Stainless Steel Reinforcement for Enhanced Durability of Reinforced Concrete Structures

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A R T I C L E

Stainless steel reinforcement: New material in the latest draft of second-generation Eurocode 2

Victoria Matres

¹

| Juan José Fern andez

²

| María L opez

³

| Luis Peir o

³

| Antonio José Madrid

⁴

| Nuria Rebolledo

⁵

| Julio Torres

⁵

| Javier S anchez

⁵

1Acerinox Europa S.A.U, Madrid, Spain

2Roldan S.A., Grupo ACERINOX, Spain

3CEDINOX, Madrid, Spain

4Structures Department (ETS ICCP-UPM) Madrid, Spain

5Durability and Safety of Structures Group (IETcc-CSIC), Madrid, Spain

Correspondence

Javier Sanchez, Durability and Safety of Structures Group (IETcc-CSIC), Madrid, Spain.

Email:[email protected]

Abstract

The durability of reinforced concrete structures exposed to aggressive environments is affected by corrosion of carbon steel reinforcement. For this reason, it is of great practical interest to look for alternative solutions guaranteeing durability through the incorporation of corrosion-resistant materials, such as stainless steel rebars. This type of steel is specially recommended for structures with a long lifespan, located in aggressive environments, with chlorides or de- icing salts, and for structures where repair, inspection and rehabilitation is complicated. This solution presents a challenge from the point of view of the materials themselves, the mechanical design and their durability. This article presents the material properties of stainless steel and their influence on concrete structures. Furthermore, it is explained how these properties have been taken into account in the latest draft of the second-generation Eurocode 2.

K E Y W O R D S

concrete, corrosion, durability, rebar, service life, stainless steel

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I N T R O D U C T I O N

One of the main causes that impairs the long-term durability of concrete structures is premature corrosion of steel reinforcement. In view of this, numerous studies have been carried out during the last decades to increase the durability of structures, especially in very aggressive environments. One of the most sustainable alternatives is the use of stainless steel reinforcement. Stainless steel is an excel- lent structural solution due to its properties in terms of cost-effectiveness, sustainability, mechanical resistance, durability and its mechanical and physical properties.^1,2

Ferrous alloys that contain at least 10.50% of chromium and less than 1.20% of carbon are referred to as

stainless steel.³ This chromium content guarantees the formation of a very thin chromium oxide layer, although other elements like silicon, nickel, manganese, and copper may also be involved. This layer covers the complete surface of the metal and is just a few atoms thick.

In oxidizing environments this layer of oxides, called passive layer, is an insoluble barrier film, and if it is dam- aged, it regenerates spontaneously by putting the surface of the metal in contact with oxidizing acids, or, in a slower way, with the oxygen in the air. Most investigators are of the opinion that the structure and composition of the thin passive film can be characterized by an inner layer of Fe3O4 under an outer layer of Fe2O3.⁴^–⁷ The inner Fe3O4 is thought to have a highly defective

This is an open access article under the terms of theCreative Commons Attribution-NonCommercial-NoDerivsLicense, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Structural Concrete.2023;24:7549–7560. wileyonlinelibrary.com/journal/suco 7549

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structure and is therefore conductive for electrons. The passive film usually has properties of a n-type semicon- ductor with excess negative charge carriers. Passive films on alloys may be enriched by some alloy elements, especially chromium, which forms a very stable oxide in comparison to iron.⁸

The development of an auto-passivating chromium oxide protective layer gives stainless steel a far superior performance with respect to resistance to corrosion, both to initiation and propagation period, compared to conventional carbon steel. Moreover, this protective layer makes stainless steel largely independent of external coatings to achieve an acceptable durability.

Concrete structures provided with stainless steel reinforcement are considered to result in a 100 years maintenance-free service life, which contributes to sustainability reducing the final life cycle cost of the work by more than 15%.⁹

These alternatives introduce new challenges in terms of material composition, mechanical design and associated durability.^10,11The state of the art in stainless steel reinforced structures and their incorporation in the latest draft version of Eurocode 2 will be addressed in the following sections.

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M A T E R I A L S

As a result of the availability of a wide range of grades of stainless steel alloys it is possible to obtain many different properties offering versatility and adaptability.^12,13 The most common grades used for reinforcement bars are explained in this section.

