Considerations when planning to cast aluminium truss in concrete blocks

Continuing my series of posts, this post will look at a specific issue, which seems common based on my observations at site inspections – the mix of aluminium truss and concrete.

Risks to consider

Aluminium and concrete can be mixed, but some considerations should be undertaken prior to the casting process.

 

Embedding untreated aluminium in fresh concrete will cause a reaction of the concrete and the aluminium, developing hydrogen gas around the exposed surfaces increasing the risk of a decrease in the homogenous structure of the hardened concrete, which could lead to potential cracking.

 

The risk of corrosion caused by the presence of calcium chloride found in wet concrete based on Portland cement, the risk of galvanic corrosion if the aluminium is coupled with any ferrous metals, like steel re-bar, as well as the risk of corrosion if the structure could be exposed to a corrosive environment (eg. near seawater or exposed to snow/salt), when coupling aluminium with ferrous metals are all worth considering during your planning.

 

Regarding the direct embedment of truss members (illustrated at the top of this post), the risk of changes to the mechanical properties due to corrosion and the risk of an increased area exposed to corrosion, caused by the lack of all-welded attachment of diagonal tubing should also be considered.

 

Some considerations should also be given to the risk of corrosion of the heat affected zones (HAZ) of the truss, as these areas all-ready have suffered a change in their mechanical properties during assembly due to the welding process.

General guidance

General guidance can’t be given as this depends on the specific aluminium alloy, but some guidelines can be given, when aluminium is to be cast into concrete:

  • Protect any exposed aluminium surfaces with either bituminous or asphalt-based paint or other applicable insulating coating.
  • Vibrate the mold during casting to release gasses and air bubbles.
  • Add non-ferrous washers in bolted connections when coupling aluminium with ferrous metals, to decrease the risk of galvanic corrosion of aluminium.
  • Do not expose non-treated aluminium to corrosive areas

I’ve added some links for your further reading on the subject, and hope this post has given some aid.

Further reading

Emmanuel Mrema, Yoshito Itoh, Akira Kaneko & Mikihito Hirohata (2018) Corrosion of aluminium alloy A6061-T6 members embedded in alkaline materials, Corrosion Engineering, Science and Technology, 53:2, 102-113, DOI: 10.1080/1478422X.2017.1402499

 

Tomcsnyi L,VargaK, Bartik I, et al. Electrochemical study of the pitting corrosion of aluminium and its alloys-II. Study of the interaction of chloride ions with a passive film on aluminium and initiation of pitting corrosion. Electrochim Acta. 1989;34(6):855–859. DOI: 10.1016/0013-4686(89)87119-1

 

Mrema E, Itoh Y, Kaneko A, et al. Galvanic corrosion study of aluminium alloy plates mounted to stainless and mild steel bolts by accelerated exposure test. Japan Society of Civil Engineers (JSCE). 2016;62A:525–536. DOI: 10.11532/structcivil.62A.525

jms