Small-strain dynamic properties of silty clay stabilized by cement and fly ash

Construction and Building Materials 237 (2020) 117646

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Small-strain dynamic properties of silty clay stabilized by cement and fly ash Lei Lang a,b, Fudong Li c, Bing Chen a,b,⇑ a

State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Department of Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China c Key Laboratory of Ministry of Education on Safe Mining of Deep Metal Mines, Northeastern University, Shenyang 110004, China b

h i g h l i g h t s
The small-strain dynamic shear modulus (G) and damping ratio (D) of CSC and CFSC were investigated by resonant column tests.
The G and D increased and decreased respectively, with an increase in the confining pressure and curing age.
The D was significantly affected by the cleanliness and cementation of soil particles.
It is desirable to use 5% cement together with 30% fly ash instead of 15% cement alone for the stabilization of silty clay.

a r t i c l e

i n f o

Article history: Received 14 June 2019 Received in revised form 7 September 2019 Accepted 17 November 2019

Keywords: Small-strain dynamic properties Resonant column test Silty clay Cement Fly ash Microstructure

a b s t r a c t This study examines the small-strain dynamic properties of cement-stabilized silty clay (CSC) and cement-fly ash-stabilized silty clay (CFSC). A series of resonant column tests were performed to investigate the effect of confining pressure, binder content and curing age on the small-strain dynamic shear modulus (G) and damping ratio (D) of CSC and CFSC. Furthermore, the maximum dynamic shear modulus (Gmax) and maximum damping ratio (Dmax) of CSC and CFSC were also evaluated. The results showed that the G of CSC and CFSC increased with the increase of confining pressure, binder content and curing age. The G decreased slowly and then rapidly with shearing strain (c), and the c of 104 was the turning point of evaluating the small-strain dynamic behavior of CSC and CFSC. Using 5% cement together with 30% fly ash as stabilizer has advantageous over using 15% cement alone in improving the stiffness properties of silty clay. The D of CSC and CFSC decreased with the increase of confining pressure and curing age, but increased with an increase in cement and fly ash content. This was attributed to the weakly cemented and rough of soil particles led to more energy dissipation of vibration wave propagating through the sample. The microstructural analysis revealed that the micro-aggregate effect and hollow structure of fly ash particles not only contributed to the improvement in G, but also effectively improve the energy dissipation performance of CFSC. Ó 2019 Published by Elsevier Ltd.

1. Introduction The subgrade is usually subjected to different vibrations resulting from earthquakes, traffic loads, pile driving and other vibration machines [1]. Soil dynamic problems involve a wide range of strains, ranging from small amplitude vibration of foundation to large strain vibration caused by strong earthquake or nuclear explosion, and the range of shear strain varies from 106 to 102. Improving the dynamic properties of geotechnical structures is

⇑ Corresponding author at: Department of Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail address: [email protected] (B. Chen). https://doi.org/10.1016/j.conbuildmat.2019.117646 0950-0618/Ó 2019 Published by Elsevier Ltd.

an ongoing problem that has been extensively studied so as to innovate foundation treatment methods. Dynamic parameters including shear modulus (G) and damping ratio (D) of soils in the small strain range of 0.0001–0.1% are essential to evaluate the geotechnical structures subjected to vibration safety or earthquake [2]. A series of laboratory tests, such as resonant column, bender element, cyclic simple shear, cyclic torsional shear and cyclic triaxial are usually used for determining the dynamic properties of soils [2–5]. The resonant column test is usually used for measuring the dynamic properties (G, D) of soils in the small range of 0.0001–0.1% [6–10]. In general, many studies have been performed to investigate the dynamic properties of soils. Anastasiadis et al. [11] and Senetakis