It should be mentioned that the manufacturing process of stainless steel rebars plays an important role as it will affect the material properties. This

manufacturing process involves three principal stages in time: Heating, hot rolling and/or cold rolling. Heat- ing temperatures vary depending on the grades of stainless steel over 800C. Hot rolling process consists of repeatedly passing the previously heated material through rolling cylinders until the required diameter is achieved. Rebars are referred to as hot-rolled when material is obtained directly from the hot-rolling process. On the other hand, to produce cold-rolled rebar, hot-rolled wire rod is passed through a drawing bench, to obtain the final rebar product. Material properties vary according to the manufacturing process, and the mechanical properties should be referred to as mentioned in Tables1, 2.

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Stainless steels used in reinforced concrete

In the latest draft of new Eurocode 2,¹⁴it is indicated that the products to be used as stainless steel reinforcement in concrete should comply to prEN 10,370Steel for the reinforcement of concrete—Stainless Steel. In this standard, it is indicated that the alloys that can be used as stainless reinforcing steels are listed in the tables of EN 10088-1 Stainless Steels. Part 1: List of Stainless Steels, and Stain- less steels and EN 10088-5 Stainless Steels—Part 5: Tech- nical delivery conditions for bars, rods, wire, sections and bright products of corrosion resisting steels for construction purposes.

Although the stainless-steel grades included in these three standards could be classified in five big families, this article concentrates on those stainless steels that are of interest to be used as reinforcement in concrete structures. These families are: Ferritic, Austenitic, and Austenitic-Ferritic.

T A B L E 1 Strength classes of reinforcing steel (source latest draft of new Eurocode 2¹⁴).

Properties for stress–strain-diagram

Reinforcing steel strength class

B400 B450 B500 B550 B600 B700

Characteristic valuefyk(MPa) 400 450 500 550 600 700

Note: All strength classes apply unless a National Annex excludes specific classes. Intermediate strength classes can be used, if included in a National Annex.

T A B L E 2 Ductility classes of reinforcing steel (source latest draft of new Eurocode 2¹⁴).

Properties for stress–strain-diagram

Reinforcing steel ductility class

A B C

Characteristic value ofk=(ft/fy)_k 1.05 1.08 1.15–1.35 Characteristic strain at maximum forceεuk 2.5% 5.0% 7.5%

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Ferritic stainless-steels

The stainless-steel grades that belong to the ferritic family are characterized by having a complete ferritic structure.

Chromium alphagenous particular action on the Fe–Cr diagram, restricts the field of existence of iron; there are also some other alphagenous elements as silicon, nio- bium, molybdenum, titanium and tantalum.¹⁵The resulting microstructure of a ferritic stainless steel is shown in Figure 1. The mechanical characteristics of these steel grades can be increased by a cold-working deformation process. The ferritic steels are magnetic in all supply conditions. The most common steel grade used for reinforcement in concrete is steel grade 1.4003, which has a chromium content of 10.50%.

The Cr-content makes stainless steels less dependent on raw-material price fluctuations and consequently steels belonging to the ferritic family become a very interesting option for use in concrete structures suffering from carbonation. However, in environments with chlorides, their corrosion resistance, although higher than that of any conventional carbon steel, is lower than austenitic and austenitic-ferritic stainless- steel grades.¹⁶

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Austenitic stainless-steels

The steel grades in this family are characterized by exhi- biting an austenitic structure at room temperature. In Figure2 this structure can be observed. Austenitic stainless steels have a minimum chromium content of 16%, with a face-centered cubic crystalline structure.

In the Ni–Fe diagram, the net gammagenic action of nickel is manifested, which unlike chromium, broadens the field of existence of gamma iron. Other elements also have gammagenic action are carbon, manganese, nitro- gen and copper.¹⁵

Molybdenum can be added to enhance the pitting corrosion resistance of stainless steel. When the chromium content amounts to at least 18%, additions of molybdenum become about three times as effective as chromium additions in improving pitting and crevice corrosion resistance in chloride-containing aqueous environments.¹⁷

In a hyperquenched state austenitic stainless steels are non-magnetic, although they do increase their magnetic permeability due to a cold-worked deformation.

