Cechar Abrasivity Index CAI

8/9/2019 Cechar Abrasivity Index CAI

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The Pennsylvania State University

The Graduate School

Department of Energy and Mineral Engineering

STUDY OF CERCHAR ABRASIVITY INDEX AND POTENTIAL MODIFICATIONS

FOR MORE CONSISTENT MEASUREMENT OF ROCK ABRASION

A Thesis in

Energy and Mineral Engineering

by

Amirreza Ghasemi

2010 Amirreza Ghasemi

Submitted in Partial Fulfillment

of the Requirementsfor the Degree of

Master of Science

August 2010


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The thesis of Amirreza Ghasemi was reviewed and approved* by the following:

Jamal RostamiAssistant Professor of Energy and Mineral EngineeringThesis Advisor

Derek Elsworth

Professor of Energy and Mineral Engineering

Robert L. GraysonProfessor of Energy and Mineral EngineeringHead of the Graduate Program

*Signatures are on file in the Graduate School


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ABSTRACT

Tool wear is an important parameter in mechanized tunneling and is highly affected by

rock abrasiveness. There are numerous tests to identify the rock abrasivity. One of the widely

used rock abrasion tests is the Cerchar abrasion index (CAI). This test is used for estimation of bit

life and wear in various mining and tunneling applications. The test is simple and can be

considered for field applications. However, there are some discrepancies in the test results related

to the equipment used, surface condition of rock samples, operator skills, and procedures used in

measurement of worn out surface (wear flat).

This study discusses the background of the test, reviews the testing parameters and their

impact on testing as noted in previous studies, and examines the impact of the different

parameters on Cerchar testing. Seven rock types ranging from abrasive to non-abrasive were in

the testing program. Pins with different hardness were used on rough and sawn surfaces of the

selected rock samples. Geomechanical properties of these samples were also measured. Cerchar

values of different pins were compared and formulas offered by some researchers for conversion

of the CAI measurements between pins of various hardness were found to be satisfactory. It was

confirmed that the rough samples have higher Cerchar values as compared to sawn-cut samples.

Good correlation between the Cerchar value and the Compressive strength and equivalent quartz

content in rough samples were achieved. Tests were performed on three rock types with different

speeds and the results proved that the test speed does not change the results significantly.

By using various loads on the pin, it was concluded that the applied load linearly affects

the Cerchar value. Meanwhile to mitigate the issue of operator sensitivity and errors associated

with measurement of the worn out surface, a new method for measuring the tip loss, which was

already developed by NTNU, was used. This method uses the side view of the pin to measure the

tip loss. It was proved that this method can decrease the operator sensitivity of the measurements.


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A new device was introduced in an attempt to address some of the shortcomings of the

Cerchar test. The test involves use of a 90-degree cone pin on a sample that is placed on a lace.

The rotation of the lace allows for control of the length of the scratch, whereas the arrangement of

the tool allows for varying the amount of load placed on the tip. Some preliminary tests were

performed on seven rock types and initial results do not show a good correlation with Cerchar

measurements. Another set of tests were performed on sawn quartzite with varying applied loads

and test durations. It was concluded that applied load is linearly correlated with the tip loss. As in

soft rocks, the pin tends to penetrate into the rock. It is more reasonable not to apply higher loads

on the pin to prevent excessive penetration and possibly failure of the sample. The results also

proved that the wear mainly occurs in the first few rotations and therefore a long test duration is

not needed.


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TABLE OF CONTENTS

LIST OF FIGURES ................................................................................................................. vii

LIST OF TABLES ................................................................................................................... ix

Chapter 1 Introduction ............................................................................................................ 1

Problem Statement ........................................................................................................... 1 Objective of the Research ................................................................................................ 2

Methodology and Approach ............................................................................................. 2

Thesis Organization ......................................................................................................... 3

Chapter 2 Background ............................................................................................................ 4

Wear and rock abrasiveness ............................................................................................. 5 NTNU rock drillability testing system ............................................................................. 6

Drilling Rate Index, DRI .......................................................................................... 7 Assessment of DRI ................................................................................................... 8

Cutter Life Index, CLI .............................................................................................. 9

Calculation of CLI .................................................................................................... 10 LCPC test ......................................................................................................................... 11

Other methods .................................................................................................................. 13 Mineral Content methods ......................................................................................... 13

Burbank test ............................................................................................................. 14

Modified Taber Abraser ........................................................................................... 14

Dynamic Impact Abrasion Index test ....................................................................... 15

Modified Schmidt hammer test ................................................................................ 16

Chapter 3 Cerchar Abrasion Index (CAI) test ........................................................................ 18

Cerchar test parameters .................................................................................................... 20

Testing Equipment ................................................................................................... 20

Pin Hardness ............................................................................................................. 21

Length of the scratch ................................................................................................ 24 Test repetition ........................................................................................................... 27

Stress dependency .................................................................................................... 27

Surface condition of the specimen ........................................................................... 28

Petrographical and Geomechanical properties ......................................................... 29

Speed of testing and its impact on the results .......................................................... 31

Measuring apparatus and methods ........................................................................... 31 Other factors affecting CAI ...................................................................................... 33

Rock Abrasiveness Classes .............................................................................................. 34

Applicability of the test .................................................................................................... 34

Repeatability & Reproducibility ...................................................................................... 35

Chapter 4 Potential modification for Cerchar test or development of a new tests for rock

abrasivity measurement .................................................................................................... 37


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Sample selection and physical property testing ............................................................... 38 Comparative CAI testing between various laboratories ................................................... 40

Impact of Pin hardness ..................................................................................................... 41

Impact of Surface condition ............................................................................................. 45

Correlation of CAI with Petrographical and Geomechanical properties ......................... 46 Impacts of Pin Speed ........................................................................................................ 50 Impact of applied load ...................................................................................................... 52

Impact of measuring apparatus and procedure ................................................................. 54

Chapter 5 Study of alternative testing configuration for measurement of rock abrasion ....... 59

Development of a new testing concept ............................................................................ 59 Discussion of the test results and its practical implications ............................................. 70

Chapter 6 Conclusions and recommendations ........................................................................ 71

Conclusions ...................................................................................................................... 71

Recommendations ............................................................................................................ 74

References ................................................................................................................................ 76


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LIST OF FIGURES

Figure 1- Impact of abrasivity ratio on abrasion wear (after Deketh, 1995) ............................ 6

Figure 2- Schematic view of the brittleness test (after Bruland, 1998) ................................... 7

Figure 3- The Sievers' miniature drill test (after Bruland, 1998) ............................................. 8

Figure 4: Calculating DRI based on S20 and SJ value (after Bruland, 1998).......................... 9

Figure 5: Abrasion testing in the NTNU rock borability index tests (after Bruland, 1998) .... 10

Figure 6: The LCPC abrasion test equipment (after Nilsen et. al., 2007) ................................ 12

Figure 7: Schematic view of Burbank abrasion test (after Bond, 1963) .................................. 14

Figure 8: Schematic view of the Dynamic Impact Abrasion Index test (after Al-Ameenand Waller, 1992) ............................................................................................................. 16

Figure 9: Indenter at the top of the Schmidt hammer and its layout (after Janach andMerminod, 1982) ............................................................................................................. 17

Figure 10: Correlation between Cerchar and LCPC abrasivity indexes (after Mathier andGisiger, 2003)................................................................................................................... 19

Figure 11: Different Testing devices for Cerchar test (after Plinninger, et al., 2003 &

Rostami, et al., 2005) ....................................................................................................... 21

Figure 12: Impact of steel type with the same hardness on CAI (after Stanford and Hagan,2009) ................................................................................................................................ 23

Figure 13: Effect of various hardness of the same steel on CAI (after Stanford and Hagan,2009) ................................................................................................................................ 23

Figure 14: Tip loss measured at different scratching lengths (after Al-Ameen and Waller,

1994) ................................................................................................................................ 25

Figure 15: Effect of testing length on CAI (after Plinninger et al., 2003) ............................... 26

Figure 16: View of pin tip wear flat measured from the side view (and what could bemeasured from the top view)............................................................................................ 33

