Hybrid Steel and Polypropylene Fibers Make Concrete 18 Times Tougher

Concrete is the second most consumed material on Earth after water, with production estimated at nearly three metric tons per person per year. Yet the material has a fundamental weakness: its direct tensile strength typically amounts to only 8 to 15 percent of its compressive strength. When slabs, pavements, and industrial floors bend under load, concrete cracks, and once cracking begins it loses stiffness and load-bearing capacity almost instantly because it has no internal mechanism to transfer stress across the fracture. A new experimental study from Ecuador suggests that the answer lies not in one type of reinforcement, but in a carefully balanced marriage of two very different fibers.

Researchers at ESPOL Polytechnic University in Guayaquil, working with colleagues in Spain, evaluated eight concrete mixtures labeled M0 through M7, including a plain control mix, single-fiber systems, and hybrid combinations of Dramix steel fibers and SikaForce-48 polypropylene macrofibers. Their goal was to identify a combination that maximized energy absorption and post-crack behavior while remaining workable and affordable for real civil engineering projects such as slabs, industrial floors, and pavements. The work, published in Frontiers in Built Environment, addresses a gap in locally generated performance data that has limited the adoption of fiber-reinforced concrete in the region.

The logic of hybridization is rooted in scale. Steel fibers, with tensile strengths near 2,300 megapascals and an elastic modulus around 207 gigapascals, act as stiff bridges that carry load across wide, established cracks and sustain residual strength after failure. Polypropylene macrofibers, by contrast, are far lighter, with a density of roughly 0.91 to 0.92 grams per cubic centimeter against steel’s 7.85, and contribute primarily to crack control and deformation stability. Combining them means the two fiber types operate at different stages of the cracking process, producing a composite whose response is more balanced than either fiber alone can deliver.

To measure performance, the team relied on EFNARC panel tests, casting 600 by 600 by 100 millimeter panels and loading them while recording complete load-deflection curves. Two indicators dominated the analysis: E25, the energy absorbed up to 25 millimeters of deflection, and R25, the residual load the panel still carries at that deflection. In total the experimental program produced 24 panels and 72 cylinders, with compressive strength tested at 7, 14, and 28 days and workability tracked through slump measurements.

The results were dramatic. The plain control mixture, M0, absorbed just 36.09 newton-meters of energy and retained a residual load of only 0.20 kilonewtons, equivalent to a mere 1.77 percent of its peak strength of 11.29 kilonewtons. In other words, conventional concrete lost more than 98 percent of its load-bearing capacity once it cracked. A mixture reinforced solely with steel fiber, M1, reached E25 of 506.03 newton-meters and retained 19.00 kilonewtons, while the all-polypropylene mix, M5, achieved 266.09 newton-meters and 7.80 kilonewtons.

The hybrids outperformed everything. Mixture M7, dosed with 11.25 kilograms of steel fiber and 3.75 kilograms of polypropylene macrofiber per cubic meter, recorded E25 of 653.63 newton-meters, R25 of 21.70 kilonewtons, a peak load of 33.89 kilonewtons, a 28-day compressive strength of 257.98 kilograms per square centimeter, and a slump of 18.33 centimeters. Compared with the plain control, that represents roughly 18 times more absorbed energy and more than 100 times greater residual load. Its sister hybrid M6 posted a marginally higher energy figure of 656.86 newton-meters, but with lower slump, lower residual load, and a material cost 13.9 percent higher.

Statistical analysis reinforced the findings. A one-way ANOVA on absorbed energy yielded an F-value of 41.41 with a p-value of 1.08 multiplied by ten to the minus eighth power, confirming significant differences among the mixtures, and Tukey’s test showed all fiber-reinforced mixes significantly outperformed the control while M6 and M7 were statistically indistinguishable from each other. Compressive strength differences were significant at 7 and 28 days, with M7 leading, although the authors note that some mixtures showed higher variability, likely tied to local differences in fiber distribution and orientation, factors the study did not independently quantify.

The economic dimension proved decisive in selecting a winner. At 179.20 US dollars per cubic meter, M7 cost nearly 29 dollars less than M6 while sacrificing less than 1 percent of energy absorption, and it delivered 12.1 percent higher residual load, 25.3 percent greater compressive strength, and better workability. The lesson is counterintuitive and important: simply loading concrete with the maximum fiber content does not produce the most efficient material. Higher dosages reduce flowability, hinder homogeneous dispersion, and raise the risk of fiber clumping, eroding the very performance gains they are meant to provide.

Beyond the laboratory, the implications reach into sustainability. Cement and concrete production is associated with roughly 7 to 8 percent of global carbon dioxide emissions, so any technology that extends service life, limits cracking, and reduces repair cycles carries environmental weight. The authors frame their contribution as a replicable, performance-oriented experimental protocol combining material characterization, EFNARC fracture testing, compressive strength verification, and cost-per-cubic-meter analysis, giving engineers a defensible multi-criteria basis for specifying hybrid fiber concrete rather than relying on peak strength alone.

The team is candid about limitations: three panels per mixture constrain statistical precision, fiber orientation was not characterized through imaging or computed tomography, and the fiber-matrix interface was not directly tested through pull-out experiments. Future work should address fatigue, impact, abrasion, chloride and carbonation durability, full-scale validation, and life-cycle cost analysis. Even so, the study demonstrates that in the unglamorous world of industrial flooring, a precisely tuned cocktail of steel and plastic fibers can turn brittle, fragile concrete into a material that bends, absorbs, and keeps carrying the load long after it has cracked.

Subject of Research: Mechanical performance and energy absorption of steel-polypropylene hybrid fiber-reinforced concrete

Article Title: Mechanical performance and energy absorption capacity of concrete reinforced with hybrid fiber systems

Article References: Briones-Bitar, J., Pinto-Ponce, B., Pacaji-Ulloa, M., Vera-Navarrete, E., Santos-Baquerizo, E., Berrezueta, E., Carrión-Mero, P., & Morante-Carballo, F. (2026). Mechanical performance and energy absorption capacity of concrete reinforced with hybrid fiber systems. Frontiers in Built Environment, 12, Article 1965114. https://doi.org/10.3389/fbuil.2026.1965114

Image Credits: AI Generated

DOI: 10.3389/fbuil.2026.1965114

Keywords: fiber-reinforced concrete, hybrid fibers, steel fibers, polypropylene macrofibers, EFNARC panel test, energy absorption, toughness, compressive strength, workability, cost analysis, pavements, construction materials