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 Requirements for the Degree of

Master of Science

August 2010

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

Jamal Rostami Assistant Professor of Energy and Mineral Engineering Thesis Advisor

Derek Elsworth Professor of Energy and Mineral Engineering

Robert L. Grayson Professor of Energy and Mineral Engineering Head of the Graduate Program

*Signatures are on file in the Graduate School

iii 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.

iv 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.

v 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

vi 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

vii

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-Ameen and Waller, 1992) ...... 16

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

Figure 10: Correlation between Cerchar and LCPC abrasivity indexes (after Mathier and Gisiger, 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 be measured 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 various sample surface conditions (Top: Sawn Surface, Bottom: Rough Surface) ...... 44

viii Figure 19: Plot of calculated CAI using HRC 41/43 pins and equation 1 vs. measured CAI 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 new equipment ...... 70

ix 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 rock types ...... 62

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

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.

3

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.

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].

Table 1- Mohs hardness scale

Talc

Topaz

Quartz

Calcite

Apatite

Mineral

feldspar

Gypsum

Diamond

Fluorspar

Corundum Orthoclase Orthoclase Mohs 1 2 3 4 5 6 7 8 9 10 Hardness

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

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

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.

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

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

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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].

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

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

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

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]:

Table 2: Rock abrasiveness classification based on LCPC test ABR (g/t) Scale 0-500 Very small 500-1000 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].

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

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

15

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

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].

17

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

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

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

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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].

6 3 5

4 3 1

A- Cerchar apparatus B- West apparatus C- CSM apparatus 1+3-Vice 2-Hand lever 4- 1-Sample vice 2-Hand crank 1+3-Vice 2-Handle 4-Testing pin Testing pin 5-Pin chuck 6- 3-Vice sled 4-Testing pin 5- 5-Pin chuck 6-Weight Weight Pin guide 6-Weight Figure 11: Different Testing devices for Cerchar test (after Plinninger, et al., 2003 & Rostami, et al., 2005)

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

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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.

23

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

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

24

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

25

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).

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

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].

26

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].

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

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].

27

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].

28

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

29

(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].

30

Suana and Peters (1982) also related CAI to the mineralogical composition of the rocks by testing monomineralic rocks and large single crystals to determine the abrasivity of rock forming minerals. By knowing their abrasiveness value, they concluded that it is possible to calculate a theoretical Cerchar Index of the rock and compare it with the measured values. Differences between the measured and calculated CAI values indicate the influence of other factors. After measuring

CAI abrasivity of the minerals, they carried out Cerchar tests on 36 rock specimens and found some deviations from predicted CAI values. The deviation was attributed to overrating of the abrasivity of hard rocks and underrating the abrasivity of softer rocks [19].

Yarali et al. (2008) studied petrographic analysis and abrasivity indices of 29 sedimentary rock samples such as sandstone, siltstone, and mudstone. They concluded that rock abrasiveness is a function of the amount of quartz and other abrasive minerals in the rock, average quartz grain size, cement type, and cementation. They mentioned that the quartz content, which is the most dominant parameter, provides a convenient measurement of rock abrasivity for a wide range of rock types.

They concluded that the effect of rock strength on rock abrasivity is less significant [20]. This could be due to narrow range of strength values tested which did not allow for extrapolation of the results to much stronger rock types.

In contrast, Al-Ameen and Waller (1994) performed extensive set of tests both on soft and hard rocks and concluded that the rock strength has dominating effect on measured CAI values, rather than abrasive mineral content. Therefore, it was suggested that it seems to be incorrect to use the term Cerchar “Abrasivity” index when a dominating factor was the rock strength [14].

Similarly, Alber (2008) performed many tests on four rock types and concluded that no significant correlation between CAI and neither the quartz content nor the equivalent quartz content was obtained [15]. Plinninger et al. (2003) also performed Cerchar test on 109 samples with a broad range of abrasiveness and strength. They concluded that the Equivalent Quartz Content alone is not suited to effectively explain the abrasion values measured by CAI Test [18].

31

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

32

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.

33

Correct CAI measurement from the side

Wrong CAI measurement 90° from the top

Figure 16: View of pin tip wear flat measured from the side view (and what could be measured from the top view) 9

0°

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].

34

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].

