Bending Reinforcement to Shape

Bar Shapes

There are several reasons for bending bars:

• Where anchorage cannot be provided to a straight length within the available concrete shape or size, it may be necessary to bend a 180° hook or 90° cog on the end. Hooks and cogs are never scheduled unless they are shown on the engineer’s drawings.
• Where continuity of strength is required between two intersecting concrete members, the bar will be bent to allow this stress transfer. Such bends are never scheduled unless they are shown on the engineer’s drawings.
• Where ties, stirrups, ligatures or spirals (called ‘fitments’ by the industry) enclose longitudinal bars in a beam or a column, the fitment will be scheduled to match the shape of the surrounding concrete. Mostly the shape is defined by the concrete surface and the specified cover. The actual shape is defined by the scheduler, provided the designer’s intentions are given in the drawings. The designer must indicate if cogged or hooked ends are required.
• Where intersecting reinforcement is likely to clash, or where parallel bars require lapping, the scheduler will decide whether or not to provide small offsets.

Standardised Bar-Bending Shapes

The standard shapes used by ARC are based on a combination of Australian and American standards to utilise the best features of each system. ARC's shapes are not subject to copyright. Appendix B of the Reinforcement Handbook contains ARC's standard shape library.

AS3600-2009 Addresses Bending of Reinforcement

The pin diameters given in Clause 17.2.3 of AS3600-2009 have been selected for very good reasons.

Steel is an elastic material, which means that when it is stretched it will return to its original length after the load is released. This is true up to the ‘yield point’. When stretched in tension beyond the yield point, the increase in length of the bar becomes permanent. The bar’s tensile strength has not been reduced however. If this ‘stretched’ bar is stretched again it may, under some circumstances, recover its elastic properties and possibly also have a new yield point.

When straight steel is bent a very limited amount, it will spring back to straight.
This is because it is still in the elastic range.

When a bar is bent to shape during processing, the steel again has been strained beyond its yield point; if it had not, it would have straightened out! Thus all steel bending changes the material from its original state and any investigation of its properties must allow for this.

A similar situation exists with bars straightened from a coil. In this case the final properties may differ from those it would have had if it had been supplied straight, but they are the properties of the steel when used in concrete and they must comply with the relevant standards.

Excessive bending can be classified as having a pin diameter at or below the bend-test diameter. This can change the metallurgical structure of the steel and can also crush the deformations thus initiating a zone of weakness.

The diameter of a bend should not be so large that the hook cannot fit inside the concrete or that it will pull out rather than act as a hook. Nor should it be so small that the pressure between the bend and the concrete will crush the concrete. A compromise value of 5db for general bending has worldwide acceptance. One of the quality control requirements for a reinforcing steel is that it will pass a bend test. For bars, this test is described in AS/NZS 4671. Despite claims made about the degree of bending which can be sustained by some steels in a laboratory, treatment on a building site can be much more severe. For this reason AS3600 prohibits the use of small diameter pins at or
below the bend test sizes. Cold weather bending and the occasional on-site ‘adjustment’ also require larger pin sizes.

Coated bars are bent about larger pins than uncoated bars. The minimum pin diameters specified for galvanised or epoxy-coated bars are based on three requirements:

(i) Firstly, particularly with epoxy-coated bars, damage to the coating is more likely with
     small pins because of the greater pressure between bar and pin.

(ii) Secondly, with galvanised bars, a small bending diameter is more likely to break
     the zinc surface coating.

(iii) Thirdly, the pickling process during hot dip galvanising can lead to hydrogen
      embrittlement of the reinforcement. The greater the cold working of the steel, the 
      more susceptible it is to hydrogen embrittlement. The larger pin diameter for
      galvanised bars and for bars to be galvanised reduces the cold working of the
      reinforcement.

Where there is a problem of fitting a hooked bar into a thin concrete section,
it must not be solved by using a smaller pin diameter. Instead, the bar must be
rotated, possibly up to 90º, to ensure adequate cover.

Cutting and Bending Bars from a Coil

Bar 16 mm and smaller is available in coil form, each coil being about 2 tonnes (approximately 2200 metres of N12 bar). Bars are cut from a coil, either singly or in pairs, after passing through a straightener. Handling is the main limit on available length.

The straightener often leaves a series of marks on the bar surface, but this does not affect the anchorage properties.

An alternative machine permits a coiled steel to be straightened and then bent in a continuous operation, after which the bent piece is cut from the coil. The shape and dimensions can be programmed.

Bending Bars Cut from Stock Lengths

After cutting, a separate operation is used to produce the required shape. Again, depending on bar diameter, one to six bars can be bent at once.

Cutting and Bending Mesh

Reinforcement mesh can be cut to size in the factory using a guillotine or cut on-site using bolt cutters. Site cutting is slow and very labour intensive.

The reinforcing fabric can be bent to suit the concrete profile. The mesh is bent to the required shape using equipment specifically designed to ensure that the bends on each wire are accurate and within construction tolerances.

Heating and Bending

Hot bending is not a normal factory operation. It is more likely to be done on-site, generally with poor supervision and inadequate quality control. Since the strength of the steel will be reduced, uncontrolled hot bending is a dangerous practice.

As a general rule, heating Grade D500N bars of any type must be avoided at all times. AS3600-2009 Clause 17.2.3.1 (b) gives a maximum temperature for reinforcement as 600° C. At 600° C the bar is only just starting to change colour. If any colour change is observed whilst heating, the reinforcement should be discarded. If the bar is heated over 450º C, the steel is softened due to changes in the crystalline structure of the metal. Once heated over 450º C the yield strength of the bar is reduced to 250 MPa. The only practical method of monitoring heat in the bar is the use of heat crayons.

Galvanised bars should not be hot bent.

Using Bars after Heating

Overheating beyond 600°C will alter the structure of the steel. 450°C has been found to be a realistic limit because above this temperature the yield stress, while under load, reduces to 250 MPa.

On-Site Rebending

Rebending or straightening bars is a common practice on-site. Instructions on a suitable procedure should be given in the structural drawings, even if it is known that such bending will not be needed. A tolerance on straightness should also be provided; an axial deviation of the centre line of one bar diameter along with a directional change of 5° is considered acceptable.

Any on-site cold bending should only be done with a proper bar bending tool. Pulling the bar against the edge of the concrete, hitting the bar with a sledge hammer or using a length of pipe damages the surface of the reinforcement, reduces its ductility, can cause breakage of the steel and may cause premature failure of the concrete element.

When reinforcement is bent about a curve smaller than the recommended minimum pin diameter, or bent against an edge, the steel is excessively strained on the compression and tension faces. An attempt to straighten a bar bent too tightly may lead to bar failure.

On-Site Bending of Reinforcing Mesh

Site bending of mesh is usually done by poking a bar through the opening in the mesh, then rolling the bar over to bend the mesh about a cross wire. This is then repeated every two or three openings for the width of the mesh. This practice is not recommended as it bends the reinforcement about an effective pin size of one bar diameter, with the cross wire acting as the bending pin. This is well below the three diameter pin size required for Ductility Class L bars or the four diameter pin size required for Ductility Class N bars in AS3600-2009, Clause 17.2.3.2. On-site bending of reinforcement also tends to be very variable and inaccurate, usually outside construction tolerances.

Final Advice on Mistreatment of Steel

It cannot be stated enough that steel cannot be expected to perform its proper function if it has been mistreated by excessive tight bending or by overheating. Site bending, welding or heating of bars should only be permitted under very strict and competent supervision. A detailed job procedure and quality control system should be employed for site bending, welding or heating of bars.