Improved composite loess roadbed material based on recycled concrete particles and preparation method

Abstract

The invention relates to the technical field of road engineering and geotechnical engineering, and discloses an improved composite loess roadbed material based on recycled concrete particles and a preparation method, the material is formed by mixing plain loess and crushed and screened recycled concrete aggregate, the particle size of the aggregate is preferably 4.75-9.5 mm, and the mixing amount is 15-25%. The preparation process comprises specific stuffy material uniform wetting and interlayer debristling treatment, so that the aggregate and the loess matrix are promoted to form a compact skeleton and filling structure. According to the invention, the rigid support system is constructed by using the recycled aggregate, so that the compaction density, unconfined compressive strength and shear strength of the loess roadbed are improved, the permeability coefficient and collapsibility coefficient of the material are greatly reduced, and post-construction settlement in a hydraulic coupling environment is effectively inhibited. According to the scheme, the engineering problems that the loess roadbed is high in water sensitivity and insufficient in bearing capacity are solved, resource utilization of the building solid waste is achieved, and remarkable economic benefits and environmental protection values are achieved.

Description

Technical Field

[0001] This invention relates to the fields of road engineering and geotechnical engineering, specifically to composite loess subgrade material and its preparation method based on recycled concrete particles. Background Technology

[0002] Loess is widely distributed in my country and is a common subgrade filler in highway and railway engineering. However, natural loess has a significant large-pore structure, weak cementation, and extremely high water sensitivity, making it a typical special soil. In engineering practice, plain loess subgrades are prone to structural disintegration and subsidence after being soaked in water, leading to subgrade settlement, cracking, and even slippage, seriously affecting the long-term service safety of transportation infrastructure. To improve the engineering properties of loess, traditional methods often use inorganic binders such as cement, lime, or fly ash for chemical modification. However, this not only increases engineering costs but also involves huge energy consumption and environmental burden, which is inconsistent with the current development trend of green transportation and circular economy.

[0003] In recent years, using recycled aggregate from crushed waste concrete to improve loess has become an environmentally friendly and economical technological approach. However, existing technologies for improving loess with recycled aggregate still face many unresolved key issues. First, regarding material composition, current technologies often lack a precise matching mechanism for aggregate particle size and dosage. If the aggregate particle size is too large or the gradation is unreasonable, it is difficult to form an effective interlocking skeleton in the loess matrix. Instead, coarse particles will be suspended among fine particles, causing internal stress concentration and failing to significantly improve the overall bearing capacity and shear strength of the subgrade. Second, regarding hydraulic stability, recycled concrete aggregate has a porous surface and high water absorption rate, which differs from the hydrophilicity of loess particles. If the microscopic interface is not tightly bonded, seepage channels can easily form between the aggregate and the soil, leading to a high permeability coefficient under hydraulic coupling, which can then cause piping or softening, failing to fundamentally solve the problem of loess water stability.

[0004] Furthermore, at the construction technology level, traditional mixed filling methods struggle to ensure the uniformity of moisture and structure within the materials. Due to the water-absorbing properties of recycled aggregates, direct compaction after mixing can easily cause the aggregates to draw moisture from the surrounding loess, leading to uneven local moisture content and strength differences. Simultaneously, conventional layered compaction processes often neglect the treatment of interlayer interfaces, resulting in smooth interfaces between upper and lower fill layers. This not only easily leads to horizontal segregation and interlayer slippage but also causes delamination under long-term loads, severely weakening the integrity and durability of the subgrade. Therefore, developing a composite loess subgrade material that optimizes aggregate gradation to construct a rigid skeleton and ensures hydraulic stability and interlayer bonding through specific processes is a pressing technical challenge. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a composite loess subgrade material and preparation method based on recycled concrete particles, which solves the problems of insufficient bearing capacity, poor impermeability, and large collapse settlement of traditional loess subgrades under hydraulic coupling environment.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] The first aspect of this invention provides a composite loess subgrade material based on recycled concrete particles.

[0008] The solid component of the composite loess subgrade material consists of loess particles and recycled concrete aggregate. Based on the total solid mass, the composition and proportion of the composite loess subgrade material are as follows: loess particles account for 75%–85% by mass, and recycled concrete aggregate accounts for 15%–25% by mass. The particle size range of the recycled concrete aggregate is strictly controlled to be 4.75mm–9.5mm.

[0009] In the above technical solution, the recycled concrete aggregate constructs a skeleton structure within the composite loess subgrade material, with loess particles filling the pores of the recycled concrete aggregate. When the particle size of the recycled concrete aggregate is between 4.75mm and 9.5mm and the dosage is within the range of 15% to 25%, an optimal dense arrangement is formed between the coarse aggregate and the fine loess particles, constituting a tightly interlocked structure. During stress, the high-strength recycled concrete aggregate bears the main stress, while the filling effect of the loess particles reduces the porosity. Microstructural analysis shows that under this mix ratio, the average equivalent diameter of the internal aggregates increases, the micropore ratio decreases, and the bonding coefficient tends to stabilize, effectively preventing the destruction of the microstructure. This results in a macroscopic increase in maximum dry density, an increase in shear strength under hydraulic coupling, and a decrease in permeability coefficient.

