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quenched and tempered reinforcing steel

Bending and Re-bending Fact Sheet

FACT SHEET 4

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The tighter the diameter of the bend the more ductility that is consumed in

the bending process. The Concrete Design Standard NZS 3101 states

the minimum bend diameters for main bars and stirrups in the two tables

which follow:

NZS 3101 Table 7.1 - Minimum diameters of bend

fy (MPa) Bar diameter, db (mm) Minimum diameter of bend

300 or 500 6 - 20 5 db

24 - 40 6 db

NZS 3101 Table 7.2 - Minimum diameters of bends for stirrups

and ties

fy (MPa) Bar diameter, db (mm) Minimum diameter of bend

Plain bars Deformed bars

300 or 500 6 - 20 2 db 4 db

24 3 db 6 db

Re-bending (or straightening of previously bent bars)

Often construction practices require the straightening of a bar. This is

essentially a second bend and further consumes the ductility of the bar.

Due to the fundamental differences between Micro-alloyed (MA) and

Quench & Tempered (QT) reinforcing the rules for straightening these two

products are different. For QT reinforcing the rules are simple re-bending

or straightening must NOT be done. The Concrete Construction Standard

NZS 3109, amendment 2 states:

Grade 500 quenched and self tempered bars shall not be straightened

or re-bent

Grade 500 Ductility Class E

To re-bend Grade 500 MA the following must rules must be applied:

The bar must be heated to cherry red (750 +/- 75 degrees Celsius).

The heated area must be over the length of the bend.

The re-bending must only happen in the same plane as the original bend.

The bending must be carried out smoothly using a continuous force.

The re-bend must not bend the bar past the original straight position.

The bar is to be cooled to ambient temperature naturally with protection

from any accelerated cooling effects such as rain or wind.

The surface of the bar is to be inspected after re-bending for cracking.

Any cracked bars are to be rejected.

Grade 300 Ductility Class E

Grade 300 Class E reinforcing is a more ductile product with a minimum

uniform elongation of 15% versus 10% for 500E. Therefore it can withstand

greater strains than the 500E and for this reason it is recommended that

300E be specified if re-bending or straightening will be required.

Grade 300 Class E reinforcing is to be re-bent once only which can be

done at ambient temperature.

General Requirements and Principles of Bending

and Re-bending.

1. Bend diameters must not be less than the values in the above table.

2. Re-bending of reinforcing should be avoided if at all possible through

design or use of appropriate construction systems.

3. Where re-bending is required Grade 300E is the preferable material

to specify.

4. When re-bending will be required designers should attempt to keep

the diameter to 16mm or below.

5. If bars are to be galvanised it is preferable that the bar is galvanised

prior to bending.

6. Bars protruding from concrete should not be bent closer than 3 bar

diameters from the face of the concrete.

7. If re-bending Grade 500 identify the method of manufacture QT

or MA before proceeding.

8. Do NOT re-bend Grade 500 Quench and Tempered reinforcing bars

find another way to solve the construction problem.

9. Specify at the design stage either Grade 300E or Grade 500E MA

if re-bending is likely to be required.

10. When rebending Grade 500E Micro-Alloyed (MA) follow

the stipulated process.

Further sources of information include the CCANZ information bulletin IB 79

and the Department of Building and Housing Practice Advisory #1.

Bending and Re-bending Reinforcing Steel has a detrimental affect on the ductility of the bar and

therefore there are strict provisions on what is allowable and what isnt allowable. The strict provisionsare laid out in the Concrete Design Standard NZS 3101 and The Concrete Construction StandardNZS 3109. Construction of designs that have obtained building consent through the use of the aboveapproved documents must, by law, follow the rules of the documents. Failure to do so is illegal.

Bending and Re-bending Fact Sheet

maqtquenched and tempered reinforcing steel

For more infomation contact Pacific Steel freephone 0800 722 783, PO Box 22 201, Otahuhu, Auckland, New Zealand

www.reinforcing.co.nz