The mechanical characteristics of austenitic stainless steels can be improved by a cold-worked deformation^12,13: both tensile strength, Rm, and yield strength, Rp0,2%, increase, whereas elongation as well as reduction of surface fracture area decreases for steel grade EN 1.4301/AISI 304.¹⁸^–²⁰

If the aforementioned steel grade has its carbon content restricted to maximum 0.030%, steel grade EN 1.4307/AISI 304L is obtained. Moreover, if a content of molybdenum between 2.0% and 2.5% is added to the lat- ter, steel grade EN 1.4404/AISI 316L will be achieved, having a significantly improved corrosion resistance.

Steel grades EN 1.4307 and EN 1.4404 are two of the austenitic steel grades most commonly used for reinforcement in concrete. Their low carbon content permits their weldability and prevents intergranular corrosion.

The austenitic steel grades are highly recommended for applications in which the working temperatures are extreme (> 550C) or in the presence of magnetic fields,

F I G U R E 1 200 microphotograph of ferritic 1.4003/UNS S40977 stainless steel, 5 mm hot rolled cold worked material.

Credit: Roldan S.A.

F I G U R E 2 200 microphotograph of austenitic 1.4301/AISI 304 stainless steel, 5 mm hot rolled cold worked material. Credit:

Roldan S.A.

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because these steel grades have a higher temperature working range and they are non-magnetic as they are obtained in a hyperquenched state. Additionally, austenitic steel grades are characterized by their higher ductility.

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Austenitic-ferritic stainless steel

The stainless-steel grades that belong to this family have a bi-phasic structure, that is, a combination of austenitic- ferritic, with a presence of approximately 50% of each. In Figure3the characteristic microstructure can be observed:

darker color for the ferrite phases and white color for the austenitic ones. The relative amounts of ferrite and austenite that are present in a mill product or fabrication of a given duplex grade depend on the chemical composition and thermal history of the steel. The ferrite/austenite phase balance in the microstructure can be predicted with multi- variable linear regression as follows (Equations (1)–(3)):

Creq¼Crþ1:73 Siþ0:88 Mo, ð1Þ Ni_eq¼Niþ24:55 Cþ21:75 Nþ0:4 Cu, ð2Þ

%Ferrite¼ 20:93þ4:01 Creq5:6 Nieqþ0:016T, ð3Þ

where T (inC) is the annealing temperature ranging from 1050 to 1150C and the elemental compositions are in weight%.¹⁷

The mechanical characteristics of duplex grades can be increased by a cold-worked deformation.

At room temperature these steel grades are magnetic, and their most representative steel grade is EN 1.4462/

UNS S32205. Nickel and molybdenum are two elements that have a significant influence on the price of the alloy.

Consequently, when their percentages do not economi- cally penalize the alloy, steel grades that maintain an austenitic-ferritic structure are frequently included in the design.

Within the austenitic-ferritic stainless-steel family, lean duplex grades can be found:

1. With low molybdenum content, like EN 1.4362/UNS S2304.

2. With low molybdenum and nickel content, like EN 1.4482/UNS S32001.

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Recommended steel grades

There are over 200 different stainless-steel grades classified in the aforementioned families. In order to ease the designer's task, Table3provides an overview of the most commonly used stainless-steel grades for reinforcement in concrete structures.

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Mechanical properties

For ease of use, the mechanical properties required according to the latest draft of the new Eurocode 2¹⁴are the same for carbon steel and for stainless steel; they have the same classes of mechanical resistance.

Two features, both characteristic of stainless steel, that distinguish them from conventional carbon steel are:

1. Fatigue characteristics; samples shall be selected and prepared in accordance with ISO 15630-1:2010, clause 4, and subjected to type testing in accordance with ISO 15630-1:2010 clause 8. While carbon steel must pass the test during 2 million cycles, stainless steel is required to pass testing 5 million cycles.²¹

2. Ductility; stainless steel has a high ductility, therefore, although it must fulfill the same values of the ductility classes as carbon steel, for design, the ratio R7%/Rp0.2%

is used; that is to say, the stress at the point of a 7%

elongation and the elastic limit 0.2% is used, instead of the Rm (tensile strength), which is employed in the calculation of carbon steel.