Figure 17: Comparison the CAI results of different laboratories ............................................ 42

Figure 18: Plot of Cerchar test results for HRC 54/56 vs. HRC41/43 pins with varioussample surface conditions (Top: Sawn Surface, Bottom: Rough Surface) ...................... 44


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Figure 19: Plot of calculated CAI using HRC 41/43 pins and equation 1 vs. measuredCAI for HRC54/56 pins ................................................................................................... 45

Figure 20: Comparison of CAI testing of rough vs. sawn surface ........................................... 46

Figure 21: Correlation between CAI on sawn surface using 41-43 HRC pins and UCS(left) and EQC (right) ....................................................................................................... 47

Figure 22: Correlation between CAI on rough surface using 41-43 HRC pins and UCS(left) and EQC (right) ....................................................................................................... 48

Figure 23: Correlation between CAI on sawn surface using 54-56 HRC pins and UCS(left) and EQC (right) ....................................................................................................... 48

Figure 24: Correlation between CAI on rough surface using 54-56 HRC pins and UCS(left) and EQC (right) ....................................................................................................... 48

Figure 25- Comparison between Actual and Predicted CAI .................................................... 50

Figure 26: CAI results with different pin speeds,(Top: Limestone, middle: Sandstone,Bottom: Quartzite) ........................................................................................................... 52

Figure 27: Correlation between CAI and applied load on stylus ............................................. 53

Figure 28: Variation of CAI measurements between operators ............................................... 58

Figure 29: Schematic view of the new equipment ................................................................... 61

Figure 30: views of the new equipment on the lathe machine ................................................. 61

Figure 31: Correlation between new tests results and CAI on seven rock types ..................... 63

Figure 32: Deep groove of the pin on sandstone ..................................................................... 64

Figure 33: Deep groove of the pin on limestone ...................................................................... 64

Figure 34: Deep groove of the pin on slate .............................................................................. 65

Figure 35: Deep groove of the pin on calcite ........................................................................... 65

Figure 36: Worn out pins after testing sandstone with different loads ................................... 66

Figure 37: Side view of worn out pins after testing sandstone with different loads ............... 67

Figure 38: Correlation between applied load, scratch length, and tip loss in new

equipment ......................................................................................................................... 69

Figure 39- Correlation between Scratch length/test duration and tip loss in newequipment ......................................................................................................................... 70


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LIST OF TABLES

Table 1- Mohs hardness scale .................................................................................................. 4

Table 2: Rock abrasiveness classification based on LCPC test ............................................... 12

Table 3: Classification of rock abrasivity based on the range of measured Cerchar Index

(after Stanford and Hagan, 2009, Michalakopoulos et. al, 2006, and Rostami et al.,2005) ................................................................................................................................ 34

Table 4: Result of Cerchar and other tests on seven selected rock types ................................. 39

Table 5: Results of Cerchar tests on the same samples in different labs ................................. 40

Table 6: Equivalent quartz content of seven rock types .......................................................... 47

Table 7: EQC and UCS of 4 added rock types ........................................................................ 49

Table 8: CAI test results for different pin speeds .................................................................... 51

Table 9: Results of different applied load on tip loss .............................................................. 53

Table 10: Two operators' measurements from the top and from the side (41/43 HRC) .......... 55

Table 11: Two operators' measurements from the top and from the side (53/54 HRC) .......... 56

Table 12: Results of the new equipment along with respected CAI value for seven rocktypes ................................................................................................................................. 62

Table 13: Results of new test on sawn -cut quartzite ............................................................... 68


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Chapter 1

Introduction

Correct estimate of the cost and rate of rock excavation in mining, tunneling, and

underground construction are of great importance for owners, clients, engineers, and contractors. In

mechanized excavation, the rate of production and related costs of the cutters, as well as the

frequency of the cutter change can be significantly affected by the time and cost of the project.

However, replacing worn out tools is a required step in the production cycle and the time needed to

stop the operation for changing the tools is the non-productive time which should be accounted for

and minimized if possible. This is because the decrease in the efficiency of the worn out tools will

impact the instantaneous production rate and increase the energy consumed in the cutting process,

which will adversely impact productivity. Thus tool replacement is necessary to keep the equipment

working under optimal operational conditions. As mechanized projects are expanding every day,

studies regarding the wear of cutting tools as well as other components of the machine that are

subject to secondary wear are getting more and more attention.

Problem Statement

The current systems and formulas for estimation of the wear on cutting tools rely on some

rock abrasivity measurement indices. One of the most widely used tests for rock abrasion is the

Cerchar Abrasivity Index. This test is commonly used in various applications including mining and

tunneling and in general rock excavation. However, there are some discrepancies in the test and

measurement system. These are mainly because there is no widely accepted standard for this test

and each lab/researcher performs tests based on their experience and preference. However, it should


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be noted that even if the tests were performed with the exact same procedure, there would be some

discrepancies based on the intrinsic shortcomings of the tests such as small scale or change in stress

conditions at the tip of the scratch pin throughout the tests. Hence, considering the widespread use

of this test, there is an urgent need for a robust standard on the test and studies regarding the other

available alternatives.

Objective of the Research

The objective is to look at the discrepancies and inherent flaws associated with the Cerchar

testing and try to find the effect of testing parameters on the final results by changing different

parameters and comparing the results from different laboratories. In addition an attempt was made

to develop means for modification of the test or alternative ways of making rock abrasion

measurement with more consistent results and less operator sensitivity.

Methodology and Approach

The approach used in this study is to perform relevant experiments on the Cerchar

Abrasivity Index to examine the previous studies and to observe the impact of various parameters

on the test results. Also, test parameters that have not been examined by other researchers will be

identified and examined. Cerchar tests will be performed on the exact same samples in different

labs as a way to verify the reason(s) for discrepancies. A new approach and setting will also be

developed to perform some initial trials towards development of a testing method that can address

some of the shortcomings of the Cerchar test and offer more consistent, repeatable results.


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Thesis Organization

In this study, rock abrasiveness as the key component for estimating wear in mechanized

tunneling is discussed. Available rock abrasivity tests are discussed in Chapter 2 which includes the

background information on this topic. The Cerchar test as one of the most widely used tests among

researchers, manufacturers, and engineers is focused in Chapter 3. In this chapter, various

parameters afecting results are discussed and discrepancies associated with the test among different

labs/researchers are covered. In chapter 4 different tests performed by the author to address some of

the discrepancies are presented. Chapter 5 introduces newly developed equipment and since the

proposed test is in early stages of design and needs more study, some preliminary tests have been

performed and the results are summarized in this chapter. Chapter 6 contains the conclusions and

recommendations for future studies on this subject.


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Chapter 2

Background

Rock abrasion is one of the characteristics of the rock that reflects the hardness of the

constituent minerals, as it pertains to tools used in rock excavation. One of the first methods to

address rock abrasion was based on a study by a German geologist, Friedrich Mohs. Mohs hardness

is typically used to express the hardness of minerals in a relative scale where the harder mineral

scratches the softer ones. The scratch represents the possibility of the softer minerals wearing or

deforming under the contact stresses with the harder objects. This scale is presented in Table 1 [1].

The numbers in table shows the relative hardness of each mineral and it does not have any

specific meaning or related to known and measureable physical properties in the minerals. The

minerals were selected based on their abundance in the nature and the corresponding numbers does

not reflect a direct proportion in their strength or hardness, meaning that they are arbitrary.

Therefore, while useful in showing the relative hardness of mineral constituent of rocks, the scale is

not accurate for quantifying the hardness of the minerals. Therefore, other methods to quantify the

hardness of minerals and rocks were needed. That was the reason for development of numerous

tests to quantify rock abrasivity. Some of these tests are more popular while many were developed

and used for specific applications. Among the rock abrasion testing methods, NTNU, LCPC, and

Table 1- Mohs hardness scale

M i n e r a l

T a l c

G y p s u m

C a l c i t e

F l u o r s p a r

A p a t i t e

O r t h o c l a s e

f e l d s p a r

Q u a r t z

T o p a z

C o r u n d u m

D i a m o n d

MohsHardness

1 2 3 4 5 6 7 8 9 10


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Cerchar Abrasivity tests have been used most frequently by the mining and civil construction

industries. These tests along with some others will be discussed in this chapter.

