Effects of freeze-thaw cycles on mechanical properties of polypropylene Fiber and cement stabilized clay

Accepted Manuscript Effects of freeze-thaw cycles on mechanical properties of polypropylene Fiber and cement stabilized clay

Maoting Ding, Feng Zhang, Xianzhang Ling, Bo Lin PII: DOI: Reference:

S0165-232X(17)30581-5 doi:10.1016/j.coldregions.2018.07.004 COLTEC 2621

To appear in:

Cold Regions Science and Technology

Received date: Revised date: Accepted date:

8 December 2017 13 July 2018 15 July 2018

Please cite this article as: Maoting Ding, Feng Zhang, Xianzhang Ling, Bo Lin , Effects of freeze-thaw cycles on mechanical properties of polypropylene Fiber and cement stabilized clay. Coltec (2018), doi:10.1016/j.coldregions.2018.07.004

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ACCEPTED MANUSCRIPT

Effects of Freeze-thaw Cycles on Mechanical Properties of Polypropylene Fiber and Cement Stabilized Clay Maoting Dinga, Feng Zhangb*, Xianzhang Linga,c, Bo Linb a

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School of Civil Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China b

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School of Transportation Science and Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China c

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School of Civil Engineering, Qingdao University of Technology, Shandong, Qingdao 266033, China *

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Corresponding author at: School of Transportation Science and Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China E-mail address: [email protected] (F. Zhang)

ACCEPTED MANUSCRIPT Abstract To investigate the effects of fiber, cement and freeze-thaw cycles on the cement-treated clay and fiber and cement stabilized clay, a series of freeze-thaw tests with no water supply condition were conducted on the stabilized soil after standard curing for 7 days subsequently, and then the unconfined compression tests were carried out. The dimension

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change ratio, stress-strain relationship curves, post-peak stress ratio, tangent modulus and

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unconfined compressive strength (UCS) were obtained and analyzed in terms of the number

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of freeze-thaw cycles, cement, and fiber contents. Results show that cement-treated soil exhibits volumetric shrinkage behavior, and the dimension shrinkage ratio increases with

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increasing cement content and freeze-thaw cycles. A majority of fiber and cement stabilized soil specimens present volumetric expansive behavior, and the dimension expansive ratio

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increases with increasing fiber content and freeze-thaw cycles. Stress-strain curves for soils subjected to freeze-thaw cycles exhibit an initial flexible stage, and a greater post-peak

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stress was observed on fiber-stabilized soils than cement-treated soils due to the ductile

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behavior of fiber. Tangent modulus increases with increasing cement content and decreases with increasing fiber content, and drops sharply by 43%~72% after one freeze-thaw cycle.

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Unconfined compressive strength increases obviously with increasing cement content, decreases sharply after the first few freeze-thaw cycles, then exhibits little change after five

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freeze-thaw cycles. The UCS reduction ratio ranges from 42% to 69% after ten freeze-thaw cycles. The addition of polypropylene fiber shows an improvement in UCS and exhibits higher UCS at fiber content of 0.20%. Finally, an empirical model for predicting UCS was proposed by considering the effects of cement content, fiber content and numbers of freeze-thaw cycles, and the predicted UCS show a good agreement with test results.

ACCEPTED MANUSCRIPT Keywords: Freeze-thaw cycle, polypropylene (PP) fiber and cement-treated clay, stress-strain relationship, unconfined compressive strength (UCS), empirical model,

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dimension change ratio

ACCEPTED MANUSCRIPT 1. Introduction Clayey soil is widely distributed in Northern China. It is often selected as a filling material for constructing the road or high-speed railway due to lack of coarse-grained soils in this area. However, clayey soil is sensitive to freezing temperature and moisture phase

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change. When the temperature drops below 0℃, the pore water partly turns to ice crystal, and may cause frost heave due to volume expansion. In contrast, when the temperature rises

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above 0℃, the ice crystal in soil melts, and the soil experiences thaw settlement and

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weakening (Andersland, et al., 2003). This repeated freeze-thaw cycle could change the macro-structure and alter the mechanical behavior (e.g. Chamberlain, 1979; Lee et al., 1995;

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Eigenbrod, 1996; Viklander, 1998; Simonsen, et al., 2002; Wang, et al., 2007; Qi et al., 2008; Ishikawa et al., 2008; Cui et al., 2014; Zhang et al., 2015; Wang et al. 2017; Feng et

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al. 2017; Wang et al., 2018).

Stabilization method is an effective technique to treat the soils to achieve satisfying

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bearing capacity and strength, and has been widely applied in infrastructure construction. Early reinforcement methodology was to add chemical additives to improve the long-term strength and stability, and several researchers investigated the Unconfined Compressive

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Strength (UCS) of cemented soil, lime soil and coal ash soil (e.g. Clough et al., 1981;

