Preventing Concrete Cancer Using Galvanised Reinforcing
Standard reinforcing steel is generally not treated to withstand atmospheric weather conditions. Why? Because the steel is protected from corrosion by being immersed in concrete which is highly alkaline.
However there are some situations where the anti-corrosive protection offered by concrete may be compromised leading to rusting and corrosion (otherwise known as concrete cancer) and in these situations, using galvanised reinforcing can prevent the structural degradation of the concrete, reducing the need for repairs.
What is galvanisation?
Steel is galvanised by adding a layer of zinc to the surface using hot-dipping. The protective zinc layer provides resistance to the surrounding environment preventing corrosive elements such as water, chloride ions, oxygen and carbon dioxide from reaching the steel.
When should you use galvanised reinforcing?
Galvanised reinforcing steel is significantly more expensive than standard reinforcing (in the Australian market, typically 50%-75% more expensive for the equivalent weight). While these costs can be significantly lower as a proportion of overall reinforced concrete costs for a number of reasons discussed below, galvanisation is generally not used unless there are specific environmental or situational factors warranting enhanced protection.
The key to economically using galvanised reinforcing is therefore to consider cost on a life-cycle basis: how much will the building owner save over the life of the structure if they don't need to perform repairs as frequently.
Galvanisation is recommended for:
Critical or high-risk construction areas and components
High corrosion environments such as marine and coastal environments
Buried structures which may be subject to corrosive ground water or fluctuating water tables. This can occur even in inland Australia where chlorides and sulphates may migrate from the soil during wet periods
Hot and humid environments
Environments where the structure will be exposed to aggressive salts, gases or electric currents
Other situations where galvanised reinforcing may be beneficial include where you are looking to achieve the following:
Longer service life of the concrete
Protecting steel prior to its embedding in concrete in corrosive environments
Aesthetic enhancement where concrete is to be exposed through reduced rust staining
Do I need to use galvanised reinforcing in the entirety of my build?
Generally, no. The Concrete Institute of Australia advises using galvanised reinforcing only in exposed surfaces and critical structural elements to reduce costs in most buildings.
In particular they recommend galvanised reinforcing be used in:
Thin precast cladding elements;
Buildings with extended design life;
Surface-exposed beams and columns;
Window and door surrounds;
Prefabricated units and modular construction;
External facades of buildings near the sea coast; and
Structures in coastal and marine environments
A few other important things to keep in mind when it comes to concrete cancer and reinforcing:
Galvanised steel isn't a substitute for making sure you have adequate concrete coverage and appropriate concrete quality for your application (particularly suitable permeability)
Galvanising must be specified in accordance with the Galvanisers Association of Australia Standard Specification and AS/NZS 4680
Mixing galvanised and black steel reinforcing can lead to reduced performance and should be avoided
Check that cutting and bending of your galvanised reinforcing hasn't damaged it and repair it or request your reinforcing provider replace it