Crushing Value Evaluation of Crushed Stone for Pavement Subbase Layers

Crushing value evaluation measures the resistance of subbase crushed stone to degradation under compressive loading, providing essential quality data for expressway, highway, and general pavement construction projects. Accredited third-party laboratories perform this testing in accordance with JTG E42, GB/T 14685, and BS 812 standards to verify that subbase aggregates meet specified acceptance limits. This article explains the sample types, test items, testing methods, quality criteria, and practical applications of crushing value assessment for subbase crushed stone.

Technical Overview

The aggregate crushing value (ACV) quantifies the ability of coarse aggregate particles to resist crushing when a compressive load is applied at a uniform, controlled rate. The test result is expressed as the percentage by mass of fines produced when a standardized test portion of crushed stone is loaded in a steel cylinder, with lower values indicating stronger, more crush-resistant particles.

In pavement engineering, the subbase layer sits directly beneath the base course and distributes traffic-induced stresses to the subgrade. If the crushed stone in this layer degrades under repeated loading, fines accumulate, the gradation migrates, drainage capacity declines, and permanent deformation or rutting may develop prematurely. Crushing value evaluation is therefore a core acceptance test for subbase materials, including cement-stabilized macadam, lime-fly ash stabilized aggregate, and unbound granular subbase.

• Primary standards: JTG E42-2005 (T 0316) for highway aggregates, GB/T 14685 for construction crushed stone, and BS 812-110 for the equivalent international method.
• Acceptance specifications: JTG/T F20-2015 (Technical Guidelines for Construction of Highway Roadbases) and related design specifications define the crushing value limits for base and subbase materials.
• Related degradation tests: Los Angeles abrasion (ASTM C131, EN 1097-2, JTG E42 T 0317) and the ten percent fines value (BS 812-111) are often performed alongside the ACV test.

Testing Samples

Natural Crushed Stone

Crushed hard rock such as limestone, granite, basalt, diabase, and dolomite quarried and processed for subbase construction. Samples are typically taken from stockpiles, conveyor belts, or loaded trucks at a frequency defined by the project quality plan, and reduced by quartering to obtain representative test portions.

Crushed and Screened Gravel

Natural gravels that have been crushed and screened to improve particle shape and angularity. These materials are widely used in secondary and lower-class road subbase, and crushing value testing verifies that the crushing and screening process has produced adequately resistant particles.

Recycled Concrete and Construction Waste Aggregate

Processed demolition concrete and masonry are increasingly used in subbase layers. Because adhered mortar raises the crushing sensitivity of recycled aggregate, ACV testing is a decisive screening test for determining whether a recycled source is suitable for the target pavement class.

Aggregate Constituents of Stabilized Mixtures

For cement-stabilized macadam or lime-fly ash stabilized subbase, the coarse aggregate fraction is sampled at the mixing plant or from the blended mixture before binder hydration, then washed, dried, and sieved to extract the test size fraction.

Sample preparation for all material types follows the same principle: the material is washed, oven-dried, and sieved to isolate the required size fraction, and a minimum of three parallel test portions is prepared so that the reported result is the mean of replicate determinations. A composite sample of roughly 20 to 50 kg per lot is normally sufficient for the crushing value test and the associated particle-size verification.

Testing Items

The core test item is the determination of the aggregate crushing value of the 9.5 mm to 13.2 mm size fraction, reported to 0.1 percent. Depending on the project specification, the evaluation is usually packaged with supporting index tests that together characterize the mechanical quality of the subbase aggregate.

• Aggregate crushing value (ACV) determination in accordance with JTG E42 T 0316 or BS 812-110.
• Crushing index for building-grade crushed stone in accordance with GB/T 14685.
• Ten percent fines value (TFV) as an alternative strength index per BS 812-111.
• Los Angeles abrasion loss (ASTM C131, EN 1097-2, JTG E42 T 0317) for complementary degradation assessment.
• Supporting characterization: particle size distribution, flakiness and elongation index, crushed particle content, water absorption, and apparent density.

Testing Methods

Apparatus

The test uses a rigid steel cylinder with an internal diameter of 150 mm, a steel plunger, a base plate, a metal tamping rod, a compression testing machine of adequate capacity, and calibrated test sieves including a 2.36 mm sieve. The apparatus must be verified and maintained under a documented quality management system.

