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A study of the shear behavior of a Portland cement grout with the triaxial test
Construction and Building Materials 176 (2018) 81–88
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A study of the shear behavior of a Portland cement grout with the triaxial test Jianhang Chen a,b,⇑, Chunhu Xu a a b
College of Resources and Safety Engineering, China University of Mining and Technology, Beijing 100083, China Open Laboratory for Deep Mine Construction, Henan Polytechnic University, Jiaozuo 454033, China
h i g h l i g h t s The shear behavior of a Portland cement grout was studied with the triaxial test. During the test, the confining pressure was ranged from a small value of 1 MPa to a high value of 35 MPa. The influence of water-to-cement ratio and confining pressure on the shear behavior of the cement grout was investigated.
a r t i c l e
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Article history: Received 28 January 2018 Received in revised form 18 April 2018 Accepted 23 April 2018
Keywords: Shear behavior Portland cement grout Triaxial test High confining pressure Shear strength envelope
a b s t r a c t Numerous research has been conducted on the axial performance of cement-based grouts. However, much less work has been focused on the shear behavior of cement-based grouts. In this study, the shear behavior of a Portland cement grout was investigated with the triaxial test. Cylindrical samples with two different water-to-cement (w/c) ratios: 0.42 and 0.35 were cast and prepared. In the test process, a series of confining pressures was applied on the samples and the confining pressure was ranged from 1 MPa to 35 MPa. The results show that there is a bi-linear relationship between the maximum vertical stress and the confining pressure independent of the w/c ratio. Low w/c ratio can effectively increase the maximum vertical stress of grouts. Mohr-Coulomb models were used to fit the shear strength envelopes of grouts. The results show that the shear strength of grout with a w/c ratio of 0.35 is much higher than that with a w/c ratio of 0.42. When a w/c ratio of 0.42 was used, the grout has a cohesive strength of 31.4 MPa and an internal friction angle of 15.1° while when the w/c ratio was decreased to 0.35, the cohesive strength of grout increases to 34.2 MPa and the internal friction angle rises to 24.3°. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Portland cement grouts have been used in the mining industry for a long time. They are mainly used as bonding agents to bond the rock reinforcement tendons, such as rock bolts and cable bolts, with the surrounding rock masses [1–9]. For example, Thorne and Muller [9] used a cement grout as the agent, bonding cable bolts with surrounding rock masses to control the roof in underground engine chambers at the Free State Geduld Mine. Davis [10] also used cement grouts, installing fully grouted cable bolts at West Coast Mines to keep the stability of open stopes. Schmuck [11] reported that cement grouts were used as the bonding agent in cable bolting at the Homestake Gold Mine. Kashiwayanagi, Shimizu [12] conducted field tests in Japan, using cement grouts ⇑ Corresponding author at: College of Resources and Safety Engineering, China University of Mining and Technology, Beijing 100083, China. E-mail address: [email protected] (J. Chen). https://doi.org/10.1016/j.conbuildmat.2018.04.189 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
to bond cable bolts with rock masses to keep the stability of an underground powerhouse. Pile, Bessinger [13] pumped cement grout into drilled boreholes and then installed cable bolts to prevent bed separation at the BHP Billiton’s San Juan Mine. Numerous laboratory tests have focused on the axial performance of Portland cement grouts. For example, Domone and Thurairatnam [10] tested the compressive performance of grouts. The influence of curing time on the compressive strength of grouts was studied. It was found that the Unconfined Compressive Strength (UCS) of grouts increased directly with the curing time rising from 1 day to 28 days. Hyett, Bawden [11] carried out a comprehensive study on the axial performance of grouts. The influence of waterto-cement (w/c) ratio and curing time on the performance of grouts was investigated. They found that both the w/c ratio and curing time have a significant effect on the UCS of grouts. Specifically, the UCS of grouts decreases from around 80 MPa to around 21 MPa when the w/c ratio increases from 0.3 to 0.7. Boumiz, Vernet [12] conducted experimental tests to evaluate the influence
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of curing time on the Young’s modulus of grouts. The results showed that with the setting time increasing from 7 h to 16 h, there is a positive linear relationship between the setting time and Young’s modulus. While when the setting time increases from 16 h to 47 h, there is a non-linear relationship between the setting time and Young’s modulus, as shown in Fig. 1. Li, Xiao [13] added nano-SiO2 to plain cement grouts and compared the performance of plain cement grouts with the gouts having nano-SiO2. It was found that adding nanophase materials is beneficial to improving the axial strength of grouts. Su and Fang [14] investigated the influence of sample size on the UCS of grouts. Cubic samples with the edge length ranging from 70 mm to 200 mm were tested. It was found that the sample size has a marked influence on the UCS of grouts and the grout UCS decreases with the sample size increasing, as shown in Fig. 2. Chen, Hagan [15] evaluated the axial performance of grouts with different w/c ratios. It was found that the w/c ratio has a negative impact on the Young’s modulus of grouts. Specifically, the Young’s modulus of grouts decreases from 11.8 GPa to 8.7 GPa with the w/c ratio rising from 0.35 to 0.45. Li, Kristjansson [16] tested the UCS of cubic samples and found that the UCS of grouts decreases apparently with the w/c ratio increasing from 0.4 to 0.5, as shown in Fig. 3. On the other hand, only a few studies have evaluated the shear performance of Portland cement grouts. Reichert [17] used triaxial tests to investigate the shear performance of grouts, finding that the grout is obviously plastic in the confined condition and the internal friction angle of the grout is quite low. Similar triaxial tests were conducted by Hyett, Bawden [18] and Moosavi [19]. The cohesive strength and internal friction angle of grouts were acquired. Furthermore, Moosavi and Bawden [20] used direct shear tests to study the shear performance of grouts, finding that there is a non-linear relationship between the shear strength of grouts and normal stresses. Chen, Hagan [21] used direct shear test to study the shear performance of a Portland cement grout under the constant normal load and constant normal stiffness conditions. The shear strength envelope of the cement grout under those two different boundary conditions was acquired. Nevertheless, little research has been conducted on the shear behavior of grouts in the high confining pressure condition. Although Reichert [17], Hyett, Bawden [18] and Moosavi [19] used triaxial tests to evaluate the shear performance of grouts, the maximum confining pressure in those tests was 20 MPa. No further attempt was conducted to test the shear behavior of grouts in a higher confining pressure condition. Previous research has already proved that the shear behavior of grouts has a significant effect on the bond failure of the cable/grout interface. Therefore, studying
Fig. 2. Influence of sample size on the UCS of grouts, after Su and Fang [14].
