Hot dip galvanizing has been used as means for prolonging the life of reinforcing steel in concrete for over 100 years.
The most common early use of galvanized reinforcing steel was in the construction of concrete water tanks where galvanized wire was used to pre-stress the tank walls. In the post-WWII period, the use of galvanized reinforcing steel became more common and by the 1960s and early 1970s a considerable tonnage of reinforcing steel was being galvanized especially for use in bridge and highway construction in the USA. In Australia, the highest profile use of hot dip galvanized reinforcing steel is in the chevron tile assemblies of the Sydney Opera House, installed from 1963 and now with around 50 years of recorded corrosion protection.
Over the last 25 to 30 years, there has been steady increase in the world-wide use of galvanized reinforcing steel in a wide variety of concrete construction and exposure conditions. For example, since 1995 all reinforcing steel on New York Thruway Authority bridge projects have been galvanized. Many bridges throughout the USA undergo periodic testing of the state of the galvanized reinforcing steel that was used in their initial construction and these have all been shown to still be in excellent condition today.
The continued success of these structures contradicts projections from the discredited accelerated testing models and is in large part responsible for the growing interest in designing with hot dip galvanized reinforcing steel.
Today, hot dip galvanized reinforcing steel is recognised as a cost-effective solution for eliminating the effects of carbonation and significantly delaying the onset of chloride-initiated corrosion compared to uncoated reinforcing steel in coastal and industrial environments.
Galvanized reinforcing steel is also ideally suited for external façades, precast panel joints and surface elements where freedom from rust staining and spalling is essential.
The cost of corrosion in Australia
The annual cost of corrosion in most developed countries is estimated at
2.5–3% of GDP.
Based on the current GDP in Australia that equates to approximately $40 billion per year. 16% of that cost relates to the maintenance, repair and replacement
of infrastructure (approximately $6.4 billion) and a large proportion of this is related to the failure and maintenance of concrete structures.
The galvanizing process
The hot dip galvanizing process begins by suspending steel articles and dipping them into a series of cleaning baths. Once cleaned, the steel is lowered at an angle into a bath of molten zinc. The molten zinc reacts with the steel to form the galvanized coating. The final step in most hot dip galvanizing processes is a quench to promote passivation of the zinc surface and to control the growth of the zinc-iron alloy layers.
Galvanized reinforcing steel is almost always galvanized in bundles rather than in individual lengths. Multiple sections of reinforcing mesh are typically hung on a jig and dipped all at once. These methods of coating the reinforcing steel make for quick turn around and complete coverage of all surfaces.