Standard test method to determine softening point of bitumen (Ring and ball apparatus)

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Job 10:

Standard test method to determine softening point of bitumen (Ring and ball apparatus)

ASTM DESIGNATION: D 36/D 36-09

SCOPE & SIGNIFICANCE:

  1. This test method covers the determination of the softening point of bitumen in the range from 30 to 157°C using the ring-and-ball apparatus immersed in distilled water [30 to 80°C] or USP glycerin (above 80 to 157°C).
  2. Bitumens are viscoelastic materials without sharply defined melting points; they gradually become softer and less viscous as the temperature rises. For this reason, softening points must be determined by an arbitrary and closely defined method if results are to be reproducible.
  3. The softening point is useful in the classification of bitumens, as one element in establishing the uniformity of shipments or sources of supply, and is indicative of the tendency of the material to flow at elevated temperatures encountered in service.
  4. This method is useful in determining the consistency of bitumen as one element in establishing the uniformity of shipments or sources of supply.
  5. Softening point is the temperature at which the bituminous binders have an equal viscosity (i.e. the consistency of all the grades will be same at the softening point e.g. if two samples have softening points of 40 °C and 80 °C respectively, both will have the same consistency at their softening point.).
  6. The test gives an idea of the temperature at which the bituminous materials attain a certain viscosity.
  7. Bitumen with higher softening point may be preferred in warmer places.
  8. Softening point should be higher than the hottest day temperature, which is anticipated in that area otherwise bitumen may sufficiently soften and result in bleeding and development of ruts.

RELATED THEORY:

SOFTENING POINT:

The softening point is defined as the mean of the temperatures at which the bitumen disks soften and sag downwards a distance of 25 mm under the weight of a steel ball.

FACTORS AFFECTING SOFTENING POINT:

Following are the main factors that affect the softening point of bitumen;

  • i)- Rate of heating
  • ii)- Grade of bitumen (harder the grade, more will be the softening point)

BLEEDING:

“Bleeding is a migration of bitumen to the surface of the pavement. It is usually caused by too much binder in the surfacing or by an unsuitable binder. Bleeding reduces surface friction and causes the road surface to become slippery”.

Asphalt expands on heating. As the temperature on roads rises, bitumen expands and comes out of the aggregate. This bitumen is unable to move back into the voids of the aggregates as it cools on the surface of the road resulting in the formation of waves on the road surfaces. This phenomenon is known as bleeding.

RUTTING:

“Rutting as illustrated in figure, is longitudinal subsidence localized in the wheel tracks caused by vehicles. The main causes of rutting are inadequate stability of the asphalt material in the surfacing, inadequate compaction of the pavement and insufficient pavement strength. Water may accumulate in the ruts during rain and expose the road users to aquaplaning. In cold climates, freezing water in ruts can be very dangerous. If water is able to penetrate the surfacing in the ruts, it may lead to cracking and breaking up of the pavement.

As a vehicle moves over the road surface, a vacuum is created under the tyre surface which results in the disintegration of the wearing surface. This disintegration starts in the form of small pot-holes on the road surface which eventually spread throughout the cross-section of the road. This phenomenon of the formation of pot-holes and their spread is known as ruts.

APPARATUS:

