Above: Bassem Andrawes, far right, with Ph.D. students Rishabh Singhvi, left, and Hang Zhao (MS 13).
Smart materials promise a better method for prestressing concrete railroad crossties, according to a current project by CEE researchers. Led by CEE Associate Professor Bassem O. Andrawes, the researchers have designed a prestressing system for concrete crossties that uses smart materials to allow a more efficient system that can be monitored and adjusted in the field.
Concrete crossties are prestressed with steel strands to provide a compressive strength that offsets the tensile stresses the ties will undergo during use on a railroad track. Conventional systems apply constant prestressing to the entire length of the crosstie to achieve the desired strength, but the reality of use leads to more loading in certain areas of the tie, depending on support conditions, loads and even climate conditions. As a result, conventional systems are inefficient and lead to earlier degradation of the ties. With the evolution of high-speed rail, concrete crossties will experience greater stresses, and the stakes will get higher for derailments, making stronger crossties an even higher priority, Andrawes said.
The new method being tested at Illinois, called the Adaptive Prestressing System (APS), promises to improve the safety and longevity of the ties by allowing the more precise, efficient prestressing of the crossties. It does this by utilizing shape-memory alloys (SMAs), a type of smart material that “remembers” its original shape, so it can be deformed as needed and then will return to its pre-deformed state when heated. Short SMA “fuses” are implanted in series with the prestress wires throughout the concrete crossties. The fuses can be triggered with an electrical current that applies the necessary heat to activate the SMAs’ memories, causing them to recover their shape and apply stress to the prestress wires.
This can be done at any time – in the plant before shipping, in the field during placement, or even at some later time in the service life of the tie. Maintenance teams can inspect and test the level of prestressing in the crossties while in service, adjusting the level of prestressing force where it is needed. The method will be cost-effective, Andrawas said, because it will utilize an alloy made of iron, nickel, cobalt and titanium, which is less expensive than the SMAs that are commonly used in the biomedical and aerospace industries.
The team is testing the new system through a combination of computer modeling and full-scale physical testing. This research is being funded by the Transportation Research Board.