Evaluation of road performance and micro-mechanism analysis of bentonite plastic concrete subgrade

This study evaluated the performance of bentonite plasticized concrete with good deformation coordination as a road base material, to address the poor deformation coordination of rigid base layers in roads. Through tests such as slump, strength, permeability, and scanning electron microscopy (SEM) and x-ray diffraction (XRD), the study analyzed the variations in workability, mechanical properties, and durability of plastic concrete as a base material under different bentonite and cement contents, as well as the underlying micro-mechanisms. The results showed that plastic concrete exhibited good workability, which improved with increased bentonite and cement content. The 28-day mechanical properties of the plastic concrete met the design criteria for road base layers and had features of higher load-bearing capacity, good toughness, slow strength attenuation, and overall integrity. In durability, the increase of bentonite content enhanced the concrete’s permeability, but decreased abrasivenes and shrinkage. From economic, performance, and engineering perspectives, the optimal bentonite and cement contents for the plastic concrete base were in the ranges of 90 kg m−3 to 120 kg m−3 and 110 kg m−3 to 150 kg m−3, respectively.


Introduction
The latest National Highway Network Planning points out that China will build 26,000 kilometers of expressways and 34,000 kilometers of ordinary highways by 2035.However, there are common problems for the current road subgrade, such as poor coordination of rigid subgrade deformation, difficulty in controlling reflective cracks in semi-rigid subgrade, and low bearing capacity of flexible subgrade, which seriously affects the quality of road surface [1].In addition, the rapid development of transportation has led to an increasing demand for materials such as cement and lime in road subgrade construction.However, the production of cement and lime emits a large amount of carbon dioxide, consumes a significant amount of resources, and causes dust pollution which leads to significant pressure on resources and the environment [2,3].Against the backdrop of the national strategies for 'carbon neutrality' and 'peak carbon emissions' and the prominent issues with road subgrade, finding new types of subgrade materials with better road performance and enriching the variety of subgrade materials are the key to solve the current problems.
Plastic concrete is an economically and environmentally friendly material made by partially replacing cement with materials such as bentonite and clay, mixing with water and aggregates.It shares some similarities with stabilizing materials using inorganic binders and lean concrete as subbase materials.In the application of dam seepage walls, plastic concrete can adapt well to soil deformations and exhibits excellent deformation coordination ability and impermeability performance [4].In addition, plastic concrete has low elastic modulus, good toughness, certain strength and low material cost [5][6][7].Plastic concrete has broad application prospects and good performance.The performance (working performance, mechanical performance, durability, etc) of plastic concrete can be flexibly prepared by adjusting the dosage of bentonite and cement according to the needs of the project [4,8].
Domestic and foreign scholars have conducted a series of studies on plastic concrete.In terms of working performance, Wu [9] found that low liquid limit red clay could improve the fluidity, water retention and cohesion of plastic concrete, and could also delay the initial setting and final setting time of concrete.In addition, some scholars pointed out that adding bentonite into the plastic concrete with high water-cement ratio could make the mixture molecules evenly arranged and prevent the aggregate from separating from the cement, thus improving the stability, plasticity and impermeability of the mixture [10,11].However, Amlashi [12,13] and others found that the addition of bentonite and fine clay to plastic concrete could lead to water absorption and expansion that caused a decrease in slump.Obviously, scholars have varying understandings of the impact of the addition of bentonite and clay on the workability of plastic concrete, which is difficult to provide a clear theoretical basis for the widespread application of plastic concrete.
In terms of mechanical properties, Song et al [14] studied the influence of the amount of plastic concrete cementitious material on the uniaxial compressive strength, and found that the axial compressive strength of plastic concrete first increased and then decreased with the addition of bentonite.However, Wang [15] was of the opinion that the elastic modulus and strength of plastic concrete were positively correlated with the cement content, and negatively correlated with the bentonite content; In the case of the same amount of cement, bentonite could improve the crack resistance of plastic concrete, and the best amount was 20% ∼ 60% of the cement amount.Kazemian [16] pointed out that the addition of bentonite would reduce the impact of compressive strength and elastic modulus of plastic concrete.Farajzadehha [17] studied the comparison of uniaxial and triaxial strength of plastic concrete containing nano-silica.The results showed that the strength of plastic concrete could be improved by adding nano-silica, and the elastic modulus was almost unchanged.He [18] modified plastic concrete with silica fume, which found that the filling effect and pozzolanic effect of silica fume could not only improve the microstructure of plastic concrete, but also improve the mechanical properties of plastic concrete.Razavi [19] found that adding polypropylene fiber into plastic concrete could greatly improve the mechanical strength.The research results of Pashang [20] showed that, in the undrained test, the changes of limiting pressure, cement and bentonite content and aggregate roughness had obvious effects on the shrinkage and expansion behavior of plastic concrete.The failure mode of plastic concrete was largely affected by the confined pressure.Under low limiting pressure, the failure mode was the same as that of brittle materials, and there was an obvious peak strength point when the sample fails; For higher confining pressures, the specimen behaved as a ductile material [21].Song et al [22] considered that the stress-strain curve of plastic concrete could generally be divided into four sections through a true triaxial compression stress test.The initial section had obvious elastic characteristics.The last section had serious plastic deformation.When the second and third principal stress levels were small, the failure mode of the specimen was similar to that of the axial compression specimen.As mentioned above, scholars have conducted a series of studies on the mechanical properties of modified plastic concrete and plastic concrete under different confining pressures.It is generally believed that the mechanical properties of plastic concrete tend to decrease with an increase in bentonite content, which provides some reference for this study.However, most of the current research on the mechanical properties of plastic concrete is based on the application context of dam seepage control projects, and there is a lack of research and evaluation specifically addressing the mechanical properties of bentonite plastic concrete in the context of road subbase engineering applications.
In terms of durability, Mahboubi [23] and others studied the influence of water-binder ratio and bentonite content on the permeability of plastic concrete.The results showed that reducing the water-binder ratio and increasing the bentonite content could improve the impermeability of plastic concrete.Mirza [8] conducted tests on mortar cubes immersed in 2% magnesium sulfate and 5% sodium sulfate solutions, which showed that with the increase of bentonite content, the sulfate corrosion resistance durability was also continuously improved.Wang [24] studied plastic concrete with bentonite content of 50% and found that its elastic modulus was similar to that of soil, and its ultimate deformation was large.Compared with ordinary concrete, it could better adapt to the deformation of soil and had better crack resistance and seismic performance.In general, plastic concrete exhibits good resistance to permeability, cracking, and ion erosion.However, further investigation is needed to explore its permeability and crack resistance properties as a road subbase material.Additionally, the performance of plastic concrete in terms of erosion resistance and drying shrinkage under different bentonite content has not been clearly evaluated, which hinders the expansion of applications in the field of bentonite plastic concrete.
Based on the above, plastic concrete has the characteristics of good deformability, environmental friendliness, and economy.However, most of the current researches on the performance of plastic concrete are based on the application context of dam seepage control projects, and there is limited research and assessment on the performance of plastic concrete as a road subbase material.To solve the problems existing in the road base course and improve the road use quality, this study intends to comprehensively evaluate the road performance of plastic concrete from the aspects of working performance, mechanical performance, durability, etc of plastic concrete base course materials by changing the amount of bentonite and cement and combining the main evaluation indicators of current road base materials.It will provide theoretical guidance and technical support for the application of plastic concrete to road bases.

