Experimental Study on Mechanical Properties of Hybrid Fiber Desert Sand Recycled Aggregate Concrete

Abstract

In response to the issues of microcrack susceptibility, high brittleness, and unstable mechanical properties of desert sand recycled aggregate concrete (DSRAC), this study experimentally investigated the mechanical performance of DSRAC reinforced with hybrid steel–FERRO fibers. By testing macroscopic properties (compressive, splitting tensile, and flexural strengths) under different desert sand replacement ratios and fiber dosages, combined with microscopic analysis, the fiber-matrix interfacial behavior and toughening mechanism were clarified. The results showed that (1) DSRAC achieved optimal compressive strength when desert sand replaced 30% natural sand, with an obvious early strength enhancement; (2) both steel fibers and FERRO fibers independently improved DSRAC’s mechanical properties, while their hybrid combination (especially F0.15-S0.5 group) exhibited a superior synergistic strengthening effect, significantly outperforming single-fiber groups; (3) the established constitutive model accurately described the stress–strain response of hybrid fiber-reinforced DSRAC; (4) microscopic observations confirmed fibers inhibited crack propagation via bridging and stress dispersion, with hybrid fibers exerting multi-scale synergistic effects. This study provided theoretical–technical support for resource utilization of desert sand and recycled aggregates, and offered practical references for localized infrastructure materials (e.g., rural road subgrades and small-span culverts) in desert-rich regions and high-value reuse of construction waste in prefabricated components, advancing eco-friendly concrete in sustainable construction.

1. Introduction

To tackle two key constraints in the development of sustainable concrete, namely natural aggregate depletion and construction waste accumulation, desert sand recycled aggregate concrete (DSRAC) was proposed as a promising eco-friendly alternative. This material reduced reliance on natural sand through partial replacement with desert sand and actualized the resource utilization of construction waste [1,2,3,4,5]. However, DSRAC exhibited inherent defects, including high susceptibility to microcracks, prominent brittleness, and unstable mechanical properties [6,7,8,9,10], which restricted its application in engineering practice. Fiber reinforcement was recognized as an effective method for improving the toughness of concrete. Nevertheless, most existing studies on DSRAC focused on single-type fibers, and the available data on the synergistic mechanism and multi-scale toughening effect of hybrid fibers remained insufficient [11,12,13,14,15]. Against this backdrop, the present study was conducted to explore the mechanical properties and toughening mechanism of hybrid fiber-reinforced DSRAC, with the aim of supplementing essential data for this understudied field.

In comparison with conventional concrete, fiber-reinforced concrete can suppress the initiation and propagation of cracks through the bridging effect, thereby effectively enhancing the flexural strength and toughness of concrete [16,17,18,19,20,21,22,23,24]. This provides an effective approach to improving the engineering applicability of DSRAC and promoting the innovation of green building material technologies. Scholars from different countries have carried out numerous studies. Bencardino et al. [25] studied cement-based mortars with paper sludge-derived cellulose fibers, analyzing the effects of fiber content (0%, 1%, 2%) and preparation methods on mechanical properties. They found that 1–2% fiber mortars under controlled conditions had comparable performance to traditional ones (flexural strength variation < 5% and compressive strength went down ~10%); 1% fiber improved flexural performance on-site via the bridging effect, and masonry walls with eco-friendly mortar showed similar compressive behavior to traditional ones (load-bearing capacity dependent on bricks). This confirms the feasibility of such fibers as sustainable additives. Kachouh et al. [26] studied the influence of steel fibers (SFs) on the properties of concrete with recycled concrete aggregate (RCA) and dune sand as aggregates. The results showed that the introduction of RCA would reduce the strength and durability of concrete. However, the addition of steel fibers (with a volume content of 1–3%) could effectively compensate for this defect, significantly improving the compressive strength, abrasion resistance, and impermeability. This effect was particularly evident at high RCA replacement rates. In addition, Kachouh et al. [27] also studied the influence of SF and RCA on the flexural properties of concrete. The results showed that the volume fraction of steel fibers had a significant impact on the flexural properties of concrete. As the amount of steel fibers added increased, both the bearing capacity of the concrete and the peak crack mouth opening displacement (CMOD) were significantly improved, and the post-peak load decay was more gradual. This was attributed to the fact that the bridging effect of steel fibers effectively inhibited the crack propagation. J. Che et al. [28] conducted research and found that desert sand concrete reinforced with polyvinyl alcohol (PVA) fibers exhibited shear ductile failure under uniaxial compression. After failure, it maintained good integrity, with fine and stable cracks, showing better ductility than ordinary concrete. In addition, the synergistic filling effect of fly ash and desert sand increased the compactness of the matrix and fracture toughness, significantly improving the tensile properties. Wang et al. [29] studied the seismic performance of reinforced concrete columns with steel fibers and aeolian sand under low cyclic repeated loading. The results showed that both the addition of aeolian sand and steel fibers could improve the seismic performance of the columns. Among them, the fiber-reinforced concrete specimens with an aeolian sand replacement rate of 30% had the best comprehensive seismic performance. Steel fibers could effectively enhance the toughness of concrete, inhibit the development of cracks, slow down the decline of bearing capacity and stiffness degradation in the later stage, make the hysteretic loop fuller, and enhance the ductility and energy dissipation capacity. The study also found that the activity and filling effect of aeolian sand could improve the properties of concrete and its bond with steel fibers, thus giving full play to the bridging effect of steel fibers. Therefore, in the case of adding steel fibers, the replacement rate of aeolian sand could be appropriately increased without reducing the seismic performance of the structure, showing good application prospects. El-Hassan et al. [30] studied the mechanical properties of steel fiber-reinforced dune sand recycled aggregate concrete. The results showed that RCA replacement rates of 30% and 70% had little effect on the compressive strength of ordinary concrete. However, by adding 2% steel fibers, even when RCA was fully replaced, the compressive strength could still be comparable to that of natural aggregate concrete. Furthermore, the addition of steel fibers significantly increased the splitting tensile strength, elastic modulus, abrasion resistance, and impermeability of the concrete while reducing water absorption and permeability. Jiang et al. [31] systematically investigated the influence of aeolian sand replacing river sand on the properties of ultra-high-performance concrete (UHPC). The study showed that aeolian sand could significantly improve the workability of UHPC. When the replacement rate was 25%, the compressive strength was optimal, and there was no significant adverse effect on the flexural strength. The addition of aeolian sand slightly enhanced the toughness and dynamic impact resistance of UHPC, confirming its application feasibility in the preparation of eco-friendly UHPC.

