Analysis of international standards

on concrete reinforcing steel bar

Jorge Madias (1), Michael Wright (2), Gustavo Behr (3), Vanessa Valladares (3)

(2) Modern Metal Consulting, Singapore

(1) metallon, San Nicolas, Argentina

(3) ADELCA, Aloag, Ecuador

Content

Introduction

Selected standards

Mechanical properties

Bending & rebending

Chemical analysis

Special tests

Conclusions

Introduction

metallon

Consulting & training company based in San Nicolas, Argentina

Modern Metal Consulting

Consulting & training company based in Singapore

ADELCA

Steel company with two plants in Ecuador

Aloag: EAF, LF, Billet caster, Rebar rolling mills

Milagro: New rebar & wire rod rolling mill recently started-up

Introduction

Standards usually reflect with a certain delay the advances of technology in product manufacturing and the growing requisites of the users

National, regional and international standards on steel reinforcing bar are analyzed

Rebar standards: low degree of globalization

Aim of the study 1) Verify trends in the quality constraints that may affect

the commercialization and application of rebar

2) Specifically, requisites related with seismic behavior and the influence of manufacturing processes (natural hardness, on-line quenching and tempering, micro-alloying)

Selected standards

Reasons for country / region selection Rebar exporting potential

High seismicity

High rebar production

Special features of the standard

In some countries, a single standard cover not only rebar but other long products for civil construction (generally round bars, but in some cases welded mesh, pre-stress concrete, etc.)

ASTM standards and their followers have usually two standards: one for reinforcing bar in general, and other for weldable rebar

Still there are in some countries standards for rebar produced by rolling rail or strip scrap (not taken into account in this study)

Selected standards

Country Standard Character Argentina IRAM-IAS U500-207 2004 Weldable

IRAM-IAS U500-528 2004 General Australia/N. Zealand AS/NZS 4671:11 General Brazil ABNT NBR7480 General Canada G30.18-09EN reaffirmed 2014 General Chile NCh 204 General China GB 1499.2 2007 General Colombia NTC 2289 2007 General Ecuador NTE INEN 102:2015 General

NTE INEN 2167:2017 Weldable Europe prEN 10080 2005 General Germany DIN 488.1 2009 Grades, properties, marking

DIN 488.2 2009 General DIN 488.6 2010 Conformity assessment

International ISO 6935-2 2015 Part 2 General Japan JIS G3112 2010 General Mexico NMX-C-407-ONNCCE-2001 General

NMX-B-457-Canacero-2013 Weldable Peru PNTP 341 031 2015 General

PNTP 339 186 2015 Weldable Spain UNE 36065 2011 High ductility UNE 36068 2011 Weldable Taiwan CNS 560 2005 General Turkey TS 708 2010 General United Kingdom BS 4429 2005 General USA ASTM A 615-14 General

ASTM A 706-14 Weldable Vietnam TCVN 1651-2 2008 General

Mechanical properties

Compared properties

Tensile and yield strength

Stress ageing ratio

Uniform elongation

Elongation to rupture

Yield strength

Present in all standards

Some standards define a minimal value, others both minimum and maximum

Lowest value 220 Mpa

Turkey, one grade

Maximum value 600 Mpa

ISO, one grade

Mechanical properties

Yield strength

Higher strength grades

0

100

200

300

400

500

600

700

Yie

ld s

tren

ggth

(M

Pa)

Mechanical properties

Yield strength Some Asian countries are

considering the introduction of 600 MPa minimum grade, as in ISO standard

Arguments favoring high strength rebar from the point of view of constructors Reducing rebar

congestion in anti seismic design, particularly in column-beam crossings

Reducing total steel mass needed for the overall construction.

Mechanical properties

Elongation Elongation to rupture is

preferred

But uniform elongation starts to make inroads Elongation measured for

the maximum load, more complex to measure (extensometer is required)

German standard changed to uniform elongation in 2009

China, Spain, Vietnam and some grades of the Turkish standard define minimum uniform elongations

Mechanical properties

Elongation

Elongation to rupture vs. yield strength

Mechanical properties

Elongation Uniform elongation

Australia / New Zealand standard: three ductility degrees low (an L is added to the bar

designation);

normal (an N is added to the bar designation)

high or seismic (an E is added to the bar designation).

Type E corresponds to 2 levels of minimum yield strength (300 and 500 MPa)

Very high uniform elongation is requested (15 and 10% respectively)

Tensile strength / yield strength ratio of at list 1.15

Important for New Zealand only

Mechanical properties

The question is “why is 5% elongation to rupture considered acceptable in higher yield strength rebar?” Experience suggests that the minimum acceptable elongation to

rupture (%) in rebar is dictated by what can reasonably be achieved in a high yield strength steel made with simple C-Mn metallurgy

But at what elongation to rupture would a metal be seen to be classed as “brittle”?

The reason for this question is that in a concrete-steel composite material, the concrete is a brittle material – if the steel is also brittle, then the entire structure can be classed as brittle. The ductile properties can only originate in the steel, so this property must be protected by Standards.

