressive strength, but tends to lack tensile strength. The reinforcing bars co-act with the concrete to provide enhanced tensile strength for the combination of materials. It has also been known to employ corrugated metal deck in combination with concrete to create a composite with similar benefits. Numerous other metal members have been embedded in concrete or provided in contact therewith to achieve enhanced benefits in the structural environment as a result of such materials. Among these additional materials are grids, beams, bolts, hold-downs and wire mesh.  
           [0006]    One problem with such construction has arisen as a result of exposure of concrete to salts, such as calcium chloride and sodium chloride, on external structural members to resist the undesired accumulation of snow and ice on bridges and other concrete paved areas such as roadways, parking lots, sidewalks and the like. While these chloride salts do provide benefits in terms of de-icing of concrete surfaces, they frequently result in the chloride solutions migrating into the concrete decks and adjacent vertical concrete surfaces, such as walls and columns, also subjecting these to chloride intrusion. Also, saline seawater may migrate into the pores of concrete exposed to seawater as in sea walls, With respect to bridge decks, in particular, an enhanced problem results from air movement under the deck creating an environment wherein the salts are aspirated into the concrete and salt laden solutions flow into the pores.  
           [0007]    Regardless of the manner in which chloride enters such concrete, the chloride, upon reaching the steel reinforcing members, tends to accelerate corrosion of the same because hydroxides of Fe 2+  frequently form and occupy porosity in the vicinity of the interface of the steel and concrete. In addition, oxides and hydroxides of Fe 3+  may also be produced. As these iron oxides and hydroxides are of greater volume than the iron metal from which they were produced, they tend to cause internal stresses which may become high enough to crack the concrete, and also degrade the desired bond between the metal reinforcing elements and the concrete.  
           [0008]    U.S. Pat. No. 5,049,412 discloses a method of re-alkatizing concrete in which carbonation has occurred. An outer layer of the concrete structure containing reinforcement which layer through exposure to air has been carbonated has an adjacent layer that remains relatively less carbonated. The patent discloses applying to the outer surface a water type adherent coating followed by introducing between the outer adjacent layers, water from a source external to the concrete structure and maintaining the concrete structure in this condition for a period of time sufficient to effect diffusion of the alkaline materials from the relatively less carbonated adjacent layer into the relatively carbonated outer layer.  
           [0009]    U.S. Pat. No. 5,198,082 discloses a process for rehabilitation of internally reinforced concrete, which includes temporary application of an adhered coating of an electrode material to surface areas of the concrete. Distributed electrodes such as a wire grid is embedded in the coating. A voltage as applied to the reinforcement and distributed to the electrode to cause migration of chloride ions from the chloride into the electrolytic coding. Among the shortcomings of this approach are the need to provide, at the local source, a source of electrical power. This electrical equipment might have to be maintained at the site for extended periods of time. This further complicates matters by establishing a risk of injury to children and others that might find the equipment at an attractive nuisance, as well as the risk of theft and vandalism. Also, such chloride extraction processes may alter the concrete microstructure by making it more porous and permeable, thereby, facilitating enhanced re-entry of chloride when de-icing salts are again applied to the exterior.  
           [0010]    It has been known to employ nitrites, such as calcium nitrite, in resisting corrosion of steel parts in concrete. It is believed that the nitrites oxidize the Fe 2+  to Fe 3+  which, in turn, precipitates as Fe 2 O 3 . The Fe 2 O 3  thus formed tends to act as a barrier to further contact between the chloride and the steel. See, generally, U.S. Pat. Nos. 4,092,109 and 4,285,733. Neither calcium nitrate nor Fe 2 O 3 , however, function to sequester chloride. The latter provides merely a barrier.  
           [0011]    There remains, therefore, a very real and substantial need for a method and related composition and structure which will resist undesired corrosion of metal structural elements contained within, or in contact with, concrete structural members.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention has met the above-described need.  
