Method for optimizing rubber particle gradation in rubberized cement stabilized macadam mixture

Abstract

The present application relates to a kind of rubber cement stabilized macadam mixtures rubber particle gradation optimization method, comprising the following steps: step 1, accurately control the gradation of aggregate;Step 2, determine rubber uniform dosage and cement uniform dosage;Step 3, select six positions of the largest size in particle size size below 9.5mm as replacement position, corresponding to the rubber particles of the same position are made;Step 4, for the aggregate position above particle size 2.36mm, the same volume of rubber particles of the same position is replaced, for the aggregate position below particle size 2.36, the rubber particle gradation is replaced using down-grade half method, obtain six groups of rubber particle gradation scheme;Step 5, prepare ordinary cement stabilized macadam test piece and six kinds of rubber cement stabilized macadam test piece under gradation scheme;Step 6, compare to obtain the best gradation scheme;The present application can realize the determination of the best rubber particle replacement scheme, help to promote the research of rubber particle mixed in ordinary cement stabilized macadam.

Description

Technical Field

[0001] This invention belongs to the field of cement-stabilized crushed stone technology, specifically relating to an optimization method for the gradation of rubber particles in rubber cement-stabilized crushed stone mixtures. Background Technology

[0002] Cement-stabilized crushed stone is the most commonly used inorganic binder stabilizing material in the construction of semi-rigid base courses in my country. Using cement-stabilized crushed stone as a base course material can effectively reduce the tensile stress at the bottom of the asphalt surface layer and significantly reduce the compressive stress at the top of the subgrade, thus ensuring road stability. However, the excessive stiffness of the semi-rigid base course also brings a series of serious consequences. On the one hand, there is a significant modulus difference between the cement-stabilized crushed stone base course and the asphalt surface layer. The higher modulus of the base course increases the shear stress within the asphalt surface layer, making it prone to rutting. Furthermore, the large modulus difference between the semi-rigid base course and the subgrade leads to uneven deformation of adjacent structural layers. This situation severely impairs the load-bearing capacity of the cement-stabilized crushed stone base course, causing stress concentration at the bottom of the base course. Under long-term vehicle loads, fatigue cracks are easily formed, ultimately leading to reflective cracks in the pavement, thus greatly reducing road durability.

[0003] One effective way to improve the toughness of cement-based materials is through modification with rubber particles. Waste rubber tires, as environmental pollutants, have good elasticity and low stiffness modulus. Applying rubber particles to cement-based materials can significantly improve their toughness, combining performance improvement with environmental benefits.

[0004] Currently, some researchers are attempting to introduce rubber particles into cement-stabilized crushed stone to improve the insufficient long-term service performance of traditional semi-rigid base courses due to excessive stiffness. For example, Sun et al. studied the mechanical properties of rubber-stabilized cement-stabilized crushed stone with three different particle sizes (40 mesh, 60 mesh, and 80 mesh) and three dosages (1.0%, 1.5%, and 2.0%). According to the results of unconfined compressive strength tests at 7d, 28d, and 90d, the addition of rubber powder has a negative impact on the early compressive strength of cement-stabilized crushed stone: under the same rubber particle size, the compressive strength of cement-stabilized crushed stone decreases significantly with increasing rubber powder dosage. For instance, Chinese patent CN111138143X uses waste rubber powder to replace part of the stone salt, with a rubber powder particle size of 40-60 mesh (i.e., 0.25-0.425 mm) and a waste rubber powder content of 2%-12%.

[0005] Current research indicates that the technology of using rubber powder as a base layer for cement-stabilized crushed stone is still in its early stages. While there are studies on the effects of rubber particle size and dosage on the performance of cement-stabilized crushed stone, existing research mostly focuses on the amount of rubber powder added as a single variable to examine its impact on the performance of rubber-stabilized cement-stabilized crushed stone. However, the dosage, particle size, gradation, density, mechanical properties, and blending method of rubber particles all have a significant impact on the performance of rubber-stabilized cement-stabilized crushed stone. Traditional mineral aggregate mix design methods typically characterize the gradation as a mass ratio. For rubber-stabilized cement-stabilized crushed stone, due to the significant difference between rubber density and aggregate density (approximately 2.4 times the density of rubber), replacing aggregate with an equal mass of rubber will cause the mixture to expand in volume, altering the original volumetric gradation and disrupting the internal interlocking structure. However, no researchers have yet investigated the effect of rubber particle size gradation on the performance of rubber-stabilized cement-stabilized crushed stone, and the relationship between the performance of rubber-stabilized crushed stone mixtures and rubber particle size remains unclear.

[0006] Therefore, it is necessary to improve the existing method of adding rubber to cement-stabilized crushed stone raw materials, design the mix proportion of rubber cement-stabilized crushed stone materials, and optimize the gradation scheme of rubber particles. This is of great significance for improving the performance of rubber cement-stabilized crushed stone semi-rigid base materials. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide an optimization method for the gradation of rubber particles in rubber cement stabilized crushed stone mixtures. The "downgrading and halving method" is used to optimize the gradation design of rubber particles in order to explore the relationship between the performance of rubber cement stabilized crushed stone mixtures and the rubber particle size.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A method for optimizing the gradation of rubber particles in a rubber-cement stabilized crushed stone mixture includes the following steps:

[0010] Step 1: Precisely control the gradation of aggregates to obtain the mineral mixture of rubber cement stabilized crushed stone; adopt a continuous gradation scheme to design aggregates, and divide them into 12 grades from high to low particle size within the range of 0-26.5mm.