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Corrosion performance

When selecting a stainless-steel grade for a specific application, it is necessary to know the corrosion performance of each of the candidate steel grades.

F I G U R E 3 500 Microphotograph of austenitic / ferritic phases of a 1.4482 duplex stainless steel, 12 mm hot rolled material.

Credit: Roldan S.A.

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To this end, it is possible to resort to the Pitting resistance equivalent number (PREN), which measures the passive layer's stability in environments containing halides, for example, chlorides. The PREN-value is obtained by using a formula that takes into account the content of some of the most relevant alloy's elements according to their performance against pitting. There are different formulae, but the most commonly employed is Equation (4):

PREN¼Crþ3:3MoþnN, ð4Þ Cr, Mo, and N in M.- %. With:n=0 for ferritic steels, n=16 for duplex steels andn=30 for austenitic steels.

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Physical properties

In Table4, some relevant physical properties of the stainless steels most commonly employed for reinforcement in

concrete that have been mentioned until now, are provided.

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D E S I G N C O N S E Q U E N C E S

In the latest draft of the new Eurocode 2¹⁴ reinforcing steel (carbon or stainless) is treated in the following sections:

1. Reinforcing steel is covered in Section 5.2.

2. Requirements of reinforcing steel assumed for validity of design provisions in Eurocode 2 are included in Section C4 of Annex C.

3. Additional or modified rules for stainless reinforcing steel are given in Annex Q.

Specific properties of reinforcing steel that are required for design are strength and ductility classes.

They are defined in Tables1, 2.

T A B L E 3 Concentration in %w. maximum values if nothing else is indicated.

Numerical

designation C Si Mn P S Cr Mo Ni N Cu Others

1.4003 0.03 1 1.5 0.04 0.03 10.5–12.5 - 0.3–1 0.03 -

1.4301 0.07 1 2 0.045 0.03 17.5–19.5 - 8–10.5 0.1 -

1.4307 0.03 1 2 0.045 0.03 17.5–19.5 - 8–10.5 0.1 -

1.4401 0.07 1 2 0.045 0.03 16.5–18.5 2–2.5 10–13 0.1 -

1.4404 0.03 1 2 0.045 0.03 16.5–18.5 2–2.5 10–13 0.1 -

1.4529 0.020 0.5 1 0.030 0.01 19–21 6–7 24–26

1.4571 0.08 1 2 0.045 0.015 16.5–18.5 2–2.5 10.5–13.5 0.15–0.25 0.5–1.5 Ti: 5xC a 0.7 1.4482 0.03 1 4–6 0.035 0.03 19.5–21.5 0.1–0.6 1.5–3.5 0.05–0.2 1

1.4362 0.03 1 2 0.035 0.015 22–24.5 0.1–0.6 3.5–5.5 0.05–0.2 -

1.4462 0.03 1 2 0.035 0.015 21–23 2.5–3.5 4.5–6.5 0.1–0.22 -

T A B L E 4 Physical properties of stainless steels.

Numerical designation

Average coefficient of linear thermal expansion between 20 and 100C (10⁶/K)

Thermal conductivity at 20C W/(m K)

Electric resistivity at 20CΩmm²/m

1.4003 10.4 25 0.6

1.4301 16 15 0.73

1.4307 16 15 0.73

1.4401 16 15 0.75

1.4404 16 15 0.75

1.4482 13 13 0.8

1.4362 13 15 0.8

1.4462 13 15 0.8

Carbon steel 10 40 0.18–0.2

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In this definition of classes, no distinction is made between stainless and carbon reinforcing steel. The same occurs in Section 5.2.4“Design assumptions,”where design stress–strain diagrams for reinforcing steel are defined.

In Annex C there are additional design values for material properties, with minimum or maximum values or an interval of values for which the design provisions of this Eurocode apply. Section C.4.1 is dedicated to carbon reinforcing steel and Section C.4.2 to stainless reinforcing steel.

From the comparison of the values proposed for both types of steel, the following conclusions can be drawn:

1. Reinforcing steel classes for design remain the same (B400, B450, B500, B550, B600, and B700)

2. Fatigue stress range is based on a different number of cycles and on a different stress ratio. Values for stainless steel are only given for B500 strength class.