[0010] Furthermore, the loess particles are loess powder that has been crushed and sieved through a screen with a aperture of no more than 2 mm. By controlling the upper limit of the loess particle size, it is ensured that the fine particles can fully fill the gaps in the aggregate skeleton of 4.75 mm-9.5 mm, avoiding the phenomenon of incomplete filling or poor gradation caused by excessively large loess particles.

[0011] Furthermore, the recycled concrete aggregate is washed and dried to constant weight. This treatment removes dust and impurities from the aggregate surface and eliminates the interference of the initial moisture content of the aggregate on the determination of the optimum moisture content of the mixture, ensuring the accuracy of the solid-liquid ratio and the bonding strength of the particle interface.

[0012] The second aspect of this invention provides a method for preparing a composite loess subgrade material based on recycled concrete particles.

[0013] The method includes the following steps:

[0014] The first step is raw material acquisition and pretreatment. Loess is acquired, crushed, and then sieved through a sieve with a mesh size of no more than 2 mm to obtain loess powder with uniform particle size. Waste concrete is acquired, crushed by a crusher, and sieved by a vibrating screen. Particles with a particle size range of 4.75 mm to 9.5 mm are selected as recycled concrete aggregate, and the recycled concrete aggregate is washed and dried to constant weight.

[0015] The second step is to determine the mix proportions and parameters. The dosage of the recycled concrete aggregate is set at 15%–25% of the total solid mass. Within this dosage range, a lightweight compaction test is used to determine the optimum moisture content and maximum dry density of the mixture. The lightweight compaction test uses a 2.5kg hammer with a drop height of 305mm. Through tests under this specific compaction energy, the moisture content parameter required for the material to reach its optimal density is determined.

[0016] The third step is mixing and curing. Accurately weigh the loess powder and recycled concrete aggregate according to the determined dosage and dry mix for 1-2 minutes to ensure initial uniformity of the aggregate and loess. Then, add the calculated amount of water and wet mix for 2-3 minutes to control the moisture content of the mixture within the optimum moisture content range of ±1.5%. Finally, place the wet-mixed mixture in a sealed environment and let it cure for 12-24 hours. This curing process allows moisture to fully penetrate into the loess particles and the surface of the concrete aggregate, balancing the internal moisture of the mixture and preventing localized density differences caused by uneven moisture distribution during subsequent compaction.

[0017] The fourth step is compaction and curing. The mixture after curing is compacted, with the compaction degree controlled to be no less than 95% of the maximum dry density. Specifically, a layered compaction process is used, with three layers of filler compacted. After each layer is compacted, the contact surfaces between layers are roughened. Roughening increases the roughness between layers, promoting the interlocking and interlocking of particles between layers, preventing interlayer slippage or delamination of the subgrade material. After compaction, curing is carried out under the following conditions: temperature controlled at 20±2℃, and relative humidity controlled at no less than 90%.

[0018] Furthermore, in the preparation process of the loess powder, natural loess is air-dried to a moisture content of no more than 5% before being crushed and sieved. Controlling the initial moisture content is beneficial to the crushing efficiency and sieve pass rate of the loess, ensuring the particle size quality of the fine aggregate.

[0019] This invention achieves a synergistic effect between recycled concrete particles and loess particles through specific particle size screening and proportion optimization, combined with a layered roughening and compaction process. The high strength of the recycled concrete particles improves the overall stiffness of the composite soil, while the optimized gradation and compaction process ensures the density and low permeability of the structure. Experimental data show that the subgrade material prepared by this method, under hydraulic coupling conditions, exhibits higher destructive normal stress (strength) and lower permeability coefficient compared to remolded loess, achieving the dual benefits of resource utilization of waste concrete and improvement of loess subgrade performance.

[0020] This invention provides a composite loess subgrade material modified with recycled concrete particles and its preparation method. It has the following beneficial effects:

[0021] 1. This invention constructs a stable coarse-grained skeleton within a loess matrix by strictly limiting the particle size of recycled concrete aggregate to 4.75mm-9.5mm and controlling its mass proportion to 15%-25%. Aggregates within this gradation range form a tightly interlocked structure with fine-grained loess, where the coarse aggregate provides the main skeleton support, and the loess powder effectively fills the skeleton voids. Compared to plain loess, this composite structure achieves a higher maximum dry density (MDD) under lightweight compaction standards, improving the material's failure normal stress and shear strength.

[0022] 2. Through optimized particle size distribution and compaction process, the present invention reduces the micropore ratio inside the composite material and stabilizes the bonding coefficient between particles. This dense microstructure effectively blocks the rapid infiltration channels of water and reduces the permeability coefficient of the material. Under the action of hydraulic coupling, the composite material can effectively inhibit the structural disintegration caused by water intrusion, thereby improving the volume stability and anti-collapse ability of the roadbed in a humid environment.

[0023] 3. This invention introduces a 12-24 hour static curing step and a layered roughening and compaction process into the preparation process. The curing process eliminates the uneven local humidity caused by the difference in water absorption rate of recycled concrete aggregates, ensuring the uniform distribution of moisture in the composite system. The roughening treatment after layered compaction increases the roughness of the interlayer contact surface, promoting the interlocking of particles between the upper and lower layers. This combination of processes effectively solves the segregation, interlayer slippage and delamination problems that are prone to occur in traditional mixtures, ensuring the integrity of the overall roadbed structure. Attached Figure Description

[0024] Figure 1 The image shows the SEM microstructure of the pure loess sample prepared in Comparative Example 1 of this invention at 75% compaction.