Materials and tests
2.1.Raw materials 2.1.1.Cement Cement is one of the important raw materials for plastic concrete, because its hydration products have an extremely significant effect on various properties of plastic concrete.In this study, Conch brand P.O 42.5 cement was used.The basic properties of cement are shown in table 1.Specifically, compressive strength tests conducted on cement at 3 days and 28 days showed measured values that far exceeded the required standard compressive strength.This demonstrates that cement has excellent compressive performance and is expected to continue to strengthen over a longer period.In addition, the flexural strength and setting time of the cement are also within the specified range.The cement's flexural strength meets the requirements, indicating that it can withstand a certain amount of bending stress.At the same time, the setting time is also within the ideal range, indicating that the cement's solidification speed is moderate.Overall, the test results show that the compressive strength, flexural strength, and setting time of the cement all meet the relevant standards and satisfy the experimental requirements.

Bentonite and clay
As shown in figure 1(a), it is a sodium-based montmorillonite produced by Yitong Mining Co., Ltd in Xinyang City, Henan Province.The chemical composition analysis of the montmorillonite in table 2 reveals that its main component is silicon dioxide, with a content as high as 70.12%.The second major component is alumina, with a content ratio of 12.29%.It also contains small amounts of metallic oxides such as sodium, potassium, calcium,   3 indicate that the montmorillonite meets the relevant standards and experimental requirements in terms of its montmorillonite content, expansion rate, water content, and fineness.The crystal structure of montmorillonite belongs to layered silicate minerals, with hexagonal crystal layers composed of aluminum, magnesium, and silicon atoms alternately arranged, interspersed with water molecules and sodium ions.Its content in the montmorillonite is as high as 81.5%, which has a significant impact on the water absorption and swelling properties of the montmorillonite.
In plastic concrete, adding an appropriate amount of clay can to some extent reduce the elastic modulus of plastic concrete, improve its impermeability, and reduce the amount of montmorillonite, thereby lowering the economic cost of plastic concrete.As shown in figure 1(b), the clay used is ordinary yellow clay.Its particle size and relative content indicators are shown in table 4. From the table, it can be seen that 94.5% of the clay particles have a particle size smaller than 0.25 mm.While the clay content with a particle size of 0.5 mm to 0.25 mm accounts for only 5.1%.Among them, particles smaller than 0.005 mm make up 30.9%.The combination of these particle sizes has a certain impact on the water absorption and cohesion of clay.

Coarse and fine aggregates
Natural river sand was used in the test.The particle sieving test was carried out by the provisions of Sand for Building (GB14684-2001) [25], and the results are shown in table 5.The fineness modulus of the natural river sand was 2.8, the apparent density was 2650 kg m −3 , and the bulk density was 1574 kg m −3 .The coarse aggregate used was continuously graded gravel with a particle size of 5-20 mm, which conformed to the standard of Pebble and Crushed Stone for Construction (GB/T14685-2011) [26].The sieve test was conducted on the aggregates used and the results are shown in table 6.

Design objectives and proportioning design 2.2.1. Design objective of base plastic concrete
In order to make the overall stress of pavement structure more coordinated, preferably adapt to various deformations of road soil foundation, cushion and surface layer without cracking, and reduce various diseases caused by stress concentration, this study is based on Technical Rules for Construction of Highway Pavement Base (JTG/T F20-2015) [27] and relevant engineering experience.The following requirements are made for the working performance and basic performance of plastic concrete for the base course: (1) The slump is 180 mm ∼ 250 mm, and the diffusion is 320 mm ∼ 450 mm; (2) 28d unconfined compressive strength: 1.5 Mpa ∼ 5 Mpa; (3) Static elastic modulus 200 Mpa ∼ 2000 Mpa; (4) Permeability coefficient 1 × 10 −6 ∼ 1 × 10 −8 cm s -1 .

Proportioning design of road plastic concrete
To explore the influence of different bentonite and cement dosages on the performance of plastic concrete for road applications and determine the optimal dosage of bentonite and cement, this experiment involved varying the dosage of bentonite and cement.Three series of 15 mix proportions were used to systematically investigate the effects of bentonite and cement dosages on the workability, mechanical properties, and durability of plastic concrete.The mix proportions are shown in table 7. The mix proportions for each cubic meter of plastic concrete were calculated by using the assumed specific gravity method, as outlined in equation (1): (W 0 -Water content of plastic concrete (kg m −3 ); C 0 -Cement content in plastic concrete (kg m −3 ); S 0 -Sand content in plastic concrete (kg m −3 ); G 0 -Amount of gravel in plastic concrete (kg m −3 ); B 0 -Bentonite content in plastic concrete (kg m −3 ); N 0 -Clay content in plastic concrete (kg m −3 ); ρ-The density of plastic concrete is assumed to be 2100 (kg m −3 ) in this study.)

