Experimental investigations on the effects of ambient freeze-thaw cycling on dynamic properties and rock pore structure deterioration of sandstone

Accepted Manuscript Experimental investigations on the effects of ambient freeze-thaw cycling on dynamic properties and rock pore structure deterioration of sandstone

Jielin Li, Rennie B. Kaunda, Keping Zhou PII: DOI: Reference:

S0165-232X(18)30181-2 doi:10.1016/j.coldregions.2018.06.015 COLTEC 2613

To appear in:

Cold Regions Science and Technology

Received date: Revised date: Accepted date:

19 April 2018 24 June 2018 29 June 2018

Please cite this article as: Jielin Li, Rennie B. Kaunda, Keping Zhou , Experimental investigations on the effects of ambient freeze-thaw cycling on dynamic properties and rock pore structure deterioration of sandstone. Coltec (2018), doi:10.1016/ j.coldregions.2018.06.015

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ACCEPTED MANUSCRIPT Experimental investigations on the effects of ambient freeze-thaw cycling on dynamic properties and rock pore structure deterioration of sandstone Jielin Li1,2,* [email protected], Rennie B. Kaunda2, Keping Zhou1 1

School of Resources and Safety Engineering, Central South University, Changsha, Hunan 410083, China;

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Department of Mining Engineering, Colorado School of Mines, 1600 Illinois Street, Golden, Colorado 80401, USA

*

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Corresponding author: Jielin Li; E-mail address:

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Abstract

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The issue of rock pore structure deterioration and dynamical mechanical properties has drawn much attention in recent years in the rock engineering community. In this study, a series of 140

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freeze-thaw cycling tests are conducted on sandstone samples. The sandstone pore structure after

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freeze-thaw cycles was detected by Nuclear Magnetic Resonance (NMR), and the dynamic load test was carried out by Split Hopkinson Pressure Bar system (SHPB). The results are: with the

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increase of freeze-thaw cycles, the saturated mass and porosity of sandstone increase, the dynamic

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peak stress of sandstone decreases; while the peak strain and total strain increase gradually, and the macroscopic damages of rocks increase. The results of T2 distribution and pore size changes

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are that the pore sizes of sandstones increase, especially that of micro-pores and macro-pores

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increase obviously after 140 freeze-thaw cycles. Under the effect of freeze-thaw cycles, the sandstone pores become connected with each other, some of the micro-pores and mini-pores are changed into meso-pores and macro-pores. The number of marco-pores increases evidently, and the largest increase is 197.23%. The rock pore structures have changed, which would lead to the change of mechanic properties. As the number of freeze-thaw cycles increases, the rock dynamical peak stress gradually decreases and the peak strain and the overall strain increase gradually. Keywords: Pore structure; Freeze-thaw cycles; NMR; Dynamic mechanics

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ACCEPTED MANUSCRIPT 1 Introduction In a cold region, the integrity of geotechnical engineering infrastructure founded on bedrock may be compromised as a result of deterioration enhanced by freeze-thaw action. For rock which is critically saturated and when the temperature is below zero degrees Celsius, a given volume of

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liquid water will increase in volume by about 9 % after freezing (Prick 1995) .Therefore the frost

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pressure will be generated on the sidewalls of pores, which will cause the pores to develop and

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expand (Omari, et al. 2015). When the temperature increases, the ice melts, and water transfers into pores, which accelerates the weakening of the cohesion between mineral particles. With

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repeated freeze-thaw cycles, the pores structure is transformed, mechanical behavior is

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deteriorated, and further leads to fracturing of rock. Further, as the rock already subjected to freeze-thaw cycles is exposed to additional dynamic disturbance such as blasting, mechanical

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crushing, earthquakes, and landslides, understanding the mechanical behavior of rock in response

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to the combined effects of these forces becomes important. In addition, rock is a complex natural material which contains joints, cracks, pores and other geological defects, which all affect the

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mechanical, physical and chemical characteristics of the rock (Xie et al. 2004). Therefore, the

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study of the rock pores structure deterioration characteristics under the effect of freeze-thaw cycles and dynamic load could help reveal the damage mechanisms for rock in cold regions. To effectively research the rock-pore structure deterioration after freeze-thaw cycling, experimental methods such as computed tomography (CT) (Tim et al.2005; Kodama et al. 2014), X-ray CT (Park et al. 2015), nuclear magnetic resonance (NMR) (Westphal et al. 2005; Li et al. 2016), scanning electron microscope (SEM) (Marco et al. 2010; Rusin et al. 2015) and ultrasonic technology (Liu et al. 2012; Murton et al. 2016; Draebing et al. 2012) have been used in previous

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ACCEPTED MANUSCRIPT studies. For example, NMR was used to evaluate the changing characteristics of T2 spectral distribution, the T2 spectrum area of rock after freeze-thaw cycles, and the characteristic of rock pore distribution (Zhou et al. 2012; Zhou et al. 2015). Cai et al. (2014) studied the effect of liquid nitrogen cooling on rock pore structure using NMR and SEM methods. Zhai et al. (2017a, 2017b)

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evaluated the action of freeze-thaw cycles on microscopic characteristics of the coal samples using

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NMR and SEM. Their results showed that freeze-thaw cycles can increase the number size and the

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connectivity of the pores, which greatly enhance the permeability of the coal. Kubicar et al. (2006)

cycles by using the pulse transient method.

