Considerations For Bending Steel Reinforcing Bar Before and After HDG
Should Steel Reinforcing Bar Be Bent Before or After HDG?
Hot-dip galvanized (HDG) steel reinforcing bars can either be bent prior to galvanizing or after the coating has been applied. Regardless, there are considerations the designer and/or specifier should review to determine whether bending before or after hot-dip galvanizing is preferred for a particular application.
Bending Reinforcing Bar Prior To HDG:
When bending rebar prior to hot-dip galvanizing, it is recommended that the bend diameter be as large as possible in order to avoid strain-age embrittlement from excessive cold forming of the steel. When rebar is cold-bent, stresses are induced into the steel. Later when the rebar is heated to the temperature of the galvanizing kettle, the change in temperature can affect the areas with the increased stress from cold working. It is when the stresses from the cold bending are excessive that it is possible for strain-age embrittlement to occur.
Besides cold working, another factor that contributes to strain aging is steel quality. Rebar is more susceptible to strain-aging because it is commonly made from lower quality steel, containing many impurities that congregate at the highly stressed points in the steel. As a result, strain-aging and fractures are more likely to occur at the toe of the bend where the highest induced stress resides.
Bend recommendations serve to limit the amount of stress introduced to the steel during cold working and decrease the chances for stain-age embrittlement. ASTM A767/A767M Table 2 provides guidelines for the recommended bend diameter based on the nominal bar size.
Minimum Finished Bend Diameters (ref. ASTM A767 Table 2)
|3, 4, 5 [10, 13, 16]||6d||6d||6d||...|
|7, 8 [22, 25]||6d||8d||8d||8d|
|9, 10 [ 29, 32]||…||…||8d||8d|
|14, 18 [43, 57]||…||…||10d||10d|
Note: d = nominal diameter of the bar.
By following these recommendations, the bend process produces much lower residual stress levels. If rebar is required to be bent tighter than these recommendations, it can be stress relieved at 900 F to 1050 F (480 C to 560 C) for one hour per inch of bar diameter to reduce the residual stress.
Bending Reinforcing Bar After HDG
When hot-dip galvanized rebar is bent after the hot-dip galvanizing process, there is no concern with strain-age embrittlement, and therefore bend diameter is not limited in any way. However, there can be an issue with cracking or flaking of the coating in the area of the bend. Furthermore, the speed at which the article is bent may affect the coating integrity, so it is best to use slower bend speeds.
When cracking or flaking of the coating occurs on the bent section of the rebar, the outer zinc layer is the only layer that cracks. Coating thickness measurements will indicate that the zinc-iron alloy layers are still intact to provide numerous years of corrosion protection. Therefore, any flaking and cracking that occurs to the coating when bending rebar after hot-dip galvanizing can be repaired per ASTM A780/A780M and is not grounds for rejection per ASTM A767/A767M Section 7.2.1:
7.2.1 When galvanizing is performed before bending, some cracking and flaking of the galvanized coating in the area of the bend shall not be grounds for rejection (Note 7).
Note 7 The tendency for cracking of the zinc coating increases with bar diameter and with severity and rate of bending.
Damage in bend areas is common and can be repaired as described in ASTM A780/A780M. However, heating the bend areas with a torch to apply zinc solder is not recommended because it can strain-age the steel due to the elevated temperature of the torch.
Because cracking or flaking of the coating is not desirable, there are some hot-dip galvanizers that have developed proprietary methods for controlling the bending process in order to eliminate or greatly minimize coating damage in the bend area. Contact your local galvanizer for more information.
Which is Best - Bending Reinforcing Bar Before or After HDG?
When hot-dip galvanizing rebar in straight lengths, later fabrication can result in cut ends, bends, and welds. These areas may require touch-up of the coating, but touch up of these areas is relatively simple. Bending after hot-dip galvanizing also provides advantages with respect to design: there are no minimum bend diameters, the material can be coated in advance for availability/inventory purposes, and large or complex rebar fabrications which may not fit within the dimensions of the galvanizing kettle can be constructed.
Although it is simple to repair small areas of the coating in accordance with ASTM A780/A780M, there are certainly advantages to fabricating prior to hot-dip galvanizing. More complex reinforcing cages for beams, columns, or foundations can be fabricated in advance so that repairs for bend, cut, and weld areas are eliminated. Although minimum bend diameters are recommended to avoid strain-age embrittlement, it is possible to bend rebar tighter than recommended. However, additional work to stress relieve the bend areas will be required.
When in doubt, be sure to discuss the hot-dip galvanizing of rebar and rebar fabrications with the galvanizer during the design phase of the project.