NO151544B - PROCEDURE FOR AA PREVENT Corrosion of the reinforcement in concrete - Google Patents

Description

The present invention relates to a method thereof

species specified in claim l's preamble.

Concretes made from hydraulic cements, of which Portland cement is the most common example, are used as structural components in various applications, such as road construction, covering bridges, as building structures, multi-storey automobile warehouses, etc. In order to strengthen the concrete's properties and enable utilization as indicated above, these materials are normally used in combination with iron or steel reinforcing structures. These reinforcing metal structures, usually in the form of metal rods or rods, are exposed to attack by various corrosive components contained in the concrete, as well as the application of external corrosive components to the structure, such as chloride salts etc., which are usually used for the removal of ice and snow from roads, bridges, pavements etc. Furthermore, various structures are located in coastal installations etc. exposed to corrosive salt attack from the environment. Repair and replacement of such structures which have deteriorated as a result of the influence of these corrosive forces is extensive and in certain cases requires complete replacement of the structure, when it is not suitable for its intended use.

In an attempt to counteract the corrosive effects that normally occur in concrete structures, as mentioned above, various corrosion-inhibiting agents have been proposed for use as additives for use in their manufacture. For example, the use of sodium nitrite is disclosed in Japanese Patent No. 33-940. According to the patent, it is stated that sodium nitrite can be added to the cement and concrete during mixing to inhibit corrosion of reinforcing iron or steel bars and frameworks. The aggregate used was sea sand.

US Patent No. 3,210,207 discloses the use of mixtures of calcium formate with minor amounts of certain nitrite or chromate salts as corrosion inhibitors, for use as accelerators in cement.

US Patent No. 3,427,175 discloses the generic use of calcium nitrite as an accelerator that partially inhibits corrosion in alite cements. The calcium nitrite used may contain smaller amounts of sodium nitrite and may be used together with calcium chloride and other accelerators.

US patent no. 3,801,338 shows the use of a mixture of calcium formate and sodium nitrite as an additive for cement which is to contain metal reinforcement. It is stated that improved compressive strength is achieved along with sulfate resistance and that "a positive corrosion inhibiting effect" is achieved.

The use of sodium nitrite, as discussed in some of the above-mentioned patents, has been found to have a destructive effect as a result of spalling and promotes alkali aggregate reaction with the concrete constituents and is consequently a poor corrosion inhibitor. The use of potassium nitrite is stated in some of the patents to be an inhibiting agent when used in any type of cement mixture. It has been found that potassium nitrite only provides minimal corrosion resistance when used as indicated according to the state of the art.

The need for an effective corrosion inhibitor or cement mixture which is essentially capable of completely inhibiting corrosion of metals, contained therein, over a longer period of time is consequently very desirable within the construction and other industries where this type of material is used.

The invention relates to a method for inhibiting corrosion of such metal parts. The mixture comprises a high-strength concrete made from a hydraulic cement and which is capable of exhibiting a compressive strength after 28 days of at least 350 kp/cm 2 and contains at least 2% calcium nitrite.

The present invention is thus aimed at a method for preventing corrosion of metal pieces contained in concrete over a longer period of time. The method is characterized by what is stated in claim i's characterizing de 1.

The cement components present in the new concretes are hydraulic cements, such as Portland cement. These cements are well known and are produced by calcining a mixture of limestone and clay to give a clinker and by grinding the clinker a fine powder is obtained. The main constituents found in Portland cement are tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminum ferrite. It is believed that the tricalcium and dicalcium silicates are the main binder components in Portland cement. When tricalcium silicate is mixed with water, calcium silicate hydrate, known as tober-morite gel and calcium hydroxide, is formed. When dicalcium silicate comes into contact with water, similar products are formed but with a lower reaction rate. Tricalcium silicate, which exhibits the greatest reaction rate, largely determines the hardening of the cement. To provide materials suitable for different purposes, Portland cements with different setting rates are available. Four general types of Portland cement are usually produced, which in principle vary with respect to the relative amounts of tricalcium silicate and dicalcium silicate. Proportions of the main components in each type of cement are shown in Table I.

Concrete formations are exposed to various corrosion environments. In some cases, the environment is an inherent part of the concrete, for example when using a calcium chloride accelerator or when using chlorine-containing materials or chlorine-containing water. Other environments can be external, for example the use of calcium chloride and/or salt for snow and ice removal, exposure to salt showers or brine etc. Such environments tend to attack and corrode the metal parts in or in contact with the concrete. The present invention provides a method which inhibits such corrosion.

