Cerchar Rock Abrasiveness Test (CRAT): A Quick Guide to Rock Abrasivity & CAI Measurement
Table of Contents
What is CRAT ?
Cerchar Rock Abrasiveness Test is a simple laboratory method which is used to determine the abrasiveness of rock. This test determines the hardness and abrasion tendency of rock materials by measuring the abrasive wear on a hardened steel stylus dragged across a prepared rock surface.
It evaluates how aggressively a rock will wear down drilling tools, cutter heads, and excavation equipment.
Birth & Adoption of Cerchar Rock Abrasiveness Test in India
This test was developed about 45 years back, in the late 1970s at the CERCHAR Research Center (France’s Centre d’Études et Recherches des Charbonnages de France) which was formally adopted during the early 1980s.
But in India it is being used for 20-25 years but gained prominence with the rise of major hydroelectric and tunnelling projects in the Himalayan region during the early 2000s.
Its adoption became more routine from 2010 onwards, particularly with metro and long tunnel projects where prediction of TBM cutter and drilling tool wear became critical. Today, the test is widely used by government agencies, academic institutions, and private geotechnical laboratories as a standard tool for assessing rock abrasivity in India.
What are its applications and where all is it being used?
It is used for Tunnel Boring Machine (TBM) cutter wear prediction, drilling cost estimation, Blast-hole drilling planning, Selection of excavation machinery, Rock mass characterization and is used worldwide in geotechnical and mining engineering.
In India, it is being used in Metro Rail Projects, Large Private Geotechnical Test Labs, Institutes dealing with Hydro Projects, Thermal Projects, Premium Educational Institutes, Railway Contractors and Consultants etc
What parameter does it measure?
This test measures the Cerchar Abrasivity Index (CAI)
CAI = Width of Steel Stylus Wear Flat
(in millimetres)
The stylus is measured under a microscope after the scratch.
The factors that affect the Cerchar Abrasivity Index include mineralogy, primarily quartz content, Grain size, Rock texture, Strength and hardness, Moisture content, and also weathering.
Typical interpretation of the test results is as follows:
| CAI Value | Abrasiveness Category | Expected Tool Wear |
| < 0.5 | Very low | Minimal |
| 0.5 – 1.0 | Low | Low wear |
| 1.0 – 2.0 | Medium | Noticeable wear |
| 2.0 – 3.0 | High | High wear on bits/cutters |
| > 3.0 | Very high | Very high tool consumption |
What makes it a popular test?
It is a quick, simple and less expensive test, which allows Good correlation with TBM and drilling tool wear, Road Headers, Drill Bits etc. It is useful for preliminary design of excavation and selection of cutter type. It requires minimal Equipment to perform the required test.
Hence, it has been popularly adopted by TBM manufacturers, mining companies, and geotechnical labs worldwide.
What are the relevant international or Indian Standards does this test conforms to?
- ASTM D7625-10 which is Standard Test Method for Laboratory Determination of Abrasiveness of Rock Using the Cerchar Method. It defines Test apparatus, Stylus specifications, Test load, Scratch procedure, Wear measurement and reporting format.
- IS 8764 (Indian Standard) which provides a general procedures related to hardness and abrasion characteristics of rocks and is commonly referenced for correlation when performing Cerchar-style hardness assessments in India.
Is Cerchar Rock Abrasiveness Test (CRAT) Apparatus easily available in India?
Apart from many International Brands who manufacture this equipment and sell in India. Many Indian Suppliers also manufacture them now in India like Aimil Ltd, which sells through its network of 15 offices across India and its distribution network. Aimil Ltd is a pioneer Instrumentation Company in India, with its head office and manufacture base at Delhi.
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