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Consoli et al., 2007; Dalla et al., 2008; Liu et al., 2010; Gullu et al., 2014; Shibi et al., 2014; Yu et al., 2015). To overcome the degradation of chemical inclusions and natural fiber in soils, synthetic fibers (i.e. polyester fiber, polyethylene fiber, glass fiber, nylon fiber, steel fiber, polyvinyl alcohol fiber and polypropylene fiber) are often used as the inclusion, the basic physical parameters and experimental studies of each fiber soil were summarized by Hejazi et al (2012). In these fibers, polypropylene (PP) fiber is the most widely used inclusion, and is often used to reduce the shrinkage and enhance the soil strength(e.g. Puppala et al., 2002; Santoni et al., 2001; Yetimoglu et al., 2005; Zaimoglu, 2010; Ghazavi et al., 2010; Tang et al., 2010; Li et al. 2018). Recently, fiber and cement are jointly used to

ACCEPTED MANUSCRIPT improve the mechanical behaviors of soils (e.g. Consoli et al., 1998, 2004; Khattak et al., 2006; Park 2009). Tang et al. (2007) investigated the effects of discrete short PP fiber on the mechanical behavior of cemented clayey soil, and revealed that bond strength and friction at the interface seem to be the dominant mechanism controlling the reinforcement benefit. Consoli et al. (2010) found that the addition of PP fibers in the cemented sandy soil increases

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the UCS, and proposed a porosity/volumetric cement content for predicting the UCS.

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Festugato et al. (2017) developed a dosage methodology based on tensile and compressive

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strength of artificially cemented fiber reinforced soils considering filament length. Park (2011) added the polyvinyl alcohol (PVA) fiber into the cemented sand, and found that 2%

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cement ratio can result in a maximum of 2.5 times increase in the UCS than non-fiber-stabilized cemented soil. Ates (2016) utilized the glass fiber and cement to

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increase the engineering properties and mechanical strength of sandy soil, and found that inclusion of 3% glass fiber and 15% cement could attain a maximum strength.

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In cold regions, the freeze-thaw cycling is one of the main factors influencing the

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structure and strength of fiber stabilized soil. Gullu and Khudir (2014) presented the effects of freeze-thaw cycles (0, 1, 2 and 3, respectively) on the UCS of fine-grained soil treated

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with jute fiber and steel fiber, and found that the UCS values decrease as freeze-thaw cycles increase except for the additions of jute fiber along. Zaimogle (2010) investigated the

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effects of randomly distributed PP fibers on the UCS and durability of a fine-grained soil subject to freeze-thaw cycles, and found that the UCS after 12 freeze-thaw cycles generally increased with increasing fiber content. Ghazavi and Roustaei (2010) performed Unconfined Undrained (UU) triaxial compression tests to investigate the effects of freeze-thaw cycles on strength properties of clayey soil stabilized with geotextile, and found that the reinforcement can reduce the effects of freeze-thaw cycles on the cohesion and resilient modulus, and the largest change in strength and height occurs at the 1st to 7th cycles. Orakoglu and Liu (2017) investigated the strength and resilient modulus on glass

ACCEPTED MANUSCRIPT and basalt fiber soil after fifth freeze-thaw cycles, and suggested basalt and glass fibers in range of 0.5% and 1% could prevent the freeze-thaw damaging effects. Li et al (2018) performed the direct tensile test by using an 8-shaped compaction mold. The tests indicate that the tensile strength decreases with the numbers of freeze-thaw cycles up to nine, most of the strength reduction occurs at the first five cycles, and then remains relatively constant

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for the 6th to 9th cycle. Kravchenko, et al (2018) investigated the mechanical behaviors of

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basalt and PP fiber clay, and found that the strength of 0.75% PP fiber reinforced soil

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increased by 70% even after fifth freeze-thaw cycles, and the strength of 0.75% basalt fiber reinforced soil remain 27.1% after fifth freeze-thaw cycles. However, there is little research

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considering the effects of freeze-thaw cycles on the mechanical and deformation behaviors of fiber and cement stabilized soil.

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This paper aims to investigate freeze-thaw effects on mechanical behaviors of PP fiber and cement stabilized clayey soil. Three groups of stabilized soil specimens with increasing

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cement ratio and fiber content were prepared, and then subjected to increasing numbers of

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freeze-thaw cycles and unconfined compression tests in sequence. The dimension change ratio, stress-strain behavior, post-peak stress ratio, tangent modulus and unconfined

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compressive strength were obtained and analyzed in terms of the number of freeze-thaw cycles and cement and fiber contents. Finally, an empirical model for predicting UCS was

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proposed and validated.

2. Materials and methods 2.1. Materials Disturbed soil sample was taken from Daqing region, Heilongjiang Province located in Northern China. The soil basic properties such as specific gravity, Atterberg limits, maximum dry density, optimum moisture content, and grain size distribution were obtained

ACCEPTED MANUSCRIPT according to Test Methods of Soils for Highway Engineering (JTG E-2007), issued by the Ministry of Transport, People’s Republic of China. This soil can be classified as clay with low plasticity (CL). Table 1 presents basic properties and Figure 1 shows the particle size distribution.

