Determination of the Crushing Value of Coarse Aggregates for Cement Concrete

The crushing value test is a fundamental quality-control method used to evaluate the mechanical strength and durability of coarse aggregates in cement concrete production. By measuring the percentage of fines generated when a standardized aggregate sample is subjected to a gradually applied compressive load, engineers can reliably predict how aggregates will perform under mixing, compaction, and long-term service stresses. This article explains the testing principle, sample requirements, equipment, procedures, applicable standards, and practical applications of aggregate crushing value determination.

Technical Overview

The crushing value, also known as the aggregate crushing value (ACV) or crushing indicator, quantifies the resistance of coarse aggregate particles to crushing under a gradually applied compressive load. In the test, a prepared mass of aggregate is confined in a rigid steel cylinder and loaded at a controlled rate. The proportion of particles crushed fine enough to pass a 2.36 mm sieve is then expressed as a percentage of the original sample mass. A lower crushing value indicates stronger and tougher aggregate that can better withstand the stresses of concrete production, placement, and long-term structural service.

Because coarse aggregate typically accounts for 60 to 75 percent of the volume of structural concrete, its mechanical integrity directly influences compressive strength, wear resistance, and long-term durability. Aggregates with high crushing values tend to degrade during handling, mixing, and compaction, generating excess fines that increase water demand, reduce workability, and compromise the quality of hardened concrete. The crushing value test is therefore a routine acceptance and quality-control requirement worldwide.

• GB/T 14685 – Pebble and crushed stone for construction (Chinese national standard for concrete aggregates)
• JTG E42 (T 0316) – Test methods of aggregates for highway engineering (Chinese highway standard)
• EN 933-4 – Tests for geometrical properties of aggregates: determination of aggregate crushing value
• BS 812-110 – Methods for determination of aggregate crushing value
• ASTM C131 / C535 – Resistance to degradation by Los Angeles abrasion, frequently used as a complementary strength index

Testing Samples

Crushed Stone Aggregate

Manufactured crushed rock such as granite, basalt, limestone, and diabase is the most commonly tested material. Samples are reduced from larger field lots using recognized splitting or quartering techniques to ensure representativeness, then sieved to isolate the specified test fraction, commonly 9.5 mm to 13.2 mm for the highway method or 10 mm to 14 mm for the European method.

Natural Gravel and Pebbles

River gravel, pebbles, and other naturally rounded aggregates are tested using the same principle. Because rounded particles tend to be smoother and may contain weathered or weaker individual stones, the crushing value test is particularly valuable for identifying unsuitable deposits before they enter concrete production.

Recycled Concrete Aggregate

Crushed demolition concrete can be evaluated with the same procedure. Recycled aggregates generally exhibit higher and more variable crushing values due to adhered mortar, so specifications typically assign them more lenient acceptance limits or restrict their use to lower-grade concrete applications.

Sample Conditioning

Test portions are washed to remove dust and dried to constant mass, normally in an oven at 105 ± 5 °C, or prepared in the air-dried condition where the governing standard requires it. A typical test portion weighs approximately 3 kg of the narrowly sized fraction.

Testing Items

The primary measured parameter is the aggregate crushing value itself, which is supported by several verification checks performed during the test:

• Crushing value (crushing indicator) expressed as a percentage to the nearest 0.1%
• Verification of the gradation of the test fraction before loading
• Mass check of the specimen compacted into the cylinder
• Sieve analysis after loading to quantify the mass passing the 2.36 mm sieve
• Mass balance verification, comparing the passing fraction with the retained fraction against the original mass

Testing Methods

Apparatus and Equipment

The test requires a compact set of standardized equipment, the geometry and hardness of which are strictly controlled to ensure comparable results between laboratories:

• Hardened steel cylinder with a nominal internal diameter of 150 to 152 mm
• Fitted steel plunger and rigid base plate
• Round steel tamping rod, typically 10 mm in diameter with a rounded end
• Compression testing machine capable of applying at least 500 kN at a controlled, uniform rate
• Balanced metal measure or straightedge for leveling the surface
• Balances readable to 1 g and, if required, 0.1 g
• Test sieves including the 2.36 mm sieve and standard drying oven

Test Procedure

The standard procedure, adapted slightly among GB, EN, BS, and JTG methods, follows these essential steps:

• Fill the cylinder with the prepared aggregate in three approximately equal layers, tamping each layer 25 times with the rod.
• Level the surface, remove any surplus particles, and determine the mass of the compacted specimen.
• Place the cylinder on the base plate, insert the plunger, and ensure it sits squarely without tilting.
• Apply the compressive load at a uniform, controlled rate. The European and highway methods typically require loading to 400 kN over approximately 10 minutes, while the building aggregate method applies 200 kN at a steady specified rate.
• Hold the maximum load for the time specified, then release it completely and remove the cylinder.
• Empty the entire tested quantity onto the 2.36 mm sieve and sieve carefully, typically for about one minute or until no further particles pass.
• Weigh the mass of fines passing the 2.36 mm sieve, and record all masses to the required precision.

Calculation and Expression of Results

The crushing value is calculated as the ratio of the mass of fines passing the 2.36 mm sieve to the total mass of the test specimen, multiplied by 100. The result is reported as a percentage to the nearest 0.1%. Duplicate determinations are performed on separate portions of the same sample, and the mean value is reported. If the two results differ by more than approximately one percentage point, a third determination is required and the median value is taken, in accordance with the precision requirements of the governing standard.

Interpretation of Results

Interpretation depends on the applicable specification and the intended concrete application. As general guidance, premium aggregates for high-strength concrete commonly require crushing values below 10 to 12 percent, general structural concrete typically accepts values up to about 20 to 25 percent, and lower-grade uses such as road base may permit values approaching 30 percent. Accredited third-party laboratories compare reported values against the class limits defined in the relevant specification for the material grade.

Applications

Crushing value determination is applied across the entire concrete and infrastructure supply chain:

• Acceptance testing of coarse aggregate for building, bridge, and precast concrete production
• Quality control of aggregate used in cement concrete pavements, where surface wear resistance is critical
• Qualification of granular base and subbase materials in highway construction
• Production monitoring at quarries and crushing plants to verify rock quality and processing consistency
• Assessment of recycled concrete aggregate for permitted reuse in new construction
• Research and development of alternative and locally sourced aggregate materials

FAQ

What is a good crushing value for concrete aggregate?

Lower values indicate stronger aggregate. Values below 10 to 12 percent are considered excellent for high-strength concrete, while most structural concrete specifications accept values up to approximately 20 to 25 percent. Exact limits depend on the aggregate class and the governing standard.

What load is applied during the test?

Most highway and European methods require loading to 400 kN over approximately 10 minutes, whereas the building aggregate method applies 200 kN at a uniform specified rate. The load, rate, and test fraction must always match the governing standard.

Which sieve defines the crushed fines?

In all major crushing value standards, particles are considered crushed when they pass the 2.36 mm sieve after loading. The passing mass forms the numerator of the crushing value calculation.

How does crushing value differ from impact value and abrasion value?

The crushing value applies a slow, gradually increasing compressive load, simulating static service stress. The aggregate impact value applies rapid dynamic impact, while the Los Angeles abrasion test measures degradation from tumbling and impact. The three tests are complementary and often specified together.

Can fine aggregate be tested for crushing value?

No. The crushing value test is designed for coarse aggregate within a specified particle size fraction. Fine aggregate quality is assessed using other methods, such as sand equivalent, methylene blue, and soundness testing.

How many samples are needed for reliable results?

A representative field sample is reduced to at least two test portions, each typically around 3 kg. Duplicate determinations must agree within the precision limits of the standard; otherwise, additional portions are tested until a valid result is obtained.