Wear and rock abrasiveness

The wear is in many ways similar to the effect of harder minerals on softer ones and is

easily represented by the scratch that the hard objects engrave in soft minerals. Plinninger et. al

(2002) depicted that “Abrasive wear” is the predominant wear process in most rock types. Abrasive

wear leads to the removal of material from the tool surfaces while it is moving against the rock.

This phenomenon is the function of hardness difference between interacting bodies. It is caused by

direct contact of tool and hard particles in the rock or contacts between tools and particles in

between rock and tool [2].

Deketh (1995) mentioned that according to the studies, when the ratio of abrasiveness of

two interacting materials exceeds 20% of their Vickers hardness, abrasive wear increases

dramatically. When the ratio is less, the abrasive wear is marginal. Figure 1 shows this phenomenon

[3].

Atkinson and Singh (1986) mentioned that various factors affect the rock abrasiveness and

they can be categorized as [4]:

Mineral composition

Hardness of mineral constituents

Grain shape and size

Type of matrix material

Physical properties of the rock including strength, hardness and toughness


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NTNU rock drillability testing system

This test was developed in 1960s at the Norwegian University of Science and Technology

(NTNU) to evaluate the drillability of percussion drilling. This test is also known as SINTEF

method. In recent years, it has been used in major international mechanized underground

construction projects, and is considered as one of the most recognized and widely used methods for

Tunnel Boring Machine (TBM) performance prediction [5]. NTNU/SINTEF method consists of a

set of laboratory tests and different indices which are described herein.

Figure 1- Impact of abrasivity ratio on abrasion wear (after Deketh, 1995)


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Drilling Rate Index, DRI

Drilling Rate Index (DRI) is evaluated based on two laboratory tests: the Brittleness test

(S20) and the Sieves’ J-miniature drill test (SJ) [5].

The Br it tleness Test

This test shows the resistance of the rock against repeated impact and crushing. It was first

developed in 1943 by N. von Maten and A. Hjeler in Sweden. Figure 2 shows the schematic view of

the test [6].

The sample comprises crushed rock with the grain size ranging between 16 and 11.2 mm

screens. The sample weight is an equivalent of 500 g for a 2.65 density rock adjusted with the

sample density. The Brittleness Value (S20) is the percentage of the rock material that passes the

11.2 mm mesh after 20 impacts of the 14 kg weight from a 25 cm height. This test should be

repeated 3-5 times and the mean should be presented in the report [6].

Figure 2- Schematic view of the brittleness test (after Bruland, 1998)


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The Sievers’ Mimiature Drill Test

This test was developed by H. Sievers in the 1950s and gives an evaluation of the surface

hardness of the rock. The test is done on a sawn sample. Figure 3 shows a schematic view of the

test. The Sievers’ J-value is the depth of the drilled hole after 200 rounds of the drill bit which is

measured in 1/10 of mm. This test should be repeated 4 to 8 times and the mean value should be

used as the final number [6].

Assessment of DRI

After measuring S20 and SJ-value in the testing equipment described, DRI can be obtained

using Figure 4. Bruland (1998) mentioned that “the Drilling Rate Index may be described as the

Brittleness Value corrected for the rock surface hardness” [6].

Figure 3- The Sievers' miniature drill test (after Bruland, 1998)


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Cutter Life Index, CLI

The Cutter Life Index is calculated based on the Sievers' J-value and the Abrasion Value of

Steel anvil or in short, AVS. The index can be used to estimate the lifetime of the TBM cutter discs

of in number of hours of machine excavating in the given rock type [6].

The Abrasion Value Steel (AVS)

In this test, rock powder in the size range of less than 1 mm is used to abrade the wear piece

made of steel from a new cutter ring. The wear piece is under 10 kg dead load to increase the

Figure 4: Calculating DRI based on S20 and SJ value (after Bruland, 1998)


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friction and contact pressure between rock grains and steel anvil. AVS is the weight loss of the wear

piece after 20 rounds (1 min) which is measured in milligrams. Figure 5 shows the abrasion test and

equipment [6].

Calculation of CLI

After identifying the AVS and SJ value, CLI can be calculated using Equation 1. This

formula is based on the real field data on actual cutter lifetime and related tested rock parameters

[6].

Equation 1

There is another index in the NTNU rock drillability testing which is called Bit Wear Index

(BWI). BWI is used to estimate the lifetime of drill bits and can be estimated using DRI along with

Figure 5: Abrasion testing in the NTNU rock borability index tests (after Bruland, 1998)


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Abrasion Value (AV). Abrasion Value (AV) test is exactly like AVS but lasts for 100 rounds (5

minutes) and tungsten carbide is used as the wear piece instead of steel [6].

As mentioned, laboratory results are usually correlated with the real projects and are

continuously updated. SINTEF benefits from nearly 3000 rock sample results [5]. However, it

should be noted that the test is relatively complex and time consuming. Also, the cost of each test is

high and the amount of sample needed for the test is quite a lot. There are only a few laboratories

which perform the tests, including the original developer of the test in Norway. The testing is very

peculiar and not much experience exists outside Norway to verify the validity of the test results and

consequently, the test is more or less exclusive to NTNU and their testing facility known as

“SINTEF”.

LCPC test

The abrasivity test of the Laboratoire des Ponts et Chaussées (LCPC) is another method for

measuring the abrasivity of the rocks and soils. This test is described in the French Standard

AFNOR P18-579. The device consists of a 750 W motor which rotates a steel impeller at the rate

of 4500 rpm for 5 minutes. The sample is 500±2 g of crushed rock with the size between 4 to 6.3

mm diameter [7]. The impeller is made of steel with a dimension of 50×25×5 mm and has to be

changed after each test. Also, it should be weighed before and after the test. Grain distribution of

the sample before and after the tests should be compared [8]. Figure 6 shows LCPC testing device

[9].


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The ratio of the steel plates’ weight loss to the tested material’s weight in grams per ton is

considered as an index for rock abrasion. This value varies between 0 and over 2000 depending on

rock abrasiveness [7].

ABR = (P0-P)/G0 Equation 2

P0 = weight of metal plate before test (g)

P = weight of metal plate after test (g)

G0 = weight of sample (t)

The abrasivity scale is given in the Table 2 [7]:

Figure 6: The LCPC abrasion test equipment (after Nilsen et. al., 2007)

Table 2: Rock abrasiveness classification based on LCPC test

ABR (g/t) Scale

0-500 Very small500-1000 Small

1000-1500 Average

1500-2000 High

>2000 Very high


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LCPC test did not gain much popularity among researchers, laboratories, and engineers,

due to the fact it does not simulate the wear process that occurs in mechanized excavation. The

speed of rotation is very high in comparison to the real cases, where as the contact stresses between

rock and the wear plate is not similar to those in field applications. Also impact has a significant

role in the wear process of this method while it is not a very important factor in the wear of

mechanized excavation machines, specifically, TBMs.

Other methods

Mineral Content methods

Geologists calculate rock abrasivity based on the abrasivity of the constituent minerals. In

this method, percentage of each mineral in the rock is calculated and multiplied by its respected

abrasivity based on different available scales [3]. Among them, Abrasive Mineral Content (AMC),

Equivalent Quartz Content (EQC), and Vickers Hardness Number for Rock (VHNR) are the most

common tests. AMC uses Mohs scratch hardness, while EQC uses Rosiwal grinding hardness and

VHNR benefits from Vickers indentation hardness (an indentation test in which the ratio of the

force to the area of the indentation is considered as an index for abrasivity of the material)[3]. In

this study, EQC will be used to evaluate the abrasiveness of the minerals in the rock samples. In

EQC method, constituent minerals of the rock will be identified either by microscopic or

macroscopic mineral evaluation methods. The Rosiwal hardness of each mineral is divided by

Rosiwal hardness of Quartz (120). This way quartz would be 100% and all other minerals hardness

will be compared to quartz. The Rosiwal hardness of the mineral will be corrected for the ratio of

each mineral in the rock sample (weighted average) and EQC of the rock is determined.