[0011] Step 2: Based on previous research, determine the critical value at which the strength of rubber cement stabilized crushed stone deteriorates sharply with the increase of rubber content, and select a value X below the critical value as the uniform rubber content; determine the uniform cement content in the rubber cement stabilized crushed stone mixture as B.

[0012] Step 3: Select the six largest particle size ranges below 9.5mm as replacement ranges and make rubber granules of the same range accordingly. The six specifications of rubber granules are numbered R1, R2, R3, R4, R5, and R6 in descending order of particle size.

[0013] Step 4: For aggregate sizes above 2.36mm, replace with rubber particles of the same size and volume. For aggregate sizes below 2.36mm, use the downgrading and halving method to grade the rubber particles, resulting in six rubber particle gradation schemes. The downgrading and halving method involves halving the rubber content of each particle size as the particle size decreases, starting from the 2.36mm size and proceeding downwards, ensuring that the rubber content of the smallest size in the gradation scheme is equal to the content of the size one or two sizes before it. Calculate the rubber content of each particle size in each gradation scheme.

[0014] Step 5: Using six rubber particle gradation schemes with a uniform rubber content X and a uniform cement content B, the maximum dry density and optimum moisture content of the rubber-cement stabilized crushed stone mixture were determined by vibration compaction technology. Ordinary cement stabilized crushed stone specimens and rubber-cement stabilized crushed stone specimens under the six gradation schemes were then prepared based on these results.

[0015] Step 6: Test and compare the performance of ordinary cement-stabilized crushed stone specimens and six types of rubber cement-stabilized crushed stone specimens to obtain the optimal rubber particle gradation scheme.

[0016] Furthermore, a downgrading method of halving is used to supplement the grades by four levels. For the first three levels, the rubber content of the highest grade in the rubber particle gradation scheme is half of the uniform rubber content X, and the rubber content of the lowest grade is half of the uniform rubber content X. N For the fourth grade, the rubber content of the largest grade in the rubber particle gradation scheme is 1 / 2 of the uniform rubber content X, and the rubber content of the smallest grade and the third to last grade is 1 / 8 of the uniform rubber content X.

[0017] Furthermore, the uniform rubber content X is 3%; the uniform cement content B in the rubber cement stabilized crushed stone mixture is 4.5%.

[0018] Furthermore, the 12 aggregate grades are 19-26.5mm, 16-19mm, 13.2-16mm, 9.5-13.2mm, 4.75-9.5mm, 2.36-4.75mm, 1.18-2.36mm, 0.6-1.18mm, 0.3-0.6mm, 0.15-0.3mm, 0.075-0.15mm, and 0-0.075mm, respectively.

[0019] Furthermore, the thicknesses of R1, R2, R3, R4, R5, and R6 are 4.75-9.5mm, 2.36-4.75mm, 1.18-2.36mm, 0.6-1.18mm, 0.3-0.6mm, and 0.15-0.3mm, respectively.

[0020] Furthermore, the six rubber particle gradation schemes are as follows:

[0021] (1) According to the uniform rubber dosage X, use the same volume of rubber particles of grade R1 to replace the aggregate of the same grade.

[0022] (2) According to the uniform rubber dosage X, replace the aggregate of the same grade with the same volume of rubber particles of grade R2.

[0023] (3) Use rubber granules of grade R2 and R3 with the same volume to replace aggregates of the same grade, according to 1 / 2 of the uniform rubber dosage X.

[0024] (4) Replace the aggregate of the same grade with rubber particles of grade R2 by 1 / 2 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber particles of grades R3 and R4 by 1 / 4 of the uniform rubber dosage X.

[0025] (5) Replace the aggregate of the same grade with rubber granules of grade R2 by 1 / 2 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber granules of grade R3 by 1 / 4 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber granules of grades R4 and R5 by 1 / 8 of the uniform rubber dosage X.

[0026] (6) Replace the aggregate of the same grade with rubber particles of grade R2 by 1 / 2 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber particles of grade R3 by 1 / 4 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber particles of grade R4 and R6 by 1 / 8 of the uniform rubber dosage X.

[0027] Furthermore, in step 5, when preparing rubber-cement stabilized crushed stone specimens, firstly, according to the determined mix proportion, weigh the corresponding aggregates, rubber, cement, and water, pour the aggregates and rubber into a vibratory mixer and vibrate and mix for 20 seconds to ensure that the rubber and aggregates are evenly mixed, then add cement and water, and vibrate and mix the mixture at a speed of 50 times / second for 90 seconds. Finally, use a vibratory compactor to prepare rubber-cement stabilized crushed stone specimens. The frequency of the vibratory compactor is set to 30Hz, and the excitation force is adjusted to 7.6kN. After the specimens are formed, demolding is carried out 24 hours later. After demolding, the specimens are transferred to a standard curing room for curing.

[0028] Furthermore, cylindrical specimens were prepared to test the unconfined compressive strength of the mixture at 7 days, 28 days, and before and after freeze-thaw cycles. The height and diameter of the specimens were both 150 mm, and the vibration compaction time for the cylindrical specimens was 90 s. Beam-type specimens were prepared for flexural strength tests, flexural resilience tests, fatigue tests, drying shrinkage tests, and thermal shrinkage tests. The length of the beam-type specimens was 400 mm, the width was 100 mm, and the height was 100 mm. The vibration time for the beam-type specimens was 120 s.

[0029] Furthermore, the uniform rubber content X and the mass of the replaced aggregate M... A They are respectively:

[0030]

[0031]

[0032] In the formula, X is the rubber content; ρ A Aggregate density (kg / m³) 3 );ρ R The density of rubber (kg / m³) 3 M R M represents the mass of rubber (kg); M represents the total mass of aggregates (kg); M A The mass (kg) of the aggregate being replaced.