[0025] Figure 2 The image shows the SEM microstructure of the pure loess sample prepared in Comparative Example 1 of this invention at 90% compaction.

[0026] Figure 3 SEM microstructure of the composite loess sample prepared in Example 2 of the present invention at 75% compaction.

[0027] Figure 4 SEM microstructure of the composite loess sample prepared in Example 2 of the present invention at 90% compaction.

[0028] Figure 5 SEM microstructure of the composite loess sample (80% compaction) prepared in Example 2 of this invention before consolidation test;

[0029] Figure 6 SEM microstructure of the composite loess sample (80% compaction) prepared for Example 2 of the present invention after undergoing a high-pressure consolidation test;

[0030] Figure 7 This is a comparison chart of the compaction characteristics of recycled concrete aggregates of different particle sizes at a 20% admixture content according to the present invention.

[0031] Figure 8 This is a trend diagram showing the influence of the amount of recycled concrete aggregate with a particle size of 4.75-9.5mm on the maximum dry density of the mixture according to the present invention.

[0032] Figure 9 This is a stress-strain relationship curve of Embodiment 2 of the present invention under a confining pressure of 300 kPa;

[0033] Figure 10 This is a curve showing the change in normal stress at failure of the composite roadbed material under different aggregate dosages according to the present invention;

[0034] Figure 11 This is a graph showing the evolution of the permeability coefficient under different bias stress levels in Embodiment 2 of the present invention;

[0035] Figure 12 This is a comparison chart showing the effect of aggregate content on the permeability coefficient of composite roadbed materials according to the present invention. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Preparation Examples 1-2:

[0038] Preparation Example 1: Preparation of Standard Loess Powder

[0039] The collected natural loess was placed in a well-ventilated indoor area to air dry naturally, turning it regularly until its moisture content dropped below 5%. The air-dried loess was then crushed using a soil pulverizer, carefully controlling the crushing intensity to break down the soil clumps without breaking the mineral particles. The crushed loess was then sieved through a standard square-hole sieve with a 2mm aperture. The undersized particles were collected, and plant roots and impurities were removed from the sieve to obtain standard loess powder with a particle size ≤2mm. This powder was then sealed and stored for later use.

[0040] Preparation Example 2: Preparation of Graded Recycled Concrete Aggregate

[0041] Waste concrete blocks are initially crushed using a jaw crusher, followed by secondary shaping and crushing using an impact crusher. The crushed products are then placed on a standard vibrating screen for grading and screening, with screen aperture combinations of 15mm, 9.5mm, 4.75mm, and 2.36mm.

[0042] After screening, aggregates of different particle size ranges are collected and defined as follows: Aggregate A: Collect particles with a particle size range of 4.75mm to 9.5mm;

[0043] (2) Aggregate B: Collect particles with a particle size range of 2.36 mm to 4.75 mm;

[0044] (3) Aggregate C: Collect particles with a particle size range of 9.5 mm to 15 mm.

[0045] The three types of aggregates collected above were placed in a stone washing machine for washing until the washing liquid became clear, in order to remove the dust and cement mortar powder adhering to the surface. Then, the washed aggregates were placed in a forced-air drying oven and dried at 105°C until constant weight (i.e., the difference between two consecutive weighings is less than 0.1%). After being removed and cooled to room temperature, they were bagged and sealed for later use.

[0046] See attached document Figure 1 -Appendix Figure 12 The present invention provides embodiments 1-3:

[0047] Example 1:

[0048] This embodiment provides a composite loess subgrade material based on recycled concrete particles and its preparation method. The solid component of the composite loess subgrade material consists of 85% by mass of standard loess powder obtained in Preparation Example 1 and 15% by mass of aggregate A obtained in Preparation Example 2. The specific preparation steps are as follows:

[0049] (1) Proportion and parameter determination: Take the standard loess powder obtained from Preparation Example 1 and the aggregate A obtained from Preparation Example 2, and mix them according to the proportion of aggregate A to 15% by mass and standard loess powder to 85% by mass. The optimal moisture content and maximum dry density under this proportion are determined by the light compaction test method. The compaction test parameters are set as follows: compaction is carried out using a compaction hammer with a hammer body mass of 2.5 kg and a drop height of 305 mm.

[0050] (2) Mixing and curing: Weigh the calculated amount of standard loess powder and aggregate A and put them into a mixer for dry mixing. The dry mixing time is controlled at 1.5 min. Then add the calculated amount of water for wet mixing. The wet mixing time is controlled at 2.5 min. The moisture content of the mixture is controlled within the range of ±1.5% of the optimal moisture content determined in step (1). Put the wet-mixed mixture into a sealed bag, remove the air and seal it. Let it stand for curing for 18 hours.

[0051] (3) Compaction molding: The mixture after curing is placed into the mold in 3 layers for compaction. The height of each layer is approximately equal. After each layer is compacted, the surface of the layer is roughened with a scraper. The roughening depth is controlled between 3mm and 5mm. Then the next layer of mixture is filled in and compaction continues until the overall compaction degree of the sample reaches 95% of the maximum dry density.

