Fiber-reinforced concrete speeds up construction and reduces costs

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Newswise – “Phoenix will be a great city when it’s finished,” said a visitor in the 1950s.

An Arizona State University engineering professor is trying to promote a method to speed up endless Valley construction projects that can reduce years to months and months to weeks. Transportation experts say the economic, safety and transit benefits could be huge.

Earlier this year, residents and businesses in South Phoenix fought against the expansion of the Valley Light Rail system in their neighborhood. Besides opposing the reduction of Central Avenue to two lanes of four, their other objection to the project was the four-year construction schedule.

Construction takes time. And the bigger the project, the longer the lead time. The Big Dig in Boston – burying a highway in the heart of the city – took 15 years. Traffic, flow of goods and services, activities carried out near construction sites, everything is disrupted.

Barzin MobasherThe magic ball of is made of fiber concrete. Mobasher and his team have come up with a certain set of calculations that engineers can use to simplify working with the material.

Every year, around the world, 10 billion tons of concrete are used: more than a ton for every man, woman and child on the planet. It is the cheapest material that can be used in the construction of roads, buildings and other things.

Designs have revolved around the fact that while concrete can handle a lot of compression, it can’t handle a lot of tension. Put some weight on it and it’s good. Separate it and it breaks. Traditionally, overcoming this defect meant putting steel rebar into the concrete. Engineers assume that the steel will carry the load that the concrete is supposed to carry. They completely ignore the issue of tension.

“Almost every structure you see — every building, every skyscraper, every bridge — is based on this assumption; take no input from concrete in tension,” said Mobasher, a professor in the School of Sustainable Engineering and the built environment of the Ira A. Fulton Schools of Engineering.

Composites have been around for 60 or 70 years. You mix steel or composite fibers into a medium (like concrete or epoxy) – typically 40 pounds of fibers into 2,000 pounds of concrete. They are very strong fibers, but also very small. When you decrease the size of something, you decrease the size of any flaws that may occur in the structure.

If the concrete tries to crack, the fibers intercept the crack and act as internal bandages. They hold it together and allow it to carry more load. It is an interlock mechanism.

“How can I make this design with this material a little bit easier so that people and engineers can adopt it and use it?” said Mobasher. “The work my team has done is come up with procedures, calculations and equations that will tell you that if you put that amount of fiber in your mix, now you can count on all of your concrete tension that you used to have. ‘ignore – now it can carry X amount of load. Alternatively, you can put enough fiber in it so you don’t have to put the rebar in it. … It suddenly changes the whole game.”

If you are building a five-story building, each floor must be designed and built one by one. Rebars must be fixed, laid out and tied together. You have costs for material, inspection, labor, logistics, safety issues, etc., when rebar is involved. Add those costs up and they’re significant.

Now remove rebar from the equation. If you only use fiber, you tell the blending plant how much and what type of fiber you want to blend in each truckload.

“That way you eliminate a lot of the ancillary costs,” Mobasher said. “You pay more for the material on a pound-per-pound basis to use fibers, but you save so much on all those extra costs.”

Mobasher decided to do a proof-of-concept experiment in his lab. He fabricated a fiber-reinforced concrete slab and subjected it to strength tests in a special machine. The parameters were: a line running seven days a week, 18 hours a day, a fully loaded three-car train running every 10 minutes for 40 years. There was a catch: in the experiment, the water eroded everything under the beam, causing it to support all the weight on its own.

Despite cracking, the demonstration showed that fiber-reinforced concrete can withstand 2 million rail traffic cycles. And one crack wouldn’t derail an entire system. Cut out the cracked section, reverse and it’s fixed.

“Basically we’re offering a solution to reduce the cost, to reduce the weight, to make the material much more ductile, earthquake resistant, corrosion tolerant – so a whole host of additional benefits that we would get,” Mobasher said. .

The methodology includes different sets of calculations for different fiber types: steel, synthetic, glass, polymer, nylon and others. “We can design structures that are much more efficient, much more durable, and actually reduce reliance on concrete materials,” he said.

There is also a durability benefit. For every ton of Portland cement – the most commonly used type – generated, one ton of carbon dioxide is released into the atmosphere.

“Our carbon footprint can be significantly reduced if we use cement efficiently,” Mobasher said.

The idea has been in the genesis for a long time. The American Concrete Institute writes the building code for concrete. From 2012 to 2018, Mobasher chaired the institute’s committee on fiber-reinforced concrete.

He has been working on the methodology since 2004. Adoption took two or three years, but progress has been slow.

“Civil engineering is a very conservative society,” he said. “Nobody wants a project to fail.”

However, at least one agency in the valley is working with the material. A Valley Metro subcontractor used fiber reinforced concrete on the light rail line near ASU.

Ram Pendyala said there are huge benefits to accelerating construction projects. Pendyla, a professor of engineering at ASU, teaches and conducts research on the planning and engineering of multimodal transportation systems.

From a transportation perspective, every construction project is a combination of disruption and nuisance, he said. Fiber-reinforced concrete could have benefits throughout the built environment.

“Construction time is a major issue in transportation, so we’re constantly looking for ways to speed up projects, minimize disruptions, make work areas as safe as possible, and part of safety is minimizing the duration of a major work area in place,” Pendyala said. “This has both transportation benefits and a safety benefit – potentially energy and emissions, as there is no ain’t no people idling in traffic that long.

“Then there are economic aspects. Every time there is a construction project, there are businesses along the corridor that are really, really worried about how it will affect their bottom line because people try to avoid the corridor and many businesses depend on passing traffic. … If this area is difficult to navigate, businesses suffer. I think this is where fiber-reinforced concrete could bring very concrete advantages from a mobility and fluidity point of view, from a safety point of view and from an economic vitality point of view.

Original article by: Scott Seckel, ASU Now Reporter: https://asunow.asu.edu/20191018-solutions-asu-engineer-fiber-reinforced-concrete-speed-construction

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