Manukau Bridge - Standing the test of time
< Back Friday, 30 April 2010
Contracted to provide approximately 25,000m³ of concrete on the Manukau Harbour Crossing (MHX), Firth Industries has been faced with a series of mix design and supply challenges.
The existing Manukau Harbour crossing is a key motorway bridge connecting Auckland with Manukau City. Built in 1984 it regularly becomes congested at peak times. A duplicate bridge is being built to the east of the existing bridge to accommodate, in conjunction with the original bridge, four traffic lanes and a shoulder lane.
"With the design life of the new bridge set at 100 years there's been a lot of engineering input required for this project," says Rob McKinnon, Materials Manager Firth Northern.
The key to future proofing is to provide a dense enough concrete to withstand penetration from chlorides so they cannot rust the reinforcing steel inside. The size of the concrete sections being poured and the saline environment has also meant the mixes on this project have needed to be more complex than they would usually be.
To mitigate cracking and enhance durability, the design of specific mixes was required to slow the onset of heat from hydration. Much of the concrete is temperature-match cured at Firth's lab to ensure accuracy and confidence in the concrete strength.
Currently most of the substructure (piles and columns) and a large portion of the superstructure are complete with over 15,000m³ of concrete having been poured so far. 8,000 to 10,000m³ of concrete is required to finish the project, the completion date being estimated to be early 2011.