MechChem Africa

In his quarterly column, Gary i. Crawford of Mettle Strategic Creativity talks about the costs of corrosion and the modern approaches being adopted to better manage the life and lifecycle costs of bridges and other structures.

Brooklyn bridgeSome disciplines seem to find a sense of stability by adhering to the practices and beliefs of the past. For example, it is not uncommon to hear bridge engineers say that no sooner have they erected a bridge that they have to start preventing it from falling down. ‘Solace from the inevitability of decay' rather than the 'positive predictability of designed-in lifespan', as it were.

Of course, the main culprit in bridge decay is corrosion of the steel components.

Corrosion converts a refined metal to a more chemically stable form, such as its oxide, hydroxide, or sulphide. It is the gradual destruction of materials by chemical and/or electrochemical reaction with their environment. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxides or salts of the original metal and results in the distinctive orange colouration. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

The primary cause of corrosion of steel bridges is exposure of the steel to atmospheric conditions. This is exacerbated by marine (salt spray) and industrial environments and the only corrosion prevention method for these structures in these environments is a barrier coating.

Until very recently little consideration was given at the design stage to ensure longevity of bridges.

According to the National Cooperative Highway Research Program (‘Bridge Life-Cycle Cost Analysis’ - NCHRP Report 483  -  2003) the United States of America has 614 387 bridges, almost four in ten of which are 50 years or older.

56 007 (9.1%) of the nation’s bridges were structurally deficient in 2016 and, on average, there were 188-million trips across these deficient bridges each day. While the number of bridges that are in such poor condition is decreasing, the average age of America’s bridges keeps going up and many are approaching the end of their design life.

The most recent estimate puts the cost of the nation’s bridge rehabilitation needs at US$123-billion and this is likely to keep increasing.

According to the U.S. Department of Commerce Census Bureau, the annual direct cost of corrosion for highway bridges is estimated to be between $6.43- and $10.15-billion, consisting of: $3.79-billion to replace structurally deficient bridges over the next 10 years; $1.07- to $2.93 billion for maintenance and capital cost of concrete bridge decks; $1.07- to $2.93 billion for maintenance and cost of capital for concrete substructures and superstructures (minus decks); and $0.50-billion in maintenance painting costs for steel bridges.

Lifecycle analysis estimates indirect costs to the user due to traffic delays and lost productivity at more than 10 times the direct cost of corrosion. In addition, it was estimated that employing 'best maintenance practices' versus 'average practices' may save 46% of the annual corrosion cost of a black steel rebar bridge deck, or $2 000 per bridge per year.

The National Cooperative Highway Research Program of 2003 was the first serious attempt to introduce lifecycle costing to the world of bridge design and maintenance. Until then, bridge repair and maintenance costs were seemingly worn as 'badges of courage' ... with costs 'proudly' communicated. For example, the George Washington Bridge, crossing the Hudson River in New York, was completed in 1931 at a cost of $75-million and maintenance to date exceeded US$1-billion.

A common rule of thumb is that maintenance costs about 4.0% of the initial construction cost per year. For a structure as old as the George Washington Bridge, that’s a lot of 4.0%’s, even though some attempts were made to build in longevity.

In 2005, the New York Times reported that repairs to the Brooklyn Bridge were $100-million over budget and the completion date had been pushed back yet again due to major cracks and holes discovered during the five years of work. Engineers discovered more than 3 000 new structural ‘flags’ on the city’s most famous span, which increased the costs of repairs and improvements from $508-million to more than $600-million.

The 1 595-foot span was originally set to fully reopen in 2006, but actually took until 2016.

Thankfully, since the publication of the NCHRP’s  ‘Bridge Life-Cycle Cost Analysis’, sanity seems to have begun to prevail, with lifecycle costing entering the world of bridges and other major structural designs.

Download and read PDF.

BANNER 8

Contact MechChem Africa

Title: Editor
Name: Peter Middleton
Email: mechchemafrica@crown.co.za or peterm@crown.co.za
Phone: +27 11 622 4770
Fax: +27 11 615 6108

Title: Editor
Name: Glynnis Koch
Email: mechchemafrica@crown.co.za
Phone: +27 11 622 4770
Fax: +27 11 615 6108

Title: Advertising Manager
Name: Brenda Karathanasis
Email: brendak@crown.co.za
Phone: +27 11 622-4770
Fax: +27 11 615-6108

saiw logo block

sassda logo block

saiche logo block

 
Full Name*
Invalid Input

Company Name*
Invalid Input

Your Email*
Invalid Input

Phone*
Invalid Input

Postal Address 1*
Invalid Input

Postal Address 2*
Invalid Input

Postal Code*
Invalid Input

Street Address 1
Invalid Input

Street Address 2
Invalid Input

Postal Code
Invalid Input

Town / City*
Invalid Input

Country*
Invalid Input

Magazine

Invalid Input

Invalid Input