International Journal of Rock Mechanics & Mi

ical

li hs i

osto

ining

GR-1

30 Ju

e 12 S

is the wear or loss of material which the rock produces

However, in spite of its popularity, there are some

(Cerchar) de France [12]. According to the original test

is shown in Fig. 1. The testing device mainly features a

where d is the wear at diameter of the stylus tip in mm.

ARTICLE IN PRESS

Corresponding author. Tel.: +30210 772 2208;

From the above description, the Cerchar test can be

classied as an abrasive wear with pressure test [15].

1365-1609/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijrmms.2005.06.009

fax: +30210 772 2191.

E-mail address: thmich@metal.ntua.gr (T.N. Michalakopoulos).quartz content, and the microstructure of the material.Abrasiveness is considered to express a behavioralcharacteristic of the rock rather than a fundamentalphysical or mechanical property. As a consequence, itis difcult to implement a testing procedure with astrong, unambiguous physical basis and a large numberof testing procedures have been proposed.A number of different rock abrasiveness tests have

been reported [1,57]. Of these tests the Cerchar test hasgained popularity as a relatively convenient andinexpensive method of measuring and comparing the

vice, 1, holding the rock sample, and a steel stylus, 4,tting into a holder, 5. On the stylus a dead weight of70N is applied, 6. The rock sample vice can be movedunder the steel stylus through a handwheel, 2, that drivesa screwthread of pitch 1mm/rev.To determine the CAI value the rock is slowly displaced

by 10mm with a velocity of approximately 1mm/s. Theabrasiveness of the rock is then obtained by measuring theresulting wear at on the tip of the steel stylus. The CAIvalue is calculated using the following formula [14]:

CAI 102d, (1)insert. [y] It is a function of mineralogy, especially the

on another material such as a TBM cutter or drill bit specication, West [13] prepared a testing device, whichTechn

The inuence of steel styabrasivenes

T.N. Michalakopoulos, V.G. Anagn

Laboratory of Excavation Engineering, Department of M

Zographou Campus,

Accepted

Available onlin

1. Introduction

During the last decades mechanical rock excavationhas gained a signicant share in the construction of bothcivil and mining engineering projects. The machinesused for mechanical rock excavation are in generalcapital intensive and, in most cases, site specic, so thata good understanding of the application conditionsplays a vital role in maximizing machine performance. Anumber of models have been developed during the lastyears to evaluate machine performance under variousapplication conditions [13]. In all such models rockabrasiveness is an intact rock property used as a majorinput parameter [1,4].According to Atkinson [5] the abrasiveness of a rockambiguities in the description of the test setup in thevarious references, especially regarding the hardness ofthe steel styli used. In this paper the inuence of the steelstyli hardness on the CAI value is examined.

2. Testing procedure

The Cerchar test has been proposed by the Laboratoiredu Centre dEtudes et Recherches des Charbonnagesning Sciences 43 (2006) 321327

note

ardness on the Cercharndex value

u, M.E. Bassanou, G.N. Panagiotou

Engineering, National Technical University of Athens,

5780 Athens, Greece

ne 2005

eptember 2005

abrasiveness of rocks [15,811]. The resulting CercharAbrasiveness Index (CAI) value is used as an inputparameter by many of the above-mentioned models forestimating mechanical excavators performance.

www.elsevier.com/locate/ijrmms

A formal description of the testing procedure isprovided in the French standard NF P 94-430-1 [14],which, to the authors knowledge, is the only formalstandard.Al-Ameen and Waller [16] and Plinninger et al. [17]

provide signicant details about the testing conditionsand their inuence on the CAI value.

3. Steel styli specications

According to the French standard NF P 94-430-1 thetip of the steel stylus applied to the rock sample isformed as a sharp conical point of cone angle equal to901731 and 1mm length. The overall length of the steelstylus should be greater than 15mm.The recommended steel styli geometry is generally the

same in the various references. However, this does notapply regarding the hardness of the steel styli. It is

obvious that any change in the hardness of the steelstyli alters signicantly the testing conditions, thusproviding incomparable CAI values. This has beendemonstrated by Al-Ameen and Waller [16], whenthey modied the test specications, in order to enablethe determination of the CAI value of certain coalsamples.In the original test setup description [12], Valantin

recommended styli made from steel of 2000MPa tensilestrength. No recommendation was made for the stylihardness.Suana and Peters [18] recommend steel styli of

ARTICLE IN PRESS

Reference Tensile strength (MPa)

T.N. Michalakopoulos et al. / International Journal of Rock Mechanics & Mining Sciences 43 (2006) 321327322Valantin [12] 2000

Suana and Peters [18] 2000

Atkinson et al. [8,9]

West [13] 2000

Atkinson [5] 2000

Al-Ameen and Waller [16]

AFNOR [14]

Plinninger et al. [17] 2000Fig. 1. The West testing device for determining the Cerchar

abrasiveness index (reproduced from [13]).

