Keywords

1 Introduction

Concrete, due to its high compressive strength, economic benefits, and the wide availability of raw materials, has become an indispensable structural material in various construction projects. Despite its many advantages [1], concrete is a non-homogeneous, long-range disordered, brittle composite material with poor tensile strength, low toughness, and a propensity to crack. This can lead to structural damage or even failure, severely impacting the normal use and service life of the project. Therefore, exploring ways to improve concrete's crack resistance is an urgent issue for engineering technicians. In response to the heterogeneous brittle characteristics of concrete [2], this paper attempts to enhance its crack resistance by utilizing waste rubber powder, fly ash, and ecological fibers [3].

A massive amount of waste rubber originates from the rapid development of the automotive industry. According to relevant data, there are currently 4 billion waste tires globally, with an annual increase of 1.1 billion. As a significant consumer of rubber, China generated 400 million waste tires in 2023 alone. Simultaneously [5], the amount of waste rubber products and scraps from other rubber industries has been steadily increasing [4]. These industrial wastes not only occupy a considerable amount of limited land but also pose pollution problems related to natural resources, energy, and the environment. Therefore, finding effective methods for the treatment and recycling of waste rubber and fly ash has become a focus in the engineering field, holding significant importance in today's era of increasingly scarce energy and resources [7].

The waste rubber powder selected in this study mainly comes from scrapped automobile tires and waste rubber products produced by the rubber industry, as well as large quantities of leftover scraps from mass production. These waste rubbers can be mechanically crushed or ground into fine particles, becoming granules and powder [8].

2 Experimental Materials and Methods

2.1 Raw Materials

(1) Cement: Saima Sign Cement Co., Ltd. in Ningxia produces P.052.5 MPa ordinary Portland cement. The cement mortar strength is tested according to the Test Method of Cement Mortar Strength (ISO method) (GB/T17671-2020). Its physical and mechanical properties, chemical composition, mineral composition, and single mineral chemical composition, as well as hydration heat detection indicators, are shown in Tables 1, 2, 3, and 4.

Table 1. Physical property test index of ordinary Portland cement by Ningxia Saima Sign P. 052.5 MPa
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Table 2. Chemical composition (by mass) of cement by Ningxia Sign P.052.5 MPa
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Table 3. Mineral composition and hydration heat test results of cement by Ningxia Saima Sign P.052.5 MPa
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Table 4. Chemical composition and mineral composition of single ore of cement by Ningxia Saima Sign P.052.5 MPa
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  1. (2)

    Fly ash: The fly ash is derived from the Grade I fly ash produced by the Lingwu Power Plant in Ningxia. Its chemical composition is shown in Table 5, and particle size distribution indicators are tested as shown in Table 6, meeting national standards.

    Table 5. Chemical composition (by mass) of Fly ash
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    Table 6. Particle size distribution of fly-ash
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  2. (3)

    Fine and coarse aggregates: Aggregates account for 70%-80% of the total volume used in concrete, with 5–150 mm as coarse aggregates and 0.16–5 mm as fine aggregates.

  1. 1)

    Physical property testing and grading analysis of fine aggregates:

Fine aggregates are natural sand from Zhenbeibao in Ningxia. The apparent density, bulk density, and dust content are evaluated, as shown in Table 7. The results of the sieve analysis of fine aggregates are shown in Table 8, and the grading curve is illustrated in Fig. 1.

Table 7. Physical properties test results of fine aggregate
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The fineness modulus of sand is calculated using the following formula:

\({\mu }_{1}= \frac{({A}_{2}+{A}_{3}+{A}_{4}+{A}_{5}+{A}_{6}) - 5{A}_{1}}{100-{A}_{1}}= 2.88\in \) (3.0 ~ 2.3) is medium sand.

Table 8. Test results of fine aggregate sieve analysis
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Fig. 1.
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Gradation Curve of Fine Aggregate

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  1. 2)

    Physical property testing and results of coarse aggregates:

Coarse aggregates play a vital role in enhancing concrete strength and preventing shrinkage. The aggregates used are artificially crushed stones with rough surfaces and good bonding properties with cement, rich in angular shapes, with particle sizes of 525 mm and 4.7516 mm. These are tested according to the standards for Pebble and Crushed Stone for Construction (GB/T14685-2001), with results shown in Table 9. Upon testing, both fine and coarse aggregates meet the national standard specifications.