Mechanical properties

Tensile strength / yield strength ratio (strain hardening ratio)

High value for some grades and low value (or no value) for others

ISO, Australia/New Zealand, Brazil, USA, UK, Mexico, China, Turkey

Only high values

Argentina, Canada, Chile, Colombia, Ecuador, Peru, Spain, Japan

Highest ratio (>1.25)

ASTM A706, Argentina, Mexico NMX-B-457, Chile, Colombia, Ecuador, Peru (Taiwan, China just for some grades)

>1.20 Spain (high ductility); Turkey (one grade); Canada

>1.15 Australia/New Zealand for seismic grades

Single low value

Germany, >1.08

Mechanical properties

Tensile strength / yield strength ratio

Relation with elongation to rupture

Mechanical properties

Bending and rebending Most standards: bending to 180º

Mandrel diameter function of bar diameter

Exception: Chile (90º bending)

Some standards prefer 90º bending only for the large diameter bars and/or the very high strength grades

Bending and rebending (with intermediate ageing) Spain, UK

Germany (for bars <40 mm)

Magnetized particles testing to assess if cracks are present after bending, common for other steel products, still not considered by rebar standards

Chemical analysis

Generally, just maximum content for some elements

Heat analysis and product analysis (usually 10% higher)

Brazil, Chile: no spec required

ASTM A615 and followers: only maximum P content

ASTM A 706 (weldable rebar) and followers: maximum values for C, Mn, P, S, Si and Ceq Same for high ductility / weldable rebar in other standards

ISO, European countries, China, Argentina: N2<0.012%, for some grades

European standard: Cu<0.80 %; Germany <0.60%

Canada: Mn, Si more restricted than usual for galvanized rebar

Internal specifications of steel plants are always more restrictive than those in the standards To make sure they obtain the required mechanical properties at

lower cost

To minimize surface and internal quality problems, as those related with high sulphur or high copper

Chemical analysis

Lowest P 0.035 ASTM (weldable rebar), Mexico, Colombia, Ecuador,

Peru

Lowest S 0,040% ISO (weldable grades), one of the Ecuador standards,

four Japanese grades, two weldable grades in Taiwan and Vietnam

Lowest C 0.22% Europe, Spain, Germany

Turkey, Argentina just for weldable rebar

Mn: <1.50 to <1.80, just some standards

Standards reflect with a long delay the advancement of steelmaking technology

Specs are relaxed; rejection is fairly uncommon

Exception: weldable / high ductility rebar C, Mn, residuals are to a certain extent under control

Trend to establish weldability / high ductility requirements

Special tests

Fatigue Not mentioned in ASTM standards and followers, and Chile, Taiwan, Turkey,

Vietnam

Required by Germany, UK. Spain for high ductility rebar

European standard: an option, but its application is very thoroughly detailed

Argentina, China, Australia / New Zealand: may be agreed between purchaser and manufacturer

Adherence Not mentioned in most standards

European standard Adherence given by the geometry of ribs, but that it can be verified by beam test or

pull test

UK: same, but beam test as the alternative testing tool

Australia / New Zealand Calculus for estimation of adherence in function of the surface of ribs

Pull test as an alternative

Fatigue Beam test Pull test

Non-standard regulations

Some governmental measures go beyond the standards regarding rebar specifications

Argentina, Ecuador: certification system for civil construction steel product In Europe this is included in the standards

Influence of the mechanism utilized by the manufacturer to achieve the required mechanical properties tends to be taken into account, as it influences behavior in use (corrosion, fire strength, seismic behavior) Department of Building and Housing of New Zealand issues Practice Advisory that

establish how to use the anti seismic grade rebar, taking into account if it was produced using microalloying or on-line quenching and tempering [36]. For example (MA means microalloyed and QT means on-line quenched and tempered) Cold re-bending/straightening Grade 500E MA and QT not permitted by NZS 3109.

Welding of Grade 500E QT is not permitted by NZS 3109.

Site welding of Grade 500E MA should be avoided.

Shop welding (but not tack welding) of Grade 500E MA is considered acceptable provided that evidence is presented that the procedures used do not affect compliance of the reinforcement with AS/NZS 4671

In certain countries, not only the producer name or logo must be rolled in the bar, but the name of the importing company, is such is the case Foreign manufacturers introducing rebar in Germany must roll in the rebar a

specific mark given by a local association

Conclusions

Trends Mechanical properties

Introduction of higher high yield grades, over 500 MPa, aiming to decrease bar congestion, particularly in column / beam crossings in high-rise buildings for seismic zones

Introduction of uniform elongation

Higher minimum elongation is high seismicity countries

Strain ageing ratio Consensus, for high seismicity countries, in >1.15 or higher

Bending European-influenced standards prefer bending and rebending.

Chemical analysis Low C and low Ceq for weldable / high ductilty grades

In some countries, particularly in Europe, only weldable / high ductility grades exist

Most critical chemistry requirements are low C (<0.22-<0.25%), and N2<120 ppm

Others Fatigue and adherence testing start to occur as a requisite in some European

standards

Grain size mentioned in the Chinese standard

Traceability more relevant, taking into account intervention of several manufacturers of different countries for a given product

Conclusions

Updating rebar standards Introduction of minimum uniform elongation, rather than elongation

to rupture, as a more fundamental requisite for seismic / high ductility purposes.

Introduction of magnetized particles testing to assess if cracks are present after bending

Maximum grain size should be considered for seismic / high ductility purposes

Lower phosphorus and sulphur contents.

As international trade of billets intended for rebar rolling becomes more important, traceability should be emphasized.

Request of conformity assessment by third parties

Hardening mechanism utilized by the manufacturer to obtain the properties becomes more important, as it influences performance (loss of properties, corrosion, seismic behavior, strength after fire)

Jorge Madias (1), Michael Wright (2), Gustavo Behr (3), Vanessa Valladares (3)

(1) metallon, San Nicolas, Argentina

(2) Modern Metals Consulting, Singapore

Modern Metal Consulting

(3) ADELCA, Aloag, Ecuador