Table 1

Recommended steel styli specicationsRockwell Hardness HRC 5456 and 2000MPa tensilestrength.Atkinson et al. [8,9] recommend styli made of EN24

or EN25 steel, heat treated to 610 Vickers Hardness(HRC 56).West [13] also recommends styli made of EN24 steel

with a tensile strength of 2000MPa, but heat treated toHRC 40.Atkinson [5] does not provide any recommendation

on steel type, but recommends steel of 2000MPa tensilestrength and HRC 5456.Al-Ameen and Waller [16] follow the recommenda-

tion of Atkinson et al. with styli made of EN24steel, heat treated to 610 Vickers Hardness, butalso provide a recommendation for modied styli madeof EN3 steel heat treated to 225 Vickers Hardness(HRC 19).AFNOR [14], which provides the only formal test

description, species that the styli must be made of steelheat treated to HRC 5456.Plinninger et al. [17] suggest the use of 115CrV4 steel

hardened to HRC 55 with a tensile strength of about2000MPa.The various recommendations can be seen in Table 1.From the authors experience, although it is clear

from the above that there is a general consensus for stylimade from steel heat treated to HRC 5456, severalequipment manufacturers, planners and contractorsrequest CAI values determined with steel styli of HRC40, as is recommended by West.

Hardness Steel type

HRC 5456

HRC 56 (HV 610) EN24 or EN25

HRC 40 EN24

HRC 5456

HRC 56 (HV 610) EN24

HRC 19 (HV 225) EN3

HRC 5456

HRC 55 115CrV4

4. Test setup and results

The objective of this study has been to investigate theinuence of steel styli hardness on the CAI valueobtained. To accomplish this objective the CAI valueof 73 rock samples has been determined for twodifferent steel styli types. Of the 73 samples, 5 wereeither extremely fragmented/weathered or too hard totest, thus results for a total of 68 samples were obtained.The samples were of 6 different rock types, which can beseen in Table 2.Each sample has been subjected to 5 individual tests

with each steel styli type, and the mean CAI value andstandard deviation of the mean corresponding to eachstyli type have been computed.The specications of the utilized steel types can be

seen in Table 3. Styli of hardness HRC 55 were usedbecause this is the hardness specied in the formal testdescription, and of HRC 40 because this is therecommendation by West, which is more frequentlyused in routine Cerchar testing.The CAI values obtained can be seen in Table 4,

where CAI55 stands for CAI values obtained with steelstyli of HRC 55, and respectively CAI40 for HRC 40. InTable 5 the absolute frequency of the obtained CAI

used. Moreover, it is observed that the CAI40 values aredistributed over a wider range.

Finally, Eq. (2) has been used to relate the CAI55 andCAI40 value classication shown in Table 8.

ARTICLE IN PRESST.N. Michalakopoulos et al. / International Journal of Rocvalues for each steel styli type can be seen. Fig. 2 showsthe corresponding bar chart.As expected, the CAI40 value was greater than the

CAI55 value for all rock samples, demonstrating the

Table 2

Sample description

Sample type Sample nos.

(i) Gabbro 13

(ii) Pyroxenite-Gabbro 4

(iii) Mylonite 516

(iv) Recrystalized massive limestone 1724

(v) Serpentinized peridotite 2552

(vi) Limestone 5368

Table 3

Utilized steel styli nominal specications [19,20]

CAI55 CAI40

Steel type CALMAX EN24 (BS970-817M40)

HRC 55 40

C % 0.60 0.40

Ni % 1.80

Cr % 4.50 0.80

Mn % 0.80 0.70

Mo % 0.50 0.25

Si % 0.35

V % 0.20 6. Conclusions

The Cerchar test has gained popularity as a relativelyconvenient and inexpensive method of measuring and5. Regression analysis