Procedure

• Prepare approximately 3,000 g of clean, dry aggregate retained between the 9.5 mm and 13.2 mm sieves for the highway method of JTG E42 T 0316.
• Fill the cylinder with the test portion, level the surface, and position the plunger so that it rests squarely on the aggregate without wedging.
• Apply the load uniformly so that a total force of 400 kN is reached in about 10 minutes; hold the load for approximately 5 seconds, then release it.
• Remove the crushed material and sieve it on the 2.36 mm sieve until no further significant passing occurs.
• Weigh the fraction passing the 2.36 mm sieve and the total specimen mass, and calculate the crushing value as the percentage of fines by total mass.

In the building aggregate method of GB/T 14685, a 9.5 mm to 19.0 mm fraction is loaded to approximately 200 kN, and the crushing index is classified into Grade I, Grade II, and Grade III categories. In BS 812-110, the standard fraction is loaded to 400 kN within 10 minutes, and the result is calculated in the same manner as the highway method.

Calculation, Precision, and Reporting

Each determination is calculated and the mean of the parallel determinations is reported. If the difference between replicates exceeds the repeatability allowance of the applicable standard, the test is repeated. The laboratory report identifies the material source, sampling location, size fraction tested, individual and mean results, the standard applied, apparatus calibration status, and any deviations from the standard procedure.

Acceptance Criteria

• Expressway and first-class highway subbase: crushing value not greater than 30 percent.
• Expressway and first-class highway base course: crushing value not greater than 26 percent.
• Secondary and lower-class highway subbase: crushing value not greater than 35 percent.
• Secondary and lower-class highway base course: crushing value not greater than 30 percent.

These limits, specified in JTG/T F20-2015 for inorganic binder stabilized materials, may be tightened by individual project specifications, particularly under heavy traffic or in regions with severe frost or moisture conditions. Test results that exceed the limit indicate weak, weathered, or excessively flaky aggregate and typically require changing the material source, adjusting the crushing process, or redesigning the mixture.

Applications

• Source qualification of quarries and aggregate suppliers before large-scale subbase construction begins.
• Incoming inspection at mixing plants producing cement-stabilized macadam or lime-fly ash stabilized subbase mixtures.
• Construction supervision and final acceptance testing for expressway, first-class, and secondary highway projects.
• Quality verification of unbound granular subbase for rural roads, airport pavement lower layers, port yards, container terminals, and heavy industrial floors.
• Feasibility assessment of recycled concrete aggregate and industrial by-product aggregates for use in lower pavement layers.
• Forensic investigation of premature subbase rutting, gradation migration, or excessive fines generation during construction and service.
• Research and development of new subbase materials and mixture designs in accredited third-party laboratories and academic institutions.

FAQ

What does the crushing value indicate about subbase performance?

The crushing value indicates how readily aggregate particles break down under compressive loading. A lower value means the crushed stone retains its particle integrity under construction compaction and traffic stress, preserving the interlocking skeleton, load-spreading capacity, and permeability of the subbase layer.

What crushing value limit applies to subbase crushed stone?

Under JTG/T F20-2015, the crushing value for expressway and first-class highway subbase must not exceed 30 percent, while secondary and lower-class highway subbase must not exceed 35 percent. Base course limits are stricter at 26 percent and 30 percent respectively. Project-specific specifications always take precedence.

How much sample is required for the test?

The test itself consumes roughly 3 kg of the 9.5 mm to 13.2 mm fraction for each determination, so three parallel tests require about 9 to 10 kg of sized material. In practice, a composite sample of 20 to 50 kg per lot is collected to allow quartering, moisture determination, and confirmatory retests.

What factors can affect the test result?

Flaky and elongated particles, weathered rock, pre-existing microcracks, excessive fines contamination, incorrect loading rate, plunger misalignment, and sieve calibration errors can all bias the result. Strict adherence to sample preparation and loading procedure is essential for reproducible data.

How does the crushing value differ from the Los Angeles abrasion value?

The crushing value measures resistance to compressive crushing and fines generation, which directly reflects behavior under static and slow rolling loads in a subbase. The Los Angeles test measures degradation from impact and abrasion in a rotating drum. The two tests evaluate different failure mechanisms and are frequently specified together for comprehensive aggregate quality control.

Can recycled concrete aggregate meet subbase crushing value requirements?

High-quality processed recycled concrete can satisfy the 35 percent limit for lower-class highway subbase, and carefully processed material can sometimes approach the 30 percent level. Testing each source is mandatory, and recycled aggregate is more commonly approved for lower pavement layers than for base courses of high-class highways.