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et al. [12] investigated the effect of rubber content on the dynamic properties of sand-rubber mixtures. Dutta et al. [13] investigated the effect of saturation on G and D of compacted clay by a series of resonant column tests, and concluded that the G decreased and D increased with the increase of saturation. Song et al. [14] investigated the low strain stiffness (G0) and damping (Dmin) of loess combining a series of torsional resonant column tests, their research results confirmed that G0 at low strain is significantly affected by water content, but the influence of water content on Dmin is small. Zhao et al. [15] evaluated the shear moduli and damping ratios of silica gel through a series of resonant column tests, and revealed that silica gel is similar to natural soils in dynamic behavior. Presti et al. [16] explored the effect of the strain level, loading rate, number of loading cycles and type of loading on the variation of G and D of two natural clays by a series of resonant column tests. Kumar and Achu [17] examined the influence of cyclic strain history and shear modulus of dry sand by a series of resonant column tests. Payan et al. [18] reported the effect of nonplastic fines content on the small-strain dynamic behavior of silty sands by means of torsional resonant column tests. Lin et al. [19] investigated the G and D of thawed saturated clay under longterm cyclic loading by a series of triaxial tests, and their results showed that the increase of confining pressure has no obvious effect on D, but has significant effect on G before the shear strain reaches a certain level. Using both resonant column and bender element tests, Jafarian et al. [20] studied the dynamic properties of calcareous and siliceous sands under the conditions of isotropic and anisotropic stress. Within a wide strain range, the dynamic properties of soils admixed rubber has been systematically explored before [21–23]. By conducting means of resonant column and bender tests, Bate et al. [24] investigated the dynamic properties of fine-grained soils containing organic, and they concluded that the total organic content and void ratio were the main factors affecting the stiffness of organoclays. Furthermore, the laboratory techniques of bender and extender elements tests have been adopted by Kumar et al. [25,26] for evaluating the dynamic properties of sand. In past decades, numerous research work has been made on the dynamic properties of sands and soft clay by conducting a series of laboratory techniques. It can be concluded from the above studies that very few investigations have been carried out on the small-strain dynamic properties of solidified silty clay, especially the effects of confining pressure, binder content and curing age on the variation of G and D are almost blank. This paper aims to perform a series of resonant column tests to fill these vacancies. Portland cement (PC) is widely used to stabilize soft soil due to its easy availability, high efficiency and reasonable price [27], and relevant scholars have done a lot of research on the dynamic properties of cement-stabilized sand [26,28,29]. The stabilization mechanism of cemented soil is through the hydration of cement, then produces the cementitious hydration products calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH), which are responsible for the improvement in mechanical properties [30–33]. However, it is noteworthy that cement production needs to consume a large number of natural resources, and produces a large number of greenhouses gases, which is not conducive to environmental protection and resource sustainability [34–38]. Fly ash (FA) is an industrial solid waste discharged from coal-fired power plants during energy production, and usually stored at coal-fired enterprises or directly dumped in landfills [39]. FA has become the largest solid waste in China, but its comprehensive utilization rate is still very low. The disposal of FA is ongoing issue for many coal-fired enterprises. If FA can be used as construction material in foundation treatment, which can not only consume a large amount of waste FA, but also save resources and protect the environment. FA has potential pozzolanic activity and is usually used as a cement admixture for the stabilization of soft soil [40–44]. Except