           [0013]    The method, in one embodiment, includes resisting corrosion in concrete containing metal reinforcing elements composed of steel, copper, galvanized steel, tin plated steel or other structurally suitable metals by introducing into fresh concrete containing metal reinforcing elements at least one compound capable of sequestering chloride ions in a low solubility compound.  
           [0014]    In connection with steel reinforcing elements, a low solubility compound within which the chloride ions are sequestered preferably also is created in a reaction that releases nitrite, which serves to oxidize Fe 2+  to thereby provide a corrosion-resisting oxide layer on the steel reinforcing elements. This, therefore, in connection with steel achieves two levels of corrosion resistance, one of which is the actual capturing or sequestering of the potentially damaging chloride ions, and the second of which provides a protective layer on the metal reinforcing elements.  
           [0015]    Among the preferred compounds for use in the method of the present invention are one or more compounds selected from the group consisting of 3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O; 3CaO.Al 2 O 3 .Ca(NO 3 ) 2 .nH 2 O; and 3CaO.Fe 2 O 3 .Ca(NO 2 ) 2 . nH 2 O; wherein n=0 to 18 and preferably 10 to 18, depending upon the relative humidity to which a compound is equilibrated. If desired, lower values of “n” may be obtained by drying at low relative humidity as by evacuation or by heating, for example.  
           [0016]    A further compound employed in another embodiment of the invention is, 3Me(II)O.R 2 O 3 .Me(II)(anion) 2 .nH 2 O wherein Me(II) is one or more divalent cations, such as CO 2  for example, R 2  is Al 2 , Fe 2  or Cr 2  anion is NO 2 , NO 3 , CO 3 , BO 4  or OH and n is 0 to 18 and preferably 10 to 18. For some formulations, the anion may be divalent. In this case the formula would be Me(II)O.R 2 O 3 .Me(II)(anion)nH 2 O wherein n is 0 to 18 and preferably 10 to 18.  
           [0017]    The invention also contemplates a concrete structure which has hydrated fresh concrete and a plurality of metal structural elements in contact with the hydrated fresh concrete with a compound which sequesters chloride ions dispersed within the concrete.  
           [0018]    The invention in another embodiment contemplates rehabilitation of existing concrete structures by providing a chloride sequestering compound in a member adjacent to the concrete structure and having a composition such that migration of chlorine ions away from metal structural elements in the concrete structure and into the adjacent overlay may be effected. In addition, if desired, release of nitrite to migrate into the concrete structure and afford corrosion protection to embedded steel.  
           [0019]    In one version, the overlay, which may be formed in situ or as a preformed panel, contains the chloride-sequestering compound. In another, a slurry may be applied to the concrete structure with or without an overlay secured thereover.  
           [0020]    It is an object of the present invention to provide a method and related compounds and structures for inhibiting corrosion of metal elements positioned within or in contact with concrete in a structural environment.  
           [0021]    It is a further object of the present invention to provide such a system wherein undesired chloride ions will, as a result of a reaction, be sequestered, thereby reducing their ability to corrode the metal elements.  
           [0022]    It is yet another object of the invention to, through a reaction effecting such sequestration of ions, to provide free nitrites which will oxidize the Fe 2+  to a Fe 3+  ion which, in turn, precipitates as Fe 2 O 3  which coats the metal element and, thereby, resists corrosion.  
           [0023]    It is yet another object of the present invention to provide such a system which employs unique compounds.  
           [0024]    It is another object of the present invention to provide such a system which will effectively and rapidly provide corrosion resistance to steel and other metals.  
           [0025]    It is yet another object of the invention to provide such a system which may be employed by merely adding one or more compounds of choice to fresh concrete without requiring substantial changes in conventional practices employed in producing and placing the concrete structure.  
           [0026]    It is a further object of the present invention to provide such a system where an existing concrete structure may be rehabilitated through by sequesting the chloride and providing a means to accumulate nitrite ions in the vicinity of the embedded steel. It is appreciated that the nitrite ions oxidize presently corroding steel to produce a protective layer. In some formulations nitrite ions may not be available and in these instances rehabilitation is the result of chloride sequestration only.  
