CGR Rebar – New Corrosion Protection Process
CGR Rebar – New Corrosion Protection Process
Reinforcing steel is used throughout the world in concrete structures of every kind. The steel often has a patterned surface to enhance the bonding with the surrounding concrete. Due to the nature of the steel used there is always a risk of corrosion if there is any ingress of water. Once the steel begins to corrode an iron oxide layer develops on the steel surface. The oxide layer has a greater volume per unit mass compared to the original steel. The result is that the surrounding concrete is subject to potentially extremely high stresses which can lead to cracking and subsequent rapid deterioration of the load bearing capacity of the steel reinforced concrete member. In addition to the potential cracking of the concrete the corroded steel will lose its tensile strength. Areas of corrosion act as points of high local stress that can lead to fatigue failure if the member is subject to any type of repetitive flexure.
Different methods have been developed to protect the reinforcing steel. Galvanized steel; epoxy coated steel; low carbon/chromium steel and hot dipped galvanised steel bar. Corrosion inhibitors can be added to the water before the concrete is mixed. A new process has been developed recently to Continuously Galvanize Reinforcing bar (CGR). The CGR Rebar process is a hot dip process and has some advantages over conventional hot dipping. CGR involves running a continuous thread of reinforcing bar through a zinc bath. The steel is immersed in the zinc for only a few seconds therefore eliminating the risk of any change to the crystal structure of the steel in question. Another advantage is the there is no limit to the length of the bar that can be continuously dipped. The treated steel can be supplied in rolls. The coils of steel can be formed into almost any shape at the construction site if the right forming equipment is available. This provides great flexibility and reduces transport costs from preforming suppliers.
Local construction site forming of CGR rebar
Standards exist for hot dipped galvanized steel bar such as:
However, there is not yet a standard for CGR Rebar. The ASTM has set up a steel committee to develop a proposed new standard for CGR. The committee is working on the new standard which is yet to be published. The work item which will eventually be published as a standard is called WK 46112. The ASTM standard is expected to be released in 2015.
In China there is not a standard for CGR Rebar. However, there is work going on to establish a national CGR Rebar standard via the National Standardization Administration.
In India there is not a standard for CGR Rebar. The development of such a standard will commence after commercial production starts in India.
Most likely it will be ASTM that first publishes a CGR standard and this will almost certainly become the reference for other nations while they develop their own national standards. There will be a need for top quality technical translations for the ASTM standard in the very near future.
Let’s take a closer look at the CGR Rebar product and process
The zinc bath contains 0.2% aluminium. The continuous hot dip coating process produces a coating that is almost pure zinc. At the interface between the steel and the zinc there is an intermediate alloy layer that is 0.1 microns thick composed of three elements – Fe2Al5-xZnx. Because the coating is so uniformly thin and adheres very well to the steel, it can be deformed and bent without any significant risk of cracking or peeling. The steel is immersed in the molten zinc (465 oC) for only four or five seconds.
The manufacturing process is composed of six or seven stages depending upon the final product specification.
CGR Rebar Process Flow Diagram
The current global production of reinforcing bar is approximately 200 million Tonnes per year. The majority of which is not treated with any corrosion protection. Currently, the most popular corrosion treatment is epoxy. However, epoxy can crack over time leaving the base steel exposed to corrosion resulting in possible reduced load bearing capability and a greater fatigue failure risk. As structures increase in length or height there will be a greater need to counter the risk of a fatigue failure. The CGR Rebar process would appear to provide the extra security for such new structures.
Comparative Costs of Different Rebar Corrosion Protection Types
The relative cost comparison is with respect to untreated black rebar.
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