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Burbank test

It is one of the tests that measure the effect of rock abrasivity on metal parts of mining and

crushing machines. This test consists of a metal paddle of the tests alloy and a container which

carries the rock samples. Container rotates at 74 rev/min and the paddle rotates in the opposite

direction with the 632 rev/min inside it. Therefore rapid wear of the paddle occurs which is an index

of the abrasivity of the rock [10]. Figure 7 shows a schematic view of the test [11].

Modified Taber Abraser

Another test which is used to evaluate the rock abrasiveness is the modified Taber Abraser.

In this test, a 6 mm thick disc form an NX core should be used. The sample rotates 400 times under

the wheel which is under a 250 g load. Debris of the rock and the abrader wheel is removed buy a

vacuum to remove the rock that could get stuck in the abrader wheel grits. The abrader weight loss

Figure 7: Schematic view of Burbank abrasion test (after Bond, 1963)


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is considered as an index for the rock abrasiveness. Moreover, weight loss of the rock is a measure

for its abrasive resistance [10].

Dynamic Impact Abrasion Index test

Al-Amen and Waller (1992) introduced the Dynamic Impact Abrasion Index test (DIAI)

which was first developed by Cassapi. This test is used for simulating the abrasive wear occurs by

fine rock particles. The result is mainly useful for transportation of materials by conveyors,

especially at transfer points and chutes.

In this test, 1000 g of crushed rock is used. The rock is blown using condensed air into a

duct. In the duct, there are steel shims with the hardness of 600 in Vickers scale. The air flow is

controlled by a rotameter flow-meter control valve at 138 l/min. The shims will be abraded and the

weight loss is measured. The weight loss is compared with the weight loss of the shims when they

are facing a standard abrasive material made of artificial corundum. Equation 3 shows how DIAI is

calculated [12].

Equation 3

K is the density correction factor. Figure 8 illustrates a schematic view of the DIAI test

[12].


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Modified Schmidt hammer test

Besides the available laboratory tests on rock abrasiveness, there are some in-situ or field

tests for abrasion measurement as well. Janach and Merminod (1982) used an M-type Schmidt

hammer for this use. They modified the front of the hammer and put a hardened steel indenter at the

top. They put the indenter at 45° and mentioned that the edge can act in a similar way to a disc

cutter of a TBM. Figure 9 shows the indenter and its layout [13].

The roller has a hardness of 62 HRC and the impact energy for each blow is 30 J. Test

should be repeated 20 to 50 times. The indenter is weighed afterwards and the weight loss for the

imposed impact energy in mg/kJ is considered as an index for rock abrasivity. As they did not have

real data, they correlated their results with miniature disc cutters and achieved reasonable results

[13].

Figure 8: Schematic view of the Dynamic Impact Abrasion Index test (after Al-Ameen and Waller,

1992)


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Figure 9: Indenter at the top of the Schmidt hammer and its layout (after Janach and Merminod,

1982)


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Chapter 3

Cerchar Abrasion Index (CAI) test

The Cerchar scratch test is one of the most commonly used tests for laboratory

measurement of rock abrasivity [14]. Cerchar Abrasivity Index (CAI) was originally developed in

1970s by Cerchar Institute in France and some of the initial publications were released in early

1980s [15]. A formal description of the testing procedure is provided in the French standard NF P

94-430-1, which is the only formal standard on the test [16] .CAI test was initially used in the

French and British coal mining industries and was gradually adopted for application in tunneling

industry [17].

Different setups are available for Cerchar testing. In general, they all consist of a vice

holding the sample while a hardened steel stylus with a 90 degree cone tip is scratched over the rock

under constant load of 70N. The lever is used to move the pin across the sample to scratch the

surface and allow the conical tip of the pin to wear under the constant load for 10 mm. As such, the

Cerchar index can be categorized as a high stress abrasion test [17]. Tip loss is measured in 1/10 of

mm using the microscope and will be reported accordingly. Each 1/10 of mm is considered as one

unit. Pins should be re-sharpened after each test.

Mathier and Gisiger (2003) performed a study on Olivine and Theoliite basalts. They

combined their results with other researcher’s data and came up with a reasonable correlation

between Cerchar and LCPC values. They mentioned that, approximately one unit of Cerchar index

is equal to 300 g/t of LCPC index. Results are shown in Figure 10 [7].


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The test is appealing to machine manufacturers since it duplicated the interaction of rock

and tool at much the same stress levels as those experienced by tools when they are excavating rock

[15]. It is fast, easy to use and cheap and can also be used in the field. As can be expected, there are

different parameters that affect the test results. These parameters can be classified into three

separate categories:

the equipment, tools, and testing procedure

the rocks samples and condition of the test surface

Measurement procedure of the worn out surface (wear flat)

Unfortunately, despite common use of this test there seems to be a lack of understanding on

the impact of many of these parameters. The impact of these parameters on the test results will be

Figure 10: Correlation between Cerchar and LCPC abrasivity indexes (after Mathier and Gisiger,2003)


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reviewed based on the results obtained by other researchers and some tests performed at the

Geomechanic laboratory at Penn State.

Cerchar test parameters

Testing Equipment

There are at least three types of testing device which are in use today. One is the first

generation machine which was suggested by Laboratoire du Centre d’Etudes et Recherches des

Charbonnages (Cerchar) de France. The other apparatus designed by West and it is the widely used

system in commercial and laboratory application. This apparatus is named after him but being

marketed by a company in the UK (Ergotech). Another testing device is made by a local machine

shop in Colorado and has been used at Colorado School of Mines (CSM) as the first lab in North

America to perform this test. The CSM device is very similar to the first generation version of CAI

testing machine [15]. There are also other local designs for the tests but they are more or less

similar to the ones mentioned before. Different apparatuses are presented in Figure 11.

In the original setup (Cerchar apparatus) the moving speed of the pins is at a velocity of 10

mm/s [18]. In contrast, the testing velocity in West apparatus could be better controlled and thus is

slower, and there is less likelihood of pin jumping over the sample by rapid uncontrolled movement

of the lever.


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Pin Hardness

AFNOR which provides the only formal test description on the test specifies that the styli

must be made of steel heat treated to Rockwell hardness HRC 54 – 56 [16]. However, the steel

qualities used in different testing sets have been varied in a wider range [18].

A- Cerchar apparatus B- West apparatus C- CSM apparatus1+3-Vice 2-Hand lever 4-

Testing pin 5-Pin chuck 6-Weight

1-Sample vice 2-Hand crank

3-Vice sled 4-Testing pin 5-Pin guide 6-Weight

1+3-Vice 2-Handle 4-Testing pin

5-Pin chuck 6-Weight

Plinninger, et al., (2003) suggested the use of 115CrV4 tool steel which was hardened to 55

HRC. He also mentioned that special care should also be taken when re-sharpening used pins to

avoid changing pin hardness. High temperatures arising from sharpening too quickly can influence

the hardness of the pin tip [18].

Alber (2008), Suana and Peters (1982), and Yarali et al. (2008) used scratching pins with

the hardness of HRC 54-56 [15, 19, and 20]

West (1989) noted that as the steel mentioned by AFNOR was unavailable in Britain, an

alternative was used in his test program. In his setup, tools were made from EN24 steel which had

been heat treated to Rockwell Hardness C40. This value was chosen after heat treating EN24 steel

Figure 11: Different Testing devices for Cerchar test (after Plinninger, et al., 2003 & Rostami, et al.,

2005)


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tools to different hardness and testing them with a specimen of Granite until a result about the same

as reported for Cerchar tools was obtained [21].

Al-Ameen and Waller (1994) mentioned that the standard EN24 stylus specified for the

Cerchar test was found to be unsatisfactory for testing of low abrasivity rocks. They also mentioned

that doubling of the hardness from 300 Vickers hardness (Hv) to 600 or from 400 Vickers hardness

to 800 resulted in an increase in abrasive wear by a factor of 1.37 [14]. This simply means that the

increase in the measured Cerchar Index in the same rock sample is proportional to square root of

pin hardness. They also noted that as their rock samples were soft and also the hardness of ordinary

Cerchar pins were significantly greater than materials used for construction of mining equipment

(except cutting tools), they used a softer stylus, made from EN3 (mild steel, 225 Hv) for their tests

[14].