[0033] Furthermore, the density of the rubber granules is taken as 1100 kg / m³. 3 The aggregate density is 2664 kg / m³ 3 The rubber content X is 3% of the total mass of the aggregate. According to formulas (1) and (2), the amount of aggregate replacement is 7.3% of the total mass of the aggregate.

[0034] The beneficial effects of this invention are:

[0035] The optimization method of this invention, through the exploration of the relationship between the performance of rubber cement stabilized crushed stone mixture and rubber particle size, can determine the optimal rubber particle replacement scheme, which helps to promote the research on the doping of rubber particles in ordinary cement stabilized crushed stone and has guiding significance for the design of rubber cement stabilized crushed stone scheme.

[0036] This invention adds rubber particles based on the principle of equal volume replacement. For aggregates with a particle size range of 2.36mm-9.5mm, rubber particles of the same grade and volume are used for replacement. For aggregates with a particle size below 2.36mm, a "downgrading and halving method" is used to optimize the gradation of the rubber particles, avoiding excessive replacement rates or even over-replacement of small-diameter aggregates. This prevents volume expansion of the cement-stabilized crushed stone mixture and maintains its original volume gradation. The invention uses finely sieved aggregates and rubber particles. By combining various rubber cement-stabilized crushed stone gradation schemes with sample test results, comparisons of rubber particles of different sizes within the rubber cement-stabilized crushed stone can be achieved. Attached Figure Description

[0037] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein:

[0038] Figure 1 Photograph of the rubber particles used in this invention.

[0039] Figure 2 This is the gradation curve of the aggregate in an embodiment of the present invention.

[0040] Figure 3 This is a flowchart illustrating the preparation process of rubber cement stabilized crushed stone specimens according to an embodiment of the present invention.

[0041] Figure 4 The dynamic flexural resilience modulus fitting curve of rubber cement stabilized crushed stone mixture in an embodiment of the present invention is shown. Detailed Implementation

[0042] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art are within the scope of protection of the present invention.

[0043] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0044] Because of the low cement content in cement-stabilized crushed stone, the rubber particles in it are difficult to form a tight bond with the cement paste, thus severely affecting the performance of cement-based materials. Existing research shows that the incorporation of rubber particles exacerbates microcracks and porosity within cement-based materials, significantly impacting their strength and damage processes. The material's gradation is also a key factor influencing mixture performance; the gradation of rubber and aggregate largely determines the performance of rubber-stabilized cement crushed stone. As a special type of aggregate, the influence of rubber particles on mixture performance is mainly reflected in their particle size distribution and blending. Therefore, designing the mix proportions of rubber-stabilized cement crushed stone and optimizing the gradation scheme of rubber particles are crucial for improving the performance of semi-rigid base materials made from rubber-stabilized cement crushed stone.

[0045] This invention optimizes the preparation of a reinforced and toughened rubber-cement stabilized crushed stone base material by replacing aggregates of the same size with rubber particles of different sizes at equal volumes. Based on indoor experiments, it reveals the influence of rubber particles of different sizes on the performance of cement-stabilized crushed stone, thereby optimizing the method of adding rubber particles to reinforced and toughened rubber-cement stabilized crushed stone based on the dispersion state of the rubber particles, and thus optimizing the preparation process of rubber-cement stabilized crushed stone mixture.

[0046] The method for optimizing the gradation of rubber particles in rubber cement stabilized crushed stone mixture of the present invention includes the following steps:

[0047] Step 1: Accurately control the gradation of aggregates to obtain a mineral mixture of rubber cement stabilized crushed stone; design aggregates using a continuous gradation scheme, dividing them into 12 grades from high to low particle size within the range of 0-26.5mm, so that the aggregate gradation coincides with the median gradation value required by the specification (Technical Details for Construction of Highway Pavement Base Course (JTG / TF20-2015));

[0048] Step 2: Based on previous research, determine the critical value at which the strength of rubber cement stabilized crushed stone deteriorates sharply with the increase of rubber content, and select a value X below the critical value as the uniform rubber content; determine the uniform cement content in the rubber cement stabilized crushed stone mixture as B.

[0049] Step 3: Select the six largest particle size grades below 9.5mm as replacement grades, and make rubber granules of the same grade accordingly. The six specifications of rubber granules are numbered R1, R2, R3, R4, R5, and R6 in descending order of particle size.

[0050] Step 4: For aggregate sizes above 2.36mm, replace with rubber particles of the same size and volume. For aggregate sizes below 2.36mm, use the downgrading and halving method for rubber particle gradation, resulting in six rubber particle gradation schemes. The downgrading and halving method involves halving the rubber content of each particle size as the particle size decreases, starting from the 2.36mm size and progressively decreasing, ensuring that the rubber content of the smallest size in the gradation scheme is equal to the content of the size one or two sizes before it. Calculate the rubber content of each particle size in each gradation scheme. The total rubber content of each particle size is the uniform rubber content X.

[0051] Step 5: Using six rubber particle gradation schemes with a uniform rubber content X and a uniform cement content B, the maximum dry density and optimum moisture content of the rubber-cement stabilized crushed stone mixture were determined by vibration compaction technology. Ordinary cement stabilized crushed stone specimens and rubber-cement stabilized crushed stone specimens under the six gradation schemes were then prepared based on these results. By limiting the uniform rubber content X, the uniform cement content B, and the maximum dry density and optimum moisture content, the rubber particle size was ensured to be the only variable in the experiment.