  1. Rings—Two square-shouldered brass rings conforming to the dimensions shown in 1(a).
  2. Pouring Plate—A flat, smooth, brass plate approximately 50 by 75 mm [2 by 3 in.].
  3. Balls—Two steel balls, 9.5 mm [3⁄8 in.] in diameter, each having a mass of 3.50 (±) 05 g.
  4. Ball-Centering Guides—Two brass guides for centering the steel balls, one for each ring, conforming to the general shape and dimensions shown in 1 (b).
  5. Bath—A glass vessel, capable of being heated, not less than 85 mm in inside diameter and not less than 120 mm in depth from the bottom of the flare.
  6. Ring Holder and Assembly—A brass holder designed to support the two rings in a horizontal position, conforming to the shape and dimensions shown in 1 (c), supported in the assembly illustrated in Fig. 1 (d). The bottom of the shouldered rings in the ring holder shall be 25 mm [1.0 in.] above the upper surface of the bottom plate, and the lower surface of the bottom plate shall be 16 (±) 3 mm [5⁄8 (±) 1⁄8 in.] from the bottom of the bath.
  7. Thermometers:
    1. An ASTM Low Softening Point Thermometer, having a range from − 2 to + 80°C, and conforming to the requirements for Thermometer 15C or 15F as prescribed in Specification E1. As an alternative, any other thermometric device used shall be at least: (1) of equal accuracy to that of the thermometer specified in Specification E1, (2) capable of indicating temperature to within 1°C, and (3) stable to within 1°C for the duration of the exposure.
    2. An ASTM High Softening Point Thermometer, having a range from 30 to 200°C, and conforming to the requirements for Thermometer 16C or 16F as prescribed in Specification E1. As an alternative, any other thermometric device used shall be at least: (1) of equal accuracy to that of the thermometer specified in Specification E1, (2) capable of indicating temperature to within 1°C, and (3) stable to within 1°C for the duration of the exposure.
    3. The appropriate thermometer shall be suspended in the assembly as shown in Fig. 1 (d) so that the bottom of the bulb is level with the bottom of the rings and within 13 mm [0.5 in.] of the rings, but not touching them or the ring holder. Substitution of other thermometers shall not be permitted. As an alternative, any other thermometric device used shall be at least: (1) of equal accuracy to that of the thermometer specified in Specification E1, (2) capable of indicating temperature to within 0.5°C, and (3) stable to within 0.5°C for the duration of the exposure.

Figure 1

REAGENTS AND MATERIALS

1.      Bath Liquids:

  1. Freshly Boiled Distilled Water.

The use of freshly boiled distilled water is essential to avoid trapping air bubbles on the surface of the specimen which may affect the results.

  1. USP Glycerin. (Warning—Glycerin has a flash point of 160°C)
  2. Release Agents:

To prevent adhesion of bitumen to the pouring plate when casting disks, the surface of the brass pouring plate may be thinly coated just before use with silicone oil or grease, a mixture of glycerin and dextrin, talc, or china clay. (Warning—Isolate silicones from other bituminous testing equipment and samples to avoid contamination, and wear disposable rubber gloves whenever handling silicones or apparatus coated with them. Silicone contamination can produce erroneous results in other tests such as those for penetration and flash point.)

TEST SPECIMENS

  1. Do not start unless it is planned to complete preparation and testing of all asphalt specimens within 6 h and all coal-tar pitch specimens within 41⁄2 h. Heat the bitumen sample with care, stirring frequently to prevent local overheating, until it has become sufficiently fluid to pour. Stir carefully to avoid incorporation of air bubbles in the sample.
    1. Take no more than 2 h to heat an asphalt sample to its pouring temperature; in no case shall this be more than 110°C above the expected softening point of the asphalt.
    2. Take no more than 30 min to heat a coal-tar pitch sample to its pouring temperature; in no case shall this be more than 55°C above the expected softening point of the coal-tar-pitch.
    3. If the test must be repeated later, do not reheat this sample; use a fresh sample in a clean container to prepare new test specimens.
  1. Heat the two brass rings (but not the pouring plate) to the approximate pouring temperature, and place them on the pouring plate treated with one of the release agents.
  2. Pour a slight excess of the heated bitumen into each ring, and then allow the specimens to cool in ambient air for at least 30 min. For materials that are soft at room temperature, cool the specimens for at least 30 min at an air temperature at least 10°C below the expected softening point. From the time the specimen disks are poured, no more than 240 min shall elapse before completion of the test.
  3. When the specimens have cooled, cut away the excess bitumen cleanly with a slightly heated knife or spatula, so that each disk is flush and level with the top of its ring.

PROCEDURE:

  1. Select one of the following bath liquids and thermometers appropriate for the expected softening point:
    1. Freshly boiled distilled water for softening points between 30 and 80°C; use Thermometer 15C or 15F. The starting bath temperature shall be 5 (±) 1°C.
    2. USP glycerin for softening points above 80°C and up to 157°C; use Thermometer 16C or 16F. The starting bath temperature shall be 30 (±) 1°C
    3. For referee purposes, all softening points up to 80°C shall be determined in a water bath and all softening points above 80°C shall be determined in a glycerin bath or as agreed to by seller and buyer.
  1. Assemble the apparatus in the laboratory hood with the specimen rings, ball-centering guides, and thermometer in position, and fill the bath so that the liquid depth will be 105 (±) 3 mm [41⁄8 (±) 1⁄8 in.] with the apparatus in place. Using forceps, place the two steel balls in the bottom of the bath so they will reach the same starting temperature as the rest of the assembly.
  2. Place the bath in ice water, if necessary, or gently heat to establish and maintain the proper starting bath temperature for 15 min with the apparatus in place. Take care not to contaminate the bath liquid.
  3. Again, using forceps, place a ball from the bottom of the bath in each ball-centering guide.
  4. Heat the bath from below so that the temperature indicated by the thermometer rises at a uniform rate of 5°C /min. Protect the bath from drafts, using shields if necessary. Do not average the rate of temperature rise over the test period. The maximum permissible variation for any 1-min period after the first 3 min shall be 65°C. Reject any test in which the rate of temperature rise does not fall within these limits.
  5. Record for each ring and ball the temperature indicated by the thermometer at the instant the bitumen surrounding the ball touches the bottom plate. Make no correction for the emergent stem of the thermometer. If the difference between the two temperatures exceeds 1°C, repeat the test.

