GB2174407A - A reinforcing steel - Google Patents

Abstract

A high-purity reinforcing steel resistant to salt and capable of retarding deterioration of concrete even when the salt content of the concrete is greater than 0.5 wt% in terms of NaCl amount in the sand. The reinforcing steel consists essentially of 0.001 to 1.0 wt% of C, less than 0.05 wt% of Si, 0.01 to 2.0 wt% of Mn, less than 0.015 wt% of P, less than 0.005 wt% of S, 1.0 to 5.5 wt% of Ni, 0.001 to 0.5 wt% of W, 0.001 to 0.10 wt% of Al, the balance being Fe and incidental impurities. Optional additions of Ca, Ce, Nb, Ti, V, Mo, Pb are possible.

Description

SPECIFICATION A reinforcing steel BACKGROUND OF THE INVENTION The present invention relates to a reinforcing steel having a good resistance to salt and capable of preventing deterioration of concrete, suitable for use in concrete structures or concrete bridges which are built on a beach, at a seashore or offshore, where they are exposed to salt particles or splashes of sea water.

Recently, various methods have been proposed and carried out which prevent cracking in various steel-reinforced concrete building construction making use of sea sand, as well as concrete structures which are situated on beaches or seashores.

The deterioration of concrete in the constructions and structures of the type mentioned above is attributable mainly to corrosion of the reinforcing steel by salt inherently contained by the sea sand or salt which has penetrated the concrete wall which is subjected to the salty atmosphere in the shore area. Specifically, the corroded steel expands to increase its volume to about 2.2 times the original volume through the creation of corrosion products, the concrete cannot withstand the expansion, and cracks are thereby caused along the reinforcing steel. When a crack grows to 0.2 mm or greater in size, oxygen, salt and carbon dioxide gas in the air penetrate into the concrete through the crack to reach the reinforcing steel. The salt promotes both the corrosion of the reinforcing steel and neutralization of concrete, thus accelerating the deterioration of the concrete.

The present inventors have made an intense study for developing reinforcing steel having improved salt resistance, through a suitable control of the chemical composition of the reinforcing steel and addition of special elements to the steel composition and succeeding in developing a reinforcing steel as shown in Japanese Unexamined Patent Publication Nos. 48054/1982 and 44457/1984. This steel is disclosed also in other literature such as "OFFSHORE GOTEBORG '81" Paper No. 42, Goteborg SWEDEN 1981, "CEMENT CONCRETE" No. 434 (1983) P. 23/31, and "Corrosion of Reinforcement in Concrete Construction" P. 419, 1983. This literature details the salt resisting mechanisms of the elements of the steel which contribute to the improvement in the salt resisting properties, and some of the steel compositions proposed in these literatures have been already put into practical use.

There is a current demand substantially to prevent the corrosion of the reinforcing steel and the resultant cracking in the concrete, both of which are attributable to penetration of salt particles and the splash of salt through concrete walls.

Problems are becoming serious in various fields in regard to cracking in concrete constructions and structures which are 10 or more years old. This is because the salt content of the concrete around the embedded reinforcing steel reaches 0.3 to 0.5 wt% when calculated in terms of NaCI amount in the sand of concrete, and has caused heavy corrosion of the reinforcing steel, resulting in crack occurrence and growth. Concrete buildings and structures of 30 or more years old often exhibit a high salt content exceeding 0.5 wt% when calculated in terms of the NaCI amount in the sand of the concrete.

It is, therefore, highly desirable to develop a technique which can prevent cracking in the concrete even when the salt content is as high as 0.5 wt% in terms of NaCI amount in the sand of concrete, and also to make it possible to remarkably retard the cracking in the concrete even when the salt content exceeds 0.5 wt% in terms of NaCI amount in the sand of concrete.

To this end, the invention provides a reinforcing steel resistant to salt and capable of preventing deterioration of concrete, essentially consisting of 0.001 to 1.0 wt% of C, not greater than 0.05 wt% of Si, 0.01 to 2.0 wt% of Mn, less than 0.015 wt% of P, less than 0.005 wt% of S, 1.0 to 5.5 wt% of Ni, 0.001 to 0.5 wt% of W, 0.001 to 0.1 wt% of Al and the balance Fe and incidental impurities.

