Aggregate Crushing Value Testing Process: Standards, Equipment and Step-by-Step Procedure

The aggregate crushing value (ACV) test measures the resistance of coarse aggregates to crushing under a gradually applied compressive load, making it one of the most important mechanical quality indicators in road construction and concrete production. This article provides a complete technical guide to the crushing value testing process, covering applicable standards, sample preparation, apparatus requirements, step-by-step procedures, calculation methods, and acceptance criteria. Understanding this standardized process helps engineers and quality inspectors verify that aggregates meet the strength requirements specified by national and international standards.

Technical Overview

The aggregate crushing value (ACV) expresses the ratio, as a percentage, of the mass of fines produced by crushing a prepared aggregate sample to the original mass of the sample. A lower crushing value indicates a stronger, harder aggregate with greater resistance to degradation under load, while a higher value signals weaker material that may break down during compaction, trafficking, or service. The test is widely specified for coarse aggregates used in asphalt pavements, cement concrete, unbound road bases, and railway construction.

The core testing standards governing this process include GB/T 14685 (construction aggregates in China), JTG E42 (test methods of aggregates for highway engineering, method T 0316), BS 812-110 (British method for determination of aggregate crushing value), and related EN specifications. Although minor details such as cylinder dimensions, particle-size fractions, and loading rates differ between standards, the fundamental principle is identical: a confined sample is loaded to a specified force, and the resulting mass of crushed fines is measured and expressed as a percentage of the original mass.

Accredited third-party laboratories typically report the crushing value as the mean of duplicate determinations, with strict rules for sample preparation, sieve fractions, tamping procedure, and loading rate to guarantee repeatability and comparability between laboratories.

Testing Samples

Crushed Stone and Gravel for Concrete

Natural gravel, crushed stone, and crushed pebble intended for use as coarse aggregate in structural concrete are tested to confirm compliance with classification limits such as those in GB/T 14685. Samples are typically taken from stockpiles or delivery trucks using quartering or riffle-splitting techniques to ensure representativeness, then washed, dried, and reduced to the required test fraction before testing.

Coarse Aggregate for Asphalt Pavement Mixtures

Aggregates used in hot-mix asphalt surface courses and binder courses must resist crushing during compaction and traffic loading. Highway specifications commonly limit the crushing value of surface-course aggregates to approximately 26% or lower for expressways and Class 1 roads, with slightly relaxed limits permitted for lower-grade pavements.

Unbound Base and Subbase Materials

Cement-stabilized, lime-stabilized, and granular base materials rely on aggregate interlock for strength. Because crushing during roller compaction reduces stability and increases fines content, base and subbase aggregates are routinely screened for crushing value before approval for use.

Railway Ballast and Specialty Aggregates

Railway ballast, riprap, and aggregates for heavy-duty industrial floors are subjected to repeated concentrated loads. Crushing value testing, often combined with Los Angeles abrasion and impact testing, provides a reliable indication of long-term durability under cyclic loading conditions.

Testing Items

The crushing value test family covers several related determinations, depending on the specification invoked:

• Aggregate crushing value (ACV) of a single particle-size fraction, reported as a percentage to one decimal place.
• Crushing value for multiple size fractions when the supplied aggregate spans a wide grading range, requiring weighted or separate determination for each fraction.
• Ten percent fines value (TFV), a complementary test that determines the load required to produce 10% fines, often specified for weaker or marginal aggregates.
• Comparative evaluation against classification limits, for example the category I, II, and III limits defined in GB/T 14685.

Testing Methods

Apparatus and Equipment

A complete crushing value test setup consists of the following items, all of which must be calibrated and verified at defined intervals:

• A hardened steel cylinder with an internal diameter of 150 mm (or 75 mm for small-scale variants), fitted with a base plate and a removable tamping rod guide collar.
• A steel plunger or piston that transmits the compressive load onto the aggregate surface.
• A compression testing machine with a capacity of at least 500 kN and a controlled loading rate, typically achieving the target force within the time specified by the standard.
• A metal tamping rod with a rounded end for compacting the sample in layers.
• Test sieves of the designated aperture sizes, generally including 13.2 mm, 9.5 mm, and 2.36 mm sieves for the standard fraction.
• A balance accurate to 0.1% of the sample mass and a drying oven set at 105 ± 5 °C.