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)

Michalakopoulos et. NTNU CSM CAI Cerchar, 1986 al classification classification (pin Value (pin hardness 54) (pin hardness 55) (pin hardness 43) hardness 56) 0.3 - 0.5 Not very abrasive Very low Not very abrasive abrasiveness Not very abrasive 0.5 - 1.0 Slightly abrasive Low abrasiveness Slightly abrasive Medium Medium Medium 1.0 - 2.0 Slightly abrasive abrasiveness to abrasiveness abrasiveness to abrasive abrasive Medium 2.0 - 4.0 Very abrasive High abrasiveness Very abrasive abrasiveness to abrasive 4.0 – 5.0: Very Extremely Extreme abrasive 4.0 - 6.0 Extremely abrasive abrasive abrasiveness 5.0 – 6.0: Quartzitic 6.0 - 7.0 Quartzitic - Quartzitic -

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].

35

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].

36

Rostami et al. (2005) asked four different laboratories to perform Cerchar test on the exact same set of specimens of 4 different rock types. Two of those labs used pin hardness of 43 while the other two used 54-56 hardness pins. For each sample, the test was run on both rough and sawn surfaces with certain interval between scratches. Then the exact same samples were shipped to the next lab for testing; where, the next set of scratches were placed between, and run in the same direction as, previous ones.

They came up with contrasting results from different labs and concluded that there is an urgent need for more consistent set of pin hardness, test procedures, measurement of wear flats, and a new or updated classification of CAI results [17].

37

Chapter 4

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

38

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.

39

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

Limestone Slate Calcite Marble Sandstone Granite Quartzite

Cerchar- 41/43 HRC- 0.2 1.3 1.7 2.1 4.0 4.6 3.3 Sawn

Cerchar- 41/43 HRC- 0.3 1.3 1.5 2.1 3.7 5.6 8.1 Rough

Cerchar- 54/56 HRC- 0.1 0.7 1.0 1.0 2.4 4.7 2.7 Sawn

Cerchar- 54/56 HRC- 0.2 0.6 0.7 1.0 3.3 4.2 5.7 Rough

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

S-Wave Velocity 2139 3475 2804 3318 2586 2691 3753 (m/s) Equivalent Quartz 3.75 1.72 3.75 3.75 54.48 58.58 100.00 Content (EQC, %)

40

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 Sub Terra, Laboratory CSM Univ. of Texas PSU Inc. Pin Hardness 54-56 55-56 41 41-43 54-56 (HRC) 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

41

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.

42

7

6

5

4

3 Other labs Other 2 Sawn 54/56 CSM 1 Sawn 55/56 UT 0 0 1 2 3 4 5 6 7 PSU- Sawn-54/56

7

6

5

4

3 Other labs Other 2

1 Rough 54/56 CSM Rough 55/56 UT 0 0 1 2 3 4 5 6 7 PSU- Rough-54/56

6

5 Sawn

- 4

3

41 HRC 41 - 2

Sub Terra Sub 1

0 0 1 2 3 4 5 6 PSU- 41/43 HRC- Sawn

Figure 17: Comparison the CAI results of different laboratories

43

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.

44

43 vs. 56 (Sawn Samples) 6.0

5.0

4.0

3.0

CAI (54/56HRC) CAI 2.0 y = 0.9273x - 0.4642 R² = 0.8561 1.0

0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0

CAI (41/43 HRC)

43 vs. 56 (Rough Samples) 9.0 8.0 Less Abrasive Samples 7.0 Abrasive Samples 6.0 5.0

4.0 y = 0.5414x + 1.2734 R² = 0.9972

CAI (54/56HRC) CAI 3.0 2.0 1.0 y = 0.4566x + 0.0254 R² = 0.9925 0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 CAI (41/43 HRC)

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

45

Calculated vs. Measured 6.0

Sawn 5.0 1 Rough :1 Line 4.0

3.0

2.0 Michalakopoulos et. al et. Michalakopoulos

1.0 Calculated CAI from 41/43 HRC using from using CAI HRC 41/43 Calculated

0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0

Measured CAI 54/56 Figure 19: Plot of calculated CAI using HRC 41/43 pins and equation 1 vs. measured CAI for HRC54/56 pins

Impact of Surface condition

Figure 20 shows the impact of surface condition on the CAI results. Dashed line corresponds to 1:1 line. As can be seen, in lower values of CAI (non-abrasive rocks) results of testing on sawn and rough surfaces are more or less the same. However, in harder rocks, the results of testing on rough surface samples are higher than sawn which confirms the conclusions of

Plinninger et al. (2004), and Rostami et al. (2005).

But there are two points in the graph that are below the line and are exceptional. These are for sandstone sample using 41/43 HRC pins and granite using 54/56 HRC pins. The reason for this discrepancy is not obvious, but as this difference is not a lot and could be attributed to location of the tests on the samples.

