DK157918B - ARMED CONCRETE CONSTRUCTION WITH CORROSION-INHIBITING PROPERTIES AND PROCEDURES FOR PRODUCING THEM - Google Patents

Description

DK 157918 B

The present invention relates to a reinforced concrete structure and to a process for its preparation which includes the use of certain concrete mixtures which have unexpectedly been found to completely inhibit corrosion of embedded reinforcing iron or ferrous metal pieces for an extended period of time. The present invention relates to a corrosion-inhibiting concrete mixture consisting of high-strength concrete, made from hydraulic cement and containing at least approx. 2% calcium nitrite, calculated on the dry weight 10 of the cement.

Concrete made from hydraulic cements, of which Portland cement is the most common example, is used as building components for various applications, such as for the construction of roads, bridge decks, building structures, 15 parking high-rise buildings and the like. To improve the properties of the concrete so that it can be used for these purposes, it is usually used in combination with iron or steel reinforcing structures. These reinforcing metal structures, usually in the form of metal bars or gratings, are subject to attack by the various corrosive elements in the concrete, as well as attacks due to the application of external, corrosive elements to the structure, such as chloride salts and the like, which are commonly used. in connection with the removal of ice and snow from roads, bridges, 25 walkways and the like. In addition, various structures in installations located on the coasts and similar sites are subject to corrosive salt attack from the surroundings. Repair and replacement of such structures, which have been weakened by the effects of such corrosive forces, are extensive and in some cases require a complete replacement of the structure as it is not suitable for the intended use.

In an attempt to counteract the corrosive effects that concrete structures are normally subjected to, as noted above, various corrosion inhibiting agents have been proposed as admixtures in the manufacture.

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By way of example, the use of sodium nitrite is described in Japanese Patent Specification No. 33-940. According to this patent, sodium nitrite can be added to cement and concrete during mixing to inhibit corrosion of reinforcing steel and steel bars and frames. Sea sand was used as an aggregate.

According to U.S. Patent No. 3,210,207, as accelerators in cement, mixtures of calcium formate with minor amounts of certain nitrite or chromate salts 10 can be used as corrosion inhibitors.

U.S. Patent No. 3,427,175 discloses the use of calcium nitrite in amounts of preferably 1-4% of the cement weight as an accelerator which partially inhibits corrosion in alitments. It is mentioned that this addition has some inhibitory effect on the corrosion tendency of the reinforcing iron, but this effect is not documented. The calcium nitrite can contain smaller amounts of sodium nitrite and can be used with calcium chloride and other accelerators.

According to U.S. Patent No. 3,801,338, a mixture of calcium formate and sodium nitrite can be used as an additive to cement containing metal reinforcement. According to this, an improved compressive strength is obtained along with a sulfate resistance, in addition to obtaining "a positive corrosion inhibition effect".

25 As described in some of the foregoing references, the use of sodium nitrite has been found to have deleterious outflow effects and to promote alkali addition reaction in the concrete mixture, and thus to be a poor corrosion inhibitor. In the above references, calcium nitrite 30 is described as being an inhibiting agent when used with any type of cement mixture. It has been found that calcium nitrite provides only a minimum of corrosion resistance when used in the manner described and known.

35 In the construction industry as well as various other industries using this type of material, there is a need 3

DK 157918 B

for a highly effective corrosion inhibition or for a cement mixture which can substantially completely inhibit corrosion of reinforcing iron or ferrous metal pieces contained herein for a prolonged period of time.

The present invention relates to reinforced concrete structures made of high strength concrete mixtures and with embedded reinforcing bars or ferrous metal pieces which are characterized by the characterizing part of claim 1. The blends comprise an inner blend of hydraulic cement, aggregate and sand and water to form a high strength concrete blend which can exhibit a compressive strength of at least 35 MPa after 28 days and with calcium nitrite in an amount of at least 2% based on the said cement's dry weight. The invention also relates to a method for producing the reinforced concrete structures of the invention, which is characterized by the characterizing part of claim 5.

