SE452454B - ARMED CONCRETE CONSTRUCTION AND PROCEDURES FOR PRODUCING THEREOF - Google Patents

Description

NO 452 454 2 which are commonly used to remove ice and snow from roads, bridges, walkways and the like. Furthermore, various structures, which are located on the coast and the like, are exposed to corrosive salt attack from the surroundings. Repair and replacement of such structures, which have been destroyed due to the effects of such corrosive forces, is expensive, and in some cases it is even required. a complete replacement of the design, ef- In order to try to counteract the corrosive effects, which normally because it is unsuitable for the intended use. come in connection with concrete structures, as discussed above, various corrosion inhibitors have been proposed for use as mixtures, which are used in the manufacture of the concrete. For example, the use of sodium nitrite is described in Japanese Patent Publication No. 53-9NO, published February 15, 1958, Application No. 30-53777, filed December 27, 1955, Kano et al. The said patent publication mentions that sodium nitrite can be added to cement and concrete during the admixture to inhibit corrosion of reinforcing iron and steel bars and frames. The ballast material used was sea sand.

U.S. Patent No. 5,210,207 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 cementitious materials.

U.S. Patent No. 3,277,175 discloses the general use of calcium nitrite as an accelerator, which partially inhibits corrosion in elite cement. Calcium nitrite may contain minor amounts of sodium nitrite and may be used in combination with calcium chloride and other accelerators.

U.S. Pat. No. 3,801,558 discloses the use of a mixture of calcium formate and sodium nitrite for addition to cement, which is intended to contain metal reinforcement. The result is stated to be improved compressive strength together with sulphate resistance, and the additive has a "positive corrosion inhibiting effect".

The use of sodium nitrite, which is mentioned in some of the above references, has been found to have detrimental effects in the form of weathering and to promote alkali-ballast reaction within the concrete composition, so that said compound is a poor corrosion inhibitor. Calcium nitrite is stated in some publications to be an inhibitory agent, when used in conjunction with any HO 3 452 454 I type of cement composition. However, it has been found that calcium nitrite provides only minimal corrosion resistance when used in the methods described in the prior art.

The need for a highly effective corrosion inhibitor for a cement composition, which inhibitor exhibits the ability to practically completely inhibit corrosion of the metal parts contained therein for long periods of time, is very great in the construction industry and other industries using this type of material.

The present invention relates to a concrete composition which exhibits practically complete corrosion inhibition with respect to metal details contained therein for a long period of time, and to a method for inhibiting corrosion of said metal parts. The composition comprises a high strength concrete formed from a hydraulic cement, has a compressive strength of at least 35 MPa after 28 days and contains at least 2% calcium nitrite. The present invention thus relates to concrete compositions which have the ability to practically completely inhibit corrosion of metal parts therein for long periods of time. The compositions comprise an intimate mixture of a hydraulic cement, aggregate and sand, which gives a concrete composition with high strength and capable of exhibiting a compressive strength of at least 35 MPa after 28 days, the composition containing calcium nitrite in an amount of at least 2 % calculated on the dry weight of the cement.

The cement components of the present concrete material are hydraulic cement materials, such as e.g. Portland cement. These cementitious materials are conventional and are made by calcining a mixture of limestone and clay to form a clinker, which is then ground to a fine powder. The main components of Portland cement are tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite. The tricalcium and dicalcium silicates are believed to be the major binding components in Portland cement. Tricalcium silicate, when mixed with water, forms a calcium silicate hydrate, known as tobermorite gel, and calcium hydroxide. When the dicalcium silicate comes into contact with water, it forms similar products but with a much lower reaction rate. The tricalcium silicate, which has the higher reaction rate, largely determines the rate of hardening of the cement.

For the production of materials suitable for various uses, Portland cementitious materials are commercially available which have a range of different bonding rates. Four general types of Portland cement, which in principle vary in relative quantities of tricalcium silicate and dicalcium silicate therein, are commonly manufactured. The proportions of the main compounds present in each type of cement are shown in Table 1.