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studied the thermophysical characteristic of dry and water saturate sandstone after freeze-thaw

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The change in mechanical behavior is caused by the rock pores structure deterioration under the freeze-thaw cycles conditions. Many researchers have carried out a large number of studies

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from laboratory tests, mechanical damage models and numerical simulations (e.g. Ruedrich et al.

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2011; Nicholson et al. 2000; Du et al. 2004; Liu, et al. 2015; Huang et al. 2018). However most of the research on the mechanical behavior of rock under the freeze-thaw cycles conditions mainly

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focuses on the physical properties and the static mechanical research (Huang et al 2018; Fu et

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al.2017; Huang et al. 2018); and only a few studies on coupled dynamic mechanical deterioration have been reported. Wang et al. (2016) studied the static and dynamic mechanical properties of sedimentary rock after freeze-thaw and thermal shock weathering, and showed that moisture content and temperature have significant effects on the process of rock freeze-thaw weathering. Yang et al. (2016) carried out dynamic mechanical testing on sandstone after freeze-thaw cycles, and the dynamic stress-strain curve, dynamic strength, peak strain and damage model were analyzed including acquisition of dynamic mechanical properties. Wen et al. (2015) used

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ACCEPTED MANUSCRIPT laboratory testing and LS-DYNA numerical simulation to obtain relationships between the rock dynamic strength and the number of freeze-thaw cycles. They found that the peak stress and the slope of the post-peak curve of the specimen before and after freeze-thaw are quite different. The specimen forms a failure plane to the axial direction and exhibiting a transverse tensile failure

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mode. Liao et al. (2016) carried out split hopkinson pressure bar system (SHPB) experiments on

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dry and saturated sandstone samples under low temperature conditions, and studied the

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relationship between rock mechanical parameters, temperature and water content at low temperatures. The results indicated that the dynamic strength properties of low-temperature rock

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specimens change very differently with static load tests. In the range of 25 ℃ to -40 ℃, the

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trend of water-saturated sandstone peak stress is initially rising slowly and dropping sharply at high strain rates, the inflection point temperature is -30 ℃. At the same time, the trend of peak

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strain is initially dropping rapidly and rising sharply, the inflection point temperature is -10 ℃.

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Ke et al. (2017) conducted NMR and dynamic load experiments on sandstone after freeze-thaw cycles, and the NMR results showed that the sandstone pore structure clearly changed with the

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increase of freeze-thaw cycles. They also presented an evolution equation for predicting the

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deterioration of dynamic compressive strength of sandstone after freeze-thaw cycles. In this study, a series of freeze-thaw cycling tests are conducted on sandstone samples. The NMR technique is used to measure the rock micro-structure, and the results of mass, porosity and T2 distribution of rock before and after freeze-thaw cycles are evaluated and discussed. The SHPB system is applied to conduct the dynamic loading experiment. The dynamic parameters of sandstone after freeze-thaw cycles are analyzed, so as to obtain the pore structure evolution and dynamic mechanical properties of rock after freeze-thaw cycling.

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ACCEPTED MANUSCRIPT 2 Experimental setup and test procedures 2.1 Experimental platform There are three components of the test system and equipment: freeze-thaw cycle test, nuclear magnetic resonance test and dynamic mechanical experiment. The specific experiment platform is

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shown in Figure 1.

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2.1.1 Freeze-thaw cycling

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A TDS-300 freeze-thaw device produced by Suzhou Donghua Test Instrument Co., Ltd. was used to conduct the freeze-thaw cycle tests. On the basis of the local temperature environment of

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the rock sampling, the rock specimens were frozen at -30 ℃ for 4 hours and then melted in water

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at 20 ℃ for 4 hours,, i.e. each freeze-thaw cycle is 8 hours. Every 20 freeze-thaw cycles were considered as one Measurement Cycle, and then the rock specimens were removed, weighed and

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carefully inspected to record any changes in appearance and characteristics. The procedure was

2.1.2 The NMR test

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repeated until after 140 freeze-thaw cycles were completed.

