Airport runways must withstand extreme concentrated loads from aircraft landing gear, with tire pressures several times higher than typical highway traffic, making the mechanical strength of coarse aggregates a critical quality parameter. Aggregate crushing value (ACV) determination quantifies an aggregate's resistance to fragmentation under a standardized compressive load and serves as a key acceptance criterion in airfield pavement engineering. This article explains the testing principle, sample requirements, procedures, applicable standards, and typical specification limits for crushing value testing of runway aggregates.
Technical Overview
The aggregate crushing value (ACV) expresses the resistance of coarse aggregate particles to crushing under a gradually applied compressive load. A prepared specimen of a specified particle-size fraction is compacted in a hardened steel cylinder, loaded to a defined force, and the mass of fines generated below a reference sieve is expressed as a percentage of the original specimen mass. A lower value indicates stronger, more crush-resistant aggregate, which is essential where particles must maintain their shape and load-transfer capability throughout the service life of the pavement.
Airport pavements are among the most demanding applications for mineral aggregates. Wide-body aircraft impose single-wheel loads that can exceed 25 tonnes, with tire pressures typically ranging from 1.2 to 1.5 MPa or higher—roughly three to four times common highway tire pressures. Combined with high-energy touchdown impacts, repeated heavy braking, and jet blast thermal cycles, weak or friable aggregate particles would progressively degrade into fines, causing rutting, raveling, loss of surface friction, and structural weakening. Crushing value testing is therefore specified during aggregate source qualification, mix design verification, and construction acceptance for runways, taxiways, and aprons.
The test is governed by several recognized standard systems, and accredited third-party laboratories routinely perform it under the following references:
- JTG E42 (T 0316), Test Methods of Aggregate for Highway Engineering, widely adopted by civil aviation pavement technical specifications for airport asphalt surfaces in China.
- GB/T 14685, Pebble and Crushed Stone for Construction, which defines a crushing index method for aggregates used in cement concrete pavements.
- BS 812-110, Testing Aggregates – Methods for Determination of Aggregate Crushing Value, applied in many international projects.
- ASTM C131 and EN 1097-2, Los Angeles abrasion methods, frequently specified as complementary degradation-resistance requirements in North American and European practice.
- Civil aviation industry standards for airport asphalt pavement design and construction, which incorporate the above aggregate test methods for airfield acceptance.
The standard apparatus consists of a steel compression cylinder with an internal diameter of 150 mm, a removable base plate, a hardened steel plunger, a steel tamping rod, a compression testing machine capable of applying the specified load accurately, a 2.36 mm test sieve, and a balance with suitable precision.
Testing Samples
Coarse Aggregates for Asphalt Surface and Binder Courses
The primary sample type is crushed stone or crushed gravel intended for hot-mix asphalt layers on runways and taxiways. Specimens are prepared from the nominal 9.5 mm to 13.2 mm fraction in accordance with T 0316, or the 10 mm to 14 mm fraction in accordance with BS 812-110. Only sound, representative particles taken from approved quarry sources are accepted, and the fraction must be washed and dried before testing.
Coarse Aggregates for Cement Concrete Pavement
For rigid runway pavements, coarse aggregate for structural concrete is evaluated using the crushing index method of GB/T 14685, typically employing particles in the 9.5 mm to 19 mm range. The result is used together with grading, cleanliness, and strength verification to qualify the aggregate for paving-grade concrete.
Granular Base and Subbase Materials
Crushed rock, graded aggregate, and cement-stabilized macadam used in pavement base and subbase layers are also tested, since particle breakdown during compaction and service reduces the structural modulus of the layer and can lead to surface distress.
Sampling and Conditioning
Field samples are taken from stockpiles, conveyor belts, or transport vehicles at frequencies defined in the project specification, then reduced by quartering or splitting to obtain representative test portions. Depending on the governing method, particles are oven-dried to constant mass at 105 ± 5 °C or brought to an air-dry condition. A typical laboratory submission comprises 15 to 20 kg of coarse aggregate to allow sieving, specimen preparation, and parallel determinations.
Testing Items
The principal measurand of this determination is:
- Aggregate crushing value (ACV), expressed as a percentage of the original specimen mass.