TABLE I Cement type. . . . . . . . . . . . . . . ................. I II III Composition, weight percent Tricalcium silicate ......... . . . . . . .. 53 47 58 Dicalcium silicate ........ . . . . . . . . .. 2% 32 16 Tricalcium aluminate. . . . . . . . . . . ..; .. 8 3 Tetracalciumaluminoferrite ......... 8 l2 In connection with the production of concrete, various corrosive environments occur. In some cases, the environment is an integral part of the concrete, e.g. when using calcium chloride accelerator or using chloride-containing materials or chloride-containing water. Other environments may be temporary, e.g. use of calcium chloride and / or salt to remove snow and ice, exposure to salt spray or saline solutions and the like. Such environments tend to attack and corrode metal details in or in contact with the concrete. According to the present invention there is provided a composition and method by which such corrosion is inhibited. The critical factors which are believed to contribute significantly to the surprising result, which means that one obtains practically complete corrosion inhibition of metal parts contained in concrete for long periods of time, is the use of a high strength concrete composition, which composition exhibits the ability to achieve a compressive strength of at least 35 MPa after 28 days, as described below, in combination with calcium nitrite, used in an amount of at least 2% by weight based on the weight of the dry concrete composition.

The present invention relates in particular to concrete compositions as opposed to cement pastes or cement mortar compositions BO HO 452 454 n ings; the cement pastes comprise a hydraulic cement and water, while the cement mortar mixtures comprise a hydraulic cement, sand and water. These materials do not exhibit the high strength and related properties required for the present composition.

The composition of the present invention requires the use of a concrete composition which exhibits a high compressive strength of at least 35 MPa after 28 days, which compressive strength is determined by means of standard tests in this technique (see e.g.

ASTM). The concrete is a mixture of hydraulic cement, sand and aggregate material as a dry mixture, which is ready to be mixed with water to achieve hydration.

The required high strength of the concrete composition can be achieved by any of a number of methods or by a combination of such methods, such as e.g. by varying (a) the ratio of water to hydraulic cement used in the production of the concrete; (b) the cement content or factor; (c) the hydraulic cement composition, especially the silicate content therein; (d) the fineness of the particle size of the hydraulic cement used; and (e) the size and distribution of the ballast material used.

High strength concrete compositions, as required by the present invention, can be prepared by maintaining as low a water to cement value as possible for the components to mix. The ratio of water to cement should be from 0.25 to 0.5 and preferably between 0.25 and 0.1 N5. The ratio can be lowered without losing the mixing ability, by using conventional water reducing agents and / or plasticizing additives ("superplasticizers") in the ways and in the amounts well known to those skilled in the art.

The concrete in question should have a high cement content or high cement factor, at least about 5-9 bags (standard bags with H3 kg) per 0.76 m3 of concrete, preferably from about 6-9 bags. The cement compositions that are suitable are hydraulic cements with a high silicate component content. The silicate components in the form of tricalcium silicate (C38) and dicalcium silicate (C25) together should make up from about 50 to 90% and preferably from about 65 to 90%.

Another factor which contributes to the high strength of the obtained concrete composition of the present invention is the fineness with respect to the particle size of the cement used. The cement should have a Blaine fineness of between about 3200 and 5000, preferably between 5200 and H000, cm 2 / gm.