In another attempt, they checked the hardness of their EN 24 and EN3 pins for Vickers

hardness and came up with surprising results. 146 EN 24 pins were tested and the hardness were

ranging from 350 Hv to 800 Hv while other 26 EN3 pins were all had the hardness between 200 and

250 Hv. They concluded that the variation in the hardness might be the reason for the inconsistent

results they got from previous tests [14].

Stanford and Hagan (2009) conducted a study involving the testing of seven different metal

types, heat treated to the same hardness level and one steel type at nine different hardness levels

from HRC 15 to 60. They concluded that CAI does not appear to be significantly affected by

changes in steel type of the stylus as long as their hardness is the same. Figure 12 shows this trend.


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They also concluded that CAI decreases linearly as hardness of the styli increases.

Therefore, an accurate estimation of CAI as a function of styli hardness might be possible. Results

of the study suggest that it might be feasible to vary the hardness of the stylus according to the rock

being tested. For example, to use a lower hardness stylus when testing softer rocks and a higher

hardness stylus when testing harder rocks. Figure 13 shows the results [22].

Figure 12: Impact of steel type with the same hardness on CAI (after Stanford and Hagan, 2009)

Figure 13: Effect of various hardness of the same steel on CAI (after Stanford and Hagan, 2009)


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Rostami et al. (2005) compared the CAI values measured on the same set of samples by

different laboratory and concluded that the labs using a softer pin can measure and report CAI

values 40 – 90% higher than those using original Cerchar pin hardness of 56 HRC. Labs using 43

HRC pins show higher sensitivity to operator skills and applied procedures. They also mentioned

that it seems using pins of hardness 56 can limit the variation in test results [17].

Michalakopoulos et al. (2006) tested sixty-eight samples from six rock types with steel styli

of both HRC 55 and 40.They came up with a linear relationship between the results (Equation 4)

[16]. Like Rostami et al., they observed that the CAI values from softer steel were distributed over a

wider range. Meanwhile, they offered the following formula to estimate the Cerchar Index for pins

of 56 HRC from the tests performed by using pins with the hardness of 40 HRC:

Equation 4

Where CAI55 is the Cerchar Abrasivity index using 55 HRC pins while CAI40 denotes using

40 HRC pins. If one calculates CAI55 and CAI40 from the formula provided by Stanford and

Hagan (2009) and substituting them into the Equation 4, it seems that there is a 0.13 difference

which is not a lot and shows that they are more or less consistent.

Length of the scratch

According to the testing procedures outlined in the original CERCHAR document, the

scratching distance on the rock sample is defined to be 10 mm [18].

Al-Ameen and Waller (1994) performed some test using various lengths (1, 2,3,5,7 and 10

mm). They concluded that within initial horizontal movement of the stylus (≈ 1mm), the cone tip

tends to deform and shear off due to the dead load and the resistance to horizontal movement, and a

flat-ended tip is formed. This flat area formed at the beginning of the test is not dependant on the


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amount of abrasive material in the host rock. It is due entirely to deformation and shear failure at

the tip of the pins, and its magnitude should be related to the rock strength and the stylus material

[14].

They also observed that the Cerchar index related to a 1 mm sliding distance is about two

thirds of the final Cerchar index at a 10 mm sliding distance for most of rock types. Approximately

30% of the Cerchar index can be attributed to the abrasion effect which corresponds to the final

9mm of sliding distance. Figure 14 shows their results. It should be noted that the pins used were

EN3 with 225 Vickers hardness (in comparison to 610 Vickeres hardness of EN24).

They mentioned that the tip wear flat diameter generated during the remaining 9 mm of the

testing distance can be related to a combination of the abrasive mineral content and the bond

strength between the minerals in the rock (i.e. rock strength). With these observations, Al-Amin et

al. concluded that the Cerchar index is mainly influenced by the rock strength and partly by the

abrasive mineral hardness [14].

Figure 14: Tip loss measured at different scratching lengths (after Al-Ameen and Waller, 1994)


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Plinninger et al. (2003) performed series of tests on identical rock samples with differing

testing lengths which confirmed the observations of Al-Ameen and Waller. The results is illustrated

in Figure 15. Their observation asserted that about 70% of the pin wear occurs during the first

millimeter of the testing length, about 85% of the CAI is achieved after 2 mm, and only 15% of the

change in CAI are achieved on the last 8mm of the testing path [18].

These findings are of great importance. They clearly show that the CAI value is highly

influenced by rock strength and also the test is not representative of the 10 mm but 1-2 mm of the

rock (although, 10 mm is also very small and questionable).

The only positive impact of this finding is that deviations in the CAI coming from the

variation of scratch length will not be very significant when the variation in testing length is kept

between 10 ± 0.5mm [18].

Figure 15: Effect of testing length on CAI (after Plinninger et al., 2003)


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Test repetition

Cerchar considers 2 – 3 single tests (pins) as sufficient for fine-grained, homogeneous rock

samples and suggests five or more tests only for samples with grain sizes of more than 1 mm [18].

West (1989) suggested that a number of measurements should be made on a single specimen of

rock to give a reasonable mean value for the rock abrasiveness. He suggested that in practice, five

tests on each specimen have been found a satisfactory number [21]. Plinninger et al. (2003) and

Yarali et al. (2008) also used five pins in their tests [18,20]. Stanford and hagan (2009) used seven

pins for each test and they exclude highest and lowest outlier measurements from their calculation

[22].

Stress dependency

Alber (2008) studied the impact of in-situ stress on Cerchar test results. The assumption

was that if a pin such as Cerchar stylus used in the test was to scratch over the surface of a rock

block in real application (i.e. underground or in drilling), it would probably wear differently than

when scratching over the rock in the laboratory. The difference would show the dependence of CAI

results on in-situ stresses and stress conditions at rock surface.

He tested 12 samples of four different rock types pressurized (from 2.5 to 12 MPa in

varying steps) in Hoek’s cell. He concluded that each rock type and each rock sample responded

with a higher CAI value when placed under confining pressure. However, the lower the CAI value

under ambient condition (sandstone < greywacke < mica schist < granite) the more pronounced the

increase in CAI appears. He mentioned that it may be concluded that the CAI may be seen as a

function of the rock porosity. He correlated the porosity of the rocks with increase in CAI per 1MPa

confining pressure and came up with a linear relationship [15].


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This could also be linked to the impact of rock strength on Cerchar test, as discussed earlier,

since the apparent strength of rock may increase under confinement. More detailed modeling of the

medium and the inter-granular boding could be performed to verify this phenomenon.

Surface condition of the specimen

Surface preparation of the specimen is not discussed in original Cerchar specification. West

(1989) suggested that the upper surface of the rock should be level. For soft rocks a flat surface is

prepared with a file and for hard rocks, a flat surface is produced by slicing the specimen with a

diamond saw [21].

Suana and Peters (1982) noted that representative results are only obtained on horizontal or

slightly inclined or curved scratch planes. Otherwise, higher force is required to move the pin across

the sample surface and thus higher CAI results are recorded [19].

Al-Ameen and Waller (1994) stated that if the rock sample is strong, the Cerchar stylus

tends to slide on the smooth rock surface, giving minimum abrasion and hence a low index value.

However, in weak rocks, the stylus tends to indent the rock, and the surface finish of the rock has

little effect on the index value. They performed some of the tests on both polished rock surfaces and

on rough rock surfaces, but the results were almost the same. Therefore, they performed the rest of

their tests on rough rock surface [14]. It should be noted that the samples used in their test program

were mainly weak rocks.

Plinninger et al. (2003) performed their tests on both saw cut and freshly broken rock

surfaces. They concluded that in rocks with low CAI values, tests on rough and saw-cut surfaces

lead to more or less equal results. This confirms the observation by Al-Amin et.al. The CAI values

on harder rock samples are about 0.5 higher on rough surfaces than the sawn surfaces. Plinninger et

al. recommend use of diamond saw cut surfaces to investigate very inhomogeneous rock types


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(such as conglomerates, coarse grained granite or schistose rock types), where broken sample

surfaces may not be suitable for a direct test [18].

