Cerchar Abrasivity Index test (CAI) is widely used for determining the abrasiveness of rocks. This test provides an index value that can be used as a gauge for the abrasiveness of rocks in order to estimation of bit life and wear in various mining and tunneling applications. In this study, changes in CAI with the changes in scratch length were investigated by the use of West apparatus to determine the effects of length on CAI. The tests were carried out on 6 different rock types (3 types of sandstone, limestone, granite and quartzite) where on rough and saw cut surfaces with a scratch length between 2mm to 20mm with an increment of 2 mm. It has been found that about 60% of the final Cerchar index (at a 20mm sliding) was observed at the first 2mm scratch and continued at a constant rate up to 16–18 mm, then rests around 20mm sliding distance. This initial high tip wears may partly depend on the deformation and shear failure at the tip of pins. It has been also observed that 87% of the final Cerchar index was reached at 10mm sliding distance while 97% of the final Cerchar index at the 15mm sliding distance. Since initial wear at the 2mm sliding distance is not dependent on the amount of abrasive material in the host rock. Approximately 35% of the Cerchar index can be attributed to the abrasion effect which corresponds to the final 8mm of sliding distance. However if the sliding distance is increased to 15mm then 45% of the Cerchar index can be attributed to the rock abrasion effects. This is very important for the tool life estimation in tunneling applications.

1 Introduction
1.1 Previous studies

Abrasion can be defined as the wearing or tearing away of particles from the surface. Abrasion is a process causing removal or displacement of material at a solid surface; which will lead to wear, especially in tools which use in mining, drilling, and tunneling applications (Atkinson 1993).

Tool wear is an important parameter in mechanized tunneling and is highly affected by rock abrasiveness. Rock abrasiveness has a direct relationship with the wear occurring in the bits of drills, road headers, tunnel boring machines and other equipment, and has been researched (Bilgin et al. 2012). Knowledge of abrasiveness of rocks in mechanical excavation (TBM, roadheader) and drilling is very important to determine pick consumption. Cerchar abrasivity index test is widely used for estimation of pick consumption in engineering projects.

There are numerous tests to identify the rock abrasivity. One of the widely used rock abrasion tests is the Cerchar abrasivity index (CAI). CAI has become widely accepted and has been used for cutter life and cost estimation for excavation machines (Plinninger et al. 2003, Yarali et al. 2008). Nevertheless, an in depth examination of specific test procedures and equipment used has highlighted the significant impact of these parameters upon results. Although the test results varying from lab to lab and the variation in values rendered by different laboratories and equipment could create major errors in related estimates (Plinninger et al. 2003, Rostami et al. 2005, Michalakopoulos et al. 2006, Stanford & Hagan 2009, Oparin & Tananio 2009, Cardu et al. 2013).

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