Szymon Grzesiak and colleagues investigated the influence of fiber addition on the properties of high-performance concrete. They showed that addition of fibers reduced compressive strength and the modulus of elasticity of the concrete. Their study recently appeared in the Journal Materials.
High performance fiber reinforced concrete (HPFRC) is developing quickly to a modern structural material with a high potential. HPFRC is a ductile concrete with high metallic fiber content (1% or more by volume), with uniform or hybrid fibers. Experimental tests showed that fiber dosage improves the energy absorption capacity of concrete and enhances the robustness of concrete elements. However, the addition of fiber does not have always have a positive effect on the mechanical properties. Besides, due to increasing costs of the produced fiber reinforced concrete there is a demand to analyze the necessary fiber dosage in the concrete composition.
Now, Szymon Grzesiak and colleagues investigated the influence of fiber addition on the properties of high-performance concrete.
“It is expected that the surface and length of used fiber in combination with their dosage influence the structure of fresh and hardened concrete.”— they said
In order to determine the influence of fiber addition, they carried out tests on a mixture with polypropylene (PP) and polyvinyl alcohol (PVA) fiber with dosages of 15, 25, and 35 kg/m³ as well as with control concrete without fiber.
They found that, due to the addition of fibers to the concrete mix, a significant difference was observed in the compressive strength of the concrete. The fiber addition of 15 kg/m³ in the concrete composition reduced the compressive strength from 83.2 MPa to 79.6 MPa. The higher fiber dosage showed a similar trend. Furthermore, it reduced bulk density and the modulus of elasticity of the concrete.
“Due to the higher air content in the fiber-reinforced mixtures compared to normal concrete, the compressive strengths differed from each other.”— they said.
They also showed that, PP and PVA fibers are effective in increasing the splitting tensile strength of concrete, which allows better utilization of material capacities and has an impact on the production costs of Fiber Reinforced Concrete (FRC) members. The comparison showed that the dosage of fibers increased from 4.0 MPa to 5.0 MPa (for 15 kg/m³), 6.7 MPa (25 kg/m³), and 6.9 MPa (35 kg/m³).
Additionally, the fiber dosage improved the flexural properties of concrete. The flexural strength increased the maximal 31% for a fiber dosage of 25 kg/m³ in comparison to the plain concrete. The bending tensile strength of concrete with added fibers also increased by up to 18% compared to materials without fibers.
Moreover, it has been shown from stress–deflection curves that long MasterFiber 235 SPA (PP) is better than short MasterFiber 401 (PVA). This is because the stress–deflection curve of fiber type MasterFiber 401 (PVA) revealed a higher increase in deflection compared to fiber type MasterFiber 235 SPA (PP), which may indicate unfavorable adhesion forces between PVA fiber and the matrix. Shorter fibers pull-out of the matrix faster than longer fibers. This is attributed to the bonding forces between the fibers and the concrete matrix.
“The 30 mm long fibers provided a better friction range than the 12 mm long fibers and also provided a better stress transfer in the matrix.”
Finally, the highest PP fiber dosage examined in the concrete composition amounted to 35 kg/m³. However, the addition of more than 25 kg/m³ of fibers to the concrete mix had less influence on the bending tensile strength of the concrete. This concrete mix had an overcritical fiber dosage and was characterized by tensile strain-hardening behavior. A comparison of the stress–deflection curves with the addition of 25 kg/m³ and 35 kg/m³ of fibers also revealed that the cracking behavior of concrete for these two fiber contents did not differ significantly.
Funding: Their research was funded by the Master Builders Solutions Deutschland GmbH.
Featured image: (A) Polypropylene fiber MasterFiber 235 SPA (PP). (B) Polyvinyl alcohol fiber MasterFiber 401(PVA) used in the present study. © Authors
Reference: Grzesiak, S.; Pahn, M.; Schultz-Cornelius, M.; Harenberg, S.; Hahn, C. Influence of Fiber Addition on the Properties of High-Performance Concrete. Materials 2021, 14, 3736. https://doi.org/10.3390/ma14133736
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