a)-   FOR MATERIALS HAVING SOFTENING POINTS 80°C OR BELOW

Assemble the apparatus with the rings, ASTM Thermometer 15 °C or I5 °F, and ball centering guides in position and fill the bath with freshly boiled water to a depth of not less than 102mm and not more than 108mm. Maintain the bath temperature at 5 ± 1°C for 15 min, placing the test container in ice water if necessary. Using forceps, place a ball, previously adjusted to the bath temperature, in each ball-centering guide.

Apply heat in such a manner that the temperature of the liquid is raised 5°C/min. Avoid the effect of drafts, using shields if necessary. (Rigid adherence to the prescribed rate of heating is absolutely essential for reproducibility of results. Either a gas burner or electric heater may be used; however, the latter must be of the low-lag, variable output type to maintain the necessary rate of heating.)

The rate of rise of temperature shall be uniform and shall not be averaged over the period of the test. The maximum permissible variation of any 1-mm period after the first 3 mm shall be ± 0.5°C. Reject all tests in which the rate of rise does not fall within these limits.

Record for each ring and ball the temperature shown by the thermometer at the instant the specimen surrounding the ball touches the bottom plate. Make no correction for the emergent stem of the thermometer. If the difference between the values obtained in the duplicate determinations exceeds 1°C repeat the test.

b)-   FOR MATERIALS HAVING SOFTENING POINTS ABOVE 80°C

Follow the same procedure as described above, except use USP glycerin instead of water and use ASTM Thermometer l6°C or 16°F. The starting temperature of the glycerin bath shall be 32°C.

CALCULATIONS:

  1. For a given bitumen specimen, the softening point determined in a water bath will be lower than that determined in a glycerin bath. Since the softening point determination is necessarily arbitrary, this difference matters only for softening points slightly above 80°C.
  2. The change from water to glycerin for softening points above 80°C creates a discontinuity. With rounding, the lowest possible asphalt softening point reported in glycerin is 84.5°C, and the lowest possible coal-tar pitch softening point reported in glycerin is 82.0°C. Softening points in glycerin lower than these translate to softening points in water of 80°C or less, and shall be so reported.
    1. The correction for asphalt is − 4.2°C, and for coal-tar pitch is − 1.7°C. For referee purposes, repeat the test in a water bath.
    2. Under any circumstances, if the mean of the two temperatures determined in glycerin is 80.0°C or lower for asphalt, or 77.5°C or lower for coal-tar pitch, repeat the test in a water bath.
  3. To convert softening points slightly above 80°C determined in water to those determined in glycerin, the correction for asphalt is + 4.2°C and for coal-tar pitch is + 1.7°C. For referee purposes, repeat the test in a glycerin bath.
  4. Under any circumstances, if the mean of the two temperatures determined in water is 85.0°C or higher, repeat the test in a glycerin bath.

VARIOUS GRADES OF BITUMEN ACCORDING TO SOFTENING POINTS

 

REPORT

  1. When using ASTM Thermometer 15C or 15F, report to the nearest 0.2°C the mean or corrected mean of the temperatures as the softening point.
  2. When using ASTM Thermometer 16C or 16F report to the nearest 0.5°C or 1.0°F the mean or corrected mean of the temperatures as the softening point.
  3. Report the bath liquid used in the test.

OBSERVATIONS & RESULTS

Sr. # SOFTENING POINT MEAN
(°C) (°C)
1 50 50.5
2 51

COMMENTS:

According to CEN specifications, this is the grade 35/50 bitumen, which is used in paving.

 

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