Experimental Study on the Shear Strength of Cement-Sand-Gravel Material

An experimental study on the shear strength development of cement-sand-gravel (CSG) material was carried out using triaxial compression tests. *e effects of the cementing agent content, aggregate content, and gradation on the shear strength of CSG material were analyzed. *e shear strength remarkably increased with increasing cementing agent content and aggregate content for a given confining pressure. *e increase in shear strength with increasing cementing agent content far exceeded that with increasing aggregate content. However, the stress-strain curves and shear strength changed only slightly when the aggregate gradation for CSGmaterial was adjusted. Based on the test data, a strength criterion for CSGmaterial is proposed as a function of the cementing agent content, aggregate content, and shear strength of the aggregate gradation.


Introduction
Like roller-compacted concrete (RCC), cement-sand-gravel (CSG) material consists of water, aggregate (rockfill material, sandy gravel material, etc.), and cementing agents such as Portland cement and fly ash.As compared to RCC, the advantages of CSG material include a lower requirement of cementing agent content, its compatibility with local aggregate, and less stringent temperature control requirements.CSG materials with varying cementing agent contents, aggregate contents, and gradations have been utilized in various infrastructure applications, such as embankments, soil treatments, reinforcement for small rural hydropower structures, and, most commonly, in dam construction [1].
A strength requirement is a basic premise in engineering applications of geotechnical materials; thus, examination of the strength characteristics of geotechnical materials is extremely important.Since the 1990s, scholars have been researching cemented sand.Some researchers [2,3] have obtained results on the strength characteristics of CSG materials from a series of compressive strength tests.e results of previous research indicate that the compressive strength of CSG material increases with increased cementing agent content, the optimal water-cement ratio is 1.2, and the strength is maximized when the fines content lies within the range of 25-30%.Kongsukprasert et al. [4] studied the effects of several factors, including the water content, cementing agent content, dry density, and curing period, on the shear strength of CSG material using triaxial compression tests conducted at a confining pressure of 19.8 kPa.Although extensive, the effects of the confining pressure on the shear strength of CSG material were not considered in these previous studies.Wu et al. [5] analyzed the effects of curing age on the shear strength of CSG material via triaxial testing and subsequently used the test data to establish a shear strength criterion as a function of the curing age and confining pressure.Sun et al. [6] conducted triaxial tests on CSG materials with cementing agent content of less than 60 kg/m 3 ; additionally, static and dynamic triaxial tests on CSG materials with cementing agent content exceeding 60 kg/m 3 were conducted by Fu et al. [7].
ese studies mainly focused on the effects of the cementing agent content on the shear strength of CSG material under different confining pressures.Amini and Hamidi [8] analyzed the effects of the cementing agent content on the cohesion c and internal friction angle φ in the Mohr-Coulomb criterion under drained and undrained conditions using triaxial compression testing.However, a shear strength criterion based on the cementing agent content and confining pressure was not proposed in these studies.Li et al. [9] conducted triaxial compression tests on artificial cemented sand, which is a type of material similar to CSG material, and proposed several novel strength criteria based on the experimental results for varying cementing agent content.Because the size of the aggregate in CSG material is significantly different from that in cemented sand, it is unclear whether a strength criterion developed for artificial cemented sand can be directly applied to CSG material.Clough et al. [10] and Wang [11] analyzed the effects of the aggregate content on the shear strength of CSG material for different confining pressures, but did not propose an aggregate content-based strength criterion.A review of the literature shows that research regarding the effects of aggregate gradation on the shear strength of CSG material is insufficient.For CSG dams, because the geological conditions and requirements of each dam project differ, the cementing agent content, aggregate content, and gradation of CSG material also vary for different dams.
In this study, triaxial compression tests were conducted to assess the effects of the cementing agent content, aggregate content, and gradation on the shear strength of CSG material.Additionally, a new strength criterion for CSG material is proposed based on the results.e purpose of the proposed strength criterion is to provide a basis for the construction of a reasonable constitutive model suitable for various types of CSG materials and to meet the engineering requirements for various infrastructure applications, including CSG dam construction.e ratio of sand to crushed stone is 1 : 4, which was the same as that used in the experimental study by Sun et al. [5].In this paper, the aggregate gradation for Material I is termed No. 1.