Fig. 3. Influence of w/c ratio on the UCS of grouts, after Li, Kristjansson [16].
the shear behavior of grouts in high confining pressure is beneficial for understanding the bond failure of the cable/grout interface and preventing cable bolts failing under high stress conditions. Therefore, this study aims at studying the shear behavior of Portland cement grouts with the triaxial test especially in the high confining pressure condition. First, the triaxial test process was illustrated. Then, the shear behavior of grouts with two different w/c ratios: 0.42 and 0.35 were given. After that, the shear behavior of grouts with those two different w/c ratios was compared and the influence of w/c ratio on the shear behavior of grouts was analyzed. 2. Process of the experiment 2.1. Sample preparation
Fig. 1. Influence of setting time on the Young’s modulus of grouts, after Boumiz, Vernet [12].
A modified Portland cement grout was used to cast samples. This cement is named Stratabinder HS which is produced by the Minova Company. It was selected in this study because it is commonly used in mining engineering. The Stratabinder HS cement is a grey powder which is composed of angular particles, as shown in Fig. 4. The bulk density of it is 1168 kg/m3. Two different w/c ratios: 0.42 and 0.35 were used to mix and cast samples. These two w/c ratios were used because field practices show that for cement grouts, UCS strength ranging from 60 MPa to 80 MPa is commonly used in cable bolting [22,23]. Previous UCS test results show that when a w/c ratio of 0.42 is used, the Stratabinder HS cement grout has a UCS of 60 MPa and when a w/c ratio of 0.35 is used, the grout has a UCS of 80 MPa [15]. Cylindrical samples with a height of 120 mm and a diameter of 42 mm were prepared, following the standard recommended by the
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Fig. 4. A photo of the Stratabinder HS cement.
International Society for Rock Mechanics [24]. After demolding, the samples were immersed in a water basin in the laboratory and the indoor temperature was around 20 °C. Under this condition, all samples cured for 28 days. Then, the samples were ground to make sure that the samples have flat surfaces. An outline of the prepared samples is shown in Fig. 5. 2.2. Triaxial test A Hoek cell was used to apply the confining load on the sample and a MTS machine was used to apply the vertical load. The loading rate was 0.003 mm/s and kept constant during the test. The confining pressure ranges from 1 MPa to 35 MPa. After the sample was installed in the Hoek cell, a base pedestal was put on the bottom of the sample and a steel cap was installed on the top of the sample, as shown in Fig. 6. During the test, the vertical force on the sample and the vertical displacement were recorded. After the test, the vertical stressstrain curves were plotted. Each test was replicated five times and the best three were averaged as the final results.
Fig. 6. Triaxial test.
3. Test results and analysis 3.1. Performance of the grout with a w/c ratio of 0.42 When a confining pressure of 1 MPa was applied on the sample, the vertical stress-strain relationships of samples are shown in Fig. 7. Test results show that there is a good consistence between
Fig. 5. Profile of prepared samples.
Fig. 7. Vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 1 MPa.
replication tests. The results of Sample 1, Sample 2 and Sample 5 were selected to represent the triaxial performance of grouts, as shown in Fig. 8. From Fig. 8, it can be seen that the grouts mixed with a w/c ratio of 0.42 is very brittle when the confining pressure is 1 MPa. Specifically, when the vertical strain is around 1.1%, the vertical stress reaches the peak of 80 MPa. Then, there is a sudden drop of the vertical stress. No residual load was acquired. It was also found that in the initial stage, the stress increases non-linearly with strain. This is because in the sample preparation, some tiny air bubbles exist in the sample. Consequently, after the sample cured, tiny voids may occur in the sample. In the test, after the sample was loaded, the tiny voids in the sample were compressed gradually, resulting in initial non-linear increasing of the stress.
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Fig. 8. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 1 MPa.
Using the same method, when a confining pressure of 3 MPa was applied on the sample, the vertical stress-strain relationships of grouts can be acquired, as shown in Fig. 9. It shows that there is still a brittle behavior of grouts. However, the maximum vertical stress of grouts is 88 MPa when the vertical strain reaches 1.3%. Similarly, the vertical stress-strain relationships of grouts when the confining pressures ranges from 4 MPa to 35 MPa can be acquired. For example, when the confining pressure is 10 MPa, 15 MPa, 25 MPa and 35 MPa, the vertical stress-strain relationships of grouts are shown in Figs. 10–13 respectively. It was found that when plotting stress-strain curves, the stress may not start from zero, as shown in Fig. 10. This is because when the assembled Hoek cell was put on the bottom plate of the MTS machine, the bottom plate was lifted gradually with the control panel until the steel cap contacted with the top plate of the MTS machine. Meanwhile, the top plate applied a small vertical load on the sample and this load was recorded by the load cell. Consequently, when plotting stress-strain curves, the initial stress was not zero and started from a very small value. The vertical stress-strain relationships of grouts in different confining pressure conditions is shown in Fig. 14. Apparently, the confining pressure has a significant effect in deciding the vertical stress of grouts. Specifically, when the confining pressure is lower than 10 MPa, the grout shows brittle behavior. However, once the confining pressure was increased to 15 MPa, the grout shows ductile behavior and after the peak, there is apparent residual load. This effect is more obvious when the confining pressure is higher than 15 MPa. When the confining pressure is 35 MPa, the grout almost shows an elastic-perfectly plastic behavior. The maximum vertical stress – confining pressure relationship is shown in Fig. 15. It shows that there is a bi-linear relationship
Fig. 9. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 3 MPa.
Fig. 10. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 10 MPa.
Fig. 11. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 15 MPa.
Fig. 12. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 25 MPa.
Fig. 13. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 35 MPa.