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The NMR can be used to measure the porosity, free fluid index, pore size distribution,

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transverse relaxation time T2 distribution and other parameters on rock specimens. NMR has been widely used to study rock pore structure. In this study, the changes of sandstone micro-structure were observed using the low-field NMR system, a type of AniMR-150 rock magnetic resonance imaging analysis system and vacuum saturation device produced by Suzhou Niumag Electronic Technology Co., Ltd. A nuclear magnetic resonance test was performed before the specimens underwent freeze-thaw cycles. After every 20 freeze-thaw cycles, the rock specimens were extracted from the machine and the surface moisture was removed by towel. Then the

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ACCEPTED MANUSCRIPT microstructure properties of sandstones before and after freeze-thaw cycling were obtained. 2.1.3 The SHPB test The rock dynamic experiments were carried out after the completion of the nuclear magnetic resonance test. An SHPB experimental system was used (Li et al. 2008; Sharma et al. 2011),

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consisting of a 40Cr alloy steel bar 50 mm in diameter. The straight bar density is 7.810 kg/cm3,

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the propagation velocity of elastic wave is 5410 m/s, the lengths of the striker bar and the

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transmission bar are 2.0m and 1.5m respectively, and the “bullet” is the spindle punch (profiled punch) type. During the test, a constant impulse pressure of 0.45Mpa was used to launch the gas

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gun. The dynamic load was loaded with a half sine wave. The data were collected through the

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strain gauges on the bars, the CS-10 ultra-dynamic signal conditioner, laptop and DL750 digital

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oscilloscope, including the dynamic parameters of sandstone under freeze-thaw cycles.

Fig.1 Experimental platform is consisted with freeze-thaw cycle test, nuclear magnetic resonance test and dynamic

mechanical experiment.

2.2 Rock specimen preparation The rock specimens were yellow sandstone, mainly composed of grit cementation which were taken from Gannan area in Gansu province of China. The homogeneity and integrity of the 6

ACCEPTED MANUSCRIPT rocks were carefully inspected and found to be reasonable. According to the requirements of the rock dynamic mechanics experiments, the rocks were prepped and cored into cylindrical specimens which have a diameter of 50 mm and a height of 50 mm, and the proportion of the height/diameter ratio was 1:1. The basic mechanical parameters of yellow stone are shown in

Uniaxial compressive

Elasticity

Poisson’s

Water

Saturated water

strength (MPa)

modulus (GPa)

ratio

absorption (%)

content (%)

62.63

15.86

7.58

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Porosity (%)

Basic physic-mechanical parameters of the sandstone

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Table 1

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Table 1.

0.22

7.35

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2.3 Experimental procedure

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The specimens were placed in vacuum saturation devices and saturated under pressure in vacuum with the pressure value of 0.1 MPa for 4 hours, then the specimens were immersed in

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distilled water for 24 hours. Next, the type of AniMR-150 nuclear magnetic resonance analysis

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system was used to conduct the NMR relaxation measurement, and the NMR results of 0 freeze-thaw cycle were obtained. After this, all the specimens were placed in a TDS-300

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freeze-thaw testing machine for the 20 freeze-thaw cycles, and NMR was performed on them again. The SHPB system was performed on the specimens in their dynamics mechanical

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characteristics of 20 freeze-thaw cycles, then freeze-thaw cycles and NMR test were repeated till 140 freeze-thaw cycles were finished. In the end, the results of NMR and dynamical mechanical properties of sandstone after 140 freeze-thaw cycles were obtained.

3 Experiment results 3.1 Pore structure alteration based on the quantity of absorbed water Table 2 shows the change of mass after 140 freeze-thaw cycles. It can be seen that with the

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ACCEPTED MANUSCRIPT increase of the number of freeze-thaw cycles, the mass of each specimen gradually increased given that during the process of freeze-thaw cycles, the water in the pores changed into ice after the specimens were frozen. The phase change of water resulted in volume expansion, which led to the expansion of pores and the generation of new pores. During the melting phase, water will

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migrate into new pores. In addition, the external moisture was also observed to migrate into the

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rock along pore channels repeatedly leading to an increase in the mass of the specimens.

Mass of rock after different numbers of freeze-thaw cycles (g)

No.

