Softening point test

The consistency of a penetration grade or oxidized bitumen can also be measured by determining its softening pOint1157’1581. In this test, a steel ball (weight 3.5 g) is placed on a sample of bitumen contained in a brass ring that is then suspended in a water or glycerin bath. The apparatus is shown in Fig. 7.2. Water is used for bitumen with a softening point of 80°C or below and glycerin is used for softening points greater than 80°C. The bath temperature is raised at 5°C per minute, the bitumen softens and eventually deforms slowly with the ball through the ring. At the moment the bitumen and steel ball touch a base plate 25 mm below the ring, the temperature of the water is recorded. The test is performed twice and the mean of the two measured temperatures is reported to the nearest 0.2°C for a penetration grade bitumen and 0-5°C for an oxidized bitumen. If the difference between the two results exceeds 1.0°C, the test must be repeated. The reported temperature is designated the softening point of the bitumen and represents an equi-viscous temperature. In the ASTM version of the softening point test, the bath is not stirred whereas in the IP version the water or glycerin is stirred. Consequently, the softening points determined by these two methods differ. The ASTM results are generally 1.5°C higher than for the IP or BS method11591.

Softening point test

As with the penetration test, the procedure for carrying out the softening point test must be followed precisely to obtain accurate results. Sample preparation, rate of heating, and accuracy of temperature measurement are critical. For example, it has been shown (‘3) that varying the heating rate from 4.5°C to 5.5°C, the tolerance permitted by the IP 58 test method, can result in a difference of 1.6°C in the measured softening point. Automatic softening point machines are available which ensure close temperature control and which automatically record the result at the end of the test. The acceptable repeatability and reproducibility quoted in IP 58 for the softening point test are as follows:

 

Repeatability °C
For penetration grade bitumens 1.0
For oxidised bitumens:
Under 80 1.5
Over 80 – 100 2.0
Over 100 – 120 2.5
Over 120 – 140 3.0
Reproducibility
For penetration grade bitumens 2.5
For oxidised bitumens 5.5

The consistency of bitumen at the softening point temperature has been measured by Pfeiffer and Van Doormaal in terms of penetration. Using a specifically prepared, extra-long penetration needle they found a value of 800 pen for many, but not all, bitumens. The exact value was found to vary with penetration index and wax content. It has also been demonstrated by direct measurement that the viscosity at the softening point temperature of the majority of bitumens is 1200 Pa.s (12,000 poise).
Three types of softening point test have been used. The temperature obtained is dependent upon the bitumen reaching a specific consistency which, expressed in terms of penetration value, is roughly as follows:
• Kramer and Sarnow- 200 dmm
• ASTM (ring and ball)- 800 dmm
• Ubbelohde- 3000 dmm