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that the grout mixed with a w/c ratio of 0.42 has a cohesive strength of 31.4 MPa and an internal friction angle of 15.1°, as tabulated in Table 1. Therefore, the shear strength of grouts mixed with a w/c ratio of 0.42 can be expressed with Eq. (3).
160
Stress (MPa)
120 1 MPa 2 Mpa 3 MPa 4 MPa 5 MPa 10 MPa 15 MPa 25 MPa 35 MPa
80
40
0 0.0%
2.0%
4.0%
s ¼ 0:27r þ 31:4
ð3Þ
where, s = shear strength of grouts, Pa; and r = normal stress, Pa. A summary of the triaxial test results when a w/c ratio of 0.42 was used is tabulated in Table 2.
6.0%
3.2. Performance of the grout with a w/c ratio of 0.35
Strain Fig. 14. Vertical stress-strain relationships of grouts mixed with a w/c ratio of 0.42 in different confining pressure conditions.
When a confining pressure of 1 MPa was applied on the sample, the vertical stress-strain relationships of grouts mixed with a w/c ratio of 0.35 are shown in Fig. 17. It shows that the vertical stress increases to a peak of 103 MPa when the strain arrives at around 1.1%. Then, the bearing capacity of grouts decreases dramatically, showing apparent brittle behavior. Similarly, the vertical stress-strain relationships of grouts with the confining pressure rising from 2 MPa to 35 MPa were acquired. For example, when the confining pressure was 10 MPa, 20 MPa, 25 MPa and 35 MPa, the vertical stress-strain curves of grouts are shown in Figs. 18–21 respectively. The influence of confining pressure on the vertical stress of grouts is shown in Fig. 22. Apparently, the confining pressure has
Table 1 Mechanical properties of grouts with a w/c ratio of 0.42.
Fig. 15. The maximum vertical stress-strain relationships of grouts mixed with a w/c ratio of 0.42 in different confining pressure conditions.
between the maximum vertical stress and confining pressure. When the confining pressure is smaller than 5 MPa, the relationship between the maximum vertical stress and confining pressure can be expressed with Eq. (1):
rmax ¼ 3r3 þ 77:4 1
ð1Þ
While, when the confining pressure is higher than 5 MPa and lower than 35 MPa, the relationship between the maximum vertical stress and confining pressure can be calculated with Eq. (2):
rmax ¼ 1:5r3 þ 87:2 1
w/c ratio
Cohesive strength (MPa)
Internal friction angle (°)
0.42
31.4
15.1
Table 2 Test results when a w/c ratio of 0.42 was used. w/c ratio
Confining pressure (MPa)
Mean value of the maximum vertical stress (MPa)
0.42
1 2 3 4 5 10 15 25 35
80 83 88 89 93 101 113 125 140
ð2Þ
A series of Mohr circles were drawn to acquire the shear strength envelope and a Mohr-coulomb model was used to fit the shear strength envelope, as shown in Fig. 16. The results show
Fig. 16. Shear strength envelope of grouts mixed with a w/c ratio of 0.42.
Fig. 17. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.35 when the confining pressure is 1 MPa.
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Fig. 18. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.35 when the confining pressure is 10 MPa.
Fig. 22. Vertical stress-strain relationships of grouts mixed with a w/c ratio of 0.35 in different confining pressure conditions.
a dominant effect in deciding the vertical stress. Larger confining pressure results in higher vertical stress. The maximum vertical stress - confining pressure relationship is shown in Fig. 23. A bi-linear model was used to depict the relationship between the maximum vertical stress and confining pressure. Specifically, when the confining pressure is smaller than 8 MPa, the maximum vertical stress can be calculated with Eq. (4):
rmax ¼ 4r3 þ 99:5 1
ð4Þ
While, when the confining pressure is higher than 8 MPa and lower than 35 MPa, the relationship between the maximum vertical stress and confining pressure can be calculated with Eq. (5): Fig. 19. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.35 when the confining pressure is 20 MPa.
rmax ¼ 2:2r3 þ 114:1 1
ð5Þ
Fig. 20. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.35 when the confining pressure is 25 MPa. Fig. 23. The maximum vertical stress-strain relationships of grouts mixed with a w/ c ratio of 0.35 in different confining pressure conditions.
Fig. 21. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.35 when the confining pressure is 35 MPa.
Fig. 24. Shear strength envelope of grouts mixed with a w/c ratio of 0.35.
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The shear strength envelope of grouts mixed with a w/c ratio of 0.35 is shown in Fig. 24. A linear line was used to fit the shear strength envelope, showing that the grout mixed with a w/c ratio of 0.35 has a cohesive strength of 34.2 MPa and an internal friction angle of 24.3°, as tabulated in Table 3. Therefore, the shear strength of grouts mixed with a w/c ratio of 0.35 can be expressed with Eq. (6).
s ¼ 0:45r þ 34:2
ð6Þ
A summary of the triaxial test results when a w/c ratio of 0.35 was used is tabulated in Table 4. 3.3. Influence of w/c ratio on the performance of the grout
Table 3 Mechanical properties of grouts with a w/c ratio of 0.35. w/c ratio
Cohesive strength (MPa)
Internal friction angle (°)
0.35
34.2
24.3
Fig. 26. Comparing the shear strength envelope of grouts with two different w/c ratios.
120
Stress (MPa)
A comparison of the maximum vertical stress of the grouts with two different w/c ratios is shown in Fig. 25. It can be seen that the w/c ratio has a significant effect in deciding the maximum vertical stress that the grouts can bear. When the confining pressure is equal, low w/c ratio can increase the maximum vertical stress. The influence of w/c ratio on the shear strength envelope of grouts with two different w/c ratios is shown in Fig. 26. It shows that decreasing the w/c ratio can effectively increase the shear strength of grouts.
80
5 MPa (w/c=0.4) 10 MPa (w/c=0.4)
15 MPa (w/c=0.4)
40
5 MPa (w/c=0.42) 10 MPa (w/c=0.42)
15 MPa (w/c=0.42)
0
0.0%
2.0%
4.0%
6.0%
Strain Fig. 27. Comparison between the results acquired in this study (solid line) and the results acquired by Hyett, Bawden [18] (dash line). Table 4 Test results when a w/c ratio of 0.35 was used.
