A recent study has revealed how particle packing models (PPMs) can significantly enhance fiber-reinforced concrete (FRC) by reducing cement content while maintaining superior strength and workability. By optimizing particle distribution, this method not only improves material efficiency but also minimizes environmental impact.
Study: Eco-Efficient Fiber-Reinforced Concrete: From Mix Design to Fresh and Hardened State Behavior. Image Credit: UNIKYLUCKK/Shutterstock.com
Background
Reducing the environmental footprint of cement-based materials is a growing priority.
One widely used approach is the partial replacement of cement with supplementary cementitious materials (SCMs) such as metakaolin, fly ash, and slag. This method not only lowers CO2 emissions but can also enhance mechanical properties and durability. Another effective strategy involves refining the particle size distribution (PSD) of powders and aggregates using advanced mix proportioning methods like PPMs.
Unlike SCMs, which chemically alter the cementitious matrix, PPMs improve the physical arrangement of particles, increasing packing density and reducing void spaces—leading to greater efficiency and lower cement consumption.
Fibers further strengthen concrete by bridging microcracks and distributing stresses; however, they can negatively impact fresh-state behavior. This study explored the potential of combining advanced mix design techniques, such as PPMs, with fiber incorporation to address this challenge.
Methods
The researchers employed the continuous Alfred model to design 12 PPM mixtures, varying the distribution coefficient (q-factor: 0.21 and 0.26). Portland cement (PC) was the primary binder, with LF partially replacing it due to its similar PSD. The fine aggregate (FA) used was natural sand, while the coarse aggregate (CA) was crushed limestone.
Different fiber parameters were tested, including fiber type (steel and polypropylene), content (0.5 % and 1.0 %), and length (38 mm and 50 mm). Two FRC mix designs were compared: PPM-designed and American Concrete Institute (ACI)-designed mixtures. Both sets of mixtures were evaluated for fresh-state properties (VeBe time, slump, and rheology) and hardened-state properties (compressive and flexural strength).
The performance of the PPM-proportioned mixtures was then compared to six conventional ACI-designed mixes with the same fiber parameters.
Fresh concrete properties were assessed immediately after mixing using standard workability and rheological tests. Cylindrical specimens were prepared for compressive strength tests, while prismatic specimens were cast for flexural strength evaluation. The researchers recorded the load-deflection response during flexural tests to assess key parameters such as peak load (PL), post-cracking behavior, and residual stress.
Results and Discussion
The study found that ACI mixtures exhibited shorter VeBe times than PPM mixtures, indicating better initial workability. However, increasing fiber content led to longer VeBe times in all mixtures, with a more pronounced effect in PPM designs.
Steel fiber mixtures generally had slightly longer VeBe times than polypropylene fiber mixtures, and longer fibers further increased these times, particularly at higher fiber contents. Additionally, PPM mixtures with a q-factor of 0.21 and higher fine content showed VeBe times similar to or longer than those with a q-factor of 0.26.
Slump values, indicative of consistency, were consistently higher in ACI mixtures compared to PPM mixtures. Increasing fiber content reduced slump in all mixtures, with the effect being most significant in PPM designs and steel fiber-reinforced mixes. Longer fibers also negatively impacted slump, particularly in PPM mixtures.
Rheological analysis revealed that ACI mixtures generally required lower torque than PPM mixtures, although apparent viscosity differences were minimal. Steel fiber mixtures required higher minimum torque than polypropylene fiber mixtures, particularly in PPM-based designs, suggesting increased resistance to flow.
Compressive strength tests demonstrated a significant advantage for PPM mixtures, with strengths ranging from 30 % to 70 % higher than ACI mixtures despite having the same water-to-cement ratio. This finding underscores the effectiveness of PPM-based mix design in optimizing mechanical properties.
Fiber content had minimal impact on compressive strength for both mix types, although ACI mixtures with long steel fibers exhibited an 18 % strength increase compared to those with short steel or polypropylene fibers. This trend was not observed in PPM mixtures.
Conclusion
This study successfully demonstrated that PPM mix proportioning can create low-carbon FRC while maintaining a balance between fresh and hardened state properties. PPM-designed FRC achieved up to 70 % higher compressive strength and up to 64 % better flexural performance than ACI-designed mixtures, despite using the same water-to-cement ratio.
The researchers recommend further refining mix proportioning by optimizing q-factors for different fiber types and dosages, exploring alternative non-inert fillers and SCMs, assessing long-term durability, and conducting large-scale validation to support real-world applications of PPM-based FRC.
Journal Reference
Bergmann, A., Eid, M. N., de Grazia, M. T., Dantas, S. R. A, & Sanchez, L. F. M. (2025). Eco-Efficient Fiber-Reinforced Concrete: From Mix Design to Fresh and Hardened State Behavior. Materials, 18(6), 1245. DOI: 10.3390/ma18061245, https://www.mdpi.com/1996-1944/18/6/124
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