Seven tests, one purpose — deciding whether a given stone is fit to build a road with. This post works through each test in turn: what it does, the apparatus, the formula, and the permissible limits you need to remember.
The Seven Tests
- Crushing test
- Abrasion test
- Impact test
- Soundness test
- Shape test
- Specific gravity and water absorption test
- Bitumen adhesion test
1. Crushing Test
One of the modes in which pavement material can fail is by crushing under compressive stress. The aggregate crushing value gives a relative measure of resistance to crushing under a gradually applied crushing load.
Procedure
- Dry aggregates passing 12.5 mm and retained on 10 mm sieves are used.
- Filled into a cylindrical measure of 11.5 cm diameter and 18 cm height, in three layers.
- Each layer is tamped 25 times with a standard tamping rod.
- The sample is weighed and placed in the test cylinder in three layers, each tamped again.
- A compressive load of 40 tonnes is applied gradually at 4 tonnes per minute — so the load takes 10 minutes to reach full value.
- Crushed aggregate is sieved through a 2.36 mm sieve.
Aggregate crushing value = (W2 / W1) × 100
where W2 is the weight of material passing the 2.36 mm sieve and W1 is the total sample weight.
Limits
| Value | Interpretation |
|---|---|
| Less than 10 | Exceptionally strong aggregate |
| Above 35 | Normally regarded as weak aggregate |
| Surface course | Should be less than 30 % |
| Base course | Should not exceed 45 % |
Note the direction: the crushing value measures how much material was crushed to dust, so a lower value means a stronger aggregate. This is true of the crushing, abrasion and impact values alike — all three are “damage” measures where less is better.
2. Abrasion Test
Carried out to test the hardness property and decide suitability for different pavement construction works.
Three Types
- Los Angeles abrasion test
- Deval abrasion test
- Dorry abrasion test
The Los Angeles abrasion test is preferred and has been standardised in India as IS 2386 Part IV.
Principle
To find the percentage wear due to the relative rubbing action between the aggregate and steel balls used as abrasive charge.
Apparatus and Procedure
| Item | Specification |
|---|---|
| Drum | Circular, internal diameter 700 mm, length 520 mm, mounted on a horizontal axis |
| Abrasive charge | Cast iron spherical balls, 48 mm diameter |
| Number of balls | Varies according to the grading of the sample |
| Quantity of aggregate | 5 to 10 kg, depending on gradation |
| Rotation speed | 30 to 33 rpm |
| Number of revolutions | 500 to 1000, depending on gradation |
| Sieve after test | 1.7 mm |
The passing fraction, expressed as a percentage of the total sample weight, is the Los Angeles abrasion value.
Limits
| Application | Maximum LA Abrasion Value |
|---|---|
| WBM base course (Indian conditions) | 40 % |
| Bituminous concrete | 35 % |
| High quality pavement material | Less than 30 % |
| Base course in WBM (relaxed) | May be 50 % |
Coefficient of Hardness
Coefficient of hardness = 20 − (Loss of weight in grams) / 3
Note that this one runs the other way — more loss means a lower coefficient, so here a higher value indicates a better aggregate.
3. Impact Test
Carried out to evaluate the resistance to impact of aggregates, that is, their toughness.
Procedure
- Aggregates passing 12.5 mm and retained on 10 mm sieve are used — the same fractions as the crushing test.
- Filled into a cylindrical steel cup of internal diameter 10.2 cm and depth 5 cm, attached to the metal base of the impact testing machine.
- The material is filled in 3 layers, each tamped 25 times.
- A metal hammer weighing 13.5 to 14 kg drops through a free fall of 38.0 cm by vertical guides.
- The specimen is subjected to 15 blows.
- The crushed aggregate passes through a 2.36 mm IS sieve.
Aggregate impact value = (W2 / W1) × 100
Limits
| Application | Maximum Impact Value |
|---|---|
| Wearing course | 30 % |
| Bituminous macadam (IRC) | 40 % |
Crushing versus Impact — Easily Confused
| Crushing Test | Impact Test | |
|---|---|---|
| Measures | Strength | Toughness |
| Load type | Gradual compressive load | Sudden blows |
| Load applied | 40 tonnes at 4 t/min | 15 blows of a 13.5–14 kg hammer falling 38 cm |
| Sieve fractions | Pass 12.5 mm, retain 10 mm | Pass 12.5 mm, retain 10 mm |
| Final sieve | 2.36 mm | 2.36 mm |
| Formula | (W2/W1) × 100 | (W2/W1) × 100 |
The two tests share the same sieve sizes and the same formula. What separates them is how the load arrives — slowly in the crushing test, suddenly in the impact test.
4. Soundness Test
Intended to study the resistance of aggregates to weathering action, by conducting accelerated weathering test cycles.
