Rebar is out, fiber is in: Valley Metro finishes light rail slabs for the latest extension
Light rail extensions disrupt business and traffic. Using steel rods to reinforce concrete beneath the tracks further slows the process while raising sustainability and worker safety concerns. So Valley Metro used an alternative on its current northwest extension.
Fiber-reinforced concrete does just what it says on the tin: It replaces a cage of half-inch rebar with sewing-pin-sized steel, glass, ceramic, carbon or polymer fibers mixed throughout the concrete.
“The area of fiber reinforced concrete is a quite a mature area; it's been in use for more than 40 or 50 years,” said project lead Barzin Mobasher of Arizona State University's School of Sustainable Engineering and the Built Environment. “And now, when we're looking at the issues of sustainability and efficiency of the concrete materials, it's become quite relevant.”
The technique saves weight, reduces corrosion, shrinks cement use by one-fifth and stops cracks at the microscopic level.
“We don't let the crack grow and widen up so that it becomes a big pothole or major separation because, once you have that, then you have corrosion problems, you have durability problems, you have significant other problems,” said Mobasher.
Valley Metro chose polypropylene and steel fibers for its 1.5 miles of track slabs, which crews completed in early May. It accepted the proposed fiber reinforcement following fatigue tests by ASU’s Structural and Materials Lab simulating 45 years of service at higher than expected loads.
Concrete stands up well to forces that squeeze or compress it, but it fails quickly against forces that flex or stretch it, such as tension. That’s a problem because concrete naturally shrinks, and track slabs support rails that undergo their own tension.
Reinforcing concrete with rebar reduces crack propagation and lets concrete carry loads after cracks appear.
“A lot of the problems on bridge decks or on highways — the noise on the highways — is, once these joints crack, they are free to move past each other,” said Mobasher. “So now you get bumps and basically you don't have a very smooth traffic flow.”
In fact, cracking is such an assumed quality of concrete slabs that builders cut control joints with diamond-bladed saws to limit their spread.
Mobasher prefers to try to stop cracks before they are visible, even as hairline fractures.
“Cracks often start as a small micro-crack,” he said. “And the opportunity here is, if I can stop these micro-cracks from connecting to one another, I can actually increase the strength of the material.”
Rebar manufacturing makes a substantial carbon footprint, and using rebar reinforcement requires weeks of crews working in precarious positions, laying down bars, welding them together and grounding components.
Mobasher says fiber-reinforced concrete is simpler, safer and takes a fraction of the time.
“It's like the difference between something that's made in the factory and delivered to the jobsite and poured instead of doing all of the work at the job site,” he said.
He added that, given the 500 million tons of rebar and 20-30 billion tons of concrete used worldwide, fiber reinforcement is really the “low-hanging fruit” of construction techniques, from a sustainability standpoint.
“And if we can develop sustainable reinforcement systems and rethink the methodologies that were developed more than 100 years ago, that's what we need to do.”