Hot-dip galvanizing is a unique process and has been a world champion in corrosion prevention for more than 150 years. No other method offers as complete protection of steel. Moreover, it is a smart and responsible choice. In the fight against climate change, circular construction plays a major role. Optimal utilization and reuse of scarce raw materials is essential here. Thanks to hot-dip galvanizing, we are going for 100% circular steel: the best protection and the most sustainable choice.
Rusting rebar is a common problem. Viaducts, tunnels and balconies are exposed to atmospheric influences, causing rebar nets and rebar to rust over time. This leads to costly repairs and the need for cathodic protection systems to prevent further damage.
Yet galvanized rebar has been used since 1950, as the benefits quickly became apparent. The zinc coating prevents rusting, while untreated steel increases in volume through oxidation – up to seven times its original size. This causes the concrete to crack, allowing water and corrosive substances such as road salt to enter, accelerating the rusting process.
When untreated rebar is coated with concrete, the highly alkaline concrete (pH 12.5 -13.2) creates a passive iron oxide layer on the steel that protects the steel. However, concrete is a non-homogeneous mix of aggregate, cement and sand, and therefore somewhat porous. This causes CO₂ to enter from the atmosphere leading to carbonation. As a result, the protective layer disappears and rust forms. In addition, chlorides from road salt, concrete mixtures and sea air can have the same effect and accelerate corrosion.
There are several ways to extend the life of concrete structures:
Nevertheless, galvanized rebar offers the most secure and cost-effective solution.
As with untreated steel, a passivating layer is created on galvanized reinforcing steel in new concrete by a reaction between zinc, calcium and water. To create this layer, about 10 microns of the metallic zinc is cut off.
The big advantage is that the decreasing pH value of concrete does not affect zinc. While ungalvanized rebar needs a safe pH of 11.5 to 13.2, galvanized steel remains stable between pH 6 and 12.5.
In addition, the resistance of galvanized rebar to chlorides is 2 to 2.5 times higher than that of untreated steel. Galvanized steel is also tough and wear-resistant, minimizing damage during transportation, storage and erection.
Zinc corrosion also does not lead to cracks in concrete. Iron rust increases in volume by a factor of 7, while zinc corrosion increases by only 1.3 times. As a result, there is no harmful pressure build-up in the concrete, and the corrosion product even migrates into the concrete composition without causing damage.
An additional benefit is that galvanized rebar is cleaner to work with. Workers do not get dirty hands and clothing due to rust, which improves working conditions on the construction site.
It is sometimes claimed that rusting rebar adheres better to concrete than galvanized steel. This is a misconception. Research shows that the passivating layer on galvanized steel actually provides 20 to 40% better adhesion than untreated steel.
In concrete, no corrosive reactions are expected to occur between the untreated and galvanized rebar. This during the time that both metals remain passive. After a while, during carbonation, it could lead to an anodic action of the zinc. To avoid this in certain applications, the choice can be made to isolate the different surfaces (zinc and steel) from each other by using polyethylene and insulating tape.
Because galvanized steel is immune to carbonation and has much higher chloride resistance, “leaner” design can be achieved. This means less use of materials and a more durable design without sacrificing structural strength and reliability.
This publication is an abridged version of the EGGA brochure “Galvanized reinforcing steel in concrete structures.