3.4. Comparison with previous research
w/c ratio
Confining pressure (MPa)
Mean value of the maximum vertical stress (MPa)
0.35
1 2 3 4 5 6 10 15 20 25 35
103 107 112 116 118 123 135 149 163 174 186
Hyett, Bawden [18] conducted triaxial tests on a traditional Portland cement grout. A w/c ratio of 0.4 was used. During the test, the confining pressure was changed from 5 MPa to 15 MPa. In this study, a w/c ratio of 0.42 was used, which is close to the w/c ratio used by Hyett, Bawden [18]. Therefore, a comparison was conducted between the results acquired in this study and the results acquired by Hyett, Bawden [18]. The vertical stress-strain relationship of the Stratabinder HS cement grout and the traditional Portland cement grout is shown in Fig. 27. It can be seen that under the same confining pressure condition, the Stratabinder HS cement grout has much higher strength compared with the traditional Portland cement grout although more water was mixed with the Stratabinder HS cement. It was also found that the Stratabinder HS cement grout shows brittle behavior while the traditional Portland cement grout shows ductile behavior. This is more apparent when the confining pressure is lower than 10 MPa. Specifically, when the strain is smaller than 1.7%, the bearing capacity of the Stratabinder HS cement grout drops dramatically. However, for the traditional Portland cement grout, when the strain is 3.5%, the grout still has a capacity higher than 30 MPa. 4. Conclusions
Fig. 25. Comparing maximum vertical stress of grouts with two different w/c ratios.
Shear behavior of a modified Portland cement grout was investigated with the triaxial test. Two different w/c ratios: 0.42 and 0.35 were used. During the test, the confining pressure was ranged from 1 MPa to 35 MPa. The results show that there is a bi-linear
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relationship between the maximum vertical stress and the confining pressure. Furthermore, low w/c ratio can effectively increase the maximum vertical stress of grouts. The Mohr-Coulomb model was used to fit the shear strength envelope of grouts. It was found that decreasing the w/c ratio can apparently increase the shear strength of grouts when the w/c ratio is ranged from 0.35 to 0.42. When a w/c ratio of 0.42 was used, the cohesive strength of grouts is 31.4 MPa and the internal friction angle is 15.1°. While when the w/c ratio decreased to 0.35, the cohesive strength of grouts is 34.2 MPa and the internal friction angle is 24.3°.
Conflict of interest The authors certify that there is no conflict of interest to report. Acknowledgements The authors would like to thank the support provided by UNSW Sydney and the grant (No. 2015KF-04) supported by the Open Laboratory for Deep Mine Construction, Henan Polytechnic University. This paper was also supported by the Fundamental Research Funds for the Central Universities (Grant No. 2018QZ06). References [1] D.J. Hutchinson, M.S. Diederichs, Cablebolting in Underground Mines, BiTech Publishers Ltd., Richmond, 1996. [2] S. Ma, J. Nemcik, N. Aziz, An analytical model of fully grouted rock bolts subjected to tensile load, Constr. Build. Mater. 2013 (49) (2013) 519–526. [3] S. Ma, J. Nemcik, N. Aziz, Z. Zhang, Analytical model for rock bolts reaching free end slip, Constr. Build. Mater. 2014 (57) (2014) 30–37. [4] J. Chen, P.C. Hagan, S. Saydam, Load transfer behavior of fully grouted cable bolts reinforced in weak rocks under tensile loading conditions, Geotech. Test. J. 39 (2) (2016) 252–263. [5] S. Ma, N. Aziz, J. Nemcik, A. Mirzaghorbanali, Bond characteristics of fully grouted rockbolts, Geotech. Test. J. 40 (5) (2017) 1–13. [6] J. Chen, P.C. Hagan, S. Saydam, Sample diameter effect on bonding capacity of fully grouted cable bolts, Tunnel Underground Space Technol. 68 (2017) 238– 243. [7] J. Chen, P.C. Hagan, S. Saydam, Parametric study on the axial performance of a fully grouted cable bolt with a new pull-out test, Int. J. Min. Sci. Technol. 26 (1) (2016) 53–58.
[8] S. Ma, Z. Zhao, W. Nie, X. Zhu, An analytical model for fully grouted rockbolts with consideration of the pre- and post-yielding behavior, Rock Mech. Rock Eng. 2017 (2017) 1–10. [9] S. Ma, Z. Zhao, W. Nie, Y. Gui, A numerical model of fully grouted bolts considering the tri-linear shear bond-slip model, Tunnel Underground Space Technol. 2016 (54) (2016) 73–80. [10] P.L. Domone, H. Thurairatnam, Cement Grouts with Specific Properties for Offshore use. Grouts and Grouting for Construction and Repair of Offshore Structure-a Summary Report, Department of Energy Offshore Technology Report OTH, London, 1988, pp. 44–58. [11] A.J. Hyett, W.F. Bawden, R.D. Reichert, The effect of rock mass confinement on the bond strength of fully grouted cable bolts, Int. J. Rock Mech. Min. Sci. 29 (5) (1992) 503–524. [12] A. Boumiz, C. Vernet, F.C. Tenoudji, Mechancial properties of cement pastes and mortars at early ages, Adv. Cem. Based Mater. 1996 (3) (1996) 94–106. [13] H. Li, H. Xiao, J. Qu, A study on mechanical and pressure-sensitive properties of cement mortar with nonophase materials, Cem. Concr. Res. 34 (3) (2004) 435– 438. [14] J. Su, Z. Fang, Experimental study on dimensional effect of mortar cubic compressive strength, Eng. Sci. 16 (2) (2012) 88–92. [15] J. Chen, P.C. Hagan, S. Saydam, Mechanical properties of cementitious grout serving in fully grouted cable bolting systems, in: S. Saydam, P.C. Hagan (Eds.), Third Australasian Ground Control in Mining Conference, UNSW Australia, Sydney, Australia, 2014, pp. 269–275. [16] C.C. Li, G. Kristjansson, A.H. Høien, Critical embedment length and bond strength of fully encapsulated rebar rockbolts, Tunnel Underground Space Technol. 2016 (59) (2016) 16–23. [17] R.D. Reichert, A Laboratory and Field Investigation of the Major Factors Influcing Bond Capacity of Grouted Cable bolts [Unpublished], Queen’s University, Kingston, 1991. [18] Hyett AJ, Bawden WF, Coulson AL. Physical and mechanical properties of normal Portland cement pertaining to fully grouted cable bolts. In: Kaiser PK, McCreath DR, editors. Rock support in Mining and Underground Construction: Proceedings of the International Symposium on Rock Support. Sudbury, Canada: A.A. Balkema; 1992. p. 341–348. [19] M. Moosavi, Load Distribution Along Fully Grouted Cable Bolts Based on Constitutive Models Obtained from Modified Hoek Cells [Unpublished], Queen’s University, Kingston, 1997. [20] M. Moosavi, W.F. Bawden, Shear strength of Portland cement grout, Cem. Concr. Compos. 