Optimization Design Parameters for Basalt Fiber Reinforced Concrete Mix Proportions

Mix proportion design is a critical step in regulating the performance of basalt fiber-Reinforced Concrete, as it determines the fiber's effectiveness and the project's cost. Current issues include a lack of effective matching between fiber parameters and matrix components, an absence of mix design standards tailored to specific environments, and a lack of standardized guidelines for engineering applications. Therefore, this paper systematically explores the application patterns ofbasalt fiber in concrete mix design from an engineering perspective and proposes reasonable optimization methods for reference.

1. Basic Principles of Mix Proportion Design

1.1 Principle of Performance Matching

Fiber parameters and matrix components must be matched based on the project's requirements for strength grade, workability, and durability. For instance, projects requiring high crack resistance necessitate higher fiber dosages, whereas those requiring high fluidity require control over fiber length.

1.2 Principle of Synergistic Enhancement

Improving fiber dispersion relies primarily on coordinating the interactions between fibers, mineral admixtures, and Chemical admixtures. For example, the morphological effect of fly ash influences fiber dispersion, while water-reducing agents can compensate for the loss of fluidity caused by fiber incorporation.

2. Optimization Patterns for Key Parameters

2.1 Optimization of the Cementitious Material System

(1) Cement Strength Grade: P.O 42.5 grade cement is selected for C30–C50 concrete, while P.O 52.5 grade cement is used for concrete grades above C60. The cement content should be no less than 300 kg/m³ to ensure the matrix adequately encapsulates the fibers.
(2) Fly Ash Dosage: The optimal dosage is 10%–20% (replacing cement); this range leverages the "ball-bearing effect" to improve fiber dispersion while simultaneously reducing the heat of hydration. Experiments confirm that concrete with a 10% fly ash dosage exhibits a slump 20 mm higher than the control group and a 40% improvement in fiber dispersion uniformity.
(3) Water-Binder Ratio Control: The water-binder ratio is 0.38–0.45 for ordinary concrete and 0.28–0.35 for high-performance concrete. For every 0.05 reduction in the water-binder ratio, the interfacial bond strength between the fibers and the matrix increases by 15%–20%, though the dosage of water-reducing admixture must be increased accordingly.

2.2 Adjustment of Aggregate Gradation

(1) Optimization of sand ratio: To fill the voids between fibers after the incorporation of basalt fibers, the sand ratio needs to be increased by 2%–4%. For instance, if the baseline sand ratio for C30 concrete is 42%, adjusting it to 44% after adding 0.6% fiber allows the slump to be restored to over 150 mm.
(2) Control of aggregate particle size: To prevent fiber filaments from tangling or clumping, the maximum coarse aggregate particle size should not exceed 25 mm, and the content of flaky and elongated particles should be no more than 10%; fibers with a length of 6 mm are optimal.
2.3 Compatibility Design for Admixtures
Selection and dosage of water-reducing admixtures: Polycarboxylate high-performance water-reducing admixtures are preferred, with a dosage range of 0.3%–0.8% relative to the binder mass. When the fiber dosage is increased from 3 kg/m³ to 6 kg/m³, the corresponding dosage of the water-reducing admixture must be raised from 2.3% to 2.8% to ensure the slump remains essentially unchanged.