Test method 2.3.1. Working performance
The fluidity of the plastic concrete mix was assessed according to the caving degree and diffusivity adopted in the Test Code for Hydraulic Plastic Concrete (DL/T 5303-2013) [28].The process of plastic concrete preparation and workability testing is shown in figure 2. Plastic concrete was loaded into the caving cylinder in three layers.Each layer was inserted and pounded 25 times, and was measured by a steel ruler.The unconfined compressive strength test of plastic concrete adopted a size of 150 × 150 × 150 mm standard cube.After reaching the curing age of 28d, measure the specimen size (1 mm accuracy).The RMT-301 strength testing machine was used for the test.The displacement control loading system was used in the test, and the loading rate was 0.02 mm s -1 .The test was stop when the test piece deformed rapidly and flaked in a large area.

Splitting tensile strength test
The splitting tensile strength test of plastic concrete shall refer to the relevant provisions of the Test Code for Hydraulic Plastic Concrete (DL/T5303-2013) [28].The specimen size is 150 × 150 × 150 mm standard cube.
The RMT-301 strength tester is also used for the 28d splitting tensile strength test of plastic concrete.The loading is controlled by displacement and the loading rate is 0.005 mm s -1 .

Bending tensile strength test
The flexural tensile strength test of plastic concrete shall refer to the relevant provisions of the Test Code for Hydraulic Plastic Concrete (DL/T5303-2013) [28].Specimen size is 100 × 100 × 400 mm small beam.RMT-301 strength testing machine is used to conduct 28d flexural strength test of plastic concrete.Displacement controlled loading system is adopted, and the loading rate is 0.005 mm s -1 .

Durability properties 2.3.3.1. Impermeability test method
The evaluation of the impermeability index of plastic concrete shall refer to the existing test method Test Code for Hydraulic Plastic Concrete (DL/T5303-2013) [28].The size of the test piece is a cone with an upper opening diameter of 175 mm, the lower opening diameter of 185 mm and the height of 150 mm.In this test, HP-4.0 automatic intelligent concrete impermeability tester is used, with the water pressure adjusted to 0.2 Mpa and the constant pressure for 4 h.After reaching the constant pressure time, use the press to take the plastic concrete sample out of the steel mold, use the press to split the sample, and measure the water seepage height.The relative permeability coefficient is calculated according to Formula (2): (K-relative permeability coefficient (cm/s); h m 2 -Average seepage height (cm); T-Constant pressure time (s); a-Water absorption of plastic concrete, (%).H-Water pressure, expressed in the height of water column (cm); 1 Mpa water pressure is equal to 10200cm water column height.)

Anti scour test method
The size of the test piece for this erosion resistance test is 150 × 150 mm cylinder that is cured for 28d and soaked in water for 24h.Take out the saturated test piece, weigh and record the mass m 1 .Use the scouring test device shown in figure 3 to conduct the test, and fix the test piece in the suspended scouring bucket with a fixture.The water surface is 5 mm higher than the top surface of the test piece.The vibration frequency of the vibration table (P-Erosion mass loss rate (%); m 1 -Saturation quality of test piece; m 2 -Scour mass.)

Dry shrinkage test method
The size of the plastic concrete dry shrinkage test specimen is 100 × 100 × 400 mm middle beam.After 28 days of curing under standard conditions, measure the initial length of the test piece and weigh the mass of the test piece.Place the test piece on the fixture, install the dial indicator, and record the initial reading.In the first 7 days, read it once a day, weigh the quality of the test piece, and then read it once every two days.After one month, put the test piece to test the water loss rate into the oven at 110 °C to dry and weigh.The drying shrinkage water loss rate, drying shrinkage strain and total drying shrinkage coefficient are calculated as follows: 1, (w i -The first water loss rate (%); δ i -The first drying shrinkage (mm); ε i -The first dry shrinkage strain (%); a di -The coefficient of the first drying shrinkage (%); m i -Mass of the first test piece (g); X i j , -The jth dial indicator reading in the test (mm); l-Length of test piece (mm); m p -Mass of the test piece after drying (g).)

Micro-morphological and compositional analysis 2.3.4.1. Scanning electron microscope (SEM)
In this study, a TESCAN MIRA4 scanning electron microscope was used for testing.Four groups of plastic concrete samples (H1-1, H1-3, H2-1, H2-3) were taken for crushing and sampling.The particles were placed in anhydrous ethanol to halt hydration and be dried until a constant weight was achieved.The dried samples were ground into powder until they could pass through a 200-mesh sieve.Subsequently, the powder was adhered to a sample holder using conductive adhesive and coated with a thin layer of gold using an ion sputter coater to improve the sample's conductivity and observe its micro-morphology better.

X-ray diffraction (XRD)
Firstly, the specimens were crushed.Secondly, samples were taken from the crushed particles.Then the sampled particles were immersed in anhydrous ethanol to halt hydration, and the samples were placed in a forced-air drying oven and dried until a constant weight was achieved at a drying temperature of 50 °C.Then, the dried samples were ground in a grinding bowl to obtain a uniformly-sized powder without any noticeable particle sensation when touched, until the powder could pass through a 200-mesh sieve.Finally, the screened powder was collected and placed in a sealed bag.Before testing, 1g of the powder was weighed and loaded onto a glass slide.Then compacted and leveled it by a scraper.The XRD instrument used was the RIGAKU Ultima IV manufactured in Japan.From the figures, it can be observed that the addition of bentonite has a certain improvement effect on the workability of the plastic concrete mixtures.In the series with the same cement dosage, as the bentonite dosage increases, there is a slight increase in both slump and flow values of the plastic concrete.This finding is consistent with the research results obtained by Iravanian [11].Compared with plastic concrete without bentonite, the maximum increase of slump of plastic concrete with bentonite in H1, H2 and H3 series is 11.9%, 13.6% and 5.4%, and the maximum increase of expansion is 20.0%, 20.6% and 21.9%.The slight increase in slump and flow values of plastic concrete with the increasing bentonite dosage is mainly influenced by the characteristics of the bentonite material.The main mineral component of bentonite is montmorillonite [(Al, Mg) 2 -(OH) 2 (Si, Al) 4 O 10 (Ca)x nH 2 O], which accounts for over 85% to 90% of its composition.On one hand, montmorillonite exhibits strong water-absorption properties, providing water retention, lubrication, and plasticizing effects.Additionally, the water absorption and expansion characteristics of bentonite lead to a decrease in the overall density of the plastic concrete [13].On the other hand, the increasing bentonite dosage has a filling effect on the voids between the sand and gravel particles, causing the aggregate to be in a suspended state, thus improving the workability of the plastic concrete [29].