46

Rough vs. Sawn Surface 9.0

8.0 1

:1 Line 7.0

6.0

5.0

4.0 54/56 HRC 3.0 CAI on rough rough on surface CAI 40/42 HRC 2.0

1.0

0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

CAI on Sawn surface

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

Correlation of CAI with Petrographical and Geomechanical properties

Equivalent Quartz Content (EQC) of the samples was calculated based on hand held mineral content evaluation and their respective Rosiwal hardness. Rosiwal hardness was calculated based on Mohs hardness of the constituent minerals using Equation 6 [24].

Equation 6

Rosiwal hardness was then multiplied by the percentage of the respected mineral and results were added. Results calculation of Rosiwal hardness and EQC are summarized in Table 6.

CAI values presented in Table 4 were correlated with other properties mentioned in Table 4 and Table 6 (physical properties and EQC of the rocks). CAI was best correlated with two of the rock properties: Uniaxial Compressive Strength (UCS) and Equivalent Quartz Content (EQC).

47

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.

Table 6: Equivalent quartz content of seven rock types

Equivalent Constituent Percentage of Mohs Rosiwal Quartz Mineral Mineral (%) hardness hardness Content Limestone Calcite 100 3 2.3 2.3 Slate Clay 100 2.25 1.1 1.1 Calcite Calcite 100 3 2.3 2.3 Marble Calcite 100 3 2.3 2.3 Quartz 40 7 104.3 Glass Sandstone (Amorphous 59 5.75 31.7 45.2 Quartz) Mica 1 2.5 1.4 Plagioclase 50 6 40.3 Granite Quartz 43 7 104.3 23.3 Biotite & 7 2.5 1.4 Muscovite Quartzite Quartz 100 7 104.3 104.3

350.0 120.00 300.0 100.00 250.0 R² = 0.4431 80.00 200.0 R² = 0.5795 60.00

150.0 EQC (%) EQC

UCS (MPa) UCS 40.00 100.0 50.0 20.00 0.0 0.00 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0

CAI- 42 HRC- Sawn CAI- 42 HRC- Sawn

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

48

350.0 120.00 R² = 0.8992 300.0 100.00 R² = 0.9334 250.0 80.00 200.0 60.00

150.0 EQC (%) EQC

UCS (MPa) UCS 40.00 100.0 50.0 20.00 0.0 0.00 0.0 2.0 4.0 6.0 8.0 10.0 0.0 2.0 4.0 6.0 8.0 10.0

CAI- 42 HRC- Rough CAI- 42 HRC- Rough

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

350.0 120.00 300.0 100.00 250.0 R² = 0.5685 R² = 0.4314 80.00 200.0 60.00

150.0 EQC (%) EQC

UCS (MPa) UCS 40.00 100.0 50.0 20.00 0.0 0.00 0.0 1.0 2.0 3.0 4.0 5.0 0.0 1.0 2.0 3.0 4.0 5.0

CAI- 54 HRC- Sawn CAI- 54 HRC- Sawn

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

350.0 120.00 300.0 100.00 R² = 0.8007 R² = 0.9741 250.0 80.00 200.0 60.00

150.0 EQC (%) EQC

UCS (MPa) UCS 40.00 100.0 50.0 20.00 0.0 0.00 0.0 2.0 4.0 6.0 0.0 2.0 4.0 6.0

CAI- 54 HRC- Rough CAI- 54 HRC- Rough

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

49

To increase the number of tests, results of 4 other rock types which were tested by SINTEF and Rostami et al. (2005) in Norway and US plus 4 other rock types which were encountered in a project in New York City were added to the set. Related rock UCS and EQC of these 8 additional samples are presented in Table 7.

Table 7: EQC and UCS of 4 added rock types Cerchar- 54 Cerchar- 54 UCS Equivalent Quartz Rock Type HRC- Sawn HRC- Rough (MPa) Content (EQC, %) Basalt 2.37 2.49 83.1 32.08 Quartzite 2.34 3.63 165.0 102.32 Tonalite 3.23 3.25 112.1 48.34 Limestone 1.15 1.02 76.8 2.35 Gneiss 2.57 2.18 61.8 70.86 Granite/Pegmatite 2.71 2.99 100.3 55.60 Amphibolite/Hornblende Schist 3.56 3.79 137.3 40.24 Aplite/Fine- grained Granite 3.12 3.29 139.5 84.34

As mentioned earlier, an attempt was made to find a relation between UCS, EQC, and CAI measured on rough sample surfaces. For statistical analysis, Minitab software was used. Two different equations for each steel type were derived (Equation 7 to Equation 10). These equations offer very good correlations between CAI and UCS and EQC. It should be noted that the first two equations are based on seven rock types while the last two benefits from 15 samples. Figure 25 shows a comparison between Actual and Calculated CAI using equations below.