However, most of the existing studies focus on the effects of single fibers on DSRAC. There is still a lack of systematic research on the synergistic mechanism of hybrid fibers and their toughening mechanism at multiple scales, thus failing to fully unleash the potential of fiber composites. Existing studies have shown that steel fibers with high elastic modulus can significantly enhance the flexural strength and post-cracking ductility of concrete. On the other hand, new composite materials such as FERRO fibers, which combine the high strength of imitation steel wire fibers and the corrosion resistance of polypropylene fibers, exhibit excellent crack resistance and potential for toughness enhancement. They can effectively inhibit the expansion of microcracks and improve the durability of the structure. Therefore, in this study, steel fibers with high elastic modulus and FERRO fibers with high toughness and corrosion resistance were introduced into DSRAC in a single-addition and hybrid-addition manner. The basic mechanical properties were evaluated through macroscopic mechanical tests. Combining with microscopic structure characterization, the fiber-matrix interface behavior and toughening mechanism were revealed. Finally, a stress–strain constitutive model for hybrid fiber-reinforced DSRAC was established to provide a theoretical basis and design support for its engineering applications.

2. Experimental Design

2.1. Material

The cementitious material was P·O 42.5 ordinary Portland cement. All of the coarse aggregates were recycled aggregates. Crushed stones with a particle size of 5–20 mm were selected from construction waste concrete after crushing, washing, and screening, as shown in Figure 1. Prior to mixing, the RCA was pre-conditioned to a saturated-surface-dry (SSD) state: dried to constant mass, cooled, immersed in water for 24 h to saturate internal pores, then surface-wiped to remove free water. This pre-treatment mitigated RCA’s water absorption, ensuring it did not alter the effective water/binder ratio during mixing. The fine aggregates were river sand with a fineness modulus of 2.75 and desert sand from the hinterland of the Taklimakan Desert in Korla, Xinjiang, China, as shown in Figure 2. The specific measured physical and mechanical performance indicators of the coarse and fine aggregates are shown in

Table 1. Chemical composition of desert sand.

Composition	SiO2	CaO	Al2O3	Fe2O3	MgO	K2O	Na2O	Ti2O	Other
Content	56.42	15.30	10.27	2.53	2.13	2.33	2.10	0.35	8.57

and

Table 2. Physical and mechanical properties of coarse and fine aggregates.

Category	Particle Size (mm)	Fineness	Apparent Density (kg/m3)	Bulk Density (kg/m3)	Mud Content (%)	Hygroscopic Rate (%)	Crushing Index (%)
Natural aggregate	5–20	-	2700	1620	0.37	1.27	13.9
Recycled aggregate	5–20	-	2650	1300	0.44	2.31	20.13
River sand	0–4.75	2.75	2580	1430	0.30	1.26	-
Desert sand	<0.18	0.26	2590	1570	0.17	1.97	-

below. The fibers incorporated were steel fibers and FERRO fibers. The steel fibers were of the wavy-milled type, measuring 30 mm in length and having a tensile strength of 600 MPa. Their tensile strength (600 MPa) met the technical requirements for “ordinary-strength steel fibers” (tensile strength ≥ 400 MPa) specified in the Chinese standard GB/T 39147-2020 Steel Fibers for Concrete [32]. The FERRO fibers were manufactured by FORTA Corporation in the United States. Their main component was high-strength polypropylene fibers. They had a length of 54 mm, a specific gravity of 0.91, and a tensile strength of 690 MPa, as depicted in Figure 3 below. The particle size distribution curves were conducted in compliance with the specifications GB/T 14685-2022 Pebbles and Crushed Stones for Construction [33] and GB/T 14684-2022 Sands for Construction [34], as presented in Figure 4 and Figure 5 below.