[0052] (4) Curing: Demold the compacted specimen, immediately wrap it with plastic wrap and seal it, and place it in a standard curing room for curing. The curing temperature is controlled at 20±2℃, the relative humidity is controlled at 95%, and the curing period is 7 days.

[0053] Example 2:

[0054] This embodiment provides a composite loess subgrade material based on recycled concrete particles and its preparation method. The solid component of the composite loess subgrade material consists of 80% by mass of standard loess powder obtained in Preparation Example 1 and 20% by mass of aggregate A obtained in Preparation Example 2. The specific preparation steps are as follows:

[0055] (1) Proportion and parameter determination: Take the standard loess powder obtained from Preparation Example 1 and the aggregate A obtained from Preparation Example 2, and mix them according to the proportion of aggregate A to 20% by mass and standard loess powder to 80% by mass. The optimal moisture content and maximum dry density under this proportion are determined by the light compaction test method. The compaction test parameters are set as follows: compaction is carried out using a compaction hammer with a hammer body mass of 2.5 kg and a drop height of 305 mm.

[0056] (2) Mixing and curing: Weigh the calculated amount of standard loess powder and aggregate A and put them into a mixer for dry mixing. The dry mixing time is controlled at 1.5 min. Then add the calculated amount of water for wet mixing. The wet mixing time is controlled at 2.5 min. The moisture content of the mixture is controlled within the range of ±1.5% of the optimal moisture content determined in step (1). Put the wet-mixed mixture into a sealed bag, remove the air and seal it. Let it stand for curing for 18 hours.

[0057] (3) Compaction molding: The mixture after curing is placed into the mold in 3 layers for compaction. The height of each layer is approximately equal. After each layer is compacted, the surface of the layer is roughened with a scraper. The roughening depth is controlled between 3mm and 5mm. Then the next layer of mixture is filled in and compaction continues until the overall compaction degree of the sample reaches 95% of the maximum dry density.

[0058] (4) Curing: Demold the compacted specimen, immediately wrap it with plastic wrap and seal it, and place it in a standard curing room for curing. The curing temperature is controlled at 20±2℃, the relative humidity is controlled at 95%, and the curing period is 7 days.

[0059] Example 3:

[0060] This embodiment provides a composite loess subgrade material based on recycled concrete particles and its preparation method. The solid component of the composite loess subgrade material consists of 75% by mass of standard loess powder obtained in Preparation Example 1 and 25% by mass of aggregate A obtained in Preparation Example 2. The specific preparation steps are as follows:

[0061] (1) Proportion and parameter determination: Take the standard loess powder obtained in preparation example 1 and the aggregate A obtained in preparation example 2, and mix them according to the proportion of aggregate A to 25% by mass and standard loess powder to 75% by mass. The optimal moisture content and maximum dry density under this proportion are determined by the light compaction test method. The compaction test parameters are set as follows: compaction is carried out using a compaction hammer with a hammer body mass of 2.5 kg and a drop height of 305 mm.

[0062] (2) Mixing and curing: Weigh the calculated amount of standard loess powder and aggregate A and put them into a mixer for dry mixing. The dry mixing time is controlled at 1.5 min. Then add the calculated amount of water for wet mixing. The wet mixing time is controlled at 2.5 min. The moisture content of the mixture is controlled within the range of ±1.5% of the optimal moisture content determined in step (1). Put the wet-mixed mixture into a sealed bag, remove the air and seal it. Let it stand for curing for 18 hours.

[0063] (3) Compaction molding: The mixture after curing is placed into the mold in 3 layers for compaction. The height of each layer is approximately equal. After each layer is compacted, the surface of the layer is roughened with a scraper. The roughening depth is controlled between 3mm and 5mm. Then the next layer of mixture is filled in and compaction continues until the overall compaction degree of the sample reaches 95% of the maximum dry density.

[0064] (4) Curing: Demold the compacted specimen, immediately wrap it with plastic wrap and seal it, and place it in a standard curing room for curing. The curing temperature is controlled at 20±2℃, the relative humidity is controlled at 95%, and the curing period is 7 days.

[0065] Comparative Examples 1-5:

[0066] Comparative Example 1: Compared with Example 2, the only difference is that no recycled concrete aggregate is added, and the solid components of the composite loess subgrade material are all the standard loess powder obtained in Preparation Example 1 (i.e., the mass ratio of loess powder is 100%). The other preparation steps and parameters are the same as in Example 2.

[0067] Comparative Example 2: Compared with Example 2, the only difference is that the recycled concrete aggregate used is aggregate B (particle size range of 2.36 mm to 4.75 mm) obtained in Preparation Example 2, which replaces aggregate A in Example 2. The other proportions, preparation steps and parameters are the same as in Example 2.

[0068] Comparative Example 3: Compared with Example 2, the only difference is that the recycled concrete aggregate used is aggregate C (particle size range of 9.5 mm to 15 mm) obtained in Preparation Example 2, which replaces aggregate A in Example 2. The other proportions, preparation steps and parameters are the same as in Example 2.

