The Confining Effect of End Roughness on Unconfined Compressive
Z. Szczepanik, D. Milne & C. Hawkes
Department of Civil and Geological Engineering, University of Saskatchewan, Canada
st Proceedings of the 1 Canada-US Rock Mechanics Symposium, Vancouver, Cana-
da, 2007, pp. 191-198.
ABSTRACT: The influence of sample end effects on the unconfined compressive strength of rock core is well recognised. ASTM standards exist to ensure that minimum standards of sample smoothness are maintained to minimise the influence of friction between the samples ends and loading platens. Sample end preparation is also
done to avoid stress concentrations on irregularities on the end surfaces. This paper describes tests that have
been conducted on relatively uniform grey granite from northern Manitoba, Canada to investigate the influence
of sample end effects. End conditions were varied by polishing the sample ends and by using loading platens with varying degrees of roughness. In one series of tests, lead foil was placed between the sample ends and the
loading platens to further decrease frictional effects. In all tests, except the lead foil tests, procedures and sam-
ple preparations were conducted within the ASTM standards for unconfined compressive strength (UCS) test-
The test results presented show that sample “hourglassing”, as measured using circumferential strain gauges
located near sample ends and at sample mid-points, resulted in strengths as low as 50% of the standard UCS values. Rougher sample ends and platens produced sample “barrelling” with strengths the same, or slightly
higher than results from standard tests. These results suggest that standard UCS tests are conducted with a
significant degree of effective sample confinement generated by sample end friction.
1 INTRODUCTION 2 SAMPLES AND PLATENS USED FOR THE
TESTING PROGRAM Research into factors influencing unconfined com-
pressive strength (UCS) tests have been conducted at 2.1 Sample Descriptions
the University of Saskatchewan for over 10 years. Samples of a medium-grained grey granite from Early testing was conducted to look at crack initia-northern Manitoba have been tested. The granitic tion and propagation in granitic samples (Eberhardt, rock was divided into two groups based on the 1998). Subsequent creep testing was conducted to P-wave velocity measured for each sample. The determine if the strength of granitic samples was re-slower velocity samples ranged from 3161 metres per duced under long term loads in excess of the theoret-second (m/s) to 4373 m/s and the faster velocities ical strength, causing unstable crack growth (Sczce-were between 4496 m/s to 5134 m/s. Two groups of panik et al., 2003). Based on the results of these sample sizes were tested as well. The smaller samples tests, research is concentrating on the influence of were 35 mm in diameter and the larger samples had sample end conditions on sample strength (Sczcepa-diameters of 61 mm. All tests had a length to diame-nik et al., 2005). Test results have shown a relation-ter ratio between 2.0 and 2.5, which is within ASTM ship between the ratio of circumferential strain at the (1987) specifications. sample mid point and sample ends versus the sample UCS, and that varying sample to platen contact fric-tion can change the resulting sample UCS by up to 2.2 Sample Instrumentation
about 100%. This paper presents the results of con-All samples were strain gauged. Circumferential tinued tests in this area. Modifications have been strain gauges were mounted 10 mm from each sam-made to test procedures to try and vary sample end ple end and at the sample midpoint. friction as much as possible.
？ For the 35 mm diameter samples, both 14mm and effects between the sample ends and platens and did 90 mm long strain gauges were used. For the not conform to ASTM standards.
samples instrumented with 14mm long gauges, 2 Three platen conditions were also used for testing. gauges were installed at each location and 2 axial In all cases the steel platens had a hardness in excess strain gauges were also used. For the tests con-of 58 (Hardness Rockwell C) HRC, as specified in ducted with 90 mm long gauges, only 1 circumfe-the ASTM standards (1987). No effort was made rential gauge was used at each location. to match the elastic properties of the rock and pla-
tens. Instead, the contact friction between the sam-？ For the 61 mm diameter tests, 90 mm long strain ple and platens was varied. Polished and striated gauges were used with circumferential gauges 10 platens were used, all of which conformed to ASTM mm from each sample end and the sample mid-standards. The polished platen was prepared on a point. Two axial strain gauges were also used. thin section polishing wheel whereas the striated pla-Two gauges were used at each location to provide ten (Figure 2) was prepared on a fine grinding wheel. redundancy. A third platen type was used that consisted of
concentric grooves with a roughness in excess of 2.3 Surface Roughness Measurements ASTM standards. This platen is shown in Figure 3.