It will be appreciated that the alloys of the invention may also comprise, in addition to the constituents specified above, one or more of the normal constituents of alloys used for reinforcement. Some of these constitutuents are mentioned elsewhere in the specification; others will be known to those skilled in the art.

The invention accordingly also provides a steel suitable for use in reinforcing concrete, and consisting essentially of 0.001 to 1.0 wt% of C, not greater than 0.05 wt% of Si, 0.01 to 2.0 wt% of Mn, less than 0.015 wt% of P, less than 0.005 wt% of S, 1.0 to 5.5 wt% of Ni, 0.001 to 0.5 wt% of W, 0.001 to 0.1 wt% of Al, the balance being Fe, incidental impurities and, if desired, any one or more incidental ingredients used in a reinforcing steel.

The invention still further provides a shaped structure, especially one of concrete, reinforced by the steel of the invention.

One of the most significant features of this reinforcing steel resides in that the resistance to salt is improved and the deterioration of concrete subjected to salt in high concentration is prevented by reducing Si and S contents in the steel and by adding Ni and W to the steel composition.

It is understood that a reduction in Si content effectively suppresses the occurrence and growth of rust and, even after rust has proceeded on the surface of the steel embedded in the concrete, the rust layer does not have high Si concentration but has a high Ni concentrated therein and uniformly contains W diffused from the reinforcing steel, thus remarkably reducing the amount of rust.

It is also understood that a large reduction in the S content significantly reduces the content of MnS which serves as cores causing rust, with a result that the corrosion resistance is drastically improved.

The reasons of limitation of the contents of respective constituents will be explained hereinunder.

The C content is limited to be 0.001 to 1.0 wt% because the steel cannot have required strength when the C content is less than 0.001 wt%, and because any content more than 1.0 wt% causes embrittlement.

Similarly, the Mn content is selected to range between 0.01 and 2.0 wt% because a Mn content less than 0.01 wt% cannot provide the required strength of the steel, while an Mn content exceeding 2.0 wt% causes embrittlement.

When the content of Ni added together with W exceeds 1 wt%, Ni is extremely enriched in the rust layer so that a remarkable rust preventing effect is obtained even when the Si content is increased to 0.05 wt% or so. This effect is further enhanced as the Si content is below 0.05 wt%. For these reasons, the upper limit of the Si content is selected to be 0.05 wt%.

Preferably, the Si content is not greater than 0.02 wt%.

The P content is limited to be less than 0.015 wt% because a P content of 0.015 wt% or higher does not produce any effect for suppressing the growth of rust but, rather, exhibits acceleration of the rusting, when the steel is used in an alkaline atmosphere such as concrete.

Because the steel of the invention contains both Ni and W, W coexisting with Ni remarkably enriched in rust layer serves to change Foe 3in Fe203 and Fe304 in the rust layer into Fe2'. This effect is remarkable even when the W content is as small as 0.001 wt% proving that the growth of the rust layer is remarkably suppressed as a result of co-existence with Ni. This effect, however, is substantially saturated when the W content is increased beyond 0.5 wt%.

In the steel of the invention, therefore, the W content ranges between 0.001 and 0.5 wt%.

Preferably, the W content ranges between 0.002 and 0.5 wt% and more preferably between 0.01 and 0.5 wt%.

In the steel of the invention, Ni is one of the most significant elements. When the rusting of reinforcing steel has processed under the influence of salt of high concentration, Ni is concentrated remarkably in the rust layer so as to substantially suppress the growth of the rust. This effect is not appreciable when the Ni content is less than 1.0 wt%, and is saturated when the Ni content is increased beyond 5.5 wt%. For these reasons, the Ni content ranges between 1.0 wt% and 5.5 wt%. Preferably, the Ni content ranges between 2.0 and 5.5 wt% and, more preferably, the Ni content is above 3 wt% but not greater than 5.5 wt%.

For the purpose of ensuring high weather resistance of the steel in the period before it is embedded in the concrete, the steel may contain a suitable element, for example Cu, by an amount of 0.01 to 0.3 wt% as well known per se.

The P content in the steel is limited to be less than 0.015 wt% because a P content not smaller than 0.015 wt% does not produce any effect on the suppression of growth of rust in an alkaline atmosphere such as concrete but rather accelerates the growth of the rust.

