Ductility in standardization

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Standard classification

The division of reinforcing steel into classes has changed after the introduction of the Eurocodes into the catalog of Polish Standards. Currently, the ductility is a parameter that determines the belonging of a steel grade to a given class. Formerly construction norms classified reinforcement on the basis of the yield point, not taking into account the ductility of the material.

These differences can be explained following the history of technological progress in steel production. In the past, the increase in the strength of the material was possible to achieve by increasing the carbon content or cold rolling. Both processes resulted in a significant decrease of ductility. It was therefore possible to obtain steel, for which the minimum yield strength was even 500 MPa, but which was much more brittle than steel with lower strengths. Therefore, recommendations and instructions for the design of structures, issued 20 years ago and earlier, to reinforce elements in which ductility plays an important role, require the use of steel A-0 or A-I, i.e. material with lower strength, but relatively good plasticity.

Currently, technological progress has made it possible to achieve high strength of reinforcing steel while maintaining good ductility parameters. The method of controlled cooling popular in the metallurgy, which involves intensive spraying of the rod with high pressure water in the last phase of rolling, makes it possible to achieve high strength without increasing the carbon content. This results in maintaining the high ductility of the steel and ensuring proper weldability.

Eurocode requirements

The European standard for the design of reinforced concrete structures PN-EN 1992-1-1 distinguishes three classes of reinforcing steel: 

Steel class Yield strength fyk [MPa] k=(ft/fy)k εuk [%]
A 400 ÷ 600 ≥ 1,05 ≥ 2,5
B ≥ 1,08 ≥ 5,0
C 1,15 ÷ 1,35 ≥ 7,5

where:

  • class A – low-ductility steel
  • class B – medium-ductility steel
  • class C – high-ductility steel

In addition, the Polish National Annexes to the Eurocodes provide explicit recommendations as to the selection of reinforcing steel for some structures:

  • The National Annex to PN-EN 1992-1-1 prescribes the use of reinforcing steel of high class C of ductility, in accordance with Polish Standard PN-H-93220, for the reinforcement securing the structure against exceptional loads and the effects of a progressive disaster, carrying objects located on mining areas as well as load-bearing reinforcement of objects exposed to dynamic loads.
  • The National Annex to PN-EN 1992-2 prescribes the use of reinforcing steel of high class C of ductility, in accordance with the relevant Polish Standards for the reinforcement of concrete bridges.

Classification according to PN-B-03264

According to the Polish Standard used in the design of reinforced concrete structures PN-B-03264:2002, the division of reinforcing steel into classes was based on the yield point. The standard defined classes from A-0, for which the characteristic yield stress of the fyk was at least 220 MPa, to A-IIIN, for which the fyk could not be less than 500 MPa. At the same time the Polish Standard stated that A-IIIN class steels show reduced ductility in relation to lower grade steels. This was closely related to the steel production process, which is mentioned above.

The PN-B-03264: 2002 standard also provided examples of reinforcing steel grades in a given class (e.g. 34GS, RB500W) and defined their weldability. It is worth noting that most of those grades are not longer available on the Polish market (producers stopped their production), moreover some of them do not meet the requirements of current design standards (e.g. incurable or plasticity grades lower than 400 MPa). This should be taken into account when designing in accordance with the "old" standard, as entering such a type of reinforcing steel into the design of the construction results in delays on the construction site, caused by the necessity of finally replacing the steel with the currently available one.