0

20

40

B21

230.27

231.08

231.71

B22

229.80

230.60

230.69

B23

229.08

229.93

230.06

B24

228.40

229.31

B25

231.26

232.19

60

80

100

120

140

231.19

231.55

231.79

232.12

231.07

230.63

230.84

231.26

231.43

230.38

229.80

229.76

230.32

230.52

229.38

229.40

229.15

228.95

229.56

229.87

232.35

232.26

232.11

232.17

232.44

232.90

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Specimens

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Table 2 Mass changes of sandstone specimens under the effect of freeze-thaw cycles

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231.56

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To discriminately observe the mass of rock specimens with the change of the numbers of freeze-thaw cycles, all the data were normalized and shown in Figure 2. It can be seen that during the first 0-20 freeze-thaw cycles, the mass of specimens underwent the most obvious change, and reached their maximum after 60 freeze-thaw cycles. The indication is that during the first phase, the cracks are caused by frost heaving. External moisture continuously migrates into the rock pores leading to an increase in the mass of rock specimens. After 80 freeze-thaw cycles, a small amount of particle spalled off from the surface of the specimens, which led to the decrease of the

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ACCEPTED MANUSCRIPT residual mass of the specimens. However, after 120 freeze-thaw cycles, the mass of moisture migrating into the rock exceeded the mass of the spalled particles, and therefore the residual mass

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of rock would continue to increase.

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Fig.2 Change in mass of sandstone after different periods of freeze-thaw cycles. During the 0-60 freeze-thaw

cycles, the mass of the specimens gradually increases with the number of freeze-thaw cycles. A small amount of

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particle spalled off from the surface of the specimens after 60 freeze-thaw cycles, and it resulted in a decrease in

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the residual quality of the specimens. The mass of the specimens began to gradually increase due to the moisture

migrating into the rock exceeded the mass of the spalled particles after 100 freeze-thaw cycles.

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3.2 Pore structure alterations characterized by NMR response

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3.2.1 Characteristics of the T2 relaxation curve NMR experiments can be used to carry out qualitative and quantitative test analyses of the rock pores, and it has the advantages of wide application range, short test time and non-destructive testing of rock samples. CPMG pulse sequential tests the complete saturation rock, with the result of the deamplification of spin echo strings. The signals are overlapped with water signals in different sized porosities. The scale of spin echo strings can be accurately fixed with the sum of a group of exponent deamplification curves. Every exponent curve has a different attenuation

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ACCEPTED MANUSCRIPT constant, and all the constants sets are composed of the crosswise relaxation time T2 distribution. According to the NMR principle, the NMR crosswise relaxation rate 1/T2 is: D( GTE )2 1 1 S   2    T2 T2 free 12  V  porosity

(1)

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where, T2free is fluid free relaxation time (ms); S is porosity surface area (cm2); V is porosity volume (cm3);  2 is crosswise surface relaxation strength (μm/ms); D is diffusion coefficient;

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 is gyromagnetic ratio (rad·/(S·T)); G is magnetic gradient (Gs/cm); TE is echo time (ms).

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If there is only one kind of fluid in the pores, the volume relaxation time is lower than that of

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the size, and the T2free could be ignored. If the magnetic field is even (the magnetic gradient G is very small) and short TE is used, the diffusion relaxation can also be ignored. And then Equation

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(1) could be simplified as (Ausbrooks et al. 1999; Matteson et al. 2000): (2)

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1 S =2   T2  V  porosity

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For the samples that can be simplified into spherical structures pores and columnar pores, Equation (2) can be further transformed into the relationship between T2 relaxation time and core

1 2 = Fs T2 rc

(3)

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pore radius, i.e.:

where, rc is the pore radius; Fs is the geometry factor (for the spherical pores Fs =3; and for the columnar pores Fs = 2). Equation (3) can be simplified as:

rc =CT2

(4)

where, C =Fs 2 , and C is called the conversion coefficient. It can be concluded from Equation (1)-(4) that the pore radius is in direct proportion to T2 (distribution can reflect the size information of porosity). The smaller T2 is, the smaller the

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ACCEPTED MANUSCRIPT represented pore becomes; conversely, the larger the pore, the larger T2 becomes. Figure 3 shows the T2 distribution of rock specimens B23# and B24# after 140 freeze-thaw cycles. From the overall morphology, during the process from 0 to 140 freeze-thaw cycles, there were mainly three to four T2 spectral peaks for the sandstone, and the relaxation time of sandstone

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was mainly concentrated in 1-100 ms. The overall morphology therefore indicates that the

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proportion of micro-pores (<0.1 μm) and mini-pores (0.1-1 μm) occupied the vast majority. With

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the increase of the number of freeze-thaw cycles, the trend of T2 spectrum was moved to the right, i.e., the NMR signal intensity of macro-pores increased. Each peak of T2 of the rock increased,

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especially for the first peak indicating that the micro-pores within the rock continuously expanded

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into meso-pores (1-10 μm) or macro-pores (10-100 μm) after freeze-thaw cycles. The expansion

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caused the rock structure to be continuously segmented resulting in freeze-thaw induced damage.

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(a) B23# specimen

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(b) B24# specimen

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Fig.3 Changes of T2 distribution of (a) B23#, (b) B24# after different freeze-thaw cycles. There were mainly three

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to four T2 spectral peaks, and the relaxation time of sandstone was mainly concentrated in 1-100 ms. The

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proportion of micro-pores and mini-pores occupied the vast majority. With the increase of the number of

freeze-thaw cycles, each peak of T2 of the rock increased and the trend of T2 spectrum was moved to the right.