In Fig. 3 the plot of CAI40 versus CAI55 values ispresented. The relation of the two CAI values has beenexamined and a linear correlation was observed, yieldingthe following equation:

CAI55 0:110914 0:587356CAI40. (2)The predicted CAI55 values and the corresponding

residuals for each rock sample are given in Table 4. Thepredicted CAI55 values are plotted versus the observedones in Fig. 4, indicating that no signicant anomaliesare present.The regression analysis is given in Table 6, and the

analysis of variance in Table 7. Since the p-value inTable 7 is less than 0.01, there is a statistically signicantrelationship between CAI55 and CAI40 at the 99%condence level.The R2 statistic indicates that the model as tted

explains 73.9251% of the variability in CAI55. Thecorrelation coefcient equals 0.859797, indicating amoderately strong relationship between CAI55 andCAI40. The standard error of the estimate shows thestandard deviation of the residuals to be 0.393807. Thisvalue has been used to construct the 95% prediction andcondence bands presented in Fig. 3.In Fig. 5 the studentized deleted residuals are plotted

versus the predicted CAI55 values. The randompattern indicates that the selected model describesadequately the observed data. Moreover, the linearityand homoscedasticity of the residuals are veried. Theabsolute studentized deleted residuals of 6 observationsare greater than 2.00 and are identied as potentialoutliers.In Fig. 6 the quantile plot of the standardized

residuals empirical distribution and the standardnormal distribution can be seen. This plot veries therequired regression assumption of normality.signicant impact of steel styli hardness on the obtainedCAI value. The reasonable general conclusion of thisinvestigation is that the CAI value of a rock sample isdecreased by an increase of the hardness of steel styli

k Mechanics & Mining Sciences 43 (2006) 321327 323comparing the abrasiveness of rocks. However, there are

ARTICLE IN PRESS

Table 4

Analysis of the observed CAI values and the linear model results

Sample no. Observed CAI40 Observed CAI55 Predicted CAI55 Observed residuals Standardized residuals Studentized

deleted residuals

Mean Std deviation

of the mean

Mean Std deviation

of the mean

1 6.35 0.68 3.58 0.28 3.840620 0.260623 0.666800 0.7189372 4.55 0.28 3.66 0.30 2.783380 0.876618 2.242813 2.359460

3 2.49 0.28 2.13 0.57 1.573430 0.556570 1.423975 1.436480

4 4.27 0.39 3.05 0.15 2.618920 0.431077 1.102904 1.118210

5 1.78 0.53 1.25 0.29 1.156410 0.093593 0.239456 0.239430

6 2.85 0.49 2.06 0.19 1.784880 0.275122 0.703895 0.701103

7 0.64 0.12 0.64 0.06 0.486822 0.153178 0.391903 0.401577

8 3.08 0.49 1.73 0.28 1.919970 0.189969 0.486033 0.4832809 1.35 0.38 1.27 0.17 0.903844 0.366156 0.936804 0.949292

10 1.35 0.19 1.45 0.11 0.903844 0.546156 1.397331 1.428140

11 2.87 0.73 1.32 0.29 1.796620 0.476625 1.219437 1.22389012 2.06 0.61 2.44 0.49 1.320870 1.119130 2.863276 3.049710

13 3.51 0.13 2.67 0.08 2.172530 0.497468 1.272764 1.282000

14 3.35 0.85 2.62 0.27 2.078560 0.541445 1.385278 1.397030

15 2.08 0.61 1.30 0.19 1.332610 0.032614 0.083442 0.08311516 5.28 0.29 3.27 0.29 3.212150 0.057848 0.148003 0.152330

17 2.87 0.14 1.75 0.21 1.796620 0.046625 0.119289 0.11838118 2.54 0.21 1.78 0.16 1.602800 0.177203 0.453371 0.450868

19 2.74 0.14 1.91 0.13 1.720270 0.189731 0.485424 0.482590

20 3.63 0.29 2.61 0.24 2.243020 0.366985 0.938925 0.941311

21 4.29 0.28 1.57 0.15 2.630670 1.060670 2.713707 2.89686022 2.64 0.25 1.52 0.25 1.661530 0.141533 0.362110 0.35978723 2.64 0.22 1.70 0.16 1.661530 0.038467 0.098417 0.097696

24 2.74 0.10 1.62 0.22 1.720270 0.100269 0.256537 0.25470825 1.83 0.25 1.45 0.12 1.185770 0.264225 0.676015 0.677586