CAI 42 HRC - Rough = -0.127 + 0.0148 UCS + 0.0411 EQC R2 = 98.1 % Equation 7

R2 = 94.6 % Equation 8

CAI54 HRC - Rough = 0.127 + 0.0103 UCS + 0.0261 EQC R2 = 85.1 % Equation 9

R3 = 89.9 % Equation 10

50

12.00

10.00

8.00

6.00 Predicted CAI Predicted 4.00

Equation 7 2.00 Equation 8 Equation 9 Equation 10 0.00 0.0 2.0 4.0 6.0 8.0 10.0 12.0 Actual CAI Figure 25- Comparison between Actual and Predicted CAI

Impacts of Pin Speed

Since the impact of Pin speed has not been examined thoroughly in past, a set of tests were performed on three different rock types ranging from non abrasive to very abrasive to evaluate the impact of testing speed. The rock types included were Limestone, Sandstone, and Quartzite.

Specimens were tested for test durations of 5, 10, 30, and 60 seconds. Results of the testing are summarized in Table 8 and related data is shown in Figure 26.

Correlation between CAI value and test duration in different test setups are shown in Figure

26. The figure shows no meaningful trend between CAI value and test speed. It can be concluded that the speed of the tests does not affect the results significantly. Meanwhile for Cerchar testing

51

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 Limestone 0.1 0.1 0.2 0.2 0.3 0.2 0.3 0.9 Sawn 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 Limestone 0.2 0.2 0.1 0.1 0.3 0.3 0.4 0.4 Rough 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

Limestone 0.9 0.8 40/42 Rough 0.7 54/56 Rough 0.6 40/42 Sawn 0.5 54/56 Sawn

CAI 0.4 0.3 0.2 0.1 0.0 0 20 40 60 Test Duration (sec)

52

Sandstone 4.5 4.0 3.5 3.0 2.5

CAI 2.0 1.5 1.0 0.5 0.0 0 20 40 60 Test Duration (sec) Quartzite 10.0 9.0 8.0 7.0 6.0

5.0 CAI 4.0 3.0 2.0 1.0 0.0 0 20 40 60 Test Duration (sec)

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

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

53

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.

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 70.0 130.0 194.8 Sawn Quartzite CAI 2.5 2.9 3.2 4.0 5.0

8.0

7.0

y = 0.0144x + 2.1442 6.0 y = 0.0087x + 2.047 R² = 0.997 R² = 0.9929

5.0 mm)

1 4.0 -

3.0 54/56 HRC

2.0 40/42 HRC Tip Loss(10 Tip

1.0

0.0 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 Applied load (N) Figure 27: Correlation between CAI and applied load on stylus

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

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 tests (varying the load) is that as long as the same stylus shape and hardness is used, the applied load is not of great importance and any load can be applied. In other words there is no loading level that can cause a shift in the behavior of the steel for the purpose of abrasion testing. The selected load level can change the absolute results, but on a comparative scale, the end results provide a basis for comparing one rock versus another.

Obviously, using higher load values means that the worn surfaces are larger and perhaps easier to measure, but at the same time, higher load applied on the pin could impose other operational difficulties, thus a reasonable balance should be struck and a nominal weight to be selected and used as standard.

Impact of measuring apparatus and procedure

Measuring of the wear flat in CAI testing, although seem to be simple, but in practice could be a source of error. West (1989) mentioned that a burr is occasionally formed on the downstream side of the wear flat during testing. He suggested that the burr should be gently removed by a soft material or the extended surface must be disregarded when measuring the wear flat [21]. However, actual attempts in the lab show that it is very difficult to remove the burr gently and consistently for measurement of the wear flat. This means that for an accurate measurement of the wear flat due to

Cerchar testing, ample amount of training and experience is needed. This can be difficult in general

55

practice in the laboratories where various technicians are assigned to perform the tests. Also distinguishing extended surface from top view is very difficult and misleading.

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 [17].

This system recently is adopted and used at the Pennsylvania State University geomechanics laboratory. To evaluate the accuracy and precision of the method proposed by

SINTEF, a series of tests were performed on seven rock types mentioned in Table 4.

Wear flat of pins were measured by two different operators using microscope from the top and also from the side. Results for both operators are summarized in Table 10 and Table 11.

