October 8, 2009
FEATURE | Concrete & masonry
Scientists look for a better way to build bridges
The National Research Council Institute for Research in Construction (NRC-IRC) has embarked on two new research projects designed to strengthen and enhance concrete bridge structures.
The first is a four-year study on the use of advanced fibre-reinforced polymers (FRPs) to protect critical concrete infrastructure against extreme shocks.
The study is part of the NRC’s cross-institute Advanced Material Initiative in concert with partners that include the NRC Industrial Materials Institute and Institute for Aerospace Research, and the University of Ottawa.
“The focus of this research project is on developing protection systems for critical concrete infrastructure,” said NRC-IRC’s Dr. Husham Almansour, a lead researcher on the project.
“By protecting safety and security-critical bridge components, we aim to reduce their risk of failure if they’re subjected to heavy truck impact or even blast impact and avoid sudden collapse.”
Advanced composites currently used in construction are typically thermoset FRPs.
The research program involves the use of thermoplastic FRPs of the type used in the aerospace industry.
Recent reductions in the cost of these materials made them more economically viable, said Almansour.
An effective protection system needs to absorb considerable impact over a short period of time.
Typically, the duration of extreme loads that can lead to structural collapse is measured in milliseconds.
“We would retrofit the critical columns from the outside using multi-layer advanced composites and a shock absorbing materials system,” said Almansour.
“The inner layers would be attached to the concrete surface of the column to distribute the load smoothly and keep fractured concrete in place. The intermediate layers would act as shock absorbing cushion, while the outer advanced composite layers face the shock pressure and possible objects and fragments generated from the shock itself or from the destruction of any nearby objects.”
Optimizing the design and performance of structures using advanced composites requires a micro-level design of the material and a macro-level design of the protection system.
This part of the research will help to determine the most efficient, practical and cost-effective configuration of the material.
Further research will involve computer simulation and lab testing to simulate and examine the impact and blast loads, the concrete structure, and the protection system.
Almansour developed a multi-scale modeling and design procedure for long-span structures made with advanced composite components.
The procedure has been used to assess the structural performance of the NRC-designed hybrid long-span, cable-stayed bridge made of both traditional construction materials and advanced composites.
Real-life data derived from Japan’s Tatara Bridge, the world’s longest traditional cable-stayed bridge, has been used to test the modeling.
“Even with the use of traditional materials, this type of bridge is one of the most difficult to design, model and build from an engineering standpoint,” said Almansour.
“If our multi-scale simulation and design procedure works with this innovative structure, and determines which parts and components of the bridge require the most protection, we can easily apply our findings to the construction of more robust bridge structures.”
The second study by NRC-IRC involves the use of ultra high-performance concrete (UHPC) to build long-life, lightweight and cost-effective bridges.
“UHPC is a new type of concrete that uses fine sand instead of aggregate and relies on additives to make a very dense mix with minimum air content,” said Almansour.
“This results in innovative concrete structures that have greater deformation capacity, longer service life, longer spans, lighter weight, reduced substructure size, improved seismic performance, and higher resistance to impact, vibration and fatigue.”
While typical concrete bridges might have a service life of 50 to 75 years, UHPC can theoretically extend it to up to 150 years.
However, because of its high cement content and low water content, UHPC is subject to significant micro-cracking.
Steel fibres have been introduced into the mix to counter the problem.
“We will investigate the risk of corrosion of steel fibres to test it against the corrosion that we typically see in Canada because of the use of de-icing salts during winter,” said Almansour.
If UHPC is successful in Canada, it could result in bridges with extended life, longer spans, lighter weight and lower maintenance costs.
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