Rostami et al. (2005) performed some tests on both surface finishes and concluded that CAI

measurements made on rough rock surface is higher than those made on sawn rock surface. They

concluded that although it is not a clear cut trend, but it seems like the difference between rough

and sawn measurements increases as rock gets harder and more abrasive. This coincides with the

finding of Plinninger et al. and Al-Amin and Waller. They also suggested that in spite of difficulties

to obtain reproducible measurements on rough rock surfaces, yet it seem to be the best choice of test

conditions since it represents a better simulation rock cutting by a tool than a sawn surface [17].

Alber (2008) performed Cerchar tests on rough surfaces as he wanted to have similar

conditions to those in situ at the face of an underground excavation [15]. Stanford and Hagan

(2009) as well as Yarali, et al. (2008) performed their tests on saw cut samples [20, 22].

Petrographical and Geomechanical properties

West (1989) measured CAI on 31 rock samples with known quartz content, using X-ray

diffraction method. The samples ranged from mudstone through siltstones to medium-grained

sandstones. The strength of the samples was in the range of 24 to 92 MPa except for two samples

with compressive strength of 140 and 173 MPa.

He compared CAI value with quartz content and obtained a reasonable correlation with the

exception of the two high strength samples which had significantly higher abrasiveness although

their quartz contents were low. Omitting these two samples from correlation, West concluded that

for in a limited range of the rocks he tested, the abrasiveness of the rocks had a nearly linear

relationship with their quartz content [21].


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Speed of testing and its impact on the results

West (1986) mentioned that the test should be completed in about 1 minute [1]. Alber

(2008) and Yarali et al. (2008) performed their tests in 1 second with the speed of 10 mm/s for

stylus traveling over rock sample [15, 20]. Michalakopoulos et al. (2006) performed their tests with

the speed of 1mm/s, resulting in the scratching movement over 10 seconds [16].

Plinninger et al. (2004) wrote that although there is a great difference in testing velocities in

Cerchar apparatus and West apparatus, the values derived from both types of testing setups are

generally estimated to be equal. Nevertheless, experience has shown that testing velocity may have

a major influence on the testing results of the Cerchar apparatus. When the testing surface is

extremely rough or coarse grains force the needle to bounce, the wear flat may be deformed and

testing velocities should be reduced to some seconds/mm [23].

Rostami et al. (2005) mentioned that the results of their limited testing showed roughly

40% higher CAI values measured on 43HRC pins at slower rate of movement. They also measured

some differences between the test results of the labs using the same machine type and stylus

hardness. The differences were attributed to the speed of running the tests where the lab running the

test at higher speed, got lower CAI results [17].

Measuring apparatus and methods

Cerchar recommends a microscopic reading method of the pin wear flat diameter but does

not describe the procedure or the equipment in detail [14]. However, a correct determination of the

start and end points of the wear flat is crucial for the accuracy of the results [15].

West (1989) suggested using a microscope with magnification factor of ×24 fitted with a

micrometer graduated to 0.01 mm but readable to 0.001 mm. Two measurements across orthogonal


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diameters are made and the mean value is used to represent the test results on one pin. Al-Ameen et

al. (1994) used travelling microscope to measure the diameter of the abraded cone end of the stylus

to an accuracy of ± 2 μm [14]. Yarali et al. (2008) used a binocular microscope with reflected light,

25 times magnifications, and a measuring ocular with micrometer scale [20].

Plinninger et al. (2003) suggested the use of a reflected light microscope and evaluation of

the wear flat with 50× magnification and a measuring ocular. This has proven to be valuable when

testing inhomogeneous, coarse to very coarse grained rock types where the wear flat is too

asymmetrical for simple and proper reading of the wear flat diameter. In such cases, two

measurements should be carried out at a 90° angle to each other and a mean value should be used

for further interpretation [18].

Rostami et al. (2005) mentioned that the difficulties associated with viewing from the top

could be considerably reduced by use of new measuring technique, recently developed at SINTEF,

Norway. This system involves analyzing digital microscope photos of the pin and wear flat from the

side and has shown very good reproducibility and correlation between different operators can be

achieved. In this system, the correct angle of the tip is determined before the actual measurement is

performed. This provides for correct determination of the start and end points of the wear flat [17].

Figure 16 shows the side view of the pin tip under the microscope use for measurement of diameter

of wear flat using this technique.


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Other factors affecting CAI

Alber (2008) concluded that the Cerchar abrasivity index may be affected by the rock

porosity with inverse relation [15]. McFeat-Smith (1977) determined that rock abrasiveness

depends on the type and degree of cementation material. The abrasiveness of rocks having cement

degree higher than 50% are high and rocks abrasiveness having cement degree less than 50% are

low [19].

Plinninger et al. (2003) concluded that a product of Young’s Modulus and the Equivalent

Quartz Content of a rock sample was best suited to interpret the CAI by means of classical rock

mechanical parameters. They mentioned that fair correlation gives rise to the supposition that the

rock’s abrasiveness determined using the CERCHAR Scratch Test is mainly influenced by its

deformability and content of abrasive minerals [18].

Figure 16: View of pin tip wear flat measured from the side view (and what could be measuredfrom the top view)

Wrong CAI measurement

from the top

Correct CAI measurement

from the side

90°


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Rock Abrasiveness Classes

There are even some discrepancies in the classification of abrasiveness of various rock

types based on the range of the measured CAI test results. There are several available classification

systems as summarized in Table 3 [16, 17, and 22].

Applicability of the test

West (1989) mentioned that CAI testing have proved suitable for most rocks except for two

types. Some soft rocks that no detectable wear can be seen and hard rocks that the tool is unable to

cut a groove, and although the steel point is blunted, it has not interacted properly with the rock to

form a genuine abrasion wear flat. He also noted that care needs to be taken in choosing the test

positions when the rock is very coarse grained, contains veins or bands, or is porphyritic [21].

Table 3: Classification of rock abrasivity based on the range of measured Cerchar Index (afterStanford and Hagan, 2009, Michalakopoulos et. al, 2006, and Rostami et al., 2005)

CAIValue

Cerchar, 1986(pin hardness 54)

Michalakopoulos et.al

(pin hardness 55)

NTNUclassification

(pin hardness 43)

CSMclassification (pin

hardness 56)

0.3 - 0.5 Not very abrasive Very low

abrasiveness

Not very abrasive

Not very abrasive0.5 - 1.0 Slightly abrasive Low abrasiveness Slightly abrasive

1.0 - 2.0Medium

abrasiveness toMedium

abrasiveness

Mediumabrasiveness to Slightly abrasive

2.0 - 4.0 Very abrasive High abrasiveness Very abrasiveMedium

abrasiveness toabrasive

4.0 - 6.0Extremelyabrasive

Extremeabrasiveness

Extremely abrasive

4.0 – 5.0: Veryabrasive5.0 – 6.0:

Quartzitic

6.0 - 7.0 Quartzitic - Quartzitic -


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If the specimen is anisotropic, banded or markedly bedded, tests at different orientations

should be made. The small size of the portion of the rock tested is seen as the main shortcoming of

the test in such conditions [21].

Al-Ameen and Waller (1994) mentioned that the test was found to give consistent results

with fine-to medium-grained rocks; however, the results were unreliable for weakly consolidated

rocks and low abrasive rocks. Furthermore, the results obtained from coarsely crystalline rocks

were likely to represent the abrasiveness of individual minerals rather than the abrasiveness of the

whole rock. They also concluded that if the applied pressure generated by the Cerchar stylus on the

sample exceeds the rock strength, the Cerchar test is valid. Therefore, only a test which generates a

visible scratch for the whole length of the test can be considered valid [14].

Atkinson et al. (1986) experienced difficulties with weak rock materials as the pin tended to

penetrate deep into the rock. They mentioned that it changes the distribution of the applied load. In

such a case, the load is not concentrated only on the peak but also on the sides which leads to lower

abrasion than expected. They cautioned the users of Cerchar test when they are dealing with

unconsolidated weak rocks [4].

