In building and civil engineering works, the mere quantity of fines present in soil or aggregate is not sufficient to judge its quality. It is the nature of these fines — and in particular the presence of swelling clays — that determines the risk of instability, cracking in concrete or subsidence of road sub-bases. Whilst the Sand Equivalent (SE) test quantifies overall cleanliness, the methylene blue test (soil blue value or SBV) specifically measures the clay content and the harmful effects of these components.
The test procedure (Standard NF EN 933-9 / NF P 94-068)
The test is based on the ability of clay minerals present in the sample to adsorb methylene blue. A solution of methylene blue is gradually added to a mixture of water and soil whilst the mixture is kept under constant agitation.
As long as the clays continue to absorb the dye, the solution remains bound. Once the specific surface area of the clays is completely saturated, free blue appears in solution. This is demonstrated by the filter paper spot test: a drop of the mixture taken up with a glass rod forms a central deposit surrounded by a persistent pale blue ring (or halo).
Equipment required in the laboratory
To carry out the test in accordance with best practice, the laboratory must have the following equipment:
- Mechanical paddle agitator with speed controller (400 to 600 rpm).
- Graduated burette for the precise injection of methylene blue solution.
- Ash-free filter paper (quantitative grade) for halo detection.
- Glass rod, 8 mm in diameter.
- Precision balance for weighing the test sample (0/2 mm or 0/5 mm fraction).
- Solution of methylene blue at 10 g/L.
Key steps in the operating protocol
- Sample preparation: Dry the material and isolate the fraction passing through the appropriate sieve (0/2 mm for aggregates in accordance with NF EN 933-9 or 0/50 mm, adjusted for soils).
- Suspension: Place the test sample in a beaker containing a specified volume of distilled water, then stir for at least 5 minutes.
- Initial dose and stepwise titration: Inject an initial dose of blue (e.g. 5 mL or 10 mL), shake for 1 minute, then take a drop with the glass rod and place it on the filter paper.
- Identifying the halo: If the central spot is surrounded by a colourless, moist ring, reinject a dose of blue (5 mL). Continue the procedure until a persistent pale blue halo appears around the central deposit.
- Confirmation that the reaction has ended: Once the halo has appeared, continue stirring without adding any more blue and repeat the test on filter paper every minute for 5 minutes. If the halo disappears, the clay was not completely saturated: add a further 2 to 5 mL of blue. Saturation is confirmed when the halo remains visible for 5 minutes in a row.
Interpreting the results and pitfalls to avoid
The blue value (VBS or MB) is expressed in grams of blue per kilogram of dry material.
- VBS < 0.2: Soil or aggregate that is impervious to water and very clean (ideal for concrete or sub-bases).
- 0.2 < VBS < 1.5: Moderately clayey soil, moderate sensitivity to water.
- VBS > 2.5: Soil that is very clayey and highly susceptible to water, requiring treatment (lime/hydraulic binders) or removal.
Common mistakes on site and in the laboratory:
- Inadequately mixed: If the mixing speed is too low, not all of the fine clay particles trapped within the granular matrix are released.
- Confusing moisture with the halo: The halo must be distinctly tinted with a light blue colour on the filter paper, and not simply a wet watermark.
- Use of tap water: The ions present in non-distilled water may affect the chemical reaction of methylene blue.
Category of test equipment: Essai au Bleu de Méthylène (VBS) - CTRMAT
Complete test kit: Kit Essai au Bleu de Méthylène (VBS) - NF EN 933-9
Consumable reagent: Bleu de Méthylène en Poudre pour Essais de Laboratoire