STEEL - Materials for Civil and Construction Engineers - page 110

Reinforcing steel is produced in the standard sizes shown in Table 3.8.
Bars are made of four types of steel: A615 (billet), A616 (rail), A617 (axle),
and A706 (low-alloy), as shown in Table 3.9. Billet steel is the most widely
used. A706 steel is often used when the rebar must be welded to structural
steel. Reinforcing steel is produced in four grades: 40, 50, 60, and 75, with
yield stresses of 276 MPa, 345 MPa, 414 MPa, and 517 MPa (40 ksi, 50 ksi,
60 ksi, and 75 ksi), respectively.
Prestressed concrete requires special wires, strands, cables, and bars. Steel
for prestressed concrete reinforcement must have high strength and low re-
laxation properties. High-carbon steels and high-strength alloy steels are
used for this purpose. Properties of prestressed concrete reinforcement are
presented in ASTM specification A416 and AASHTO specification M203.
These specifications define the requirements for a seven-wire uncoated steel
strand. The specifications allow two types of steel: stress-relieved (normal-
relaxation) and low-relaxation. Relaxation refers to the percent of stress re-
duction that occurs when a constant amount of strain is applied over an
extended time period. Both stress-relieved and low-relaxation steels can be
specified as Grade 250 or Grade 270, with ultimate strengths of 1725 MPa
(250 ksi) and 1860 MPa (270 ksi), respectively. The specifications for this ap-
plication are based on mechanical properties only; the chemistry of wires is
not pertinent to this application. After stranding, low-relaxation strands are
subjected to a continuous thermal-mechanical treatment to produce the re-
quired mechanical properties. Table 3.10 shows the required properties for
seven-wire strand.

3.8

Mechanical Testing of Steel

Many tests are available to evaluate the mechanical properties of steel. This
section summarizes some laboratory tests commonly used to determine prop-
erties required in product specifications. Test specimens can take several
shapes, such as bar, tube, wire, flat section, and notched bar, depending on
the test purpose and the application.
Certain methods of fabrication, such as bending, forming, and welding,
or operations involving heating, may affect the properties of the material
being tested. Therefore, the product specifications cover the stage of manu-
facture at which mechanical testing is performed. The properties shown by
testing before the material is fabricated may not necessarily be representa-
tive of the product after it has been completely fabricated. In addition, flaws
in the specimen or improper machining or preparation of the test specimen
will give erroneous results (ASTM A370).

■

3.8.1 Tension Test

The tension test (ASTM E8) on steel is performed to determine the yield
strength, yield point, ultimate (tensile) strength, elongation, and reduction