Steels_ Metallurgy and Applications, Third Edition

Low-carbon structural steels 179

Reinforcing bars

Large amounts of steel are used for the reinforcement of concrete in build-

ings, bridges and marine structures and reinforcing bars therefore constitute a

competitor to structural steel plates and sections. At one time, reinforcing bars

were regarded as low-grade, undemanding steel products and were often produced

from diverted casts that were out of specification for the originally intended order.

However, with the trend towards higher-strength steels and the requirement for

good fabrication characteristics, reinforcing steels are now made to high quality

standards. Whereas some reinforcing bars are supplied in the form of plain carbon

steel with a yield strength of 250 N/mm 2, extensive use is now made of higher-

strength steels with yield strengths up to 500 N/mm 2.

Standard specifications

The UK standard for steel reinforcement is BS 4449:1988 Carbon steel bars
for the reinforcement of concrete. It covers grades with minimum yield strength
levels of 250 and 460 N/mm 2 in the form of plain rounds and deformed (ribbed)
bars respectively. The chemical compositions of these grades are only broadly
defined, as indicated in Table 2.15.
In order to provide a reasonable level of weldability, maximum carbon equiv-
alent values of 0.42% (Grade 250) and 0.51% (Grade 460) must be observed,
based on the following formula:
Mn Cr + Mo + V Ni + Cu
CE = C + --~ + 5 + 15 wt%

To ensure adequate ductility, minimum elongation values of 22% (Grade 250)

and 12% (Grade 460) are required but the materials must also be capable of

being bent through 180* around formers of the following proportions:

9 Grade 250- 2 x nominal bar diameter

9 Grade 460 - 3 x nominal bar diameter

The specification also includes a rebend test requirement which was introduced

initially to determine the susceptibility of the materials to strain age embrittle-

ment. This involves bending bars initially through 45* around formers of the

following proportions.

9 Grade 250- 2 x nominal bar diameter

9 Grade 460- 5 x nominal bar diameter

Table 2.15

Grade 250 Grade 460

C% 0.25 max. 0.25 max.

S% 0.060 max. 0.050 max.

P% 0.060 max. 0.050 max.

N% 0.012 max. 0.012 max.

, ,