as cement admixture in soft soil stabilization, FA is also used in pavement and subgrade treatment [45,46]. Despite several investigations on the use of FA in applications such as soft soil stabilization, pavement and subgrade treatment, the small-strain dynamic properties of using FA together with cement in silty clay improvement application, is currently limited. This paper presents a laboratory study on the small-strain dynamic properties of cement-fly ash-stabilized silty clay (CFSC) by conducting a series of resonant column tests. The small-strain G and D of CFSC are studied by comparison with that of cementstabilized silty clay (CSC) and the samples with different mix proportions, confining pressures and curing ages are prepared for each. Furthermore, the maximum dynamic shear modulus (Gmax) and damping ratio (Dmax) are introduced to evaluate the effectiveness of using FA in silty clay improvement. To further elaborate the findings, the microstructures of typical samples were analyzed by scanning electron microscopy (SEM) tests. The results of this study will encourage the use of FA, in the future foundation treatment projects on silty clay areas, with the additional benefit of consuming waste FA stored in landfills. 2. Materials and methods 2.1. Raw materials The raw materials used in this study are silty clay, fly ash (FA) and Portland cement (P.O42.5). The silty clay was taken from Shenyang in Northeast China. According to a series of materials characterization tests, the silty clay had the natural water content of 30.43%, liquid limit of 26.08% and plastic limit of 11.18%. Furthermore, the bulk density of silty clay was 1.91 g/cm3 and the optimum water content determined by compaction test was 20%. The particle size distribution of silty clay is presented in Fig. 1. FA as an industrial by-product was used as cement admixture for stabilizing silty clay. Based on the scanning electron microscopy (SEM) tests, the microstructures of silty clay and FA were shown in Fig. 2. It could be seen that FA particles are hollow spherical glass beads, which is conducive to fill the voids between soil particles. This observation is in line with the prior studies [5,47]. Portland cement (P.O 42.5) was used as soil stabilizer in this study. The chemical compositions of raw materials are shown in Table 1. 2.2. Mix design and testing program The purpose of this study is to investigate the effects of confining pressure, binder content and curing age on the small-strain dynamic properties of CSC and CFSC. According to the compaction tests, it was found that the optimum water content (OWC) of the CSC and CFSC mixtures was 18% and 15%, respectively. The similar experimental phenomena were also found by Zentar et al. [48], and the reason was that the FA lubricated soil particles and filled the voids. Additionally, the other possible reason might be that the water absorption of FA was less than that of cement. Combined with the compaction test results, the designed water content of CSC and CFSC was determined 18% and 15%, respectively. The details of mix design and testing program were given in Table 2.

Fig. 1. The particle size distribution of silty clay.

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Fig. 2. The microstructures of (a) fly ash and (b) silty clay.

Table 1 Chemical compositions of raw materials. Raw materials

Percentage content of the sample (%)

SC PC FA

SiO2

MgO

Al2O3

SO3

Fe2O3

CaO

K2O

Na2O

Loss

57.02 21.60 54.90

3.68 1.06 1.80

16.42 4.13 25.80

0.05 1.74 0.60

6.79 4.57 6.90

3.63 64.44 8.70

3.59 0.56 0.10

0.81 0.11 0.30

6.43 0.76 0.20

Notes: SC: silty clay; PC: Portland cement; FA: fly ash.

Table 2 Summary of mix design and testing program. Series

PC (%)

FA (%)

SC (%)

WC (%)

CA (days)

CP (kPa)

1 2 3 4 5 6

8 12 15 5 5 5

0 0 0 20 30 40

92 88 85 75 65 55

18 18 18 15 15 15

3, 3, 3, 3, 3, 3,

50, 50, 50, 50, 50, 50,

7, 7, 7, 7, 7, 7,

28 28 28 28 28 28

100, 100, 100, 100, 100, 100,

Symbol 150 150 150 150 150 150

C8 C12 C15 CF14 CF16 CF18

Notes: PC: Portland cement; FA: fly ash; SC: silty clay; WC: water content; CA: curing age; CP: confining pressure; C8: the cement content is 8% (C12 and C15 are similar); CF14: the mass ratio of cement to fly ash is 1:4 (CF16 and CF18 are similar). 2.3. Sample preparation

2.5. Test procedures

The silty clay was first dried in the oven under the temperature of 105 ±5 , then crushed into powder and passed through 2 mm sieves. Based on the mix design, the required mass of raw materials and water were weighted. Poured the weighted raw materials into the mixer and stirred for 5 min, then gradually added water and continued stirring for 10 min to obtain a uniform mixture. The cylindrical steel moulds with the diameter of 50 mm and height of 100 mm were used for sample preparation. Noting that the Vaseline was applied on the inner of the mould so as to facilitate later demoulding. The sample preparation process was conducted by artificial vibration, which was divided into four sequential layers, and each layer had the same number of vibrations to ensure uniform compaction. The surface of each layer should be scraped after compaction in order to avoid delamination effect. After sample preparation, demoulding was carried out and the samples were wrapped with fresh-keeping film and maintained until testing. The sample preparation process is shown in Fig. 3.