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3.2.2 Quantification of rock porosity with NMR measurements

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Porosity is the percentage of pore volume to the total volume of the matrix, and reflects the pores condition of the matrix. Figure 4 shows the porosity changes of three samples B22 #, B23#

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and B24# after different numbers of freeze-thaw cycles. It can be seen that with the increase of the

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number of freeze-thaw cycles, the porosity of sandstone becomes larger. After 140 freeze-thaw cycles, the porosity of B22#, B23# and B24# increased by 31.17%, 32.18% and 31.34% respectively, and the growth rate is clear. The inference is that the frost heaving force which exists within the sidewall of pores is far greater than the cohesion between the crystal grains inside the rock under the condition of freeze-thaw cycles, which promotes the pores expansion and connection, and changes the mutual effect between mineral particles, leading to a substantial increase in porosity.

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ACCEPTED MANUSCRIPT It should be noted that the porosity changes in each phase of freeze-thaw cycle are different. During the freeze-thaw cycles from 0 to 20, the porosity of rock increases rapidly. During the freeze-thaw cycles from 20 to 100, the increase in the porosity of the rock slowed down. Especially between 60-80 freeze-thaw cycles, the porosity decreased. However, the porosity

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increased faster during freeze-thaw cycles from 100 to 140. The result shows that during the

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process of freeze-thaw cycles, the accumulation of the pore volume increases caused the mineral

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particulate matter inside the rock to be compacted, and the cohesion between the mineral particles gradually increased. The phenomena could also effectively eliminate part of the frost heaving

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force, and then limit the porosity expansion. On the other hand, water flowed into the pores, and

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thus the rapid expansion of pores was somewhat contained by the static pressure of water. Between 60-100 freeze-thaw cycles, a small amount of particles spalled off from the surface of the

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specimens, which led to the decrease of the residual mass of the specimens (as shown in Figure 2).

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With this spalling, the pores that originally existed between the particles on the rock surface disappeared, which made it impossible for fluids to be present in these pores. As the NMR

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porosity only responded to the fluid in the pores, the porosity decreased. After 100 freeze-thaw

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cycles, the pore size of the rock began to increase, implying that the rock pores developed and the degree of expansion began to increase. The results show that the frost heaving force accumulated on the sidewall of pores repetitively and continued to weaken the cohesion between mineral particles. The accumulation made the frost heaving force gradually grow larger than the cohesion between the mineral particles, which resulted in a sharp increase in porosity.

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Fig.4 Changes of porosity of B22#, B23# and B24# increased by 31.17%, 32.18% and 31.34% respectively after

140 freeze-thaw cycles. The porosity changes in each phase of freeze-thaw cycle are different. The frost heaving

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force which exists within the sidewall of pores is leading to a substantial increase in porosity.

condition of freeze-thaw cycles

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3.3.1 Dynamic stress-strain curve

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3.3 Evolutionary characteristics of rock dynamic mechanical parameters under the

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Figure 5 shows the dynamic compressive stress-strain curves of sandstone after different numbers of freeze-thaw cycles. Compared with the static stress-strain curve, the dynamic

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compressive stress-strain curve does not have a clear compaction phase (Zhou et al. 2015; Yang et

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al. 2016), which is generally divided into three phases of elasticity, yielding and failure. The difference in behavior is due to the fact that the pores in the sandstone are too late to close under the dynamic load conditions, and resistance to deformation has improved. The shape of each curve in Figure 5 is similar and can be described by these three phases. With the increase of the number of freeze-thaw cycles, the dynamic stress-strain curve of rock tends to shrink to the X-axis as a whole. The dynamic peak stress of rock gradually decreases and the peak strain and total strain increase gradually.

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ACCEPTED MANUSCRIPT Based on the changes of porosity and T2 spectrum, after 140 freeze-thaw cycles, a large number of new micro-pores were formed in the rock, and the initial pores were expanded by the frost heave force, such that the peak strain of the rock substantially increased under the dynamical load. This phenomenon shows that the freeze-thaw action would make the pores and fissures in

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the sandstone expand, leading to micro-structural damage in the rock. With the increase of

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freeze-thaw cycles, this kind of damage continually accumulated, resulting in the decrease of its

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mechanical properties, deterioration of the brittleness of the rock and the increase of the strain, which eventually led to the damage of rock. Therefore, under the dynamic load the rock specimens

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the continuous increase of the total strain.