2.2. Mix Design

In this study, the volume fraction of fibers was employed as the primary variable parameter to investigate its enhancing effect on the mechanical properties of DSRAC. Initially, a preliminary experiment was conducted to determine the influence of the desert sand replacement ratio (0%, 30%, 50%, 70%, and 100%) on the 7-day and 28-day compressive strengths of DSRAC. The mix proportions of the experiments are presented in

Table 3. Mix proportions of DSRAC in the preliminary DS replacement ratio screening test.

Number	W/C	Material Consumption (kg/m3)					Specimen Size (mm)
		C	W	S	DS	RCA
DS0	0.53	414	219	764	0	1342	150 × 150 × 150
DS30	0.53	414	219	534.8	229.2	1342	150 × 150 × 150
DS50	0.53	414	219	382	382	1342	150 × 150 × 150
DS70	0.53	414	219	229.2	534.8	1342	150 × 150 × 150
DS100	0.53	414	219	0	764	1342	150 × 150 × 150

Note: In “DS30”, the “30” denotes that the replacement ratio of desert sand was 30%. The same logic applies to other designations. Here, W/C represented the water/cement ratio, where C refers to cement, W refers to water, S represents natural sand, DS represents desert sand, and RCA represents recycled aggregate.

. The preliminary parametric study (results presented in Section 3.1) evaluated the effect of desert sand (DS) replacement ratios on DSRAC compressive strength, demonstrating that the 28-day compressive strength reached its peak (39.0 MPa) at a 30% DS replacement ratio. To establish a stable, high-performance baseline matrix for subsequent fiber-reinforcement tests, 30% was selected as the optimal DS replacement ratio. Subsequently, specimens were cast to systematically explore the impact of varying fiber contents on the compressive strength, splitting tensile strength, and flexural strength of DSRAC. The detailed design of fiber contents is provided in

Table 4. Mix ratio design of fiber-reinforced desert sand recycled concrete.

Number	Material Consumption (kg/m3)					Fiber Content (%)
	C	W	S	DS	RCA	Steel Fiber	FERRO
S0.75	414	219	534.8	229.2	1342	0.75	-
S1	414	219	534.8	229.2	1342	1	-
F0.3	414	219	534.8	229.2	1342	-	0.3
F0.15	414	219	534.8	229.2	1342	-	0.15
F0.15-S0.5	414	219	534.8	229.2	1342	0.5	0.15
F0.2-S0.5	414	219	534.8	229.2	1342	0.5	0.2
F0.3-S0.5	414	219	534.8	229.2	1342	0.5	0.3
F0.3-S0.75	414	219	534.8	229.2	1342	0.75	0.3

Note: In the notation “F0.15-S0.5”, “F0.15” signifies that the volumetric dosage of FERRO fibers in desert sand recycled aggregate concrete (DSRAC) was 0.15%, while “S0.5” indicates that the volumetric dosage of steel fibers in DSRAC was 0.5%.

. The target water/binder ratio was set at 0.45. No explicit extra water was added during batching, as RCA was pre-conditioned to the SSD state (Section 2.1) to neutralize its water absorption—this maintained the effective water content of the mix and ensured consistent workability across all groups.

2.3. Specimen Preparation

All the desert sand, recycled aggregate, and concrete specimens were cast and mixed using an HJW-60 single-shaft horizontal mixer. First, desert sand, natural sand, recycled aggregate, and fibers were added to the mixer and stirred for 60 s. Then, 60% of tap water was added, followed by cement and another 30 s stir. After that, the remaining water was added. Once the mixing was completed, the concrete was quickly poured into the molds. A vibrating table with a frequency of 50 Hz ± 3 Hz was used to vibrate the concrete until it was compacted, and then the surface was finished. After the specimens were cast, plastic wrap was used to cover their surfaces. They were horizontally placed and left standing indoors at a temperature of 20 ± 5 °C for 24 h. Once the concrete had reached a certain strength, the molds were removed, and serial numbers were marked. Subsequently, the specimens were placed in a standard curing room for curing. The curing temperature was maintained at 20 ± 2 °C, and the relative humidity was greater than 95%. After 28 days of curing, the relevant specimens were taken out for various tests. The slump values of each specimen are shown in

Table 5. The slump values of specimens in each group.

Sample	Slump (mm)	Sample	Slump (mm)
DS0	85	F0.3	67
DS30	80	F0.15	74
DS50	75	F0.15-S0.5	70
DS70	65	F0.2-S0.5	66
DS100	58	F0.3-S0.5	60
S0.75	77	F0.3-S0.75	57
S1	75	-	-