Test results and analysis
In the series with the same bentonite content, the workability of plastic concrete is also improved with the increase of cement content.Cement can increase the powder and granular materials in the plastic concrete, which can improve the concrete particle gradation, and the sand and gravel materials are completely wrapped and suspended.Therefore, with the increase of cement content, the slump and expansion of plastic concrete will increase.As observed from the data in figure 4, the slump range of the mixture is within 178-288 mm, and the flow values range from 325-540 mm, indicating good fluidity and plasticity.Therefore, the flowability of the plastic concrete with bentonite addition generally meets the design requirements for plastic concrete base layers as specified in section (2.2.1).

Mechanical performance 3.2.1. Unconfined compressive strength
In order to study the influence of bentonite content and cement content on the strength of plastic concrete, 15 groups of plastic concrete are designed.It is divided into three series: H1 (cement: 110 kg m −3 ), H2 (cement: 150 kg m −3 ) and H3 (cement: 190 kg m −3 ).The bentonite content in each series is 0kg/m 3 , 30 kg m −3 , 60 kg m −3 , 90 kg m −3 and 120 kg m −3 .According to the test results shown in figure 5(a), with the increase of bentonite content, the compressive strength of plastic concrete with different cement content shows a downward trend.In the three series, the content of bentonite increased from 0 to 30 kg m −3 , and the compressive strength of plastic concrete decreased significantly by 0.32 Mpa, 0.67 Mpa and 0.51 Mpa, respectively, while the decrease was relatively slow in the range of 30 ∼ 120 kg m −3 .The compressive strength of the H1 series decreased slightly, while that of the H2 series and H3 series decreased greatly.H1, H2 and H3 decreased by 0.44 Mpa, 1.03 Mpa and 1.07 Mpa respectively.
As an auxiliary cementing material of cement, the pozzolanic activity of bentonite is much lower than that of cement, and its cementation ability is weak.The compressive strength of plastic concrete is mainly determined by the cementation strength of cement hydration products.Therefore, the partial replacement of concrete with bentonite will lead to a decrease in the strength of the plastic concrete, which is consistent with the findings of several other researchers [30][31][32].The reduction rate of compressive strength is different due to different bentonite content, mainly because the addition of bentonite changes the internal microstructure of plastic concrete and the distribution of hydration products.Bentonite has a remarkable wrapping effect.When bentonite wraps on the surface of cement particles, it forms a layer of water-blocking film, which hinders the formation of cement hydration and strength.In addition, bentonite has strong dispersibility.Hydration products such as C-S-H gel (hydrated calcium silicate), AFt (ettringite), and CH (calcium hydroxide) are dispersed by bentonite particles, which is sparsely distributed, leading to a decline in cementation capacity.Therefore, when the bentonite content is 0 to 30 kg m −3 , the strength of plastic concrete decreases rapidly.
Before the bentonite content is 30 kg m −3 , the strength of plastic concrete is dominated by cement.However, the strength decreases slowly when the content of bentonite exceeds 30 kg m −3 , mainly because the hardening process of plastic concrete gradually becomes a process of interaction between the 'solidification' of cement and the 'plasticization' of bentonite [33].In addition, bentonite also has a certain pozzolanic activity, which can react with cement hydration products for secondary hydration.The products not only have a certain cementation effect, but also can fill the capillary pores and improve the pore structure of plastic concrete [34].The bentonite particles themselves also have the filling function.They expand with water.Some pores are closed largely under the extrusion of the internal expansion force so that the plastic concrete has a good integrity.
Figure 5(b) shows that the unconfined compressive strength of plastic concrete increases significantly with the increase of cement content.The content of bentonite is 0 kg m −3 , and the unconfined compressive strength of plastic concrete increases linearly with the increase of cement content.For other series, when the cement content exceeds 150 kg m −3 , the unconfined compressive strength of plastic concrete increases greatly.This is mainly because the more cement is added, the higher the content of hydration products such as C-S-H gel, CH and AFt, and the stronger the cementation ability.In addition, these hydration products are closely combined to form a more compact structure, further reducing the number of harmful pores.Therefore, with the increase of concrete content, the strength of plastic concrete also increases significantly.
The experiments show that the strength of the plastic concrete with bentonite addition ranges from 1.9 to 5 Mpa, which generally meets the design requirements for road base layers under the conditions of secondary and lower-level highways and light traffic as specified in Technical Guidelines for Construction of Highway Roadbases (JTG/T F20-2015) [27].The specified design requirements in the document range from 2.0 to 4.0 Mpa.The content of bentonite has a negative correlation with the unconfined compressive strength of plastic concrete.The higher cement content, the higher strength of plastic concrete.Before the dosage of bentonite is 30 kg m −3 , the unconfined compressive strength of plastic concrete is determined by the degree of cement hydration and the distribution characteristics of hydration products.When the dosage exceeds 30 kg m −3 , the strength formation is determined by the joint action of cement and bentonite.Therefore, in the actual project, the amount of bentonite can be adjusted to replace cement, which will prepare a plastic concrete base that meets the strength requirements and is economical and environmentally friendly.