Material II
Cement: 42.5 grade ordinary silicate cement from the Anhui Digang Hailuo Cement Co., Ltd.Fly ash: Type I fly ash from the Nanjing market.Sand and gravel: One aggregate gradation (No. 2) and a second gradation (No. 3), which are listed in Table 1.
Water: tap water.

Mix Proportions of CSG Material and Test programs.
For Material I, the water-cement ratio was 1.0 [12], and the cementing agent contents were 20 kg/m For Material II, the ratio of cement to coal ash was 1 : 1, and the water-cement ratio was 1.0 [12].One aggregate gradation (No. 2) samples of Material II were subjected to drained triaxial shear tests under various confining pressures (300 kPa, 600 kPa, 900 kPa, and 1200 kPa) and varying cementing agent content (20 kg/m 3 , 80 kg/m 3 , and 100 kg/m 3 ).In addition, to assess the effects of aggregate gradation on the strength characteristics of CSG, a second gradation (No. 3) was tested in Material II samples.ey were subjected to drained triaxial shear tests under different confining pressures (300 kPa, 600 kPa, 900 kPa, and 1200 kPa).Test programs conducted on Material II, to investigate the effects of cementing agent content and aggregate gradation, are presented in Table 3.

Equipment and Test Methods Used in Drained Triaxial
Shear Tests.Drained triaxial shear tests of CSG material samples were conducted using a TYD-1500 dynamic triaxial tester, as is shown in Figure 1. e mixing materials used to produce CSG material samples for a series of large-scale triaxial tests are shown in Figure 2(a).
e materials were compacted in steel molds of 30 cm in diameter and 70 cm in height (Figure 2(b)).e samples were cured in a laboratory at a temperature of 20 ± 2 °C for 28 days.
e triaxial tests for determination of the shear strength of the CSG materials were conducted in accordance with China Standard SL237-1999 [13].
e samples were first saturated and then subjected to one of the four levels of confining pressures (300, 600, 900, or 1200 kPa) for 10 min prior to axial loading.Axial loading at a strain rate of 2 mm/min was then applied and stopped when the axial strain reached 15%.
To improve the accuracy of the results, two samples were prepared and tested for each test group.To prevent damage to the tester due to particles falling from damaged samples, the samples were covered with rubber sleeves that were securely fastened.

Shear Strength versus Cementing Agent Content.
e results of the drained triaxial shear tests performed on the samples of Material I and Material II are presented in Figures 3 and 4. As these figures show, when the cementing agent content is low, the q − ε a curves of CSG material comprise of three stages: an initial stress increase, a slowing  Advances in Materials Science and Engineering stress increase, and a peak stress similar to that of the rock ll material in the CSG material.e in uence of the cementing agent content on the strain-softening behavior of the material is apparent.e stress-strain curves consist of ve stages: an initial stress increase, a slowing stress increase, a peak stress, plastic softening, and a residual strength that approaches that of RCC material when the cementing agent content is increased to 100 kg/m 3 .e maximum stress and stress at a given axial strain both signi cantly increase with increasing cementing agent content at each con ning pressure considered in this study (300 kPa, 600 kPa, 900 kPa, and 1200 kPa).
is is because cementation between the particles in the CSG material increases with the cementing agent content, thus causing the internal bearing capacity mechanism to change from friction between particles, as in rock ll material, to gradually increasing internal cohesive strength.is is consistent with the results of Li et al. [9], who reported that the cementing agent content is the main factor in uencing the strength of arti cial cemented sand, which is similar to CSG material.
Figure 5 illustrates the shear strength, which is the maximum stress shown in the curves in Figures 3 and 4 under varying con ning pressure and cementing agent content.As Figure 5 shows, the shear strength of CSG material ranges from 1200 to 12,000 kPa and increases with increasing cementing agent content and con ning pressure.
e relationship between the peak strength and con ning pressure is approximately linear for a given cementing agent content. is is consistent with the observations of Fu et al. [7] and other researchers [5] who have reported that increasing the cementing agent content is highly e ective in enhancing the shear strength of CSG and various other  Advances in Materials Science and Engineering materials, such as cemented sand and polyurethane foam adhesive rock ll materials.