[0052] Step 6: Test and compare the performance of ordinary cement-stabilized crushed stone specimens and six types of rubber cement-stabilized crushed stone specimens to obtain the optimal rubber particle gradation scheme.

[0053] The downgrading and halving method, mathematically speaking, means that when progressively downgrading from the 2.36mm grade, the downgrading increment is N grades. For the first three grades, the rubber content of the largest grade in the rubber particle gradation scheme is half of the uniform rubber content X, and the content of the smallest grade is half of the uniform rubber content X. N For the fourth grade, the rubber content of the largest grade in the rubber particle gradation scheme is 1 / 2 of the uniform rubber content X, and the rubber content of the smallest grade and the third to last grade is 1 / 8 of the uniform rubber content X.

[0054] If only R1 and R2 graded rubber granules have a particle size greater than 2.36mm, then the six rubber granule gradation schemes are as follows:

[0055] (1) According to the uniform rubber dosage X, use the same volume of rubber particles of grade R1 to replace the aggregate of the same grade.

[0056] (2) According to the uniform rubber dosage X, replace the aggregate of the same grade with the same volume of rubber particles of grade R2.

[0057] (3) Use rubber granules of grade R2 and R3 with the same volume to replace aggregates of the same grade, according to 1 / 2 of the uniform rubber dosage X.

[0058] (4) Replace the aggregate of the same grade with rubber particles of grade R2 by 1 / 2 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber particles of grades R3 and R4 by 1 / 4 of the uniform rubber dosage X.

[0059] (5) Replace the aggregate of the same grade with rubber granules of grade R2 by 1 / 2 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber granules of grade R3 by 1 / 4 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber granules of grades R4 and R5 by 1 / 8 of the uniform rubber dosage X.

[0060] (6) Replace the aggregate of the same grade with rubber granules of grade R2 by 1 / 2 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber granules of grade R3 by 1 / 4 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber granules of grades R4 and R6 by 1 / 8 of the uniform rubber dosage X.

[0061] For the last gradation scheme, if the rubber particle gradation is still halved at this point, it means that the content of the two smallest particle sizes of rubber particles is 1 / 16, which is the uniform rubber content of 3% * 0.0625 = 0.1875%. This will result in the rubber particle content being too small. Therefore, a skip-gradation design is adopted, skipping the rubber particles in the R5 grade and using the rubber particles in the R6 grade and the rubber particles in the R4 grade to implement 1 / 8 of the uniform rubber content X, so that the rubber particles in the smallest grade can be kept at 0.375% of the uniform rubber content, so as not to be too small.

[0062] Among them, the uniform rubber content X and the mass of the replaced aggregate M A They are respectively:

[0063]

[0064]

[0065] In the formula, X is the rubber content; ρ A Aggregate density (kg / m³) 3 );ρ R The density of rubber (kg / m³) 3 M R M represents the mass of rubber (kg); M represents the total mass of aggregates (kg); M A The mass (kg) of the aggregate being replaced;

[0066] The density of the rubber granules is 1100 kg / m³. 3 The aggregate density is 2664 kg / m³ 3The rubber content X is 3% of the total mass of the aggregate. According to formulas (1) and (2), the amount of aggregate replacement is 7.3% of the total mass of the aggregate.

[0067] The invention will now be described in detail with reference to specific examples.

[0068] The mix design of rubber cement stabilized crushed stone is based on the traditional cement stabilized crushed stone mix design, but introduces gradation-optimized rubber particles into the cement stabilized crushed stone to prepare a strong and toughened semi-rigid base material that meets engineering requirements. Its raw materials are cement, aggregates, rubber particles, and water.

[0069] Cement is the inorganic binder that acts as a bond in cement-stabilized crushed stone. Through hydration, cement produces a series of hydrates, such as calcium silicate hydrate, which bind the rubber particles and coarse and fine aggregates into a stable, integral material. The cement dosage in cement-stabilized crushed stone is generally 3%-6%, much lower than that in cement concrete, meaning that it is difficult for the rubber particles to form a tight bond with the cement paste. Therefore, the amount of rubber particles should not be too high. The cement used in this embodiment was provided by Zhuzhou Huaxin Cement Co., Ltd., with a grade of 32.5 and a dosage of 4.5% to ensure quality and performance. All physical and mechanical properties of this cement meet the requirements of "Road Silicate Cement" (GB1369-2005).

[0070] This invention uses tap water as the mixing and curing water for rubber cement stabilized crushed stone base material.

[0071] Aggregates include coarse and fine aggregates. Coarse aggregates typically act as a skeleton in the mixture through interlocking and bonding. Compared to cement concrete, which relies on both cement and coarse aggregates for strength, cement-stabilized crushed stone with a lower cement content relies solely on the skeleton structure of the coarse aggregates for strength. This study uses mechanically crushed basalt as coarse aggregate, categorized into three sizes: 4.75-9.5 mm, 9.5-19 mm, and 19-26.5 mm. The coarse aggregate particles in each size range have smooth surfaces and sharp edges, exhibiting good bonding with cement paste. Fine aggregates include natural sand and manufactured sand, primarily serving as fillers in the mixture, filling the voids between coarse aggregates. This embodiment uses manufactured limestone sand as fine aggregate, with a particle size range of 0-4.75 mm. Furthermore, since aggregates smaller than 0.075 mm contain a large amount of soil and dust impurities, S95 mineral powder was selected as the fine aggregate for this size range. In this invention, the mineral aggregate (aggregate) adopts a continuous gradation scheme with a particle size range of 0-26.5 mm. It is divided into 12 grades from highest to lowest particle size: 19-26.5 mm, 16-19 mm, 13.2-16 mm, 9.5-13.2 mm, 4.75-9.5 mm, 2.36-4.75 mm, 1.18-2.36 mm, 0.6-1.18 mm, 0.3-0.6 mm, 0.15-0.3 mm, 0.075-0.15 mm, and 0-0.075 mm. To accurately control the composite gradation of the aggregate, all aggregates are finely screened. The composite gradation of the mineral aggregate coincides with the median gradation value required by the specification (Technical Details for Construction of Highway Pavement Base Course (JTG / TF20-2015)). The resulting design gradation is shown in Table 1, and the gradation curve is shown in Table 2. Figure 2 The proportion of aggregates of each particle size is shown in Table 2.