3. For both materials, the same ductility classes are defined for design.

Annex Q, a normative annex, is dedicated to stainless reinforcing steel. The criterion is that provisions of this new Eurocode apply to stainless reinforcing steel unless modified in this Annex. Table Q1 gives a series of provisions (eight in total). Of these, those that provide some technical content are the following:

4. The design value of the modulus of elasticity Es for stainless steel may be assumed to be 200,000 MPa, for ease of use equal to the value for carbon steel. How- ever, in the text of EC2 a note is added: TheE-modulus of stainless steel depends on the alloy and can range between 150,000 and 200,000 MPa.

5. Requirements to producers of stainless steel are partly different to carbon steel reinforcement.

Based on the above, when a designer uses this new version of the Eurocode for the dimensioning of rein- forcements (e.g., bending, shear, punching, cracking verifications,…), no difference will be found whether carbon steel or stainless steel is chosen for these reinforce- ments. This was probably the intention of the Eurocode editorial team.

There are also no differences in the use of carbon or stainless reinforcing steel for all aspects concerning detailing (anchorage lengths, lap types and lengths, etc.).

In the treatment of durability differences are encoun- tered when stainless steel is used, specifically with respect to corrosion caused by carbonation (exposure classes XC) or chloride ingress (exposure classes XD- chlorides from sources other than from sea water, and exposure classes XS-chlorides from sea water).

The approach of the Eurocode 2 involves defining a sufficient concrete cover thickness to control the effect of attacks on the reinforcement resulting from corrosion due to both chloride ingress and carbonation. The approach is based on the definition of Exposure Resis- tance Classes (ERC). ERC are defined in accordance with the quality of the concrete when it is been built.

The nominal cover c^nom is defined as a minimum cover cmin plus an allowance in design for deviation, Δc^dev(Equation5):

cnom¼cminþΔcdev: ð5Þ Durability requirements are included in the termc^min. The deviation term Δcdev depends on execution specifications.

As is known, the concrete cover has three major roles:

6. The protection of the steel against corrosion (durability).

7. The safe transmission of bond forces.

8. An adequate fire resistance (covered in prEN 1992-1-2).

Requirements for both durability and bond have to be met byc^minvalue, defined by Equation (6):

cmin¼max cmin,durþX

Δc;cmin,b; 10 mm

n o

, ð6Þ

where cmin,duris the minimum cover required for environmental conditions. Its value depends on design service life, exposure class and ERC. The environmental exposure conditions are those chemical, physical and biologi- cal conditions to which the structure will be exposed.

Exposure classes XC are related to corrosion induced by carbonation, classes XD are related to corrosion induced by chlorides other than from seawater, classes XS are related to corrosion induced by chlorides from seawater, XF-classes are related to freezing and thawing attack, XA-classes are related to chemical attack, XM-classes are related to mechanical attack by concrete abrasion and exposure class X0 is applied in case of exposure conditions without any durability related risks. Minimum cover values for carbonation and chlorides attacks for carbon reinforcing steel are defined in Tables 6.3 and 6.4 of the new draft version of Eurocode 2. P

Δc is the sum of several applicable reductions and additions to the minimum cover. These reductions depend on several factors (design life of 30 years or less, superior compaction or improved curing, prestressed concrete, additional concrete protection, etc.). The use of stainless reinforcing steel does not modify any of them.cmin,bis the minimum

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cover for bond requirement. As in the previous case, the use of stainless steel does not make any difference to this requirement.

For stainless reinforcing steel, in Annex Q a classification of corrosion resistance of stainless reinforcing steel dependent on the PREN is included (see Table5).

In Table6values forc^min,durare defined, according to exposure classes and stainless steel resistance classes

(SSRC). Values are given for a design service life of 50 years. There are correction values for a 100 years design life, and also for combined action of carbonation and chloride induced corrosion.

In Table 6 a column has been added to the right to compare minimum concrete cover values for stainless steel with correspondent values for carbon steel (values from latest draft of new Eurocode 2,¹⁴Tables 6.3 and 6.4).