2003 (25) (2003) 729–735. [21] J. Chen, P.C. Hagan, S. Saydam, Shear behaviour of a cement grout tested in the direct shear test, Constr. Build. Mater. 166 (2018) 271–279. [22] P. Craig, M. Holden, In situ bond strength testing of Australian cable bolts, in: N. Aziz (Ed.), 14th Coal Operators’ Conference, The University of Wollongong, Wollongong, Australia, 2014, pp. 147–155. [23] P.C. Hagan, J. Chen, Optimising the selection of fully grouted cable bolts in varying geotechnical environments, Australian Coal Association Research Program, Sydney, 2015, pp. 1–94. [24] K. Kovari, A. Tisa, H. Einstein, J.A. Franklin, Suggested methods for determining the strength of rock materials in triaxial compression: revised version, Int. J. Rock Mech. Min. Sci. Geomech. Abstracts 20 (6) (1984) 285–290.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A study of the shear behavior of a Portland cement grout with the triaxial test Jianhang Chen a,b,⇑, Chunhu Xu a a b
College of Resources and Safety Engineering, China University of Mining and Technology, Beijing 100083, China Open Laboratory for Deep Mine Construction, Henan Polytechnic University, Jiaozuo 454033, China
h i g h l i g h t s The shear behavior of a Portland cement grout was studied with the triaxial test. During the test, the confining pressure was ranged from a small value of 1 MPa to a high value of 35 MPa. The influence of water-to-cement ratio and confining pressure on the shear behavior of the cement grout was investigated.
a r t i c l e
i n f o
Article history: Received 28 January 2018 Received in revised form 18 April 2018 Accepted 23 April 2018
Keywords: Shear behavior Portland cement grout Triaxial test High confining pressure Shear strength envelope
a b s t r a c t Numerous research has been conducted on the axial performance of cement-based grouts. However, much less work has been focused on the shear behavior of cement-based grouts. In this study, the shear behavior of a Portland cement grout was investigated with the triaxial test. Cylindrical samples with two different water-to-cement (w/c) ratios: 0.42 and 0.35 were cast and prepared. In the test process, a series of confining pressures was applied on the samples and the confining pressure was ranged from 1 MPa to 35 MPa. The results show that there is a bi-linear relationship between the maximum vertical stress and the confining pressure independent of the w/c ratio. Low w/c ratio can effectively increase the maximum vertical stress of grouts. Mohr-Coulomb models were used to fit the shear strength envelopes of grouts. The results show that the shear strength of grout with a w/c ratio of 0.35 is much higher than that with a w/c ratio of 0.42. When a w/c ratio of 0.42 was used, the grout has a cohesive strength of 31.4 MPa and an internal friction angle of 15.1° while when the w/c ratio was decreased to 0.35, the cohesive strength of grout increases to 34.2 MPa and the internal friction angle rises to 24.3°. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Portland cement grouts have been used in the mining industry for a long time. They are mainly used as bonding agents to bond the rock reinforcement tendons, such as rock bolts and cable bolts, with the surrounding rock masses [1–9]. For example, Thorne and Muller [9] used a cement grout as the agent, bonding cable bolts with surrounding rock masses to control the roof in underground engine chambers at the Free State Geduld Mine. Davis [10] also used cement grouts, installing fully grouted cable bolts at West Coast Mines to keep the stability of open stopes. Schmuck [11] reported that cement grouts were used as the bonding agent in cable bolting at the Homestake Gold Mine. Kashiwayanagi, Shimizu [12] conducted field tests in Japan, using cement grouts ⇑ Corresponding author at: College of Resources and Safety Engineering, China University of Mining and Technology, Beijing 100083, China. E-mail address: [email protected] (J. Chen). https://doi.org/10.1016/j.conbuildmat.2018.04.189 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
to bond cable bolts with rock masses to keep the stability of an underground powerhouse. Pile, Bessinger [13] pumped cement grout into drilled boreholes and then installed cable bolts to prevent bed separation at the BHP Billiton’s San Juan Mine. Numerous laboratory tests have focused on the axial performance of Portland cement grouts. For example, Domone and Thurairatnam [10] tested the compressive performance of grouts. The influence of curing time on the compressive strength of grouts was studied. It was found that the Unconfined Compressive Strength (UCS) of grouts increased directly with the curing time rising from 1 day to 28 days. Hyett, Bawden [11] carried out a comprehensive study on the axial performance of grouts. The influence of waterto-cement (w/c) ratio and curing time on the performance of grouts was investigated. They found that both the w/c ratio and curing time have a significant effect on the UCS of grouts. Specifically, the UCS of grouts decreases from around 80 MPa to around 21 MPa when the w/c ratio increases from 0.3 to 0.7. Boumiz, Vernet [12] conducted experimental tests to evaluate the influence
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of curing time on the Young’s modulus of grouts. The results showed that with the setting time increasing from 7 h to 16 h, there is a positive linear relationship between the setting time and Young’s modulus. While when the setting time increases from 16 h to 47 h, there is a non-linear relationship between the setting time and Young’s modulus, as shown in Fig. 1. Li, Xiao [13] added nano-SiO2 to plain cement grouts and compared the performance of plain cement grouts with the gouts having nano-SiO2. It was found that adding nanophase materials is beneficial to improving the axial strength of grouts. Su and Fang [14] investigated the influence of sample size on the UCS of grouts. Cubic samples with the edge length ranging from 70 mm to 200 mm were tested. It was found that the sample size has a marked influence on the UCS of grouts and the grout UCS decreases with the sample size increasing, as shown in Fig. 2. Chen, Hagan [15] evaluated the axial performance of grouts with