Plinninger et al. (2004) noted that simple model tests, like the Cerchar test have some

weaknesses that suggests even with more and better data sets, predicting the tool wear would be a

rough estimations [23].

Repeatability & Reproducibility

West (1989) examined repeatability of the test. He asked two technicians to perform the

tests on the same surface of the same specimen. He concluded that the two operators obtained

almost the same mean value for the abrasiveness [21].


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Chapter 4

Potential modification for Cerchar test or development of a new tests for rockabrasivity measurement

As can be seen, there are numerous discrepancies associated with the test which has to be

addressed to achieve repeatable and reliable results and to minimize the difference in values CAI

measured by various laboratories. As it stands the Cerchar test has become a more or less standard

tests of rock abrstivity for various applications such as use of Tunnel Boring Machines (TBM),

roadheaders, and in general in tunnel industry. As such, variation in test results could cause

differences in estimated cost of projects and many construction claims. Thus there is a need to

develop standard testing procedures for this test or alternatively, introduce a new test that can be

used and inherently has less variation and can produce more reliable/repeatable results. In General,

the discrepancies in test results can be classified into two major categories:

1. The issues related to the fact that there is no uniform and widely acceptable set of

standards for Cerchar test and as a result, each lab/researcher performs the test

according to their available equipment, tools, experience, and judgment which

makes the results of testing somewhat different from laboratory to laboratory.

2. Other problems associated with intrinsic shortcomings of the test such as scale of

the test (short distance), consistency of the pin material and preparation, surface

condition of the samples, speed of test, and impacts of minor variations in the stylus

hardness that could result in a shift in measurements.

The first category of issues can be solved by a comprehensive literature review and

performing a set of detailed Cerchar tests on different rock types using various tools and test

procedures to develop a coherent set of standards for equipment, pins, and procedures.. The second


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group of issues needs more study to meet the demand for modification of the test or developing a

new test. This involves development of a test that is simple, repeatable, less operator sensitive, and

possibly the equipment being portable.

Sample selection and physical property testing

In this study, a set of tests were performed on different rock types to evaluate the impacts

of various testing parameters. As part of this investigation, Cerchar test was performed on seven

rock types both on sawn and rough surfaces using two set of styli with different Rockwell hardness

(41-43 HRC and 54-56 HRC). Equipment used was similar to the one used by West which is

currently manufactured by ErgoTech Company of UK. The selected suit of rock samples were

subjected to a series of physical and geomechanical tests, along with hand held mineral content

evaluation. Geomechanical tests included Uniaxial Compressive Strength (UCS), Brazilian Tensile

Strength (BTS), Ultrasonic wave velocity, Young’s modulus, and Poisson’s ratio. These tests were

performed to allow for further correlation of the Cerchar test results with rock physical and

mechanical properties. This could provide an alternative for estimation of the CAI for cross

checking the results, or estimation of a CAI value where the other test results are available but

running CAI is, for any reason, not an option. Cerchar tests performed with five styli for each test.

For most geomechanical tests, three specimens were used to have the better representative value for

different parameters. Results of the tests are summarized in Table 4. For all Cerchar tests, five pins

were used and the average is presented.


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Table 4: Result of Cerchar and other tests on seven selected rock types

Limestone Slate Calcite Marble Sandstone Granite Quartzite

Cerchar- 41/43 HRC-

Sawn0.2 1.3 1.7 2.1 4.0 4.6 3.3

Cerchar- 41/43 HRC-

Rough0.3 1.3 1.5 2.1 3.7 5.6 8.1

Cerchar- 54/56 HRC-Sawn

0.1 0.7 1.0 1.0 2.4 4.7 2.7

Cerchar- 54/56 HRC-

Rough0.2 0.6 0.7 1.0 3.3 4.2 5.7

Dry Density 2.04 2.75 2.71 2.68 2.62 2.64 2.63

Porosity (%) 18.78 0.91 0.20 0.33 2.51 0.59 0.51

E (Gpa) 17.25 75.34 40.85 67.12 20.98 49.00 69.94

Poisson's ratio(ν) 0.19 0.17 0.22 0.25 0.25 0.18 0.09

UCS (MPa) 35.5 120.6 67.6 132.8 127.1 183.5 290.7

BTS (MPa) 3.4 ---- 5.1 11.1 8.6 7.8 17.9

P-Wave Velocity

(m/s)4174 7504 4249 4989 3528 5129 6856

S-Wave Velocity(m/s)

2139 3475 2804 3318 2586 2691 3753

Equivalent Quartz

Content (EQC, %)

3.75 1.72 3.75 3.75 54.48 58.58 100.00


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Based on the results of the CAI and geomechanical testing, an analysis was performed to

evaluate the effect of various testing parameter on the end results. Details of this analysis will be

discussed later in this chapter.

Comparative CAI testing between various laboratories

Other laboratories in US and other countries were also invited to participate in the research

by conducting Cerchar tests on the aforementioned 7 rock samples. The exact same samples which

were tested at PSU were sent to other labs. This way it is possible to calibrate the equipments in

different labs. However, it also takes a long time to have all the results as the samples should be

tested in one lab and then transferred to the other one. Because of practical implications, only three

of the laboratories have performed the tests so far. Results of these tests are presented in Table 5.

The samples are under testing in the 4th laboratory and are destined to travel to two more labs in the

US and several labs in other parts of the world.

Table 5: Results of Cerchar tests on the same samples in different labs

Laboratory CSM Univ. of TexasSub Terra,

Inc.PSU

Pin Hardness

(HRC)54-56 55-56 41 41-43 54-56

No. Rock Type Sawn Rough Sawn Rough Sawn Sawn Rough Sawn Rough

1 Slate 1.1 1.1 1.8 1.6 1.9 1.3 1.3 0.7 0.6

3 Calcite 0.8 0.9 2.6 3.0 2.3 1.7 1.5 1.0 0.7

3 Quartzite 3.6 5.8 4.7 5.5 4 3.3 7.1 2.7 5.7

4 Granite 4.2 4.4 4.9 4.8 4.3 4.6 5.6 4.7 4.2

5 Sandstone 3.7 4.2 4.3 4.1 4.5 4.0 3.7 2.4 3.3

6 Limestone 0.5 0.4 0.6 0.5 0.3 0.2 0.3 0.1 0.2

7 Marble 1.0 1.0 3.1 2.8 2.6 2.1 2.1 1.0 1.0


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As can be seen, the results of the Cerchar testing on exact same rock samples using the

same pin in different labs are not consistent. For more accurate analysis of the results and to make

any observation on the trends and to draw sensible conclusions, additional data is needed. However,

based on the test results so far, a graph is drawn and shown in Figure 17. The dashed line is 1 to 1

slope line.

In the following sections, results of some additional CAI tests which were performed in this

study to see the impact of some of the testing parameters are presented. An attempt was made to

categorize the test variables based on the disputed parameters mentioned earlier.

Impact of Pin hardness

As mentioned earlier, Michalakopoulos et al. (2006) tested sixty-eight samples from six

rock types with steel styli of both HRC 55 and 40.But did not mention the surface condition of their

specimen. They came up with a linear relationship (Equation 5) between the measured values of

Cerchar Abrasivity for pins of different hardness as follows [16].

Equation 5

Where CAI55 is the Cerchar Abrasivity Index using 55 HRC pins while CAI40 denotes

using 40 HRC pins.


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Figure 17: Comparison the CAI results of different laboratories

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6 7

O t h e r l a b

s

PSU- Sawn-54/56

Sawn 54/56 CSM

Sawn 55/56 UT

0

1

2

3

4

5

6

7

0 1 2 3 4 5 6 7

O t h e r l a b s

PSU- Rough-54/56

Rough 54/56 CSM

Rough 55/56 UT

0

1

2

3

4

5

6

0 1 2 3 4 5 6

S u b T e r r a - 4 1 H R C - S a w n

PSU- 41/43 HRC- Sawn


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To examine the validity of this equation, a series of tests were performed and the results are

shown in Figure 18. Note that, dashed line illustrates 1 to 1 slope. As anticipated, in both sawn and

rough specimen, CAI with pins 41/43 HRC are higher than CAI 54/56 (except for sawn Granite

which is almost the same value).

