Physical Properties of Rocks: A Reference Guide for Engineers

Physical Properties of Rocks: A Reference Guide for Engineers

Every crushing plant decision — chamber selection, liner alloy, motor sizing, screen aperture, conveyor belt class — traces back to a small set of physical properties of the rock being processed. Density tells you how the material will load a feeder. Hardness predicts how a manganese steel jaw plate will wear. The Bond Work Index (Wi) translates feed-to-product reduction into kilowatt-hours per tonne. Los Angeles abrasion loss (LA) signals whether an aggregate will hold up in a wearing course or crumble before commissioning.

This reference consolidates the eight rock characteristics that govern crusher sizing, wear-part selection and aggregate qualification, drawing directly on test methods and value ranges set out in international standards — ASTM C131, EN 1097-2, EN 933-4, ASTM D 7625-10 and the French P18-579 LCPC procedure. Each property is presented with its definition, the standard test that produces it, the formula behind the result, and a typical range for the rocks engineers encounter in quarrying, mining and aggregate production.

The Master Reference Table at the end of this guide consolidates Bond Work Index, density, bulk density, abrasion index, LA loss and uniaxial compressive strength for more than twenty common rocks — the same dataset used by MEKA application engineers when sizing crushing and screening equipment.

The Eight Key Physical Properties of Rocks

Eight measurable physical properties drive virtually every engineering decision in a crushing and screening circuit. They are summarised below; each is examined in detail later in this guide.

Why Engineers Need These Values

Plant designers use the Bond Work Index to estimate the connected motor power per tonne of throughput, then layer in a safety factor and a drive efficiency to size the crusher motor. Wear-part engineers translate the Abrasion Index into a lifespan factor — directly determining how often a jaw plate, blow bar or cone mantle needs to be replaced. Aggregate producers rely on LA loss, flatness index and shape index to qualify their product for concrete, asphalt or unbound base layers under EN, ASTM and DIN specifications.

Skipping these tests, or relying on values from a similar-looking quarry, is a common cause of underpowered crushers, premature wear-part failure and rejected aggregate batches. Where a property cannot be measured in a laboratory at the start of a project, the typical ranges in the Master Reference Table below provide the next-best engineering estimate.

Hardness (Mohs Scale)

Hardness, in the rock mechanics context used for crusher sizing, is the relative resistance of a mineral to scratching. The standard scale was established in 1812 by the German mineralogist Friedrich Mohs and ranks ten reference minerals from talc (1) to diamond (10). Every rock-forming mineral can be located on this scale, and the dominant mineral typically determines the abrasive behaviour of the rock as a whole.

Quick Reference — Mohs Scale

Common field references for quick scratch testing:

• Fingernail hardness ≈ 2.5
• Copper coin hardness ≈ 3
• Steel blade or glass plate hardness ≈ 5.5
• Steel file or steel nail hardness ≈ 6

Because Mohs hardness is ordinal — not linear — the difference between corundum (9) and diamond (10) is far larger than between fluorite (4) and apatite (5). For quantitative wear prediction, engineers use the Abrasion Index Ai (see Abrasiveness Tests below), not Mohs alone. For a full treatment of Mohs hardness and its relationship to the Vickers scale, see [link to: /blog/mohs-hardness-scale].

Specific Gravity and Bulk Density

Two distinct density values appear in every rock data sheet, and they are routinely confused. Solid density (specific gravity) describes the rock material itself; bulk density describes a heap of crushed aggregate. Both are reported in tonnes per cubic metre (t/m³), but they answer different engineering questions.

Specific Gravity (Solid Density): Definition and Units

Specific gravity — also called solid density — is the ratio of the weight of a solid to its volume, with no air voids included. It is expressed in t/m³ (numerically equal to g/cm³ and to specific gravity referenced to water at 4 °C). For typical silicate rocks, solid density falls between 2.6 and 3.0 t/m³. Iron ores such as hematite and magnetite reach 5.1–5.7 t/m³ because of their high iron content.