To provide a complete mechanical quality profile for runway aggregates, the crushing value is commonly determined together with the following companion tests:
- Los Angeles abrasion loss (ASTM C131 / EN 1097-2)
- Apparent density, bulk density, and water absorption
- Flakiness index and elongation index of particles
- Soundness under sodium sulfate or magnesium sulfate cycles
- Polished stone value for surface friction courses
Testing Methods
The general procedure for ACV determination under the steel cylinder method is as follows:
- Prepare the test portion by sieving the sample to isolate the specified size fraction and conditioning it to the required moisture state; determine the mass of aggregate needed to fill the measure by compaction in layers with the tamping rod.
- Transfer the specimen into the crushing cylinder so that the surface is level and the depth of aggregate is uniform, avoiding any segregation or vibration that could alter the packing state.
- Insert the plunger carefully, ensuring it sits horizontally on the aggregate surface without initial wedging against the cylinder wall.
- Place the assembly in the compression machine and apply the load smoothly so that the total force reaches 400 kN over a period of approximately 10 minutes.
- Hold the maximum load steady for about 5 seconds in accordance with the governing method, then release the load and remove the plunger.
- Discharge the entire specimen, sieve the complete quantity on the 2.36 mm test sieve, and weigh the material passing the sieve.
- Calculate the crushing value as the mass of passing fines divided by the original specimen mass, multiplied by 100, and report the result to the precision stated in the standard.
Two parallel determinations are normally performed on separate specimens, and the mean value is reported. The repeatability of duplicate results is typically required to agree within approximately 2 percentage points; otherwise, additional specimens must be tested. It should be noted that the crushing index method of GB/T 14685, used for concrete aggregates, applies a lower total load of 200 kN to a different specimen configuration, and results from the two methods must never be compared directly. Laboratory quality control includes regular verification of load calibration, sieve condition, and cylinder geometry to ensure traceable, reproducible results.
Applications
Crushing value determination for runway aggregates supports decision-making across the full life cycle of airfield pavements, including:
- Qualification of quarry sources and crushed stone suppliers before runway construction or overlay projects.
- Verification of hot-mix asphalt aggregate suitability for surface, binder, and friction courses on runways and taxiways.
- Acceptance testing of coarse aggregate for jointed or continuously reinforced cement concrete airfield pavements.
- Quality control of graded aggregate and cement-treated base materials during staged construction.
- Independent compliance verification by accredited third-party laboratories on behalf of airport authorities, contractors, and supervision engineers.
- Forensic investigation of pavement distress, where excessive aggregate breakdown is suspected as a contributing cause.
- Performance assessment of recycled concrete aggregate or locally available materials proposed for lower pavement layers.
FAQ
What crushing value is acceptable for airport runway aggregates?
For aggregates used in asphalt surface and binder courses of airport runways, specifications commonly require a crushing value not greater than 26%. For base and subbase materials, limits between approximately 22% and 30% are typical depending on layer function and traffic classification. For cement concrete pavement aggregates, GB/T 14685 specifies crushing index limits of 10% for Class I, 20% for Class II, and 30% for Class III crushed stone, with premium airfield projects generally demanding the stricter classes.
Why is the 2.36 mm sieve used to define the crushed fraction?
The 2.36 mm sieve marks the boundary between the original coarse particle sizes and the fines generated by crushing. Weighing only the material that passes this sieve isolates the degradation caused by the applied load, allowing a direct, reproducible measure of particle strength.
How does the crushing value test differ from the Los Angeles abrasion test?
The crushing value test applies a static compressive load and measures fragmentation resistance, while the Los Angeles abrasion test measures degradation from impact and abrasion in a rotating steel drum with abrasive charges. The two mechanisms are complementary, and many runway specifications require both results before aggregate approval.
What sample quantity should be submitted for testing?
A submission of approximately 15 to 20 kg of coarse aggregate is normally sufficient, which allows the laboratory to sieve out the required size fraction, prepare replicate specimens, and retain material for repeat testing if the precision check fails.
Can crushing value results from different standards be compared directly?
No. Differences in specimen fraction, mold configuration, applied load, and sieving endpoints mean results are method-specific. The governing standard for the project specification must be followed exactly, and accredited third-party laboratories report the standard designation alongside every result.
How does high crushing value affect runway performance?
Aggregates with excessive crushing values break down under compaction and aircraft loading, generating fines that reduce internal friction, accelerate rutting and raveling, increase dusting, and shorten maintenance intervals. Early detection through testing prevents premature pavement failure and costly rehabilitation.