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Table 1: Basic properties of soil used in this study

Basic properties

Values

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Specific gravity

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Liquid limit (%)

Plasticity index Maximum dry density (g/cm3)

15.0 1.92 14.0

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Optimum moisture content (%)

31.8 16.8

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Plastic limit (%)

2.71

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100

60 40

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Cumulative passing (%)

80

20

0

10

1

0.1

0.01

0.001

Grain size (mm)

Figure 1 Grain size distribution of Daqing clay, China

32.5R cement was used as the cementing agent, and the specific gravity of the cement grains is 3.15. The cement content used in the experiment was no more than 12% by weight of the dry soil. The fiber used in this investigation is PP fiber, because of its hydrophobic, and non-corrosive behavior and resistance to alkalis, chemicals, and it affordability. It is

ACCEPTED MANUSCRIPT also the most widely used inclusion in soil reinforcement (Hejazi, et al. 2012). The PP fiber herein is monofilament Y type. They were of 9 mm in length and 0.031mm in diameter. The fibers have a specific gravity of 0.91, a tensile strength of 350 MPa, an elastic modulus of 3.5 GPa, and a breakage elongation of 30%. In the experiments, the fibers content was no more than 0.30% by weight of the dry soil. Distilled water was used to prepare and cure the

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specimens for the unconfined compression tests.

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2.2 Specimen preparation

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Three groups of stabilized samples were prepared as shown in Table 2. In Group I, four

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types of samples with no PP fiber and cement content of 3%, 6%, 9% and 12%, respectively, were prepared. In Group II, four types of samples with 6% cement and fiber content of

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0.05%, 0.10%, 0.20% and 0.30%, respectively, were prepared. In Group III, four types of samples with 0.20% PP fiber content and cement content of 3%, 6%, 9% and 12%,

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respectively, were prepared. All specimens were prepared at the optimum moisture content.

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The preparation procedures are described below in detail.

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Test Group

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Table 2 Fiber content, cement content and freeze-thaw cycles of the soil specimens

Sample No.

Fiber content (%)

Cement content (%)

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0

3.0

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0

6.0

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0

9.0

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0

12.0

N05

0.05

6.0

N06

0.10

6.0

N07

0.20

6.0

N08

0.30

6.0

N09

0.20

3.0

N07

0.20

6.0

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Number of freeze-thaw cycles

0, 1, 5, and 10, respectively

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0.20

9.0

N11

0.20

12.0

2.2.1. Soil preparation The air-dried clay soils were passed through sieve with mesh opening of 0.5 mm for

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preparing the cemented soil, the PP fiber stabilized soil, and the PP fiber stabilized cemented soil. The sample preparation procedure was performed according to the Specification of

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Stabilized Soil Test Method (JTG E51-2009), issued by the Ministry of Transport, People’s

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Republic of China.

Cemented soil was hand-mixed thoroughly with appropriate amount of distilled water to

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achieve the optimal water content, and was placed into a container and sealed for at least 12

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hours to ensure uniform moisture distribution. Then the appropriate amount of cement was added into the prepared soil and mixed thoroughly. The cement content Cc is defined as

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follows:

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Cc 

Wc 100% (1) Wds

where Wc is the weight of cement, and Wds is the weight of dry soil.

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For PP fiber stabilized soil, the appropriate amount of fibers was divided into five parts.

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One part was added into air-dried soil and then mixed thoroughly. This procedure was repeated for all five parts until all fibers were added and mixed well. The fiber content C f is defined as follows:

Cf 

Wf Wds

100%

(2)

where W f is the weight of PP fiber. After the fiber is mixed thoroughly with raw soil, the distilled water was added for thorough mixing. Then the PP fiber stabilized soil was placed into a container and sealed for 12 hours.

ACCEPTED MANUSCRIPT The PP fiber stabilized cemented soil was prepared by adding PP fiber first, then the appropriate amount of cement was added for thorough mixing. 2.2.2. Compaction of specimens The soil samples were compacted at the maximum dry density of 1.92 g/cm3 using

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static compaction method according to the standard for Disturbed Sample Preparation (JTG E-2007). The prepared soil samples were divided into three parts for placement in the molds.

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The inner wall of cylindrical steel molds was lubricated first. Each layer was compacted by

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using displacement-controlled static compression machine, and the load was maintained for

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one minute. Then, the layer interface was roughed up to ensure good surface-to-surface contact. After compaction, the specimens were extracted from the mold with a stripper

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machine. According to the Test Methods of Materials Stabilized with Inorganic Binder for Highway Engineering (JTG E51-2009) issued by Ministry of Transport of China, all

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2.2.3 Specimens curing

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specimens were prepared with a diameter of 100 mm and a height of 100 mm.

The prepared soil specimens were sealed with a plastic membrane and placed into plastic

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bags to avoid moisture loss. According to the standard of Curing Method of Stabilized Soil

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(JTG E51-2009), the specimens were put into a standard moisture room with temperature maintained at 20±2℃ and relative moisture above 95% for curing. The curing duration was determined to be seven days, as the strength of stabilized soil on the seventh day is typically used for road design (JTG E51-2009). 2.3 Freeze-thaw cycling After

curing for

seven

days,

the sealed specimens were placed into a

temperature-controlled freezer to conduct the freeze-thaw cycling. During freeze-thaw cycling, no water was supplied into the stabilized specimens in a closed-system (Jones,

ACCEPTED MANUSCRIPT 1987). According to the field temperature monitoring on the road subgrade in Harbin, China, the lowest temperature in surface subgrade is around -10℃ (Xu et al., 2013). Therefore, for each freeze-thaw cycle, the sealed specimens were kept at -10℃ for 18 hours, and then allowed to thaw at 23±2℃ for 6 hours. Figure 2 shows the variation temperature at specimen center during one freeze-thaw cycle. This freeze-thaw process was repeated for 1, 5 and 10

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times, respectively. In this study, the maximum number of freeze-thaw cycle was ten, as

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many previous studies show that physical and mechanical behaviors of most soil would not

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change after six to eight freeze-thaw cycles (e.g. Wang et al., 2007; Qi et al., 2008;

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Ghazavi et al., 2010; Yu et al., 2010; Chang et al., 2014; Li et al. 2018).