Axial compressive strength
Compression strength tests were conducted on plastic concrete samples with different bentonite and cement dosages.The test results are shown in figure 6.As observed from figure 6(a), the 28-day compressive strength of the three series of samples generally decreased with increasing bentonite dosage from 0 kg m −3 to 120 kg m −3 .The respective reductions were 9.1%, 35.5%, and 30.2%.These results are consistent with other research findings [13].This phenomenon indicates that as the bentonite dosage increases, the encapsulation and dispersion effects have a detrimental impact on the strength of the plastic concrete.However, as the bentonite dosage continues to increase, the reduction in strength becomes less pronounced.Nonetheless, the strength of the three series of plastic concrete samples is still greater than the design target of 1.5 Mpa.They range from 1.6 Mpa to 1.76 Mpa, 2 Mpa to 3.1 Mpa, and 3 Mpa to 4.3 Mpa, respectively.Therefore, they still meet the basic requirements of road base design for secondary and lower-level highways and light traffic conditions as specified in Technical Guidelines for Construction of Highway Roadbases (JTG/T F20-2015) [27], which require a strength of 2.0 Mpa to 4.0 Mpa.
It can be seen from figure 6(b) that the axial strength of 28d plastic concrete increases significantly with the increase of cement content.With the same bentonite content, the strength of plastic concrete in different series increased by 144.3%, 127%, 122.2%, 102.5% and 87.5% respectively with the increase of cement content from 110 kg m −3 to 190 kg m −3 .When the cement dosage increased from 110 kg m −3 to 150 kg m −3 , the compressive strength of plastic concrete samples increased by 76.1%, 53.3%, 42.0%, 39.2%, and 25.0% in the respective series.When the cement dosage increased from 150 kg m −3 to 190 kg m −3 , the compressive strength of the plastic concrete samples increased by 38.7%, 48.0%, 56.5%, 45.5%, and 50.0%.In the series without bentonite, the compressive strength of the plastic concrete samples increased linearly with the increase in cement dosage.However, in other series, the increment of compressive strength of plastic concrete samples became larger when the cement dosage exceeded 150 kg m −3 .Beyond this threshold, the cement dosage has a definitive impact on the strength.Therefore, 150 kg m −3 can serve as a reference limit for cement dosage in plastic concrete.In conclusion, the cement dosage has a decisive influence on the strength of plastic concrete.The higher cement dosage, the higher compressive strength of plastic concrete in the corresponding series.

Splitting tensile strength
The splitting tensile strength is an important reference for plastic concrete to be used as pavement base material, which can well reflect the fracture resistance of materials.It can be seen from the fracture surface of the sample in figure 7 that the aggregate is still complete without fracture.Only some of the aggregate surfaces have friction marks.It can be seen the complete pits left after the aggregate stripping .It shows that the weakness of plastic concrete lies in the interface between aggregate and cementitious material.
The splitting tensile test was conducted for plastic concrete with different bentonite content and cement content.The results are shown in figure 8(a).It can be seen from the figure that, with the increase of bentonite content, the splitting tensile strength of H1 series plastic concrete shows a small decrease overall.The strength ranges from 0.26 to 0.29 MPa, with a decrease of 10.3%.The splitting tensile strength of H2 series and H3 series plastic concrete shows a significant linear downward trend.The splitting tensile strength of plastic concrete is 0.31 ∼ 0.47 Mpa and 0.46 ∼ 0.63 Mpa, respectively, with a decrease of 34% and 27%.
The addition of bentonite has a significant impact on the splitting tensile strength of plastic concrete, especially when the cement content is greater than 150 kg m −3 .On the one hand, the increase of bentonite content will lead to agglomeration.There are more cracks and larger pore diameter in the aggregate, resulting in fewer contact surfaces with the aggregate, further reducing the friction resistance, which is consistent with the phenomenon of splitting section that the aggregate remains intact; On the other hand, with the increase of the number of harmful pores, the stress concentration at the edge of the pores increases that leads to crack expansion.Finally leads to the destruction of the specimen.
It can be seen from figure 8(b) that the splitting tensile strength of plastic concrete increases significantly with the increase of cement content.The splitting tensile strength of plastic concrete increased by 61.7%, 59.4%, 36.3%, 20.0% and 18.7% respectively when the cement content increased from 110 kg m −3 to 150 kg m −3 ; From 150 kg m −3 to 190 kg m −3 , the splitting tensile strength of plastic concrete increased by 33.3%, 30.9%, 44.5%, 59.9% and 47.9% respectively.When the cement content exceeds 150 kg m −3 , the increase of splitting tensile strength of plastic concrete becomes larger.This shows that the cement content of 150 kg m −3 is an important limit.When the cement content exceeds this limit, it will have a decisive impact on the splitting tensile strength.When the bentonite content is 0 kg m −3 and 30 kg m −3 , the splitting tensile strength of plastic concrete increases  linearly with the cement content.The reason is that the more cement is added, the higher the content of cement hydration products CH, AFt, and C-S-H gel is.These hydration products interlace and lap together to form a denser structure, further reducing the number of harmful pores, thus improving the strength of plastic concrete.Therefore, the greater the cement content, the higher the splitting tensile strength of plastic concrete.

Bending tensile strength
The test results are shown in figure 9(a).With the increase of bentonite content, the flexural tensile strength of H1 series plastic concrete shows a slight fluctuation overall, with a gentle trend.The flexural tensile strength is between 0.24 and 0.27 Mpa, with a maximum decrease of 11.1%;The flexural tensile strength of H2 series and H3 series plastic concrete shows a significant linear downward trend.The flexural tensile strength of plastic concrete is 0.38 ∼ 0.58 Mpa and 0.61 ∼ 0.82 Mpa, respectively, with a decrease of 34.5% and 25.6%.When the content of bentonite in the three series is 0 ∼ 60 kg m −3 , the downward trend is more intense, and then tends to be gentle.Therefore, the addition of bentonite has a significant impact on the flexural tensile strength of plastic concrete, especially when the cement content is greater than 150 kg m −3 .
It can be seen different series of plastic concrete with the same bentonite content from figure 9(b).When the cement content increases from 110 kg m −3 to 150 kg m −3 , the flexural tensile strength of plastic concrete increases by 114.8%, 85.2%, 72.8%, 70.4% and 46.9% respectively; when the cement content increases from 110 kg m −3 to 190 kg m −3 , the flexural tensile strength of plastic concrete increases by 203.7%, 166.7%, 177.0%, 166.7% and 135.4% respectively; when the cement content increases from 150 kg m −3 to 190 kg m −3 , the flexural tensile strength of plastic concrete increased by 41.4%, 44.0%, 60.2%, 56.5% and 60.2% respectively.
The increase of cement content can greatly improve the flexural strength of plastic concrete.The flexural tensile strength of H3 series plastic concrete is greater than that of H2 series plastic concrete, while the flexural tensile strength of H2 series plastic concrete is greater than that of H1 series plastic concrete.When the cement content is increased from 110 kg m −3 to 150 kg m −3 , the flexural strength of plastic concrete is increased by the largest extent, which indicates that 150 kg m −3 can be used as the reference value of the best cement content of plastic concrete under the premise of considering the engineering economy.

