The specification ASTM A 767/A 767M is applicable exclusively to the hot-dip galvanizing of reinforcing steel, otherwise known as rebar, as seen in Figure 13, and is applicable to all types of rebar, both smooth and deformed. However, wire is not included.
The requirements in ASTM A 767/A 767M are also intended to produce a high quality zinc coating for corrosion protection.
ASTM A 767/A 767M Requirements
- Identity – the galvanizer is responsible for consistent material tracking if necessary
- Coating Thickness/Weight – material category and steel thickness
- Chromating – to prevent reaction between cement and recently galvanized material
- Finish – continuous, smooth, and uniform
- Appearance – free from uncoated areas, blisters, flux deposits and gross dross inclusions as well as having no heavy zinc deposits that interfere with intended use
- Adherence – should be tightly adherent throughout intended use of the product
- Bend Diameters – flaking and cracking due to fabrication after the hot-dip galvanizing process are not rejectable
Once rebar is delivered to be hot-dip galvanized, it is the galvanizer’s responsibility to track and maintain the identity of the product throughout the hot-dip galvanizing process until shipment of the finished product. Again, the analogous coating requirements in the areas of coating thickness, finish, and adherence are present in ASTM A 767/A 767M. However, this single product specification introduces a few new requirements that apply solely to hot-dip galvanized rebar. In ASTM A 767/A 767M, the coating requirement is given in “mass of the zinc coating per surface area”. A summary of the table given in ASTM A 767/A 767M and the minimum required coating thickness / weight of the classes is seen in Table 4.
This specification also introduces a new requirement to the galvanized coating known as chromating. Newly galvanized steel can react with wet cement and potentially form hydrogen gas as a product. As this evolved hydrogen gas travels through the concrete matrix toward the surface, voids can be created which weaken the bonding with the concrete or disturb the smoothness of the concrete surface. In order to help prevent and suppress this reaction, hot-dip galvanized rebar is dipped into a weak chromate quench solution after being removed from the galvanizing kettle.
The finish requirement for rebar is along the same lines as the finish requirements given in specifications ASTM A 123/A 123M and A 153/A 153M. The coating is intended for corrosion protection, so deficiencies that affect the coating’s corrosion performance are grounds for rejection. In addition, since rebar is handled frequently during its installation, any tears or sharp spikes that make the material dangerous to handle are grounds for rejection.
Rebar is commonly bent prior to the hot-dip galvanizing process. The table below gives recommendations for bend diameters based upon the bare steel bar diameter before coating. Steel reinforcing bars that are bent cold prior to hot-dip galvanizing should be fabricated to a bend diameter equal to or greater than the specified values. However, steel reinforcing bars can be bent to diameters tighter than specified in Table 5 providing they are stress relieved at a temperature of 900 to 1050 F (480 to 560 C) for one hour per inch (25 mm) of diameter.