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Figure 2 Temperature measured at the specimen center during one freeze-thaw cycle.

2.4 Unconfined compression tests

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MTS-810 (i.e., Material Test System 810) was employed to conduct the unconfined compression tests. The test procedure follows the Specification of Soil Test (JTG E51-2009), and all unconfined compression tests were carried out at indoor temperature of 23±2 ℃. To reduce friction between the sample and loading head, vaseline was applied on top and bottom of samples. Loading was applied on the specimen at a constant displacement rate of 2 mm per minute, and continued until sample failure or 8% of the axial strain. During loading, the axial displacement and vertical load were recorded at five data points per second.

ACCEPTED MANUSCRIPT 3. Experimental results and analyses 3.1 Dimension change In order to determine dimension change of treated soil subjected to freeze-thaw cycles, the diameter and height of each specimen were measured using a caliper with an accuracy of

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0.01mm after each freeze-thaw cycle. The diameter was measured at the top, middle and bottom of specimens, and the height was measured at three azimuth angles, i.e. 0°, 120° and

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240°, respectively. Table 3 shows the mean diameter and height of stabilized soil specimens

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after varying number of freeze-thaw cycles.

Diameter (mm) Sample No.

0

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5

10

99.76

99.74

100.40

99.74

101.52

101.52

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N01

100.00

99.88

N02

100.04

99.78

N03

100.00

99.72

N04

100.00

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Number of freeze-thaw cycles

10

99.70

99.68

100.40

99.70

101.88

101.80

99.64

99.60

101.00

101.30

101.42

101.40

99.60

99.55

99.54

102.40

101.78

100.80

100.82

100.00

100.02

100.02

100.04

102.00

101.30

101.06

101.06

N06

100.00

100.08

100.10

100.18

101.80

102.40

102.42

102.46

N07

100.02

100.44

100.80

100.95

101.00

100.44

103.42

103.80

N08

100.04

100.52

100.80

100.82

101.02

101.46

101.80

101.82

N09

100.00

100.40

100.72

100.80

102.00

101.62

101.40

101.44

100.00

100.30

100.66

100.65

101.40

102.04

101.20

101.30

100.00

100.15

100.50

100.52

102.00

99.64

101.04

101.12

N10 N11

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Height (mm)

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1

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0

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Number of freeze-thaw cycles

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Table 3 Mean diameter and height of stabilized specimens after varying freeze-thaw cycles

To evaluate the dimension change of treated soil due to freeze-thaw cycles, the dimension change ratio R is defined as follows: R

DN  D0 100% D0

(3)

ACCEPTED MANUSCRIPT Where, DN is the sample dimension of the Nth freeze-thaw cycle, and D0 is the initial dimension without freeze-thaw cycles. Generally, the dimension herein could be represented by the length, diameter or volume of the soil sample. When dimension change ratio is negative, it indicates shrinkage behavior. Otherwise, it indicates expansive behavior. Figure 3 shows effects of freeze-thaw cycles on diameter change ratio for three groups of

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stabilized soils. As shown in Figure 3(a), all of the cement-treated soils subjected to

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freeze-thaw cycles experienced shrinkage in the diameter direction. The diameters of

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cement-treated soil with same dosage shrank with increasing number of freeze-thaw cycles, and exhibited little change after the 5th cycle. For the same number of freeze-thaw cycles, the

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diameter change ratio of cement-treated soil increased when cement content increases from 3% to 12%. It can be seen that after five freeze-thaw cycling, the dimension change ratio of

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soil with 12% cement is 0.45%, which is almost twice of that of the specimen with 3% cement (i.e. 0.24%). Figure 3(b) shows the diameter change ratio of specimens with 6%

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cement and varying fiber content after 1, 5 and 10 freeze-thaw cycles. One can observe the

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expansive behavior on PP fiber- and cement-stabilized soils after freeze-thaw cycling. The diameter change ratio increased with increasing fiber content. The diameter change ratio with

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fiber content of 0.20% and 0.30% is noticeably greater than that with fiber content of 0.05% and 0.10%. Furthermore, the diameter of samples increased sharply for the 1st and the 5th

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freeze-thaw cycle, and then exhibited little additional change at 10th freeze-thaw cycle. This trend was also revealed on PP fiber reinforced clay (Ghazvi and Roustaie, 2010) and unreinforced clay (Wang et al., 2007). Figure 3(c) shows the diameter change ratio of specimens with 0.20% fiber and varying cement content after 1, 5 and 10 freeze-thaw cycles. One can see that the diameter change ratio increased with increasing number of freeze-thaw cycles and decreasing cement content. The diameter change ratio exhibited a stable stage after five freeze-thaw cycles. As the cement content increase, more pore water is consumed in the hydration reaction, resulting in less pore water content in the soil given all specimens

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during freezing, and hence less diameter change with increasing cement content.