The small-strain dynamic properties of each sample, namely the G and D were determined by drawing the stress–strain hysteresis loop and vibration attenuation curve obtained from the fixed-free resonant column test system as per to the ASTM D4015-07 standard [49]. Fig. 4 presents the details of test procedures. The tested sample was firstly vacuum-pumped for one hour, and then gradually injected with distilled water until the sample was completely immersed and started the saturation process. Using rubber sheath to cover the sample and installed the pervious stone, the bottom of sample was fixed on the base and both ends needed to be covered with leather bands to ensure the sealing. After sample installation, the sample was covered with a steel plastic housing and distilled water was slowly injected into so that no bubbles produced. The top-cap was positioned on the top of the sample and the drive system should be carefully aligned to ensure free movement of each magnet within the coils. Then, the sample was isobarically consolidated until the pore pressure dissipates and the strain was constant. The initial drive voltage was set to 0.01 V, adjusted the excitation voltage during the test to ensure that the shear strain was controlled between 106 and 104. Based on the resonance frequency, sample density (q), geometric dimensions and boundary conditions of samples, the G was extrapolated by the following expression.

2.4. Test apparatus The fixed-free resonant column test system was used to measure the small-strain G and D of CSC and CFSC, and its testing accuracy can reach 1010. In the process of testing, the cylindrical sample was excited in torsion mode by a drive system, which was made up of a four-arm rotor and a support cylinder. The theoretical basis of resonant column test is to vibrate the tested sample in a basic pattern of vibration, and then produces the torsion or flexure. By testing the motion of free end, the wave velocity and energy dissipation performance of the sample can be obtained, and then the D of the tested sample could be measured directly. Combining with the velocity and density (q), the G of the tested sample can be determined.

G ¼ qð2pfh=bÞ

2

ð1Þ

where f is resonance frequency; h is 100 mm; b is eigenvalue of vibration equation. The D was directly recorded in the system, and it was based on the freevibration decay curve. The logarithmic decrement (d) of the decay curve is calculated by the following equation.

d¼

1 lnðh1 =hnþ1 Þ n

ð2Þ

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Fig. 3. Sample preparation process: (a) cylindrical steel mould; (b) vibration compaction forming; (c) sample after demoulding; (d) some samples for testing.

where n represents the cycles betweenh1 andhnþ1 ; hi represents the amplitude after excitation. Usually, between 10 and 50 cycles were used in calculation, and D was determined from the d using:

D¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d2 4 p 2 þ d2

 100%

ð3Þ

2.6. Microstructure The microstructural analyses of typical samples were conducted by employing the scanning electron microscopy (SEM) tests. A COXEM EM-30 Plus SEM was used in this study for SEM testing. The tested sample pieces which not exceeding 7 mm in size were prepared. Before testing, the sample pieces were coated with gold on surface, and then loaded into the equipment for capturing SEM images.

3. Results and discussion 3.1. Small-strain dynamic shear modulus (G) The effects of confining pressure, binder content and curing age on the variation of G with changes in shearing strain (c) are shown in Figs. 5–7. Due to a lot of figures are look similar, the results with typical features were selected to be described.