As cement components of the present concrete types, hydraulic cements such as Portland cement are used. These 20 cements are well known in the art and are prepared by calcining a mixture of limestone and clay to form clinker and by grinding the clinker into a fine powder. The most important compounds in Portland cement are tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminum ferrite. The tricalcium and dicalcium silicates are believed to be the major binding constituents of the Portland cement. Tricalcium silicate in admixture with water forms a calcium silicate hydrate gel, known as tobermorite gel and calcium hydroxide.

The dicalcium silicate, when in contact with 30 water, forms similar products, but at a much lower reaction rate. The tricalcium silicate, which has the greatest reaction rate, largely determines the rate of cementing of the cement. In order to provide materials suitable for various applications, Portland cements having different bonding rates have been commercially produced. Four common types of Portland cements, as per

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4 varies substantially with respect to the relative amounts of tri-calcium silicate and dicalcium silicate present are generally produced. The relationships between the major compounds present in each type of cement are shown in Table I.

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Table I

Cement type I II III IV

Composition, weight%:

Tricalcium silicate 53 47 58 26

Dicalcium Silicate 24 32 16 54

Tricalcium aluminate 8382

Tetracalcium aluminum ferrite 8 12 8 12 15 _

Concrete structures are exposed to various corrosion environments. In some cases, the environment is a solid component of the concrete, e.g. such as using a calcium chloride accelerator or chloride-containing materials or chloride-containing water. In other cases, the environment may be an outside, e.g. use of calcium chloride and / or salt against snow and ice, exposure to salt mist or salt water, and the like. Such environments tend to attack and corrode metal parts in or in contact with the concrete.

The decisive factors, which are primarily thought to contribute to the unexpected result of achieving virtually complete corrosion inhibition of reinforcing iron or ferrous metal pieces in concrete over a longer period, is the use of a high strength concrete mixture which can achieve a compressive strength of at least 35 MPa in 28 days, as described below, in combination with calcium nitrite used in amounts of at least 2% by weight, based on the weight of the dry cement.

In particular, for the present invention concrete mixes are used, unlike cement adhesives or mortar mixtures, wherein

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the cement adhesives are composed of a hydraulic cement and water, and the mortar mixtures are composed of a hydraulic cement, sand and water. These materials do not exhibit the high strength and related properties required by the composition described herein.

The present invention requires the use of a concrete mixture capable of exhibiting a compressive strength of at least 35 MPa in 28 days, determined from standard tests in this field, cf. eg. ASTM. The concrete is a mixture 10 of hydraulic cement, sand and aggregate in the form of a dry mix which is ready to mix with water so that hydration takes place.

The required high strength of the concrete mixture can be obtained by any of a number of methods 15 or by a combination of such methods, such as by varying a) the ratio of the water to the hydraulic cement used in the preparation of the concrete, b (c) the hydraulic cement composition, especially the silicate content; (d) the particle size travel properties of the hydraulic cement used; and (e) the size and distribution of the aggregate used.

High strength concrete mixtures can be made by keeping the water-cement ratio as low as possible when mixing the components should be possible. The water / cement ratio should be from 0.25 to 0.5, preferably between 0.25 and 0.45. The ratio can be lowered without loss of mixing ability using ordinary water / cement ratio reduction agents and / or superplasticizers according to methods and in amounts known in the art.

The concrete in question should have a high cement content or a high cement factor, ie. at least approx. 7 to approx. 12 sacks (standard 43 kg), ie. from approx. 301 to approx. 516 kg of cement per m ^ concrete, preferably from about 8 to about 12 bags, 35 ie. from approx. 344 to approx. 516 kg of cement. Suitable cement mixtures are hydraulic cements with a high content of silica.

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6 cat components. The silicate components in the form of tricalcium silicate (C3S) and dicalcium silicate (C2S) should be present in a total content of approx. 50% to approx. 90%, preferably from approx.

65% to approx. 90%.

Another factor contributing to a high strength of the resulting concrete mix used in accordance with the invention is the particle size fineness of the cement used. The cement should have a Blaine fineness between approx. 3200 cm 2 / g and 5000 cm 2 / g, preferably between 3200 cm 2 / g and 4000 cm 2 / g.

10 The sand and grout must conform to the specifications of Publication No. 211 of the American Concrete Institute (ACI).