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were broken into many small fragments, which led to the loss of carrying capacity, and resulted in

In addition, as shown in Figure 5, with the increase of the number of freeze-thaw cycles, the

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elastic phase of the dynamic stress-strain curve of sandstone is prolonged, and the elastic

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characteristics are obvious. The dynamic elastic modulus of the rock is gradually reduced, and the gradient of the stress-strain curve of elastic phase was negatively correlated with the number of

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freeze-thaw cycles. The analysis shows that the effect of freeze-thaw cycle on pore structure is a

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process of damage accumulation. In the elastic deformation stage, as the number of freeze-thaw cycles increases, the micro-cracks in the rock start to grow and expand steadily. And the linear elastic deformation characteristics of rocks become clearer (Yang 2016). The gradient of the line decreases, implying that the micro-cracks in the sandstone have appeared in large quantity and expand from stable expansion to unstable expansion. The internal damage of sandstone gradually accumulated under the condition of freeze-thaw cycles, and then led to a reduced initial modulus. Under the dynamical load, the main cracks grow rapidly to break through the rock specimen,

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which eventually leads to the macroscopic damage of the rock.

Fig.5 Stress-strain curves of sandstone under dynamic load after 140 freeze-thaw cycles. The dynamic compressive

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stress-strain curve does not have a clear compaction phase. With the increase of the number of freeze-thaw cycles,

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the dynamic peak stress of rock gradually decreases, and the peak strain and total strain increase gradually. The

sandstone exhibits tensile failure in the axial direction, and the size distribution of rock fragments is closely related

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to the number of freeze-thaw cycles.

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3.3.2 Failure mode of sandstone under the action of freeze-thaw cycles and dynamic load It can be seen from the morphology of the rock specimens in Figure 5 that under the dynamic

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load conditions, the sandstone exhibits tensile failure in the axial direction. The size distribution of

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rock fragments is closely related to the number of freeze-thaw cycles. With an increase in the number of freeze-thaw cycles, the volume of rock fragments decreases, the number of fragments increases and the degree of damage becomes larger. When the number of freeze-thaw cycles reaches 140, the B22 # specimen had more fragments, and more uniform and smaller distributions. At the same time, a large amount of powdery debris was generated, and the degree of breakage was larger. The failure mode of rock showed the characteristics of elastic transitioning to plastic deformation. The transformation provides an indication that a change in rock micro-structure

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ACCEPTED MANUSCRIPT affects the macroscopic mechanical characteristics. The freeze-thaw action leads to an increase in the number of large-sized pores in the rock specimens. The pores with similar size show a centralized distribution, which leads to a uniform distribution of relatively smaller fragments. Under static load conditions, the sandstone after freeze-thaw cycles mainly display a shear

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failure mode, and fracturing along the weak joint fissures (Zhang et al. 2013). With the increase in

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the number of freeze-thaw cycles, damage inside the rock continuously accumulates, and the rock

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failure model is changed from a single oblique shear plane into an "X" -like conjugate shear plane, and the number of fragments gradually increased. In contrast, under the dynamic load conditions,

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sandstone mainly exhibits tensile failure characteristics along the axial direction. As the number of

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freeze-thaw cycles increases, the fragments are uniformly and finely distributed, and the role of freeze-thaw cycling aggravates the damage in the rock and the macro damage continues to

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4 Discussions

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increase.

4.1 Evolution of rock pores under freeze-thaw influence

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Freeze-thaw cycles can change the size, quantity and distribution of the pores in the rock, and

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thus directly affecting the rock mechanical properties. Given that the T2 distribution of NMR actually reflects the rock pore structure, the rock pores structure can be obtained through the distribution of NMR T2. According to the correspondence between the pore radius and the T2 in Equations (3) and (4), and considering that the C value of the conversion coefficient is regionally empirical, (i.e. the C values are unique for sand in different regions, and the C value of the most of sandstones is in the range of 0.01-0.15  m / ms in China (Li et al. 2006)), authors assumed a C value of 0.1, leading to a simplification of Equation (4) as:

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ACCEPTED MANUSCRIPT rc  0.1T2

(5)

Therefore, the T2 distribution of rock can be transformed into the pore size distribution curve. In order to observe the effect of freeze-thaw cycles on the changes of pores structure, The T2 distribution and pore size distribution of B23# and B24# specimens after 0 freeze-thaw cycle and

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140 freeze-thaw cycles were selected and analyzed, as shown in Figure 6.

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Fig.6 T2 distribution and pores distribution curve of (a) B23#, (b) B24# specimens. Four categories of pore size were classified, and the number of meso-pores and mini-pores dominate in the initial sandstone. After 140

freeze-thaw cycles, the pore growth and expansion, and the pores are interconnected. The number of macro-pores

of (a) B23#, (b) B24# specimens increased about 3 and 2.5 times respectively.

