With budgets tight and demands for infrastructure spending growing, getting the best bang out of every buck remains paramount.
Determining how to extend the lifecycle of bridges, for example, is both beneficial to the public purse and the environment since snarled traffic and resulting disruption only means more wasted fossil fuel resources.
Still, it's a daunting challenge.
"About one third of highway bridges in North America are structurally or functionally deficient with a short remaining service life," said Jieying Zhang, P.Eng., research officer, critical concrete infrastructures at the National Research Council (NRC) Canada and adjunct professor at Carleton University.
The questions Zhang, her colleague Shiyuan Qian and their team have been asking are: How do we build better concrete structures and avoid corrosion of reinforcing concrete? How do we develop better investigative technologies to monitor existing bridges and anticipate what will need remedial action? And what type of repair works best for what damage?
Most recently, the work has focused on the type of steel rebar and where it is best placed within a concrete bridge.
While hot dipped galvanized steel has been used for a while, Zhang said, it's worthwhile to look at options of stainless steel and what grade of stainless presents the best price-value proposition.
ASTM A1035 steel and stainless steels, for example, resist salt corrosion and can help extend the lifecycle of structures.
To that end, the team looked at the performance characteristics of ASTM A1035 steel, galvanized steel and black carbon steel.
ASTM A1035 is the current state-of-the-art bridge rebar, known as MMFX, and is considered to resist corrosion five times better than conventional steel.
Out of the six stainless steels tested, Zhang said, three are relatively new in terms of being used in concrete structures, SS 2101, SS 2304 and UNS S24100. These stainless steels are alloyed with manganese (2~12 per cent) and nitrogen, which compensates for their nickel content and makes them less expensive than the commonly used stainless steel alloys.
The more traditional grades, SS 316LN, SS 304LN, both low nitrogen molybdenum steel, and SS 2205, a nitrogen enhanced steel, were also tested. The results broke out over three groups.
Using electrochemical techniques to simulate corrosion over a long period with chloride, the team found SS 2205, 316LN, and 304LN performed best in retaining their integrity. Grades SS 2304 and UNS S24100 ranked second while SS 2101 ranked third.
Zhang said as expected the stainless steel vastly outperformed the carbon steel, but using stainless steel throughout a project as a default is cost prohibitive and the price-value curve has to be considered.
"It is important to note that even the stainless steel ranking third among the six had an average chloride threshold value 33 times higher than that of the black carbon steel," said Zhang in a summary report. "As a comparison, ASTM A1035 steel was found to have a chloride threshold value five times higher than black carbon steel. This means that stainless steel provides corrosion protection to concrete structures exposed to very severe corrosive environments, while ASTM A1035 steel may be considered to use in moderate to heavy corrosive environments."
The research also found hot-dip galvanized reinforcing steel, a perennial choice for years, will continue to protect the underlying steel even after corrosion begins in other parts. This is true even if there is severe corrosion within the concrete itself. As a result, this steel can last more than 50 years and may represent the best value in most cases.
It's not necessarily rocket science, admits Zhang, more common sense.
Building a bridge in Arizona, for example, allows a different approach to corrosion protection than a bridge over Highway 401 in the heart of Toronto with all the weather and road salt challenges.
"And no matter what material you use you have to follow the construction code," Zhang said.
As a result, the best practices will vary as to where the bridge is built, the environment and road salt anticipated, the type of construction and the form of concrete.
"Of course we can easily stop corrosion, if there was an unlimited budget," she said. "We can put a man on the moon, so this would be easy. So, we have to look at how best to prevent corrosion with safety first in mind, then optimization."
Zhang's other methods include chemicals in the mix to resist corrosion, which looks promising, while thicker concrete is also preferred. Permeable concrete, predictably, is not preferred. Also proving useful is a cathodic/anodic protection technique to actively prevent the steel from corroding.
The Ontario Ministry of Transportation (MTO) currently has a cathodic/anodic installation at the Warden Avenue bridge over Highway 401 powered by a solar panel array on the northwest side of the cloverleaf.
"MTO has been quite innovative and advanced in their approach," she said, noting NRC has also rigged a small facility to test steel in concrete simulating weather and aging effects.
On deck now is an investigation into better technologies to predict and detect corrosion and developing guidelines for owners and engineers to assess corrosion damage.
She said accurate field corrosion inspection provides better assessment and more reliable repair/rehabilitation results in longer service life. For different concrete structures and environments, an optimization of the approaches and strategies can be achieved through life-cycle cost analysis.
A concern is that the existing ASTM guidelines designed for carbon steel can lead to "adverse misinterpretation of corrosion conditions" if applied to galvanized steel installations.
Zhang has been carrying out this research work since arriving at the NRC in 2004 and says there's still more work to be done.
"For example, we need to investigate uses of other materials other than steel for reinforcement in concrete structures, such as fibre-reinforced polymers," she said. "They don't corrode, but age and degrade as affected by temperature and water. And some of these aren't in the construction or highway bridge code yet."