Impact of basalt fiber on mechanical properties and durability of shotcrete

2.1. Materials

In this test, the grade of shotcrete was configured as C30. Both the coarse aggregates (nominal size: 5-12 mm) and the fine aggregates (natural river sand with a fineness modulus of 2.6) were employed. The length of SF is 25 mm. The admixtures include a high-range water reducer and an alkali-free liquid flash setting admixture.

2.2. Proportioning of shotcrete

The shotcrete mix proportions are shown in Table 1 and Table 2. The dosage of water reducing admixture (WRA) and flash setting admixture (FSA) is 5.5 and 33.6 kg/m³, respectively.

2.3. Test methods

The specimens used in this test were made from shotcrete slabs cut and cured to a specified age for testing. Cubic specimens measuring 100×100×100 mm were fabricated in accordance with GB/T 50081-2019 and tested for compressive strength and splitting tensile strength (Fig. 1). The compressive strength and splitting strength results were averaged from three specimens.

Fig. 1Splitting strength test (Hanghang Wang took the photo, Chang’an University, and 2025.06.03)

The compressive strength and splitting tensile strength are calculated according to Eqs. (1, 2):

where $F$ denotes the failure load of the specimen, and $A$ denotes the loaded area of the specimen.

The rapid freeze-thaw test was conducted in accordance with GB/T 50082-2024, and the performance was evaluated by the mass loss rate and the relative dynamic elastic modulus. The mass loss rate and relative dynamic elastic modulus results were averaged from three specimens.

The mass loss rate is one of the most important characterization indicators of concrete's own frost durability, and the mass loss rate is calculated according to Eq. (3):

where $∆W_n$ denotes the mass loss rate of specimens after $n$ freeze-thaw cycles, $m_0$ denotes the quality of specimens before freeze-thaw cycles, and $W_n$ denotes the mass of specimen after $n$ freeze-thaw cycles.

The dynamic modulus of elasticity of concrete reflects the compactness of its internal structure, and higher values indicate greater structural density. The relative dynamic modulus is calculated according to Eq. (4):

where $P$ denotes the relative dynamic modulus of elasticity of specimens after $n$ freeze-thaw cycles, $f_n$ denotes the transverse fundamental frequency of specimens after freeze-thaw cycles, and $f_0$ denotes the transverse fundamental frequency of specimen before n freeze-thaw cycles.

3.1. Compressive strength

The compressive strength enhancement factor was calculated according to Eq. (5):

where $f_{cu}$ denotes the compressive strength of fiber-reinforced shotcrete, $f_{cum}$ denotes the compressive strength of ordinary shotcrete.

As indicated in Fig. 2, the compressive strength of shotcrete peaks at a 6 mm fiber length across all curing ages. At a fiber length of 16 mm (BF-16), the 28 d compressive strength reaches 43 MPa, representing a 16.2 % improvement over plain shotcrete. This enhancement is attributed to the denser internal structure through BF incorporation, which improves crack resistance. However, when the BF length increases to 50 mm (BF-50), the 28 d compressive strength decreases due to fiber agglomeration caused by inadequate dispersion within the matrix.

According to Fig. 3, SF also enhance the compressive strength of the shotcrete, achieving a 28 d value of 38 MPa. Nevertheless, this represents an 11.6 % reduction compared to the BF-16 group, indicating that SF provides a relatively weaker enhancement effect on compressive strength. The high stiffness of steel fibers makes them prone to balling and non-uniform dispersion during the mixing process, may result the lower compressive strength.

Fig. 2Effect of BF length on compressive strength (NF, non-fiber reinforced; SF, steel fiber reinforced; BF-6, 6 mm basalt fiber reinforced; BF-16, 16 mm basalt fiber reinforced; BF-50, 50 mm basalt fiber reinforced)

Fig. 3Effect of fiber type on 28d compressive strength

3.2. Splitting tensile strength

The variation of splitting tensile strength with fiber length is presented in Fig. 4. The splitting strength similarly follows a trend of initial increase followed by decrease. The optimum BF length of 16 mm yields a 28 d splitting tensile strength of 3.5 MPa, corresponding to a 9.4 % improvement over the reference shotcrete. This improvement stems from the appropriate fiber length enhancing matrix connectivity and utilizing the inherent tensile properties of the fibers. However, the splitting strength decreased at 50 mm BF length (BF-50), confirming that excessive fiber length diminishes the reinforcement effect.

Fig. 4Effect of BF length on splitting tensile strength

As illustrated in Fig. 5, the SF exhibit the most pronounced enhancement in splitting tensile strength, achieving a 28 d value of 3.9 MPa. This represents a 21.9 % improvement over plain shotcrete, indicating that SF provides superior enhancement compared to BF fibers for this particular mechanical property. This is likely due to the high elastic modulus and excellent anchorage performance of SFs, which enhance the crack resistance of shotcrete, thereby resulting in higher splitting tensile strength.

In summary, BFs significantly enhance the early splitting tensile strength of shotcrete, with 16mm fibers demonstrating optimal performance. However, SFs also exhibit substantial improvement in splitting tensile strength under standardized curing conditions.

Fig. 5Effect of fiber on 28d splitting tensile strength

3.3. Frost resistance

3.3.1. Rate of mass change

As illustrated in Fig. 6, a progressive rise in mass loss rate is observed with increasing freeze-thaw cycles. Notably, 50 mm BFs exacerbate mass loss during freeze-thaw cycles. Specimens containing 50 mm fibers exhibited dramatically higher mass loss – approximately 12 times and 5 times greater than conventional shotcrete after 25 and 100 cycles, respectively. This detrimental effect is attributed to the excessively high aspect ratio, which creates extensive interfacial contact areas with the shotcrete matrix. These interfaces possess relatively lower strength compared to both the concrete bulk material and the fibers themselves, rendering them particularly vulnerable to freeze-thaw damage. In contrast, 16mm BFs effectively reduce mass loss compared to ordinary shotcrete, demonstrating their potential for enhancing frost resistance in shotcrete applications.

Fig. 6Effect of BF on rate of mass change

3.3.2. Relative dynamic modulus of elasticity

As shown in Fig. 7, the relative dynamic modulus exhibited a clear negative correlation with the number of freeze-thaw cycles, demonstrating accelerated material deterioration. When the number of cycles reaches 50, initial damage emerges within the concrete matrix, activating the reinforcement mechanisms of both basalt and steel fibers. The 6 mm BFs (BF-6), with their low length-to-diameter ratio, effectively suppress microcrack initiation during early freeze-thaw stages. The 16 mm and 50 mm basalt fibers demonstrate intermediate effectiveness, while steel fibers show the least improvement. Beyond 50 cycles, 16 mm BFs deliver optimal performance, increasing the relative dynamic elastic modulus by 32 % compared to ordinary shotcrete after 100 freeze-thaw cycles. Although both BF types enhance the dynamic modulus, SF provides inferior reinforcement due to corrosion susceptibility and limited operating temperature range. In summary, 16 mm BF significantly enhance concrete frost resistance, supporting enhanced durability for concrete in cold regions of northern China.

Fig. 7Effect of BF on relative dynamic modulus of elasticity