26 1.04 0.17 1.04 0.05 0.721764 0.318236 0.814202 0.829071

27 2.28 0.46 1.45 0.20 1.450080 0.000085 0.000217 0.00021628 1.58 0.35 1.22 0.08 1.038940 0.181064 0.463249 0.465120

29 1.55 0.31 1.20 0.10 1.021320 0.178685 0.457163 0.459210

30 1.73 0.54 1.37 0.18 1.127040 0.242961 0.621612 0.623550

31 5.67 0.33 3.51 0.22 3.441220 0.068779 0.175971 0.183568

32 4.67 0.20 3.76 0.24 2.853870 0.906135 2.318332 2.453100

33 5.05 0.09 3.61 0.34 3.077060 0.532940 1.363518 1.414910

34 3.91 0.37 2.16 0.32 2.407470 0.247475 0.633161 0.63438435 4.17 0.67 2.69 0.28 2.560190 0.129813 0.332124 0.333268

36 2.92 0.64 1.75 0.24 1.825990 0.075993 0.194426 0.19298437 2.64 0.16 0.79 0.17 1.661530 0.871533 2.229803 2.30173038 1.93 0.27 1.25 0.23 1.244510 0.005490 0.014045 0.014011

39 1.70 0.45 0.97 0.24 1.109420 0.139419 0.356701 0.35724940 2.08 0.29 1.12 0.33 1.332610 0.212614 0.543969 0.54303441 1.93 0.28 0.74 0.12 1.244510 0.504510 1.290781 1.30442042 2.90 0.40 1.30 0.15 1.814250 0.514245 1.315687 1.32301043 3.35 0.47 1.83 0.25 2.078560 0.248555 0.635924 0.63385444 1.40 0.31 0.69 0.13 0.933212 0.243212 0.622254 0.62753545 2.13 0.37 1.37 0.12 1.361980 0.008018 0.020515 0.020424

46 2.26 0.34 1.37 0.36 1.438340 0.068338 0.174841 0.17392147 1.42 0.39 0.64 0.08 0.944959 0.304959 0.780233 0.78793548 3.00 0.53 1.83 0.23 1.872980 0.042981 0.109966 0.10913949 1.52 0.07 0.79 0.17 1.003690 0.213695 0.546735 0.54983950 3.33 0.22 1.57 0.20 2.066810 0.496808 1.271075 1.27868051 2.36 0.14 1.19 0.13 1.497070 0.307073 0.785641 0.78448652 2.67 0.34 1.37 0.26 1.679150 0.309154 0.790965 0.78879653 3.48 0.21 1.32 0.14 2.154910 0.834912 2.136109 2.20226054 3.23 0.14 2.06 0.09 2.008070 0.051927 0.132855 0.131952

55 2.97 0.13 1.81 0.13 1.855360 0.045360 0.116054 0.11517756 3.71 0.10 2.03 0.09 2.290000 0.260004 0.665216 0.66517557 3.38 0.14 1.83 0.10 2.096180 0.266176 0.681007 0.67922158 2.87 0.10 1.93 0.22 1.796620 0.133375 0.341238 0.338905

T.N. Michalakopoulos et al. / International Journal of Rock Mechanics & Mining Sciences 43 (2006) 321327324

ARTICLE IN PRESS

ed CA

f RocTable 4 (continued )

Sample no. Observed CAI40 Observed CAI55 Predict

Mean Std deviation

of the mean

Mean Std deviation

of the mean

T.N. Michalakopoulos et al. / International Journal osome ambiguities regarding the recommended steel stylihardness.In this paper the inuence of steel styli hardness on

the CAI value has been investigated. Sixty-eight samplesof various rock types have been tested with steel styli ofboth HRC 55 and 40. The relation of the two CAI

