New design approach may help slash the price of ultra-durable concrete
Penn State
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A team of engineers at Penn State is researching approaches to make ultra-high-performance more affordable. They tested their mixtures using a series of lab experiments, including a unique device that can effectively measure the tensile strength of concrete under stress.
view moreCredit: Poornima Tomy / Penn State
UNIVERSITY PARK, Pa. — Concrete, although the most common building material in the world, is brittle and can easily crack under tension. Ultra-high-performance concrete (UHPC) is a special class of concrete known for its dense structure and extreme durability. This class uses internal metallic fibers to flex and resist cracking — the downside being these fibers can lead the material to cost up to 30 times more than traditional concrete. A team of researchers, led by engineers from Penn State, is laying the foundation to help builders get more bang for their buck when using this specialized compound.
The team used a series of tests to measure the physical strength and ductility, or ability to flex and bend without cracking, of different UHPC mixtures, including experimental types reinforced with both metallic and non-metallic fibers. Testing identified several key characteristics that can be optimized to reduce the material’s price — cutting costs by up to 75% — while maintaining its remarkable strength, ductility and durability. The team developed a new design approach based on their assessments, which they said could help material producers, infrastructure owners and construction companies around the globe not just save money, but develop stronger and more environmentally friendly concrete. They published their work in Cement and Concrete Composites.
UHPC has become critical for building large, durable structures like bridges, high-rise buildings or coastal infrastructure like floodgates due to its high strength, ductility and exceptional durability, according to study co-author Farshad Rajabipour, the John and Harriette Shaw Professor of Civil and Environmental Engineering and head of the Department of Civil and Environmental Engineering at Penn State. Specifically, Rajabipour said UHPC is the key to accelerated bridge construction, which helps streamline bridge building and repairs that used to take months to just days or weeks.
“Elements of the bridge are prebuilt in a factory, brought to a site and then put together almost like Lego pieces,” Rajabipour explained. “The main portions of the bridge are built out of traditional concrete, but the grout that bonds each piece and holds them in place is UHPC. It’s not meant to replace traditional concrete, but to instead support high-strength applications.”
The material’s strength and ductility come from within. Thousands of tiny steel fibers are encased inside a larger matrix of cement, water, aggregates and additives, with each fiber measuring only 13 millimeters, about half an inch long, and 0.2 millimeters, or less than eight thousandths of an inch thick. By mechanically latching onto the cement matrix they are encased in, they form a material that is flexible in the face of extreme tension.
However, Rajabipour said these fibers are the major culprit behind the price spike. Despite only making up about 2% of the material’s total volume, they’re responsible for about 70% of the cost. Additionally, UHPC is commercially sold as pre-bagged, proprietary mixtures, further increasing the price of use. According to Rajabipour, the key to making the material more commercially accessible and affordable is optimizing the fibers.
To do this, the team produced 15 different mixtures of UHPC. Nine of the 15 mixtures used metallic fibers, employing different concentrations and designs to see if the same performance could be obtained for less material and, in turn, a lower price.
“We tried different lengths, widths and shapes of fiber — indenting them, twisting them, adding tiny hooks to help them better latch onto the cement matrix,” Rajabipour said. “The thought is that if we can get the same or better performance using less material, we can massively reduce the price.”
The team also tested six mixtures using non-metallic fibers made of fibrillated, or very thin, glass strands; a type of stone known as basalt; as well as polymer reinforced with glass or carbon fibers. Although these materials are not as strong as traditional steel fibers, Rajabipour said that identifying a non-metallic fiber that can provide similar performance could be a major leap in reducing the overall price of UHPC.
The individual samples were then subjected to a series of tests that the team developed to assess key mechanical characteristics of the different concrete mixtures. They evaluated each mixture’s flowability in liquid form, which Rajabipour said is important to understand when considering use for rapid, high-quality construction projects; compressive strength, or durability under forces pushing on the material; tensile strength, or durability under forces pulling the material; ductility; and bond strength, or the force it took for the internal fibers to come disconnected from the cement matrix.
The team observed that two of the metallic fibers tested — microsteel and striated steel — were able to maintain their performance, even when the total fiber volume was cut in half.
Additionally, the team identified which characteristics have the biggest impact on the UHPC’s performance. Fibers with higher length-to-diameter ratios exhibit much improved tensile performance. The team reported that engineering the bond so that the fibers pull out from the concrete before snapping under stress is critical to maintaining the strong performance. They also noted that although commercial non-metallic fibers still do not perform like steel fibers, better designs could produce fibers that offer metal-like performance for a fraction of the cost.
“Because of these tests, we have clear, number-based data on what characteristics should be optimized to save money, while not impacting performance,” Rajabipour said. “We knew that all these factors mattered — the volume fraction of fibers in the material matters, the total number of fibers matters and the bonding strength of the fibers matters — but we didn’t have clear quantitative information on how critical they each were.”
Going forward, the team plans to conduct further research on different fiber makeups, explore new non-metallic fibers and optimize existing manufacturing approaches. Additionally, the researchers plan to continue studying different opportunities to reduce the carbon dioxide that is released during UHPC manufacturing, greatly improving the sustainability of this material.
“We’ve shown pathways so that concrete producers can reliably make UHPC, instead of it being limited to a few proprietary formulations,” Rajabipour said. “The fibers are not only the biggest contributor to cost; they’re also the biggest contributor to emissions. We are not only presenting a pathway to reducing the cost of this material, but reducing their environmental impact, as well.”
Rajabipour holds additional affiliations with the Larson Transportation Institute and the Materials Research Institute. Other co-authors on the work include Rajabipour’s doctoral candidate advisees: Abdullah Al Moman, a structural design engineer at Dutchland who received his doctorate in civil engineering from Penn State; Deepika Sundar, research scientist at CalPortland who received her doctorate in civil engineering from Penn State; and Amir Alarab, structural engineering at AECOM, who earned his doctorate in civil engineering from Penn State.
Additional co-authors include Shaohua Chu, assistant research professor of civil engineering; and Jovan Tatar, associate professor of civil, construction and environmental engineering at the University of Delaware.
This research was supported by the U.S. Department of Transportation and the Pennsylvania Department of Transportation.
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Above is a look at several different types of internal fibers highly magnified through a microscope — including fibrillated basalt fibers tightly bundled together (a), a fiber reinforced polymer, with internal glass fibers glued together with resin (b), the magnified end of a hook steel fiber (c) and a single striated steel fiber (d). These internal fibers are the key to giving ultra-high-performance concrete its strength.
Credit
Provided by Farshad Rajabipour
Journal
Cement and Concrete Composites
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Tailoring fiber-matrix interaction to achieve Strain-Hardening Ultra-High-Performance Concrete (SH-UHPC) at low fiber volumes
