What is Hot-Dip Galvanized Rebar?

The hot-dip galvanizing is a process of protecting the reinforcing steel from corrosion by immersing them into molten zinc. As per ASTM A767/767M, all those steel bars that meet the requirements the following steel types can undergo hot dip galvanizing: 

1.  ASTM 615/A615M “Standard specification for Deformed and Plane Carbon-steel bars for Concrete Reinforcement.”
2. ASTM 706/A706M “Standard specification for deformed and plain low alloy steep bars for concrete reinforcement.” 
3. ASTM 996/A996M “Standard Specification for Rail steel and Axle Steel deformed bars for concrete reinforcement.”. 

The recommendations for hot-dip galvanizing of rebars are followed as per ASTM 767/A767M “Zinc coated or galvanized steel bars for concrete reinforcement.”. The coating must be performed before the fabrication process. 

Sizes of Hot-Dip Galvanized Reinforcement Bars

• In  United States, the bar sizes are available in #11, #14, #18 and #20. Metric sizes available from 10M, 15M, 20M, 25M, 30M, 35M, 45M, and 55M.
• The galvanized bars are available in length of 40 feet. Special requirement can make available larger lengthened bars.
• The designers and engineers must try maximum to demand for rebars with length less than 40 feet. The galvanization of a higher length greater than 40 feet demands quality issues.

Thickness of Galvanization of Reinforcement Bars

The reinforcement bars must be coated with hot dip galvanization for a minimum thickness. This thickness is specified in ASTM A767/A767M in two classes, namely Class 1 and Class 2. 

• Minimum Coating weight of Class 1 = 1.1 kg/m2
• Minimum Coating weight of Class 2 = 0.61 kg/m2

The process of galvanization increases the weight the reinforcement by 6 to 8% than the original weight of bars. Hence, this weight must be considered during the design process.

Effect of Hot-Dip Galvanization on Reinforcement Bars

In general, a significant change in the properties of reinforcement bars is observed, when they are heated for their processing and their cooling process at an ambient temperature. They are mainly heated for a average temperature of 650 degree Celsius to bring any alteration to their microstructure and mechanical properties.

The zinc bath or hot-dip galvanization process of reinforcement is performed at a temperature of 840 degree celsius. This temperature hence is not sufficient to bring any change to the properties of the treated steel. The cold worked steel have some change during hot dip, which is not found as a big concern. 

Subsequent galvanization of fabricated bars result is strain age embrittlement. In such cases, as per ASTM A767/ A 767M, the steel must be subjected to a stress relieving process prior to hot dip.

High strength steel with yield strength ranging upto 1000 MPa is subjected to galvanization without any issue. These when exposed to heat and hydrogen, sometimes results in hydrogen embrittlement.

So, it is always necessary to test the steel after the galvanizing process for better assurance.

Hot-dip Galvanization Protection to Reinforcement Bars

The hot-dip galvanization coating act as a sacrificial protection and as a barrier for external detrimental conditions. The coat isolates the reinforcement bars from the cement matrix. The zinc coating protects the iron steel. The corrosion must first finish off the zinc coating to reach the steel bar.

The zinc corrodes slowly compared to steel bars. This hence delays the corrosion. The zinc coating is metallurgical bonded to the base steel. This way, there is no chance for under film corrosion.

Also Read:

1. What is Concrete Cover for Reinforcement?
2. Corrosion of Concrete Structures- Causes and Mechanism