The aggregate crushing value test is one of the most important quality indicators for cement-stabilized crushed stone used in semi-rigid pavement base courses, directly governing the load-carrying capacity, durability, and cracking resistance of highways and airfields. This technical article explains the testing standards, sample requirements, laboratory procedures, calculation methods, and acceptance criteria for cement-stabilized crushed stone crushing value testing, providing a practical reference for engineers, laboratory technicians, and quality inspectors.
Technical Overview
Cement-stabilized crushed stone, also known as cement-stabilized macadam, is a semi-rigid base material produced by mixing graded crushed aggregate with a controlled proportion of cement and water, then compacting the mixture to high density. It is the dominant base course material for expressways, highways, and airport pavements because of its high stiffness, excellent load-spreading capacity, and strong resistance to rutting.
The aggregate crushing value (ACV) quantifies the ability of coarse aggregate to resist crushing under a gradually applied compressive load. In this test, a specified quantity of 9.5–13.2 mm aggregate is placed in a steel cylinder and loaded to 400 kN; the percentage by mass of crushed material passing a 2.36 mm sieve is reported as the crushing value. A lower result indicates stronger, more durable aggregate.
For cement-stabilized mixtures, this index is decisive. Aggregate with a high crushing value degrades during plant mixing, paving, and vibratory compaction, generating excess fines, disturbing the designed gradation, reducing the 7-day unconfined compressive strength, and increasing shrinkage cracking potential of the finished base. Consequently, the crushing value is a mandatory acceptance item in Chinese highway specifications such as JTG/T F20 (Technical Guidelines for Construction of Highway Roadbases) and JTG E42 (Test Methods of Aggregate for Highway Engineering), with equivalent requirements in international practice such as BS 812-110.
Testing Samples
Representative sampling is the precondition of a valid crushing value result. Samples are typically drawn from the following sources and delivered to accredited third-party laboratories or project site laboratories in sealed, labeled bags with full chain-of-custody documentation.
Quarry-Produced Coarse Aggregate
Crushed limestone, granite, basalt, diabase, and dolomite aggregates sampled from production belts or finished stockpiles following the standardized sampling procedure of JTG E42 (T 0301). Every new source, and every significant change in rock formation or crushing process within a source, requires retesting.
Stockpiles at the Mixing Plant
Fractional stockpiles feeding the stabilized-soil mixing plant, commonly 19–31.5 mm, 9.5–19 mm, and 4.75–9.5 mm fractions, are sampled per production lot. Because the crushing value is a property of the parent rock, the coarsest fraction is usually the most critical indicator for the entire blend.
Recycled and Reclaimed Aggregates
Recycled concrete aggregate and milled pavement materials are increasingly evaluated for reuse in stabilized subbase layers. The crushing value is a primary screening index because recycled particles often contain attached mortar that reduces resistance to crushing.
Field Verification Samples
Material extracted from constructed base layers or obtained from cores may be retested to verify that the aggregate actually placed conforms to the approved mix design and to support acceptance or dispute resolution.
Testing Items
The core test item is the aggregate crushing value of the coarse aggregate used in the cement-stabilized crushed stone mixture. In practice, it is carried out together with a suite of companion index tests that jointly define aggregate conformity for semi-rigid base construction:
- Aggregate crushing value determined in accordance with JTG E42 T 0316
- Los Angeles abrasion loss or road-stone abrasion value as complementary hardness indices
- Flakiness and elongation particle content, which influences particle breakdown during compaction
- Soundness against sodium sulfate or magnesium sulfate cycling
- Sieve analysis and 0.075 mm fines content of the composite gradation
- Apparent relative density and water absorption of the coarse aggregate
- Clay lump, deleterious substance, and soft particle content
- 7-day unconfined compressive strength of the cement-stabilized mixture as the integrated performance check
Testing Methods
Applicable Standards and Specifications
- JTG E42-2005 (T 0316-2005), Test Method for Resistance to Crushing of Coarse Aggregate, the primary method for highway engineering in China
- JTG/T F20-2015, which defines material acceptance limits for cement-stabilized roadbase construction
- GB/T 14685, the crushing index method for crushed stone used in concrete, which employs a different apparatus and is applied to building aggregates rather than highway base materials
- BS 812-110, the internationally recognized determination of aggregate crushing value, which is technically equivalent and uses a 10–14 mm test fraction
- ASTM C131 and BS EN 1097-2, Los Angeles abrasion methods commonly requested as supplementary durability indices
Test Apparatus
- Hardened steel crushing cylinder with an internal diameter of 150 mm, matched plunger, and rigid base plate
- Compression testing machine of at least 400 kN capacity with a controllable, uniform loading rate
- Standard test sieves including 9.5 mm, 13.2 mm, and 2.36 mm apertures
- Balance readable to 1 g, sample trays, stiff brushes, and a ventilated drying oven
Sample Preparation
The laboratory sample is dried and sieved, and the 9.5–13.2 mm fraction is retained for testing. Flaky, elongated, and visibly weathered particles are excluded as specified by the method so that the specimen represents the sound parent rock. The fraction is reduced by splitting to the mass required to fill the test cylinder, and duplicate specimens are prepared for parallel determination.