, Figure 6 and Figure 7. Figure 6 depicts the scenario where, in the absence of fiber addition, the slump of concrete gradually declined as the replacement rate of desert sand increased. Specifically, the slump of the reference group DS0 measured 85 mm. This indicated that when natural sand and recycled coarse aggregate (RCA) were pre-wetted to the saturated-surface-dry (SSD) and were utilized, the concrete demonstrated favorable fluidity. As the replacement rate of desert sand escalated from 30% to 100% (ranging from DS30 to DS100), the slump gradually decreased from 80 mm to 58 mm. This trend could be primarily ascribed to the characteristics of desert sand. The desert sand particles possessed a high degree of angularity and a relatively large specific surface area. During the mixing process, these particles adsorbed and retained a portion of the mixing water that was initially intended to lubricate the aggregates. Consequently, this led to an increase in the viscosity of the cement paste and the internal frictional resistance within the mixture, ultimately resulting in a reduction in the concrete’s workability. Figure 7 further illustrates the influence of varying fiber dosages on the slump of concrete. The findings were as follows: Steel fibers (designated as S0.75 and S1) exerted a relatively minimal impact on the slump, registering values of 77 mm and 75 mm, respectively. These values exhibited only a slight reduction compared to the DS30 reference group, suggesting that at low dosages, their interference with the fluidity of the concrete was limited. In contrast, the effect of FERRO fibers was considerably more pronounced. The addition of F0.15 caused the slump to decrease to 74 mm, and with the addition of F0.3, the slump further dropped to 67 mm. This indicated that the high surface roughness and strong interfacial adsorption capacity of FERRO fibers significantly augmented the resistance of the cement paste. Hybrid fiber combinations, such as F0.15-S0.5, F0.2-S0.5, and F0.3-S0.5, exhibited a superimposed effect. The slump values decreased to 70 mm, 66 mm, and 60 mm, respectively. When the dosage of FERRO fibers reached 0.3% and was combined with 0.75% steel fibers (F0.3-S0.75), the slump measured 57 mm. Collectively, these results indicated that FERRO fibers had a far more substantial inhibitory effect on the workability of concrete compared to steel fibers. The underlying mechanism might be closely associated with their physical morphology and surface properties. Without significantly impeding the water available for hydration, FERRO fibers reduced the fluidity of the concrete by increasing the viscosity of the cement paste and the frictional forces between the fibers and the matrix. Notably, the slump of all mix proportions exceeded 55 mm, fulfilling the requirements for laboratory vibration compaction. This ensured sufficient compaction and provided favorable conditions for subsequent hydration processes.

2.4. Test Methods for Mechanical Properties

According to the Chinese standard GB/T 50081-2019 [35], the mechanical property tests were conducted using an electro-hydraulic servo universal testing machine with a maximum load capacity of 2000 kN. For the compressive strength test, the specimen size was a 100 × 100 × 100 mm cubic block, and the loading rate was 0.5 MPa/s. For the splitting tensile strength test, the specimen size was also a 100 × 100 × 100 mm cubic block, with a loading rate of 0.05 MPa/s. For the flexural strength test, the specimen size was a 100 × 100 × 400 mm prismatic specimen. The load was continuously and uniformly applied to the specimen at a loading rate of 0.05 MPa/s. The strain was obtained using strain gauges and the TDS540 static strain acquisition system from Tokyo Measuring Instrument Laboratory, Japan. The test equipment and loading device are shown in Figure 8 below.

The splitting tensile strength was calculated according to Equation (1).

$$f_{\mathit{ts}}={\displaystyle \frac{2F}{\mathit{\pi }A}}=0.637{\displaystyle \frac{F}{A}}$$

(1)

where $f_{\mathit{ts}}$ is the splitting tensile strength of concrete (MPa); $F$ is the failure load of the specimen (N); and A is the splitting surface area of the specimen (mm²).

The flexural strength was calculated according to Equation (2).

$$f_{\mathrm{f}}={\displaystyle \frac{F\cdot l}{b\cdot h^2}}$$

(2)

where $f_{\mathrm{f}}$ is the flexural strength of concrete (MPa); F is the maximum failure load of the specimen (N); l is the Span between supports of the bending test setup (mm); b is the width of the specimen cross-section (mm); and h is the height of the specimen cross-section (mm).

3. Results and Discussion

3.1. Compressive Strength

The test results are shown in

Table 6. Experimental results of compressive strength for DSRAC.

Sample	Maximum Load (kN)		Compressive Strength (MPa)		Average Compressive Strength (MPa)		Standard Deviations
	7d	28d	7d	28d	7d	28d	7d	28d
DS0-1	596.03	843.77	26.5	37.5	26.8	37.4	0.8	0.3
DS0-2	590.09	833.71	26.2	37.1
DS0-3	624.04	845.08	27.7	37.6
DS30-1	700.18	889.02	31.1	39.5	30.2	39.0	1.2	0.4
DS30-2	691.74	869.95	30.7	38.7
DS30-3	649.43	876.24	28.9	38.9
DS50-1	663.68	816.12	29.5	36.3	28.4	36.3	1.1	2.4
DS50-2	641.26	764.52	28.5	34.0
DS50-3	614.24	870.64	27.3	38.7
DS70-1	686.67	811.07	30.5	36.0	30.0	35.9	1.0	0.2
DS70-2	649.36	804.85	28.9	35.8
DS70-3	689.30	813.31	30.6	36.1
DS100-1	689.58	792.12	30.6	35.2	28.8	35.3	2.0	0.1
DS100-2	654.68	796.55	29.1	35.4
DS100-3	600.72	794.59	26.7	35.3

Note: In “DS50-1”, “50” indicates that the replacement rate of desert sand is 50%, and “1” represents the first test block. The same principle applies to the others.