[0069] Comparative Example 4: Compared with Example 2, the only difference is that no roughening treatment was performed during the compaction process in step (3). Specifically, the mixture after curing was placed into the mold in 3 layers for compaction. After each layer was compacted, the next layer of mixture was directly filled in and compacted. No roughening treatment was performed between layers. The remaining preparation steps and parameters were the same as in Example 2.

[0070] Comparative Example 5: Compared with Example 2, the only difference is that no static curing treatment was performed during the mixing and curing process in step (2). Specifically, after the calculated amount of standard loess powder and aggregate A were mixed evenly by dry and wet mixing, they were not sealed and allowed to stand, and were directly put into step (3) for compaction and molding. The remaining preparation steps and parameters were the same as those in Example 2.

[0071] Test Examples 1-4:

[0072] Test Example 1: Verification of Compaction Characteristics and Compaction Mechanism

[0073] This test case aims to determine the compaction characteristics of mixtures with different mix proportions and aggregate sizes through indoor light compaction tests, to determine the optimum moisture content (OMC) and maximum dry density (MDD), and to verify the influence of the mix proportions and aggregate size ranges described in this invention on soil density.

[0074] Test method:

[0075] The dried mixtures prepared in Examples 1-3 and Comparative Examples 1-3 were selected as test subjects. The tests were conducted according to the light compaction test method in the "Standard for Geotechnical Testing Methods" (GB / T50123-2019).

[0076] The specific operating steps are as follows:

[0077] For each group, five samples with different moisture contents were prepared. The moisture contents were adjusted by increasing or decreasing the estimated optimum moisture content in increments of 2% to 3%.

[0078] After mixing with water, the sample is placed in a sealed bag and left to stand for at least 12 hours to allow the moisture to be fully and evenly distributed.

[0079] A lightweight compactor was used for compaction. The compaction cylinder had an inner diameter of 102 mm and a volume of 947.4 cm³. 3 The layered compaction method is adopted, with material loaded in 3 layers, each layer compacted 25 times. The weight of the compaction hammer is strictly controlled at 2.5kg, and the drop height is controlled at 305mm. During the compaction process, the hammer is kept to fall freely and vertically.

[0080] After compaction, the top surface of the compaction cylinder is leveled, the wet density of the sample is measured, and a sample is taken from the center of the sample to determine its actual moisture content.

[0081] Calculate the dry density based on the wet density and moisture content, and plot the relationship curve between dry density and moisture content. The horizontal axis corresponding to the peak value of the curve is the optimum moisture content (OMC) (%), and the vertical axis is the maximum dry density (MDD) (g / cm³). 3 ).

[0082] The test results are shown in Table 1:

[0083] Table 1. Summary of compaction characteristic test results for each group of mixtures

[0084]

[0085] Summarize:

[0086] Comparing Examples 1, 2, and 3 with Comparative Example 1, it can be seen that as the amount of recycled concrete aggregate increases, the maximum dry density of the mixture first increases and then decreases. Comparative Example 1 (pure loess) has the lowest dry density (1.715 g / cm³). 3 ).

[0087] When the dosage was 15% (Example 1), the dry density increased to 1.812 g / cm³. 3 When the dosage reached 20% (Example 2), the dry density reached a peak of 1.884 g / cm³. 3 However, when the dosage was further increased to 25% (Example 3), the dry density actually decreased slightly to 1.867 g / cm³. 3 Mechanism analysis: At low admixture concentrations, the aggregate is suspended in the loess matrix and does not form a contact skeleton. The increased density is mainly due to the aggregate's density being greater than the soil density. When the admixture concentration reaches around 20%, the 4.75-9.5mm aggregate forms a spatial skeleton structure in the soil, while the loess particles fully fill the pores between the skeletons, achieving an optimal dense arrangement and minimum void ratio.

[0088] When the admixture content is too high (such as 25% or higher), the increased contact between aggregates leads to a significant voiding effect. The amount of loess filling is insufficient to fill the increased voids between aggregates, resulting in a decrease in the overall macro density.

[0089] At the same dosage (20%), the maximum dry density of Example 2 (4.75-9.5 mm) was significantly higher than that of Comparative Example 2 (2.36-4.75 mm) and Comparative Example 3 (9.5-15 mm).

[0090] Mechanism Analysis: Comparative Example 2 used finer aggregates, which have a large specific surface area and high water demand (OMC of 13.15%). Furthermore, the fine particles are difficult to form an effective rigid support skeleton, and are prone to relative displacement with the soil during compaction, failing to generate a strong interlocking effect. Comparative Example 3 used coarser aggregates. Although the skeleton strength was high, the large volume of individual aggregate particles resulted in voids between the aggregates exceeding the effective filling range of the loess aggregates. Larger pores were easily generated at the interface, thus reducing the overall density. The 4.75-9.5mm particle size range defined in Example 2 complemented the optimal gradation of the loess particles, achieving a synergistic effect between the coarse-particle skeleton and the fine-particle filling.

[0091] Data shows that as the aggregate content increases, the optimum moisture content of the mixture gradually decreases from 16.42% to 12.15%. This is because the specific surface area of ​​recycled concrete aggregate is much smaller than that of loess particles, and its water absorption capacity is relatively weak (after pretreatment by washing and drying). This characteristic indicates that the composite material requires less water during compaction construction, which helps to reduce construction costs and minimize the risk of shrinkage deformation caused by subsequent moisture evaporation.

