⛽ Softening point of bitumen by the Ring and Ball method

Petrochemistry PN-EN 1427

Overview

The standard PN-EN 1427 specifies a method of determining the softening point of bitumens and bituminous binders in the range 28–150 °C; it also applies to binders recovered from asphalt mixtures, for example by extraction. The softening point is the temperature at which the material reaches a prescribed consistence under standardised conditions. The present description is based on the edition PN-EN 1427:2015-08, withdrawn and replaced by PN-EN 1427:2026-07 — the changes in the new edition are to be checked before implementation.

The 2015 edition gave up the mercury thermometer as the normative reference thermometer, improved the description of the establishment of the temperature gradient and recognised that in glycerol a uniform gradient is hard to establish in the range 30–60 °C, and for that reason the requirement for the gradient applies from 60 °C (Annex B with examples). The standard warns that at softening points above 100 °C the replacement of mercury thermometers by electronic ones may have changed the conditions of the measurement; below about 100 °C the difference of the readings lies within the repeatability of the method.

The softening point together with the penetration (PN-EN 1426) classifies bitumens and determines the conditions of their transport, pumping and storage and of the production and service of pavements. The result in water is about 4 °C lower than in glycerol, and for that reason the standard lays down protocols for results near 80 °C (8.8.2–8.8.5). Precision: r = 1.0 °C and R = 2.0 °C (unmodified bitumen, water); r = 1.5 °C and R = 3.5 °C (polymer-modified, water); r = 1.5 °C and R = 5.5 °C (oxidised, glycerol).

Method principle

Two horizontal discs of binder, cast in shouldered brass rings, are heated at a controlled rate in a liquid bath, each carrying a steel ball of (9.50 ± 0.05) mm and (3.50 ± 0.05) g. The softening point is given as the mean of the temperatures at which the two discs have softened enough for each ball, enveloped in binder, to fall (25.0 ± 0.4) mm. The liquid of the bath: freshly boiled and cooled distilled or deionised water for softening points of 28–80 °C (a starting temperature of (5 ± 1) °C) or glycerol of a density of (1250 ± 10) kg/m³ and a purity of 99 % for 80–150 °C (a starting temperature of (30 ± 1) °C). Heating from below with stirring, a rise of 5 °C/min: after the first 3 min (water) or from 60 °C (glycerol) the rise in each minute must be 4.4–5.6 °C, and the total rise at the end of the test must be within ± 1 °C of the number of minutes × 5 °C — otherwise the test is rejected.

Applications

• Classification of paving and polymer-modified bitumens (together with the penetration)
• Control of deliveries of binders in the road laboratories of GDDKiA
• Testing of binder recovered from an asphalt mixture
• Determination of the conditions of transport, pumping and storage of bitumen
• Assessment of the effect of ageing on the consistence of a binder

Key parameters

Standard

Standard number

PN-EN 1427:2015-08

Title (PL)

Asfalty i lepiszcza asfaltowe — Oznaczanie temperatury mięknienia — Metoda Pierścień i Kula

Title (EN)

Bitumen and bituminous binders — Determination of the softening point — Ring and Ball method

Step-by-step procedure

1. Preparation of the samples

   Take the sample according to EN 58 and prepare it according to EN 12594 (modified bitumen: enough for four rings in case the test has to be repeated). Heat the rings (not the plate) to a temperature not higher than 100 °C above the expected softening point and set them on the plate coated with release agent.

2. Casting of the discs

   Pour a slight excess of the heated binder into each ring and cool in air for at least 30 min (materials that are soft at room temperature: ≥ 30 min in air at a temperature of at least 10 °C below the expected softening point). Complete the test within 4 h of pouring.

   ⏱ Time: ≥ 30 min

3. Cutting off the excess

   After cooling, cut off the excess binder level with the upper edge of the ring with a heated knife or blade — immediately before the rings are placed in the assembly.

4. Choice of the liquid and of the starting temperature

   An expected softening point of 28–80 °C: water, a reading of 0.2 °C, a starting temperature of (5 ± 1) °C. Above 80 °C up to 150 °C: glycerol, a reading of 0.5 °C, a starting temperature of (30 ± 1) °C.

5. Assembly

   Assemble the rings, the ball-centring guides and the sensor (the bottom of the sensor level with the bottom of the rings, within 13 mm, without touching them), fill the bath so that the surface of the liquid is (50 ± 3) mm above the upper edge of the rings; place the balls with tongs in the bath or in a separate vessel at 5 °C or 30 °C.

6. Conditioning

   Cool the bath in iced water or in a thermostat to (5 ± 1) °C (water) or heat it to (30 ± 1) °C (glycerol) and hold for 15–20 min. Then dry the outside of the bath, transfer it quickly into the apparatus and place the balls in the guides with tongs.

   ⏱ Time: 15–20 min

7. Heating

   Check the starting temperature (outside the range — stop). Stir and heat from below so that the rise settles at 5 °C/min; shield it from draughts. After the first 3 min (water) or from 60 °C (glycerol; 60 ± 1 °C after 6 min) the rise in each minute must be 4.4–5.6 °C, and the total rise at the end within ± 1 °C of the minutes × 5 °C; otherwise reject the test.

8. Reading and result

   For each ring record the temperature at the moment when the binder enveloping the ball touches the bottom plate (the manual method) or cuts the light beam (an automatic apparatus). Repeat the test where the difference exceeds 1 °C (below 80 °C) or 2 °C (above 80 °C); for modified bitumens also where the ball breaks through the film or the binder partly detaches from the ball. Result: the mean rounded to 0.2 °C (≤ 80 °C) or 0.5 °C (> 80 °C), with the protocols of 8.8.2–8.8.5 for results near 80 °C.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Glycerol has a flash point of 160 °C; above 100 °C it may contain water, spit onto the heater and catch fire — heat with care and have extinguishing means at hand
• Hot binder and a hot knife — heat-resistant gloves, a face shield
• An electric propeller stirrer must be safely earthed; shield the gas burner from draughts
• Do not apply an excess of release agent — particles in the liquid disturb the light beam of an automatic apparatus

Frequently asked questions