. The relationship between the desert sand replacement rate and the average compressive strength is presented in Figure 9. As can be seen from the figure, with the increase in the desert sand replacement rate, the 28-day compressive strength of concrete generally showed a downward trend. However, when the replacement rate was 30%, both the 7-day and 28-day cube compressive strengths reached their maximum values. The 28-day compressive strength was 4.3% higher than that of the reference group (DS0). When the replacement rate was 100% (DS100), the 28-day compressive strength was 6% lower than that of DS0. This phenomenon could be attributed to the fact that the incorporation of an appropriate amount (30%) of desert sand effectively optimized the particle gradation of concrete. The voids between aggregates of different particle sizes were fully filled, thus improving the compactness. However, an excessive amount of desert sand disrupted the original gradation, introducing more voids and resulting in a decrease in compactness and strength. In addition, the 7-day compressive strengths of all specimens with desert sand incorporation were higher than that of DS0, with increases of 12.7%, 6.0%, 11.9% and 7.5%, respectively, indicating that desert sand had a certain early strength effect. This was mainly due to the fine particles and high water absorption rate of desert sand. By absorbing part of the mixing water, the actual water/cement ratio was effectively reduced, thus promoting the development of early strength. The results of this study showed that desert sand could partially replace natural sand to improve the mechanical properties of recycled concrete. Based on the above results, this study selected 30% as the optimal desert sand replacement rate, and on this basis, subsequent mechanical property tests of hybrid fiber-reinforced desert sand recycled concrete were carried out.

The test results of the compressive strength of DSRAC with different fiber types and dosages are shown in

Table 7. Compressive strength test results of fiber-reinforced DSRAC.

Sample	Maximum Load (kN)	Compressive Strength (MPa)	Average Compressive Strength (MPa)	Standard Deviations
FS0-1	324.06	32.4	32.0	0.4
FS0-2	315.52	31.6
FS0-3	321.28	32.1
S0.75-1	376.04	37.6	35.7	2.2
S0.75-2	363.04	36.3
S0.75-3	332.67	33.3
S1-1	405.881	40.6	38.0	3.0
S1-2	347.31	34.7
S1-3	386.74	38.7
F0.3-1	417.43	41.7	37.2	2.2
F0.3-2	364.13	36.4
F0.3-3	333.93	33.4
F0.15-1	370.55	37.1	34.7	4.2
F0.15-2	327.16	32.7
F0.15-3	343.74	34.4
F0.2-S0.5-1	330.55	33.1	33.9	1.1
F0.2-S0.5-2	345.54	34.6
F0.2-S0.5-3	340.25	34.0
F0.15-S0.5-1	409.95	41.0	42.1	0.8
F0.15-S0.5-2	422.97	42.3
F0.15-S0.5-3	430.79	43.1
F0.3-S0.5-1	390.07	39.0	38.7	2.1
F0.3-S0.5-2	405.56	40.6
F0.3-S0.5-3	365.30	36.5
F0.3-S0.75-1	381.95	38.2	36.5	2.3
F0.3-S0.75-2	374.25	37.4
F0.3-S0.75-3	338.75	33.9

and Figure 10. The results showed that in the reference group (FS0) without fiber addition, the average compressive strength of 30% DSRAC was 32.0 MPa, providing a performance comparison benchmark for this study. The addition of fibers significantly improved the compressive performance of DSRAC. When the volume dosages of steel fibers were 0.75% (S0.75) and 1.0% (S1), the average compressive strengths reached 35.7 MPa and 38.0 MPa, respectively, indicating that steel fibers had an obvious strengthening effect on the concrete matrix, and the strength showed an upward trend with the increase in fiber dosage. On the other hand, the addition of FERRO fibers also showed a similar effect: when the volume dosages were 0.3% (F0.3) and 0.15% (F0.15), the average compressive strengths were 37.2 MPa and 34.7 MPa, respectively, indicating that this fiber could effectively improve the compressive performance of concrete at different dosages. Particularly noteworthy was that the hybrid fiber group (F0.15-S0.5) exhibited the best overall performance. Its average compressive strength reached 42.1 MPa, which was significantly superior to that of the single-fiber reinforcement groups. This result demonstrated that by using a combination of different types and dosages of fibers, their synergistic strengthening effect could be fully exploited, and the mechanical properties of desert sand recycled concrete could be improved more effectively.

Figure 11 shows the typical failure modes of fiber-reinforced DSRAC. It could be seen that the fibers mainly enhanced the toughness and ductility of the concrete through the bridging effect. Relying on the bonding force between the fibers and the matrix, the crack propagation was effectively suppressed.

For the steel fiber-reinforced DSRAC, its failure mode is usually presented as a spindle-shaped pattern with contraction in the middle and expansion at both ends. This characteristic could be attributed to the high tensile strength and elastic modulus of the steel fibers, which enabled them to play a bridging role at the initial stage of crack appearance and restrict the development of cracks. As the load continued to increase, the fibers were gradually pulled out or broken, resulting in an increase in the width of local cracks, while the damage in other areas was relatively minor. Eventually, a typical spindle-shaped failure morphology was formed. This failure mode reflected the significant effect of steel fibers in suppressing crack expansion and improving the overall toughness of the concrete.

In contrast, the specimens reinforced with FERRO fibers did not show a similar spindle-shaped failure pattern. This was mainly attributed to the different interfacial bonding characteristics between the FERRO fibers and the concrete matrix. The FERRO fibers were more likely to disperse stress evenly, avoiding stress concentration, thus forming a more evenly distributed pattern of fine cracks throughout the specimens. Thanks to its material properties and surface morphology, the FERRO fiber could effectively disperse stress at the microcrack stage and suppress the concentrated development of macrocracks. Therefore, its failure mode was manifested as a large number of fine and uniform cracks, reflecting the advantages of this fiber in enhancing the toughness and deformation ability of concrete.