The sand and ballast material should meet the requirements of American Concrete Institute (ACI) Publication 2ll, the contents of which are hereby incorporated into the present text. High-strength concrete is obtained by using a maximum amount of large ballast material with constantly decreasing ballast and sand particles down to a value close to the particle size of the cement, in order to obtain practically complete elimination of voids in the finished concrete structure. clean. The mixture should be made practically homogeneous and consolidated. Calcium nitrite, when used in certain amounts and in combination with the concrete described above, forms a composition which, in a surprising manner, practically completely eliminates corrosion of metal parts contained therein over a long period of time, thus giving extended service life. and elimination of repairs of concrete structures formed by such compositions. The required calcite size is from at least 2% by weight to 3% by weight based on the weight of the concrete composition. Higher amounts can be used, if this is economically defensible. Quantities in excess of 5% by weight are, however, considered unnecessary. Calcium nitrite can be added to the concrete in different ways. Calcium nitrite can be added to cement clinkers before grinding and can be mixed thoroughly with the cement component during the grinding step. The calcium nitrite can also be added to the dry concrete mixture and mixed thoroughly so that it is homogeneously dispersed therein. The calcium nitrite can be dissolved in the water used to prepare the concrete composition. The concrete mixture can be premixed with water and then mixed or brought into contact with the calcium nitrite. In general, it is possible to use any mixing method which enables practically homogeneous mixing of the calcium nitrite with the concrete mixture before it forms a hardening composition.

Other conventional mixtures may be added to the present composition in a manner and in amounts well known to those skilled in the art. Such mixtures can e.g. include water reducing agents such as calcium lignosulfonate, glucose polymers, polysaccharides and the like, or superplasticizing additives such as polynaphthalene sulfonate, polymelaminiformaldehyde sulfonate and the like. Other conventional means may be used in known manner provided that they contribute to the superior properties of the concrete obtained and do not impair the compressive strength to a value which is below the previously described required value.

The following examples are given for illustrative purposes only and are not intended to limit the invention other than as set forth in the appended claims. All parts resp. percentages refer to parts by weight resp. weight percent, unless otherwise stated. c EXAMPLE 1 Concrete made from a finished mixture (maximum size of the aggregate material 2.5 cm) with a cement factor of 6 bags per 0.76 m5 was mixed with 2% calcium nitrite, calculated on the weight of the cement, so that a drop of 10 cm was obtained. , an air content of U, 8% and a compressive strength after 28 days of 39.2 MPa, whereupon the mixture was placed in a mold with the dimensions 1.8 x 0.6 m and with a thickness of 15 cm, which mold contained a double mat of reinforcing bars (l5.9 mm diameter), 5 cm in the center along the length of the mold and 2.5 cm below the level of the brushed concrete.

Seven days after casting, keeping the board covered with hemp fabric and plastic at all times and keeping it wet, the mold was removed, and the plate was placed on pillars 0.9 m above the ground. For the next five weeks, a dust was constructed on the upper surface, and at the beginning of the seventh week, 3% sodium chloride solution (1200 ml) was spread over the surface daily. open circuit potentials were often measured according to the procedures described in ACI Publication SP-H9, pages 71-82 (1975), California Transportation Laboratory Research Report CA-DOT-TL-5116-12-75-03 January 1975, and Uhlig, Corrosion & Corrosion Control, p. M5 (1971). It is found that the surface which was subjected to corrosion (with a more negative potential than -0.550 volts relative to a copper-After approximate six months vis-à-vis copper-sulfate (CCS) reference electrode) containing calcium nitrite and 36, H % of the total area of a was zero on the floor comparative sample made of the same components but without calcium nitrite, so that the concrete had a depression of ll, l cm, an air- \ .1 ~ | Ul I ..

C. J 452 454 s content of 5.5% and a needle fascia of 31.0 MPa after 28 aagaí-fl.

EXAMPLE 2 For comparison, a concrete containing 2% calcium nitrite was manufactured in a 0.28 m3 mixer with a ballast material with a maximum size of 16 mm, 6 bags per 0.76 m3 cement, a water / cement ratio of 0.56. a drop of 8.3 cm, an air content of U, 6% and a compressive strength after 28 days of 51.1 MPa.

After about 6 months, this concrete, which had been cast in the form of a slab in the same manner as in Example 1, showed corrosion to the extent of 93%, while the zero sample (no calcium nitrite, a water / cement ratio of 0) 57, a drop of 12.1 cm, 4.51 air and a compressive strength after 28 days of 2 Å, 8 MPa) showed 100% corrosion.