There are many kinds of classification for the pore size of sandstones. Some scholars divided the pore size into two ranges of large pores (≥50 μm) and small pores (<50 μm) (Xiao 1998), and four categories were classified by the reference laboratory capillary pressure measurement of pore radius (R ≥ 0.1 μm, 0.1 μm ≤ R ≤ 1 μm, 1 μm < R ≤ 10 μm and R> 10 μm) (Shao et al. 2013) and

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ACCEPTED MANUSCRIPT classified the low permeability sandstone with macro-pores (≥1 μm), meso-pores (0.1 - 1 μm) and mini-pores (<0.1 μm) (Yan et al. 2016). Based on the pore size distribution of sandstone in this study, the authors classified the pore size into four categories: micro-pores (<0.1 μm), mini-pores (0.1-1 μm), meso-pores (1-10 μm), macro-pores (10-100 μm). By segmenting the pore distribution curve in Figure 6, the distribution proportion of the different pore radiuses after 140 freeze-thaw

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cycles can be obtained, as shown in Tables 3 and 4.

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Table 3 Proportional variations of pore sizes in core 23#

Proportion of pores in cores (%)

40

60

Micro-pores

2.68

3.56

5.52

3.68

Mini-pores

39.56

34.84

32.91

Meso-pores

53.20

52.05

Macro-pores

4.56

9.54

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20

80

100

120

140

5.65

4.55

3.55

4.37

33.26

34.20

32.73

31.05

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0

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Pore size

32.92

51.47

50.01

49.28

51.92

51.01

10.14

11.93

11.08

11.98

11.80

13.56

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51.43

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Table 4 Proportional variations of pore sizes in core 24#

Pore size

20

40

60

80

100

120

140

9.03

5.07

5.70

5.45

5.18

6.01

5.59

3.82

Mini-pores

39.51

36.38

35.67

35.89

36.22

36.20

34.47

34.27

Meso-pores

47.30

50.99

50.61

50.28

50.25

48.44

51.22

51.68

Macro-pores

4.16

7.56

8.02

8.38

8.35

9.36

8.72

10.23

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0

Proportion of pores in cores (%)

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Micro-pores

In the sandstone pore structures after 0 and 140 freeze-thaw cycles, the largest number of pore structures is meso-pores, followed by mini-pores, then macro-pores; and the last one is the smallest number of micro-pores. The number of meso-pores and mini-pores dominate. The

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ACCEPTED MANUSCRIPT percentages of rock samples B23# after 0 and 140 freeze-thaw cycles are 92.76% and 82.06% respectively. And the percentages of rock samples B24# after 0 and 140 freeze-thaw cycles are 86.81% and 85.95% respectively. After 140 freeze-thaw cycles, the pore size of the two specimens increased, especially the number of macro-pores. The number of macro-pores of B23# and B24# specimens increased about 3 and 2.5 times respectively. Due to rock heterogeneity, there are different changes of pore

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structure between two specimens after 140 freeze-thaw cycles: the B23# specimen has both

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expansion of pores and generation of pores, and as is shown by the results the numbers of micro-pores and macro-pores, increased by 62.97% and 197.23% respectively. In the meantime,

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the numbers of mini-pores and meso-pores decreased by 21.49% and 4.11% respectively. That is,

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the distribution of T2 spectrum moved to both the left and right. However, the number of micro-pores and mini-pores in B24# samples decreased by 57.65% and 13.26% respectively. The

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number of meso-pores and macro-pores increased by 9.25% and 145.99% respectively, i.e., the distribution of T2 spectrum moved to the right.

The analysis above indicates that there are many mini-pores and meso-pores in the initial

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sandstone, and the number of micro-pores and macro-pores is small. With the increase of the

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number of freeze-thaw cycles, the pore growth and expansion are caused by the freeze-thaw action, and the pores are interconnected. Finally, part of micro-pores and mini-pores expand into

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meso-pores and macro-pores, especially the number of macro-pores which increased significantly. Consequently the changes of the rock pore structure result in changes in mechanical properties.

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4.2 Relationship between the degradation of dynamic peak strength and the number of freeze-thaw cycles Figure 7 shows that the peak strength of sandstone decreases with the number of the freeze-thaw cycles. As we can see from Figure 7, the dynamic strength of the sandstone gradually decreases with the increase of the number of freeze-thaw cycles, and there are some different variations in different freeze-thaw phases. Especially in the 0-60 freeze-thaw cycles, the peak strength changes slowly. This could be because the accumulation of the volume increase of the initial pores caused the particles inside the rock to be compressed during the process of