Bulk Density: Measurement Method

Bulk density is the weight of a unit volume of aggregate — not solid rock, but a heap of crushed material with air voids between particles. The standard procedure dries the aggregate at 110 °C until it is segregation-free, then fills a clean container of known volume V to the brim. The excess is struck off with a straight-edged board, the container is weighed, and bulk density is calculated as the ratio of aggregate mass to container volume.

Why the Two Differ

Bulk density is always lower than solid density for the same rock because of the air voids between particles. The ratio between the two is governed by three factors: porosity (intrinsic void volume of the rock), particle shape (spheres pack at ≈64 % whereas elongated chips pack lower), and particle size distribution (well-graded mixes pack tighter than single-size chips). For most quarried aggregates, bulk density is 1.5–1.8 t/m³ while solid density of the same rock is 2.6–3.0 t/m³ — the difference is the air.

Bulk density determines how much material a feeder, bin or conveyor will actually move — not the solid density. Sizing a stockpile or a silo on solid density alone overstates capacity by 35–40 %.

Crushability (CR)

What CR Tells You

Crushability (CR) is a percentage that indicates how easily a rock can be reduced in size. It is determined together with the LCPC abrasiveness test (described below) and reported as the friability percentage — the proportion of the original test sample that passes a 1.6 mm sieve after 5 minutes of impact in the LCPC apparatus.

A higher CR value means the rock breaks more readily under impact — useful information when selecting between crusher types (impact crushers favour high-friability rocks; jaw and cone crushers handle the full range). Friability is grouped into four classes:

Abrasiveness Tests

Abrasiveness measures the wear that a rock inflicts on a reference test piece. Three tests are routinely used in the crushing and tunnelling industries, each developed in a different region and each measuring slightly different aspects of the same underlying behaviour. They are not interchangeable, and a complete rock characterisation often reports all three.

ABR — LCPC Abrasivity Coefficient (P18-579)

The LCPC test was developed by the Laboratoire Central des Ponts et Chaussées and is governed by French standard P18-579. It produces both the LCPC Abrasivity Coefficient (ABR, in g/t) and the friability index (CR, %) in a single 5-minute test.

The testing apparatus consists of a rotor that holds a test plate approximately 25 mm × 50 mm in size and 5 mm in thickness, with a Brinell hardness of approximately 60–75. The plate is used for measuring abrasiveness and friability. The material to be tested is placed in a container measuring Ø90 × 100 mm. Within the container, 500 grams of material in the size range of 4–6.3 mm is placed.

The test plate is rotated at a speed of 4500 rpm for 5 minutes. After this duration, the material inside the container is emptied and sieved through a test sieve with a gap of 1.6 mm. The test piece and the material sieved through 1.6 mm are then weighed.

The LCPC Abrasivity Coefficient is then calculated as:

ABR = (m0 − m) / M

Where:

• ABR: LCPC Abrasivity Coefficient, g/t
• m0: Weight of the test plate before the test, grams
• m: Weight of the test plate after the test, grams
• M: Weight of the material subjected to the test, 0.00005 tons

The friability index (CR) is calculated from the same test as:

CR (%) = (M1.6 / M) × 100

• M1.6: Weight of material sieved through the 1.6 mm sieve, grams
• M: Weight of material subjected to the test, grams

Cerchar Abrasivity Index (CAI) — ASTM D 7625-10

The Cerchar Abrasivity Index is determined by fixing the rock sample tightly in a clamp. A heat-treated alloy steel tip, with a 7 kg dead weight on a 90° conical needle-shaped pin, is lowered onto the sample. The needle is moved 10 mm over the sample within a 1.5 second period.