Table 10: Two operators' measurements from the top and from the side (41/43 HRC) 1st operator Rock Microscope from the top Microscope from the side Type Sawn Rough Sawn Rough Limestone 0.1 0.2 0.2 0.3 Slate 1.6 1.3 1.3 1.3 Calcite 2.0 1.3 1.7 1.5 Marble 2.8 3.0 2.1 2.1 Sandstone 4.5 4.0 4.0 3.7 Granite 5.2 5.7 4.6 5.6 Quartzite 4.2 8.1 3.3 8.1 2nd operator Rock Microscope from the top Microscope from the side Type Sawn Rough Sawn Rough Limestone 0.6 0.9 0.2 0.3 Slate 1.7 1.5 1.4 1.3 Calcite 2.7 2.1 1.7 1.4 Marble 3.0 3.7 2.1 2.1 Sandstone 4.4 3.9 4.3 3.8 Granite 5.9 7.1 4.7 5.9 Quartzite 4.3 8.0 3.4 8.0

56

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

1st operator Rock Microscope from the top Microscope from the side Type Sawn Rough Sawn Rough Limestone 0.1 0.2 0.1 0.2 Slate 1.0 0.7 0.7 0.6 Calcite 1.3 0.7 1.0 0.7 Marble 1.3 1.1 1.0 1.0 Sandstone 3.2 3.6 2.4 3.3 Granite 5.3 4.1 4.7 4.2 Quartzite 3.4 5.9 2.7 5.7 2nd operator Rock Microscope from the top Microscope from the side Type Sawn Rough Sawn Rough Limestone 0.2 0.2 0.1 0.2 Slate 1.0 0.8 0.7 0.6 Calcite 1.4 0.9 1.1 0.7 Marble 1.3 1.3 0.9 1.0 Sandstone 2.8 3.8 2.6 3.3 Granite 4.8 5.1 4.7 4.0 Quartzite 3.5 6.3 2.8 5.6

As mentioned earlier, tests were performed both on sawn and rough surface with two different sets of pins with the hardness of 41/43 HRC and of 54/56 HRC. Test speed was 1 mm/sec or 10 second per pin scratch. Results of these tests are summarized in Figure 28.

As can be seen, variation decreased significantly when measuring the diameter of the wear flat from side view. This indicates that the use of proposed method can reduce the operator sensitivity of the test and yield more accurate and repeatable results.

57

1.2 Top view- 54/56 Rough 1.0 Side view- 54/56 Rough

0.8

0.6 operators 0.4

0.2 Measuremnt difference difference between Measuremnt 0.0 1 2 3 4 5 6 7 Rock types 0.6 Top view- 54/56 Sawn 0.5 Side view- 54/56 Sawn

0.4

0.3 operators 0.2

0.1 Measuremnt difference difference between Measuremnt

0.0 1 2 3 4 5 6 7 Rock types 1.6 Top view- 40/42 Rough 1.4 Side view- 40/42 Rough 1.2

1.0

0.8

operators 0.6

0.4

Measuremnt difference difference between Measuremnt 0.2

0.0 1 2 3 4 5 6 7 Rock types

58

0.8 Top view- 40/42 Sawn 0.7 Side view- 40/42 Sawn 0.6

0.5

0.4

operators 0.3

0.2

Measuremnt difference difference between Measuremnt 0.1

0.0 1 2 3 4 5 6 7 Rock types

Figure 28: Variation of CAI measurements between operators

Chapter 5

Study of alternative testing configuration for measurement of rock abrasion

Development of a new testing concept

As mentioned earlier, one of the drawbacks of the Cerchar tests is the limited scratch distance of the test. In another words, the tests scale is not a good representative of the rock, especially in course grained rocks. While the errors caused by grain size of the rock could be mitigated by increasing the number of tests, there are other inherent shortcomings in the testing system that requires considering possible modifications in the equipment, procedures, or the testing configuration altogether.

Another significant shortcoming of the test is that the applied stress on the pin is not constant throughout the test. This is because of the geometry of the tool which is a 90° sharp cone with theoretical zero diameter at the beginning of the test. As the tests progresses, the tip wears off but the applied load is constant, so naturally the magnitude of the stress at the pin tip decreases.

Therefore, depending the abrasivity of the rock and hence the flat area, the actual contact stress at the pin tip differs between various samples.

Due to the issues noted earlier, an attempt was made to design and develop a new equipment to address some of the shortcomings and possibly introduce a new device for measurement of rock abrasivity that can yield more accurate and consistent results. Different approaches were considered and one concept was selected for preliminary testing. The configuration of the proposed new test was selected to utilize available equipments in the lab, easy to run, and higher probability of producing more consistent and repeatable results.

The new equipment simply consists of a handle mounted on a frame which can transfer vertical applied load into the horizontal load. Different applied loads can be achieved by applying

60

different weights or just by changing the location of the dead weight. Figure 29 shows a schematic view of the equipment while Figure 30 illustrates the equipment while is in work.

