WO1991014660A1

WO1991014660A1 - Inhibiting corrosion of reinforcements in concrete

Info

Publication number: WO1991014660A1
Application number: PCT/AU1991/000073
Authority: WO
Inventors: Olaf Fritz Dielenberg
Original assignee: Olaf Fritz Dielenberg
Priority date: 1990-03-20
Filing date: 1991-03-06
Publication date: 1991-10-03

Abstract

A corrosion-inhibiting additive for use in a concrete-forming mixture is provided to inhibit the corrosion of steel reinforcements in a concrete structure cast from the concrete-forming mixture. The additive consists of a hexavalent chromium compound added to the mixing water of the concrete forming mixture in a preferred quantity of above 200 parts per million of the mixing water. CrO3 has been found to be particularly effective at concentrations at or above an optimum concentration of about 300 parts per million of the mixing water.

Description

INfflBITING CORROSION OF REINFORCEMENTS IN CONCRETE Field of the Invention

This invention relates to additives for use in concrete-forming mixtures to inhibit the corrosion of steel reinforcements in reinforced concrete structures, and also relates to a method of forming steel reinforced concrete structures wherein corrosion of steel reinforcements is inhibited by such a corrosion-inhibiting additive.

Background of the Invention

For many years, the corrosion of steel reinforcements embedded in concrete structures has been a serious problem. Such corrosion takes place at varying rates depending on local conditions, but is particularly serious in coastal areas. It is believed the initial stage of corrosion in a reinforced concrete structure is an electrochemical process causing the formation of oxidation products around the steel reinforcements. The initial corrosion can then lead to cracks in the concrete structure which, in adverse conditions such as a chloride environment, can accelerate the corrosion of the steel reinforcements and the formation of further and larger cracks in the concrete. Attempts at remedial work, such as the injection of epoxy resins into cracks in the concrete structures, are never wholly satisfactory, and often badly affected buildings or structures have to be pulled down completely and rebuilt.

It is known to use galvanised steel as a reinforcing element for a concrete structure and to protect a steel reinforcing element with a passivating coating to prevent or inhibit corrosion of the reinforcing element when it is embedded in a concrete structure. However, the cost of using galvanised steel or of providing a passivating coating on a steel reinforcement can be prohibitive for all but a few special constructions.

It is therefore desirable to provide a method of forming reinforced concrete structures so as to prevent or inhibit the steel reinforcements from suffering from corrosion, and thus reduce the liklihood of cracks appearing in the concrete structure.

It is also desirable to provide an additive for use in a concrete-forming

SUBSTITUTE SHEET mixture to inhibit corrosion of steel reinforcements in a reinforced concrete structure formed from the concrete-forming mixture.

It is also desirable to provide a relatively inexpensive corrosion-inhibiting additive, preferably in a form which may be readily added to a concrete-forming mixture for a reinforced concrete structure.

Background Art

Australian Patent Specifications AU-B-56119/80 and AU-B-22227/83 of W.R. Grace & Co. disclose the use of calcium nitrite in a concrete composition capable of inhibiting corrosion of metal pieces contained therein and as a major proportion of a corrosion-inhibiting additive for Portland cement compositions. Australian Patent Specifications AU-B-40231/85 and ATJ-B-40232/85 of Onada Corporation both disclose processes for inhibiting the corrosion of steel material within reinforced concrete which processes include the step of subjecting the surface of the reinforced concrete to an aqueous solution of calcium nitrite to impregnate the reinforced concrete with the solution. The use of calcium nitrite, however, has not been wholly successful and it is more convenient to use a corrosion-inhibiting additive initially in a concrete-forming solution than to apply a solution to the surface of reinforced concrete in the processes proposed by the Onada Corporation.

Summary of the Invention

It has now been discovered that CrO₃, a hexavalent chromium compound known as chromium trioxide but also commonly called chromic acid, is effective in inhibiting the corrosion of steel reinforcements in concrete, particularly when used in certain specific concentrations as a corrosion-inhibiting additive to a concrete-forming mixture.

According to a first aspect of the invention there is provided an additive for use in a concrete-forming mixture to inhibit the corrosion of steel reinforcements in a structure formed from the concrete-forming mixture, characterised in that the additive comprises a hexavalent chromium compound, preferably CrO₃.

SUBSTITUTE SHEET The corrosion-inhibiting additive of the invention may be added either in solid form to the mixing water of a concrete-forming mixture or as a ready-mixed solution containing a required concentration of additive in the solution. When CrO₃ is to be added in solid form to the mixing water of a concrete-forming mixture, it is preferably added as anhydrous chromium trioxide which is readily soluble in the mixing water.