Table 9. Physical properties test results of coarse aggregate
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  1. (1)

    Mixing and curing water: Tap water is used, and its quality meets the standards required for concrete mixing water.

  2. (2)

    Additives: FDN naphthalene-based water reducer produced by Beijing Muhu Additives Co., Ltd. is used, achieving a water reduction rate of 25%-30%, with a dosage of 0.3%-0.5% of the cementitious material.

  3. (3)

    Ecological fiber: UltraFiber 500, a product from Shanghai Luoyang New Material Technology Co., Ltd., is used with a dosage of 0.6–1.2%. Performance indicators are shown in Table 10, and the actual product is illustrated in Fig. 3.

  4. (4)

    Rubber powder: Waste rubber powder is produced from waste rubber products through mechanical processes and then processed into powders of various fineness according to different uses. Particles smaller than 1.5 mm are rubber powder, and 8–20 mesh are granules, used for underlays, lawns, road base layers, tracks, elastic layers for roads, and sports field paving; 30–40 mesh is coarse rubber powder, used for producing activated rubber powder, reclaimed rubber, paving, and rubber boards; 40–60 mesh is fine rubber powder, used for plastic modification, rubber product production; 60–80 mesh is fine rubber powder, applied in car tires, rubber products, and building materials; 80–120 mesh is microfine rubber powder, used in rubber products, military products; 200–500 mesh is ultrafine rubber powder, used for high polymer roll material modification. In this experiment, 60 mesh rubber powder produced by Henan Jiaozuo Hongrui Rubber Co., Ltd. is used, with a bulk density of 0.375 g/cm3 and a density of 1.22 g/cm3. The actual product is shown in Fig. 2.

Table 10. Performance index (UltraFiber 500) of Bokai ecological fiber
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Fig. 2.
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60-mesh waste rubber powder

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Fig. 3.
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BoKai 500 ecological fiber produced in Shanghai

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2.2 Experimental Methods and Mix Design

The experimental design utilizes an L18 (35) orthogonal array, with the concrete designed for a strength class of C30. The strength and forming tests are conducted according to the Standard for Test Method of Concrete Physical and Mechanical Properties (GB/T 50081–2002) GB/T1767. A 10 × 10 × 10 cm3 model is chosen. Five factors at three levels, namely water-cement ratio, fly ash, waste rubber powder, sand ratio, and ecological fibers, are identified as the influential factors in the experimental design, with the orthogonal experiment factor level table shown in Table 11.

Following the specifications of the Standard for Test Method of Concrete Physical and Mechanical Properties (GB/T 50081–2002) GB/T1767, the orthogonal experiment plan is outlined in Table 12. The total volume of binder (C + F) in the concrete is set at 450 kg/m3, with fly ash replacing an equivalent amount of cement. Composite ecological fibers are mixed in as a percentage of the volume of the binder materials. The designed slump for the concrete is 30–50 mm. Specimens are molded and maintained in the initial setting with the mold for 24 h, demolded, and then cured under standard conditions until the age of testing [9]. The test results and comparisons are presented in Table 13.

Table 11. Orthogonal test table of factor levels L18 (35)
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Table 12. Design table of orthogonal scheme
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Table 13. Analysis of orthogonal test results and comparison of brittleness coefficient
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3 Experimental Results and Analysis

  1. (1)

    Orthogonal experiment results and brittle coefficient analysis

Comparing results from Table 13, the baseline concrete has the highest brittle coefficient of 9.85, which is 37.8% higher than the average brittle coefficient of 7.16 in the modified mixtures. The inclusion of rubber powder, fly ash, and fibers does not significantly affect the compressive strength or may slightly decrease it. However, the tensile strength is significantly increased, and the brittle coefficient is considerably reduced, as shown in tests 3, 7, 8, 10, 15, 16, 17, and 18, where the optimal crack resistance is achieved with 1.5% - 4.5% rubber powder and 0.9% -1.2% fiber content. This indicates that rubber powder, fly ash, and fibers can enhance the toughness of concrete and play a vital role in crack resistance.

