Galvanized Steel Reinforcement for Concrete

Hot dip galvanizing is a viable means of protecting reinforcement, particularly where the durability of concrete cannot be guaranteed. Its use should be considered for harsh exposure conditions, precast construction and prestige facades where long life, freedom from rust staining and low maintenance are important criteria.  Rust-stained surfaces and cracking and spalling of concrete in recently completed structures demonstrate the wide need to protect steel reinforcement.

Current Practice Note 17 published by and available from the Concrete Institute of Australia concludes that “Wherever there are serious doubts that (impermeable concrete) will be achieved and maintained for the design life of the structure, then galvanizing should be given serious consideration”.

Galvanized coatings provide important advantages for the protection of reinforcement.

Research and practical experience since the 1950s have shown the corrosion resistance of galvanized steel reinforcement to be greatly superior to uncoated steel, while the bond strengths of galvanized and black steel bars to concrete are not significantly different.

The corrosion protection of the galvanized coating ensures that the design strength of concrete is maintained and the possibility of surface rust staining and eventual corrosion of reinforcement and spalling of concrete is removed.

Steel accessories for use in reinforced concrete structures, particularly fittings and inserts which may be partially exposed, are susceptible to the effects of corrosion and should be galvanized.

Where only parts of a reinforced concrete element require the reinforcement to be galvanized, such as the external mesh of a precast panel or the top mat of a slab, and black steel is to be used elsewhere in the element, it is vital that the steel be placed in strict compliance with the design requirements.

If galvanized bars are placed in contact with black bars in areas prone to corrosion, there is a likelihood that the galvanizing will attempt to sacrificially protect the uncoated bars, resulting in a reduction in the life of the galvanized coating. However, this effect is likely to be observed only in situations where the galvanic couple – the connection between the galvanized and the black steel – is prone to corrosion such as in areas of reduced cover to the reinforcement or poor compaction or cracking (i.e. overall poor quality) of the concrete.

To ensure that this is not a durability concern, it is recommended that where particular parts of RC elements are to utilize galvanizing, all steel in that area should be galvanized including tie wire, inserts and bar chairs. Alternatively, plastic coated ties should be used. Further, any point of connection to uncoated steel should be deeply embedded in the concrete to ensure that both the steel and the galvanized coating are maintained in their respective passive state. Under these conditions, neither the steel nor the galvanizing will be prone to corrosion.

 

Corrosion of reinforcement

Corrosion of steel reinforcing bars inevitably weakens concrete members, reducing load bearing capacity and safety factors. In extreme cases, failure of reinforced concrete members can occur, partly because of loss of strength due to corrosion of the reinforcement itself, and partly because of the breaking up of the concrete surrounding the reinforcement. 

When steel reinforcement corrodes, the corrosion product occupies more than three times the volume of the original steel, exerting great disruptive tensile stress on the surrounding concrete, leading to further cracking, more weather access and further corrosion. In mild cases rust staining occurs.  In more serious cases, severe spalling of concrete may occur and ultimately concrete members may fail completely.

 

Steps in the corrosion of uncoated steel reinforcing bars.  Galvanized rebar is not subject to this effect and retains full bond strength to concrete.

 

In normal circumstances uncoated steel reinforcing bars give satisfactory service provided the following requirements are maintained:

1. The design provides for adequate concrete cover over the steel reinforcement.

2. Precise placement of reinforcement is maintained.

3. Uniformly high quality concrete is used.

4. Complete compaction of concrete is attained with no voids or pockets.

It is sometimes impractical or impossible to achieve all these requirements and depending on exposure conditions, corrosion of uncoated reinforcement may begin.

The benefits of galvanizing reinforcement include:

• Protection to the steel during storage and construction prior to placing the concrete.

• Diminished effect of variations in concrete quality.

• Safeguards against poor workmanship, especially misplacement of reinforcement, poor compaction, and inadequate curing.

• Delayed initiation of corrosion and the onset of cracking.

• Reduced likelihood of surface staining.

• Increased structural life of concrete, particularly where chloride contamination is likely.

 

Galvanized Reinforcement for Concrete

For a copy of the complete technical manual "After Fabrication Hot Dip Galvanizing: A Practical Reference" please click here and request a copy.

For a copy of "Galvanized Rebar: It Works" please click here to download a copy.