It is believed that there is an optimum concentration of CrO₃ of approximately 300 parts per million of the mixing water for a concrete-forming mixture which is particularly effective in inhibiting corrosion of steel reinforcements in reinforced concrete structures. Concentrations of CrO₃ well below 300 parts per million of mixing water do not appear to be nearly so effective in inhibiting corrosion of steel reinforcements in concrete structures formed from the concrete-forming mixture, although it is believed that concentrations of at least 200 parts per million are to some extent effective. On the other hand, concentrations well above 300 parts per million may be at least as effective in inhibiting corrosion as the optimum concentration, but if the concentration of CrO₃ is too high (say in excess of 500 parts per million) an adverse discoloration of the concrete in the form of yellow stains may occur and it is not desirable to have too high a concentration of hexavalent chromium compounds for environmental reasons. Also, it is believed in practice that there is apparently little significant increase in the corrosion-inhibiting effect when concentrations above 300 parts per million are used and so it would appear to be wasteful to use concentrations much in excess of that amount.

Thus, in accordance with a second aspect, the present invention comprises the use of an additive in accordance with the first aspect of the invention in a concrete-forming mixture characterised in that the hexavalent chromium compound, preferably CrO₃, is added to or present in the mixing water of the concrete-forming mixture in a quantity of at least 200 parts per million of the mixing water, and preferably a quantity of at least about 300 parts per million of the mixing water. In a preferred embodiment, the CrO₃ is added to or present in the mixing ^"water in a quantity of approximately 300 parts per million.

According to a third aspect of the present invention there is provided a

SUBSTITUTE SHEET method of forming a reinforced concrete structure comprising the steps of: providing a concrete-forming mixture; providing one or more steel reinforcement elements for the structure; and casting the concrete-forming mixture around the steel reinforcement element or elements to form the reinforced concrete structure, characterised by the step of adding or including a hexavalent chromium compound, preferably CrO₃, in the concrete-forming mixture as a corrosion- inhibiting additive.

In a preferred method, the CτO₃, or other hexavalent chromium compound is present in, or added to mixing water for the concrete-forming mixture in a quantity of at least 200 parts per million of the mixing water and, preferably, in a quantity of at least about 300 parts per million of the mixing water.

In the method of the present invention the CrO₃ may be added as anhydrous chromium trioxide to the mixing water of the concrete-forming mixture, or as a ready-mixed solution of CrO₃.

Description of a Preferred Example

60g of anhydrous CrO₃ was added to 200 litres of mixing water for a concrete-forming mixture to provide a CrO₃ concentration of about 300 parts per million of the mixing water. The concrete-forming mixture was then cast around steel reinforcement elements to form a reinforced concrete block having a volume of approximately lm³.

The concrete block was placed in a corrosive atmosphere for approximately one year before being broken up. An examination of the steel reinforcement elements of the concrete block revealed no corrosion on the elements. In a similar test undertaken with a CτO₃ concentration of 200 parts per million of mixing water, rust was starting to appear on the steel reinforcements after one year.

Description of Simulated Test of CrO, as a Suitable Corrosion-Inhibitor Introduction

Tests were carried out on an inhibitor formulation to determine its suitability for use to protect steel reinforcements from corrosion in concrete. A

SUBSTITUTE SHEET solution of calcium hydroxide was used to obtain results in a shorter time than would be possible with casting concrete blocks. The solution is an approximation to concrete pore water although pore water does have additional alkali components. The exposure used involved air access to the stagnant solution which would be a reasonable approximation to young concrete mixes. However, once concrete cures, the interior has very restricted oxygen access while the test cell has some oxygen access from an open but unagitated solution surface. Notwithstanding these provisos, calcium hydroxide solution has been used instead of concrete pore water by groups researching corrosion of steel in concrete.

Four solutions were made up to 500mL using distilled water. The comparisons are given below.

The calcium hydroxide level was chosen as about 50% saturation. If the solution were saturated, steel would be expected to remain in a passive "non- corroding" state, the calcium chloride level was chosen to give a ratio of hydroxide to chloride of 1:1. This will be a corrosive solution as literature gives a criteria of exceeding a ratio of 0.6 as liable to cause corrosion. Weight loss coupons and electrochemical electrode probes were constructed from sections cut from black reinforcing bar. The electrochemical probes were cast in mounting resin and the exposed steel face polished.

Potential measurements were made over a 45 day period using a saturated calomel electrode and a digital multimeter. The data is reported in Table 1. Weight loss measurements were made on four, virtually identical coupons after

SUBSTITUTE SHEET 15 days exposure in the four solutions. The results are reported in Table 2.

TABLE 1 Potential Measurements of Steel in Ca (OH)₂, Ca CI₂, CrO₃ Solutions

Potential vs. SCE (mV)

-420 -373 -35* -576 -324 36 -559* -265 •212 -500 -241 -1S8

-464 -276 ^■25 -451 -364* •3a -477 -411 ^• ~~

-594 -483 Note: * At this time a distinct, rusty crevice was observed on the electrode at the epoxy/steel interface.