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Figure 3 Effects of freeze-thaw cycles on diameter change ratio for stabilized soils. (a)

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cement-treated; (b) 6% cement + varying fiber content; and (c) 0.20% fiber + varying cement content.

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Figure 4 shows effects of freeze-thaw cycles on volumetric change ratio for varying stabilized soil. As shown in Figure 4(a), the cement-treated soils after one freeze-thaw cycle

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shown shrinkage behaviors. However, after 5 freeze-thaw cycles, the soils with 3% and 6% cement exhibited volumetric expansion of 0.63% and 0.79%, respectively, whereas the soils with 9% and 12% cement exhibited volumetric shrinkage ratio of 0.31% and 2.44%, respectively. Similar phenomenon could be observed after ten freeze-thaw cycles. Figure 4(b) shows the volumetric change ratio of 6% cement-treated soils with varying fiber content after 1, 5 and 10 freeze-thaw cycles. It indicate that cement-treated soils with 6% and 0.05% PP fiber presented volumetric shrinkage, and the volume reduced by 0.65%, 0.88% and 0.84% for freeze-thaw cycle of 1, 5, and 10, respectively. However, the 6%

ACCEPTED MANUSCRIPT cement-treated soils with PP fiber content of 0.1%, 0.2% and 0.3% presented volumetric expansion with increasing freeze-thaw cycles from one to ten, and the greatest expansion ratio was 4.69% occurred on specimens with PP fiber content of 0.3% after 10 freeze-thaw cycles. Figure 4(c) shows volumetric change ratio of 0.2% fiber and varying cement content after 1, 5 and 10 freeze-thaw cycles. Specimens except the ones with 0.2% fiber and 12%

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cement subjected to one freeze-thaw cycle demonstrated increasing volume with increasing

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freeze-thaw cycles.

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Figure 4 Effects of freeze-thaw cycles on volumetric change ratio for stabilized soils. (a) cement-treated; (b) 6% cement + varying fiber content; and (c) 0.20% fiber + varying cement content.

3.2 Stress-strain behavior The stress-stain behaviors provide an understanding the deformation and strength characteristics of stabilized soil. Figures 5, 6 and 7 show selected stress-strain curves of

ACCEPTED MANUSCRIPT cement-treated soils and fiber and cement-treated soil corresponding to different number of freeze-thaw cycles. Figure 5 shows the stress-strain curves for cement-treated soil after different freeze-thaw cycles. As shown in these figures, the stress-strain curves for cement-treated soil non experienced freeze-thaw cycling exhibited four stages: linear elastic stage, plastic

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yield stage, strain-softening stage and residual stage. However, the cement-treated soils

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experienced freeze-thaw cycling showed an initial flexible compaction stage beside these

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four stages. Post-peak stress corresponding to large axial strain (e.g. 8%) was much smaller than peak stress and close to zero for some specimens. It is clear that the peak strength at a

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specific number of freeze-thaw cycles increased as the cement content increases from 3% to 12%. This trend can be attributed to the cementation between soil particles formed by

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hydration reaction of cement.

Figure 5 Stress-strain curves for cement-treated soil after different freeze-thaw cycles (a) 3% cement; (b) 6% cement; (c) 9% cement; (d) 12% cement

ACCEPTED MANUSCRIPT Figure 6 and 7 show the stress-strain curves for fiber stabilized cemented soil after different freeze-thaw cycles. In these figures, Figure 6 presents stress-strain curves for the soils with 6% cement and varying fiber content, while Figure 7 shows stress-strain curves for 0.20% fiber soil with varying cement content. As shown in Figure 6, the peak stress of fiber stabilized soil with 6% cement increased slightly when the fiber content increased from

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0.05% to 0.30%. But for the cemented soil with 0.20% fiber shown in Figure 7, the peak

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stress increased significantly when the cement content increased from 3% to 12%.

Figure 6 Stress-strain curves for 6% cement-treated soil with varying fiber content after different freeze-thaw cycles. (a) 6% cement + 0.05% fiber; (b) 6% cement + 0.10% fiber; (c) 6% cement + 0.20% fiber; (d) 6% cement + 0.30% fiber.

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Figure 7 Stress-strain curves for 0.2% fiber soil with varying cement content after different freeze-thaw cycles. (a) 3% cement + 0.20% fiber; (b) 6% cement + 0.20% fiber; (c) 9% cement +

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0.20% fiber; (d) 12% cement + 0.20% fiber.