High-strength concrete is prepared by using the largest possible amount of a large aggregate with smooth aggregate and sand particle gradation down to about the cement particle size 15 to achieve a near complete elimination of pores in the final concrete structure.

The mixture should be made practically homogeneous and uniform.

When calcium nitrite is used in certain quantities and in combination with the above-mentioned concrete, it forms a mixture which unexpectedly eliminates corrosion of reinforcing iron or ferrous metal pieces in the concrete for a prolonged period, thus allowing an extended life and elimination of repair of concrete structures prepared from such mixtures. The required amount of calcium nitrite is at least 2 to 3% by weight, based on the dry weight of the cement. Larger amounts can be used if economically feasible. Quantities greater than 5% by weight must be deemed unnecessary.

The calcium nitrite can be added to the concrete according to various methods. The calcium nitrite can be added to the cement clinker prior to grinding and can be thoroughly mixed with the cement component during the grinding step. The calcium nitrite can also be added to the dry concrete mix and can be thoroughly mixed to achieve a uniform dispersion. The calcium nitrite can be dissolved in the water used to make concrete 7

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mixture. The concrete mixture can be pre-mixed with water and then mixed or contacted with calcium nitrite.

In general, any bonding method can be used which allows a practically uniform mixture of the calcium nitrite with the concrete mixture before forming a cured mixture.

Other conventional admixtures may be added to the subject blend according to methods and in amounts well known in the art. Such admixtures may e.g.

10 may be agents for reducing the water / cement ratio such as calcium lignin sulfonate, glucose polymers, polysaccharides and the like, or also superplasticizers such as poly naphthalene sulfonate, polymelamine formaldehyde sulfonate and the like. Other conventional means can be used according to known methods and to the extent that they contribute to the extra good properties of the concrete formed and do not impair the required compressive strength as stated above.

The following examples serve to illustrate the invention. All parts and percentages are by weight unless otherwise stated.

Example 1

Concrete from a supplier of finished mixes (maximum 25 aggregate size 25 mm) with a cement factor of 8 sacks per unit. m3, is mixed with 2% calcium nitrite on the basis of the dry weight of the cement so as to achieve a set dimension of 10 cm, 4.8% air content and a compressive strength after 28 days of 40 MPa, and placed in a mold measuring 183 cm x 61 cm and has a thickness of 30 cm, which contains a double mat of reinforcing bars (16 mm in diameter) 5 cm "on center" extending in the longitudinal direction of the mold and 2.5 cm below the concrete surface which is trimmed with a broom.

Seven days after casting, during which the plate has been continuously covered with sackcloth and plastic and kept wet, the mold is removed and the plate is placed on pellets

DK 157918B

s 1 m above the ground. Over the next five weeks, an upholstery is constructed on the top and from the beginning of the seventh week a 3% sodium chloride solution (1200 ml) is poured daily over the surface.

5 Open circuit potentials are frequently measured according to the methods of ACI, Publication No. SP-49, pp. 71-82 (1975), California Transportation Laboratory Research Report, CA-DOT-TL-5116-12-75-03, January 1975, and Uhlig, Corrosion & Corrosion Control, p. 45 (1971). After approx. six months 10 it was found that the area subjected to the corrosion (which had a more negative potential than -0.350 volts over a copper-copper sulphate (CCS) reference electrode) was 0% on the cover plate with calcium nitrite and 36.4% of the total area of the blank prepared from the same components, but without calcium nitrite, so that the concrete had a set dimension of 11 cm, 5.5% air content and a 28 day strength of 31.5 MPa.

Example 2 In comparison, in a 0.3 m 3 mixer, a concrete containing 2% calcium nitrite is prepared and with an aggregate size of a maximum of 16 mm, 8 sacks per unit. m3 cement, a water / cement ratio of 0.56, a set dimension of 8 cm, 4.6% air and with a 28 day compressive strength of 31.6 MPa.

25 After approx. At 6 months, this concrete, which had been cast in a slab in the same manner as described in Example 1, was attacked by corrosion equivalent to 93%, despite the blank (no calcium nitrite, a water / cement ratio of 0.57, a set target of 12 cm, 4.5% air and a 28 day compressive strength of 25.2 MPa) had a degree of attack of 100%.