Linear relationship can be observed in the results, especially in rough samples. It appears

that CAI measurements in rough sample surfaces can be distinguished in two different groups; One

for less abrasive rocks and the other one for more abrasive rocks (perhaps the breaking point

between the abrasive and non-abrasive could be CAI of 3). The slopes of both lines are more or less

similar to the slope of Equation 5. Figure 19 shows the results of tests corrected for pin hardness

using Equation 5, to transfer CAI41/43 to CAI54/56 domain for the purpose of comparing the

impacts of the surface conditions.

It can be observed that for lower values of CAI, results show a better correlation with

Equation 5. It also seems like the slope of the more abrasive rough samples is the same as 1 to 1

line, meaning that the slope of the Equation 5 is reasonable and only the intercept is different.

Therefore, it might be possible to have two different equations with the same slope but different

intercepts for abrasive and non-abrasive rocks. It should be noted that in Michalakopoulos et al.

study, among 66 tests, only six of them had the value of CAI 55 greater than 3 with the maximum

value of 3.76 which shows that most rocks were not very abrasive. This could explain the reason for

larger differential between the values of CAI54-56 predicted by Equation 5 and that of measured

values in this study.


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Figure 18: Plot of Cerchar test results for HRC 54/56 vs. HRC41/43 pins with various samplesurface conditions (Top: Sawn Surface, Bottom: Rough Surface)

y = 0.9273x - 0.4642

R² = 0.8561

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

C A I ( 5 4 / 5 6 H R C )

CAI (41/43 HRC)

43 vs. 56 (Sawn Samples)

y = 0.4566x + 0.0254R² = 0.9925

y = 0.5414x + 1.2734

R² = 0.9972

0.01.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

C A I ( 5 4 / 5 6 H R C )

CAI (41/43 HRC)

43 vs. 56 (Rough Samples)

Less Abrasive Samples

Abrasive Samples


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Correlation between CAI and UCS and EQC are depicted in Figure 21 to Figure 24. As can be seen,

CAI values from rough samples have better correlation both with UCS than EQC. Therefore, an

attempt was made to find a relation between CAI and combination of UCS and EQC in CAI tests on

rough sample surfaces.

R² = 0.4431

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

0.0 1.0 2.0 3.0 4.0 5.0

U C S ( M P a )

CAI- 42 HRC- Sawn

R² = 0.5795

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.0 1.0 2.0 3.0 4.0 5.0

E Q C ( % )

CAI- 42 HRC- Sawn

Table 6: Equivalent quartz content of seven rock types

Constituent

Mineral

Percentage of

Mineral (%)

Mohs

hardness

Rosiwal

hardness

EquivalentQuartzContent

Limestone Calcite 100 3 2.3 2.3

Slate Clay 100 2.25 1.1 1.1

Calcite Calcite 100 3 2.3 2.3Marble Calcite 100 3 2.3 2.3

Sandstone

Quartz 40 7 104.3

45.2Glass

(AmorphousQuartz)

59 5.75 31.7

Mica 1 2.5 1.4

Granite

Plagioclase 50 6 40.3

23.3Quartz 43 7 104.3

Biotite &Muscovite

7 2.5 1.4

Quartzite Quartz 100 7 104.3 104.3

Figure 21: Correlation between CAI on sawn surface using 41-43 HRC pins and UCS (left) and

EQC (right)


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R² = 0.8992

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

0.0 2.0 4.0 6.0 8.0 10.0

U

C S ( M P a )

CAI- 42 HRC- Rough

R² = 0.9334

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.0 2.0 4.0 6.0 8.0 10.0

E Q C ( % )

CAI- 42 HRC- Rough

R² = 0.4314

0.0

50.0

100.0

150.0

200.0

250.0

300.0350.0

0.0 1.0 2.0 3.0 4.0 5.0

U C S ( M P a )

CAI- 54 HRC- Sawn

R² = 0.5685

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.0 1.0 2.0 3.0 4.0 5.0

E Q C ( % )

CAI- 54 HRC- Sawn

R² = 0.8007

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

0.0 2.0 4.0 6.0

U C S ( M P a )

CAI- 54 HRC- Rough

R² = 0.9741

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.0 2.0 4.0 6.0

E Q C ( % )

CAI- 54 HRC- Rough

Figure 22: Correlation between CAI on rough surface using 41-43 HRC pins and UCS (left) and

EQC (right)

Figure 23: Correlation between CAI on sawn surface using 54-56 HRC pins and UCS (left) andEQC (right)

Figure 24: Correlation between CAI on rough surface using 54-56 HRC pins and UCS (left) andEQC (right)


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using the screw feeder for movement of the pin against the rock surface (as in the Ergotech unit) it

seems like 10-20 seconds for the test duration (0.5-1 mm/sec) could be a reasonable rate since it is

more convenient for the operator. This allows for more consistency in testing between the operators.

Table 8: CAI test results for different pin speeds

Pin Hardness 54-56 HRC 41-43 HRC

Test Duration (s) 5 10 30 60 5 10 30 60

Sawn

Limestone 0.1 0.1 0.2 0.2 0.3 0.2 0.3 0.9

Sandstone 2.9 2.4 2.7 2.5 4.2 4.0 4.1 4.0

Quartzite 3.1 2.7 2.8 2.6 3.3 3.3 3.5 3.3

Rough

Limestone 0.2 0.2 0.1 0.1 0.3 0.3 0.4 0.4

Sandstone 2.7 3.3 2.5 2.7 3.7 3.7 3.2 3.3

Quartzite 5.2 5.7 5.1 5.2 6.8 8.1 8.6 7.5

0.0

0.10.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 20 40 60

C A I

Test Duration (sec)

Limestone

40/42 Rough

54/56 Rough

40/42 Sawn

54/56 Sawn


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Impact of applied load

A brief review of the available literature shows that there is no systematic study of the

impact of applied load on CAI value. As the first steps toward developing a new test, it was deemed

necessary to evaluate the effect of varying load and applied force on the pin, meaning varying

Figure 26: CAI results with different pin speeds,(Top: Limestone, middle: Sandstone, Bottom:

Quartzite)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 20 40 60

C A I

Test Duration (sec)

Sandstone

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0 20 40 60

C A I

Test Duration (sec)

Quartzite


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contact stress between the pin tip and the rock on the wear and deformation of the tip and

development of wear flat in general, and on CAI results in particular. Therefore, a set of tests was

performed on sample of quartzite with sawn surface. These tests were performed using two

different steel and varying applied loads. Results of these tests summarized in Table 9 and are

illustrated in Figure 27.

As can be seen, a linear relationship seems to exist between the applied load and tip loss.

This trend is true both steel types and hardness and means that the wear flat, especially in harder

Table 9: Results of different applied load on tip loss

54/56 HRC

Applied Load (N) 25.5 50.7 70.0 130.0 194.8 302.1 410.9 509.9 603.2

Sawn Quartzite CAI 2.1 2.4 2.8 3.3 3.8 4.8 5.4 6.5 7.3

40/43 HRC

Applied Load (N) 25.5 50.7 70.0 130.0 194.8Sawn Quartzite CAI 2.5 2.9 3.2 4.0 5.0

Figure 27: Correlation between CAI and applied load on stylus

y = 0.0087x + 2.047

R² = 0.9929

y = 0.0144x + 2.1442

R² = 0.997

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0

T i p L o s s ( 1 0 - 1 m m )

Applied load (N)

54/56 HRC

40/42 HRC


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54

rocks, is a direct function of contact stresses at the tip of the stylus. In other words, this clearly

explains the reason for observing the maximum removal of the tip at the first mm of scratch where

the contact stress is very high. The observed phenomenon of early development of the wear flat in

CAI testing can be simply attributed to yielding of the tip at the point of contact.

The results of this test can be used for the development of a new test of rock abrasion using

a similar concept of running a hardened steel pin or blade against rock to measure the abrasivity.

The practical implication of these test

Documents

Cechar Abrasivity Index CAI