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What is ductility?

Ductility is the ability of a material to undergo a visible enduring deformation through elongation. This article describes the methods for testing the ductility of bitumen.

Ductility and brittleness

Ductility is the ability of a material to undergo a visible enduring deformation through elongation (decreasing of cross section area). It expresses the extent to which the material can be plastically deformed without fracture. The opposite material behavior is brittleness.

Typical for a brittle fracture is a very low plastic deformation. For the determination of ductility and brittleness tension tests and bending tests are used.

The ductility and brittleness depend on the special cohesive characteristics of the sample. The behavior depends on the temperature. For example, bitumen shows a brittle fracture at low temperatures. To determine this, a special Fraass breaking point bending test (EN 12593) is used. At room temperature, a ductile fracture occurs. For a ductility text explanation, please see point 3. For more information on Fraas Breaking Point Testerers, click here.

What is cohesion?

Cohesion, in physics, is the intermolecular attractive force acting between two adjacent portions of a substance, particularly of a solid or liquid. It is this force that holds a piece of matter together. Intermolecular forces also act between two dissimilar substances in contact, a phenomenon called adhesion. These forces originate principally because of coulomb (electrical) forces. When two molecules are close together, they are repelled; when further apart, they are attracted; and when they are at an intermediate distance, their potential energy is at a minimum, requiring the expenditure of work to either approximate or separate them. Thus, work is required to pull apart two objects in intimate contact, whether they be of the same or different material.[2] More simply expressed, cohesion is the holding together of particles within the same material. An example: sticking bitumen would be very cohesive, while dry beach sand has no cohesion whatsoever.

The terms ductility and cohesion are often used in a related manner. A ductility material test is a special option for the evaluation of cohesiveness of a material. A typical sample with ductility behavior is bitumen.

How to test the ductility of bitumen

For many decades, the different ductility methods have proved to be a reliable indicator for the rheological characteristics of bitumen and bituminous binders at medium temperature.

The basic principle of the test is that a bitumen sample is prepared in special molds between two clips. The sample is then pulled apart horizontally at a uniform speed and at a specified temperature in a long water-filled bath.

Figure 3: Three bitumen samples in the start position in the ductility meter

Figure 4: Different types of molds

The ductility is used as a performance-based specification criterion for bitumen in product specification standards.
Typical standardized material specifications which require a ductility test are:

  • Paving grade bitumen
  • Hard paving grade & industrial bitumen
  • Polymer-modified bitumen (PmB)
  • Oxidized bitumen
  • Asphalt rubber binder
  • Chemically modified asphalt cement
  • Trinidad lake asphalt modified binder
  • Emulsified asphalt
  • Cationic bituminous emulsion
  • Cut-back & fluxed bituminous binders

Such ductility tests are performed in the bitumen laboratory of refineries, modified bitumen mixing plants, control labs of road construction companies, technical universities (asphalt technology), wall and roof insulation material plants, government control departments and independent road control labs.

For more information on Digital Ductility Meter, see here.

Three different methods of ductility test are used in practice.

Elongation test

The bitumen is cast into a brass mold. The briquette is then pulled apart in a water bath at a defined temperature until it ruptures. The rupture distance indicates the cohesiveness and general tensile properties of the material. The maximum pulling length in case of very cohesive material goes up to 150 cm.

The standardized tests acc. to ASTM D 113, DIN 52 013, JIS K 2207, and AASHTO T51 are almost identical. The main test conditions are a pulling speed of 5 cm/min and a temperature of 25 °C in the water bath.

Elastic recovery test

The elastic recovery is measured by the recoverable strain determined after a specified time after severing an elongated briquette specimen. The sample is pulled to a specified distance and cut with a pair of scissors in the middle. After a fixed period of recovery time the distance reduction in relation to the total distance is measured.

The test is useful in confirming that a component has been added to the bitumen to provide a significant elastomeric characteristic.

In order to determine the particular properties of polymer-modified bitumen (PmB), the method of elastic recovery has turned out to be more expedient than the elongation test.

Figure 5: Sample cut in the middle

Figure 6: Sample after a recovery time of 30 min

Typical test conditions are a pulling speed of 5 cm/min and a temperature of 25 °C in the water bath.

Test conditions acc. to ASTM D6084 specify a pulling distance of 10 cm and a recovery time of 60 min.Test conditions acc. to EN 13 398 specify a pulling distance of 20 cm and a recovery time of 30 min.

Force ductility test

Figure 7: Typical curve of a force ductility test with polymer-modified sample

The tensile properties are measured on the basis of the performed force and the specimen strain until the briquette ruptures or a specified strain is achieved.

The deformation energy is another significant characteristic for the cohesive power, especially of polymer-modified bitumen. The energy is determined from the recordings of the tensile curves by calculating the area below the curve. Typical is the area between 20 cm to 40 cm pulling lengths.

The related standards are EN 13589, EN 13703, and AASHTO T300.

Typical conditions are a test pulling speed of 5 cm/min and a temperature of 4 °C acc. to AASHTO – US standard and 5 °C acc. to the EN standard tests.