different w/c ratios. It was found that the w/c ratio has a negative impact on the Young’s modulus of grouts. Specifically, the Young’s modulus of grouts decreases from 11.8 GPa to 8.7 GPa with the w/c ratio rising from 0.35 to 0.45. Li, Kristjansson [16] tested the UCS of cubic samples and found that the UCS of grouts decreases apparently with the w/c ratio increasing from 0.4 to 0.5, as shown in Fig. 3. On the other hand, only a few studies have evaluated the shear performance of Portland cement grouts. Reichert [17] used triaxial tests to investigate the shear performance of grouts, finding that the grout is obviously plastic in the confined condition and the internal friction angle of the grout is quite low. Similar triaxial tests were conducted by Hyett, Bawden [18] and Moosavi [19]. The cohesive strength and internal friction angle of grouts were acquired. Furthermore, Moosavi and Bawden [20] used direct shear tests to study the shear performance of grouts, finding that there is a non-linear relationship between the shear strength of grouts and normal stresses. Chen, Hagan [21] used direct shear test to study the shear performance of a Portland cement grout under the constant normal load and constant normal stiffness conditions. The shear strength envelope of the cement grout under those two different boundary conditions was acquired. Nevertheless, little research has been conducted on the shear behavior of grouts in the high confining pressure condition. Although Reichert [17], Hyett, Bawden [18] and Moosavi [19] used triaxial tests to evaluate the shear performance of grouts, the maximum confining pressure in those tests was 20 MPa. No further attempt was conducted to test the shear behavior of grouts in a higher confining pressure condition. Previous research has already proved that the shear behavior of grouts has a significant effect on the bond failure of the cable/grout interface. Therefore, studying
Fig. 2. Influence of sample size on the UCS of grouts, after Su and Fang [14].
Fig. 3. Influence of w/c ratio on the UCS of grouts, after Li, Kristjansson [16].
the shear behavior of grouts in high confining pressure is beneficial for understanding the bond failure of the cable/grout interface and preventing cable bolts failing under high stress conditions. Therefore, this study aims at studying the shear behavior of Portland cement grouts with the triaxial test especially in the high confining pressure condition. First, the triaxial test process was illustrated. Then, the shear behavior of grouts with two different w/c ratios: 0.42 and 0.35 were given. After that, the shear behavior of grouts with those two different w/c ratios was compared and the influence of w/c ratio on the shear behavior of grouts was analyzed. 2. Process of the experiment 2.1. Sample preparation
Fig. 1. Influence of setting time on the Young’s modulus of grouts, after Boumiz, Vernet [12].
A modified Portland cement grout was used to cast samples. This cement is named Stratabinder HS which is produced by the Minova Company. It was selected in this study because it is commonly used in mining engineering. The Stratabinder HS cement is a grey powder which is composed of angular particles, as shown in Fig. 4. The bulk density of it is 1168 kg/m3. Two different w/c ratios: 0.42 and 0.35 were used to mix and cast samples. These two w/c ratios were used because field practices show that for cement grouts, UCS strength ranging from 60 MPa to 80 MPa is commonly used in cable bolting [22,23]. Previous UCS test results show that when a w/c ratio of 0.42 is used, the Stratabinder HS cement grout has a UCS of 60 MPa and when a w/c ratio of 0.35 is used, the grout has a UCS of 80 MPa [15]. Cylindrical samples with a height of 120 mm and a diameter of 42 mm were prepared, following the standard recommended by the
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Fig. 4. A photo of the Stratabinder HS cement.
International Society for Rock Mechanics [24]. After demolding, the samples were immersed in a water basin in the laboratory and the indoor temperature was around 20 °C. Under this condition, all samples cured for 28 days. Then, the samples were ground to make sure that the samples have flat surfaces. An outline of the prepared samples is shown in Fig. 5. 2.2. Triaxial test A Hoek cell was used to apply the confining load on the sample and a MTS machine was used to apply the vertical load. The loading rate was 0.003 mm/s and kept constant during the test. The confining pressure ranges from 1 MPa to 35 MPa. After the sample was installed in the Hoek cell, a base pedestal was put on the bottom of the sample and a steel cap was installed on the top of the sample, as shown in Fig. 6. During the test, the vertical force on the sample and the vertical displacement were recorded. After the test, the vertical stressstrain curves were plotted. Each test was replicated five times and the best three were averaged as the final results.
Fig. 6. Triaxial test.
3. Test results and analysis 3.1. Performance of the grout with a w/c ratio of 0.42 When a confining pressure of 1 MPa was applied on the sample, the vertical stress-strain relationships of samples are shown in Fig. 7. Test results show that there is a good consistence between
Fig. 5. Profile of prepared samples.
Fig. 7. Vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 1 MPa.
replication tests. The results of Sample 1, Sample 2 and Sample 5 were selected to represent the triaxial performance of grouts, as shown in Fig. 8. From Fig. 8, it can be seen that the grouts mixed with a w/c ratio of 0.42 is very brittle when the confining pressure is 1 MPa. Specifically, when the vertical strain is around 1.1%, the vertical stress reaches the peak of 80 MPa. Then, there is a sudden drop of the vertical stress. No residual load was acquired. It was also found that in the initial stage, the stress increases non-linearly with strain. This is because in the sample preparation, some tiny air bubbles exist in the sample. Consequently, after the sample cured, tiny voids may occur in the sample. In the test, after the sample was loaded, the tiny voids in the sample were compressed gradually, resulting in initial non-linear increasing of the stress.
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Fig. 8. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 1 MPa.