Elastic modulus
According to the test results in figures 10(a) and (b), the bentonite content has a significant effect on the elastic modulus of plastic concrete.The compressive elastic modulus of prism is greater than that of cube.Due to the difference in specimen size, there is a large difference in numerical value, but the change rule is basically the same.It can be seen from figure 10(a) that the elastic modulus of plastic concrete corresponding to the three series decreases by about 11% ∼ 42%.The dosage of bentonite is 0 ∼ 60 kg m −3 , and the elastic modulus of plastic concrete decreases most rapidly.When the content of H1 and H2 bentonite exceeds 60 kg m −3 , the influence of the content of bentonite on the elastic modulus of plastic concrete decreases significantly.The amount of bentonite has an important influence on the elastic modulus of plastic concrete.The larger the amount of bentonite, the lower the elastic modulus of plastic concrete under the corresponding series.This is mainly because the bentonite particles swell to dozens of times after absorbing water [35], which has a certain plasticizing effect.With the increase of bentonite content, the plasticity of plastic concrete increases, and the elastic modulus decreases.
As shown in figures 11(a) and (b), the change rules of elastic modulus of prism and cube are basically consistent under different cement content.It can be seen from figure 11(a) that with the increase of cement content, the elastic modulus of plastic concrete increases significantly, almost linearly.With the increase of cement content, the elastic modulus of plastic concrete increased by 125.7%, 187.0%, 169.4%, 119.3% and 103.9% respectively.When the bentonite content is 0kg/m 3 and 30 kg m −3 , the elastic modulus of plastic concrete increases linearly with the cement content.While the cement content of other series exceeds 150 kg m −3 , the elastic modulus of plastic concrete increases greatly.The cement dosage has a decisive influence on the elastic modulus of plastic concrete.The higher the cement dosage, the higher the elastic modulus of plastic concrete in the corresponding series.This finding is consistent with the research conducted by Sina [16].Therefore, the cement consumption should be reasonably controlled in the mix design.

Analysis of deformation and failure characteristics
The side of the cube after failure is shown in figure 12.And figure a is crushing failure, mainly consisting of vertical and oblique fine cracks, which are distributed in a network under the load.figure b shows that under the continuous action of load, the main cracks are mainly wide vertical cracks, and the cube is divided into strips.As shown in figure c, the failure mode is shear failure, and the main cracks are vertical cracks and diagonal cracks running through.When the plastic concrete cube is compressed, the typical failure mode in the test process is  that there are small vertical cracks near the side surface of the test block waist and the upper and lower side corners.With the increase of the load, the vertical cracks at the waist and upper and lower corners penetrate, forming a splayed crack that is connected vertically and vertically.At the same time, there are many small diagonal and vertical cracks in the middle of the side, and the side of the test piece bulges outward in the form of chaps.
As shown in figure 13, when the plastic concrete prism sample is compressed, the typical failure mode during the test is that small vertical cracks first appear at the upper and lower side corners.With the increase of load, several small vertical cracks appear in the middle of the diagonal of the side, and then the small vertical cracks run through from head to tail, forming a diagonal main crack.At the same time, plastic concrete volume expansion occurs (bulges outward).The side of the specimen after failure is shown in figure 13. Figure a is the most typical shear failure type, and the specimen is divided into upper and lower parts by a through main oblique fine crack.The failure pattern in figure b is less, and the shape of the main crack is similar to X.Under the continuous action of the load, the waist of the test piece starts to peel off pieces.While the integrity of the two compression ends of the test piece is good, and the restraint effect of the friction force between the upper and lower end faces is obvious.The failure pattern in figure c is seldom seen.The specimen breaks in the middle, and the oblique crack breaks only when it develops to the middle of the specimen.
The typical stress-strain curve of plastic concrete under uniaxial compression is shown in figure 14.The typical stress-strain curve is divided into five stages: OA (primary crack closure stage), due to the shrinkage difference between aggregate and mortar materials and the micro stress field generated by uneven temperature field, micro-cracks appear on the bonding interface of plastic concrete aggregate surface, and under the effect of the initial load, micro-cracks close, and the specimen is compacted.AB (linear elastic stage), under the action of load, the stress-strain curve rises linearly, and its slope is the elastic modulus, and the midpoint is the proportional limit point.BC (stable fracture stage), the increased rate of axial strain decreases gradually, mainly  because new cracks begin to appear in the plastic concrete.CD (rapid crack growth stage), after point C, the new cracks develop rapidly in the plastic concrete, and the cracks continue to accumulate until the plastic concrete is finally destroyed.DF (surface crack penetration stage): after point D of the peak stress, the internal cracks of the plastic concrete gradually penetrate, and the first thin and short longitudinal crack roughly parallel to the compression direction appears on the external surface of the plastic concrete.With the continuous action of the load, several approximately parallel vertical cracks gradually appear, and the vertical cracks gradually expand, forming oblique through cracks, until the oblique cracks become wider, and the volume on both sides rapidly expands outward.Therefore, the stress-strain curve gradually decreases after point D. When the internal damage of plastic concrete becomes more and more serious, it still has a high bearing capacity.
It can be seen from figure 15 that there is a large difference between the uniaxial stress-strain curve of the prism and the uniaxial stress-strain curve of the cube.Although the overall characteristics of the curves are similar, the key points are obviously different.The OA section of the cube test block rises more gently and the curve is longer.The elastic stage AB is not obviously different.Due to the size effect, the elastic segment of the prism is steeper.The most obvious difference is in the DF section.At this time, the crack penetrates.The stressstrain curve of the cube remains stable and the stress decreases slowly under the constraint of the end face because the cube is shorter than the prism.At this time, the whole cube test block is cracked without scattering,  and its integrity is good, indicating that it has good load-holding capacity and deformation capacity [4,24].While the middle of the prism test block peels off quickly, its strength decays rapidly.At the same time, the peak stress and peak strain of the cube are also significantly higher than those of the prism.