[0072] Table 1. Mineral gradation table of rubber cement stabilized crushed stone.

[0073]

[0074] As shown in Tables 1 and 2, the proportion of aggregates of different particle sizes in the mineral mixture is uneven, with finer aggregates accounting for a smaller proportion. If single-size rubber particles are used for equal-volume replacement, it can easily lead to an excessive replacement rate or even over-replacement of small-size aggregates, significantly altering the original gradation of the mineral mixture and even causing extreme situations such as discontinuous gradation. For example, if a uniform rubber content of 3% is used for equal-volume replacement of the same size in the cylinder section, then 7.3% of the total aggregate mass of the same size needs to be replaced, as shown in Table 3. When replacing aggregates below 1.18mm, the replacement rate of the current size of the mineral aggregate will be greater than 100%, resulting in excessive or over-replacement. However, the replacement particle size of the rubber and aggregate is not equal, causing a change in the original gradation of the mineral mixture.

[0075] Table 2 shows the proportion of aggregates of each particle size.

[0076] Aggregate size (mm) 19-26.5 16-19 13.2-16 9.5-13.2 4.75-9.5 2.36-4.75 Percentage (%) 16 8 7.5 11 17.5 13.5 Aggregate size (mm) 1.18-2.36 0.6-1.18 0.3-0.6 0.15-0.3 0.075-0.15 0-0.075 Percentage (%) 9 6 4 2.5 1.5 3.5

[0077] Table 3. Proportion of different sizes of aggregates in mineral aggregates

[0078] Aggregate size (mm) Proportion(%) Aggregate replacement rate (%) 4.75-9.5 17.5 7.3 2.36-4.75 13.5 7.3 1.18-2.36 9.0 7.3 0.6-1.18 6.0 7.3 0.6-0.3 4.0 7.3 0.15-0.3 2.5 7.3

[0079] The rubber granules are made from waste tires. The processing of waste tire rubber can be achieved through cryogenic technology or room-temperature mechanical crushing. Cryogenic technology refers to cooling waste tires to -87 to -198°C with liquid nitrogen to overcome the glass transition temperature of the rubber polymer, and then introducing them into a hammer mill with high impact energy for processing. Room-temperature mechanical crushing mainly involves cutting waste tires into smaller rubber granules. In this embodiment, the rubber granules were directly purchased from the 1688 website, from the Lingshou County Yuetai Mineral Products Processing Plant. Six specifications of rubber granules were selected, with particle sizes from largest to smallest: 4.75-9.5mm, 2.36-4.75mm, 1.18-2.36mm, 0.6-1.18mm, 0.3-0.6mm, and 0.15-0.3mm, named R1 to R6 respectively. The performance parameters of the six specifications of rubber granules are shown in Table 4; the performance parameters of the five specifications of rubber granules other than 0.3-0.6mm are shown in Table 4. Figure 1 As shown, Figure 1 In the middle, (a) are 4.75-9.5mm rubber granules, (b) are 2.36-4.75mm rubber granules, (c) are 1.18-2.36mm rubber granules, (d) are 0.6-1.18mm rubber granules, and (e) are 0.15-0.3mm rubber granules.

[0080] Table 4 Basic Performance Indicators of Rubber Granules

[0081] index Test results standard unit density 1.11 1.10-1.13 g.cm-3 Acetone extract content 13.3 ≤15 ％ Moisture 0.96 ≤1.0 ％ Ash 9.2 ≥10 ％ Tensile strength 6.2 ≥4.5 MPa Elongation at break 860 ≥510 ％

[0082] As a special type of aggregate, rubber granules must maintain their volumetric gradation during the introduction of cement-stabilized crushed stone. Therefore, the replacement ratio between rubber and aggregate cannot be simply represented by an equal mass ratio. This invention employs a rubber-aggregate equal-volume replacement method, replacing aggregate with rubber granules of the same volume while ensuring that the replaced aggregate and rubber granules have the same particle size, replacing the aggregate portion in the original gradation with rubber granules. This ensures that the rubber and aggregate involved in the replacement have the same gradation, maintaining the original gradation of the mixture before and after rubber replacement. For smaller grades of rubber granule gradation, this invention uses a "downgrading and halving method" for rubber granule gradation optimization design; the smaller the particle size of the rubber granules, the lower their dosage. This effectively avoids excessive replacement of fine aggregate during aggregate replacement and prevents significant differences in the replacement rate of aggregates with different particle sizes.