Using the same method, when a confining pressure of 3 MPa was applied on the sample, the vertical stress-strain relationships of grouts can be acquired, as shown in Fig. 9. It shows that there is still a brittle behavior of grouts. However, the maximum vertical stress of grouts is 88 MPa when the vertical strain reaches 1.3%. Similarly, the vertical stress-strain relationships of grouts when the confining pressures ranges from 4 MPa to 35 MPa can be acquired. For example, when the confining pressure is 10 MPa, 15 MPa, 25 MPa and 35 MPa, the vertical stress-strain relationships of grouts are shown in Figs. 10–13 respectively. It was found that when plotting stress-strain curves, the stress may not start from zero, as shown in Fig. 10. This is because when the assembled Hoek cell was put on the bottom plate of the MTS machine, the bottom plate was lifted gradually with the control panel until the steel cap contacted with the top plate of the MTS machine. Meanwhile, the top plate applied a small vertical load on the sample and this load was recorded by the load cell. Consequently, when plotting stress-strain curves, the initial stress was not zero and started from a very small value. The vertical stress-strain relationships of grouts in different confining pressure conditions is shown in Fig. 14. Apparently, the confining pressure has a significant effect in deciding the vertical stress of grouts. Specifically, when the confining pressure is lower than 10 MPa, the grout shows brittle behavior. However, once the confining pressure was increased to 15 MPa, the grout shows ductile behavior and after the peak, there is apparent residual load. This effect is more obvious when the confining pressure is higher than 15 MPa. When the confining pressure is 35 MPa, the grout almost shows an elastic-perfectly plastic behavior. The maximum vertical stress – confining pressure relationship is shown in Fig. 15. It shows that there is a bi-linear relationship
Fig. 9. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 3 MPa.
Fig. 10. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 10 MPa.
Fig. 11. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 15 MPa.
Fig. 12. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 25 MPa.
Fig. 13. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.42 when the confining pressure is 35 MPa.
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that the grout mixed with a w/c ratio of 0.42 has a cohesive strength of 31.4 MPa and an internal friction angle of 15.1°, as tabulated in Table 1. Therefore, the shear strength of grouts mixed with a w/c ratio of 0.42 can be expressed with Eq. (3).
160
Stress (MPa)
120 1 MPa 2 Mpa 3 MPa 4 MPa 5 MPa 10 MPa 15 MPa 25 MPa 35 MPa
80
40
0 0.0%
2.0%
4.0%
s ¼ 0:27r þ 31:4
ð3Þ
where, s = shear strength of grouts, Pa; and r = normal stress, Pa. A summary of the triaxial test results when a w/c ratio of 0.42 was used is tabulated in Table 2.
6.0%
3.2. Performance of the grout with a w/c ratio of 0.35
Strain Fig. 14. Vertical stress-strain relationships of grouts mixed with a w/c ratio of 0.42 in different confining pressure conditions.
When a confining pressure of 1 MPa was applied on the sample, the vertical stress-strain relationships of grouts mixed with a w/c ratio of 0.35 are shown in Fig. 17. It shows that the vertical stress increases to a peak of 103 MPa when the strain arrives at around 1.1%. Then, the bearing capacity of grouts decreases dramatically, showing apparent brittle behavior. Similarly, the vertical stress-strain relationships of grouts with the confining pressure rising from 2 MPa to 35 MPa were acquired. For example, when the confining pressure was 10 MPa, 20 MPa, 25 MPa and 35 MPa, the vertical stress-strain curves of grouts are shown in Figs. 18–21 respectively. The influence of confining pressure on the vertical stress of grouts is shown in Fig. 22. Apparently, the confining pressure has
Table 1 Mechanical properties of grouts with a w/c ratio of 0.42.
Fig. 15. The maximum vertical stress-strain relationships of grouts mixed with a w/c ratio of 0.42 in different confining pressure conditions.
between the maximum vertical stress and confining pressure. When the confining pressure is smaller than 5 MPa, the relationship between the maximum vertical stress and confining pressure can be expressed with Eq. (1):
rmax ¼ 3r3 þ 77:4 1
ð1Þ
While, when the confining pressure is higher than 5 MPa and lower than 35 MPa, the relationship between the maximum vertical stress and confining pressure can be calculated with Eq. (2):
rmax ¼ 1:5r3 þ 87:2 1
w/c ratio
Cohesive strength (MPa)
Internal friction angle (°)
0.42
31.4
15.1
Table 2 Test results when a w/c ratio of 0.42 was used. w/c ratio
Confining pressure (MPa)
Mean value of the maximum vertical stress (MPa)
0.42
1 2 3 4 5 10 15 25 35
80 83 88 89 93 101 113 125 140
ð2Þ
A series of Mohr circles were drawn to acquire the shear strength envelope and a Mohr-coulomb model was used to fit the shear strength envelope, as shown in Fig. 16. The results show
Fig. 16. Shear strength envelope of grouts mixed with a w/c ratio of 0.42.
Fig. 17. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.35 when the confining pressure is 1 MPa.
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Fig. 18. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.35 when the confining pressure is 10 MPa.
Fig. 22. Vertical stress-strain relationships of grouts mixed with a w/c ratio of 0.35 in different confining pressure conditions.
a dominant effect in deciding the vertical stress. Larger confining pressure results in higher vertical stress. The maximum vertical stress - confining pressure relationship is shown in Fig. 23. A bi-linear model was used to depict the relationship between the maximum vertical stress and confining pressure. Specifically, when the confining pressure is smaller than 8 MPa, the maximum vertical stress can be calculated with Eq. (4):
rmax ¼ 4r3 þ 99:5 1
ð4Þ
While, when the confining pressure is higher than 8 MPa and lower than 35 MPa, the relationship between the maximum vertical stress and confining pressure can be calculated with Eq. (5): Fig. 19. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.35 when the confining pressure is 20 MPa.
rmax ¼ 2:2r3 þ 114:1 1
ð5Þ
Fig. 20. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.35 when the confining pressure is 25 MPa. Fig. 23. The maximum vertical stress-strain relationships of grouts mixed with a w/ c ratio of 0.35 in different confining pressure conditions.