Weight is placed on the horizontal top surface of the handle and as the handle rotates around the hinge, the dead weight (constant load in vertical direction) translates to a constant horizontal load as the vertical side of the triangular shape handle pushes the tool/pin against rock sample. Sample is held by the chuck of a lathe machine and can be turned at different RPMs.

Therefore by changing the duration of the test, RPM, and also distance from center of the core different scratch lengths can be achieved.

For the pin tip configuration, various scenarios were examined. One alternative was to use a constant cross-section tool instead of cone shaped tip. The constant cross section offers a constant level of contact stress between the stylus and the rock. However, a preliminary calculation of the sizes and required loads indicated that a complicated mechanism needs to be designed and fabricated to allow for application of reasonable contact stresses at the pin tip. Consequently, for the preliminary testing, the same tip configuration was adopted, where a 10 mm round stylus with a

90 degree cone tip, or standard Cerchar pin configuration was selected for testing.

For the initial phase of studies on the proposed device, it was decided to use the same 54/56

HRC pins which are used in Cerchar. This hardness was selected because at the first stages of the testing the results of new test could be compared with the available Cerchar results.

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Applied Distance between applied load Load and rotation point

Rotation Point Handle’s Weight

Rock Pin Sample

Frame

Figure 29: Schematic view of the new equipment

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

62

A set of preliminary tests were performed on the seven rock types which were discussed earlier in this study examine the effect of the new test configuration on the pin. The initial test results were also used to seek possible relation/correlation between the proposed test and Cerchar test, so as to provide a basis for transition to new testing system. All tests were performed on sawn- cut surfaces. Distance between pin tip and core center was 1.524 cm (~0.6 inch) for all tests. RPM was 52 rev/min and test duration was 5 minutes. Scratch length can be calculated and is 2.49 m (100 inches). Three different settings were tested per sample. In one setting no weight was put on the handle, meaning that the weight of the handle itself was used as the applied load on the pin. Based on the geometry of the handle, unit weight of the material (Aluminum 6061), thickness (2.54 cm), and distance of the pin from the center of rotation, the applied load on the pin tip can estimated as

2.829 kg. Results of these tests are presented in Table 12.

Table 12: Results of the new equipment along with respected CAI value for seven rock types

Average tip CAI 54-56 CAI 54-56 loss (micron) rough sawn Limestone 82 0.16 0.137 Slate 793 0.629 0.703 Calcite 216 0.731 1.003 Marble 218 0.953 0.964 Sandstone 1647 3.321 2.379 Granite 821 4.215 4.741 Quartzite 885 5.69 2.729

Figure 31 shows the correlation between the results of new test with CAI of respected rock type. As can be seen, no meaningful correlation can be seen.

63

6

5

4

3 CAI CAI rough 2 CAI sawn

1

0 0 500 1000 1500 2000

New Test tip Loss (micron)

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

The lack of correlation could be attributed to the fact that pin tends to penetrate deep into the rock, especially for softer rocks. The impact of the penetration of the pin in different rocks was monitored carefully during the testing. In all soft rocks, a deep penetration occurred in the first few seconds of the test. Figure 32 to Figure 35 show a deep penetration of the pin on sandstone, limestone, slate, and calcite. For the sandstone sample, different load levels were used but due to high level of penetration, it was decided not to increase the applied load. Thus the subsequent tests were run using weight of the handle itself.

64

Figure 32: Deep groove of the pin on sandstone

Figure 33: Deep groove of the pin on limestone

65

Figure 34: Deep groove of the pin on slate

Figure 35: Deep groove of the pin on calcite

Figure 36 shows the worn out pins used on sandstone sample with various loads. This clearly shows that the stress level decreases dramatically throughout the tests as an increased contact area develops very quickly due to the penetration of the pin into the rock sample. Therefore,

66

wear occurs in lower levels of stress. Obviously, wear occurs not only at the tip of the pin but also on the sides of the pin in radial the direction relative to the rotation of the sample.

Amount of nominal Load

13253 g 9778 g 6304 g 2829 g

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

As pin cannot rotate during the test, wear occurs on two opposite sides of the tip. This shows that the method of evaluating wear by measuring the tip loss by microscope cannot be directly applied here. That is because the higher magnitude of wear on the tip and also the shape of the wear surfaces which is not flat. Therefore other methods like weight loss should be considered for future tests. The side view of the pin using the microscope was used for measurement of the wear surface but at lower magnification. Also an imaginary flat surface was used in a way that represents the volume loss of the pin. Figure 37 shows a view of the tip of the stylus from the side view and the measurement method for that.