3.2. Splitting Tensile Strength

The test results are shown in

Table 8. Splitting tensile strength test results of fiber-reinforced DSRAC.

Sample	Maximum Load (kN)	Split Tensile Strength (MPa)	Average Split Tensile Strength (MPa)	Standard Deviations
FS0-1	44.557	2.84	2.91	0.1
FS0-2	44.285	2.82
FS0-3	48.335	3.08
S0.75-1	50.196	3.20	3.26	0.1
S0.75-2	49.314	3.14
S0.75-3	53.885	3.43
S1-1	54.400	3.47	3.72	0.3
S1-2	56.980	3.63
S1-3	63.888	4.07
F0.3-1	59.895	3.82	4.04	0.2
F0.3-2	60.253	3.84
F0.3-3	70.061	4.46
F0.15-1	47.823	3.05	3.00	0.3
F0.15-2	49.683	3.16
F0.15-3	43.885	2.79
F0.2-S0.5-1	49.980	3.18	3.40	0.4
F0.2-S0.5-2	51.617	3.29
F0.2-S0.5-3	58.680	3.73
F0.15-S0.5-1	57.037	3.63	4.13	0.3
F0.15-S0.5-2	69.845	4.45
F0.15-S0.5-3	67.692	4.31
F0.3-S0.5-1	71.438	4.55	4.14	0.4
F0.3-S0.5-2	58.322	3.71
F0.3-S0.5-3	65.485	4.17
F0.3-S0.75-1	46.801	2.98	3.14	0.1
F0.3-S0.75-2	50.394	3.21
F0.3-S0.75-3	52.015	3.24

and Figure 12. By analyzing the test data of the splitting tensile strength of DSRAC with different fiber dosages, the following conclusions could be drawn.

Firstly, the average splitting tensile strength of the control group (FS0) without fiber addition was 2.91 MPa. When steel fibers were introduced, the splitting tensile strength increased significantly with the increase in dosage. Specifically, when the dosage was 0.75% (S0.75), the average splitting tensile strength reached 3.26 MPa, which was approximately 12.0% higher than that of the control group. When the dosage was 1.0% (S1), the average splitting tensile strength further increased to 3.72 MPa, with an increase rate of 27.8%. This indicated that, relying on its high strength and good bonding performance, steel fibers could effectively enhance the tensile capacity of concrete, restrain crack propagation, and thus significantly improve the overall toughness of the material.

Secondly, the addition of FERRO fibers also showed a significant strengthening effect. When the dosages were 0.3% (F0.3) and 0.15% (F0.15), respectively, the average splitting tensile strengths reached 4.04 MPa and 3.00 MPa, respectively, which were approximately 31.9% and 3.1% higher than those of the control group. Due to its unique material properties, FERRO fibers performed well in dispersing stress concentration points and could effectively reduce crack formation and delay their propagation, thereby improving the mechanical properties of concrete.

Particularly notable was that the samples with the hybrid fiber combination (such as F0.15-S0.5) showed the best splitting tensile strength. The average value was as high as 4.31 MPa, which was approximately 48.1% higher than that of the control group. This result indicated that by reasonably selecting and mixing different types and proportions of fibers, not only could the splitting tensile strength of DSRAC be further enhanced, but also its overall mechanical properties could be significantly improved. The synergistic effect of hybrid fibers could disperse stress more effectively, enhancing the toughness and ductility of the material. This was of great significance for the structural safety and durability in practical engineering applications.

Figure 13 shows the typical failure modes of fiber-reinforced DSRAC in the splitting tensile test. As can be seen from the figure, the specimens reinforced with steel fibers (Figure 13a) mostly exhibited a failure mode of being clearly split into two halves. The fracture surface was relatively neat, reflecting the characteristic of rapid crack propagation under high tensile stress. Relying on their high tensile strength and good bonding performance with the matrix, steel fibers played a bridging and crack-arresting role to a certain extent, but failed to completely prevent the penetration of macrocracks.

In contrast, the specimens with FERRO fibers added (Figure 13b,c) did not undergo complete splitting. Their failure mode was characterized by a uniformly distributed and fine microcrack network. The width of the main crack was small, and its development was significantly inhibited. This indicates that FERRO fibers can more effectively disperse local stress, delay the generation and propagation of cracks, thus significantly improving the toughness and ductility of concrete and making the failure process slower and more controllable.

3.3. Flexural Strength

The flexural strength of desert sand recycled aggregate concrete (DSRAC) with different fiber dosages was tested through four-point bending tests. The test results are shown in

Table 9. Flexural strength test results of fiber-reinforced DSRAC.