[0083] Specifically, R1, R2, R3, R4, R5, and R6 are 4.75-9.5mm, 2.36-4.75mm, 1.18-2.36mm, 0.6-1.18mm, 0.3-0.6mm, and 0.15-0.3mm, respectively. Preliminary experiments revealed that as the size of the rubber replacement particles decreases, the strength of the rubber cement stabilized crushed stone gradually decreases. When replacing the rubber particles with R5 grade (0.3-0.6mm), the strength of the rubber cement stabilized crushed stone also decreases. Therefore, in this embodiment, the rubber particle gradation 5 using the R5 grade only reflects the results of previous experiments. The six rubber particle gradation schemes are as follows:

[0084] Grade 1: According to the uniform rubber content X, replace the aggregate of the same grade with rubber particles of the same size with the same volume of 4.75-9.5mm grade;

[0085] Grading 2: According to the uniform rubber content X, use rubber particles of the 2.36-4.75mm grade to replace aggregates of the same grade with the same volume. Because the amount of aggregates of the 4.75-9.5mm and 2.36-4.75mm grades is sufficient to replace 3% of rubber particles with the same volume, it is not necessary to replace rubber particles of other sizes with rubber particles of the same grade. If rubber particles of the same size are used to replace aggregates of other smaller grades with the same volume, it will cause excessive or excessive replacement of that grade, which will cause changes in the volume gradation of rubber cement stabilized crushed stone.

[0086] Grade 3: According to 1 / 2 of the uniform rubber content X, replace the aggregate of the same grade with the same volume of rubber particles of the 2.36-4.75mm and 1.18-2.36mm grades;

[0087] Grade 4: Replace aggregates of the same grade with rubber granules of the 2.36-4.75mm grade at 1 / 2 of the uniform rubber content X; replace aggregates of the same grade with rubber granules of the 1.18-2.36mm and 0.6-1.18mm grades at 1 / 4 of the uniform rubber content X.

[0088] Grade 5: Replace aggregates of the same grade with rubber granules of the 2.36-4.75mm grade at 1 / 2 the uniform rubber content X; replace aggregates of the same grade with rubber granules of the 1.18-2.36mm grade at 1 / 4 the uniform rubber content X; replace aggregates of the same grade with rubber granules of the 0.6-1.18mm and 0.3-0.6mm grades at 1 / 8 the uniform rubber content X.

[0089] Grade 6: Replace aggregates of the same grade with rubber granules of the 2.36-4.75mm grade at 1 / 2 of the uniform rubber content X; replace aggregates of the same grade with rubber granules of the 1.18-2.36mm grade at 1 / 4 of the uniform rubber content X; replace aggregates of the same grade with rubber granules of the 0.6-1.18mm and 0.15-0.3mm grades at 1 / 8 of the uniform rubber content X.

[0090] Under the optimization method of this invention, two adjacent gradation schemes have a significant overlap in their gradation composition. By comparing the performance of rubber cement stabilized crushed stone between two adjacent schemes, the influence of rubber particles of different sizes outside the overlap on the performance of the mixture can be qualitatively studied. For example, comparing gradation 2 and gradation 3, the overlap between the two schemes is 50% of 2.36-4.75mm rubber particles. It can be considered that these rubber particles play the same role in the mixture. The discussion of the two schemes focuses on the part outside the overlap, that is, the comparison between 50% of 2.36-4.75mm rubber particles and 1.18-2.36mm rubber particles.

[0091] Specifically, to ensure the accuracy of the experimental results, the cement content was uniformly adjusted to 4.5% in this paper. According to the "Test Procedure for Inorganic Binder Stabilized Materials in Highway Engineering" (JTG / E51-2009), vibration compaction technology was used to determine the maximum dry density and optimum moisture content of the mixture. Ordinary cement-stabilized crushed stone is represented by the symbol CSM, and the rubber-cement stabilized crushed stone corresponding to the six rubber particle gradation schemes is represented by the symbols RCSM-G1, RCSM-G2, RCSM-G3, RCSM-G4, RCSM-G5, and RCSM-G6, respectively. The test results are shown in Table 5.

[0092] Table 5 Results of Vibration Compaction Test

[0093] Mixture Optimal moisture content (%) Maximum dry density (g·cm⁻³) CSM 6.0 2.413 RCSM-G1 5.6 2.279 RCSM-G2 5.4 2.298 RCSM-G3 5.3 2.314 RCSM-G4 5.2 2.335 RCSM-G5 5.1 2.358 RCSM-G6 5.0 2.378

[0094] Based on the results of vibration compaction tests, the optimum moisture content and maximum dry density of different types of mixtures were determined, and ordinary cement-stabilized crushed stone specimens and rubber cement-stabilized crushed stone specimens under six gradation schemes were prepared accordingly. The material composition and component ratio of various mixtures are shown in Table 6.

[0095] Table 6. Distribution ratio of each component in the mixture

[0096]

[0097] Due to the significant density difference between rubber and mineral aggregates, rubber particles tend to float to the surface of the sample during mixing. Vibratory mixing ensures that the rubber particles are evenly dispersed in the cement-stabilized crushed stone, guaranteeing a more uniform distribution of the components in the base pavement material and preventing stratification. Therefore, all mixture samples were mixed using a vibratory mixer. To prepare rubber-cement-stabilized crushed stone specimens, the appropriate amounts of aggregate, rubber, cement, and water were weighed according to the determined mix proportions. The aggregate and rubber were poured into the vibratory mixer and vibrated for 20 seconds to ensure uniform mixing. Then, cement and water were added, and the mixture was vibrated at 50 times per second for 90 seconds. Finally, a vibratory compactor was used to prepare the rubber-cement-stabilized crushed stone specimens. The frequency of the vibratory compactor was set to 30 Hz, and the excitation force was adjusted to 7.6 kN. The specimens were demolded 24 hours after molding and then transferred to a standard curing room for further curing.