           [0027]    It is yet another object of the present invention to provide such a system wherein an overlay, which contains a composition which may be of the type employed in other embodiments of the invention and facilitates sequestering of chloride and corrosion protection of metal structural elements. In another version, a slurry containing the compound of interest may be applied to the concrete structure with an overlay material either formed in situ or as a preformed panel secured thereover.  
           [0028]    It is yet another object of the present invention to provide such a system for rehabilitation of existing concrete structures without requiring a source of electrical energy to be present on an ongoing basis during the performance of the method.  
           [0029]    These and other objects of the invention will be more fully understood from the following description of the invention with reference to the drawings appended hereto. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a schematic cross-sectional illustration of a concrete bridge deck containing metal reinforcing elements.  
         [0031]    [0031]FIG. 2 is a schematic cross-sectional illustration similar to FIG. 1, but showing a construction having an overlay containing the chloride sequestering composition.  
         [0032]    [0032]FIG. 3 is a schematic cross-sectional illustration similar to FIG. 2 except that the overlay consists of a slurry adjacent to the concrete structure and an overlaying material.  
         [0033]    [0033]FIG. 4 illustrates a cross-sectional illustration looking downward on a concrete piling which is to be rehabilitated through the system of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]    As employed herein the term “concrete structure” refers to an existing structure which is composed in at least significant part of concrete which has set and hardened, as contrasted with “fresh concrete” as defined herein and shall expressly include, but not be limited to, bridges, roadways, parking lots, sidewalks, parking garages, floors, support columns, piers, marine structures, piling, conduits and other concrete structures whether located inside or outside, and whether subject to vehicular or foot traffic thereover or not.  
         [0035]    As employed herein, the term “fresh concrete” means concrete which is in a plastic state.  
         [0036]    As employed herein reference to “introducing” a compound into fresh concrete shall be deemed to include introducing the compound in solid form and in slurry form with or without other ingredients such as minerals and additives into fresh concrete and shall also embrace admixing or blending the composition in dry form with dry cement and/or other ingredients prior to water being added.  
         [0037]    As employed herein, the term “metal elements” means metal elements placed within or in contact with concrete for various purposes including, but not limited to, structural purposes and shall expressly include, but not be limited to, reinforcing bars, grills, beams, metal deck hold downs and wire mesh.  
         [0038]    As shown schematically in FIG. 1, a layer of concrete  2 , overlies and is supported by a deck member  4 . The concrete in the form shown has a plurality of elongated, generally parallel, reinforcing bars  6 ,  8 ,  10 ,  12 ,  14 ,  16 ,  18 . This assembly may be created in a conventional manner to provide the desired structure which, in the form shown, may be a bridge deck having an undersurface  22 , exposed to air  24  and an upper surface  26 , which may have undesired snow deposited thereon or ice formed thereon. Application of calcium chloride, sodium chloride or other chloride containing salts to the upper surface  26 , or the overlying ice and snow (not shown) results in chloride penetration into the concrete interior and, if not inhibited, contact with the metal reinforcing bar  6 - 18  (even numbers only) which will generally be composed of steel to create the undesired corrosion.  
         [0039]    For convenience of reference herein, the use of metal elements such as steel reinforcing bars  6 - 18  (even numbers only) will be discussed. It will be appreciated that corrosion inhibition of other types of metal elements such as those made of or coated with copper, tin or zinc, for example, may benefit from the present invention.  
         [0040]    In one embodiment of the invention, there is not only provided free nitrite, which oxidizes ferrous (Fe 2+ ) to ferric (Fe 3+ ) ion to thereby effect precipitation of Fe 2 O 3  to form an iron oxide barrier, but also provides means to sequester chloride which enters the concrete porosity by capturing the same in low solubility compounds.  
         [0041]    As employed herein the term “low-solubility compounds” means, chloride-containing compounds exhibiting solubilities substantially below those of sodium chloride or calcium chloride, and shall include, but not be limited to, chloride-containing compounds, which at saturation in aqueous solutions permit less than about 1 kg of soluble chloride per cubic meter of concrete. A chloride level of about 1 kg/m 3  is considered the threshold level for corrosion.  