The Al content is determined to range between 0.001 wt% and 1.0 wt% in consideration of both deoxidation effect and strength. Namely, an Al content less than 0.001 wt% is insufficient for converting the oxygen to the form of stable Al oxides, whereas an Al content more than 0.1 wt% allows large inclusions to be formed causing embrittlement of the steel.

The S content is limited to be less than 0.005 wt%, aiming at reducing the content of MnS which serves as cores causing rust. The reduction in the S content is accomplished by adding a desulfurizer such as a Ca compound, rare earth metal or the like. Such a desulfurizer converts MnS into, for example, (Mn, Ca)S to thereby appreciably increase the corrosion resistance of the steel. The addition of such a desulfurizer is well known in this field, so that trace amounts of Ca and Ce are usually included. Such elements, however, do not adversely affect the corrosion resistance of the steel when their contents range between 0.001 and 0.05 wt%. Preferably, the S content is not greater than 0.003 wt%.

The steel of the invention can contain 0.01 to 0.2 wt% of one, two or more elements selected from a group consisting of Nb, Ti, V and Mo, for the purpose of improving the strength and toughness of the reinforcing steel. The use of such elements also is well known in this field.

When the steel is required to have a specific property such as a high machinability or cutting property, the steel may contain Pb of 0.01 to 0.5 wt%.

The steels of the invention having the compositions as described above can be produced by melting in a converter or an electric furnace followed by ingot-making or blooming or, alterna tively, continuously cast and rolied followed by a suitable heat treatment such a patenting. The steel is then drawn to become reinforcing steel.

The steel of the invention can have a composite structure composed of a hardened surface region and a toughened core region, as the occasion demands.

If necessary, the reinforcing steel of the invention can be lined with a zinc plating layer or coated by an organic coating material.

The salt-resistant effect of the reinforcing steel of the invention can be equally enjoyed even when the steel is shaped into an H-shaped steel which is embedded in the concrete. Thus, the reinforcing steel of the invention can be used in the form other than round steel bars.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an illustration of a test piece of a steel-reinforced concrete having steel bars embedded therein, showing the shape and size of the test piece, as well as the arrangement of the reinforcing steel bars; Figure 2 is a graph showing the conditions under which a test was conducted for the purpose of investigation of rust promotion; There is no figure numbered 3 in the present specification.

Figure 4a and 4b are sketches and photographs both showing the state of cracking in a concrete reinforced with a reinforcing steel in accordance with the invention in comparison with that in a concrete reinforced with a comparison reinforcing steel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Samples of reinforcing steel in accordance with the invention were produced by preparing materials of compositions specified by the invention, melting in a converter, ingot-making, blooming and drawing. Similarly, samples of known reinforcing steels of conventional compositions were prepared by melting in an electric furnance. The compositions of the samples and the progress of corrosion of sample steels and degradation of the concrete embedding these sample steels are shown in the following Table.

The samples of reinforcing steels shown in the Table are hot-rolled steel bars of 9 mm dia.

Afteer machine-grinding followed by degreasing, the reinforcing steel bars were embedded in a concrete mortar having a water-to-cement ratio of 0.60 and a salt content of 1.0 wt% in terms of NaCI amount in the sand of the concrete, and the pieces of concrete mortar were formed into test pieces of the shape and size as shown in Fig. 1.

After 28 days of curing, the concrete test pieces were places in a therom-hygrostat and subjected to repetitional cyles having a period of one week constituted by 48-hour wetting, 24hour drying, 48-hour wetting and 48-hour drying, and the state of cracking was observed after 56 days and 70 days from the start of the test.

In Fig. 1, a reference numeral 1 denotes a concrete test piece, 2 denotes the embedded reinforcing steel bars of 9 mm dia., and 3 denotes an epoxy seal on the mortar. A symbol "L" denotes the depth of the reinforcing-steel bars as measured from the top surfaces.

The testing cycles as shown in Fig. 2 create very severe conditions comprising repetitional drying and wetting steps at high temperature of 80"C at which the amount of oxygen dissolved in steam is maximized, thus promoting the corrosion of the embedded reinforcing steel bars.

Changes in the depth of neutralization by carbon dioxide gas and corrosion loss of the embedded reinforcing steel bars were also measured in relation to time.

The sizes of the cracks in the concrete test pieces were measured by a crack gauge.