The critical factors believed to contribute significantly to the unexpected result with regard to achieving essentially complete corrosion inhibition of the metal parts contained in the cement over a longer period of time is the use of a concrete mixture with a high compressive strength of at least 350 kp/cm 2 during 28 day, as described below in combination with calcium nitrite used in an amount of at least 2% by weight, calculated on the dry weight of the concrete mixture.

The present method relates in particular to concrete mixtures as opposed to cement pastes or mortar mixtures. Cement pastes consist of hydraulic cement and mortar mixes consist of a hydraulic cement, sand and water. These materials do not exhibit the high strength and associated properties required by the new composition described below.

The method according to the invention requires the use of a concrete mixture which is capable of exhibiting a high compressive strength of at least 350 kp/cm 2 after 28 days, determined according to standard test methods, (for example ASTM). The concrete is a mixture of hydraulic cement, sand and aggregates in the form of a dry mixture that is ready to be mixed with water to cause hydration.

The required high strength property of the concrete mixture can be achieved in a number of different ways or combinations of such ways, for example by varying (a) the ratio of water to hydraulic cement during the manufacture of the concrete, (b) the cement content or factor, (c) the composition of the hydraulic cement, particularly its silicate content, (d) particle size fineness of the hydraulic cement used, and (e) size distribution of the aggregate used.

The concrete mixtures, with the necessary high strength required according to the invention can be prepared by using a ratio of water to cement at as low a value as is achieved during a simultaneous mixing of the components. The water to cement ratio should be 0.25 - 0.5, preferably 0.25 - 0.45. The ratio can be lowered without loss of mixing ability by utilizing conventional water-reducing agents and/or superplasticizers in such ways and in quantities that are well known in the state of the art.

The concrete should have a high cement content or factor, i.e. at least 2 80 - 500 kg of cement per m 3 concrete, preferably 3 35 - 50 0 kg. The cement mixtures that are suitable are hydraulic cements with a high content of silicate. The silicate components in the form of tricalcium silicate (C2S) and dicalcium silicate (C2S) should have a combined content of 50 - 90%, preferably 65 - 90%.

Another factor which contributes to the high strength of the obtained concrete mixture, necessary according to the present invention, is the particle fineness of the cement used. The cement should have a "Blaine" fineness in the range of 3,200 - 5,000 cm 2 /g, preferably 3,200 - 4,000 cm 2 /g.

The sand and aggregate should meet the requirements of American Concrete Institute (ACI) publication 211. High-strength concrete is achieved by using a maximum amount of large aggregates with a uniform gradation of the aggregates and sand particles down to the particle size of the cement for essentially to completely eliminate voids in the finished concrete construction.

The mixture should be essentially homogeneous and consolidated.

Calcium nitrite used in certain amounts and in combination with the above-described concrete provides a mixture which unexpectedly substantially eliminates corrosion of metal pieces contained therein over a longer period of time and thus allows for an extended life and elimination of repair of concrete structures made from such mixtures . The amount of necessary calcium nitrite is at least 2-3% by weight, calculated on the weight of the concrete mixture.

Larger quantities can be used if it is economically justifiable. Amounts exceeding 5% by weight are considered unnecessary.

Calcium nitrite can be added to the concrete using different techniques. Calcium nitrite can be added to the cement clinker before painting and can be carefully mixed with the cement component during the mixing step. Calcium nitrite can also be added to the dry concrete mixture and carefully mixed with it for an even distribution therein. Calcium nitrite can also be dissolved in the water used to prepare the concrete mixture. The concrete mixture can be premixed with water and then mixed or brought into contact with calcium nitrite. In general, any mixing method can be used which enables a substantially uniform mixing of calcium nitrite with the concrete mixture before it hardens.

Other conventional additives can be added to the present mixture in such ways and amounts as are well known in the art. Such additives can, for example, include water-reducing agents such as calcium lignosulfonate, glucose polymers, polysaccharides, etc., or superplasticizers such as polynaphthalene sulfonates, polymelamine formaldehyde sulfonates, etc. Other conventional means can be used in a known manner to the extent that they contribute to improved properties of the concrete obtained and do not reduce the necessary compression strength described above.

The invention is illustrated in the following examples in which all parts and percentages are per weight, unless otherwise indicated.