T A B L E 5 Classification of corrosion resistance of stainless reinforcing steel dependent on the pitting resistance equivalent number PREN (source latest draft of new Eurocode 2¹⁴).

Stainless steel resistance class

Pitting resistance

equivalent number PREN^a Description

Informative examples EN 10088-1 Ferritic Duplex Austenitic

SSRC0 0–9 Carbon steel reinforcement - - -

SSRC1 10–16 Chromium steels 1.4003 - -

SSRC2 17–22 Chromiumn steels - 1.4482 1.4301

1.4307

SSRC3 23–30 Chromium nickel steels with molybdenum - 1.4362 1.4401

1.4404 1.4571

SSRC4 ≥31 Steels with increased content of

chromium and molybdenum

- 1.4462 1.4529

aCalculation of the pitting resistance equivalent: PRE=Cr+3.3Mo+nN; Cr, Mo, and N in M.- %. With:n=0 for ferritic steels,n=16 for duplex steels andn=30 for austenitic steels. The calculation of the pitting resistance equivalent is a useful/practical indication when classifying stainless steels. Other factors can negatively influence the corrosion resistance. The results of the test could be used as input parameters for the service life design.

T A B L E 6 Minimum concrete covercmin,dur(in mm) to stainless steel reinforcement for 50 years design service life (source latest draft of new Eurocode 2¹⁴).

Exposure class

Exposure resistance class ERC

Stainless steel resistance class

Carbon

reinforcing steel

SSRC1 SSRC2 SSRC3 SSRC4

XC1 ≤XRC7 0 0 0 0 15

XC2 0 0 0 0 25

XC3 ≤XRC4 0 0 0 0 25

≤XRC7 15 0 0 0 40

XC4 ≤XRC4 15 0 0 0 25

≤XRC7 20 0 0 0 45

XD1, XS1 ≤XRDS0,5 10 0 0 0 20

≤XRDS1,5 20 10 0 0 25

≤XRDS3 25 15 10 0 30

≤XRDS6 35 25 15 0 40

≤XRDS10 45 35 25 15 50

XD2, XD3, XS2, XS3 ≤XRDS0,5 15 10 10 0 20–30

≤XRDS1,5 25 20 15 0 30–40

≤XRDS3 35 30 20 10 40–55

≤XRDS6 50 40 30 20 65 for XS2 otherwise

≤XRDS10 65 50 40 30 -

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From comparison, differences can be noted in the case of corrosion induced by carbonation (exposure class XC, first six rows of the table). In the case of corrosion induced by chlorides (exposures classes XD and XS, last six rows of the table), differences are smaller for the lower SSRC and become more significant for the higher classes.

An informative Annex P provides an alternative approach to design cover for durability without use of ERC. It is the traditional procedure as used in EN 1992-1-1:2004. A National Annex allows to choose between use of ERC according to Clause 6.4 or use of Annex P.

In this Annex P,c^minis defined by Equation (7):

cmin¼ maxncmin,durþΔcdur,γΔcdur,stΔcdur,add; cmin,b; 10 mmo

ð7Þ

wherecmin,duris the minimum cover due to environmental conditions. In Annex P, Table P.1 gives rules to select a structural class. For each structural class, and depending on the exposure class, recommended values ofc^min,dur are given in Tables P.2 (carbon reinforcing steel) and P.3 (prestressing steel).Δc^dur,γis the additive safety element.

Its recommended value is 0 mm.Δcdur,stis the reduction of minimum cover for use of stainless steel. The recommended value, without further specification, is 0 mm. As durability formulation of this Annex P is the same as that of the UNE-EN 1992-1-1:2013 currently applicable in Spain, it can be considered that Table AN/5 of the National Annex would apply. This table is reproduced as Table7.

where Δcdur,add is the reduction of minimum cover for use of additional protection (e.g., coating). The recommended value, without further specification, is 0 mm.

c^min,b is the minimum cover due to bond requirement, previously commented.