Calcium nitrite exerted a certain effect in inhibiting corrosion, in a measurement in the same way as described in * Example 1 above. The range with an open circuit potential between -D, 500 and -0.550 relative to the CCS electrode was 0.3% for the plate containing calcium nitrite and 7.0% for the plate or floor without calcium nitrite.

JWLL ". _. .__. _- .. .. .v- ¿_-___» ¿; ____ = ¿-__. G_, The range for which a value was recorded between -O, ü5O V and -0,500 V relative to the CCS electrode, was 9, ü% for the calcium nitrite plate and 18.5% for the plate without calcium nitrite, Nevertheless, the corrosion inhibiting effect in this series was far from as pronounced as in Example 1.

EXAMPLE 3 Also for comparison purposes, low strength concrete compositions were prepared in substantially the same manner as described in Example 2, which compositions exhibited the following parameters: Inhibited Zero Sample 6 Cement factor, sacks / 0.76 m3 6 12.7 Immersion, cm 10.2 E 0 , 59 Water / cement 0.57 14.8 Air, zi 14.8 2% Calcium nitrite 0 71, Å%% of the surface included in corrosion 100 Although the corrosion was reduced by the use of calcium nitrite, it was not surprisingly practically full. - constant inhibitions.

EXAMPLE U Concrete made essentially as described in Example 299 452 454 I e but with the addition of 8.9 cl of calcium lignosulfonate-based water reducing agent gave the following parameters: Inhibited Zero Test 6 Cement factor, sacks / 0.76 m3 6 lü, 0 Immersion, cm 14.0 0.55 Water / cement 0.53, 2 Air,% 5.8 ü3, l Compressive strength, 28 days, MPa 35.3 2% Calcium nitrite O 0%% of the surface included in corrosion after 6 months 27.6 % zero% of the surface including corrosion after 19 months 100% A test in one month is roughly equal to one year of winter in Kansas.

From these samples it can be seen that, although both the blank and the samples containing calcium nitrite showed high compressive strength, only the latter showed virtually no corrosion at all after a long period of time.

Claims (6)

452 454 10 PATENT REQUIREMENTS 1. Reinforced concrete structure formed by a concrete mixture with high strength and with metal details embedded in it, characterized in that the concrete mixture is formed of hydraulic cement, sand, aggregate material and water with the addition of at least 2 percent calcium nitrite, calculated on the dry weight of the cement content in the concrete mixture, the proportions between the cement, the sand, the aggregate material and the water being selected in a manner known per se so that the resulting concrete after 28 days of hardening has a compressive strength of at least 35 MPa. Reinforced concrete structure according to claim 1, characterized in that the calcium nitrite is present in from 2 to 3 percent based on the dry weight of the cement. Reinforced concrete structure according to claim 1 or 2, characterized in that it also contains a plasticizing additive in the form of a so-called superplasticizer. 4. A process for producing a reinforced concrete structure formed from a high strength concrete mixture and having metal details embedded therein, characterized in that a non-hardened concrete composition is prepared by substantially homogeneously mixing a concrete mixture formed by hydraulic cement, sand, aggregate and water, with at least 2% calcium nitrite, calculated on the dry weight of the cement content of the concrete mix, the proportions between the cement, sand, aggregate and water being selected in a manner known per se so that the resulting concrete after 28 days hardening has a compressive strength of at least 35 MPa, embeds metal details in the uncured concrete structure, while forming a shaped structure, and allows the composition to harden. 5. A process according to claim 4, characterized in that from 2 to 3 percent of the calcium nitrite is mixed into the concrete mixture. ' Method according to one of Claims 4 to 5, characterized in that a plasticizing additive in the form of a so-called so-called additive is mixed into the non-hardened concrete composition. superplasticizer. _