The variation in platen types was used to vary Both sample end conditions and platen conditions sample to platen contact friction. The three types of were varied and a method of quantifying the surface sample end finish and three platen types that were conditions of the sample ends and platens was used are summarized in Table 1, along with corres-adopted. A portable surface roughness tester (profi-ponding ranges in average roughness. The sample lometer) was used to measure sample end roughness tested using lead foil are also listed in this table. The to the nearest 0.01 μm. The roughness tester meas-thickness of these foils were 0.015 mm, 0.03 mm, or ured roughness along a 12.5 mm profile length. Av-in three cases, 1 mm. erage surface roughness, Ra, was recorded. Ra is calculated by first determining an average straight
profile to represent the surface trace. The areas
above and below the profile are calculated and added
together. This total area is divided by the straight line
profile length to determine the average profile
roughness parameter Ra. To account for any direc-
tional anisotropy in the roughness of the sample
ends, roughness was measured at 60? increments on
the sample surface to produce a “roughness rosette”.
An average roughness value for each sample end was
obtained by averaging the roughness rosette values.
Figure 1 shows how the Ra value is calculated.
jFigure 2. Striated platen showing scratch marks. Scale bar
shows 1 cm boimpacgnfe hdlkCentrelineRoughnessaverage (Ra)
Figure 1: Average roughness, Ra. (after Hebert,
2.4 Platen and Sample End Preparation
Three sample groups were prepared with rough,
standard and polished end conditions. All of these
sample end conditions were within ASTM standards
(ASTM, 1987). An additional suite of samples was
tested with lead foil placed between the platens and Figure 3. Rough platen showing concentric grooves. Scale bar the rock surface. This was done to reduce frictional shows 1 cm
Table 1. Platen types and sample end finishes used in labora-ends and mid-point allows the frictional effect to be tory UCS tests indirectly quantified. Figure 4 shows measured sam-ple UCS versus the sample circumferential strain ra-Platen type / Sample end finish Average Roughness
tio. Four groups of test results are shown. Tests con-？ Polished platens 0.17 μm<Ra<0.21 μm ducted with lead foil on the sample ends have been ？ Smooth / striated platens 0.8 μm<Ra<1.0 μm circled. The granite samples with the lower compres-？ Concentric grooved platens 4.0 μm <Ra<4.6 μm sional wave velocities were expected to be weaker ？ Rough sample end condition 3.8 μm <Ra< 4.3 μm and in general a slightly lower strength is obtained ？ Standard / smooth sample 2.4 μm <Ra< 3.0 μm for these samples at a given circumferential strain ra-end condition tio (Figure 4). ？ Polished sample end condition 0.6 μm <Ra< 1.2 μm Tests with a mid-point to end-point circumferen-tial strain ratio greater than 1.0 are deforming to a barrel shape, showing significant end friction. In this 3 TEST RESULTS circumferential strain ratio range, relatively high ASTM testing procedures were followed for sample sample strengths between 200 and 250 MPa were testing unless otherwise stated. The sample loading consistently measured. At circumferential strain ra-rate was kept constant at 8.1 MPa per minute, which tios less than 1.0 the samples developed an hourglass failed the samples in the ASTM recommended time shape, signifying reduced end friction. Sample period of 5 to 15 minutes for all but the strongest strengths in this circumferential strain ratio range test results. Load and strain values were conti-show a distinct trend of reduced strength with in-nuously recorded. creased hourglassing. Strengths range from just over After each test, the sample strains at 50% of the 200 MPa to under 100MPa. Of the 16 samples that sample UCS were assessed. The ratio of the circum-show some degree of hourglassing, 8 were tested ferential strain at the sample mid-point to the average with lead foil, 7 were tested with polished ends and circumferential strain at the sample ends was calcu-polished platens, and one was tested with polished lated. The smallest value for this ratio represents the platens and standard smooth ends. It is interesting sample end that experienced the greatest strain, to note that the smooth striated platens produced which is interpreted as the end with the minimum some of the highest sample strengths and the largest surface to platen confining friction. degree of sample barrelling. The striated platen was Table 2 summarizes the ratios of minimum mid-finished on a fine grained aluminum oxide polishing point strain to average end-point strain measured at wheel that left very shallow scratches on the platen 50% of the sample UCS. It is interesting to note that surface. The scratched surface produced a low Ra the polished samples tested on the striated platens roughness value of 0.8 to 1.0 μm, however, the show the highest circumferential strain ratio, suggest-scratched or striated surface produced an apparent ing that the contacts between polished samples and high friction between the samples and platen. striated platens