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

AIM

To determine the softening point of a bituminous material. The softening point is defined as the temperature at which the bitumen reaches a specific degree of softness under controlled conditions, causing a steel ball to fall a set distance through a disk of the material. This test provides a measure of the temperature susceptibility of the bitumen, indicating its stiffness at elevated temperatures.

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THEORY

Bitumen is a viscoelastic material, meaning it behaves like a viscous liquid at high temperatures and an elastic solid at low temperatures. Unlike crystalline solids, it does not have a sharp, well-defined melting point. Instead, it gradually softens as the temperature increases. The softening point test, using the ring and ball method, is an empirical test that assigns a single temperature value to this gradual process. It is a crucial parameter for pavement design, as it helps engineers predict how the bitumen binder will behave under hot climatic conditions.

A higher softening point indicates a more temperature-resistant or less temperature-susceptible bitumen. This is desirable for road construction in hot climates, as it helps the pavement resist rutting and deformation under heavy traffic and high temperatures. Conversely, a lower softening point suggests a softer bitumen, which is suitable for colder regions to maintain flexibility and prevent cracking.

The principle is straightforward: a disk of bitumen, cast in a brass ring, is heated in a liquid bath at a uniform rate. A steel ball is placed on the surface of the bitumen. As the temperature of the bath rises, the bitumen softens, and the ball, under its own weight, sinks through the ring. The temperature at which the ball, along with the bitumen, touches a bottom plate placed at a specified distance is recorded as the softening point.

APPARATUS

1. Ring and Ball Apparatus: A test apparatus consisting of a support for two brass rings and a bottom plate. The distance between the bottom of the rings and the top of the bottom plate must be exactly 25 ± 0.4 mm.
2. Brass Rings: Two tapered brass rings with a specific depth and internal diameter.
3. Steel Balls: Two steel balls, each with a diameter of 9.53 mm and a mass of 3.50 ± 0.05 grams.
4. Ball Guides: Two guides that center the steel balls on top of the bitumen rings.
5. Heat-Resistant Beaker: A beaker, typically a 600 ml glass beaker, to serve as the liquid bath.
6. Thermometer: A calibrated thermometer with a range appropriate for the expected softening point. A low-range thermometer (-2°C to 80°C) is used for softening points below 80°C, and a high-range thermometer (30°C to 200°C) for values above 80°C.
7. Heating Source: A hot plate or gas burner with a stirrer to ensure uniform heating of the liquid bath.

8. Liquid Bath:

• Distilled water for softening points up to 80°C.
• Glycerine for softening points above 80°C.
9. Steel Plate: A metal plate for casting the bitumen samples.
10. Timing Device: A stopwatch.

PROCEDURE

1. Sample Preparation:
• Carefully heat the bitumen sample to a fluid consistency, typically between 75°C and 100°C above its approximate softening point. Stir it to make it homogenous and remove any air bubbles.
• Heat the brass rings to a temperature similar to the molten bitumen. Place them on a metal plate that has been coated with a releasing agent (e.g., a mixture of glycerin and dextrin) to prevent the bitumen from sticking.
• Pour the molten bitumen into the rings until it slightly overflows.
• Allow the rings with the bitumen to cool in ambient air for 30 minutes. Then, with a slightly heated, sharp knife, trim off the excess bitumen to get a flush, flat surface.

2. Test Setup:

• Assemble the ring and ball apparatus with the prepared bitumen rings.
• Fill the beaker with the appropriate liquid bath (distilled water or glycerine) to a height of at least 50 mm above the top of the rings.
• Place the entire assembly in a refrigerator or ice bath to cool the liquid to a starting temperature of 5 ± 1°C for water or 35 ± 1°C for glycerine. Maintain this temperature for 15 minutes.
• Using forceps, place a steel ball on the center of each bitumen disk in the rings.

3. Testing:

• Position the beaker on the heating source and place the thermometer so its bulb is at the same level as the rings but not touching them.
• Start heating the bath at a uniform rate of 5 ± 0.5°C per minute. The rate of temperature rise must be controlled precisely throughout the test.
• As the temperature rises, the bitumen softens, and the balls will begin to sink.
• Record the temperature at the exact moment each ball, along with its envelope of bitumen, touches the bottom plate.

OBSERVATION/RESULTS

Record the temperatures at which each of the two balls touches the bottom plate. The softening point of the bitumen is the average of these two temperatures. The two readings should not differ by more than 1°C for results up to 40°C, and 1.5°C for results up to 60°C. If the difference is greater, the test should be repeated.

Example Observation:

Ball No.	Temperature at which it touches the bottom (°C)
1	48.5
2	49.0
Average Softening Point	48.75°C

PRECAUTIONS

• Maintain the prescribed heating rate of 5 ± 0.5°C/minute strictly throughout the test.
• Ensure the liquid bath is free of air bubbles, which can affect the temperature and the descent of the ball.
• The apparatus must be free from vibrations during the test.
• The thermometer bulb should be placed correctly to accurately measure the temperature of the liquid near the rings.
• Use a fresh sample and a clean apparatus for each test.

CODES/IS STANDARDS

• IS 1205:1978: Methods for Testing Tar and Bituminous Materials - Determination of Softening Point.
• ASTM D36: Standard Test Method for Softening Point of Bitumen (Ring-and-Ball Apparatus).

APPLICATIONS/USES

• Bitumen Specification: The softening point is a key parameter for specifying the grade of bitumen used in different climatic conditions. For example, a bitumen with a higher softening point (e.g., > 50°C) is preferred for hot climates to prevent premature pavement failure.
• Quality Control: It's a standard quality control test at refineries to ensure the consistency of bitumen produced for various applications.
• Material Selection: It helps in selecting the appropriate bitumen binder for various applications, such as road pavements, waterproofing, and roofing. A higher softening point is also indicative of the bitumen's ability to withstand higher service temperatures without becoming too soft.

This video demonstrates the procedure for the ring and ball test for softening point as per IS 1205.