The presence of clayey impurities or plastic fines in fine aggregates severely compromises binder-aggregate adhesion in asphalt mixtures and exponentially increases the water/cement ratio in structural concrete. The Sand Equivalent (SE) test is the most effective rapid quality control test to quantify the relative proportion of harmful dust and colloidal elements in the fine fraction. This article analyzes the standardized experimental procedure, calculation equations, main sources of laboratory error, and practical application in material acceptance.
1. What is the test and what problem does it solve?
The Sand Equivalent test expresses, as a percentage, the relative proportion of the volume occupied by sandy elements (clean granular particles) relative to the total volume occupied by the suspension of flocculated soil or fine aggregate.
In the preparation of cement concrete and asphalt mixtures, the fine aggregate's gradation alone does not distinguish whether particles passing the 0.063 mm (or No. 200) sieve are harmless crushing dust (quartz/limestone) or highly detrimental expansive clays. The Sand Equivalent test solves the problem of evaluating the chemical and physical quality of the fine fraction, allowing the detection of clay contamination before its application in the manufacture of concrete or paving layers.
2. What is it used for and when should it be applied?
The determination of the SE value is a normative requirement in the following situations:
• Reception and technological control of river sands and crushed sands for structural and precast concrete.
• Evaluation of the suitability of fine aggregates for the formulation of asphalt mixtures (asphalt concrete, asphalt macadam, and cold micro-surfacing).
• Quality control of unbound crushed aggregate sub-bases and bases (ABNT) in road infrastructure.
• Periodic inspection in concrete plants and asphalt plants to identify contamination by overburden soils during the crushing process.
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3. Reference standards
The test must be carried out strictly in accordance with the normative reference adopted by the project:
• NP EN 933-8: Tests for geometrical properties of aggregates - Part 8: Assessment of fines - Sand equivalent test.
• ASTM D2419: Standard Test Method for Sand Equivalent Value of Soils and Fine Aggregate.
• AASHTO T 176: Standard Method of Test for Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalent Test.
• LNEC E 199: Aggregates - Determination of sand equivalent.
• DNIT 054/2004 - ME: Fine aggregate - Determination of sand equivalent.
4. Sample preparation
Sampling and material preparation determine the reproducibility of the test:
- Collection of a representative sample of the fine aggregate batch by manual quartering or using a riffle splitter.
- Sieving the sample through a 4.75 mm (ASTM No. 4) or 5.0 mm (according to NP EN 933-8) sieve, discarding the retained fraction.
- Homogenization of the material passing the sieve and reduction until at least two test aliquots with a standardized volume of approximately 120 cm³ each are obtained (using the calibrated measuring cup).
- Sample moisture control: the test should preferably be performed at natural moisture content or after air drying; drying at temperatures above 60 °C should be avoided to prevent altering the expansion properties of clays.
5. Equipment and materials
Equipment must comply with the dimensions and tolerances stipulated in the standards:
• Transparent cylindrical graduated cylinders made of acrylic or glass, with an engraved scale and an internal diameter of 32 mm ± 0.5 mm.
• Concentrated working wash solution (composed of anhydrous calcium chloride, purified glycerin, and formaldehyde or 40% formalin as a preservative) diluted in the standardized proportion with distilled water.
• Complete wash siphon with a rigid stainless steel wash tube and an internal diameter of 1.5 mm.
• Mechanical cylinder shaker calibrated to operate at a frequency of 175 ± 2 cycles per minute with a stroke of 203 ± 1.0 mm (or standardized manual shaking of 90 cycles in 30 seconds).
• Weighted measuring piston with a calibrated mass of 1 kg ± 5 g and a perforated seating disc at the base.
• Volumetric measuring cup of 120 cm³, wide-mouth funnel, and stopwatch with 1-second accuracy.
6. Step-by-step procedure
The operational sequence for performing the test is described below:
- Siphon the diluted wash solution into the acrylic cylinder until it reaches the lower graduated mark (100 mm ± 2.5 mm).
- Pour the fine aggregate aliquot (120 cm³) into the cylinder with the aid of the wide-mouth funnel. Gently tap the base of the cylinder with the palm of the hand to expel trapped air and loosen the material.
- Leave the cylinder at rest in a vertical position for 10 minutes ± 1 minute to ensure complete wetting of the sample.
- Close the cylinder with the rubber stopper and secure it in the mechanical shaker. Perform continuous shaking for 30 seconds ± 1 second (corresponding to 90 complete cycles).
