AIM
To determine the ductility of a bituminous material, which is a measure of its ability to deform under tension without fracturing. The ductility value, expressed as the distance a standard bitumen sample can be stretched before breaking, is a critical parameter for evaluating the binder's adhesive properties and its ability to withstand temperature-induced movements and traffic loads.
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THEORY
Ductility is the property of a material that allows it to undergo significant plastic deformation before rupture. For bitumen, it's a measure of its cohesion (internal strength) and adhesion (ability to stick to aggregates). In a flexible pavement, the bitumen binder forms a thin film around the aggregates, and this film must be sufficiently ductile to accommodate slight movements and stresses caused by temperature variations and repeated traffic loading. If the bitumen has low ductility, it will become brittle and prone to cracking, especially in colder weather.
The test is performed on a standard briquette-shaped specimen of bitumen. The specimen is submerged in a water bath at a constant, specified temperature (typically 25°C or 27°C). It is then stretched at a uniform speed until it breaks. The distance the specimen elongates before rupture is its ductility, measured in centimeters. A higher ductility value indicates a more flexible and less brittle bitumen, which is generally more desirable for pavement construction.
The test conditions are strictly controlled because ductility is highly sensitive to temperature and the rate of pulling. The standard temperature and speed ensure consistent and comparable results.
APPARATUS
- Ductility Testing Machine: A machine consisting of a constant temperature water bath and a pulling device. It's designed to pull a standard briquette specimen at a uniform horizontal speed (usually 50 mm/minute) without undue vibration. The machine includes a scale to measure the distance of elongation.
- Briquette Mould: A brass mould with detachable sides and end clips that, when assembled, forms a briquette specimen of specified dimensions. The minimum cross-section is 10 mm x 10 mm.
- Water Bath: A bath with a capacity of at least 10 liters, equipped with a thermostat to maintain the test temperature with a tolerance of ±0.5°C. The water must be deep enough to fully submerge the specimen by at least 25 mm both above and below.
- Thermometer: A calibrated thermometer with a range appropriate for the test temperature and a resolution of at least 0.2°C.
- Brass Plate: A flat, level brass or metal plate on which the mould is assembled for casting the specimen.
- Releasing Agent: A mixture of equal parts of glycerin and dextrin, or talc, to coat the mould surfaces and prevent the bitumen from sticking.
- Heating Plate/Oven: For softening the bitumen sample.
- Hot, Straight-Edged Putty Knife or Spatula: For trimming the excess bitumen from the mould.
PROCEDURE
- Sample Preparation:
- Heat the bitumen sample to a pouring consistency, typically 75°C to 100°C above its approximate softening point. Stir it to make it uniform and remove any air bubbles.
- Coat the interior surfaces of the briquette mould and the brass plate with the releasing agent. Assemble the mould on the brass plate.
- Pour the heated bitumen into the mould in a thin, continuous stream until it is slightly overfilled.
- Allow the mould assembly to cool at room temperature for 30-40 minutes.
Temperature Conditioning:
- Place the mould assembly in the water bath, which is maintained at the specified test temperature (e.g., 27°C) for 30 minutes.
- After 30 minutes, remove the assembly and carefully trim the excess bitumen with a heated knife or spatula to make the surface level with the top of the mould.
- Return the mould and the brass plate to the water bath and condition it for an additional 85-95 minutes to ensure the specimen reaches thermal equilibrium.
Testing:
- After conditioning, remove the mould from the brass plate and carefully detach the side pieces, leaving the bitumen briquette attached to the end clips.
- Hook the end clips of the briquette to the fixed and movable pins of the ductility machine inside the water bath. Ensure the specimen is fully submerged.
- Set the machine's pointer to zero or note the initial reading.
- Start the machine and pull the clips apart at a uniform rate of 50 ± 2.5 mm per minute until the bitumen thread ruptures.
- Record the distance on the scale at the moment of rupture.
Repetitions:
- Perform the test at least three times. A valid test is one where the specimen pulls out to a point or a thin thread and ruptures at its minimum cross-section.
- If the specimen breaks near the clips or its cross-section is not uniform at the point of rupture, the test is invalid.
OBSERVATION/RESULTS
The ductility is the distance, in centimeters, to which the specimen elongates before breaking. The final reported value is the average of at least three valid test results. The individual results should not differ by more than a specified percentage (e.g., 10% of the mean for repeatability as per IS standards).
Example Observation:
PRECAUTIONS
- The test temperature must be maintained very precisely, as even slight variations can significantly affect the results.
- The briquette specimen must be free of air bubbles, as these can cause premature failure and inaccurate results.
- The mould must be coated thoroughly with the releasing agent to prevent sticking, which could affect the elongation.
- The pulling speed must be strictly uniform throughout the test.
- The specimen must remain fully submerged in the water bath at all times.
CODES/IS STANDARDS
- IS 1208:1978: Methods for Testing Tar and Bituminous Materials - Determination of Ductility.
- ASTM D113: Standard Test Method for Ductility of Bituminous Materials.
- AASHTO T 51: Standard Method of Test for Ductility of Bituminous Materials.
APPLICATIONS/USES
- Pavement Design: Ductility is a key indicator of a bitumen's suitability for flexible pavement construction. High ductility is essential for a durable pavement that can withstand thermal expansion and contraction and resist cracking under traffic loads.
- Quality Control: The ductility test is used to ensure that a bitumen binder meets the required specifications for a particular project. A minimum ductility value is often specified for different grades of bitumen. For example, some specifications require a minimum ductility of 75 cm for VG-10 grade bitumen.
- Material Specification: It helps in the selection of the correct grade of bitumen. A highly ductile bitumen is preferred for areas with wide temperature fluctuations, while a less ductile, harder bitumen might be used in extremely hot climates where rutting is the primary concern.
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