Abstract
Self-compacting concrete (SCC) incorporating silica fume is characterized by a dense microstructure and reduced permeability; however, the influence of polypropylene fiber length on simultaneously ensuring self-compactability and frost resistance remains insufficiently investigated. The objective of this study was to evaluate the effect of low-modulus polypropylene fiber length (6–20 mm) on the rheological behavior, durability performance, and microstructure of SCC. The mixture composition and superplasticizer dosage were kept constant; only the fiber length was varied at a fixed dosage of 1.0 kg/m³. Rheological properties were assessed using slump flow, T500 time, V-funnel, L-box, and J-ring tests. Durability was evaluated in terms of water absorption and frost resistance, while phase composition and morphology were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results indicate that increasing fiber length led to reduced flowability of the mixture: slump flow decreased from 700 to 600 mm, T500 time increased from 2.1 to 5.8 s, V-funnel flow time increased from 6.3 to 13.8 s, the L-box ratio decreased from 1.00 to 0.82, and the J-ring blocking step increased from 8 to 26 mm. The optimal combination of properties was achieved at a fiber length of 12–15 mm, at which water absorption decreased to 4.1–4.2%, and frost resistance reached F350 (compared to 4.8% and F300 for the control mixture). XRD analysis did not reveal the formation of new crystalline phases, while SEM observations indicated the development of a dense fiber–matrix interfacial transition zone. The findings substantiate the suitability of 12–15 mm polypropylene fibers for achieving a balanced combination of rheological performance, durability, and service-related properties in silica fume-modified SCC under the tested conditions.