- Remove the cylinder from the shaker, remove the stopper, and lower the wash tube inside. Wash the internal walls of the cylinder by lowering the tube to the bottom, injecting the wash solution with turbulence to suspend the fines.
- Raise the cylinder, keeping the liquid level at the upper graduated mark (380 mm ± 2.5 mm). Remove the wash tube without interrupting the flow until the final level is reached.
- Leave the cylinder in absolute rest on a vibration-free bench for exactly 20 minutes ± 15 seconds.
- After the resting time, visually read the height of the top of the clay flocculate (h1) on the cylinder scale.
- Gently lower the weighted measuring piston inside the cylinder until its seating base rests on the settled sand. Read the height at the top of the piston next to the indicator ring (h2).
7. Calculations and results processing
The calculation of the Sand Equivalent (SE) value is expressed as a percentage by the following formula:
SE = (h2 / h1) * 100 (%)
Where:
• h1 = Height of the top of the flocculated suspension (upper level of the clay sediment), measured visually in mm.
• h2 = Height of the top of the pure sand sediment, measured with the weighted piston rod in mm.
Rounding and presentation rule:
The result is rounded to the next whole number (or to one decimal place depending on the standard reference) and reported as the average of at least two parallel determinations. If the difference between the two tests exceeds 4 percentage units, the test must be repeated with new aliquots.
8. Interpretation of results
The obtained SE value classifies the purity of the fine aggregate according to the normative project specifications:
• SE < 50%: Fine aggregate with high clay content and plastic fines. Unsuitable for structural concrete and asphalt mixtures; high risk of shrinkage cracking and loss of adhesion.
• 50% <= SE < 70%: Aggregate of medium purity. Acceptable for leveling concrete, non-structural mortars, or fills, but subject to restrictions in high-grade mixtures.
• 70% <= SE < 80%: Good quality fine aggregate. Meets standard requirements for reinforced concrete and crushed aggregate road sub-bases.
• SE >= 80%: Excellent quality fine aggregate, practically free of plastic impurities. Ideal for high-strength concrete (HPC) and heavy-traffic asphalt wearing courses.
9. Common errors and how to avoid them
Operational factors that frequently cause distortions in SE values:
• Wash solution temperature outside the range (20 °C to 23 °C): Higher temperatures increase the flocculation rate and alter height h1. Solution: Keep the solution reservoir in a climate-controlled room.
• Bench subject to vibration during the 20 minutes of rest: Vibrations caused by centrifuges or sieves cause premature settling of the flocculate. Solution: Provide an isolated anti-vibration bench for cylinder rest.
• Error in manual shaking: Shaking with amplitude or speed outside the standard alters the disaggregation of clays. Solution: Preferably use a calibrated mechanical shaker.
• Abrupt lowering of the measuring piston: Dropping the piston with impact penetrates the sand layer and provides a falsely low h2 reading. Solution: Lower the piston extremely gently until it rests on the sand.
• Wash solution expired or without preservative: Bacterial proliferation or glycerin degradation alters viscosity and flocculating power. Solution: Prepare fresh working solutions and respect the concentrate's validity.
10. QA/QC and traceability
Within the scope of the laboratory quality management system (ISO/IEC 17025):
• Dimensional calibration of the acrylic cylinder: Verify the internal diameter and linearity of the engraved markings annually.
• Verification of the measuring piston mass: Ensure the piston assembly weighs exactly 1 kg ± 5 g.
• Solution quality control: Verify the pH and density of the concentrated and diluted solution.
• Batch traceability: Attach to the record sheet the solution temperature, concentrate batch code, shaking time, and individual h1 and h2 readings signed by the laboratory technician.
11. Practical application on site
The SE value acts as a direct criterion for aggregate acceptance or rejection:
• In ready-mix concrete plants, an abrupt drop in the SE value signals an increase in mixing water demand, which would necessitate an increase in superplasticizer dosage or a reduction in 28-day mechanical strength.
• In asphalt paving works, an SE value lower than specified (usually min SE = 55% to 65%) causes stripping, where the bitumen film detaches from the aggregate due to the presence of clay.
12. Limitations and points requiring confirmation
Methodological limitations requiring attention from the responsible technician:
• The Sand Equivalent test measures the amount of flocculate but does not identify the mineralogy of the clays (e.g., montmorillonite vs. kaolinite). In case of doubt about the harmfulness of fines, the Methylene Blue test (NP EN 933-9) should be performed.
• For marine sands or industrial wash sands, the presence of soluble salts can interfere with the action of the wash solution; the sample's conductivity should be confirmed.
• SE specification limits vary depending on the Project Specifications and the traffic class or exposure of the structure.
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