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D U R A B I L I T Y C O N S E Q U E N C E S

A major cause of decreased durability of reinforced concrete structures is corrosion of steel reinforcement, especially for structures that are exposed to a chloride environment (XD and XS exposure classes).^22–24 It is in these environments that stainless steel reinforcement becomes an viable alternative to traditional carbon steel reinforcement. There are numerous studies on the amount of chlorides that initiate corrosion of carbon steel reinforcement.²⁵^–²⁹ However, for stainless steel rebars, fewer studies have been published and they cover only

some of the stainless steel grades.^30–33 Figure 4 shows indicative values for the threshold concentration of total chlorides in wt% of cement for 50% and 10% probability.

The values have been taken as a function of the confi- dence interval.³⁰

These critical concentration values allow the service life assessment to be obtained for the particular case of a structure exposed to a known environment and with a concrete that is also characterized in terms of durability parameters.³⁴^–³⁷ The service life of a structure is subdi- vided into an initiation period (ti), which in this case is the time necessary for the chlorides to reach the critical concentration at the level of the reinforcement and a con- secutive propagation period (t^p) during which corrosion attack of the reinforcement develops, with implications for the mechanical behavior of the reinforcement^38,39and the structural performance of the concrete element.

A simulated case of assessment of the initiation period of a structure for the case where different grades of steel quality are used is shown below. If it is assumed that the transport of chlorides in concrete occurs mainly by diffusion,⁴⁰^–⁴² it is possible to calculate the initiation period by solving Fick's second law. In this theoretical framework, a scenario can be considered in which: (i) the concrete aging factor remains zero in the long term, and therefore (ii) the diffusion coefficient (Dns) remains constant over time, (iii) the initial chloride concentration (C0) is zero, and (iv) the surface concentration (CS) remains constant over time. Note that in the approach of CEN an aging factor >0 is used, with its value dependent on the type of cement and the prevailing exposure conditions. As the aging factor is known to have a dramatic impact on the outcome of this calculation exercise will be over-conservative.

The values obtained in previous studies are then applied to the case of a structure located in a port in Spain.⁴³ Given the scope of this work, it has been approached deterministically by assigning a constant value to each model parameter. Figure 5shows the predicted profile of chloride concentrations after 100 years for the following deterministic parameter values from⁴³: Dns=5.6*10¹²m²/s andCS=4.7%cem. This figure also shows the range of concentrations from which depassiva- tion occurs for the different grades of reinforcement in Figure 4. In Figure 6, the minimum concrete cover has been calculated to achieve a durability of 100 years. As the alloy quality, that is, the corrosion resistance of the reinforcement, increases, the minimum concrete cover decreases significantly, to the extent that EN 1.4462 steel the minimum concrete cover would reduce to 0 to fulfill the durability requirements.

In the case where the concrete cover is fixed, either for construction reasons or in existing structures, it is

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possible to repeat the above calculation to estimate the initiation period. Figure 7 shows the evolution of the chloride concentration at the level of the reinforcement for the case of a 60 mm cover, maintaining the same values of Dns andCSas for the previous case. The same figure shows the values of the critical chloride concentrations for the different steel grades. The cut-off point of both curves determines the corrosion initiation time (see Figure8). As can be seen, it is predicted that the use of carbon steel will result in an initiation time of less than 5 years. To guarantee a service life of 50 years it is necessary to use a quality equivalent to EN 1.4362 and to guarantee a service life of 100 years it is necessary to use a stainless steel of EN 1.4462 quality.

The propagation period begins with the onset of reinforcement corrosion. During this period, the corrosion

rate gradually increases with increasing chloride concentration at the reinforcement surface. Previous studies have shown the evolution of the corrosion rate as a function of the chloride concentration.³⁰Taking into account

T A B L E 7 Δcdur,stvalues (mm) in Spanish National Annex (source UNE-EN 1992-1-1:2013).

Exposure class X0 XC1 XC2 XC3 XC4 XD1 XS1 XD2 XS2 XD3 XS3

Δcdur,st Others 0 0 0 0 0 25 25 Requires specific study

verification durability LS

Suitable cement 0 0 0 0 0 5 5 10

F I G U R E 4 Threshold concentration of total chlorides in wt%

of cement for 50% and 10% probability of being exceeded. Credit:

IETcc-CSIC.

F I G U R E 5 Predicted chloride concentration profile for t=100 years,Dns=5.6 10¹²m²/s andCS=4.7%cem. Distance to exposed concrete surface. Credit: IETcc-CSIC.