Impermeability
According to the results in figure 16(a), the relative permeability coefficient of plastic concrete decreases with the increase of bentonite content.It can be seen from the figure that with the increase of bentonite content, the relative permeability coefficient of the three series decreases by 21.6% ∼ 93.4%.The relative permeability coefficient of plastic concrete is 5.39 × 10 −9 ∼ 1.15 × 10 −7 cm s −1 , basically meeting the impermeability requirements of plastic concrete base.Moreover, the bentonite content is in the range of 0 ∼ 90 kg m −3 , and the decline rate is sharp.When the bentonite content exceeds 90 kg m −3 , the decline trend is relatively gentle.Therefore, the optimum content of impermeable bentonite in plastic concrete is 90 kg m −3 .
From the graph, it can be observed that the higher the bentonite dosage, the better the permeability resistance of plastic concrete in the corresponding series.This is mainly attributed to the continuous filling of pores by the water absorption and expansion of bentonite [8,24].The improvement of impermeability of bentonite under different series is different (H1 > H2 > H3), which shows that the improvement of impermeability of bentonite will be more obvious in plastic concrete with lower cement content.The reason is that the cement stone skeleton of plastic concrete with higher cement content is dense and the internal macropore structure is less than that of plastic concrete with lower cement content.When the cement content is low, there are many macropores.The bentonite can just fill the internal pores, so the improvement effect of impermeability of low cement content series plastic concrete with the same bentonite content will be more obvious [5].
It can be seen from figure 16(b) that when the cement is increased from 110 kg m −3 to 150 kg m −3 , the relative permeability coefficient of the two series with bentonite content of 0 kg m −3 and 30 kg m −3 decreases greatly, while the relative permeability coefficient of the corresponding series of plastic concrete with bentonite content of 60 ∼ 120 kg m −3 decreases slightly.When the cement content increases from 150 kg m −3 to 190 kg m −3 , the influence on the relative permeability coefficient of plastic concrete under each content is not obvious, and the overall trend is slightly downward.
The addition of bentonite has both physical and chemical effects on the improvement of impermeability of plastic concrete.The seepage is carried out through the connecting pores and microcracks in the plastic concrete.Montmorillonite and kaolin in bentonite belong to layered structure and have expansibility with water.In the process of molding, they can fill the initial cracks in the plastic concrete.Part of the pores are closed largely under the extrusion of the internal expansion force, making the internal structure of the plastic concrete more compact, thereby improving the impermeability of the plastic concrete.At the same time, bentonite particles can change the free water in the system into bound water through the adsorption of electric charges, thus improving the impermeability of concrete.In addition, the bentonite particles have the secondary hydration ability, which generates C-S-H gel through secondary hydration, thus filling the pores and micro-cracks, refining the large pores, improving the pore structure, and reducing the porosity and pore connectivity of concrete [34].Moreover, the higher the bentonite dosage, the better the permeability resistance of plastic concrete in the corresponding series.This is primarily due to the continuous filling of pores by the water absorption and expansion of bentonite [8,24].
The increase of cement and bentonite can improve the impermeability of plastic concrete, and the impermeability is significant.There is an optimal economic combination ratio between bentonite and cement.On the premise that the working performance and mechanical performance meet the engineering requirements, the cement content of plastic concrete shall not exceed 150 kg m −3 .

Erosion resistance
It can be seen from figure 17 that under the H1 and H2 series, the erosion quality loss of plastic concrete increases exponentially with the increase of bentonite content, and the fitting degree can reach 0.777 and 0.979 respectively.Under the H1 and H2 series, the erosion mass loss of plastic concrete is 0.005% ∼ 0.091% and 0.005% ∼ 0.064% respectively.
The test results show that when the bentonite content exceeds 60 kg m −3 , the erosion mass loss increases greatly.This is because the addition of bentonite reduces the cohesion of plastic concrete.Under the impact of water vibration, the outer fine particles begin to peel off first.When the fine particles around the sand peel off to a certain extent, the fine sand will peel off.With the increase of bentonite content, the cohesion between particles decreases, and the erosion resistance of plastic concrete decreases rapidly.After water seeps into the gap between the base course and the surface course, it is easy to accumulate, so that the interface between the layers becomes the most direct part of scouring.In order to ensure the stability of the erosion resistance of the base course, the bentonite content should be controlled at about 60 kg m −3 .The appropriate increase of cement content can significantly improve the erosion resistance of plastic concrete.

Shrinkage property
From figures 18 and 19, it can be found that the water loss rate and dry shrinkage strain of plastic concrete under the five bentonite contents of H1 and H2 are basically similar.The water loss rate and dry shrinkage strain increase with the increase of test time, and increase with the increase of bentonite content; The cumulative water loss rate of H1 and H2 is 8.3% ∼ 12.4%, 9.9% ∼ 12.4%, and the cumulative shrinkage strain is 310 ∼ 855 με, 536 ∼ 909 με, The increased range is large, that is, the greater the bentonite content is, the greater the dry shrinkage is; Comparing H1 and H2 series, it can be found that the H2 series with higher cement content has higher water loss rate and dry shrinkage strain than the H1 series with the same bentonite content, indicating that the increase of cement content in plastic concrete will increase its dry shrinkage to a certain extent, which is consistent with the effect of cement content on the dry shrinkage of ordinary concrete and water stabilized materials.
It can also be seen from figure 18 that the water loss of the test piece is particularly fast in the first 7 days, reaching about 70% of the total water loss rate.In the next 7 to 21 days, the growth rate of the water loss rate is slow, and then tends to be stable.However, the dry shrinkage strain increases slowly in the first 5 days of the test, increases rapidly in the next 5 ∼ 17 days, and increases slowly and gradually becomes stable after 17 days.
It can be seen from figure 20 that the variation law of the dry shrinkage coefficient of H1 and H2 plastic concrete is basically the same, and the dry shrinkage coefficient gradually increases with the increase of bentonite content.The dry shrinkage coefficient of bentonite free series is 37.16 × 10 −6 /% ∼ 69.22 × 10 −6 /%, 54.15 × 10 −6 /% ∼ 74.35 × 10 −6 /%, with the maximum increase of 86.3% and 37.3% respectively.The addition of bentonite will make the dry shrinkage performance of plastic concrete worse, and the greater the amount of bentonite, the greater the impact.This is mainly because the larger the bentonite content is, the more pores in the plastic concrete are, and the larger the free water content is, resulting in the larger the drying shrinkage coefficient.The higher the cement content is, the greater the drying shrinkage coefficient is, which is consistent with the research conclusions of many scholars.According to the comparison between figures 21(a), (b) and (e), (f), and (c), (d) and (g), (h), it can be seen that figures 21(e)-(h) has higher cement content, needle bar AFt crystal, flocculent C-S-H gel has higher distribution density, and the number of macropores and small pores is relatively reduced, which shows that the microstructure is denser.
Through micro-morphological analysis and previous studies, it has been observed that as the bentonite admixture increases, more needle-like AFt (ettringite), C-S-H (calcium-silicate-hydrate) gel, and CH (calcium hydroxide) are enveloped and dispersed by bentonite particles.This significant wrapping effect is not conducive to the hydration process of cementitious minerals, resulting in a lower degree of hydration and weaker strength of the formed cementitious minerals.This dispersion of hydration products by bentonite also hampers the cementitious ability of the cement, which helps to explain why the addition of bentonite admixture improves the mechanical properties of plastic concrete but decreases its resistance to erosion.The enhanced impermeability and deformation coordination ability of plastic concrete can be attributed to the filling effect of fine bentonite particles on some of the pores.Furthermore, increasing the dosage of cement helps reduce the number of internal pores in plastic concrete, optimize pore diameter, and increase pore curvature, leading to a denser internal structure [5].

