Ultra-high-performance concrete (UHPC) is widely used in projects that demand exceptional strength and durability. Designers and engineers specify it for bridges, high-rise buildings and coastal infrastructure because it performs well under extreme conditions. However, UHPC costs far more than conventional concrete. The specialised fibres inside the material account for most of that difference.

Researchers at Penn State have developed a new design approach that could reduce the cost of UHPC by up to 75%. Their study shows how manufacturers can optimise fibre reinforcement while maintaining strength, ductility and durability. The findings were published in Cement and Concrete Composites.

Testing Different Fibre Designs

UHPC contains thousands of short fibres embedded in a dense cement matrix. These fibres help the material resist cracking and improve its ability to bend under load. Although the fibres make up only about 2% of the material’s volume, they represent around 70% of its total cost. They also contribute significantly to its carbon footprint.

To identify more efficient solutions, the researchers produced 15 different UHPC mixtures. Nine contained steel fibres with different shapes, sizes and concentrations. The remaining six replaced steel with non-metallic alternatives, including fibrillated glass fibres, basalt fibres and fibre-reinforced polymer composites containing glass or carbon fibres.

The team then tested every mixture for flowability, compressive strength, tensile strength, ductility and bond strength. These tests revealed which fibre properties have the greatest influence on overall performance.

Better Performance With Less Material

The results showed that fibre design plays a major role. Two steel fibre types, microsteel and striated steel, delivered similar mechanical performance even after researchers reduced the fibre content by half. This suggests that manufacturers could lower material use without reducing structural performance.

The researchers also found that fibres with higher length-to-diameter ratios improved tensile strength. They observed another important factor. Fibres should pull out of the cement matrix before they break. This behaviour allows the concrete to absorb more energy and remain more resistant to cracking.

The non-metallic fibres did not yet match the performance of steel. Even so, the researchers believe future designs could achieve similar results at a much lower cost.

Lower Costs And Lower Emissions

The study provides practical guidance for manufacturers that want to produce more affordable UHPC. By optimising fibre design, producers could reduce both material costs and carbon emissions. Steel fibres are the largest contributor to both.

The research team will continue exploring new fibre materials and improved manufacturing methods. They will also investigate additional ways to reduce the carbon emissions associated with UHPC production. Their goal is to make this high-performance material more accessible for infrastructure and other demanding construction projects.

Source: Penn State
Photo: Poornima Tomy / Penn State