experienced the highest friction. The failure mechanism, as well as strength, be-Rough samples tested on the grooved platens, how-tween hourglassing and barrelling samples appears to ever, show approximately 1-to-1 mid-point to end-be different (Figures 5 and 6). Figure 7 shows the point circumferential strain ratios, indicating signifi-location of the circumferential strain gauges. Figure 5 cantly less friction. It appears that the relatively shows a typical failed sample that deformed to a bar-rounded groove surfaces of the concentric grooved relled shape before failure. The sample ends were platens produce less rock-to-platen friction under relatively intact after failure and a shear type failure loading than the polished or striated platens, even developed in the sample. Figure 6 shows a typical though these have much lower roughness (Ra) val-failed sample that deformed to an hourglass shape ues. before failure. Axial fracturing developed in the sam-Additional tests were also conducted on the large ple and the UCS was significantly lower than for the samples with the lower compressional wave velocity. samples that deformed to a barrel shape prior to fail-Brazilian tests were conducted, according to ASTM ure. The three test results for the lower velocity sam-standards (ASTM, 1995), to obtain an estimate of ples with the greatest degree of hourglassing before the tensile strength of the rock. 8 samples were failure had circumferential strain ratios between 0.48 tested and gave an average tensile strength of 12.1 and 0.50 (Table 2) and an average UCS of 98 MPa MPa, with a standard deviation of 1.0 MPa. (Figure 6). It is interesting to note that the average unconfined compressive strength for these three samples is 8.1 times the tensile strength. This is 4 TEST INTERPRETATION remarkably close to the theoretical 8:1 ratio between UCS and tensile strength predicted with Griffith’s The frictional bond between the loading platens and crack theory (Griffith, 1924). sample ends is not well understood, however, the application of circumferential strain gauges at sample
Table 2. Minimum ratio of sample mid-point to end-point circumferential strain
Sample / platen description tial strain ratio strain ratio for tial strain ra-
for rough sam-smooth / stan-tio for po-
ple ends dard ends lished sample
61mm diameter samples:
Concentric grooved platens 1.113
Smooth striated platens 1.449
Polished platens 1.802
0.482 lead foil
0.502 lead foil
0.489 lead foil
35mm diameter samples:
Polished platens 1.065 0.799 0.743
1.12 0.838 0.862
1.063 0.845 0.505
0.637 lead foil*
0.818 lead foil
0.524 lead foil
1.552 lead foil
0.790 lead foil
* One sample end strain gauge failed for these tests – the remaining sample mid to end
circumferential strain ratio is shown
Sample BarrellingSample Hourglassing
Samples Tested with Lead Foil150
UCS (MPa) 100
High Velocity Small Samples
High Velocity Small Samples - 1 End Gauged50 Low Velocity Large Samples High Velocity Large Samples
Figure 4. Sample UCS versus the minimum mid-point to end-point circumferential strain ratio.
Figure 5. Post-failure photograph of a sample that showed relatively high strength with pronounced sample barrelling before fail-
Circumferential Strain Gauges
Figure 6. Post-failure photograph of a sample that showed rel-Figure 7. Strain gauged sample showing circumferen-
atively low strength with sample hourglassing before failure. tial gauge locations.
Testing has been conducted on relatively uniform medium grained granitic rocks. The samples tested were divided into two groups based on compression-al wave velocity, and the lower velocity samples ap-peared to have a slightly lower strength. Circumfe-rential strain gauges applied at centre of the sample and 10 mm from the each sample end provided an in-direct measure of the friction end confinement due to the contact between the platens and the rock. Sample hourglassing resulted in sample strength dropping to less than 50% of the maximum UCS values obtained. In half the cases of sample hourglassing, lead foil had been placed between the loading platens and the sample ends. Hawkes and Mellor (1970) reported similar testing procedures with the application of pa-per or Teflon between the loading platens and sam-ples. It was reported that thicker layers, in excess of 0.5 mm, extruded under loading and the failed sam-ples exhibited axial cleavage. It was thought that the axial cleavage was evidence that the material placed between the sample ends and platens was inducing a tensile stress at the samples ends and this approach was not recommended. The tests reported in this paper suggest that rock failure by axial cleavage can be obtained simply by reducing end friction during testing and that this can significantly influence the re-ported unconfined compressive strength of the rock. Additional testing is planned at the University of Saskatchewan to test other rock types and to better quantify sample end constraint and friction effects. REFERENCES
ASTM D2938-86. 1987. Standard test method for unconfined compressive strength of intact core specimens, ASTM, Philadelphia, Pennsylvania, USA.
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