Fig. 5 shows the effect of confining pressure on the variation of G with c. It could be found that the G increased with the increase of confining pressure. This is due to the increase of confining pressure compressed the voids between the soil particles, and then the compactness of sample increased. The increase of compactness was responsible for the increase of G. With the development of torsional vibration, the internal structure of sample loosened until it was destroyed, showing that the G decreased nonlinearly with c. As illustrated in Fig. 5, the variation of G with c can be described by a two-stage model: gradual decrease and rapid decrease. When c was between 106 and 104, G decreased gradually; when c increased to 104, G decreased sharply. This observation is in line with the nonlinearity and hysteresis of dynamic stress–strain relation of soils. It could be presumed that the c of 104 is the turning point, which is important to the small-strain dynamic evaluation of CSC and CFSC. Fig. 6 presents the effect of binder content on the variation of G with c. The G of CSC and CFSC increased with the increase of cement and FA, respectively. By comparison, the G of CFSC with CF16 was more than that of CSC with C15. This indicates that using 5% cement together with 30% FA instead of 15% cement alone for stabilizing silty clay can effectively improve the G. The stabilization mechanism of cemented soil is through the cement hydration, and

L. Lang et al. / Construction and Building Materials 237 (2020) 117646

5

Fig. 4. The test procedures of resonant column test (a) sample vacuum pumping and saturation; (b) sample installation; (c) drive system installation; (d) waveform of torsional excitation.

Fig. 5. Effect of confining pressure on the variation of dynamic shear modulus with shearing strain at 28 days (C15 and CF18).

then produces the calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH), which are responsible for the improvement in bond strength between soil particles. Furthermore, the secondary hydration reaction between Portlandite (Ca(OH)2) and clay minerals (mainly SiO2 and Al2O3) produced more CSH and CAH, which are responsible for long-term strength development. For CFSC, the filling effect of FA also improved the compactness and

Fig. 6. Effect of binder content on the variation of dynamic shear modulus with shearing strain at 28 days (150 kPa).

then increased the G. Therefore, using cement together with FA as stabilizer for improving the G of silty clay, which has more advantage over using cement alone. Fig. 7 shows the effect of curing age on the variation of G with c. It is evident that the G increased with the increase of curing age. The longer the curing age was, the more fully the cement hydration was, and the more hydration products (CSH and CAH) were

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Fig. 7. Effect of curing age on the variation of dynamic shear modulus with shearing strain under the confining pressure of 150 kPa (C15 and CF18).

Fig. 8. Relationship between Gmax and confining pressure of CSC and CFSC at 28 days.

produced. Generally, the cement hydration is conducive to the increase of early strength development, while the secondary hydration reaction (pozzolanic reaction) also had significant improvement in the later strength development. It is well known that FA has good potential pozzolanic activity, thus using cement together FA can simultaneously improve the early and long-term dynamic strength development of solidified silty clay. 3.2. Maximum dynamic shear modulus (Gmax) It could be concluded from the variation of G with c that the G reached the maximum at a strain level of 106, and then decreased nonlinearly with the increase of c. The dynamic stress–strain relations of CSC and CFSC were fitted based on the Hardin-Drevich (H-D) model [50], and then the maximum dynamic shear modulus (Gmax) can be evaluated as per to the following expression:

G Gmax

¼

1þ

1  a c cr

ð4Þ

where G=Gmax is normalized dynamic shear modulus; cand cr are shearing strain and reference shearing strain, respectively; ais the curvature parameter relating to the variation of curve-fitting. Usually, cr is determined by the following equation.



cr ¼ cr1

r0m pa

Fig. 9. Relationship between Gmax and binder content of CSC and CFSC.



of silty clay. Furthermore, the Gmax of CFSC with CF14 is slightly higher than that of CSC with C8, but lower than that of CSC with C12. Fig. 10 represents the effect of curing age on the variation of Gmax under the confining pressure of 150 kPa. It is conspicuous that the increasing curing age contributed to the increase of Gmax. This was attributed to the hydration products gradually formed with the increase of curing age. The increase rate of Gmax in the first

