Galrebars - Why Galvanised Bars

·	Corrosion products of steel are over three times the volume of original steel.

·	Higher volume of corrosion products exerts greater tensile disruptive stress on the surrounding concrete.

·	The structure would have spalling on concrete, suffer loss of durability and serviceability and would require early remediation, repair and even demolition.

·	Corrosion of steel reinforcement bars is a key reason for damage to concrete.

·	Many regions in India accounting for 20-25% of rebar use require corrosion protection that is not provided by the concrete cover.

·	Of the several corrosion protection techniques tried and tested over long periods, galvanised reinforcement bar have emerged technically superior and cost effective. Further galvanised reinforcement bars are economical.

·	Galvanisation increases reinforcement bar life by more than 300%.

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.

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.

Corrosion protection provided by galvanising:

In areas where the reinforcement may be exposed accidentally due to thin or porous concrete, cracking, or damage to the concrete, the galvanised coating provides extended protection. Since the corrosion product of zinc occupies a smaller volume than the corrosion products of iron, any small degree of corrosion, which may occur to the galvanised coating, causes little or no disruption to the surrounding concrete mass.

Corrosion levels are very high in India, specially the coastal regions of India, including our major metros Mumbai, Chennai, Calcutta. To see the comparative corrosion level map of India click here.

Studies were made at the Structural Engineering Materials Laboratory, University of California, Berkeley California, of the effects of corrosion on reinforced concrete test prisms.

Prisms 300 x 100 x 100 mm were axially-reinforced with 19 mm diameter galvanised or black steel bars. A 12.5 mm deep notch was cut at the mid section of each prism to enforce formation of a crack at the notch, should corrosion products exert sufficient disruptive stresses. Prisms were placed in loading frames and the steel reinforcing bars stressed to 140 MPa. Prisms were then subjected to alternate immersion/drying cycles in a 4% NaCl solution.

Cracks occurred in test prisms reinforced with uncoated steel bars in less than ten months exposure. Large crack areas had developed by about 18 months and were still increasing at 24 months. No cracks were observed in prisms reinforced with galvanised bars until almost 16 months exposure. These crack areas were very small compared to those in prisms reinforced with uncoated steel bars and crack development ceased after a further two and a half months exposure.

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