         [0042]    In general, the invention contemplates the addition of any compound into which chloride ions would enter to produce a low solubility compound that sequesters the chloride.  
         [0043]    An example of a preferred reaction of the present invention, which accomplishes both the objective of creating an iron oxide barrier and the sequestering of chloride, is shown in reaction (1).  
         3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O+2Cl −           3CaO.Al 2 O 3 .CaCl 2 .nH 2 O+2NO 2   − .   (1)  
         [0044]    In this example 3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O wherein n=10 is added to fresh concrete as a particulate solid. The reaction that occurs is the chloride from the de-icing salts used on the hardened concrete reacts to produce Friedel&#39;s salt, which sequesters the chloride and, in addition, serves to release nitrite in order to oxidize any Fe 2+ . In adding the particulate compound, 3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O, is added to the fresh concrete, it is preferred that in general about 3 to 88 pounds of the particulate solid will be added per cubic yard of hydrated fresh concrete, and preferably about 22 to 66 pounds per cubic yard. The exact amount will be influenced by the anticipated rates of chloride ingress into the concrete having the usual range of water-to-cement ratios, e.g., 0.35 to 0.50. The admixture may, if desired, be employed in concrete having lower water-to-cement ratios such as 0.25 to 0.35, for example, or higher ratios such as 0.5 to 0.9, for example. In general, the higher the anticipated rate of chloride ingress, the larger the amount of particulate composition employed. The compound is admixed with the hydrated fresh concrete to achieve substantially uniform distribution thereof. When the concrete sets, this constituent will be present in the concrete to receive and interact with chlorine from the icing salts that penetrates the pores of the concrete. This compound (3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O) is generally stable over the range of pH values normally encountered in concrete. The resultant compound 3CaO.Al 2 O 3 .CaCl 2 .1OH 2 O is a low solubility compound within which the chloride is sequestered. This compound, is more stable than the nitrite. Chloride will exchange for the nitrite thereby freeing the nitrite and sequestering the chloride. As a result, the concentration of chloride in the concrete at the surface of the steel, such as re-bars  6 - 18  (even numbers only) will be reduced as compared with concrete not containing the compound. This same reaction may be employed with the same result substituting Fe 2 O 3  for Al 2 O 3  in the starting material. This would result in the reaction  
         3CaO.Fe 2 O 3 .Ca(NO 2 ).nH 2 O+2Cl −           3CaO.Fe 2 O 3 .CaCl 2 .nH 2  O+2NO 2   −   
         [0045]    In lieu of providing the compound such as 3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O in dry particulate form, it may be presented as a slurry with a pH of about 10 or greater with the particulate being present in the slurry in the range of about 5 to 60 weight percent and preferably about 10 to 35 weight percent. The slurry then would be admixed with the hydrated fresh concrete.  
         [0046]    In lieu of introducing the particulate solid or slurry into hydrated fresh concrete, if desired, one may admix the particulate solid or slurry with one or more of the dry components of the concrete such as the cement, for example.  
         [0047]    In lieu of the compound employed in reaction (1), other compounds may be used to create essentially the same reaction with the following differences. Among these compounds are, 3CaO.Fe 2 O 3 .Ca(NO 2 ) 2 .nH 2 O wherein n=0 to 24;  
         [0048]    3CaO.Al 2 O 3 .Ca(NO 3 ) 2 .nH 2 O wherein n=0 to 24;  
         [0049]    and 3CaO.Fe 2 O 3 .Ca(NO 3 ) 2 .nH 2 O wherein n=0 to 24.  