  1. (2)

    Analysis of the effect of mineral components on the brittle coefficient

The mineral components of the binder materials significantly impact the toughness of the concrete. In cement, the mineral components C2S and C4AF reduce the heat of hydration and drying shrinkage. C4AF also functions to reduce brittleness and improve toughness. In Table 14, when C2S at 14% and C4AF at 2%, and C2S at 17% and C4AF at 2% are added (groups JC11 and JC12), the 7-day brittle coefficients are respectively 32.5% and 15.3% lower than the baseline concrete, and 28-day brittle coefficients are 10% and 8% lower. A reduction in the brittle coefficient indicates an increase in toughness.

  1. (3)

    Analysis of performance test results from baseline and optimized composites

Results from Tables 15 and 16 indicate that the 7-day and 28-day brittle coefficients of baseline concrete (JZH) are higher than those of the composite groups JC5, JC9, JC10, and JC11. With the addition of single minerals, rubber powder, fly ash, and fibers, the brittle coefficient shows a decreasing trend. Performance data from Table 16 reveals that splitting tensile strength and columnar tensile strength are on the rise, while the modulus of elasticity gradually decreases, and the tensile modulus shows an increase. The mechanical performance tests indicate that rubber powder, fly ash, and fibers can enhance the crack resistance of concrete, playing a significant role in extending the service life of engineering projects.

Table 14. Effect of mineral composition on brittleness coefficient
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Table 15. Strength test results of optimized composite mortar
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Table 16. Test results of partial concrete performance after composite of reference and optimization
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4 Microscopic Testing and Analysis

  1. (1)

    Comparative analysis of Figs. 4 and 5 shows that, after failure under compression, the baseline concrete specimens exhibit significant damage (as shown in Fig. 4). In contrast, concrete specimens with a blend of a single mineral, rubber powder, and fibers maintain their integrity and do not disintegrate upon failure (as shown in Fig. 5), demonstrating entirely different failure morphologies.

  2. (2)

    The concrete with composite binder materials, after 28 days of curing, shows continuous strength growth. Dense crystals and binder reaction products gradually form around the fly ash spheres, as illustrated in Figs. 6(a), (b), (c), and (d). Some fly ash spheres remain exposed, and as curing time progresses, reactions on the surfaces of the particles continue, producing hydration products of varying sizes and increasingly complete polymer encapsulation layers. This demonstrates the micro-level grading and microfiller effects of polymers. Therefore, the inclusion of multiple components is beneficial for both the strength and crack resistance of concrete, consistent with macroscopic test results.

  3. (3)

    Figure 7 presents concrete with a mix of single minerals, rubber powder, fibers, and fly ash. The concrete shows low early strength but significant gains in the later stages, particularly when the hydration products of fibers, rubber powder, and single mineral combine to form a dense structure, significantly reducing porosity. This indicates that the combined addition of these materials has a synergistic effect. The chaotic nature of the fibers helps prevent the propagation of cracks, and the elastic properties of rubber powder, the micropowder effect of fly ash spheres, and the toughness of single mineral C4AF all contribute to the composite effect. Coupled with the testing and calculations of the mechanical properties of the concrete, these findings confirm that the combined performance meets the expected targets for crack resistance.

Fig. 4.
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Baseline concrete failure mode

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Fig. 5.
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Single mineral rubber powder fiber concrete failure mode

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Fig. 6.
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SEM characteristics of standard curing for 28 days Baseline Concrete

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Fig. 7.
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SEM condition of optimized composite concrete after 28 days

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5 Mechanism Analysis

  1. (1)

    Benefits of fly ash micropowder micro-grading morphological characteristics

Incorporating an appropriate amount of fly ash in concrete can effectively enhance the workability of the concrete mix. Replacing cement with fly ash not only reduces cement usage and cost but also decreases the heat of hydration in concrete, which is beneficial for large-volume concrete structures. Fly ash particles are molten aluminosilicate spherical vitreous bodies that are dense and elastic with a smooth surface, which favorably impacts concrete's crack resistance. Fly ash contains large amounts of SiO2 and Al2O3, which react with the hydration product Ca(OH)2 of cement, forming C-S-H and C-A-H gel materials. These materials gradually transform into fibrous crystals over time, increasing in number and interlocking to form a chained structure. This fills the voids in the mixture, playing the role of microfiller grading and micro-framework, positively influencing the strength enhancement and significantly contributing to the later strength of concrete, making the structure of hardened concrete denser [10].