ð5Þ

where cr1 is reference shearing strain under the effective average stress (r0m ) of 100 kPa; pa is equal to 100 kPa; kis the correlation coefficient. The typical relationship between Gmax and confining pressure of CSC and CFSC is shown in Fig. 8. It is evident that the Gmax increased almost linearly as the increase of confining pressure, implying that increasing confining pressure has obvious improvement in the development of G. This obviation is in line with the results by prior researchers [15,28,33]. Fig. 9 shows the variation of Gmax with binder content of CSC and CFSC. For CSC, the Gmax increased with the increase of cement content and reached the maximum under the cement content of 15%. Similarly, the Gmax of CFSC also increased with an increase in FA content. This is consistent with the previous test results, indicating the rationality of H-D model in evaluating Gmax. As illustrated in Fig. 9, the Gmax of CFSC with CF16 is higher than that of CSC with C15, this further confirming that using 5% cement together with 30% FA as stabilizer has advantage over using 15% cement alone in improving the Gmax

Fig. 10. Relationship between Gmax and curing age of CSC and CFSC under the confining pressure of 150 kPa.

L. Lang et al. / Construction and Building Materials 237 (2020) 117646

7 days was higher than that in the later stage, implying the rapid early strength development caused by cement hydration. 3.3. Small-strain damping ratio (D) Damping ratio (D) of soils reflects the energy dissipation properties subjected to the cyclic or dynamic load. In this study, the effects of confining pressure, binder content and curing time on the variation of D with c of CSC and CFSC were investigated. Similarly, only some typical results were discussed, and the results are applicable to other situations. Fig. 11 shows the effects of confining pressure on the variation of D with c of CSC and CFSC at 28 days. It could be found that D

Fig. 11. Effect of confining pressure on the variation of damping ratio with shearing strain at 28 days (C15 and CF18).

7

decreases with the increase of confining pressure. With the increase of confining pressure, the internal structure of CSC and CFSC became denser and the bonding between soil particles was strengthened, then vibration waves propagated more evenly from top to the bottom of sample. When c ranged from 106 to 104, D increased approximately linearly with c, while the D increased rapidly at a strain level of 104. This can be explained that with the continuous of torsional vibration, the bonding between soil particles was gradually destroyed the path of vibration wave propagation in sample became more complex, leading to the increase of energy consumption of vibration wave passing through the sample. Fig. 12 presents the effects of binder content on the variations of D with c of CSC and CFSC. In the first 7 days, it is clearly found that D increased with the increase of cement and fly ash content, but as the curing age increased to 28 days, this change trend was not obvious. Usually, the stiffer the material is, the lower the D is. So, the above-described results seemed unexpected at first sight. This observation is in line with the results of Saxena et al. [29]. Due to the variation of D is related to the energy loss during wave propagation through the sample, the energy loss of wave propagating through a weakly cemented sample is larger than propagating through a clean and un-cemented sample. Namely, the cleanliness and cementation degree of soil particles in sample determined the energy dissipation. This is attributed to the energy spent by the wave to propagate through the sample was used for the rearrangement of soil particles and the scattering caused by rough surface. When cement hydration was not completed, the soil particles in the sample were in a weak cementation state. The energy spent on the samples in weak cementation and rough soil particle surface should be larger than that in strong cementation and smooth soil particle surface. With the increase of curing age, the cementation strength between soil particles increased gradually. It could be

Fig. 12. Effect of binder content on the variation of damping ratio with shearing strain under the confining pressure of 150 kPa and at (a) 3 days; (b) 7 days; (c) 28 days.

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presumed that with the continuous increase of curing age and binder content, the D of CSC and CFSC will decrease obviously. Fig. 13 shows the effect of curing age on the variation of D with c. The D of CSC and CFSC decreased with the increase of curing age. Compared with CSC, in the first 7 days, the curing age had less impact on the D of CFSC, while as the curing age achieved 28 days, the D of CFSC was obviously lower than that of curing for 3 and 7 days. This can be explained that the hydration rate of cement was higher than that of FA, and the hydration products were

formed in short time. It is well anticipated that the effect of curing age on the D will gradually weaken with the increase of curing age. 3.4. Maximum damping ratio (Dmax) The maximum damping ratio (Dmax) of CSC and CFSC was investigated in this study, and it is related to the normalized dynamic shear modulus (G/Gmax) as per to the results of prior studies [51– 53]. The following expressions have been proposed by the previous researchers [54,55], which were used to evaluate the variation of Dmax with confining pressure, binder content and curing age:

G 1 ¼ Gmax 1 þ c=cr

ð6Þ

  D c=cr ¼ Dmax 1 þ c=cr

ð7Þ

and then

 m D G ¼ 1 Dmax Gmax

Fig. 13. Effect of curing age on the variation of damping ratio with shearing strain under the confining pressure of 150 kPa (C15 and CF18).

