Aggregate crushing value testing is a fundamental quality control procedure for asphalt pavement construction, measuring the resistance of coarse aggregates to crushing under gradually applied compressive loads. This test directly impacts the load-bearing capacity, rutting resistance, and long-term durability of asphalt pavement surfaces. This article provides a comprehensive overview of crushing value testing standards, sampling requirements, testing procedures, and acceptance criteria for highway and road engineering projects.
Technical Overview
The aggregate crushing value (ACV) test evaluates the ability of coarse aggregates used in asphalt pavement to resist crushing under compressive loading. During the service life of an asphalt pavement, aggregates in the load-bearing skeleton are subjected to repeated traffic stresses. If the aggregate particles are weak and fracture under load, the internal structure of the mixture degrades, leading to rutting, raveling, and premature pavement failure. A lower crushing value indicates stronger, more durable aggregate that can better maintain the integrity of the asphalt mixture skeleton.
In Chinese highway engineering, the crushing value test for aggregates in asphalt pavements is performed in accordance with JTG E42-2005 (Test Methods of Aggregate for Highway Engineering), specifically test method T 0316 for the crushing value of coarse aggregate. The acceptance limits are specified in JTG F40-2004 (Technical Specification for Construction of Highway Asphalt Pavements). Comparable international references include BS 812-110 for the determination of aggregate crushing value and GB/T 14685 for construction aggregates. Typically, coarse aggregate for expressway and first-class highway surface courses must meet a crushing value of no more than 26%, while other pavement layers and lower-grade roads may permit slightly higher values, commonly up to 28% or 30% depending on the layer and road classification.
Testing Samples
Coarse Aggregate from Production Stockpiles
Samples are typically drawn from crushed stone or gravel stockpiles at the aggregate production site or the asphalt mixing plant. Representative sampling following JTG E42-2005 procedures requires collecting material from multiple locations and depths of the stockpile, then reducing the combined sample by quartering or splitting to obtain the required test mass of approximately 3,000 grams of the 9.5 mm to 13.2 mm size fraction.
Aggregates Extracted from Asphalt Mixtures
For quality verification of produced hot-mix asphalt, coarse aggregate can be recovered from loose mixture samples through extraction or ignition furnace procedures in accordance with JTG E20-2011 (Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering). The recovered aggregate is washed, dried, and sieved to obtain the specified test fraction before crushing value determination.
Aggregates from Field Cores
In acceptance testing and failure investigation, aggregates may be recovered from pavement cores drilled from the completed asphalt layer. After bitumen removal and sieve analysis, the extracted coarse aggregate is evaluated to verify whether the in-place material complies with the design crushing value requirements.
Testing Items
The core testing item is the crushing value of coarse aggregate, expressed as the percentage by mass of fines generated when a standard sample is loaded to 400 kN under controlled conditions. In a comprehensive aggregate quality inspection program for asphalt pavement, the crushing value test is commonly performed alongside the following related evaluations:
- Los Angeles abrasion loss, assessing resistance to impact and abrasive degradation
- Apparent relative density, bulk density, and water absorption of coarse aggregate
- Flat and elongated particle content, influencing mixture stability and compactability
- Soundness of aggregate against sodium sulfate or magnesium sulfate attack
- Polished stone value for surface course friction performance
- Aggregate adhesion with bitumen binder
- Soft particle content and deleterious material content
Testing Methods
The standard crushing value test is conducted on aggregate passing the 13.2 mm sieve and retained on the 9.5 mm sieve. The test specimen is placed in a steel cylindrical mold, and a uniform load is applied through a plunger at a constant rate until the total force reaches 400 kN. The crushed material is then sieved over a 2.36 mm sieve, and the mass of fines passing the sieve is expressed as a percentage of the original sample mass. The general procedure is as follows:
- Prepare the sample by sieving, washing, and drying to constant mass, then weigh approximately 3,000 g of the 9.5 mm to 13.2 mm fraction.
- Fill the steel cylinder in three layers, compacting each layer with 25 blows of the tamping rod, and level the surface of the aggregate.
- Insert the plunger, position the assembly in the compression testing machine, and apply force at a uniform rate of 1 kN per second until the total load reaches 400 kN.
- Hold the load momentarily, release it, remove the crushed aggregate, and sieve the entire specimen over a 2.36 mm sieve.
- Weigh the material passing the 2.36 mm sieve and calculate the crushing value as the ratio of passing mass to original sample mass, multiplied by 100.
- Perform duplicate determinations and report the mean value; results from parallel tests should generally not differ by more than the reproducibility limits stated in the applicable standard.
Accredited third-party laboratories perform this test using calibrated compression machines, verified molds and plungers, and certified sieves, with all equipment traceable to national metrology standards, ensuring that reported results are legally defensible for acceptance and dispute resolution purposes.
Applications
Crushing value testing for asphalt pavement aggregates supports a wide range of engineering and quality assurance activities:
- Source qualification of quarries and aggregate suppliers before large-scale procurement for asphalt paving projects
- Mix design verification for dense-graded, stone mastic asphalt, and open-graded friction course mixtures
- Routine quality control at asphalt mixing plants during production of surface, intermediate, and base asphalt layers
- Construction acceptance testing for expressways, arterial highways, urban roads, and airport pavements
- Investigation of premature pavement distress such as rutting, aggregate degradation, and raveling
- Research and development of recycled asphalt pavement applications, where reclaimed aggregate strength must be confirmed
- Tender and contractual compliance documentation for public and private road infrastructure projects
FAQ
What is a qualified crushing value for asphalt pavement coarse aggregate?
Under JTG F40-2004, coarse aggregate used in expressway and first-class highway surface courses generally requires a crushing value no greater than 26%, while lower layers and other road classes may allow up to 28% or 30%. The specific limit depends on the pavement layer, traffic loading level, and project specifications, so the governing contract documents should always be checked.
How is the crushing value different from the Los Angeles abrasion test?
The crushing value test applies a slow, static compressive load of 400 kN and measures the fines generated, reflecting resistance to crushing under overburden and traffic pressure. The Los Angeles abrasion test subjects aggregate to impact and abrasion in a rotating drum with steel balls, measuring degradation from attrition. The two tests evaluate different failure mechanisms, and asphalt specifications typically require both.
What sample size and particle fraction are required for the test?
The standard test uses aggregate passing the 13.2 mm sieve and retained on the 9.5 mm sieve, with a specimen mass of approximately 3,000 grams. Oversized and undersized particles must be removed by dry sieving before the specimen is prepared.
What causes an excessively high crushing value in aggregates?
Common causes include weak or weathered parent rock, high contents of schistose or laminated particles, excessive flat and elongated grains, micro-cracking from poor crushing operations, and contamination with soft or deleterious particles. Corrective actions involve changing the rock source, adjusting crusher settings, or applying selective screening and washing.
How frequently should crushing value testing be performed?
For continuous aggregate supply, typical quality control practice requires testing at a defined frequency such as one test per designated tonnage or per production batch, along with mandatory retesting whenever the aggregate source changes, visual abnormalities appear, or during initial qualification of a new supplier.
Can the test be performed on aggregate recovered from compacted pavement?
Yes. Cores can be extracted from the finished pavement, the bitumen can be removed by extraction solvents or an ignition furnace, and the recovered coarse aggregate can be sieved and tested. This approach is widely used in acceptance audits and failure investigations to verify that the aggregate actually incorporated into the pavement meets the specified crushing value requirements.