EXAMPLE 1

Concrete produced by a ready-mix supplier (maximum aggregate size 2.5 cm) and with a cement factor of 335 kg per m<3> was mixed with 2% by weight of calcium nitrate, calculated on the cement and to have a slump of 10 cm, an air content of 4.8% and a compressive strength after 28 days of 398 kp/cm 2, be introduced into a mold which measured 180 x 60 cm and with a thickness of 15 cm containing a double matted anchoring iron (diameter 1.6 cm), 5 cm from the center and extending the length of the mold and 2.5 cm below the level of the plastered concrete. The cement used had a "Blaine" fineness of 3600 +

300 cm <2>/g and the water/cement ratio was approx. 0.5.

Seven days after casting, where the slab was covered with sacks and plastic and kept wet, the mold was removed and the slab placed on piles 1 m above the ground. During the next five weeks a pond formed on the top surface and at the beginning of the seventh week a 3% sodium chloride solution (1.2 L) was distributed daily over the surface.

Open circuit potentials were measured frequently according to the procedure described in ACI publication SP-49, pages 71 - 82,

(1975), (California Transportation Laboratory Research Report CA-DOT-TL-5116-12-75-03 January 1975, and Uhlig, Corrosion & Corrosion Control, page 45 (1971)). After approx. At 6 months, the area exposed to corrosion (with a potential more negative than -0.350 volts relative to the copper-copper sulfate (CCS) reference electrode) was found to be zero on the plate containing calcium nitrite and 36.4% of the total area for a blank sample made from the same ingredients without calcium nitrite so that the concrete had a slump of 11.1 cm, an air content of 5.5% and a compressive strength after 28 days of 315 kp/cm^.

EXAMPLE 2

For comparison purposes, a concrete containing 2% calcium nitrite was prepared in a 280 liter mixer with a maximum aggregate size of 1.6 cm, 335 kg cement per m 3 of concrete, a water/cement ratio of 0.56 and a slump of 5.2 cm, 4.6% air and a compressive strength after 28 days of 316 kp/cm 2.

After approx. At 6 months, this concrete, cast in the same manner as described in Example 1, showed 93% involvement in corrosion, while a blank (no added calcium nitrite, a water/cement ratio of 0.57, a slump of 12.0 cm, compression strength after 28 days, 252 kp/cm 2) was 100% involved.

Calcium nitrite showed some effect in inhibiting corrosion, measured in the same way as described in Example 1 above. The area that had an open circuit potential of between -0.500 V and -0.550 V in relation to the CCS electrode was 0.3% for the plate containing calcium nitrite and 7.4% for the plate without calcium nitrite. The area recorded between -0.450 V and -0.500 V in relation to the CCS electrode was 9.4% for the plate containing calcium nitrite and 18.5% for the plate without calcium nitrite. However, the corrosion inhibiting effect for this experiment was not as clearly expressed as in Example 1.

EXAMPLE 3

Additionally, for comparison purposes, low strength cement mixtures were prepared in substantially the same manner as described in Example 2, with the following parameters:

Although the corrosion was less when using calcium nitrite, the unexpected complete inhibition was not achieved.

EXAMPLE 4

Concrete was prepared as indicated in Example 2, but with the addition of 120 ml of a calcium lignosulphonated based water-reducing agent gave the following parameters:

A trial period of one month corresponds to approx. 1 year of Kansas winter.

Of these samples, it can be seen that although the blank sample and the sample containing calcium nitrite showed high compressive strength, only the latter showed essentially no corrosion over a longer period of time.

Claims (4)

1. Method for preventing corrosion of the reinforcement in a calcium nitrite containing aggregate metal-reinforced concrete based on cement, characterized in that at least 2% by weight of calcium nitrite, calculated on the dry weight of the cement, is added to the concrete mixture, which has a total silicate content in the range of 50-90% by weight and that the weight ratio between cement, sand, aggregate and water is set in a manner known per se so that the concrete after hardening exhibits a compressive strength of at least 350 kp/cm after hardening for 28 days . 2. Method according to claim 1, characterized in that calcium nitrite is mixed in an amount of 2-3% by weight calculated on the dry weight of the cement. 3. Method according to claim 1, characterized in that the hydraulic cement is used in an amount of 275-510 kg per m and where the cement has a "BLAINE" fineness in the range 3200-5000 cm<2>/g and that the water to cement ratio is set at 0.25-0.5, which mixture after 28 days exhibits a compressive strength of at least 350 kp/cm2 . 4. Method according to any one of the preceding claims, characterized in that a superplasticizer and/or water-reducing agent is added to the uncured mixture in a manner known per se.