20

ACCEPTED MANUSCRIPT freeze-thaw cycles, which made it more difficult for water to move into the pores, thus limited the pore expansion. So the evolution of the internal damage of the sandstones slowed down, and the freeze-thaw effect on the dynamic strength of the sandstones decreased. Between 60 freeze-thaw cycles and 100 freeze-thaw cycles, the rate of decrease of dynamic strength of sandstone tends to be rapid. From the NMR results, the porosity of the sandstone slowed down during this phase, indicating that the porosity is not the only factor to affect the dynamic strength. After 100

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freeze-thaw cycles, the pores structure in sandstone accelerates to change and decrease the peak

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strength most evidently. With the results of the T2 spectrum and the pore size distribution, it can be concluded that some pores in the sandstone were interconnected and expanded into secondary

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fractures; and the characteristic of multi-scale distribution started to show from the size of the

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pores structure. The multi-scale distribution with micro-pores, mini-pores, meso-pores and macro-pores is more conducive to the interconnecting of the pores when sandstone is subjected to

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dynamic loading. The interconnection resulted in a sharp drop in dynamic peak strength.

Fig.7 Relationship between the dynamic peak strength σ and the freeze-thaw cycles N. The dynamic strength of

the sandstone gradually decreases with the increase of the number of freeze-thaw cycles, and there are some

different variations in different freeze-thaw phases.

4.3 Relationship between dynamic peak strain and freeze-thaw cycles As the number of freeze-thaw cycle increases, the peak strain under dynamic loading gradually increases, although the overall growth rate is relatively gentle, as is shown in Figure 8.

21

ACCEPTED MANUSCRIPT The observed behavior occurs as a result of less deformation given that the loading rate is high during the dynamic test, and the pores or cracks within the rock are not completely closed within a short time. During each phase of freeze-thaw cycling, the trends of dynamic strain change are nonuniform. Between 0 and 20 freeze-thaw cycles, the peak strain increases nearly linearly. However, the strain increases slowly during the 20-100 freeze-thaw cycles, and increases gradually after 140 freeze-thaw cycles due to the change of pore structure, and an increase in

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macro-pores under the action of freeze-thaw cycling. The freeze-thaw cycling resulted in

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stress-relief, loss of rock strength, and significant deterioration of mechanical properties. Therefore there is a high likelihood that the rock specimens experienced crushing during

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deformation.

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Fig.8 Relationship between dynamic peak strain and freeze-thaw cycles. The peak strain under dynamic loading

gradually increases. the trends of dynamic strain change are nonuniform during each phase of freeze-thaw cycling.

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5 Conclusions

In summary, this study investigated the coupling effects of freeze-thaw and dynamic loading and found that they had a significant influence on the microstructure of sandstone rock specimens. The coupling effects are supported by observations that rock specimens under dynamic loading conditions responded differently from the rock specimens under static loading conditions. Prior to exposure to freeze-thaw cycling, the rock specimens account for the different types of pore structure observed, with significant implications for bulk permeability and macroscopic mechanical properties. The following conclusions can be made:

22

ACCEPTED MANUSCRIPT (1) As the number of freeze-thaw cycles increased, the mass and porosity of rock specimens also increased. The rock specimens were damaged during the freeze-thaw cycling; and pores developed and expanded continuously with the increase of the number of freeze-thaw cycles. (2) The distribution of T2 spectrum of sandstone mainly shows three peaks. Between 0 and 140 freeze-thaw cycles, the T2 distribution of the specimen increased, especially the micro-pores and macro-pores, indicating that micro-pores in rock continuously developed and expanded under

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the action of freeze-thaw leading to damage occurrence in rock. According to T2 distribution of

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sandstone after freeze-thaw cycles, the pore sizes are classified into four categories: micro-pore, mini-pores, meso-pores and macro-pores. The number of meso-pores and mini-pores dominate in

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the sandstone, with the highest proportion accounting for 92.76%, and the number of meso-pores

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and micro-pores being small. With the increase of the number of freeze-thaw cycles, the freeze-thaw cycles cause the pores to develop, expand, and to interconnect. Finally, some

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micro-pores and mini-pores are transformed into meso-pores and macro-pores, especially the number of macro-pores with a maximum increase of 197.23 %. The rock pore structure changed

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and resulted in stress relief, and finally changed the mechanical properties.

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(3) As the number of freeze-thaw cycles increased, the dynamic peak stress gradually decreased while the peak strain and the overall strain gradually increased. The sandstone mainly

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shows tensile failure modes under dynamic loading. With the increase in the number of freeze-thaw cycles, the number of large-size pores in the specimen also increases. The pores of

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similar size show a centralized distribution, and the macro-damage increases continuously.

Acknowledgement This work was supported by the National Natural Science Foundation of China (Nos. 41502327, 51474252 and

51774323), National Major Scientific Instruments and Equipment Development Projects (2013YQ17046310). The

first author would like to acknowledge the Chinese Scholarship Council for financial support to the visiting scholar

at the Colorado School of Mines.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8