The depth of neutralization by carbon dioxide gas was determined by spraying a solution of phenolphthalein to the concrete test piece and measuring the depth of the point at which the color was changed from red to colorless from the test piece surface.

The corrosion loss was determined by crushing the, concrete test piece, chemically removing the rust from the exposed reinforcing steel, measuring the weight of the steel after the removal of the rust, and subtracting the measured weight from the weight before the corrosion. Thus, the corrosion loss is expressed in terms of loss of weight per 28 cm length.

The results of these measurements are shown in the Table.

The salt content of the concrete was measured by collecting powders of crushed concrete around the reinforcing steel bar and conducting measurement in accordance with the methods as specified by Japanese Concrete Engineering Association and Japanese Cement Association; namely, partly by measuring Cl in accordance with a nitric acid decomposition method of chemical analysis and partly by measuring Cl in accordance with cold-water extraction method.

The thus measured values of Cl were converted into salt content and are expressed in terms of the NaCI amount (wt%) in the sand of the concrete.

The concrete test pieces reinforced with the reinforcing steel sample Nos. 2-1, 2-2, 2-4, 2-5 and 2-6 shown in the Table were placed in the aforementioned thermostat thermohydrostat and were subjected to the wetting/drying cycle test. The salt contents of the concrete around the reinforcing steel bars and the amount of free salt extracted by cold water were chemically analyzed and determined in terms of the NaCI amount in the sand of the concrete, after 56 days and 70 days from the start of the test. The salt contents and the amounts of free salt were about 1.0 wt9to and about 0.6 wt%, respectively, in all test pieces.

These test results show that the reinforcing steel in accordance with the invention exhibits an extremely small rate of corrosion as compared with the conventional reinforcing steel, even when the salt content of the concrete is as great as 1.0 wtWo in terms of NCI amount in the sand of concrete and, hence, remarkably delays the deterioration of the concrete. Thus, the reinforcing steel in accordance with the invention can effectively prevent deterioration of concrete from occurring even under such severe conditions that the salt is finally concentrated to a high value of 1.0 wt% in terms of NaCI amount in the sand of the concrete.

Figs. 4a and 4b illustrate the states of deterioration of concrete test pieces reinforced with the reinforcing steel sample Nos. 2-1, 24 and 2-18 appearing in the Table. Fig. 4b carries photo graphs of the test pieces used as the basis for the sketches in Fig. 4a.

The reinforcing steel in accordance with the invention is capable of ensuring high durability of concrete constructions and structures which are used under salty conditions, well satisfying the current demand for high salt-resistance of steel-reinforced concrete constructions and structures.

Thus, the reinforcing steel of the invention ensures longer service life and higher stability of concrete constructions and structures and finds wide use in various fields.

Compositions ($) No. C Si Xn P S Ni W ::0 R 2-1 2-1 0.14 0.13 0.65 0.017 0.023 0.08 2 m 2-2 0.13 0.08 0.55 0.023 0.017 ca arn 2-5 0.21 0.050 0.30 0.011 0.002 3.48 0.005 2-6 0.20 0.040 0.29 0.010 0.001 3.50 0.002 2-7 0.05 0.010 0.80 0.012 0.001 3.38 0.010 o 2-8 0.05 0.008 0.65 0.008 0.001 3.52 0.007 0 2-9 0.05 0.007 0.69 0.010 0.004 3.50 0.005 a) > 2-10 0.05 0.006 0.75 0.011 0.003 3.10 0.006 o 2-11 0.06 0.01 0.50 0.009 0.001 3.46 0.001 0 2-12 0.05 0.008 0.60 0.010 .004 3.90 0.050 a) 2-13 0.06 0.007 0.48 0.011 0.001 3.52 0.100 2-14 0.78 0.01 0.28 0.008 0.003 3.51 0.080 2-15 0.75 0.01 0.30 0.010 0.001 3.10 0.070 2-16 0.15 0.02 0.60 0.010 0.0048 3.72 0.31 2-17 0.22 0.008 0.32 0.010 0.0038 2.06 0.31 2-18 0.21 0.016 0.31 0.011 0.001 3.57 0.32

Table (Cont'd)