Porous aggregates subjected to freezing and thawing are likely to disintegrate prematurely. Specified in IS 2386 Part V.
Procedure
- Aggregates of specified size undergo cycles of alternate wetting in a saturated solution of either sodium sulphate or magnesium sulphate for 16 to 18 hours.
- Then dried in an oven at 105 to 110 °C to constant weight.
- After five cycles, the loss in weight is determined by sieving out undersized particles and weighing.
Limits
Loss should not exceed 12 % with sodium sulphate
and 18 % with magnesium sulphate
The salt solution simulates frost. Salt crystals grow inside the pores of the stone and force it apart — exactly as ice does — but far faster, which is why the test is called accelerated.
5. Shape Test
The particle shape of an aggregate mass is determined by the percentage of flaky and elongated particles in it. Such particles are detrimental to higher workability and stability of mixes.
Flakiness Index
The percentage by weight of aggregate particles whose least dimension is less than 0.6 times their mean size.
Elongation Index
The percentage by weight of particles whose greatest dimension (length) is 1.8 times their mean dimension.
| Flakiness Index | Elongation Index | |
|---|---|---|
| Dimension considered | Least (thickness) | Greatest (length) |
| Factor | Less than 0.6 times mean size | 1.8 times mean dimension |
| Shape identified | Thin, flat particles | Long, slender particles |
Flaky is thin; elongated is long. The factors 0.6 and 1.8 are frequently swapped in exams — remember that the flakiness factor is less than one because it identifies particles that are too thin, while the elongation factor is greater than one because it identifies particles that are too long.
Angularity Number
This represents the degree of packing.
Angularity number = 67 − (W / C Ga) × 100
| Symbol | Meaning |
|---|---|
| W | Weight of aggregate in a cylinder |
| C | Weight of water in the same cylinder |
| Ga | Specific gravity of aggregate |
Understanding the 67
- 67 represents the volume of solids, in percent, of the most rounded gravels in a well compacted state — which therefore have 33 % voids.
- The angularity number therefore measures the voids in excess of 33 %.
- The higher the angularity number, the more angular the aggregate.
- The range for construction aggregates is 0 to 11.
The logic is elegant. Perfectly rounded stones pack most efficiently, leaving 33 % voids — so they score zero. Anything more angular will not pack as tightly and leaves more voids, and that excess is the angularity number.
6. Specific Gravity and Water Absorption
Specific gravity is the ratio of the mass of a solid to that of an equal volume of distilled water at a specified temperature.
Because aggregates contain water-permeable voids, two measures are used:
Apparent Specific Gravity
Computed on the basis of the net volume — that is, excluding water-permeable voids.
Gapp = (MD / VN) / W
MD = dry mass of aggregate
VN = net volume, excluding the volume of absorbed matter
W = density of water
Bulk Specific Gravity
Computed on the basis of the total volume, including water-permeable voids.
Gbulk = (MD / VB) / W
VB = total volume including the volume of absorbed water
Since VB is larger than VN, the bulk specific gravity is always lower than the apparent specific gravity.
Water Absorption
The difference between the apparent and bulk specific gravities is nothing but the water-permeable voids of the aggregate.
Their volume is measured by weighing the aggregate dry and in a saturated, surface dry condition with all permeable voids filled with water. The difference is MW.
Water absorption = (MW / MD) × 100
Typical Values
| Property | Typical Range |
|---|---|
| Specific gravity of road aggregates | 2.5 to 2.9 |
| Water absorption for road surfacing aggregates | 0.1 to about 2.0 % |
7. Bitumen Adhesion Test
- Bitumen adheres well to all normal types of road aggregates provided they are dry and free from dust.
- In the absence of water there is practically no adhesion problem. The problem occurs when the aggregate is wet and cold.
- It can be dealt with by removing moisture from the aggregate by drying and increasing the mixing temperature.
- The presence of water causes stripping of binder from coated aggregates, which occurs when the bitumen mixture is permeable to water.
Static Immersion Test
The static immersion test is specified by IRC and is quite simple. Its principle is to immerse aggregate fully coated with binder in water maintained at 40 °C for 24 hours.
IRC maximum stripping value: should not exceed 5 %
All Tests, Formulas and Limits
| Test | Formula | Key Limits |
|---|---|---|
| Crushing | (W2/W1) × 100 | < 30 % surface course; ≤ 45 % base course |
| Los Angeles abrasion | % passing 1.7 mm sieve | 40 % WBM base; 35 % bituminous concrete |
| Impact | (W2/W1) × 100 | 30 % wearing course; 40 % bituminous macadam |
| Soundness | Loss after 5 cycles | 12 % sodium sulphate; 18 % magnesium sulphate |
| Flakiness index | Least dimension < 0.6 × mean size | — |
| Elongation index | Greatest dimension = 1.8 × mean | — |
| Angularity number | 67 − (W/CGa) × 100 | Range 0 to 11 |
| Water absorption | (MW/MD) × 100 | 0.1 to 2.0 % |
| Stripping value | Static immersion, 40 °C, 24 hr | Not more than 5 % |
Quick Revision Notes
- Crushing test: 40 tonnes at 4 t/min, sieve 2.36 mm; below 10 exceptionally strong, above 35 weak.
