Method of resisting corrosion in metal reinforcing elements contained in concrete and related compounds and structures

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. Ser. No. 10/010,581, filed Nov. 13, 2001 and now U.S. Pat. No. 6,610,138.

BACKGROUND OF THE INVENTION

1. Field of the Invention

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.

2. Description of the Prior Art

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 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.

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.

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 the oxidation of the metal metallic iron to Fe

2+

is catalyzed by the chloride. Also, oxides and 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.

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.

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

The present invention has met the above-described need.

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.

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.

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.

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 Ca 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 24, and preferably 10 to 24. 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.

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.

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.

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.

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.

It is yet another object of the present invention to provide such a system which employs unique compounds.

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.

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.

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

FIG. 1

is a schematic cross-sectional illustration of a concrete bridge deck containing metal reinforcing elements.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As employed herein, the term “fresh concrete” means concrete which is in a plastic state.

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.

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.

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.

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.

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.

As employed herein the term “low-solubility compounds” means, chloride-containing compounds exhibiting solubilities substantially be 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.

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.

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)

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'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

−

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.

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.

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;

3CaO·Al

2

O

3

·Ca(NO

3

)

2

·nH

2

O wherein n=0 to 24;

and 3CaO·Fe

2

O

3

·Ca(NO

3

)

2

·nH

2

O wherein n=0 to 24.

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.

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.

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.

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

In order to provide more detailed information regarding the manner of synthesizing the compounds, examples will be provided.

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.

In employing 3CaO·Al

2

O

3

the following process of synthesis may be employed:

In employing CaO·Al

2

O

3

the following process of synthesis may be employed:

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.

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.

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.

In using 3CaO·Al

2

O

3

as a starting material, the following process can be employed.

Employing CaO·Al

2

O

3

as the starting material, the following process can be employed.

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

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'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

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.

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.

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.

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.

Certain preferred compounds have been disclosed herein, along with their method of use and resultant structure.

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.