TABLE 2 Weight Loss Coupons after 15 days

Appearance before^* Cleaning

Etched patch in centre Etched region at shadowed edge Black scale in parts of shadowed face

No major attach but slight etch Note: Typical coupon weight was 30gm.

Discussion

The time limitation oh reporting meant that it was not possible to make up concrete blocks and test steel embedded in concrete. However, it is well known that steel in any environment does not corrode if its potential, ie., the voltage measured between the steel and a standard reference cell, is in a passive or immune range. (Passive conditions mean a thin inert film has formed on the surface of the steel while immune means the steel is not affected at all). The reference cell may be one of several types, e.g., copper/copper sulphate,

SUBSTITUTE SHEET silver/silver chloride or calomel (SCE). Prediction of the presence or absence of corrosion by measuring potentials is the basis of a technique used world wide to assess whether reinforcement in concrete is corroding. The technique has been codified by the American Society for Testing Materials in ASTM standard C876. The potential measurements show clearly that, after an initial, brief stabilisation period, the inhibitor in its recommended optimum (300 ppm) concentration holds the steel in a passive condition for more than a week. In the case of double concentration inhibitor, the inhibiting effect persists for at least two weeks. The half strength inhibitor is ineffective probably because the solution contained chlorides from the initial mixing. However, the electrode (and weight loss) samples in Solution 2, apparently suffered worse attack than in the non-inhibited solution. This promotion of localised attack is known in cooling water with chromate inhibitors for solutions with high chloride to chromate rations. This phenomenon may also contribute to the eventual corrosion in the crevice between the electrode and its mounting compound as such crevices tend to concentrate chlorides. This could eventually overwhelm the local chromate concentration.

The test environment is similar to steel in concrete but the difference in oxygen access and particularly the presence of chlorides form the initial mixing means these tests are very severe. There is clear evidence that the inhibitor is effective at and above its recommended optimum concentration.

Conclusions

Potential measurements, weight loss and visual assessment shows that chromium trioxide is an effective inhibitor for steel in chloride-rich, concrete like solutions particularly if it is dosed above 300 ppm.

There appears to be a corrosion accelerating effect at a concentration of 150 ppm but this may not be applicable to steel in concrete as the test conditions, particularly oxygen access and chloride levels, may not be representative.

SUBSTITUTE SHEET

Claims

oCLAIMS

1. An additive for use in a concrete-forming mixture to inhibit the corrosion of steel reinforcements in a structure formed from the concrete-forming mixture, characterised in that the additive comprises a hexavalent chromium compound.

2. An additive according to claim 1 characterised in that said hexavalent chromium compound is CrO₃.

3. The use of an additive according to claim 1 or claim 2 in a concrete- forming mixture characterised in that the hexavalent chromium compound is added to or present in mixing water for the concrete-forming mixture in a quantity of at least 200 parts per million of the mixing water.

4. The use of an additive according to claim 3 characterised in that the hexavalent chromium compound is added to or present in the mixing water of the concrete-forming mixture in a quantity of at least about 300 parts per million of the mixing water.

5. The use of an additive according to claim 3 characterised in that the hexavalent chromium compound is CrO₃ and is added to or present in the mixing water of the concrete-forming mixture in a quantity of approximately 300 parts per million of the mixing water.

6. A method of forming a reinforced concrete structure comprising the steps of: providing a concrete-forming mixture; providing one or more steel reinforcement elements; and casting the concrete-forming mixture around the steel reinforcement element or elements to form the reinforced concrete structure, the method being characterised by the step of adding or including a hexavalent chromium compound in the concrete-forming mixture.

SUBSTITUTE SHEET

7. A method according to claim 6 characterised in that the hexavalent chromium compound is present in the concrete-forming mixture in a quantity of at least 200 parts per million of mixing water for the concrete-forming mixture.

8. A method according to claim 7 characterised in that the hexavalent chromium compound is present in the concrete-forming mixture in a quantity of at least about 300 parts per million of the mixing water.

9. A method according to claim 7 characterised in that the hexavalent chromium compound is present in the concrete-forming mixture in a quantity of approximately 300 parts per million of the mixing water.

10. A method according to any one of claims 6 to 9 characterised in that the hexavalent chromium compound is CrO₃.

11. A method according to claim 10 characterised in that the CrO₃ is added in solid form to mixing water in the concrete-forming mixture.

12. A method according to claim 10 characterised in that the CrO₃ is dissolved in solution prior to being added to the concrete-forming mixture.

13. A concrete-forming mixture for use in the method of claim 6 characterised in that the mixture includes CrO₃ in solution present in a quantity of approximately 300 parts per million.

SUBSTITUTE SHEET