         [0050]    Also, 3Me(II)O.R 2 O 3 .Me(II)(anion) 2 .nH 2 O wherein Me(II) is one or more cations, R 2  is Al 2 , Fe 2  or Cr 2 , anion is NO 2 , NO 3  or OH and n=0 to 24 may be employed. These approaches, in many instances, involve a substitution in the compound employed in equation (1) for the aluminum, for the calcium or the nitrite. As to the substitution for the nitrite, this would be replaced by nitrate in equation (1) 3CaO.Fe 2 O 3 .Ca(NO 3 ) 2 .nH 2 O or (3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O. As stated hereinbefore, the anion may be divalent in which case the formula would be 3Me(II)O.R 2 O 3 .Me(II)(anion)nH 2 O wherein n is 0 to 18 and preferably 10 to 18. In other compositions, nitrite could be replaced by carbonate, borate or other anions.  
         [0051]    The nitrites have the advantage of sequestering chloride in addition to liberating a species capable of rapidly oxidizing ferrous (Fe 2+ ) ions near the surface of corroding seal to ferric (Fe 3+ ) ions to facilitate the formation of a protective layer of ferric oxide or hydroxide on the steel.  
         [0052]    It is understood that the value of “n”, meaning the number of waters of hydration, may vary, depending on the relative humidity to which the compounds are exposed.  
         [0053]    Among the preferred compounds for use in the invention are, 3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O and 3CaO.Fe 2 O 3 .Ca(NO 2 ) 2 .nH 2 O in terms of effectiveness for both chloride sequestration in concrete and protective oxide layer formation of metal embedded or in contact with concrete. It is preferred that n=0 to 24.  
       EXAMPLE 1  
       [0054]    In order to provide more detailed information regarding the manner of synthesizing the compounds, examples will be provided.  
         [0055]    In the synthesis of 3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O wherein n=0 to 24, the following procedure may be followed.  
         [0056]    In employing 3CaO.Al 2 O 3  the following process of synthesis may be employed:  
                         
 
         [0057]    In employing CaO.Al 2 O 3  the following process of synthesis may be employed:  
                         
 
         [0058]    The presence NaOH does not appear to interfere with sequestration of chloride or with the action of nitrite on steel and, as a result, it is not necessary to remove the NaOH by washing the product compounds. Alternatively, the 3CaO.Al 2 O 3  and Ca(NO 2 ) 2 .nH 2 O can be crystallized and separated from the NaOH solution.  
         [0059]    In each of these two examples, the Ca(OH) 2  and calcium aluminate were employed as fine powders. Ca(NO 2 ) 2  and NaNO 2  are commercially available and highly soluble in water. While there are no critical particle size distributions, in general, it is preferred to have a particle size such that 99% of the powder passes through a 325 mesh sieve. Commercially available Ca(OH) 2  was employed as was commercially available CaO.Al 2 O 3  with the latter being employed as a refractory cement. The synthesis in each case was carried out at room temperature by mixing the reactives with approximately 10 times their weight of water in suitable sealed containers. Their reaction occurred more rapidly if the contents of the containers were stirred or agitated. Optionally, if desired, grinding media such as Zirconia media, for example, may be placed in the containers.  
         [0060]    The nitrate chloride sequestering compound 3CaO.Al 2 O 3 .Ca(NO 3 ) 2 .nH 2 O wherein n=0 to 24 can be produced in the manner described in the foregoing two examples employing tri-calcium aluminate or mono-calcium aluminate and calcium hydroxide.  
         [0061]    In using 3CaO.Al 2 O 3  as a starting material, the following process can be employed.  
                         
 
         [0062]    wherein n=0 to 24.  
         [0063]    Employing CaO.Al 2 O 3  as the starting material, the following process can be employed.  
                         
 
         [0064]    wherein n=0 to 24.  
         [0065]    The presence NaOH does not appear to interfere with sequestration of chloride or with the action of nitrite on steel and, as a result, it is not necessary to remove the NaOH by washing the product compounds. Alternatively, the 3CaO.Al 2 O 3 .Ca(NO 3 ) 2 .nH 2 O and Ca(NO 3 ) 2  can be crystallized from the NaOH solution.  