[0098] Cylindrical specimens were prepared to test the unconfined compressive strength of the mixture at 7 days, 28 days, and before and after freeze-thaw cycles. The specimens were 150 mm high and 150 mm in diameter, and the vibration compaction time for the cylindrical specimens was 90 s. Beam-shaped specimens were prepared for flexural strength tests, flexural resilient modulus tests, fatigue tests, drying shrinkage tests, and thermal shrinkage tests. The beam-shaped specimens were 400 mm long, 100 mm wide, and 100 mm high, and the vibration time for the beam-shaped specimens was 120 s. The preparation process for rubber cement stabilized crushed stone specimens is as follows: Figure 3 As shown.

[0099] The properties of ordinary cement-stabilized crushed stone specimens and rubber cement-stabilized crushed stone specimens under six different gradation schemes are shown below:

[0100] Table 7 Results of the 7-day unconfined compressive strength of the mixture

[0101]

[0102] Table 8 Results of the flexural strength test of the mixture

[0103]

[0104] Table 9 Results of flexural tensile ultimate failure strain test of cement-stabilized crushed stone

[0105]

[0106]

[0107] Table 10 Results of Dynamic Flexural Resilient Modulus of the Mixture

[0108]

[0109] Table 11 Fitting parameters for the fatigue equation of the mixture based on stress ratio

[0110]

[0111] The above results indicate that:

[0112] (1) After the addition of rubber particles, the unconfined compressive strength and flexural strength of rubber cement stabilized crushed stone both decreased. RCSM-G3 showed the best strength retention effect and could meet the strength requirements specified in the code. The flexural tensile ultimate strain of rubber cement stabilized crushed stone was significantly higher than that of ordinary cement stabilized crushed stone, meaning that the addition of rubber particles reduced the stiffness of cement stabilized crushed stone and improved the deformation capacity of the semi-rigid base material. Among all mixtures, RCSM-G3 had the strongest ultimate deformation capacity, which was 1.83 times higher than that of ordinary cement stabilized crushed stone. According to Table 11, the k-value of the CSM fatigue equation was smaller than that of other RCSM fatigue curves, indicating that under the same nominal stress ratio, the fatigue life of rubber cement stabilized crushed stone was higher than that of ordinary cement stabilized crushed stone; and the fatigue resistance of the combined particle size rubber gradation scheme (RCSM-G3, RCSM-G4, RCSM-G6) was slightly better than that of the single particle size gradation scheme (RCSM-G1, RCSM-G2).

[0113] (2) Due to the excessive stiffness of the semi-rigid base course, on the one hand, there is a large modulus difference between the cement-stabilized crushed stone base course and the asphalt surface course. The higher modulus of the base course will increase the shear stress in the asphalt surface course, which is prone to causing rutting. Due to the large modulus difference between the semi-rigid base course and the subgrade, the deformation of adjacent structural layers is uneven. This situation will damage the bearing capacity of the cement-stabilized crushed stone base course, causing stress concentration at the bottom of the base course. Under long-term vehicle load, fatigue cracks are prone to occur, which will eventually lead to reflective cracks in the pavement, thus greatly reducing the durability of the road. Adding rubber particles will reduce the resilient modulus of cement-stabilized crushed stone and effectively improve the toughness of the material. The dynamic and static flexural resilient modulus of RCSM-G3 mixture is reduced by more than half compared with ordinary cement-stabilized crushed stone. Therefore, by changing the gradation of rubber particles, the adjustable and designable modulus of cement-stabilized crushed stone can be achieved within a certain range. According to Table 10, the dynamic flexural resilient modulus fitting curve of the mixture is shown in the figure. Figure 4 As shown, the relationship between the dynamic flexural elastic modulus and gradation variation of cement-stabilized crushed stone can be obtained. During highway design, different rubber gradations are selected according to the modulus requirements of the cement-stabilized crushed stone layer of the pavement structure, and different moduli of the cement-stabilized crushed stone base layer are designed to achieve the coordination of stiffness of each structural layer of the pavement, thereby extending the service life of the highway.

[0114] It should be noted that this optimization method, as a research method for the amount of rubber particles in rubber cement stabilized crushed stone, adopts a relatively high rubber particle replacement rate of 3% in order to clearly explore the impact of rubber particle doping gradation on the performance of rubber cement stabilized crushed stone. However, in actual construction, due to problems such as segregation, the construction quality is not easy to control, and a smaller replacement rate is generally used, usually around 1%. It is understood that the above description is only exemplary, and the embodiments of this application do not limit it. The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are within the scope of protection of the pending claims of the present invention.