Sample	Maximum Load (kN)	Flexural Strength (MPa)	Average Flexural Strength (MPa)	Standard Deviations
FS0-1	13.94	4.2	4.1	0.1
FS0-2	13.95	4.2
FS0-3	13.50	4.1
S0.75-1	15.88	4.8	4.5	0.3
S0.75-2	14.87	4.5
S0.75-3	14.29	4.3
S1-1	18.02	5.4	5.5	0.1
S1-2	17.91	5.4
S1-3	18.71	5.6
F0.3-1	19.26	5.8	5.3	0.4
F0.3-2	17.21	5.2
F0.3-3	16.23	4.9
F0.15-1	18.45	5.5	5.6	0.5
F0.15-2	19.92	6.0
F0.15-3	17.65	5.3
F0.2-S0.5-1	18.08	5.4	5.6	0.5
F0.2-S0.5-2	20.74	6.2
F0.2-S0.5-3	17.36	5.2
F0.15-S0.5-1	17.97	7.2	6.9	0.5
F0.15-S0.5-2	18.01	7.2
F0.15-S0.5-3	15.96	6.4
F0.3-S0.5-1	18.07	5.4	5.7	0.3
F0.3-S0.5-2	19.89	6.0
F0.3-S0.5-3	19.06	5.7
F0.3-S0.75-1	14.57	5.8	5.5	0.5
F0.3-S0.75-2	12.25	4.9
F0.3-S0.75-3	14.27	5.7

, and the comparison of the average flexural strength is presented in Figure 14. The average flexural strength of the reference group (FS0) without fiber addition was 4.1 MPa.

The addition of fibers significantly enhanced the bending performance of DSRAC. When the volume fraction of steel fibers rose from 0.75% (S0.75) to 1.0% (S1), the average flexural strength increased from 4.5 MPa to 5.5 MPa, registering an increase of 34.1%. This indicated that increasing the dosage of steel fibers could effectively augment the flexural-tensile bearing capacity of concrete. FERRO fibers likewise exhibited excellent strengthening effects. When the volume fraction was 0.15% (F0.15), the average flexural strength attained 5.6 MPa, representing a 36.6% increment compared to the reference group. This demonstrated its potential to enhance the bending toughness of the material even at a limited dosage. Notably, the hybrid fiber group (such as F0.15-S0.5) exhibited optimal flexural performance. The average strength reached 6.9 MPa, showing a significant increase of 68.3% compared to the reference group. This result highlighted the positive hybrid effect generated by different fibers when resisting bending loads, indicating that the fiber combination could more comprehensively improve the bending properties of DSRAC.

Figure 15 presents the typical failure modes of fiber-reinforced DSRAC in the flexural test. As can be seen from the figure, the specimens reinforced with steel fibers (Figure 15a) did not undergo complete brittle fracture. They demonstrated certain crack-control capabilities and ductile characteristics, indicating that steel fibers, through the bridging effect, restricted the crack propagation to a certain extent and delayed the ultimate failure of the specimens.

The specimens reinforced with FERRO fibers (Figure 15b) exhibited a more uniform and finer crack distribution pattern. The main crack had a smaller width, and there were many branched cracks, reflecting that this type of fiber had excellent stress-dispersion capabilities. It could effectively suppress the concentrated propagation of cracks and significantly improve the toughness and deformation properties of concrete.

The specimens reinforced with hybrid fibers (Figure 15c) had a more uniform crack distribution and better integrity retention, reflecting the synergistic strengthening effect of steel fibers and FERRO fibers in suppressing crack propagation. This failure characteristic indicated that hybrid fibers could more comprehensively improve the flexural performance and failure mode of concrete.

It was acknowledged that the hybrid fiber system in this study exhibited optimal comprehensive performance. The F0.15-S0.5 group achieved a 68% flexural strength improvement compared to the control group. This was consistent with the well-established synergistic mechanism of combining stiff (steel) and ductile (synthetic) fibers. Steel fibers resisted early crack propagation, and FERRO fibers enhanced post-crack energy absorption. Regarding the practical considerations of fiber volume content, the following was noted: for groups with a total fiber volume ≤1% (e.g., F0.15-S0.5 at 0.65% total volume), observations during mixing and casting showed that the fresh DSRAC mixture had good flowability and cohesion. It filled molds uniformly and eliminated air bubbles via standard vibration (50 Hz ± 3 Hz, 30 s) without additional process adjustments. For the group with fiber content exceeding 1% (S1.0-F0.3 at 1.3% total volume), the mixture exhibited increased viscosity. It required slightly extended vibration time to ensure full compaction, though it remained workable for construction. In terms of cost, the F0.15-S0.5 group only incurred a minimal incremental fiber cost relative to the total DSRAC material cost, achieving a balance between performance enhancement and economic feasibility. The >1% fiber group, however, saw a notable rise in fiber-related expenses. It was more suitable for high-performance scenarios (e.g., earthquake-resistant components) rather than for general engineering. Thus, the F0.15-S0.5 hybrid fiber ratio (0.65% total volume) was recommended as a performance and cost-efficient option for widespread application.

3.4. Compressive Stress–Strain Curve

Scholars from different countries have achieved numerous research outcomes regarding the curve of the uniaxial compressive stress–strain relationship of concrete. Among these, there were models proposed by Hognestad, Guo, Saenz, Popovics, Hajime, etc. [36,37,38,39,40,41,42]. Among them, the constitutive model proposed by Guo [37] has been widely applied.