Shear Strength versus Aggregate Content.
Figure 6 shows the stress-strain curves for CSG materials with respect to the aggregate content, obtained via drained triaxial shear testing.
As Figure 6 shows, the aggregate content has little e ect on the shape of the stress-strain curve, but the peak stress increases as the aggregate content increases.is is attributed to an increase in the aggregate content reinforcing the internal bearing capacity of the CSG material, which is a result of increased particle contact area.
Figure 7 shows the shear strength, which is the maximum stress in the curves shown in Figure 6, for varying con ning pressure and aggregate content.As these gures show, the shear strength increases with increased con ning pressure and aggregate content. is is consistent with the results of Wang [11] regarding the changes in the strength characteristics of CSG material with relative density and a con ning pressure below 300 kPa.In comparison with the cementing agent content, the aggregate content yields less e ect on the shear strength of CSG material.

Shear Strength versus Aggregate Gradation.
Figure 8 shows the stress-strain curves for CSG materials with different aggregate gradations, obtained via drained triaxial shear testing.e e ects of aggregate gradation on the stressstrain behavior of CSG material are not notable for any of the con ning pressures considered (300 kPa, 600 kPa, 900 kPa, and 1200 kPa).
is is similar to the minimal e ect of

Strength Criterion
e Mohr-Coulomb theory, which serves as the basis for the strength criterion for CSG material in this study, is commonly used to describe the stress-strain response of materials [5][6][7][8].It can be expressed as follows: where c is the cohesion of the material, φ is the angle of the internal friction, τ f is the shearing stress, and σ is the normal stress.
To describe the relationships between the peak strength and con ning pressure for the varying cementing agent content and aggregate content shown in Figures 6 and 7, the Mohr-Coulomb criterion represented by the principal stress in (1) can be expressed as follows: where q m is the peak strength, σ 3 is the con ning pressure for drained triaxial shear testing, c is the cohesion, and φ is the angle of the internal friction (shearing resistance).
Based on (2) and the shear strength test results obtained for di ering cementing agent content and aggregate content, values for the cohesion c and angle of internal friction (shearing resistance) φ were extracted for this analysis, as is shown in Tables 4 and 5.
e Mohr-Coulomb theory is based on the assumption that the cohesion and angle of shearing resistance in (1) are constant.However, for CSG materials used in practical engineering applications, the cohesion and angle of shearing resistance vary according to the cementing agent content and aggregate content.
is means that the original Mohr-Coulomb strength theory expression is not suitable for CSG materials with varying cementing agent content and aggregate content.us, a new strength criterion for the shear strength of CSG material is proposed.is criterion is a function of the cementing agent content and aggregate content.4), the relationship between cohesion and the cementing agent content can be expressed as follows:  Advances in Materials Science and Engineering

Cohesion c. Based on the cohesion values obtained for di erent cementing agent contents (listed in Table
where H 0 is the parameter related to the composition of the CSG material and C c is the cementing agent content.When the cementing agent content in (3) is low, the cohesion of the CSG material is near zero, which is close to the cohesion of rock ll material, as calculated by Sun et al. [5]. Figure 9 shows a comparison of the test data and results calculated using (3) for CSG material [5,6,8], PFA-reinforced rock ll material [14], cemented sand [9], and cemented soil [15] for di erent cementing agent contents.As Figure 9 shows, the calculated results for CSG material, PFA-reinforced rock ll material, and cemented sand t the experimental results well; this con rms that (3) yields a reasonable description of the cohesion of those cemented and bonded materials as a function of the cementing agent content.However, because the soil in the cemented soil studied by Baxter et al. [15] had some viscosity and a cohesion value greater than zero, (3) is not suitable for this type of cemented soil.
Based on the cohesion values obtained for di erent aggregate contents (Table 5), curves of cohesion as a function of aggregate content were developed, as shown in Figure 10.
ese curves show that the cohesion of CSG material increases with increasing aggregate content.However, compared to the in uence of the cementing agent content, the in uence of the aggregate content on the cohesion of CSG is lower.
e relationship between cohesion and aggregate content can be formulated as follows: where H g is the parameter related to the type of aggregate in the CSG material and ρ g is the aggregate content.By where H z is the parameter related to the composition and type of aggregate for CSG material.According to the results of the drained triaxial shear tests described above, H z 0.005.

