The Lugeon Test: Methodology, Interpretation, and Applications in Geotechnical Engineering

AIM

To determine the solubility of a bituminous material in a specified organic solvent, typically carbon disulfide (CS2​) or trichloroethylene (C2​HCl3​). The test quantifies the percentage of pure bitumen in the sample, indicating the amount of inert mineral matter or other organic impurities present.

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THEORY

Bitumen is an organic material that is almost completely soluble in certain organic solvents. The solubility test is a fundamental purity test that distinguishes between the active binding agent (bitumen) and any non-bituminous, inert impurities like sand, clay, or other mineral contaminants. It is a critical quality control measure to ensure that the material being used is a pure hydrocarbon product and not an adulterated mix.

The principle of the test is simple: a known mass of bitumen is dissolved in a solvent. The insoluble portion, which consists of any impurities, is then separated by filtration, dried, and weighed. The percentage of soluble material is calculated by subtracting the mass of the insoluble residue from the initial mass of the bitumen. A high percentage of solubility (typically over 99%) indicates a high-purity bitumen. A low solubility value suggests the presence of unwanted contaminants, which could compromise the performance of the final product, such as hot mix asphalt. For example, inert mineral matter does not contribute to the binding properties of the bitumen, reducing the effective binder content in the mix.

The use of carbon disulfide ($C{S}_2$) is the historical standard due to its high solvent power for bitumen, but due to its toxicity and flammability, less hazardous alternatives like trichloroethylene ($C_{2}HC{l}_3$) are now commonly used.

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APPARATUS

1. Conical Flask: A glass flask (capacity 100 ml) with a ground glass stopper.
2. Filter Apparatus: A Gooch crucible (or a filter paper) with an asbestos mat (or glass fiber filter). A Gooch crucible is a small porcelain crucible with a perforated bottom. The asbestos mat is prepared by placing a fine layer of asbestos fibers on the perforated plate to act as the filter.
3. Filtration Flask: A suction flask with a side arm for vacuum filtration.
4. Vacuum Pump: To create suction for rapid filtration.
5. Desiccator: A sealed container with a desiccant to dry the filter crucible and the residue to a constant mass.
6. Hot Plate/Oven: For heating the bitumen and solvent.
7. Balance: An analytical balance capable of weighing to the nearest 0.0001 gram.
8. Stirring Rod: A glass rod for stirring the mixture.
9. Solvent: Carbon disulfide (CS2​) or Trichloroethylene (C2​HCl3​).

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PROCEDURE

1. Preparation of Filter Crucible:
• Clean a Gooch crucible and place an asbestos mat or a glass fiber filter on its perforated bottom.
• Place the crucible in an oven at 105°C for 20 minutes to dry.
• Cool the crucible in a desiccator to room temperature.
• Weigh the cooled crucible to the nearest 0.0001 gram. This is the initial mass of the crucible ($W_1$).

2. Preparation of Bitumen Sample:

• Heat the bitumen sample carefully to a fluid state.
• Weigh out approximately 2 grams of the molten bitumen into a small beaker or container to the nearest 0.0001 gram. This is the initial mass of the bitumen sample ($W_2$).
• Add about 50 ml of the solvent (CS2​ or C2​HCl3​) to the bitumen and stir until the bitumen is completely dissolved.

3. Filtration and Washing:

• Set up the vacuum filtration apparatus by placing the weighed Gooch crucible on the suction flask.
• Pour the dissolved bitumen-solvent solution through the filter crucible.
• Use a wash bottle with the solvent to wash the beaker and ensure all insoluble material is transferred to the crucible.
• Wash the residue in the crucible with fresh solvent until the filtrate (the liquid that passes through the filter) becomes colorless.

4. Drying and Final Weighing:

• Disconnect the vacuum and place the crucible with the insoluble residue in an oven at 105°C for about 30 minutes to remove all traces of the solvent.
• Cool the crucible and residue in a desiccator for about 30 minutes.
• Weigh the crucible with the dry insoluble residue to the nearest 0.0001 gram. This is the final mass ($W_3$).

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OBSERVATION/RESULTS

The percentage of insoluble material and the percentage of solubility are calculated based on the masses recorded.

Calculation:

• Mass of insoluble material = Final mass of crucible with residue (W3​) - Initial mass of empty crucible (W1​)
• Percentage of Insoluble Matter = $\frac{(W_3-W_1)}{W_2}\times 100$
• Percentage of Solubility = $100-(\text{Percentage of Insoluble Matter})$

Example Observation:

Measurement	Mass (g)
Mass of empty crucible ($W_1$)	25.4325
Mass of bitumen sample ($W_2$)	2.0560
Mass of crucible + residue ($W_3$)	25.4340
Mass of insoluble matter ($W_3-W_1$)	0.0015
Percentage of insoluble matter	$\frac{0.0015}{2.0560}\times 100=0.073\%$
Percentage of solubility	$100-0.073=99.927\%$

The reported solubility is 99.927%. For most road-paving bitumens, the minimum solubility is specified as 99.0% for grades VG-10 and VG-20, and 99.5% for VG-30 and VG-40.

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PRECAUTIONS

• The solvent is highly flammable and toxic. The test must be conducted in a well-ventilated laboratory, preferably under a fume hood.
• The bitumen must be completely dissolved in the solvent before filtration.
• The residue in the crucible must be completely dry before final weighing; otherwise, the mass will be inaccurate.
• The balance must be highly sensitive and calibrated to ensure precise mass measurements, as the insoluble residue is often very small.
• Handle the chemicals and apparatus with care and wear appropriate personal protective equipment (PPE).

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CODES/IS STANDARDS

• IS 1216:1978: Methods for Testing Tar and Bituminous Materials - Determination of Solubility in Carbon Disulphide.
• IS 9381:1979: Methods for Testing Tar and Bituminous Materials - Determination of Solubility in Trichloroethylene.
• ASTM D2042: Standard Test Method for Solubility of Asphalt Materials in Trichloroethylene.

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APPLICATIONS/USES

• Purity Check: This is a fundamental test for determining the purity and quality of a bitumen binder. A low solubility indicates contamination, which can be a sign of adulteration or poor manufacturing practices.
• Quality Control: The test is used at refineries and construction sites to ensure that the supplied bitumen meets the required specifications for purity.
• Performance Indicator: While not a direct measure of pavement performance, a low solubility value is a strong indicator of a poor-quality binder that may lead to inferior pavement durability due to reduced adhesive properties and binding efficiency.
• Material Specification: The solubility value is a key parameter in material specifications for bitumen, with a minimum percentage often prescribed to ensure a high-quality product.