For the stress–strain relationship of hybrid fiber-reinforced DSRAC, this descriptive approach had offered a high level of accuracy. It could more precisely reflect the actual behavior of the material under different conditions, thereby providing a more reliable foundation for evaluating and predicting the performance of hybrid fiber-reinforced DSRAC. The specific model was presented in Equation (3).

$$y=\left\{\begin{array}{c}ax+(3-2a)x^2+(a-2)x^3,0\le x\le 1\\ {\displaystyle \frac{x}{b{(x-1)}^2+x}},x\ge 1\end{array}\right.$$

(3)

where, in, $x=\epsilon /{\epsilon }_c,y=\sigma /{\sigma }_c$, ${\epsilon }_c$ represents the peak strain, and ${\sigma }_c$ represents the peak stress.

Figure 16 depicts the constitutive model curves of hybrid fiber-reinforced desert sand recycled concrete, and

Table 10. Constitutive model parameters of hybrid fiber-reinforced DSRAC.

Sample	a	R12	b	R22
FS0	2.333	0.9995	7.6638	0.9912
S0.75	4.279	0.9997	3.5537	0.9924
S1	2.5118	0.9998	2.0865	0.9941
F0.15	6.2792	0.9982	3.6971	0.9993
F0.3	2.5878	0.9975	2.5645	0.9887
F0.15-S0.5	2.0654	0.9996	2.2028	0.9997
F0.2-S0.5	0.2372	0.9994	4.3748	0.9912
F0.3-S0.5	2.7612	0.9996	2.4284	0.9987
F0.3-S0.75	6.3208	0.9997	2.8867	0.9986

lists the relevant parameters. As could be deduced from the figures and tables, the fitting determination coefficients R₁² and R₂² of all fiber-reinforced groups had exceeded 0.99 (Table 10). This indicated that the model could accurately describe the plastic behavior and damage evolution process of hybrid fiber-reinforced DSRAC under compressive loading conditions. In comparison with ordinary concrete, the incorporation of fibers had significantly altered the profile of the descending branch of the stress–strain curve. The curve of the reference group (FS0) had exhibited a relatively steep decline, demonstrating certain brittle characteristics. In contrast, for the fiber-reinforced groups, particularly for the hybrid fiber groups (such as F0.15-S0.5), the descending segment of the curve had become notably gentler. This implied that the bridging effect of fibers had effectively retarded the stress degradation process after the failure of concrete, thereby enhancing the material’s toughness and energy absorption capacity. Regarding the influence of fiber types, both steel fibers and FERRO fibers could enhance the ductility of concrete, yet their underlying mechanisms differed. Steel fibers primarily restricted crack propagation by physically bridging macroscopic cracks, while FERRO fibers delayed damage accumulation by more uniformly dispersing microscopic cracks. The hybrid fiber groups (such as F0.3-S0.5, F0.15-S0.5, etc.) had displayed a more favorable elastic–plastic stage and a stable damage development stage in terms of the curve profile. This suggested that the two types of fibers had possessed a synergistic strengthening effect in suppressing cracks at different scales.

4. Microscopic Research

Figure 17 presents scanning electron microscope (SEM) images of fiber-reinforced DSRAC. From these qualitative images, a visible trend could be observed: under load, FERRO fibers showed apparent elongation and bending deformation. This visual phenomenon suggested the potential formation of an effective bond between FERRO fibers and the concrete matrix, which might have enabled the fibers to participate in stress transfer and crack control processes. At the microscope scale, the observed deformation behavior of the fibers provided a preliminary insight into their potential bridging role: when microcracks were initiated within the concrete, the fibers seemed to span both sides of the cracks, and their deformation trend implied a possible ability to resist further crack propagation via interfacial interactions. Quantitative validation (e.g., via image analysis software for void counting or X-ray-computed tomography for 3D crack mapping) will be conducted in future studies to further confirm these trends.

5. Conclusions

Based on the foregoing macroscopic mechanical tests, microscopic structure characterizations, and constitutive model investigations conducted on hybrid fiber-reinforced desert sand recycled concrete, this study systematically revealed the mechanism underlying the enhancement of its mechanical properties and the failure patterns. The main conclusions were as follows.

(1) Partially replacing natural sand with desert sand could improve the mechanical properties of recycled concrete. When the replacement rate was 30%, the compressive strength was the highest, which was 4.3% higher than that of the reference group, and it exhibited an obvious early strength effect.

(2) The incorporation of both steel fibers and FERRO fibers could significantly enhance the compressive, splitting tensile, and flexural strengths of desert sand recycled concrete. Moreover, as the fiber content increased, the strengthening effect became more pronounced.

(3) The hybrid fiber group (such as F0.15-S0.5) demonstrated optimal comprehensive mechanical properties. The compressive strength, splitting tensile strength, and flexural strength were increased by 31.6%, 48.1%, and 68.3%, respectively, compared to the group without fiber addition, confirming the synergistic strengthening effect of steel fibers and FERRO fibers at both macroscopic and mesoscopic scales.

(4) Microstructural analysis indicated that fibers effectively inhibited crack propagation through the bridging effect. FERRO fibers were more conducive to the uniform distribution of stress, while steel fibers were more suitable for macroscopic crack control. The combination of hybrid fibers further optimized the interfacial properties and the damage evolution process.

(5) The established constitutive model could well describe the stress–strain relationship of hybrid fiber-reinforced DSRAC, with a high fitting accuracy (R² > 0.9), providing a reliable theoretical model for its engineering applications.