Interbloc utilises Reid Construction System's Reidbar system for vertical refinancing. Reidbar is available as plain steel bar, or hot dipped galvanised bar.
Galvanisation
Galvanisation is a process that coats steel with a layer of zinc. This layer of zinc protects the steel from corrosion by sacrificing itself to the elements. The thicker the zinc coating, the longer the steel will be protected from corrosion.
Reidbar is galvanised to meet the requirements of AS/NZS 4680 1999. This means that the nominal coating mass on Reidbar is 600 g/m2. This equates to a surface zinc thickness of approximately 0.10 mm (100 microns).
The Galvanizers Association of Australia handbook gives the anticipated life of 600 g/m2 of hot dipped coating at 50 years in a mild coastal environment and 25 years in a marine environment.Environment Limitations
The effectiveness of galvanisation is affected by the environment in which the steel is exposed. In some environments, galvanisation may not provide adequate corrosion protection.
For example, galvanisation will give minimal protection for pH values less than 6.5 to 7.0. This means that galvanised steel should not be used in acidic or alkaline environments.
Unprotected galvanised systems should not be used with acid solutions below pH 6.0 or alkaline solutions above pH 12.5. Additionally, additional protection is required when galvanised steel is in contact with chemically treated timber.
Cement Grouts and Concrete
Cement grouts and concrete provide an environment where the pH is typically 9.5 to 13.5. In this environment, a passive film forms on the steel that protects it from corrosion.
However, the loss of this protective alkalinity around the steel, or the presence of aggressive ions, notably chloride, in the grout or concrete, can lead to corrosion.
Hot Dip Galvanising and Development Length
Hot dip galvanising will have no significant effect on the development length of reinforcing bars. The development length is the length of bar that must be embedded in concrete to develop the full tensile strength of the bar.
Corrosion Protection of Reidbar for Highly Corrosive Environments
In certain situations, Interbloc is used to construct bulk materials storage for materials that are highly corrosive, such as fertiliser. In these situations, it is important to extend the corrosion protection of Reidbar.
From a code compliance perspective, the relevant code is Clause B2 of NZBC, which in turn refers to NZS3101 part 1 Section 3. Looking deeper into the standard, the only section that we might be able to seek guidance on protection of our vertical reinforcing will be Section 3.13 of NZS 3101 -- see screenshot below.
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It is self-explanatory that only "conventional" surface protection methodology is included in this section. This conventional surface protection is hot dipped galvanising. However, we know that historical data has shown that none of the protection methods has been able to offer a satisfactory solution for a fertiliser store environment.
As a result, a second layer (or sacrificial, depending on how you look at it) of defence, i.e. a lanolin-based product, can be applied. Ideally the client has direct participation / input in the selection of the exact material based on their past experience.
Unlike HD Galvanising, the effectiveness of the lanolin-based material is highly dependent on the site application QA QC process.
Conclusion
Galvanisation is a highly effective way to protect steel from corrosion. However, it is important to consider the environment in which the steel will be exposed when specifying galvanisation.
In some environments, galvanisation may not provide adequate corrosion protection. In these cases, it may be necessary to use additional protection methods.
Partly sourced information from Reidbar Design Manual, August 2023.