Steels_ Metallurgy and Applications, Third Edition

180 Steels: Metallurgy and Applications

The bars are then immersed in boiling water for at least 30 minutes. On cooling

to ambient temperature, the bars must be capable of being bent back towards

their original shape through an angle of at least 23*.

Traditional reinforcing steels

In the UK, high-strength reinforcing steels have been produced traditionally by:

1. The cold twisting of plain carbon steels.


2. The use of vanadium-bearing micro-alloy steels.

At one time, the former was specified in a separate standard (BS 4461), but
the cold-twisted product was incorporated in BS 4449 when this standard was
revised in 1988. Although these steels employ different strengthening mecha-
nisms, namely work hardening as opposed to precipitation strengthening, their
properties are sufficiently similar to be covered by a single set of requirements
for tensile and bend-rebend properties.
Whitely 26 reports that the cold-twisted product can be welded, with little loss of
strength, by employing high heat inputs for short periods of time. Such practices
restrict the area of the heat-affected zones and the quenching effect of adja-
cent material leads to the formation of strong, low-temperature transformation
products. Whitely also indicates that the cold-twisted product provides adequate
elevated-temperature tensile properties which are pertinent to the fire resistance
of reinforced concrete structures.
Although not specified in BS 4449, the impact strength of reinforcing bars is of
interest in relation to the potential damage that can be caused by the crashing of
vehicles into the supporting columns and deck parapets of concrete road bridges.
In collaboration with the Department of Transport, extensive work on this topic
was carded out by British Steel Technical. 27 Using a large pendulum impact
machine, with an impact velocity of 7.6 m/s, impact transition temperature curves
were developed on full-section cold-twisted bars. It was shown that unnotched
bars remained fully ductile at temperatures down to at least -65"C but, with
the introduction of a sharp notch, the impact transition temperature was raised
significantly. However, when impact tests were carded out on large reinforced
concrete beams, the effect of notching the bars was shown to be significant only in
the case of cold-twisted material. This is illustrated in Figure 2.20, which shows
that very little loss of impact strength was observed in mild steel (Grade 250)
bars and hot-rolled (micro-alloy, Grade 460) bars.

Controlled-cooled bars

In the mid-1970s, the CRM laboratories at Liege in Belgium published details of
an in-line heat treatment process for the production of high-strength reinforcing
bars. 28 Designated the Tempcore process, the application of controlled cooling
after rolling results in the formation of an outer layer of martensite which is
tempered subsequently by conduction of heat from the core of the bars. The
main aim of the process is to produce weldable, high-strength reinforcement