Technical Note: Multiplate Culverts – Aluminium vs Steel

CSP’s range of corrugated metal culverts are available in both aluminium and steel. This article helps explain the key differences between the two materials to assist specifiers when selecting which product best suits requirements.

Steel Multiplate culverts feature hot dip galvanised mild steel plate walls. The layer of zinc deposited by the galvanising process offers cathodic (sacrificial) protection to steel. Once the zinc has been expended, which in normal environmental conditions in New Zealand can take 10 to 20 years, corrosion accelerates as it starts to attack the steel leading to a rusty appearance, loss of the wall thickness and, eventually, perforation. Galvanised steel culverts in New Zealand are designed using the ‘sacrificial wall thickness’ method meaning that some loss of wall thickness is acceptable without compromising the structural integrity of the structure.

Steel culverts have the following advantages over aluminium:

• Mild steel is up to 40% stronger than aluminium plate of the same thickness. As the residual steel wall thickness decreases, as a result of corrosion, this advantage diminishes for culverts designed for a service life longer than 25 years.
• Bolted connections in steel plates have higher capacity than aluminium meaning for larger spans less bolts are required.
• Mild steel is three times stiffer than aluminium which, due to its stiffness, reduces culvert deformation during backfilling and in service.
• Better availability of steel plate means that lead times will normally be shorter than for aluminium.

Aluminium Multiplate and Multiplate 100+ culverts are made of marine grade 5052-H34 aluminium alloy. When in contact with an aggressive environment aluminium forms a protective layer of oxide which gives it corrosion resistance superior to steel. Unlike galvanised steel, which corrodes faster once the zinc has been expended, the rate of corrosion in aluminium does not increase over time. While aluminium culverts are also designed using the ‘sacrificial wall thickness’ method the rates of wall thickness loss are much lower than for steel (in low corrosion environments there is no loss of thickness at all).

The advantages of aluminium over galvanised steel culverts:

• Superior durability which negates the price premium over galvanised steel for culverts when the design life is longer than 25 years.
• Aluminium is three times lighter than steel which leads to substantial savings in freight, handling and installation.
• Aluminium oxides are dull grey in colour so there is no ‘rusty’ appearance in service.
• Aluminium is cheaper than equivalent strength galvanised steel culverts designed for 50+ years of service life.