Maximum crack width in concrete test piece (nix) (including drop ped portion) Al Ca,Ce Others 56 days 70 days 0.005 Cu 0.27 1.0 8.0 0.004 Cu 0.44 1.4 9.0 0.023 Cu 0.23 0.2 1.0 0.023 Ca 0.0002 0.11 0.29 0;;022 Ca < 0.0002 0.10 0.30 0.020 Ca < 0.0002 0.08 0.20 0.021 Ca 0.0001 0.06 0.20 0.018 Ca < 0.0002 0.08 0.18 0.023 Ca 0.0001 0.11 0.31 0.026 Ca < 0.0002 0.10 0.32 0.025 Ca 0.0001 Nb 0.02 0.04 0.15 0.019 Ca < 0.0002 V 0.03 0.12 0.31 0.021 Ca 0.0001 Nb 0.03, Cu 0.05 0.04 0.16 0.018 Ca 0.0001 0.08 0.18 0.020 Ca < - 0.0002 0.06 0.15 0.024 Ca < 0.0002 0.04 0.41 0.023 Ca < 0.0002 0.04 0.20 0.015 Ca < 0.0002 0 0.06 Table (Cont'd)

Maximum corrosion loss Co2 Penetration of embedded depth steel fmm) (g/9 mm# x 28 cm) 56 days 70 days 56 days 70 days 7.2 12.4 6.5 10.8 9.0 11.9 6.9 10.4 3.8 5.7 3.3 5.2 2.0 4.1- 2.9 4.8 2.1 4.3 3.0 5.0 2.0 3.6 2.6 . 4.2 2.1 3.8 2.7 4.3 1.8 3.7 2.5 4.1 2.2 4.4 2.9 4.4 2.1 4.0 3.1 4.8 1.8 3.9 2.7 4.1 2.3 4.4 3.1 4.9 1.7 3.6 2.5 4.0 1.8 3.7 2.7 4.3 1.6 3.2 2.5 4.1 1.6 4.7 2.4 5.8 1.7 3.8 2.6 4.3 0.5 1.5 2.1 2.6

Claims (9)

1. A high-purity reinforcing steel resistant to salt and capable of remarkably retarding deterioration of concrete even under such a severe corrosive condition that the salt content of said concrete exceeds 0.5 wt% in terms of NaCI amount in sand of concrete, said reinforcing steel essentially consisting of 0.001 to 1.0 wt% of C, not greater than 0.05 wt% of Si, 0.01 to 2.0 wt% of Mn, less than 0.015 wt% of P, less than 0.005 wt% of S, 1.0 to 5.5 wt% of Ni, 0.001 to 0.5 wt% of W, 0.001 to 0.01 wt% of Al and the balance Fe and incidental impurities.

2. A reinforcing steel according to claim 1, further containing 0.0001 to 0.005 wt% of at least one selected from a group consisting of Ca and Ce.

3. A reinforcing steel according to claim 1, further containing 0.01 to 0.2 wt% in total of at least one selected from a group consisting of Nb, Ti, V and Mo.

4. A reinforcing steel according to claim 1, further containing 0.01 to 0.5 wt% of Pb for improving the cutting property.

5. A steel suitable for use in reinforcing concrete, and consisting essentially of O.q01 to 1.0 wt% of C, not greater than 0.05 wt% of Si, 0.01 to 2.0 wt% of Mn, less than 0.018 wt /ó of P, less than 0.005 wt% of S, 1.0 to 5.5 wt% of Ni, 0.001 to 0.5 wt% of W, 0.001 to 0.1 wt% of Al, the balance being Fe, incidental impurities and, if desired, any one or more incidental ingredients used in a reinforcing steel.

6. A reinforcing steel, the composition of which is substantially as described with reference to any one of the examples herein Nos. 2-4 to 2-18.

7. A steel consisting essentially of 0.001 to 1.0 wt% of C, not greater than 0.05 wt% of Si, 0.01 to 2.0 wt% of Mn, less than 0.015 wt% of P, less than 0.005 wt% of S, 1.0 to 5.5 wt% of Ni, 0.001 to 0.5 wt% of W, 0.001 to 0.1 wt% of Al and the balance Fe and incidental impurities.

8. Concrete, reinforced by a steel as claimed in any one of claims 1 to 7.

9. Any new feature hereinbefore described or any new combination of hereinbefore described features.