AZ Researchers Replace Rail Project Rebar
Recent work from Arizona State University has helped to cut down on time and costs on the construction of the Metro Phoenix light rail extension project by utilizing fiber-reinforced concrete instead of rebar.
The project extended the light rail by 1.5 miles and was reportedly completed last month.
About the Research
The collaboration between ASU, Phoenix Valley Metro Regional Transportation Authority and Kiewit-McCarthy, the project’s construction firm, began with a proposal from Barzin Mobasher, an ASU professor of structural engineering in the school of Sustainable Engineering and the Built Environment.
Mobasher stated that over 60% of the volume of concrete has no tensile efficiency and is unable to “carry load.” Reportedly, as a result the concrete in load-bearing structures is susceptible to cracks that begin very small and then grow until there is a fault in the structure.
According to Mobasher, using rebar, or reinforcing bar, that is embedded in the concrete, provides the loadbearing strength necessary for most concrete-based construction. The downsides, however, reportedly involve high costs, “dramatic” carbon footprints, worker safety risks and overall take a long time to complete.
For the study, Mobasher reportedly considered both steel and polymeric fibers added into the concrete, which eliminated the need for rebar. Steel fibers were selected by Valley Metro for the extension project.
Awesome – an @ASU project found that using fiber-reinforced concrete cuts construction costs while increasing worker safety and efficiency.— Kyrsten Sinema (@kyrstensinema) May 30, 2023
It’s no wonder ASU ranks first in the nation for innovation. ?? https://t.co/fF2NcrTbZg
ASU reports that testing involved creating full-size mock-ups for both rebar-reinforced concrete and fiber-reinforced concrete, in the same ratios as full-size sections. Side-by-side testing allowed comparisons of strength and flexibility, as well as documentation of concrete cracking and fatigue susceptibility.
The process for testing also reportedly projected cost and construction time savings of over 100 days and around $12 million, respectively.
“The idea of taking several long rebars that are half an inch in diameter, separated by 12 to 18 inches and built into a cage that is 12 inches above ground and replacing them with a fiber material, which is 2 inches long and only 1/32nd of an inch in diameter and mixed in with the concrete, might seem on scale non-competitive,” Mobasher said.
“But if you have thousands of those small fibers distributed in there, they become much more effective in arresting the cracks—working as small Band-Aids to keep the cracks closed and transfer the load. [Fiber-reinforced concrete] can be designed to bear up to an unprecedented 40% of the tensile load capacity of concrete.”
“We did the fatigue tests to simulate conditions for up to 45 years of service at much higher expected loads as proof of concept, and they accepted the proposed approach,” said Mobasher of the approvals from Valley Metro and the city of Phoenix.
“It’s been a tremendous experience for them to save the amount of materials used and, at the same time, to be able to meet the project at costs much lower than the original budget and in a much faster time frame.”
This project will reportedly serve as a prototype for similar light rail upgrades across the country and is also expected to be presented at an international Fiber-Reinforced Concrete Workshop hosted by the ASU in September.
Using fiber-reinforced concrete as a replacement for rebar reportedly helps to significantly reduce the disruption of surrounding neighborhoods to weeks, and sometimes, even days.
Project manager for Jacobs Engineering in Tempe, Florida, Andrew Haines said: “There’s an accepted way of doing reinforced concrete in the United States, especially with regard to light rail. I think engineers get into this track of just, ‘We’ve got to do it a certain way, that’s how it’s always been done,’ and it’s been very difficult to change that—to accept something new.
“The placement of the concrete with the fibers has been very simple. There’s no reinforcement—there’s no bars in the track slab for workers to try to walk on and perhaps slip on. So, it’s just the prepared earth and the rails are in place and the concrete gets placed around it—the reinforcement is integral with the concrete.”
According to Haines, the ability to develop material samples and test them in the ASU labs was a major component in starting the change.
Mobasher said that fibers are added to the concrete mix at the plant before it is transported to “ready-mix” trucks at the construction site. The entire mix reportedly self-consolidates after discharge, leaving a smooth, finished concrete surface.
“The work that used to take weeks to be done is finished in a matter of hours because we don’t need a crew laying up the steel rebars, connecting them, making sure they are all adequately welded together and that the components are all grounded,” Mobasher said.
He added that one of the missions with this research is to eventually make this sustainable concrete technology available nationwide for other cities and communities to use. Similar formulas for concrete have reportedly been employed around the world, though often coming with proprietary restraints.
“In our laboratory, we provide a scientific basis for the design validation of structural components by combining the design codes, analytical and computer simulation design tools,” Mobasher said.
“Then we go a step further to verify the results with full-scale tests under the same loads the designers are concerned about. This approach gives us the ability to dial in the level of over-strength and conservativeness the engineers are comfortable with for the service life.”
According to testing, the new mixture validates stability for over 45 years and is likely to have a service life of 100 years due to Phoenix’s climate. The purpose of sustainable engineering is reportedly to design the material at a different level that “may not sound intuitive but has load-bearing qualities that enhance longevity while reducing the carbon footprint.”
“We want to show that the (fiber-reinforced concrete) construction process can be a do-it-yourself project for local communities working with local construction companies,” Mobasher said. “We can show that collaboration between municipalities, industry, government agencies and universities can come together to share resources, cut costs and increase sustainability.”
Additionally, Mobasher explained that the method provides enhanced worker safety and sustainability.