       EXAMPLE 2  
       [0066]    The phase 3CaO.Fe 2 O 3 .CaCl 2 .nH 2 O wherein n=10 has been created by reacting the precursors 3CaO.Fe 2 O 3 .Ca(NO 2 ) 2 .nH 2 O and 3CaO.Fe 2 O 3 .Ca(NO 3 )  2 .nH 2 O with chloride. This indicates that chloride ions can be sequestered in the Fe analog of Friedel&#39;s salt (3CaO.Al 2 O 3 .CaCl 2 .10H 2 O). The compounds 3CaO.Fe 2 O 3 .Ca(NO 2 )  2 .nH 2 O and 3CaO.Fe 2 O 3 .Ca(NO 3 ) 2 .nH 2 O have also been produced employing 2CaO.Fe 2 O 3 . in the presence of supplementary Ca from Ca(OH) 2  and nitrite or nitrate from their calcium and/or sodium salts. 2CaO.Fe 2 O 3  may be produced by blending Fe 2 O 3  and CaCO 3  in a molar ratio of about 2:1 followed by sintering this mixture at 1150° C. for approximately 1.5 hours. The mixture of CaO and 2CaO.Fe 2 O 3  is produced by calcining 3 moles of CaCO 3  with 1 mole of Fe 2 O 3  at 1100° C. for approximately 1.5 hours. A variety of reaction times and temperatures can be used in the synthesis of this compound or this mixture. After cooling the 2CaO.Fe 2 O 3  or the mixture of 2CaO.Fe 2 O 3  and CaO were ground to an average particle size of approximately 10 microns using known comminution techniques.  
       EXAMPLE 3  
       [0067]    The compounds 3CaO.Fe 2 O 3 .Ca(NO 3 ) 2 .nH 2 O may be produced by calcining 1 mole of CaCO 3  with 3 moles of Fe 2 O 3  at 1100° C. for about 1.5 hours. This produces a mixture of CaO and 2CaO.Fe 2 O 3 . This mixture is then ground and reacted with either NaNO 3  or Ca(NO 3 ) 2  under basic conditions. In the event that NaNO 3  is used, it is preferred to add supplemental calcium. This may be added as CaO or Ca(OH) 2  for example.  
         [0068]    With respect to compound 3Me(II)O.R 2 O 3 .Me(II)(anion) 2 .nH 2 O wherein R 2  is Al 2 , Fe 2  or Cr 2 , anion is NO 2 , NO 3  or OH and n is 0 to 24 where Me(II) is a cation such as Ca, but may be partially substituted by other divalent cations or may be completely substituted by other divalent cations such as Ba, Sr, Mn, Zn, for example. For some compositions divalent anions such as carbonate or borate may be used.  
         [0069]    Referring to FIG. 2, wherein an existing concrete structure  2  having reinforcing metal elements  6 - 18  (even numbers only) is shown with an underlying deck member  4 , which may or may not be present in connection with the rehabilitation of existing concrete structures as provided in this embodiment of the invention. An overlay  30 , which in the form illustrated, it is concrete containing a compound usable in the present invention to sequester chloride ions with or without the capability of releasing nitrites to establish an oxide coating on the metal reinforcing member  6 - 18  is shown. This overlay  30  preferably has a porosity similar, or in excess of, to that of the concrete in the structure so as to permit free movement of chloride ions and nitrites therebetween. The thickness T of the overlay  30  may be in the order of 0.5 to 10 inches with a preferred thickness being about 1-4 inches.  
         [0070]    The overlay  30  may be established in situ and self-bonded to the upper surface  32  of the concrete structure. In the alternative, the overlay  30  may be a preformed panel containing the compound which may be secured to the concrete structure  2  by any desired means such as an adhesive material preferably provide a continuously between the overlay  30  and the concrete layer  2  without interfering meaningfully with porosity in the interchange between the two structural elements or may be provided in certain locations leaving other areas for surface-to-surface contact between the overlay  30  and the concrete member  2 . A suitable adhesive for this purpose is latex.  