As presented in Figures 5, 6 and 7, the freeze-thaw cycles had significant impact on the

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stress-strain relationships. It is clear that the initial slope related to the initial modulus and elastic modulus reduced greatly (Gullu and Khudir, 2014; Yu et al, 2015), and the peak

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stress dropped when freeze-thaw cycles increased from one to ten. The reduction ratio of peak stress varies with cement and fiber content. The reduction is likely due to the structural damage of stabilized soil. During the freezing stage, the pore water turns into ice crystals, and expands by 9% in volume. The original structure of soil particle and fiber assembly, cementation bondages between soil particles and soil particles and fibers are broken and partly separated by ice crystals. When the environment temperature rises above 0℃, the ice crystals melt and the location of soil particles are memorized and partly kept. After the freeze-thaw cycles, many new voids and macro-cracks are formed. This new structure would

ACCEPTED MANUSCRIPT affect the mechanical behaviors, as also observed in other related experimental studies of un-stabilized soil (Wang et al, 2007; Zhang et al, 2015; Wang et al, 2017; Wang et al, 2018), fiber stabilized soil (Zaimoglu, 2010; Ghazavi and Roustaie, 2010; Gullu and Khudir, 2014) and cement stabilized soil (Liu et al, 2010; Shibi and Kamei, 2014). Additionally, a comparison of Figure 6 and 7 with Figure 5 shows that fiber stabilized

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cemented soil obviously exhibited a greater post-peak stress and residual stress even at a

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fiber content of 0.05% (the lowest in the study), and the post-peak stress at the axial strain of

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8% could reach a half of the peak stress. This behavior could be attributed to the ductile behavior of fiber added to the soil (Tang et al, 2007; Gullu and Khudir, 2014). To evaluate

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the ductile or brittle behaviors for stabilized soil, the post-peak stress ratio R pp is defined

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herein as Eq. (4) to describe the stress level after the soil failure. R pp 

qref  qf

(4)

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Where, q f is the failure stress, and qref  is the post-peak stress value at specific reference strain. One can observe from the stress-strain curve that the stress at strain of 6% exhibited a relative stable value. Therefore, qref  was obtained from the stress-strain data

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at 6% strain in this study.

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Figure 8 shows the effects of freeze-thaw cycles on post-peak stress ratio for stabilized soil. It can be seen from Figures 8(a) that cement-treated soil exhibited smaller post-peak stress ratio than fiber stabilized cement-treated soil. The post-peak stress ratio of cement-treated soil ranged from 17% to 60%, and most of them are smaller than 40%. For the fiber- and cement stabilized soils as shown in Figure 8(b) and (c), the post-peak stress ratio ranges from 48% to 100%, and most of them are greater than 80%. It could be concluded that the fiber contributes more to post-peak stress than cement due to the ductile behavior of fiber. Moreover, on the basis of the results of post-peak stress ratio in these

ACCEPTED MANUSCRIPT figures, freeze-thaw cycles show a notable effect on the post-peak stress ratio. The post-peak stress ratio for non-freeze-thaw cycles soil shows a lower stress level compared

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to the soil experienced 1, 5, 10 freeze-thaw cycling.

Figure 8 Effects of freeze-thaw cycles on post-peak stress ratio for varying stabilized soil. (a)

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cement-treated; (b) 6% cement + varying fiber content; (c) 0.20% fiber + varying cement content.

Modulus is used to evaluate the behavior of deformation. Many researchers analyzed

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the modulus of soil based on the unconfined compression test (Lee et al. 1995; Wang et al. 2007; Ghazavi and Roustaei 2013; Gullu and Khudir, 2014;). Tangent modulus in this research is determined by the linear stage from the stress-strain curves. Figure 9 shows the effects of freeze-thaw cycles on tangent modulus for all stabilized soil. As shown in Figure 9(a), the tangent modulus of cement-treated soil increased with increasing cement content, and decreased with increasing freeze-thaw cycles. Tangent modulus of cemented soil without freeze-thaw ranged from 46 MPa to 85 MPa, and dropped by 55%~70% after one freeze-thaw cycle, then kept relatively stable after five and ten freeze-thaw cycles. Figure

ACCEPTED MANUSCRIPT 9(b) exhibited decreasing tendency of tangent modulus with increasing fiber content, the similar conclusions were drawn by Park (2011), Gullu and Khudir (2014). After one freeze-thaw cycle, the tangent modulus reduced from 49 MPa~70 MPa to 17 MPa~28 MPa, the reduction percent ranged from 43 % to 72 %. Figure 9(c) shown similar tendency with Figure 9(a), but the tangent modulus of cement-treated soil was greater than fiber soil with

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the same cement content. The main reason maybe that the fiber reduces the contact

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behaviors between cemented particles.

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Figure 9 Effects of freeze-thaw cycles on tangent modulus for varying stabilized soil. (a) cement-treated; (b) 6% cement + varying fiber content; (c) 0.20% fiber + varying cement content.

3.3 Unconfined compressive strength According to the Test Standard of JTG E51-2009, the peak stress of strain-softening stress-strain curves or stress of strain-hardening stress-strain curves at specific strain was determined as the UCS. Table 4 shows the UCS of all stabilized soils after varying number of freeze-thaw cycles.

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content

content

(MPa)

(%)

(%)

0

1

5

10

N01

0

3

0.48

0.29

0.14

0.15

N02

0

6

0.63

0.31

0.19

0.22

N03

0

9

0.87

0.41

0.38

N04

0

12

1.05

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0.31

0.66

0.51

0.54

N05

0.05

6

0.60

0.47

0.22

0.33

N06

0.10

6

0.65

0.59

0.30

0.29

N07

0.20

6

0.77

0.57

0.41

0.45

N08

0.30

6

0.67

0.32

0.34

0.24

N09

0.20

3

0.66

0.32

0.20

0.27

N010

0.20

9

1.00

0.97

0.49

0.57

N011

0.20

12

1.12

0.88

0.48

0.48

Sample No.