67

2829 g 6304 g

9778 g 13253 g

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

Another set of tests was performed on quartzite to study the impact of tests duration/scratch length and different load levels. For all tests, distance between the tip and core center was 1.524 cm

(~0.6 inch) as before. Also RPM was kept constant at 52 rev/min. Tests were performed in six different durations (0.5, 1, 2, 5, 10, and 30 min). Tests were carried out at four different loading levels: without any additional weight except for the weight of the handle, 2279 g weight at the distance of 7.62 cm, 15.24, and 22.86 cm from rotating point. (the selected weight was nominal and based on available steel pieces machined to the shape for mounting on the handle). Three pins were used for each test (repetition) and the average value of the measured wear flats is presented in Table

13Table 2.

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Table 13: Results of new test on sawn -cut quartzite Test Duration Scratch Estimated Load Average Tip No. (min.) Length (cm.) on the pin (gr.) Loss (micron) 1 0.5 249 2829 734.5 2 1 498 2829 749.7 3 2 996 2829 739.2 4 5 2490 2829 746.6 5 10 4979 2829 864.5 6 20 9959 2829 929.3 7 0.5 249 6304 933.7 8 1 498 6304 871.0 9 2 996 6304 1008.7 10 5 2490 6304 1042.8 11 10 4979 6304 1078.8 12 20 9959 6304 1303.3 13 0.5 249 9778 1124.3 14 1 498 9778 1004.0 15 2 996 9778 1087.8 16 5 2490 9778 1165.7 17 10 4979 9778 1382.3 18 20 9959 9778 1414.7 19 0.5 249 13253 1113.0 20 1 498 13253 1125.2 21 2 996 13253 1191.7 22 5 2490 13253 1278.8 23 10 4979 13253 1375.8 24 20 9959 13253 1633.5

Estimated applied load was correlated with tip loss. The result is shown in Figure 38. As can be seen, it seems like there is more or less a linear relationship between the applied load and the tip loss as was seen in Cerchar tests. It suggests that the magnitude of load is not of great importance in both tests and any load can be used as the standard for future tests. In other words, changing loading level does not make a shift in the material behavior and any loading level can be

69

used for this purpose as long as the applied load at the tip is consistent between the test set ups if this system was to be adopted in the future for rock abrasion testing.

1800.0

1600.0

1400.0 30 sec 1200.0 1 min. 1000.0 2 min. 800.0 5 min

Tip Loss (micron) Loss Tip 600.0 10 min 20 min 400.0

200.0

0.0 0 2000 4000 6000 8000 10000 12000 14000

Estimated Applied Load (gr.)

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

Figure 39 shows the correlation between scratch length/test duration with the tip loss. AS can be seen, a good correlation was not achieved but this more or less shows that in the beginning of the test, a dramatic wear occurs and during the testing, the curve flattens. This also coincides with the results of the Cerchar tests and suggests the use of other tools rather than a cone shape pin for the future tests.

70

1800.0

1600.0

1400.0

1200.0

1000.0

800.0

Tip Loss(micron) Tip 600.0

400.0 without applied load load of 2279 gr at 3 in. 200.0 load of 2279 gr at 6 in. load of 2279 gr at 9 in. 0.0 0 2000 4000 6000 8000 10000 12000

Scratch Length (cm.) Figure 39- Correlation between Scratch length/test duration and tip loss in new equipment

Discussion of the test results and its practical implications

As mentioned earlier, the proposed new test is in its first stages and more tests should be performed. So far, it seems that the magnitude of applied load is not of great importance in this test as linear trend was achieved between the load and the tip loss. Moreover, deep penetration and intrusion into the sample was seen when applied load increased. Therefore it seems reasonable not to add any weight to the weight of the handle.

Considering the fact that increase in scratch length/test duration also does not increase the tip loss significantly and the curve is more or less flat, it is recommended that a test duration of over

30 seconds would be sufficient for reaching a reasonable wear flat on the sample. Therefore, a test duration of 60 sec or 1 minute for future tests.

Next stage of the tests should be performed using the constant cross-section rods. But for measuring the magnitude of occurred wear another method rather than microscope measurement should be developed. Perhaps, using weight loss on the pin can be considered for this purpose.

Chapter 6

Conclusions and recommendations

Conclusions

Rock abrasivity affects rate and cost of production of mechanized excavations. There are numerous tests to quantify rock abrasion. Cerchar Abrasivity Index test is widely accepted throughout the world to represent rock abrasion as it pertains to tool wear and life in mining, tunneling, and construction activities. Despite good merits and simplicity of Cerchar test, there are some discrepancies in the test results that are due to the type of equipment, condition of rock samples, operator skills, procedures used in testing, and final measurement of the wear flat. These discrepancies are the result of two main issues: first the lack of standard in performing the tests, and second intrinsic shortcomings of the test.

