Does Bending Rebar Weaken It? Rebending, Heat and Field Bends

A single cold bend made around the correct minimum bend diameter doesn't weaken rebar in any way that matters to the design. Canadian reinforcing bar is made to pass a bend test, a full 180° bend around a specified pin without cracking on the outside of the bend, and the minimum bend diameters in CSA A23.1 are set so normal bends stay within that capacity. What does damage rebar is bending too tightly, bending the same spot more than once, straightening and rebending, and heating the bar. Those are the practices to avoid on site.

Why a proper bend doesn't hurt the bar

When you bend a bar, the outside of the bend stretches and the inside compresses. Steel can take a lot of this permanent deformation before it cracks. That ability is called ductility, and it's one of the properties reinforcing steel is tested for.

Under CSA G30.18, the Canadian rebar standard, bars must pass two tests that matter here:

• Elongation in the tensile test, a measure of how far the steel stretches before it breaks.
• A bend test, in which the bar is bent 180° around a pin of specified diameter and must show no cracking on the outside of the bend.

The bend diameters used for real hooks and bends, set out in CSA A23.1 and the RSIC Manual of Standard Practice, depend on the bar size and grade. Larger bars need larger bend diameters, and stirrups and ties have their own rules. Bending to those diameters keeps the strain on the outside of the bend at a level the bar is proven to handle. For the actual values, see our rebar bending chart and rebar hooks and bend radius.

The bend does change the steel locally: it work-hardens, becoming a little stronger and less ductile right at the bend. That's expected, and it's why the standard bend details exist. Your job on site is simply to bend to the right diameter, once.

What actually damages rebar

Bending too tightly

Bending around a pin smaller than the minimum diameter concentrates the strain on the outside of the bend. The result can be cracking there, sometimes as fine surface cracks that are easy to miss. Hammering a bar over a sharp form edge or bending it around another bar does the same thing. Minimum bend diameters differ by bar size and between regular (R) and weldable (W) grades, so use the value for the bar you actually have. The bar's grade is rolled into it and shown on the mill certificate.

Rebending and straightening

Can rebar be bent twice? Bending a bar to a shape it was designed for is one cycle. Bending it back, straightening a hook, or bending it again in the same place is where problems start. Each cycle uses up more of the bar's ductility at that point, and the bar can crack on what was the inside of the first bend.

The RSIC Manual of Standard Practice is direct about it: reinforcing bars should not be bent or straightened in a manner that will damage or fracture the material. It also warns about a common site habit, "springing" closed stirrups open to drop longitudinal bars in, which can cause breakage or microscopic cracks that nobody notices. That's why detailers often split closed stirrups into two pieces where bars must be threaded in.

Bending bars already cast in concrete

Dowels and starter bars that stick out of a pour are the bars most often bent in the field, either to get them out of the way or to fix a location mistake. CSA A23.1 has long addressed this. In the 2004 edition of the standard, bars partially embedded in concrete must not be field bent except as shown on the drawings or as permitted by the owner. The 2024 Ontario Building Code references the 2019 edition of CSA A23.1, so confirm the current wording with your engineer, but the principle hasn't changed: don't bend embedded bars without the engineer's say-so.

Bending a bar right at the concrete face is especially risky, because there's no room for a proper bend diameter and the concrete edge can spall. If dowels are in the wrong place, ask the engineer for a fix rather than bending them over. Options often include drilled-in dowels or mechanical connections; our post on rebar couplers vs lap splices covers the coupler side.

Heating the bar

Bending rebar with heat is tempting with large bars or in cold weather, but heat changes the steel. Rebar gets its strength and ductility from its chemistry and from how it was rolled and cooled at the mill, and heating it with a torch can change those properties in an uncontrolled way. Uneven heating also leaves you guessing about what the bar's properties are afterwards.

The 2004 edition of CSA A23.1 required bars to be bent at temperatures between 10 °C and 100 °C unless the owner permitted otherwise. That covers both ends of the problem: heating bars beyond that range, and bending them when they're very cold. Again, check the edition on your project. The RSIC manual describes standard fabrication as cold bending to the specified dimensions.

In practice: don't heat bars to bend them unless the engineer has approved a procedure, and be cautious about bending bars outdoors in an Ontario winter. Ordering bent shapes from the shop avoids both problems.

Coated and non-steel bars

Bending rules get stricter for bars with coatings or made from other materials:

• Epoxy-coated rebar: field bending is best avoided because the coating can crack, and any damage has to be repaired.
• GFRP (fibreglass) rebar: bends are formed by the manufacturer before the resin cures. Field bending isn't possible, and every hook, corner and stirrup has to be ordered as a bent piece. The RSIC manual states plainly that field bending of FRP bar is not permitted.

Is rebar hard to bend?

Small bars, such as 10M, can be bent with a hand bender, though getting consistent dimensions takes practice. From 15M up, hand bending gets hard quickly, and larger bars need a powered bender with the right mandrel for the bend diameter. Hand bending also makes it easy to bend too tightly or overshoot and bend back, which is exactly what damages the bar.

That's the real argument for shop bending. A fabrication shop bends on machines set to the correct pin size for each bar, to the dimensions on the bar list, within standard fabricating tolerances. Our article on bending stirrups on site vs shop fabrication compares the two for the most common bent shape.

A practical checklist for site crews

• Bend loose bars only, and only to the minimum bend diameter for that size and grade.
• Bend once. Don't straighten and rebend.
• Don't bend embedded dowels or starter bars without the engineer's approval.
• Don't heat bars to bend them unless the engineer has approved a procedure.
• Don't spring closed stirrups open. Ask for two-piece stirrups if bars must be threaded in.
• Don't field bend epoxy-coated bar, and never try to bend GFRP.
• Inspect bends you weren't sure about. Cracks on the outside of a bend mean the bar should be replaced.

Get your bars bent in the shop

Procut Steel cuts and bends rebar from 10M to 55M in our shop to your bar lists, in all standard shapes, then tags and bundles it so your crew can place it without sorting or bending on site. We also prepare bar lists and placing drawings from your structural drawings, supply fabricated dowels and stirrups, and supply GFRP rebar. For the ordering side, see how to order cut and bend rebar.

Send us your drawings or bar list for a free quote, or see our rebar products.