Fig. 21. Selected vertical stress-strain curves of samples mixed with a w/c ratio of 0.35 when the confining pressure is 35 MPa.
Fig. 24. Shear strength envelope of grouts mixed with a w/c ratio of 0.35.
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The shear strength envelope of grouts mixed with a w/c ratio of 0.35 is shown in Fig. 24. A linear line was used to fit the shear strength envelope, showing that the grout mixed with a w/c ratio of 0.35 has a cohesive strength of 34.2 MPa and an internal friction angle of 24.3°, as tabulated in Table 3. Therefore, the shear strength of grouts mixed with a w/c ratio of 0.35 can be expressed with Eq. (6).
s ¼ 0:45r þ 34:2
ð6Þ
A summary of the triaxial test results when a w/c ratio of 0.35 was used is tabulated in Table 4. 3.3. Influence of w/c ratio on the performance of the grout
Table 3 Mechanical properties of grouts with a w/c ratio of 0.35. w/c ratio
Cohesive strength (MPa)
Internal friction angle (°)
0.35
34.2
24.3
Fig. 26. Comparing the shear strength envelope of grouts with two different w/c ratios.
120
Stress (MPa)
A comparison of the maximum vertical stress of the grouts with two different w/c ratios is shown in Fig. 25. It can be seen that the w/c ratio has a significant effect in deciding the maximum vertical stress that the grouts can bear. When the confining pressure is equal, low w/c ratio can increase the maximum vertical stress. The influence of w/c ratio on the shear strength envelope of grouts with two different w/c ratios is shown in Fig. 26. It shows that decreasing the w/c ratio can effectively increase the shear strength of grouts.
80
5 MPa (w/c=0.4) 10 MPa (w/c=0.4)
15 MPa (w/c=0.4)
40
5 MPa (w/c=0.42) 10 MPa (w/c=0.42)
15 MPa (w/c=0.42)
0
0.0%
2.0%
4.0%
6.0%
Strain Fig. 27. Comparison between the results acquired in this study (solid line) and the results acquired by Hyett, Bawden [18] (dash line). Table 4 Test results when a w/c ratio of 0.35 was used.
3.4. Comparison with previous research
w/c ratio
Confining pressure (MPa)
Mean value of the maximum vertical stress (MPa)
0.35
1 2 3 4 5 6 10 15 20 25 35
103 107 112 116 118 123 135 149 163 174 186
Hyett, Bawden [18] conducted triaxial tests on a traditional Portland cement grout. A w/c ratio of 0.4 was used. During the test, the confining pressure was changed from 5 MPa to 15 MPa. In this study, a w/c ratio of 0.42 was used, which is close to the w/c ratio used by Hyett, Bawden [18]. Therefore, a comparison was conducted between the results acquired in this study and the results acquired by Hyett, Bawden [18]. The vertical stress-strain relationship of the Stratabinder HS cement grout and the traditional Portland cement grout is shown in Fig. 27. It can be seen that under the same confining pressure condition, the Stratabinder HS cement grout has much higher strength compared with the traditional Portland cement grout although more water was mixed with the Stratabinder HS cement. It was also found that the Stratabinder HS cement grout shows brittle behavior while the traditional Portland cement grout shows ductile behavior. This is more apparent when the confining pressure is lower than 10 MPa. Specifically, when the strain is smaller than 1.7%, the bearing capacity of the Stratabinder HS cement grout drops dramatically. However, for the traditional Portland cement grout, when the strain is 3.5%, the grout still has a capacity higher than 30 MPa. 4. Conclusions
Fig. 25. Comparing maximum vertical stress of grouts with two different w/c ratios.
Shear behavior of a modified Portland cement grout was investigated with the triaxial test. Two different w/c ratios: 0.42 and 0.35 were used. During the test, the confining pressure was ranged from 1 MPa to 35 MPa. The results show that there is a bi-linear
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relationship between the maximum vertical stress and the confining pressure. Furthermore, low w/c ratio can effectively increase the maximum vertical stress of grouts. The Mohr-Coulomb model was used to fit the shear strength envelope of grouts. It was found that decreasing the w/c ratio can apparently increase the shear strength of grouts when the w/c ratio is ranged from 0.35 to 0.42. When a w/c ratio of 0.42 was used, the cohesive strength of grouts is 31.4 MPa and the internal friction angle is 15.1°. While when the w/c ratio decreased to 0.35, the cohesive strength of grouts is 34.2 MPa and the internal friction angle is 24.3°.
Conflict of interest The authors certify that there is no conflict of interest to report. Acknowledgements The authors would like to thank the support provided by UNSW Sydney and the grant (No. 2015KF-04) supported by the Open Laboratory for Deep Mine Construction, Henan Polytechnic University. This paper was also supported by the Fundamental Research Funds for the Central Universities (Grant No. 2018QZ06). References [1] D.J. Hutchinson, M.S. Diederichs, Cablebolting in Underground Mines, BiTech Publishers Ltd., Richmond, 1996. [2] S. Ma, J. Nemcik, N. Aziz, An analytical model of fully grouted rock bolts subjected to tensile load, Constr. Build. Mater. 2013 (49) (2013) 519–526. [3] S. Ma, J. Nemcik, N. Aziz, Z. Zhang, Analytical model for rock bolts reaching free end slip, Constr. Build. Mater. 2014 (57) (2014) 30–37. [4] J. Chen, P.C. Hagan, S. Saydam, Load transfer behavior of fully grouted cable bolts reinforced in weak rocks under tensile loading conditions, Geotech. Test. J. 39 (2) (2016) 252–263. [5] S. Ma, N. Aziz, J. Nemcik, A. Mirzaghorbanali, Bond characteristics of fully grouted rockbolts, Geotech. Test. J. 40 (5) (2017) 1–13. [6] J. Chen, P.C. Hagan, S. Saydam, Sample diameter effect on bonding capacity of fully grouted cable bolts, Tunnel Underground Space Technol. 68 (2017) 238– 243. [7] J. Chen, P.C. Hagan, S. Saydam, Parametric study on the axial performance of a fully grouted cable bolt with a new pull-out test, Int. J. Min. Sci. Technol. 26 (1) (2016) 53–58.
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