- Los Angeles drum: 700 mm diameter, 520 mm length; balls 48 mm; 30–33 rpm; 500–1000 revolutions; sieve 1.7 mm.
- Coefficient of hardness = 20 − (loss in grams)/3.
- Impact test: cup 10.2 cm dia, 5 cm deep; hammer 13.5–14 kg, fall 38 cm, 15 blows.
- Both crushing and impact use pass 12.5 mm, retain 10 mm and a 2.36 mm final sieve.
- Soundness: 16–18 hours in sulphate solution, dried at 105–110 °C, 5 cycles; limits 12 % sodium and 18 % magnesium sulphate.
- Flakiness: least dimension < 0.6 × mean size. Elongation: greatest dimension 1.8 × mean.
- Angularity number = 67 − (W/CGa) × 100; 67 is the solid volume of the most rounded gravels, which have 33 % voids; range 0 to 11.
- Bulk specific gravity is always less than apparent specific gravity.
- Specific gravity of road aggregates 2.5 to 2.9; water absorption 0.1 to 2.0 %.
- Static immersion test: 40 °C for 24 hours; stripping value not more than 5 %.
Mistakes Students Commonly Make
- Swapping 0.6 and 1.8 between flakiness and elongation index.
- Confusing the crushing test and impact test. Both share sieve sizes and formula — the difference is gradual load versus sudden blows.
- Thinking a higher crushing, abrasion or impact value is better. All three measure damage, so lower is better.
- Forgetting that the coefficient of hardness runs the opposite way — higher is better.
- Swapping the soundness limits. Sodium sulphate 12 %, magnesium sulphate 18 %.
- Saying bulk specific gravity exceeds apparent. It is lower, because it includes void volume.
- Using the wrong final sieve. Crushing and impact use 2.36 mm; Los Angeles uses 1.7 mm.
- Forgetting that the angularity number of the most rounded gravel is zero, not 67.
Conclusion
Seven tests, each guarding one property. The crushing and impact tests look almost identical on paper but differ in how the load arrives; the Los Angeles test measures wear from rubbing; the soundness test accelerates years of weathering into five cycles of salt crystallisation; the shape tests catch particles too thin, too long or too poorly packing; specific gravity and water absorption reveal the internal pore structure; and the static immersion test checks whether bitumen will stay stuck when water arrives. Learn the numbers in groups — sieve sizes together, limits together, formulas together — and the whole set becomes manageable.
Frequently Asked Questions
What load is applied in the aggregate crushing test?
A compressive load of 40 tonnes, applied gradually at the rate of 4 tonnes per minute, so it takes 10 minutes to reach full value.
What is the aggregate crushing value formula?
Crushing value = (W2/W1) × 100, where W2 is the weight passing the 2.36 mm sieve and W1 is the total sample weight.
What crushing value is permitted for surface and base courses?
Less than 30 percent for surface course, and not exceeding 45 percent for base course.
What are the dimensions of the Los Angeles machine?
A circular drum of 700 mm internal diameter and 520 mm length, mounted on a horizontal axis.
What is the principle of the Los Angeles abrasion test?
To find the percentage wear due to the relative rubbing action between the aggregate and steel balls used as abrasive charge.
What is the difference between the crushing test and the impact test?
Both use aggregate passing 12.5 mm and retained on 10 mm and both express the result as the percentage passing a 2.36 mm sieve. The crushing test applies a gradual compressive load of 40 tonnes, while the impact test applies 15 sudden blows from a 13.5 to 14 kg hammer falling 38 cm.
What is the flakiness index?
The percentage by weight of aggregate particles whose least dimension is less than 0.6 times their mean size.
What is the elongation index?
The percentage by weight of particles whose greatest dimension is 1.8 times their mean dimension.
What does the number 67 represent in the angularity number?
The volume of solids, as a percentage, of the most rounded gravels in a well compacted state, which therefore contain 33 percent voids. The angularity number measures voids in excess of that 33 percent.
What is the difference between apparent and bulk specific gravity?
Apparent specific gravity is based on the net volume excluding water-permeable voids, while bulk specific gravity is based on the total volume including them. The difference represents the water-permeable voids.
What is the maximum stripping value permitted by IRC?
5 percent, determined by the static immersion test in which coated aggregate is immersed in water at 40 °C for 24 hours.