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UCS at varying freeze-thaw cycles

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Cement

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Fiber

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Table 4: Unconfined compressive strength of stabilized soil after varying freeze-thaw cycles

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Figure 10 shows the effect of freeze-thaw cycles on the UCSs of cement-treated soil. For the cemented soil without freeze-thaw, the UCS increased from 0.48 MPa to 1.05 MPa and exhibited a linear growth trend when the cement content increased from 3% to 12%. It can

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be attributed to the cementation formed by cement during hydration. When more cement

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was added into the soil, more extensive and strong cementation was produced, resulting in greater UCS (Consoli et al.2007). After 1, 5 and 10 freeze-thaw cycles, the UCS of specimens with cement content of 3%, 6%, 9% and 12% decreased from 0.48 MPa to 0.15 MPa (69% reduction), from 0.63 MPa to 0.22 MPa (65% reduction), from 0.87 MPa to 0.38 MPa (56% reduction), and from 1.05 MPa to 0.54 MPa (49% reduction) respectively. Moreover, it is easy to see that the majority of UCS decreased occurs after the 1st freeze-thaw cycle, and the value of UCS stabilized after the 5th freeze-thaw cycle. The main reason is that the freeze action could cause the pore between soil particles expand due to phase change of water into ice. When cemented soil thaws, the pore cannot be recovered fully. This repeated

ACCEPTED MANUSCRIPT freeze-thaw cycle break the initial structure between soil particles, and produce a new structure. Once the internal structure of stabilized soil stabilizes due to the repeated freeze-thaw cycling,

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so is the strength (Wang et al, 2007; Zhang et al, 2015).

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Figure 10 Effects of freeze-thaw cycles on UCS for cement-treated clay

The cemented soil is stiffer and stronger than un-stabilized soil, but can exhibit a brittle

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behavior with tensile cracking as a major failure mode. Therefore, fiber was added to overcome this shortcoming. Figure 11 shows the effects of freeze-thaw cycle on UCS for 6%

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cement-treated soils with varying fiber content. As seen in this figure, the UCS of specimens

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with 6% cement and fiber content of 0.05%, 0.10%, 0.20% and 0.30% with non-freeze-thaw cycles was 0.60 MPa, 0.65 MPa, 0.77 MPa and 0.67 MPa, respectively. The UCS increased

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by 28% when the fiber content increases from 0.05% to 0.20%, but dropped by 13% when

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the fiber content was 0.30%. Similar observation was presented by Park (2011) and Gullu (2014), and they found that the UCS increases when the fiber content (polyvinyl alcohol fiber, jute fiber and steel fiber) did not exceed 1.0%. After 1, 5 and 10 freeze-thaw cycles, the UCS of 6% cement-treated soil with fiber content of 0.05%, 0.1%, 0.2% and 0.3% decreased from 0.60 MPa to 0.33 MPa (45% reduction), from 0.65 MPa to 0.29 MPa (55% reduction), from 0.77 MPa to 0.45 MPa (42% reduction), and from 0.67 MPa to 0.24 MPa (64% reduction), respectively. Comparing with results of soils with 6% cement shown in Figure 10, one can see that the UCS of fiber stabilized specimens with 6% cement was generally greater than that of specimens with the same cement content but no fiber

ACCEPTED MANUSCRIPT reinforcement. The specimen with 6% cement and 0.20% PP fiber had the highest UCS in

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Group II.

Figure 11 Effects of freeze-thaw cycles on UCS for 6% cement-treated clay with varying fiber

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content

Figure 12 shows effect of freeze-thaw cycles on UCS for 0.20% fiber stabilized soil

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with varying cement content. The UCS of the specimens with 0.20% fiber and the cement content of 3%, 6%, 9% and 12% without freeze-thaw cycles was 0.66 MPa, 0.77 MPa, 1.00

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MPa and 1.12 MPa, respectively. When the soils experienced five freeze-thaw cycles in

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sequence, the UCS decreased to 0.20 MPa, 0.41 MPa, 0.49 MPa and 0.48 MPa. The reduction ratio was 69%, 47%, 51% and 57%, respectively. When the soils experienced ten

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freeze-thaw cycles, the UCS further decreased except for the specimens with 9% cement

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and 0.20% PP fiber. Compared with Figure 10, one can see that the UCS of most specimens with 0.20% PP fiber was greater than that of the specimen with the same cement content but no fiber reinforcement. This echoes with Tang et al (2007) that the addition of fiber improved the internal strength.

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Figure 12 Effects of freeze-thaw cycles on UCS for 0.2% fiber clay with varying cement content

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In addition, Figure 10, 11 and 12 indicate that the UCS of most stabilized soil decreased after one and five freeze-thaw cycles, but increases slightly after ten freeze-thaw cycles. It is

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well known the strength of soil treated by cement is dependent on the curing time. In this study, all test samples were cured for seven days, and then subject to freeze-thaw cycling and

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unconfined compression tests. However, for the soil samples that were subject to ten freeze-thaw cycles, the curing time was longer than seven days due to the time required for

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the freeze-thaw cycles, which likely contributed to the slight increase in the UCS.

