EXPERIMENT NO. 12
SOFTENING POINT OF BITUMEN (RING-AND-BALL APPARATUS).
[ASTM Designation: D 36-76, AASHTO Designation: T 53-89]
SOFTENING POINT: Softening Point of Bitumen
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.
SCOPE & SIGNIFICANCE:
1- This method is useful in determining the consistency of bitumen as one element in establishing the uniformity of shipments or sources of supply.
2- 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.).
3- The test gives an idea of the temperature at which the bituminous materials attain a certain viscosity.
4- Bitumen with higher softening point may be preferred in warmer places.
5- 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.
Ring — A brass shouldered ring.
Ball— A steel ball, 9.53 mm (3/8”) in diameter, weighing between 3.45 and 3.55 grams.
Ball Centering Guide — A guide for centering the ball and made of brass.Ring Holder — The rings shall be supported on a brass ring holder.
Rings shall be supported in a horizontal position with the bottoms of the rings 25 mm above the upper surface of the bottom plate and a distance of at least 13 mm and not more than 19 mm between the bottom plate and the bottom of the bath.
The thermometer shall be suspended so that the bottom of the bulb is level with the bottom of the rings and within 13 mm of the rings but not touching them.
Brass Pouring Plate — A flat, smooth brass plate approximately 75 by 50 mm that has been treated to prevent the bituminous material from adhering to it. A suitable treatment is to coat the plate just before use with a thin layer of a mixture of glycerin and dextrin, talc, or china clay.
Bath — A glass vessel, capable of being heated, not less than 85 mm in diameter and not less than 120 mm in depth from the bottom of the flare.
Thermometers — having a range from -2 to +80°C.
Heat the sample with care to prevent local overheating, with constant stirring until it has become sufficiently fluid to pour. In no case, however, shall the temperature be raised to more than 55°C above the expected softening point for tar pitch, or to more than 110 °C above the expected softening point for asphalt. Avoid incorporating air bubbles in the sample.
Bring an asphalt sample to the pouring temperature in not more than 2 hrs. Bring coal tar pitch to the desired pouring conditions in minimum time, not exceeding 30 min.
Pour a slight excess of the heated sample into two rings, preheated to approximately the pouring temperature. While being filled the rings shall rest on the brass pouring plate, previously treated to prevent bituminous materials from adhering to it.
Cool the specimen disks for a minimum of 30 min. In no case shall more than 240 min elapse before completion of the test. Cool specimens that are soft at room temperature for at least 30 min at a minimum of 8 °C below the expected softening point.
After cooling, cut the excess material off cleanly with a slightly heated knife or spatula. In case the test is repeated, use a clean container and fresh sample to prepare the test specimen.
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.
OBSERVATIONS & RESULTS
The experiment has been performed successfully and the softening point came out to be 46.4oC. Bitumen having this softening point should not be used in Lahore as the city’s highest maximum temperature was 48.3 °C recorded on May 30, 1944 And 48 °C was recorded on June 10, 2007. At the time the meteorological office recorded this official temperature in the shade, it reported a heat index in direct sunlight of 55 °C. So to avoid the problems of Rutting and bleeding this bitumen should not be used in Lahore.
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