This study was aimed to identify the impact of some of the common test parameters on the test results and offer potential solutions for more uniform testing and more consistent/repeatable results. Also a new device was introduced to examine the possibility of substituting Cerchar test with another rock abrasion index that could produce better results. Early stages of development have been covered and more in depth research and additional studies is needed to evaluate the potentials of the proposed testing system.

Based on this study and performing multiple series of targeted Cerchar tests using different procedures, pin material, surface condition of the samples, test speed, various applied load, and measurement methods following conclusions could be drawn.

1. Cerchar testing has some inherent shortcomings and a level of operator sensitivity

that needs to be carefully evaluated when issuing test results or developing standard

72

procedures for performing the test. This was proven by many researchers as well as

the limited comparative testing performed as part of the current study between

various testing laboratories.

2. Petrographical and geomechanical properties of the samples were measured to

evaluate the relationship between measured CAI and some other rock properties for

cross checking the end results of Cerchar tests. Comparison of the measured CAI

values and the physical properties of the rock indicated that CAI on rough

specimen show more consistent results and better correlation with UCS and EQC

values.

3. In comparing the result of testing using different pin hardness the equation

proposed by Michalakopoulos et al. seems to produce a reasonable estimate for

rocks with low abrasivity, however for more abrasive rocks, the intercept of the

proposed equation should be changed.

4. Studies regarding the surface condition of the samples confirmed the findings of

other researchers. It showed that in softer rocks, the surface condition does not

influence the results while in harder rocks, rough surface tends to yield higher CAI

values in comparison to sawn surface.

5. The study showed that pin speed does not have any systematic and meaningful

impact on the test results. Meanwhile, a testing duration of 10-20 second seem to be

reasonable from operational viewpoint.

6. Testing various levels of applied load on the pins also showed that while the end

results of measured wear flat will increase linearly with applied load, it does not

seem to offer better or more consistent results at higher load for performing Cerchar

test.

73

7. To address the issue of operator sensitivity on the measurement of wear flat,

variation of test results are minimal if the diameter of wear flat is measured on a

side view/picture of the pin tip. The use of proposed test setting and measurements

can reduce the operator sensitivity of the test and yield more accurate and

repeatable results.

8. A new device was introduced to address two main shortcomings of Cerchar test:

small scale of the test (length of scratch) and change of stress during the test. The

preliminary tests focused on the scale issue and the impact of extended length of

the scratch. A set of preliminary tests were performed on a newly developed testing

device. Scratch length tests show that the tip loss occurs in the first few seconds of

the tests and becomes more or less steady afterwards. The test results shows similar

trends relative to the length of scratch, but the observed problem is the creation of a

grove on the test sample that can increase the area of contact and thus reduce

effective contact stresses between the pin tip and the rock.

9. Similar to Cerchar testing, different applied load in the proposed new test

configuration seems to have a linear relation with tip loss. Measurement of tip loss

with microscope in new test could be unsuccessful and an alternative method,

perhaps weight loss should be considered for future tests. This relates to the rather

non-uniform shape of the tip loss during the testing which is determined by the size

and shape of the grove into the rock surface.

It should be noted that the conclusions on the proposed new testing system is very preliminary and needs more in depth study and some follow up testing to evaluate the merits and potentials of the proposed testing system.

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Recommendations

More variant set of rock types should be tested. Geomechanical and mineral composition of the rocks should be examined and the relation between CAI, EQC, and UCS should be re-evaluated.

Moreover, the

Following is a brief list of recommendation for continuation of this study.

1. The rock types selected for the initial testing included a set of 7 samples with a

wide range of properties. However, an extended set of rock samples should be

tested to verify the trends found in this limited study. Geomechanical and mineral

composition of the rocks should be examined and the relation between CAI, EQC,

and UCS should be re-evaluated. Moreover, the relation between different pins and

rock surface with CAI has to be studied.

2. Other labs should be invited to perform CAI tests on the seven samples to see the

operator sensitivity of the test and their results should be compared and the reason

for the discrepancies should be discussed.

3. It might be useful to perform some Cerchar tests using the scratch lengths more

than 10 mm and see the effect of the scratch lengths more than 10 mm on the

results. It can be done using different set of applying loads.

4. A standard procedure for measuring the tip loss based on the side view should be

defined and introduced to the industry.

5. Set of tests should be performed using 41/43 HRC pins on the new equipment, and

weight loss of the stylus to be used as a substitute for measurement of the wear flat

to see possibility of better correlations.

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6. Further studies of the proposed equipment should consider the use of Constant

cross-section pins with different cross section area/applied load and different

hardness to see the possibility of a more consistent and repeatable set of results.

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