ð8Þ

where m is the test parameter, and others are explained as above. Fig. 14 presents the relationship between Dmax and confining pressure. After curing for 3 and 7 days, the Dmax decreased slightly with the increase of confining pressure, but the change trend of abnormal increase and sharp decrease existed at the same time. This indicates that when curing age was short, the Dmax decreased with the increase of confining pressure, but the change trend was unstable. This is attributed to the hydration degree of cement and FA and bonding between soil particles. As the curing time

Fig. 14. Relationship between maximum damping ratio and confining pressure of CSC and CFSC at (a) 3 days; (b) 7 days; (c) 28 days.

L. Lang et al. / Construction and Building Materials 237 (2020) 117646

increased to 28 days, Dmax decreased slowly at first and then sharply with the increase of confining pressure. Therefore, the more hydration the cement is, the more obvious the effect of the increase of confining pressure on the decrease of the Dmax. Fig. 15 shows the relationship between Dmax and binder content of CSC and CFSC. When the curing age was 3 and 7 days, the Dmax of CSC and CFSC increased with the increase of cement and fly ash content. Furthermore, as illustrated in Fig. 15, the 3-day Dmax of CSC with C15 was more than 20%, which was slightly lower than that of CFSC with CF18. This indicates that after curing for 3 days, the energy dissipation performance of CFSC was better than that of CSC. The 7-day Dmax of CSC with C158 was higher than that of CFSC with CF18. Generally, the Dmax of CSC with C12 was similar to the that of CFSC with CF16. However, as the curing age achieved 28 days, it is interesting that the Dmax of CSC and CFSC decreased slightly with the increase of cement and FA content. This change trend is consistent with that of the variation of D with binder content, implying that the empirical model used in this study is feasible. Therefore, with the continuation of hydration, it could be presumed that the Dmax of CSC and CFSC will increase at first and then decrease with the increase of binder content. Fig. 16 shows the variation of Dmax versus curing age of CSC and CFSC. For the CSC with C15, the Dmax decreased linearly with the increase of curing age. This indicates that when cement content is more than 15%, the Dmax of CSC will decrease linearly with the curing age. Except for the CSC with C15, the Dmax of others mixes decreased rapidly at first and then slowly with the increase of curing age. Especially for CSC with C8 and CFSC with CF14, the 7-day Dmax was almost equal to the 28-day Dmax. Therefore, it can be concluded that when binder content is small, the Dmax of CSC and CFSC change little after 7 days of curing. 3.5. Microstructure analysis The SEM micrographs, taken at magnification of 1000 times, for CSC and CFSC at 28 days are presented in Fig. 17. It is evident that the hydration products calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) can be found in all the CSC and CFSC mixes, confirming that the CSH and CAH are the main hydration products in CSC and CFSC. For CSC with C8, as shown in Fig. 17 (a), the pores among soil particles were clearly seen, and the amount of CSH and CAH was relatively small. With the increase of cement content, more hydration products were formed and the integrity became better (Fig. 17(b) and (c)). The CSH and CAH not only filled up the pores, but also improved the interconnection between the soil particles. Compared with CSC with C8, the pores

Fig. 15. Relationship between maximum damping ratio and binder content of CSC and CFSC.

