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

AIM

To determine the loss in mass of a bituminous material due to the evaporation of volatile components when heated under specified conditions. This test simulates the hardening and aging of bitumen that occurs during the hot mixing process in an asphalt plant. The test also provides a residue that can be used for further tests, such as penetration or viscosity, to evaluate the change in material properties.

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THEORY

When bitumen is heated during the production of hot mix asphalt, it is exposed to high temperatures and air. This exposure causes certain volatile components to evaporate, and the bitumen undergoes oxidation. Both of these effects lead to a change in the physical properties of the binder, primarily causing it to harden and become more brittle. This process is known as short-term aging.

The Loss on Heating Test is an empirical method that quantifies this change by measuring the percentage of weight loss of a bitumen sample after being subjected to a controlled heating process. A low loss on heating indicates that the bitumen is relatively stable and has a low content of volatile compounds. This is desirable as it suggests the material will not harden excessively during the mixing and paving process, which could lead to premature pavement failure due to cracking.

The test is performed in a Thin Film Oven (TFO), where a thin film of bitumen (approximately 5 mm deep) is heated in a controlled oven at a constant temperature for a specific duration. The rotation of the shelf ensures that the entire sample is exposed to uniform heating.

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APPARATUS

1. Thin Film Oven: A thermostatically controlled oven with a rotating perforated shelf. The oven must maintain a constant temperature of 163 ± 1°C throughout the test. The shelf must rotate at a speed of 5 to 6 revolutions per minute (rpm).
2. Sample Containers: Cylindrical, flat-bottomed metal or glass containers with a diameter of 55 mm and a depth of 35 mm.
3. Thermometer: An ASTM Loss on Heat thermometer, with a range of 155°C to 170°C and a precision of 0.5°C. The thermometer is typically mounted to the oven's rotating shelf.
4. Balance: An analytical balance capable of weighing to the nearest 0.001 gram.
5. Heating Plate/Oven: For softening the bitumen sample.

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PROCEDURE

1. Sample Preparation:
• Heat the bitumen sample to a fluid consistency, typically at least 75°C to 100°C above its approximate softening point. Do not heat to a temperature that may cause premature volatilization.
• Stir the sample thoroughly to ensure a homogeneous mixture and to remove any entrapped air bubbles.
• Weigh the empty sample container to the nearest 0.001 gram.
• Pour approximately 50 ± 0.5 grams of the molten bitumen into the container.
• Allow the sample to cool to room temperature, then weigh the container with the bitumen sample to the nearest 0.001 gram. This is the initial mass ($W_1$).
2. Heating:
• Pre-heat the thin film oven to a temperature of 163°C.
• Place the container with the weighed bitumen sample on the rotating shelf of the oven.
• Close the oven door and start the shelf rotation.
• Maintain the temperature at 163 ± 1°C for a duration of 5 hours. The timer begins once the oven temperature has recovered to 162°C after the sample has been placed inside.
3. Final Weighing:
• After 5 hours, remove the container from the oven.
• Allow the container and the heated bitumen residue to cool to room temperature.
• Weigh the container with the residue to the nearest 0.001 gram. This is the final mass ($W_2$).
4. Calculation:
• Calculate the mass loss as a percentage of the original sample mass.
• The test should be performed in duplicate. If the results differ by more than a specified limit, the test should be repeated.

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OBSERVATION/RESULTS

The loss on heating is expressed as a percentage of the original mass. The average of the duplicate test results is reported.

Calculation:

$$\text{Loss on Heating, % by mass} = \frac{(W_1 - W_2)}{W_1} \times 100$$

Where:

• W1​ = Initial mass of the bitumen sample (g)
• W2​ = Final mass of the bitumen sample after heating (g)

Example Observation:

Sample No.	Initial Mass ($W_1$)	Final Mass ($W_2$)	Mass Loss ($W_1-W_2$)	% Loss
1	50.125 g	49.850 g	0.275 g	0.548%
2	50.110 g	49.835 g	0.275 g	0.549%
Average Loss				0.5485%

The reported loss on heating is 0.549%. According to standards, this value is typically limited to a maximum of 1% for most grades of bitumen.

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PRECAUTIONS

• The oven temperature must be maintained with high precision.
• The sample must be weighed accurately using a calibrated balance.
• Ensure no water is present in the bitumen sample before testing, as it can cause a significant and inaccurate mass loss.
• The oven door should be opened for the minimum time possible to avoid temperature fluctuations.

• The rotating shelf must operate smoothly at the specified speed.

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CODES/IS STANDARDS

• IS 9382:1979: Methods for testing tar and bituminous materials - Determination of effect of heat and air by thin film oven test.
• ASTM D6: Standard Test Method for Loss on Heating of Oil and Asphaltic Compounds.
• ASTM D1754: Standard Test Method for Effect of Heat and Air on Asphaltic Materials (Thin-Film Oven Test).

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APPLICATIONS/USES

• Quality Control: The Loss on Heating Test is a critical quality control test for bitumen. It helps determine if the material meets specifications related to its volatility.
• Predicting Pavement Performance: The test simulates the short-term aging of bitumen during the hot mix asphalt production. A high percentage of mass loss and corresponding hardening can indicate a bitumen that is prone to becoming brittle, which can lead to premature cracking in the pavement.
• Material Specification: It is a key parameter in specifying the properties of different grades of bitumen, especially in modern performance-graded systems where the properties of the aged binder are as important as those of the original binder.

• Comparative Analysis: The residue from this test is often used for subsequent tests (e.g., penetration, viscosity) to quantify the change in properties. A high percentage change in penetration after the test indicates that the bitumen is highly susceptible to hardening.