[0092] Test Example 2: Triaxial Dissolution-Shear Characteristics Test under Hydraulic Coupling

[0093] This test case uses the SLB-1 stress-strain controlled triaxial shear permeability tester to conduct triaxial permeability shear tests. The aim is to simulate the strength characteristics and deformation law of the roadbed under the combined action of groundwater infiltration and superstructure load (hydraulic coupling) during actual service, and to verify the strengthening mechanism of recycled concrete aggregate on loess matrix.

[0094] Test method:

[0095] The samples prepared in Examples 1-3 and Comparative Examples 1-3 were selected as test objects. The sample size was 61.8 mm in diameter and 125 mm in height.

[0096] Sample saturation and installation: The sample is saturated using a vacuum saturation device for 8-10 hours. The saturated sample is then installed in the pressure chamber of an SLB-1 triaxial apparatus, along with permeable stones and a latex membrane.

[0097] Consolidation stage: Confining pressures were set at 100 kPa, 200 kPa, and 300 kPa. Isotropic compression consolidation was performed until the pore water pressure dissipated and stabilized. Subsequently, deviatoric stress was applied for deviatoric consolidation, with the deviatoric stress level set at 50% of the non-filtration limiting deviatoric stress.

[0098] Infiltration stage (hydraulic coupling): Under the condition of maintaining constant confining pressure and deviatoric stress, the infiltration system is turned on. Set the counterpressure 1 (top) to 20 kPa and the counterpressure 2 (bottom) to 40 kPa, and use the constant pressure difference to infiltrate and scour the compressed soil. Record the axial strain and volumetric strain during the infiltration process until the strain rate stabilizes (≤0.01% / day).

[0099] Shear stage: After infiltration, maintain drainage conditions (CD test) and apply shear loading at a rate of 0.002-4 mm / min until the specimen fails (axial strain exceeds 15% or the peak deviatoric stress falls back).

[0100] Data acquisition: The computer automatically acquires stress-strain curves, calculates the failure normal stress, and plots the Mohr circle based on the Mohr-Coulomb theory to solve for the cohesion and internal friction angle under hydraulic coupling.

[0101] The test results are shown in Table 2:

[0102] Table 2. Summary of test results of mechanical properties and water stability of mixtures in each group

[0103]

[0104] Summarize:

[0105] Under harsh conditions of 300 kPa confining pressure and 50% bias, the failure normal stress of Example 2 (20% admixture) reached 1086.2 kPa, which was 140% higher than that of Comparative Example 1 (pure loess). The data show that although long-term infiltration (leaching) attempts to destroy the soil structure, the addition of recycled concrete aggregate significantly improves the material's resistance.

[0106] Mechanism analysis: Under the scouring of water flow and the action of deviatoric stress, pure loess particles undergo reorganization and softening, resulting in a significant decrease in strength. In Example 2, however, the 4.75-9.5mm aggregate forms a stable rigid skeleton, which can still bear the main deviatoric stress even if the loess matrix softens locally after immersion in water.

[0107] Triaxial shear data showed that the cohesion in the permeable section increased significantly (reaching 62.8 kPa). This is because the cementing and interlocking between the aggregate and loess particles were fully developed under the combined compaction effect of confining pressure and permeable water pressure.

[0108] During the infiltration stage (simulating the long-term effects of groundwater), Comparative Example 1 exhibited significant creep deformation (axial strain 4.85%, volumetric strain 7.12%), demonstrating obvious collapsing and softening characteristics. In contrast, the axial strain of Example 2 was only 1.52%.

[0109] Mechanism analysis: As the aggregate content increases (from 0% to 20%), the permeability deformation shows a non-linear decreasing trend. This is because the hard aggregate not only fills the space but also restricts the displacement and reorganization of loess particles under the action of water flow. When the content reaches 20%, the interlocking effect of the skeleton reaches its optimum, effectively suppressing the tendency of piping or structural collapse caused by permeability force.

[0110] Test Example 3: Permeability and Anti-seepage Stability under Hydraulic Coupling

[0111] This test case aims to quantitatively evaluate the impermeability of the composite loess subgrade material described in this invention under complex stress and hydraulic coupling environment. By measuring the permeability coefficient and critical hydraulic gradient under different conditions, its ability to resist seepage damage (such as soil erosion and piping) is verified.

[0112] Test method:

[0113] The same SLB-1 stress-strain controlled triaxial shear permeability tester as in Test Example 2 was used. Samples prepared in Examples 1-3 and Comparative Example 1 (pure loess) were selected, and the maximum dry density was controlled to 100% compaction.

[0114] Sample saturation and consolidation: After vacuum saturation and back pressure saturation tests (B≥0.95), the samples were isotropically consolidated under a preset confining pressure, and then biased stress was applied for biased consolidation to simulate the actual stress state of the roadbed.

[0115] Constant head permeability test: The top back pressure is set to 20 kPa and the bottom back pressure to 40 kPa to form a constant head difference. Under the condition of constant confining pressure and deviatoric stress (hydraulic coupling state), the amount of water flowing through the sample is measured.