The diameter of the worn tip is measured precisely under a microscope in terms of 1/10th of mm. The Cerchar Abrasivity Index is then calculated as:

CAI = 10 × d

Where:

• CAI: Cerchar abrasion index
• d: Diameter of the wear flat, mm

The hardness of the pin should be 54–56 Rockwell C. For accurate measurements, 5 measurements should be taken with 5 pins. The CAI is widely used in mechanised tunnelling — it correlates strongly with disc cutter wear in TBM operations — and as a quick laboratory cross-check on LCPC results.

Abrasion Index (Aᵢ) and Friability Class — Reference Table

Combining the LCPC and Cerchar approaches, the table below groups common rock classes by abrasion resistance level, friability class and corresponding friability range. It is the quickest way to estimate the duty class of a feed material in the absence of laboratory data.

How to Use Aᵢ for Wear-Part Lifespan Prediction

For a more accurate calculation of the worn jaw and side liner lifespans in jaw and cone crushers, a lifespan factor of L1 = 1 is taken to correspond to an abrasion index of 0.5. The chart below — derived from field data on common quarry rocks — plots the lifespan factor (Lf) against the rock's abrasion index (Ai).

Reading the chart for a representative selection of rock types:

A limestone wear part might run 5–15 times the reference life of the same component in a granite quarry. This factor feeds directly into spare-part stocking, planned maintenance windows and total cost of ownership calculations.

Los Angeles Abrasion Loss (LA)

Los Angeles abrasion loss is the most widely specified single number in aggregate qualification. It appears in highway specifications, railway ballast standards and concrete aggregate codes worldwide. The test method applied to rocks is determined by ASTM C131 and EN 1097-2.

Test Method

The test device consists of a cylinder with an inner diameter of Ø711 mm and a length of 508 mm, rotating at a speed of 30–33 rpm. The axis of the cylinder is in a horizontal position. Inside the cylinder, there is one shelf with a thickness of 25 mm and a height of 90 mm. The purpose of this shelf is to lift the material and steel balls to a certain height, ensuring their fall.

Inside the cylinder, steel balls with a diameter of Ø48 mm and a quantity ranging from 6 to 12, depending on the gradation of the test material, are charged with 5000 g of test material. After rotating the cylinder 500 times, the contents are emptied into a tray. After separating the balls, the test material is sieved through a sieve with an opening of 1.70 mm. The material remaining on the sieve is dried and weighed.

LA Formula

The Los Angeles abrasion loss is calculated as:

LA = (5000 − G) / 50

Where:

• LA: Los Angeles Abrasion Loss, %
• G: Weight of rocks remaining above the 1.70 mm sieve, measured in grams

Typical LA Values for Common Rocks

Specifications for high-traffic asphalt wearing courses commonly require LA ≤ 25 %; concrete aggregates typically allow LA ≤ 35–40 %. Indicative ranges from the MEKA database:

Basalt and diabase, with their tightly interlocked fine-grained matrices, sit firmly inside the strictest highway specifications. Limestone and dolomite — the workhorses of cement and base aggregate production — cluster around the threshold for unbound layers and require careful source selection for asphalt use.

Bond Work Index (Wᵢ)

Of all the values in this guide, none affects equipment selection more directly than the Bond Work Index. Developed by Fred C. Bond at the Allis Chalmers test centre and accepted worldwide, the Bond Work Index is the indicator showing the energy required to perform a crushing operation.

What Wᵢ Measures

The Bond Work Index (Wᵢ) expresses, in kilowatt-hours per ton (kWh/t), the work input required to reduce a rock from a theoretically infinite particle size down to 80 % passing 100 microns. The Bond formula uses Wᵢ to estimate the actual energy required to crush a given feed to a given product size, which is then translated directly into installed motor power.

The Bond Test Setup

30 cubically shaped rocks are needed for the test. These rocks will pass through a 70 × 70 mm square mesh but will not pass through a 55 × 55 mm square mesh. The rock is placed on the anvil between two pivoting hammers; the hammers are released from increasing heights until the rock breaks. The lifting distance at the moment of breakage records the impact energy absorbed by the rock.