F I G U R E 6 Minimum concrete cover to guarantee a 100-year service life, corrosion initiation. Credit: IETcc-CSIC.

F I G U R E 7 Evolution of chloride concentration for a 60 mm concrete cover,Dns=5.6 10¹²m²/s andCS=4.7%cem. Credit:

IETcc-CSIC.

F I G U R E 8 Initiation period for a 60 mm concrete cover, Dns=5.6 10¹²m²/s andCS=4.7%cem. Credit: IETcc-CSIC.

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Faraday's law, it is possible to estimate the evolution of the loss of reinforcement diameter over time.^44–46 Two further assumptions were made:

1. The corrosion rate remains constant where properties such as saturation do not change significantly during concrete service life. The corrosion rate remains negli- gible when the chloride concentration is below the threshold value.

2. As corrosion takes place locally, a magnitude of the corrosion penetration rate, P^corr, [μm/year] recommended by RILEM TC 154-EMC and a constant pitting factor of 10 are then applicable.^22,45^–⁴⁷

The P^corr findings, that is, the predicted loss in bar diameter, are shown in Figure 9for the steel grades stud- ied. According to the predictions, grades EN 1.4482/2001 and EN 1.4307/AISI 304-L will exhibit similar behavior because their threshold chloride concentrations are assumed to be likewise similar. Corrosion tolerance or resistance was also similar for EN 1.4404/AISI 316-L and EN 1.4362/2304 duplex stainless steel, whilePcorrwas insig- nificant under these conditions in EN 1.4462/2205. For 2 mm corrosion penetration, for instance, for carbon steel the service life is predicted to be 20 years, over 60 years for EN 1.4307/AISI 304-L and EN 1.4482/2001, more than 100 years for EN 1.4362/2304 and EN 1.4404/AISI 316-L and much more than 100 years for EN 1.4462/2005.

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C O N C L U S I O N S

The use of stainless steel reinforcement makes it possible to increase the durability of reinforced concrete structures, especially in very aggressive environments. For this reason it is of interest to look for alternative solutions guaranteeing durability, through incorporation of reinforcing materials other than carbon steel, such as stainless steel rebars.

The change of material in the manufacture of reinforcement involves a modification in the mechanical design parameters and in the concrete covers associated with the different exposure classes. These changes will be incorpo- rated in the latest draft of the new Eurocode 2.¹⁴The cur- rent paper has described the different types of stainless steel in chapter 2, the suitability of the use of stainless steel in the design of structures in chapter 3 and the implications for the durability of structures in chapter 4.

D A T A A V A I L A B I L I T Y S T A T E M E N T

The data that support the findings of this study are avail- able from the corresponding author upon reasonable request.

O R C I D

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A U T H O R B I O G R A P H I E S

Victoria Matres, Acerinox Europa

S.A.U., Madrid, Spain.

Email: victoria.matres@acerinox.

com.

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Juan José Fernandez, Roldan S.A., Grupo ACERINOX, Madrid, Spain. Email: juanjose.fernandez@

acerinox.com.

María Lopez, CEDINOX, Madrid, Spain. Email: maria.lopezalvarez@

acerinox.com.

Luis Peiro, CEDINOX, Madrid, Spain. Email: luis.peiro@

acerinox.com.

Antonio José Madrid, Proes Tech- nical Director. Professor in Struc- tures Department (ETS ICCP-UPM)

Madrid, Madrid, Spain.

Email:[email protected].

Nuria Rebolledo, Durability and Safety of Structures Group (IETcc-CSIC), Madrid, Spain.

Julio Torres, Durability and Safety of Structures Group (IETcc-CSIC), Madrid, Spain. Email: juliotorres@

ietcc.csic.es.

Javier Sanchez, Durability and Safety of Structures Group (IETcc-CSIC), Madrid, Spain.

How to cite this article:Matres V, Fernandez JJ, Lopez M, Peiro L, Madrid AJ, Rebolledo N, et al.

Stainless steel reinforcement: New material in the latest draft of second-generation Eurocode 2.

Structural Concrete. 2023;24(6):7549–60.https://

doi.org/10.1002/suco.202201138