Claims

1. A method for optimizing the gradation of rubber particles in a rubber-cement stabilized crushed stone mixture, characterized in that, Includes the following steps: Step 1: Precisely control the gradation of aggregates to obtain the mineral mixture of rubber cement stabilized crushed stone; adopt a continuous gradation scheme to design aggregates, and divide them into 12 grades from high to low particle size within the range of 0-26.5mm. Step 2: Determine the critical value at which the strength of rubber cement stabilized crushed stone deteriorates sharply with the increase of rubber content, and select a value X below the critical value as the uniform rubber content; determine the uniform cement content in the rubber cement stabilized crushed stone mixture as B. Step 3: Select the six largest particle size ranges below 9.5mm as replacement ranges, and produce rubber granules of the same range accordingly. The six specifications of rubber granules are numbered R1, R2, R3, R4, R5, and R6 in descending order of particle size; R1, R2, R3, R4, R5, and R6 are 4.75-9.5mm, 2.36-4.75mm, 1.18-2.36mm, 0.6-1.18mm, 0.3-0.6mm, and 0.15-0.3mm, respectively. Step 4: For aggregate sizes above 2.36mm, replace with rubber particles of the same size and volume. For aggregate sizes below 2.36mm, use a downgrading and halving method for rubber particle gradation. For the first three sizes, the rubber content of the largest size in the rubber particle gradation scheme is 1 / 2 of the uniform rubber content X, and the rubber content of the smallest size is 1 / 2 of the uniform rubber content X. N For the fourth grade, the rubber content of the largest grade in the rubber particle gradation scheme is 1 / 2 of the uniform rubber content X, and the rubber content of the smallest grade and the third to last grade is 1 / 8 of the uniform rubber content X, resulting in six sets of rubber particle gradation schemes; calculate the rubber content of each particle size in each grade gradation scheme. Step 5: Using six rubber particle gradation schemes with a uniform rubber content X and a uniform cement content B, the maximum dry density and optimum moisture content of the rubber-cement stabilized crushed stone mixture were determined by vibration compaction technology. Ordinary cement stabilized crushed stone specimens and rubber-cement stabilized crushed stone specimens under the six gradation schemes were then prepared based on these results. Step 6: Test and compare the performance of ordinary cement-stabilized crushed stone specimens and six types of rubber cement-stabilized crushed stone specimens to obtain the optimal rubber particle gradation scheme.

2. The method for optimizing the gradation of rubber particles in rubber-cement stabilized crushed stone mixture according to claim 1, characterized in that: The uniform rubber content X is 3%; the uniform cement content B in the rubber cement stabilized crushed stone mixture is 4.5%.

3. The method for optimizing the gradation of rubber particles in rubber-cement stabilized crushed stone mixture according to claim 1, characterized in that: The 12 aggregate sizes are as follows: 19-26.5mm, 16-19mm, 13.2-16mm, 9.5-13.2mm, 4.75-9.5mm, 2.36-4.75mm, 1.18-2.36mm, 0.6-1.18mm, 0.3-0.6mm, 0.15-0.3mm, 0.075-0.15mm, and 0-0.075mm.

4. The method for optimizing the gradation of rubber particles in rubber-cement stabilized crushed stone mixture according to claim 1, characterized in that: The six rubber particle gradation schemes are as follows: (1) According to the uniform rubber dosage X, replace the aggregate of the same grade with the same volume of rubber particles of grade R1. (2) According to the uniform rubber dosage X, replace the aggregate of the same grade with the same volume of rubber particles of grade R2; (3) Use rubber particles of grade R2 and R3 with the same volume to replace aggregates of the same grade, according to 1 / 2 of the uniform rubber dosage X; (4) Replace the aggregate of the same grade with rubber particles of grade R2 by 1 / 2 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber particles of grades R3 and R4 by 1 / 4 of the uniform rubber dosage X. (5) Replace the aggregate of the same grade with rubber particles of grade R2 by 1 / 2 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber particles of grade R3 by 1 / 4 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber particles of grade R4 and R5 by 1 / 8 of the uniform rubber dosage X. (6) Replace the aggregate of the same grade with rubber particles of grade R2 by 1 / 2 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber particles of grade R3 by 1 / 4 of the uniform rubber dosage X; replace the aggregate of the same grade with rubber particles of grade R4 and R6 by 1 / 8 of the uniform rubber dosage X.

5. The method for optimizing the gradation of rubber particles in rubber-cement stabilized crushed stone mixture according to claim 1, characterized in that: In step 5, when preparing rubber-cement stabilized crushed stone specimens, firstly, weigh the corresponding aggregates, rubber, cement, and water according to the determined mix proportions. Pour the aggregates and rubber into a vibratory mixer and vibrate for 20 seconds to ensure uniform mixing of the rubber and aggregates. Then, add cement and water and vibrate the mixture at a speed of 50 times / second for 90 seconds. Finally, use a vibratory compactor to prepare rubber-cement stabilized crushed stone specimens. The frequency of the vibratory compactor is set to 30Hz, and the excitation force is adjusted to 7.6kN. After the specimens are formed, demolding is performed 24 hours later. After demolding, the specimens are transferred to a standard curing room for curing.

6. The method for optimizing the gradation of rubber particles in rubber-cement stabilized crushed stone mixture according to claim 1, characterized in that: Cylindrical specimens were prepared to test the unconfined compressive strength of the mixture at 7 days, 28 days, and before and after freeze-thaw cycles. The height and diameter of the specimens were both 150 mm, and the vibration compaction time for the cylindrical specimens was 90 s. Beam-type specimens were prepared for flexural strength tests, flexural resilience tests, fatigue tests, drying shrinkage tests, and thermal shrinkage tests. The length of the beam-type specimens was 400 mm, the width was 100 mm, and the height was 100 mm. The vibration time for the beam-type specimens was 120 s.

7. The method for optimizing the gradation of rubber particles in rubber-cement stabilized crushed stone mixture according to claim 1, characterized in that: Rubber uniform dosage X, mass of replaced aggregate They are respectively: Equation (1) Equation (2) In the formula, X represents the rubber content; Aggregate density (kg / m³) 3 ); Rubber density (kg / m³) 3 ); The mass of rubber (kg); Total mass of aggregate (kg); The mass (kg) of the aggregate being replaced.

8. The method for optimizing the gradation of rubber particles in rubber-cement stabilized crushed stone mixture according to claim 7, characterized in that: The density of the rubber granules is 1100 kg / m³. 3 The aggregate density is 2664 kg / m³ 3 The rubber content X is 3% of the total mass of the aggregate. According to formula (1) and formula (2), the amount of aggregate replacement is 7.3% of the total mass of the aggregate.

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