Sample Preparation

The laboratory sample is air-dried or oven-dried, then sieved to isolate the specified particle-size fraction, commonly 9.5 mm to 13.2 mm for highway aggregate testing. The mass of the test portion is determined so that, after compaction in three approximately equal layers with 25 tamping blows per layer, the depth of the aggregate in the cylinder reaches the level indicated on the inside of the mold. Excess material is removed with a straightedge so the surface is flush with the top of the cylinder, and the filled cylinder is then weighed.

Step-by-Step Test Procedure

• Place the filled cylinder, with the plunger positioned on the aggregate surface, centrally on the compression machine platen.
• Apply the load uniformly at the specified rate, commonly around 1 kN per second, until the total force reaches the standard requirement, typically 400 kN, maintained for approximately 5 seconds before release.
• Remove the crushed sample from the cylinder and sieve it over the 2.36 mm sieve, taking care to avoid additional mechanical degradation during handling.
• Weigh the material passing the 2.36 mm sieve to the nearest gram, and record both the original mass and the fines mass.
• Repeat the entire determination on a second prepared test portion from the same laboratory sample.

Calculation and Expression of Results

The aggregate crushing value is calculated as the mass of fines passing the 2.36 mm sieve divided by the original mass of the test portion, multiplied by 100, and reported to one decimal place. Where duplicate determinations are performed, the result is expressed as the mean value, provided the difference between the two determinations does not exceed the repeatability limit stated in the governing standard; otherwise, additional determinations are required. The final value is compared directly against the specification limit applicable to the intended end use.

Quality Control and Influencing Factors

Key factors affecting accuracy include particle-size distribution of the test fraction, moisture condition of the sample, uniformity of tamping, alignment of the plunger, loading rate control, and sieve condition. Laboratories must control these variables through documented operating procedures, regular apparatus verification, and participation in interlaboratory comparison programs to maintain valid accreditation.

Applications

Crushing value data directly support engineering decisions across multiple sectors:

• Highway and airport pavement design, where surface-course aggregates with low crushing values preserve skid resistance and texture depth over the service life.
• Ready-mixed and precast concrete production, where strong aggregates ensure the durability of structural elements.
• Unbound and stabilized base construction, where maintaining the gradation of crushed aggregates is essential for achieving design density and stiffness.
• Railway engineering, where ballast integrity under repeated axle loads governs track geometry stability.
• Quarry and mine product certification, where crushing value testing forms part of routine factory production control and third-party product conformity verification.

FAQ

What is a typical acceptance limit for aggregate crushing value?

Acceptance limits depend on the specification and end use. For concrete aggregates under GB/T 14685, category I materials are generally limited to 16%, category II to 20%, and category III to 30%. For asphalt surface courses on expressways, a limit of 26% is commonly applied, while base and subbase materials may be permitted slightly higher values.

Why is the 2.36 mm sieve used to define crushed fines?

The 2.36 mm aperture separates the fines generated by crushing from particles that essentially retain their original shape and strength. The proportion passing this sieve therefore quantifies the actual crushing degradation caused by the applied load, giving a direct and reproducible measure of aggregate toughness.

How is the crushing value test different from the Los Angeles abrasion test?

The crushing value test applies a static, gradually increasing compressive load and measures fines generation, whereas the Los Angeles abrasion test uses impact and abrasion forces from rotating steel balls. The two tests assess different degradation mechanisms and are frequently specified together for a comprehensive mechanical evaluation.

How many test portions are required for a valid result?

Most standards require at least two determinations from the same laboratory sample, with the reported value being the mean, subject to a repeatability check. If the two results differ beyond the permitted tolerance, the test must be repeated on fresh portions.

How should samples be taken and transported for testing?

Samples should be taken using recognized sampling procedures such as quartering or riffle splitting to obtain a representative portion, sealed in clean containers to prevent contamination and moisture change, labeled with full identification details, and delivered promptly to an accredited third-party laboratory together with the required test specification.