Internal Friction Angle φ.
Figure 11 illustrates the internal friction angle values obtained for CSG material [5,6,8], PFA-reinforced rock ll material [14], cemented sand [9], and cemented soil [15] for di erent cementing agent contents.As Figure 11 shows, the internal friction angle of CSG material, PFA-reinforced rock ll material, cemented sand, and cemented soil ranges from 30 °to 50 °, which moderately di ers from the range of 25 °to 65 °for gravel [16].e reason for this di erence is that the cementing agents in the cemented materials limit the slippage angle of the aggregate.e internal friction angle of CSG material and PFA-reinforced rock ll material for various cementing agent contents is approximately 39.5 °.
Similarly, the internal friction angle of CSG material is approximately 39 °for various aggregate contents, as presented in Table 5.Based on these results, the internal friction angle value for CSG material is taken as 39.3 °for a range of cementing agent contents and aggregate contents.

Strength Criterion according to Cementing Agent Content and Aggregate Content.
Based on the results summarized above, the following expression for the shear strength of CSG material as a function of the cementing agent content and aggregate content is proposed:

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where H z is a parameter associated with the type and composition of the CSG material.
For a given value of c, (6) describes the peak strength of CSG material for varying aggregate content.For a given value of ρ g , ( 6) describes the peak strength of CSG material for varying cementing agent content.
According to the results of triaxial testing on Material I for various cementing agent contents and ( 6), H z 0.005 and φ 39.3 °.Equation ( 6) can also be expressed as follows: To verify (6), drained triaxial shear tests on samples with a cementing agent content of 60 kg/m 3 and aggregate content of 2110 kg/m 3 were conducted under con ning pressures of 300 kPa, 600 kPa, 900 kPa, and 1200 kPa.e test results and calculated results are shown in Figure 12. e calculated results t the experimental results well, thereby demonstrating that (6) can be used to describe the shear strength of CSG material as a function of the cementing agent content and aggregate content.

Conclusions
e e ects of the cementing agent content, aggregate content, and gradation on the shear strength of CSG material were investigated by means of drained triaxial shear testing.
e conclusions drawn from the test results can be summarized as follows.The calculated results The test results

Figure 3 :
Figure 3: Stress-strain curves of Material I with regard to the e ect of cementing agent content under various con ning pressures: (a) 300 kPa; (b) 600 kPa; (c) 900 kPa; (d) 1200 kPa.

Figure 7 :
Figure 7: Relation between peak strength and con ning pressure under di erent aggregate contents.
(a) e in uence of the cementing agent content on the shear strength of CSG material is much more signi cant than the in uence of the aggregate content and gradation.(b) e cohesion of CSG increases with increasing cementing agent content and aggregate content, whereas the internal friction angle changes only slightly.e e ects of the aggregate gradation on cohesion and the internal friction angle are negligible.(c) A strength criterion for CSG material is proposed based on an analysis of the strength characteristics of the material as a function of the cementing agent content, aggregate content, and aggregate gradation.Overall, strength model ts the test data well of CSG material and can provide evidence for numerical calculation of CSG dam.

Figure 12 :
Figure 12: Comparison of the test results and calculated results.

Table 1 :
Aggregate gradation for Material II.

Table 2 :
Test programs on Material I to investigate effects of cementing agent content and aggregate content.

Table 3 :
Test programs of Material II to investigate effects of cementing agent content and aggregate gradation.

Table 4 :
Cohesion and internal friction angle under di erent cementing agent contents.

Table 5 :
Cohesion and internal friction angle under di erent aggregate contents.