(2) Elastic effects of micro-spring bodies in rubber powder.

  1. 1)

    Experimental results indicate that the strength of rubber powder concrete is slightly reduced compared to the baseline concrete. Rubber powder, being an elastic organic polymer material, has relatively weak bonding with cement paste. This reduces the interfacial bond strength, increasing the number of weak points within the concrete and thus reducing its compressive strength. However, the high elasticity of rubber complements the brittleness of concrete, enhancing its ductility and effectively reducing brittleness and the onset of cracking.

  2. 2)

    Rubber powder acts as micro-spring bodies distributed within the concrete. External forces cause the cement matrix around the rubber particles to crack due to stress concentration. Rubber itself has excellent tensile properties that hinder the propagation of cracks, thereby maintaining the integrity of the specimens by preventing cracks caused by compression from becoming continuous.

  3. 3)

    There is an optimal content of rubber powder for enhancing crack resistance, as shown in the orthogonal test results, where the best crack resistance effects are evident at rubber powder content ranges between 1.5% and 4%. In comparative experiments, the strength of concrete with optimal rubber content decreases less compared to the baseline concrete. As part of the aggregate mix, rubber powder optimizes aggregate grading and fills voids within the concrete.

  4. 4)

    The combined effect of rubber powder and fly ash, along with coarse and fine aggregates and cement, where each component's average particle size exists on different scales, is advantageous. This composite arrangement allows for complementarity: voids in coarse aggregates are filled by fine aggregates and rubber powder, voids in fine aggregates by cement particles, and cement particle voids by the effect of fly ash, optimizing micro-aggregate grading, hindering crack propagation, enhancing concrete's toughness, reducing brittleness, and improving the density and crack resistance of the structure, with a combined effect superior to single additions [6].

  1. (3)

    Crack resistance effect of composite ecological fibers’ random distribution.

From Fig. 7, it can be observed that the hydration particles are abundant, and the structure is relatively dense. Due to the reaction of SiO2 in fly ash with Ca(OH)2, the edges become irregular, and the formation of CSH not only fills the internal voids of the concrete but also intersects with the boundary fibers. In the concrete interface transition zone, the Ca(OH)2 crystals are smaller, porosity is reduced, and the CSH gel forms a dense network structure. The small diameter of fibers and their random orientation within the concrete allow them to bear some of the tensile stress caused by shrinkage in the cement stone, alleviating stress concentration at the tips of micro-cracks and preventing the formation and propagation of micro-cracks. They cross each other, forming a more complex multidimensional random distribution structure within the concrete, making the microstructure of the concrete dense, with few pores and micro-cracks, aligning with macroscopic test resulst [11]. When the fiber content reaches the optimal range of 0.6–0.9%, its resistance to columnar indices RL282.97 MPa and split tensile modulus E reach maximum values of 38.88 GPa.

6 Conclusion

  1. (1)

    The combined use of multiple additives exhibits various synergistic effects. Rubber powder, single minerals, fibers, fly ash, coarse and fine aggregates, and cement have average particle sizes at several different scales. Their combined use facilitates complementary benefits, optimizing the functions and effects of each material. This arrangement restricts crack propagation, enhances the toughness of the concrete, reduces brittleness, and increases the density of the structure, which is beneficial for improving crack resistance. The overall effect is superior to using single additives.

  2. (2)

    Concrete with only rubber powder as an additive has a lower compressive strength compared to the baseline concrete, indicating that rubber powder has an adverse effect on the growth of concrete's compressive strength. However, the rubber itself possesses excellent tensile properties that can hinder the progression of cracks, preventing the cracks caused by compression from penetrating through, thus maintaining the integrity of the concrete specimens.

  3. (3)

    Concrete with a composite mixture of rubber powder, fibers, single minerals, and fly ash demonstrates superior overall performance. Experimental results and calculated data on various crack resistance indices show that the performance of the composite mixture surpasses that of single additives and achieves the anticipated expectations.