XRD
Figure 22(a) shows the XRD diffraction pattern of bentonite, from which it can be seen that the main components of bentonite are SiO 2 , Al 2 O 3 , CaO, etc The mineral phase was analyzed by Jade software.From the graph in figure 22(b), it can be seen that the XRD patterns of H1-1(bentonite: 0 kg m −3 , cement 110 kg m −3 ), H1-4(bentonite: 90 kg m −3 , cement 110 kg m −3 ), and B1-1(bentonite: 60 kg m −3 , cement 130 kg m −3 ) slurry hydration products are just different in intensity, and no new characteristic peaks appear.The hydration products mainly include CH, AFt, C-S-H gel, etc, of which C-S-H gel is an amorphous substance in the form of gel, mainly in the form of steamed bread peak, and there is no obvious characteristic peak.It is difficult to identify in the atlas.
It can be seen from figure 22(b) that the diffraction peak of CH in the sample with bentonite is obviously weaker than that in the sample without bentonite.There are two possible reasons.On the one hand, due to the high dispersion of bentonite, cement particles react quickly with water.In the process of cement hydration, bentonite accumulates around cement particles under the action of water molecules, forming a layer of bentonite film, which prevents the penetration of water particles, thus delaying the cement hydration process.On the other hand, CH (Calcium Hydroxide) is mainly obtained by the hydration of C 2 S (Dicalcium Silicate) and C 3 S (Tricalcium Silicate).The partial substitution of cement by bentonite leads to a decrease in the content of C 2 S and C 3 S, as well as a reduction in the generation of hydrates.As a result, the mechanical properties also decline.

Conclusion
In conclusion, bentonite plastic concrete, as an economical and environmentally friendly material, meets the relevant standards for workability, mechanical properties, and durability in road subbase design, except its poor resistance to erosion and shrinkage.At present, the plastic concrete base course material is in the preliminary exploration stage, and there are still many problems worthy of further research.Based on this study, further research can be carried out around the road performance of plastic concrete base course material, and long-term observation can be carried out by paving test roads.In this paper, through designing plastic concrete with different bentonite and cement content, the change in plastic concrete performance of bentonite is studied, and the conclusions are as follows: (1) Increasing the content of bentonite and cement can improve the workability of plastic concrete.The maximum increase in slump is 13.6%, and the maximum increase in spread is 21.9%.
(2) Although the incorporation of bentonite reduces the mechanical properties of plastic concrete, the various mechanical indicators at 28 days still meet the design requirements for the subbase of plastic concrete.On the contrary, the optimal content of cement determined by the study is 110 ∼ 150 kg m −3 .
(3) Plastic concrete demonstrates significant resilience, exhibiting a gradual decline in strength during the process of failure.Furthermore, following failure, it tends to fissure rather than disintegrate, displaying commendable integrity.Therefore, it possesses notable load-bearing capacity as well as deformation capability.
(4) As the content of bentonite increases, the corresponding series of plastic concrete exhibits improved impermeability.The optimal for the content of bentonite to enhance impermeability is suggested to within 90 kg m −3 to 120 kg m −3 Similarly, an increase in the cement content can also significantly enhance the impermeability of plastic (5) The addition bentonite has a detrimental effect on the erosion resistance of plastic concrete, as mass rate of plastic concrete increases exponentially with the increase in bentonite content.Conversely, increasing the cement dosage can significantly the erosion of plastic concrete.
(6) With an increase in the cement and bentonite content, the drying shrinkage and water loss rate of plastic concrete also increase.

3. 1 .
Working performance 3.1.1.Slump and expansion The variations in slump and flow values of plastic concrete mixtures are shown in figures 4(a) and (b).

Figure 4 .
Figure 4. Effect of bentonite content on the workability of plastic concrete.

Figure 6 .
Figure 6.Change rule of axial compressive strength.

Figure 10 .
Figure 10.Variation of elastic modulus with bentonite content.

Figure 11 .
Figure 11.Variation of elastic modulus with cement content.

Figure 15 .
Figure 15.Typical Stress Strain Relationship of Prism and Cube.

Figure 17 .
Figure 17.Variation of scouring mass loss with bentonite content.

3. 4 .
Microscopic morphology and composition analysis 3.4.1.SEM Figures 21(a) and (b).It can be seen from figure 21(c) and (d) that when bentonite is added, bentonite particles are inlaid and wrapped around needle like AFt, C-S-H gel and CH, which better fill the pores of gel, with a decrease in porosity and good integrity of microstructure.But AFt, C-S-H gel, CH, etc are dispersed more thinly by bentonite particles.

Figure 18 .
Figure 18.Variation of drying loss rate of plastic concrete with time.

Figure 19 .
Figure 19.Variation of drying shrinkage strain of plastic concrete with time.

Figure 20 .
Figure 20.Variation of Shrinkage Coefficient with Bentonite Content.

Table 1 .
Basic performance indexes of cement.

Table 2 .
Chemical composition of bentonite.

Table 3 .
Main physical performance indexes of bentonite.

Table 4 .
Clay particle size and relative content.

Table 5 .
Fine aggregate screening test results.

Table 6 .
Results of screening test for coarse aggregate.