59 2.92 0.11 1.42 0.18 1.825990

60 3.15 0.16 1.91 0.21 1.961080

61 2.57 0.11 1.58 0.09 1.620420

62 2.85 0.17 1.73 0.15 1.784880

63 2.89 0.21 1.86 0.10 1.808370

64 3.12 0.15 1.86 0.15 1.943460

65 3.13 0.12 2.39 0.18 1.949340

66 3.20 0.26 1.91 0.18 1.990450

67 1.93 0.36 0.91 0.07 1.244510

68 4.67 0.11 2.62 0.20 2.853870

Table 5

Absolute frequency of the observed CAI values

Value range CAI55 CAI40

0.300.50 0 0

0.501.00 8 1

1.002.00 41 15

2.004.00 19 42

4.006.00 0 9

46.00 0 1

0

10

20

30

40

50

0.3-0.5 0.5-1.0 1.0-2.0 2.0-4.0 4.0-6.0 >6.0CAI value

Abs

olut

e fre

quen

cy

CAI40CAI55

Fig. 2. Distribution of observed CAI values.I55 Observed residuals Standardized residuals Studentized

deleted residuals

k Mechanics & Mining Sciences 43 (2006) 321327 325values has been examined and a linear correlation wasobserved. The yielded equation can be used to convertCAI40 values to CAI55, which are required by the formaldescription of the test.

0.405993 1.038727 1.0392600.051084 0.130698 0.1297690.040418 0.103409 0.1026740.054878 0.140403 0.1393410.051628 0.132090 0.131088

0.083464 0.213541 0.2120470.440663 1.127429 1.130130

0.080452 0.205835 0.2044490.334510 0.855838 0.8586100.233865 0.598340 0.607421

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00Predicted CAI55

Obs

erve

d CA

I 55

Fig. 4. Plot of predicted versus observed CAI55 values.

0.00

1.00

2.00

3.00

4.00

5.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

CAI40

CAI 5

5

Fig. 3. Plot of CAI40 versus CAI55 values.

ARTICLE IN PRESS

Table 6

Regression analysis

Linear model: y a bxDependent variable (y): CAI55Independent variable (x): Observed CAI40Data sets: 68

Parameter Estimate Standard error t statistic p value

Intercept (a) 0.110914 0.132092 0.839671 0.4041

Slope (b) 0.587356 0.042938 13.6791 0.0000

Table 7

Analysis of variance

Source Sum of

squares

Degrees of

freedom

Mean

square

F ratio p value

Model 29.0188 1 29.0188 187.12 0.0000

Residual 10.2355 66 0.155084

Total 39.2543 67

Correlation coefcient 0.859797R2 73:9251%Standard error of estimate 0.393807

-4.00

-2.00

0.00

2.00

4.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00Predicted CAI55

Stud

entiz

ed d

elet

ed re

sidua

ls

Fig. 5. Plot of predicted CAI55 versus studentized deleted residuals

values.

0.00

0.25

0.50

0.75

1.00

-3.00 -2.00 -1.00 0.00 1.00 2.00 3.00Standardized residual value

Cum

ulat

ive

prob

abili

ty

Standardized residualsStandard normal distribution

Fig. 6. Standardized residuals normality test.

T.N. Michalakopoulos et al. / International Journal of Rock Mechanics & Mining Sciences 43 (2006) 321327326References

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Table 8

Cerchar abrasiveness index (CAI) classication

Classication CAI55 CAI40a

Very low abrasiveness 0.300.50 0.320.66

Low abrasiveness 0.501.00 0.661.51

Medium abrasiveness 1.002.00 1.513.22

High abrasiveness 2.004.00 3.226.62

Extreme abrasiveness 4.006.00 6.6210.03

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de Information Techniques de creusement, Luxembourg 1973:

13740.

[13] West G. Rock abrasiveness testing for tunneling. Int J Rock Mech

Min Sci Geomech Abstr 1989;26:15160.

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1: Essai de rayure avec une pointe (NF P 94-430-1). Paris:

AFNOR; 2000.

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1978;15:8997.

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[18] Suana M, Peters Tj. The Cerchar Abrasivity Index and its relation

to rock mineralogy and petrography. Rock Mech 1982;15:17.

[19] Uddeholm. CALMAXPlastic mould and cold work steel.

http://utab.uddeholm.com/b_97.php (accessed 27 February,

2005).

[20] NAMTEC. Low alloy steel, AISI 4340 (annealed). http://

www.namtec.co.uk/metals/samples/ferrous/sample1.html (accessed

27 February, 2005).

ARTICLE IN PRESST.N. Michalakopoulos et al. / International Journal of Rock Mechanics & Mining Sciences 43 (2006) 321327 327

The influence of steel styli hardness on the Cerchar abrasiveness index valueIntroductionTesting procedureSteel styli specificationsTest setup and resultsRegression analysisConclusionsReferences