Test Procedure
- Fill the steel cylinder with the prepared aggregate, level the surface, and seat the plunger so that it rests evenly without tilting
- Center the complete assembly in the compression machine with the load applied axially
- Apply the load smoothly and uniformly so that the full 400 kN is reached within approximately 10 minutes
- Hold the load for about 5 seconds, then release it completely
- Discharge the specimen and sieve the entire quantity on the 2.36 mm sieve until no further fines pass
- Weigh the material passing the 2.36 mm sieve (m1) and record the original dry mass of the test specimen (m0)
Calculation and Precision
The crushing value is calculated as CV = (m1 / m0) × 100% and reported to the nearest 0.1%. Two parallel determinations are performed and averaged; if the difference between duplicates exceeds the reproducibility limit of the method, typically 1 to 2 percentage points, additional specimens are tested and the outlying result is investigated. Consistent loading rate, plunger alignment, and complete sieving are the main sources of bias and must be strictly controlled.
Acceptance Criteria and Evaluation
Under JTG/T F20-2015, the crushing value of aggregate for cement-stabilized base courses of expressways and first-class highways is commonly limited to no more than 26%, with a somewhat relaxed limit of around 30% permitted for subbase layers or lower classification roads, and stricter project-specific limits frequently imposed for extremely heavy traffic corridors. Because excessive fines generated by weak aggregate cannot be offset simply by increasing cement dosage, aggregates exceeding the specified limit are generally rejected or restricted to lower pavement layers.
Applications
Crushing value testing of aggregates for cement-stabilized crushed stone supports quality decisions at every stage of pavement infrastructure delivery:
- Source approval and routine quality control at quarries and aggregate production plants
- Mix design verification for semi-rigid base and subbase of expressways, arterial highways, and rural roads
- Lot-by-lot acceptance testing at the stabilized-soil mixing plant prior to paving
- Airfield pavement, municipal road, port yard, and heavy industrial pavement base construction
- Feasibility screening of recycled concrete and reclaimed pavement aggregates for sustainable reuse
- Failure investigation, dispute resolution, and research on long-term performance of semi-rigid pavements
FAQ
What is a good crushing value for cement-stabilized crushed stone?
For base courses of expressways and first-class highways, a crushing value of 26% or less is typically required; values below 20% indicate very strong aggregate such as basalt or high-quality granite, while values approaching the limit suggest marginal material that warrants closer control during compaction.
Why is the 9.5–13.2 mm fraction tested?
This narrow fraction provides a standardized, repeatable particle population that reflects the parent rock strength while excluding both very coarse particles, which bridge and orient inside the small cylinder, and fine particles, which would crush disproportionately. BS 812-110 uses the equivalent 10–14 mm fraction for the same reason.
Can aggregate exceeding the crushing value limit still be used?
Only where the project specification allows, typically in subbase or lower-class road layers under lighter traffic. Increasing cement content raises strength of the mixture but does not prevent particle degradation during compaction or under traffic, so it is not accepted as compensation for nonconforming aggregate.
How often should crushing value testing be performed?
Testing is required at initial source approval, whenever the aggregate source or crushing process changes, and at a defined frequency per production lot during construction, commonly one test per designated tonnage or daily production batch as stipulated by the project quality plan.
Does higher cement dosage offset a weak aggregate?
No. A high crushing value means particles continue to break down during mixing, compaction, and service, generating fines that raise shrinkage cracking risk and degrade the gradation. Specifications therefore treat aggregate strength and cement dosage as independent requirements.
