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 an eighth 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.