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As discussed above, the UCS of stabilized clay soil depends on the cement content, fiber content and number of freeze-thaw cycle. It is assumed that these three influence

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factors contribute to the UCS. Therefore, an empirical model for estimating the UCS qucs

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was proposed as follows,

qUCS  a1Cc  a2C 2f  b2C f  e( a3 N

FT

b3 )

c

(5)

Where, qucs is the unconfined compressive strength (MPa), Cc , C f and NFT is cement content (%), fiber content (%) and number of freeze-thaw cycle, respectively. a1 is the regression coefficient effected by cement, a2 and b2 are the regression coefficients effected by fiber, a3 and b3 are the regression coefficients effected by freeze-thaw cycles, and c is the comprehensive regression coefficient.

ACCEPTED MANUSCRIPT In Equation 5, the UCS of stabilized clay includes four parts: the contribution of cement exhibits a linear function as shown in the first term, the contribution of PP fiber presents a second order convex polynomial as shown in the second to third term, the reduction of freeze-thaw cycle exhibits a natural exponential function as shown in the fourth term, and the last term could be deemed as a correction coefficient. After fitting

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Equation 5 using the multivariate nonlinear regression analysis, all of the regression

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coefficients for empirical model and the correlation coefficients were obtained and are

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shown in Table 5.

Table 5. Regression coefficient and correlation coefficients for empirical model of UCS Regression coefficient

a1Cc

a1 a2

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a2C 2f  b2C f

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Fitting function

-5.43 1.78

a3

-0.86

b3

-0.81

c

-0.085

0.89

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R2

0.048

b2

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e( a3 N b3 )

Values

Figure 13 shows the comparison of UCS between the predicted and tested results. As

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shown in Figures 13(a), (b) and (c), all of the tested UCSs are shown by bar figures, and the

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predicted UCSs used Equation 5 are shown by surface meshes. Figure 13(d) shows the predicted results versus tested results. Comparison between the test data and model predicted results indicated that the predicted UCSs by the empirical model agree favorably with tested results. Equation 5 can be employed to predict and determine the UCS of stabilized clay by considering the cement content, the fiber content and numbers of

cycles.

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freeze-thaw

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Figure 13 Comparison of UCS between predicted results and tested result: (a) cement-treated; (b) 6% cement + varying fiber content; (c) 0.20% fiber + varying cement content; and (d) tested UCS versus predicted UCS.

ACCEPTED MANUSCRIPT 4. Conclusions Freeze-thaw cycles and unconfined compression tests were performed on the polypropylene fiber stabilized cemented clayey soil, and the effects of freeze-thaw cycles on dimension changes, stress-strain behaviors and unconfined compressive strength were

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analyzed and discussed. The main findings based on the study results can be summarized as follows:

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(1) The freeze-thaw cycle, fiber content and cement content have important effects on the

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dimension change of stabilized soil. The cement-treated soil exhibited volumetric shrinkage, and the dimension shrinkage ratio increased with increasing cement content and freeze-thaw

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cycles. A majority of fiber stabilized cement-treated soils exhibited volumetric expansion, and the dimension expansion ratio increased with increasing fiber content and freeze-thaw

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cycles.

(2) Stress-strain curves of stabilized soil without freeze-thaw cycling exhibit an initial

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linear stage, but exhibit an initial compaction stage after 1, 5 and 10 freeze-thaw cycles. Fiber stabilized cement-treated soils exhibit greater post-peak stress than cemented soil due to the ductility of fiber, and most of post-peak stress are at least 80% of the peak stress.

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Tangent modulus increases with increasing cement content and decreases with increasing fiber content. After one freeze-thaw cycle, the tangent modulus drops sharply by 43-72%,

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and then exhibits little change at the 5th and the 10th freeze-thaw cycle. (3) Unconfined compressive strength general increases with increasing cement and fiber contents. The addition of 3% to 12% cement has more influence on the UCS than the addition of 0.05% to 0.30% fiber, and the addition of 0.20% fiber exhibits a higher UCS than other fiber content. After the first few freeze-thaw cycles, the UCS decreases sharply, and then exhibits little change after five freeze-thaw cycles. The reduction ratio of UCS ranges from 42% to 69% after ten freeze-thaw cycles.

ACCEPTED MANUSCRIPT (4) An empirical model for prediction UCS of cement-treated and fiber soils was proposed and validated, and it can be used to estimate the UCS of stabilized clay by considering the effects of cement content, fiber content and numbers of freeze-thaw cycles.

Acknowledgments

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The research was supported the National Natural Science Foundation of China (No.

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51578200, 51408163 and 41430634), the Key Natural Science Foundation of Heilongjiang

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Province (No. ZD201218), and the State Key Laboratory of Road Engineering Safety and

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Health in Cold and High-altitude Regions (No. YGY2017KYPT-04).

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ACCEPTED MANUSCRIPT Highlights

Freeze-thaw cycles were performed on prepared fiber and cement stabilized clay.

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Dimension changes of stabilized soil due to F-T cycles were observed and discussed.

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A series of unconfined compression tests were conducted on stabilized clay

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Stress-strain curves and UCS effected by F-T, fiber and cement were discussed

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An empirical model for prediction USC was proposed and validated.

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