The calcium nitrite exerted a certain effect in inhibiting corrosion measured in the same manner as described in Example 1. The area, which had an open current potential between -0,500 volts and -0.550 volts over the CCS electrode, was 0.3% for the calcium nitrite plate and 7.4% for the calcium nitrite cover plate. The area that showed between -0,450

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9 volts and -0.500 volts relative to the CCS electrode were 9.4% for the calcium nitrite plate and 18.5% for the plate without calcium nitrite. Nevertheless, the corrosion inhibition effect in this series was not as pronounced as in Example 1.

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Example 3

Also, for comparison, low-strength concrete mixtures, prepared in practically the same manner as described in Example 2, provided the following parameters: .................

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Inhibited Blind sample 8 cement factor, sacks / m3 8 13 set dimensions, cm 10 0.59 water / cement 0.57 15 4.8 air,% 4.8 2 calcium nitrite,% 0 71.4 corrosion attack area,% 100

Although the corrosion was reduced by using calcium nitrite, the unexpected, virtually complete inhibition was not obtained.

Example 4

Concrete made practically as in Examples 2.25 but with the addition of 90 ml of calcium lignin sulfonate-based water reducing agent gave the following parameters:

Inhibited Blind Test 8 cement factor, sacks / m3 8 30 14 set dimensions, cm 14 0.55 water / cement 0.53 5.2 air,% 5.8 43.7 28 days compressive strength, MPa 35.8 2 calcium nitrite,% 0 35 0 area affected by corrosion of 27.6 6 months,%

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10 0 corrosion attack area of 100 19 months,%

A one-month trial is almost equal to a one-year Kansas winter.

5 These tests show that although both the blind sample and the calcium nitrite sample exhibited high compressive strengths, only the latter showed virtually no corrosion after a prolonged period.

Claims (8)

1. Reinforced concrete structure made of a high-strength concrete mixture and with cast iron or ferrous metal pieces, characterized in that the concrete mixture is made from hydraulic cement, sand, aggregate and water and that this mixture contains at least 2% calcium nitrite, based on the dry weight of its cement content and that the mixture exhibits a compressive strength of at least 35 MPa after 28 days of curing. Construction according to claim 1, characterized in that the content of calcium nitrite is 2 to 3%, based on the dry weight of the cement. Construction according to claim 2, characterized in that the concrete mixture is made from a hydraulic cement containing a total silicate component amount of approx. 50% to approx. 90%, and with a cement content of at least 301 to 516 kg / kg. m3, as well as a water / cement ratio of 0.25 to 0.5 and with a Blaine fineness of between approx. 3200 cm 2 / g to approx. 5000 cm 2 / g in such a combination that the resulting concrete mixture exhibits a compressive strength of at least 35 MPa after 28 days. Construction according to claim 3, characterized in that it additionally contains a superplasticizer and / or other agents which reduce the water demand. Process for the manufacture of a reinforced concrete structure according to claim 1, characterized in that a non-bonded concrete mixture is prepared by practically uniform mixing of a concrete mixture consisting of hydraulic cement, sand, aggregate and water and containing at least 30 2% by weight of calcium nitrite, based on the dry weight of the cement content of the concrete mixture, and that this concrete mixture, after bonding, exhibits a compressive strength of at least 35 MPa after 28 days of curing, the mixture is allowed to decompose. Process according to claim 5, characterized in that the content of the admixed calcium nitrite is from 2 to approx. 3%. Process according to claim 6, characterized in that the unbound concrete mixture is made from 5 from a hydraulic cement having a total silicate content of approx. 50% to approx. 90%, a cement content of at least 301 to 516 kg per kg. m3, and a water / cement ratio of 0.25 to 0.5, and a Blaine fineness of between approx. 3200 cm 2 / g and approx. 5000 cm 2 / g in such a combination that the resulting concrete mix 10 exhibits a compressive strength of at least 35 MPa after 28 days. Process according to claim 7, characterized in that the unbound concrete mixture contains a super-plasticizer and / or other agents which reduce the water demand.