Dual Surface-Engineered Steel Fibers in Cementless UHPAAC

Surface-roughened steel fibers embedded in a highly alkaline, cementless matrix resist pullout differently once the surrounding paste shifts from Portland cement hydration products to C–A–S–H–dominated reaction gels, and that change in interfacial chemistry immediately alters how tensile cracks initiate and spread. Ultra-high-performance alkali-activated concrete (UHPAAC) promises compressive strengths comparable to ultra-high-performance concrete while reducing cement-related carbon emissions, yet its tensile behavior still trails that of cement-based counterparts. The disparity has less to do with bulk strength than with the fiber–matrix interface, where lower equivalent bond strength limits strain hardening and crack control. In UHPAAC, replacing Portland cement with ground granulated blast-furnace slag and silica fume modifies the chemistry of the interfacial transition zone, and that modification weakens the anchorage mechanisms that high-performance fiber-reinforced systems depend upon. Previous attempts to compensate have largely focused on adjusting mixture proportions or increasing fiber content. Raising the steel fiber volume fraction does improve flexural and tensile resistance, but it also reduces flowability and promotes fiber congestion, which in turn disturbs orientation and dispersion. Geometric modifications to fibers, including hooked or corrugated shapes, can increase mechanical anchorage; however, excessive anchorage risks local matrix damage and unstable crack propagation. The tension, then, is evident: improving bond strength without inducing brittle failure or sacrificing workability.

Surface engineering of straight steel fibers offers a more targeted path. Chemical roughening treatments using EDTA-based electrolytes generate longitudinal indentations aligned with the cold-drawn microstructure of pearlitic steel, which can create micron-scale surface damage that increases frictional resistance during pullout. Functional coatings, including calcium carbonate and nano-silica, alter the interfacial chemistry and micro-mechanics by filling voids or reacting with matrix phases. However, these strategies have mostly been studied in cement-based ultra-high-performance systems. How they operate within alkali-activated matrices, and whether combining roughening with secondary coatings yields cooperative or redundant effects, are still unknown. A recent research paper published in Cement and Concrete Composites and conducted by Dr. Soonho Kim, Dr. Seong Yun Woo, Dr. Rongzhen Piao and professor Doo-Yeol Yoo from the Yonsei University working together with Professor Nemkumar Banthia from the University of British Columbia, the researchers developed dually modified straight steel fibers combining EDTA-induced surface roughening with secondary CaCO₃ or nano-silica coatings for use in cementless ultra-high-performance alkali-activated concrete. They integrated these fibers at controlled volume fractions and evaluated tensile response using direct tension coupled with digital image correlation.

The research team fabricated UHPAAC mixtures using ground granulated blast-furnace slag and silica fume as binders, activated by a sodium silicate–sodium hydroxide solution at a water-to-binder ratio of 0.3. They introduced straight steel fibers of 0.2 mm diameter and 19.5 mm length, first roughening them with an EDTA-electrolyte treatment that generated longitudinal micron-scale indentations. They then applied secondary coatings: calcium carbonate through controlled precipitation and nano-silica through a sol–gel process and by adjusting pH and aging conditions, they synthesized either micrometer-scale CaCO₃ particles or uniformly distributed nano-silica layers on the roughened surfaces. The authors found using scanning electron microscopy that EDTA treatment produced aligned indentations, while CaCO₃ coatings formed distinct crystalline morphologies, including rhombic submicron clusters when EDTA was present during precipitation.

The investigators cast dog-bone specimens with 1.5% and 2.0% fiber volume fractions and conducted direct tensile tests under displacement control, coupling the tests with digital image correlation to track crack evolution in real time. At 1.5% fiber content, specimens with dual-treated fibers displayed clear strain-hardening behavior and higher post-cracking stresses compared with both the control and solely roughened fibers. EDTA-CCE fibers achieved tensile strengths exceeding 12 MPa and markedly increased strain energy density, while also producing a denser distribution of microcracks. The researchers observed that CaCO₃ particles fractured and accumulated within the interfacial zone during pullout, and increased frictional resistance; nano-silica, by contrast, contributed through secondary reactions that stiffened the interface. According to the authors, these distinct mechanisms explain why CaCO₃ coatings favored strain hardening, whereas nano-silica enhanced initial stiffness. At 2.0% fiber content, performance still improved relative to the control, but the incremental benefit of dual treatments diminished. The authors linked this attenuation to fiber congestion and orientation effects: higher volume fractions reduce dispersion quality, which constrains the mechanical advantage conferred by surface modifications. That trade-off is instructive. Increasing fiber content strengthens bridging capacity in principle, yet excessive volume undermines alignment and reduces the effective contribution of engineered interfaces. Digital image correlation confirmed that dual-treated fibers at 1.5% produced smaller average and maximum crack widths across increasing strain levels, while at 2.0% the crack-control advantage narrowed.

The study by Kim, Woo, Piao, Banthia, and Yoo looks carefully at what happens when steel fibers in cementless UHPAAC are modified twice at the surface—first by roughening, then by adding a secondary coating. For practicing engineers, the work shifts attention away from the usual response of just adding more fibers toward changing how the fiber actually grips the matrix. Additionally, increasing fiber volume does raise tensile capacity, but anyone who has mixed these systems knows the trade-offs. When workability drops, fibers cluster and orientation become unpredictable. In the paper, instead of pushing the dosage upward, the researchers altered the fiber surface so that pullout resistance develops differently. Roughening creates micro-scale indentations; coatings such as CaCO₃ or nano-silica modify the interfacial zone. Together, these changes adjust bond stress evolution during crack opening. That bond behavior, in turn, governs how cracks distribute, how wide they become, and how long strain hardening can be sustained. From a structural standpoint, serviceability, permeability, and corrosion risk are all tied to crack width and the data of professor Doo-Yeol Yoo and colleagues show that dual-treated fibers allow larger tensile strains while keeping crack widths below 50 μm and 100 μm—values often used as durability benchmarks. So the material can deform more, yet still remain within acceptable crack limits. That matters in chloride exposure, in cyclic loading, in thin sections where redistribution capacity is limited. One detail we found important is the performance at 1.5% fiber volume. Surface-engineered fibers reached or exceeded the tensile behavior of higher-volume control mixes. That suggests a realistic pathway to reduce fiber content without sacrificing mechanical response. For precast elements, bridge link slabs, marine panels, overlays, even impact-resistant components, that balance between ductility and crack control is not abstract. It is design-critical.