[0116] Evaluation index calculation:

[0117] Permeability coefficient: Calculated based on Darcy's law.

[0118] Anti-seepage stability: Calculate the critical hydraulic gradient and compare it with the actual hydraulic gradient to determine whether seepage failure has occurred.

[0119] The test results are shown in Table 3:

[0120] Test conditions: confining pressure = 300 kPa, bias stress level = 0 (simulating hydrostatic pressure environment) and bias stress level = 0.5 (simulating high bias coupling environment).

[0121] Table 3. Test results of specimen strength decay under wet-dry cycling.

[0122]

[0123] The seepage stability analysis (critical hydraulic gradient) is based on the physical parameters measured in the experiment to calculate the critical hydraulic gradient range of each group under a confining pressure of 100-300 kPa.

[0124] Comparative Example 1 (pure loess): the critical hydraulic gradient range is 25.1-35.2.

[0125] Example 2 (20% aggregate): The critical hydraulic gradient range is 28.7-41.8.

[0126] Conclusion: The critical hydraulic gradient of Example 2 is significantly higher than that of pure loess, indicating that under the same water head, the material of the present invention is less prone to soil erosion or piping failure.

[0127] Summarize:

[0128] Comparing the data in Table 3, the permeability coefficient of Example 2 (20% doping) remained on the order of 10⁻⁷ cm / s, which is an extremely low permeability material.

[0129] Mechanism Analysis: Although recycled aggregates themselves may contain micro-cracks, this invention uses 4.75-9.5mm graded aggregates to form a dense skeleton and filling structure with the loess matrix. Under high confining pressure and bias pressure, the aggregates, acting as a rigid core, bear the stress, forcing the loess particles to further compact (structural reorganization) within the skeleton gaps, effectively cutting off seepage channels. In contrast, pure loess (Comparative Example 1) is prone to pore expansion under water flow, resulting in a higher permeability coefficient.

[0130] The experiment found that as the bias stress level increased from 0 to 0.5, the permeability coefficient of each group of samples showed a decreasing trend.

[0131] Mechanism analysis: This verifies that the material of the present invention has excellent compaction effect. Under hydraulic coupling, the application of deviatoric stress did not lead to shear failure of the sample structure and the generation of cracks (which would usually result in a sharp increase in the permeability coefficient), but instead promoted further closure of the internal microstructure. In particular, for Example 2, its permeability coefficient steadily decreased with increasing stress, indicating that the material system can effectively utilize the upper load to enhance its impermeability barrier capacity.

[0132] Combining the strength data from Test Example 2 with the permeability data from this Test Example, it can be seen that Example 2 achieved the highest shear strength while maintaining an extremely low permeability coefficient.

[0133] Conclusion: This invention overcomes the contradiction in traditional geotechnical materials where increased strength is often accompanied by increased brittleness or decreased impermeability due to fracture development. The addition of recycled aggregate not only provides mechanical support but also enhances the hydrodynamic stability of the subgrade by improving particle size distribution and limiting soil deformation.

[0134] Test Example 4: Microstructure Morphology and Enhancement Mechanism Analysis

[0135] This test case utilizes a Quanta 250 environmental scanning electron microscope (SEM) at 1000x magnification to observe the microstructure of samples under different working conditions, aiming to reveal the reinforcement mechanism and structural evolution law of recycled concrete aggregate on loess matrix from a microscopic level.

[0136] Test method:

[0137] Sample grouping:

[0138] Group A (Compaction Degree Influence Group): Samples of pure loess (without aggregate) and composite loess (with aggregate) were selected at compaction degrees of 75% (loose state) and 90% (dense state) respectively to analyze the synergistic effect of compaction work and aggregate.

[0139] Group B (Load Influence Group): Composite loess samples were selected at 80% compaction. One group was left untreated, while the other group underwent consolidation compression tests to analyze the skeleton stability under load.

[0140] Observation steps: After drying, cross-section preparation and surface gold spraying, the sample is placed in high vacuum mode for SEM scanning, focusing on observing particle contact relationship, pore distribution characteristics and skeleton formation.

[0141] Test results and graph analysis:

[0142] Synergistic densifying effect of compaction degree and aggregate:

[0143] like Figure 1 As shown in the diagram (75% compaction, pure loess), pure loess under low compaction exhibits a typical porous structure with loose particles and strong connectivity of large pores. Figure 2 As shown in the diagram (90% compaction, pure loess), as the compaction degree increases, the spacing between pure loess particles decreases and the number of large pores decreases. However, obvious intergranular pores can still be observed, and the particle contact is mainly point contact, lacking rigid support. This explains why pure loess is prone to compression deformation under high pressure.

[0144] Skeleton filling effect of recycled aggregate:

[0145] like Figure 3 As shown in (75% compaction, composite loess), even at low compaction, the added recycled concrete aggregate changed the microstructure of the soil, and a clear soil particle aggregation zone was formed around the aggregate.