         [0071]    In lieu of the concrete material employed in overlay  30 , other suitable materials having the desired strength, porosity and other characteristics needed for the present invention, may be employed. Among these are asphaltic materials, clay and clay-like materials and other cement materials including but not limited to Portland cements, blends of Portland cement with other materials such as fly-ash, slag or silica fume, calcium aluminate cements and mortars.  
         [0072]    The overlay  30  provides a number of beneficial actions, which facilitate rehabilitation of the existing concrete structure  2 . First of all, chloride will migrate out of the concrete  2  in response to the concentration gradient produced in the pore structure of the concrete  2 , the pore structure across the interface with the overlay  30  and with the pore structure of the overlay  30  itself. The admixture in the overlay  30  sequestered chloride ions that enter the overlay  30 . Nitrite will migrate from the overlay  30  into the concrete  2  and toward the reinforcing steel  6 - 18  (even numbers only) in response to the concentration ingredient produced in the pore structure of the concrete itself, in the pore structure across the interface at surface  32  between the concrete  2  and overlay  30  and within the pore structure of the overlay  30  itself. The nitrite facilitates formation of a protective coating on the metal reinforcing elements  6 - 18 , which may be composed of steel. All of this is accomplished without requiring prior art external electric current application. The system, therefore, results in passive chloride extraction.  
         [0073]    If desired, in order to enhance the efficiency of maintaining the desired continuous moisture path, through which the chloride ions and nitrite can move, additional wetting may be applied and a low porosity overlay (not shown) overlying the upper surface  33  of the overlay  30  may be provided in order to seal the moisture in the structure. Also, rain may enhance such moisture paths. The low porosity overlay  30  may be applied as a self-bonding coating established in situ or as a preformed element secured to surface  33 .  
         [0074]    In employing the process in connection with FIG. 2 and the embodiment describing in connection with FIG. 3, the compounds previously disclosed herein may be employed. It will be understood that those compounds which both sequester chloride ions and release nitrite will result in both the sequestration of chloride ion and releasing of nitrite serving to create the protective oxide layer around the metal reinforcing members  6 - 18  in the manner described herein.  
         [0075]    Referred to FIG. 3, there is shown an embodiment similar to that of FIG. 2 except that the overlay  30  has a lower portion which is a separately formed slurry  34  disposed between the upper surface  32  of existing concrete structure  2  and the upper portion of overlay  30  with the overall thickness of the overlay  30  remaining within the range of thickness T. The slurry will be porous to facilitate migration of chloride ions and nitrite between it and the underlying concrete structure  2 . The porosity of the slurry  34  will be such as to maintain communication with the underlying concrete  2 . The slurry  34 , which may be employed alone (not shown) or in combination with another portion of overlay  30  as shown in FIG. 3, will contain the compound employed to effect the objectives of the invention and may also include cements and sand as desired. In cases where slurry  34  is employed preferably alone it has a thickness of about ⅛ inch to 4 inches. In general, it will have a water to solids ratio of the slurry will facilitate its being pumpable or spreadable with the capability of hardening with the consumption of free water during formation of and 3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O. Wherein n=0 to 24. The water to solids ratios may be about 0.25-5 and preferably about 0.4 to 1.0. The slurry is pumped, sprayed, troweled or otherwise placed on the surface  32  to create slurry layer  34 . The thickness of the slurry preferably will be in the range of about 0.125 to 4 inches and if sand is not present in the composition, will preferably be in the range of about 0.25 to 0.5 inch. With sand, the range is preferably about 0.5 to 1.0 inch. It will be appreciated that if in lieu of the composition previously recited in this paragraph, the composition CaO.Al 2 O 3 .Ca(NO 3 ) 2 .nH 2 O, wherein n=0 to 24 were employed as nitrate is not regarded as a corrosion inhibitor in the sense of creating an oxide protective coating on the metal elements, this compound would provide solely a means for removing chloride ions from the concrete, but not inhibition of corrosion of embedded steel or other metal. The amount of the compound employed in a specific installation can be determined by the amount of chloride that has entered the concrete structure and can be determined readily by those skilled in the art.  