9

Fig. 16. Variation of maximum damping ratio versus curing age of CSC and CFSC under the confining pressure of 150 kPa.

gradually reduced in the CSC with C12 and C15. Furthermore, the cementation degree became stronger as the increase of cement content, which leading to the larger G. However, as illustrated in Fig. 17(b) and (c), the overlapping of cemented soil led to the scattering and refraction of the vibration waves, then the wave propagation became more complex and resulted in more energy dissipation. For CFSC, as shown in Fig. 17(d)–(f), the more compact microstructures shown in CFSC over CSC might be attributed to the micro-aggregate effect of FA. In addition, more hydration products produced because of the pozzolanic reaction between Ca(OH)2 and SiO2 and Al2O3. It is worth noting that FA particles are hollow spherical glass beads, the energy dissipation of vibration wave passing through the hollow structure was more than that passing through the compact structure. From a microscopic point of view, the stabilization of silty clay with cement and FA can not only improve the stiffness behavior, but also improve the energy dissipation performance. 4. Conclusions In this paper, the small-strain dynamic properties of cementstabilized silty clay (CSC) and cement-fly ash-stabilized silty clay (CFSC) were studied by conducting a series of resonant column tests. The effects of confining pressure, binder content and curing age on the small-strain dynamic shear modulus (G) and damping ratio (D) of CSC and CFSC were systematically investigated. Furthermore, the microstructures of CSC and CFSC samples were analyzed by conducting scanning electron microscope (SEM) tests. The following findings can be drawn: (1) The G of CSC and CFSC increased with the increase of confining pressure, binder content and curing age. The hydration products and compactness were responsible for the improvement in G of CSC and CFSC. The variation of G can be divided into two stages of gradual and rapid decrease with shearing strain (c), and the c of 104 was the turning point of evaluating the small-strain dynamic behavior of CSC and CFSC. (2) The Hardin-Drevich model was used to evaluate the maximum dynamic shear modulus (Gmax) of CSC and CFSC. The Gmax of CFSC with CF16 is higher than that of CSC with C15, implying that using 5% cement together with 30% cement fly ash as stabilizer has advantageous over using 15% cement alone in improving the Gmax of silty clay. Furthermore, the Gmax of CFSC with CF14 is slightly higher than that of CSC with C8, but lower than that of CSC with C12.

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L. Lang et al. / Construction and Building Materials 237 (2020) 117646

(a)

(b)

Pore

CAH CSH

CAH

CSH

(c)

CAH

(d)

CAH CSH

CSH

FA

CSH

(e)

CAH

(f) CSH CAH

FA FA CSH

Fig. 17. The microstructure images of CSC and CFSC with the mixes of (a) C8, (b) C12, (c) C15, (d) CF14, (e) CF16 and (f) CF18 at 28 days.

(3) The D of CSC and CFSC decreased with the increase of confining pressure and curing age. The D increased almost linearly with the c ranging from 106 to 104, but increased rapidly at the strain level of 104. Interestingly, the D increased with the increase of binder content. The weakly cemented and rough of soil particles led to more energy dissipation of vibration wave propagating through the sample. (4) The effect of confining pressure on the variation of maximum damping ratio (Dmax) is related to the curing age. When cured for 3 and 7 days, the Dmax of CSC and CFSC decreased rapidly with the increase of cement and fly ash content. However, as the curing age achieved 28 days, the Dmax of CSC and CFSC decreased slightly with the increase of cement and fly ash content. (5) The SEM images showed that the CSH and CAH were the main hydration products in CSC and CFSC. The pore structure weakened the G and the CSH and CAH improved the G of CSC and CFSC. The micro-aggregate effect of fly ash improved the compactness of CFSC, leading to the improvement in the G. Additionally, the hollow structure of fly ash particles not

only improved the stiffness properties, but also improved the energy dissipation performance of CFSC. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This study was supported by the National Natural Science Foundation of China (Grant No. 51972209). References [1] A.C. Cherian, J. Kumar, Effect of vibration cycles batches on shear modulus and damping of dry sand, J. Geotech. Geoenviron. Eng. 143 (9) (2017) 06017007. [2] A.C. Cherian, J. Kumar, Effects of variation cycles on shear modulus and damping of sand using resonant column tests, J. Geotech. Geoenviron. Eng. 06016015 (2016).

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