[0146] like Figure 4 As shown in the figure (90% compaction, composite loess), this represents the ideal microstructure of the present invention. (Compared to...) Figure 2 Compared to (pure loess with the same degree of compaction), Figure 4 The microstructure within the material is extremely dense. A tight interlocking structure is achieved between the recycled concrete aggregate (rough, blocky material) and the loess matrix. Under high-pressure compaction, loess particles are forcefully compressed into the micropores on the aggregate surface and into the voids between aggregate particles, essentially eliminating interconnected large pores larger than 10 μm in diameter. This structure not only utilizes the aggregate to bear the main skeletal stress but also uses the loess to fill the voids, achieving a dual improvement in density and strength.

[0147] Structural Reorganization and Strengthening under Load:

[0148] Initial state before consolidation: Figure 5 The results show the state of the composite loess under 80% compaction without load, with relatively uniform particle distribution and a small amount of native micropores.

[0149] The strengthened state after consolidation: Figure 6 The morphology of the sample after high-pressure consolidation is shown. Figure 5 In comparison, the following changes can be clearly observed:

[0150] Contact mode transformation: Under the action of deviatoric stress, particle contact changes from point-to-point contact to more stable surface-to-surface contact, and the soil matrix exhibits layered or blocky characteristics.

[0151] Aggregate locking effect: No aggregate breakage was observed in the image, indicating that the strength of the recycled concrete aggregate is higher than that of the surrounding soil. The aggregate restricts the slippage and displacement of surrounding soil particles, and this microscopic locking effect effectively resists shear deformation caused by external loads.

[0152] Pore ​​closure: The consolidation process is essentially a process in which microscopic pores are compressed and closed. Figure 6 Almost no interconnected pores were observed, which microscopically confirms the mechanism behind the significant decrease in permeability coefficient in Test Example 3.

[0153] Summarize:

[0154] Microscopic analysis reveals that the reinforcement mechanism of the subgrade material described in this invention lies in the fact that the recycled concrete aggregate constructs a rigid load-bearing skeleton during compaction, while the loess particles, under optimal moisture content and compaction work, act as a highly dense filler. With increasing compaction degree (from 75% to 90%) and the application of external loads (consolidation), the system spontaneously evolves into a stable microstructure characterized by low porosity, high interlocking, and surface contact, thereby endowing the subgrade material with excellent mechanical strength and impermeability.

Claims

1. Composite loess subgrade material based on recycled concrete particle improvement, characterized in that, The composite loess roadbed material has a skeleton and filling dense structure, which is formed by compaction of the following components with mass percentages: Loess particles: 75%-85% by mass; Recycled concrete aggregate: 15%-25% by mass; The particle size of the loess powder is ≤2 mm, and the particle size range of the recycled concrete aggregate is 4.75-9.5 mm. In the skeleton and filling dense structure, the recycled concrete aggregate forms a rigid support skeleton that bears the load, the loess powder is densely filled in the voids of the rigid support skeleton and the micropores on the surface of the aggregate as a filling body, and the contact interface between the recycled concrete aggregate and the loess powder presents a micro-inlaid occlusion state.

2. A method for preparing a composite loess subgrade material based on recycled concrete particles, characterized in that, The method comprises the following steps: Obtaining and pretreating raw materials: crushing loess blocks to obtain loess powder; Breaking and screening waste concrete to obtain recycled concrete aggregate; Determination of the mixing ratio and parameters: determining that the dosage of the recycled concrete aggregate is 15%-25% of the total solid mass; Determining the optimum water content and maximum dry density under the dosage by using a light compaction test method; Mixing and curing: accurately weighing the loess powder and the recycled concrete aggregate according to the dosage for dry mixing, then adding a calculated amount of water for wet mixing, and placing the mixed material after wet mixing for 12-24 hours for curing; Compaction molding and curing: compacting and molding the mixed material after curing, with a compaction degree requirement of ≥95% of the maximum dry density, and then curing.

3. The method for preparing a composite loess subgrade material improved with recycled concrete particles according to claim 2, characterized in that, In the step of obtaining and pretreating raw materials, the particle size of the loess powder is ≤2 mm; After breaking and screening the waste concrete, the recycled concrete aggregate with a particle size range of 4.75-9.5 mm is selected.

4. The method for preparing composite loess subgrade material based on recycled concrete aggregate improvement according to claim 2, characterized in that, The loess powder is prepared by drying natural loess to a water content of ≤5%, and then crushing and screening.

5. The method for preparing composite loess subgrade material based on recycled concrete aggregate improvement according to claim 2, characterized in that, The dry mixing time in the mixing and curing step is controlled to be 1-2 min, and the wet mixing time is controlled to be 2-3 min.

6. The method for preparing composite loess subgrade material based on recycled concrete aggregate improvement according to claim 2, characterized in that, The compaction molding in the compaction molding and curing step specifically includes: compacting the material in 3 layers, and performing a shaving treatment after each layer is compacted.

7. The method for preparing composite loess subgrade material based on recycled concrete aggregate improvement according to claim 2, characterized in that, The curing condition in the compaction molding and curing step is: the temperature is controlled to be 20±2℃, and the relative humidity is controlled to be ≥90%.

8. The method for preparing composite loess subgrade material based on recycled concrete aggregate improvement according to claim 2, characterized in that, In the mixing and curing step, when wet mixing the loess powder and the recycled concrete aggregate, the water content of the loess powder and the recycled concrete aggregate is controlled to be within the range of the optimum water content±1.5%.

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