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

AIM

To determine the viscosity of a bituminous material at specified temperatures, which is a measure of its resistance to flow. The viscosity value is a critical parameter for classifying bitumen into viscosity grades and for determining the appropriate temperatures for mixing, laying, and compaction of hot mix asphalt.

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THEORY

Viscosity is the internal friction or resistance to flow of a fluid. For bitumen, a viscoelastic material, viscosity is highly dependent on temperature. The viscosity test provides a more fundamental and accurate measure of its consistency compared to the penetration test, especially at elevated temperatures relevant to construction. The higher the viscosity, the stiffer the bitumen.

Bitumen is now predominantly graded by viscosity (VG grades) instead of penetration. This is because viscosity provides a better indication of a binder's performance at service temperatures. The test is typically performed at 60°C (Absolute Viscosity) and 135°C (Kinematic Viscosity). The 60°C temperature represents the maximum pavement temperature reached in hot climates, which is critical for predicting resistance to rutting. The 135°C temperature is representative of the mixing and compaction temperatures in asphalt plants and on site, ensuring proper workability and aggregate coating.

The test is performed using a vacuum capillary viscometer for absolute viscosity and a glass capillary viscometer for kinematic viscosity. The principle is to measure the time taken for a fixed volume of bitumen to flow through a capillary tube of a known diameter under a controlled head and temperature. This time, known as the efflux time, is then used to calculate the viscosity.

APPARATUS

1. Vacuum Capillary Viscometer: A glass tube with a U-shape, a reservoir, a capillary, and timing marks. The viscometer is selected based on the expected viscosity range of the bitumen.
2. Constant Temperature Bath: A bath with a capacity of at least 10 liters, capable of maintaining the test temperature (60°C or 135°C) with an accuracy of ±0.1°C. It must allow for complete submersion of the viscometer.
3. Vacuum System: A system that can maintain a constant vacuum of 30 ± 0.05 cm of mercury for the absolute viscosity test. It includes a vacuum pump and a mercury manometer.
4. Thermometer: A calibrated thermometer with a range appropriate for the test temperature (e.g., 58°C to 62°C or 133°C to 137°C) and a precision of 0.1°C.
5. Stopwatch: An accurate timing device with a resolution of at least 0.5 seconds.
6. Heating Plate/Oven: For softening the bitumen sample prior to testing.
7. Sample Containers: Glass jars or beakers for preparing and holding the sample.
8. Solvent: An appropriate solvent (e.g., toluene or trichloroethylene) for cleaning the viscometer after the test.

PROCEDURE

1. Sample Preparation:

• Heat the bitumen sample to a pouring consistency, typically 75°C to 100°C above its approximate softening point. Do not overheat, as this can alter the properties of the material.
• Stir the sample to make it homogeneous and remove any entrapped air bubbles or moisture.
• Filter the hot bitumen through a 75-micron sieve to remove any foreign particles.

2. Charging the Viscometer (Absolute Viscosity):

• Select a clean and dry vacuum capillary viscometer of a suitable size.
• Pour the heated bitumen sample into the viscometer through the filling tube until the level is slightly above the filling mark.
• Place the charged viscometer in an oven or bath maintained at 135 ± 5.5°C for 10 ± 2 minutes to allow air bubbles to escape.
• Transfer the viscometer to the test bath, which is maintained at a precise 60 ± 0.1°C. Allow the viscometer to stand for 30 ± 5 minutes to reach thermal equilibrium.

3. Testing:

• Connect the vacuum system to the viscometer's outlet tube and apply a vacuum of 30 cm of mercury.
• Release the flow and use the stopwatch to measure the time (efflux time) required for the leading edge of the bitumen meniscus to pass from the first timing mark to the second timing mark.
• The flow time should be at least 60 seconds to ensure accurate results. If the time is too short, repeat the test with a viscometer of a smaller capillary size.
• For kinematic viscosity at 135°C, a vacuum is not required, as the bitumen flows under gravity. The procedure is similar, but the temperature of the bath is maintained at 135 ± 0.1°C.

OBSERVATION/RESULTS

The viscosity of the bitumen is calculated by multiplying the efflux time (in seconds) by the calibration constant of the viscometer. The final reported value is the average of at least three consistent readings.

Calculation:

• Absolute Viscosity ($n_{abs}$) = Viscometer Constant (K) × Efflux Time (t)
• The result for absolute viscosity is reported in Poise (P) or Pascal-seconds (Pa·s). (1 Pa·s = 10 P).
• Kinematic Viscosity ($n_{kin}$) = Viscometer Constant (C) × Efflux Time (t)
• The result for kinematic viscosity is reported in Centistokes (cSt) or $m{m}^2$/s. (1 cSt = 1 mm2/s).

PRECAUTIONS

• Ensure precise temperature control of the water bath throughout the test, as viscosity is extremely sensitive to temperature changes.
• Clean and dry the viscometer thoroughly before each use to prevent contamination.
• Avoid local overheating of the bitumen during sample preparation.
• Ensure that no air bubbles are present in the bitumen sample within the capillary tube during the test.
• The viscometer must be perfectly vertical in the bath.

CODES/IS STANDARDS

• IS 1206 (Part 2): 1978: Methods for Testing Tar and Bituminous Materials - Determination of Absolute Viscosity.
• IS 1206 (Part 3): 1978: Methods for Testing Tar and Bituminous Materials - Determination of Kinematic Viscosity.
• ASTM D2171: Standard Test Method for Viscosity of Asphalt by Vacuum Capillary Viscometer.
• ASTM D2170: Standard Test Method for Kinematic Viscosity of Asphalts (Bitumens).

APPLICATIONS/USES

• Viscosity Grading: This test is fundamental to the modern viscosity-based grading system for bitumen (e.g., VG-10, VG-30), which provides a more performance-oriented classification than the older penetration grading.
• Mix Design: The viscosity value at 135°C is used to determine the mixing and compaction temperatures for asphalt concrete mixes. This ensures the bitumen is fluid enough to uniformly coat the aggregates and that the mix can be compacted to the required density.
• Quality Control: It's a key quality control test at refineries and construction sites to ensure the consistency of the bitumen binder meets project specifications.
• Pavement Performance Prediction: By testing at both 60°C and 135°C, engineers can predict a pavement's susceptibility to high-temperature rutting and workability during construction. A low viscosity at 60°C may lead to rutting, while an overly high viscosity at 135°C can make the mix difficult to compact.