         [0076]    Referring to an embodiment wherein the vertical concrete structural be remediated, FIG. 4 shows a piling  40  which is generally vertically oriented and may be located under water. It has a plurality of elongated steel reinforcing members  42 ,  44 ,  46 ,  48 ,  50  embedded therein. A continuous clamshell  60  has been placed around the piling  40  to create an annular region  64  within which a slurry of the present invention may be introduced. The clamshell  60  may be in segments which are longitudinally adjacent to each other and secured to each other. They may be joined by bolts or other suitable mechanical means such as cables, or clamps. The annular region  64  has the slurry introduced after the clamshell  60  is placed in the space with the slurry being pumped in to displace water within an annular region  64 . In other respects, the system of the invention performs in the identical manner as previously described herein.  
         [0077]    It will be appreciated that depending upon the specific nature of the concrete structure to be remediated and the location and nature of the environment in which it is being employed, certain preferred refinements of this embodiment of the invention may be employed. For example, in situations where vehicular or foot traffic may be imposed on the concrete structure and an overlay with high strength should to be provided. Also, for example, in situations were the concrete structure will be subjected to a freeze-thaw cycles certain preferred approaches may serve to minimize the effects of the same. For example, an air-entrained admixture may be provided in slurry  34  of FIGS.  3  to counteract the effects of the freeze-thaw cycles. Such an approach might involve adding a chemical in a small amount, such as about 0.1% of the weight of the concrete, for example, to produce small bubbles when the concrete freezes the water in the porosity migrates into the bubbles and freezes harmlessly.  
         [0078]    An alternate way of minimizing the effect of the freeze-thaw cycle would be maintain a high ionic strength liquid in the porosity of the slurry. The more ions dissolved in water the lower the freezing temperature. For example, soluble nitrite salts such as calcium nitrite, calcium nitrite, sodium nitrate, or sodium nitrite may be employed for this purpose and function to increase the concentration ingredient in nitrite and thereby facilitate movement of nitrite into the concrete.  
         [0079]    Another compound suitable for use in the present invention would involve the use of the source of aluminum not coming from cement. This would result from the use of sodium aluminate NaAlO 4 . This may be accomplished by the following approaches.  
         2NaAlO 4 +3Ca(OH) 2 +Ca(NO 2 ) 2 →3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O+2NaOH  
         [0080]    wherein n=0 to 24 and preferably 0 to 12  
         [0081]    or  
         2NaAlO 4 +4Ca(OH) 2 +2NaNO 2 →3CaO.Al 2 O 3 .Ca(NO 2 ) 2 .nH 2 O +4NaOH  
         [0082]    wherein n=0 to 24 and preferably 0 to 12.  
         [0083]    It will be appreciated, therefore, that the present invention has provided an effective method and related compounds and structure for incorporating into concrete containing metal elements a class of compounds which will effectively resist undesired corrosion of the metallic compounds by both sequestration of chloride ions and provide a coating on the metallic elements, in some instances such as reactions that release nitrite. Other reactions, such as those which release nitrate alone, occur without providing such a coating.  
         [0084]    It will be appreciated that the compositions of the present invention may be combined with fresh concrete as defined herein in many ways. For example, the composition may be combined in solid form (a) with concrete in a plastic state (b) with ready mix concrete at a job site (c) at the time of batching or (d) inter-blended with mineral admixtures of materials such as slag, fly ash, or silica fume, or (e) may be interblended with cement, for example. It may also be combined in slurry form in a suitable liquid such as Ca(OH) 2  solution at the time of batching, for example. These approaches are all within the scope of the present invention.  
         [0085]    It will further be appreciated that the present invention provides a system for rehabilitation of an existing concrete structure through an overlay which contains compounds which serve to sequester chloride ions. It may also establish an oxide barrier layer on metal structural members associated with the concrete structure.  
         [0086]    Certain preferred compounds have been disclosed herein, along with their method of use and resultant structure.  
         [0087]    Whereas particular embodiments have been described herein for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details may be made without departing from the invention as defined in the appended claims.