Fraass breaking point
The Fraaß breaking point is a measured value for the classification of bitumen . This is the temperature at which a film made of bitumen, which has been melted or pressed onto a test sheet, breaks when the test sheet cools down and sags as prescribed. The experiment is carried out in a standardized breakpoint device.
The test sheets used are 41 ± 0.05 millimeters long, 20 ± 0.2 millimeters wide and 0.15 ± 0.02 millimeters thick and are made of polished cold steel of the types Ck75H + A or Ck101H + A. Trays with rust spots must not be used. Bitumen is melted onto the sheet metal when the ring and ball softening point does not exceed 100 ° C. Otherwise the bitumen layer will be pressed on.
Prepare the sample
Melting
When melting, the sheet metal is placed on a magnetic block and the block is heated to a maximum of 70–80 ° C above the softening point. When the bitumen has become liquid, spread it over the entire sheet by carefully tipping the block back and forth. If air bubbles are trapped, fan with a small flame until the air bubbles have escaped. This creates a smooth surface. This process should be finished after 5–10 minutes. After the sheet has cooled to 18–28 ° C, protected from dust, it is weighed to ensure that it is within the specified weight range. It is then left to rest for 30 minutes before using it for the experiment. In this method, 0.4 * σ 25 ± 0.01 grams of bitumen on the sheet aufgebracht.σ 25 is the density of the binder in g / cm³.
Pressing on
When pressing onto the test sheet, 2 grams of bitumen are applied between two separating foils. The sheet is then placed between two pressure blocks heated to 20 ° C below the softening point of the bitumen and pressed together for 1 minute. The tray is then allowed to cool to room temperature and then placed in ice water. After two minutes, the upper separating film is removed and the edges of the sample trimmed. The weight of the bitumen after pruning may only be 0.41 * σ 25 ± 0.01 grams of bitumen.
The difference between the factors 0.40 and 0.41 is due to the fact that no meniscus is created on the edge when the material is pressed open . This is taken into account with the various factors.
Carrying out the exam
Before the test, the test sheet with the specimen is heated to at least 15 ° C above the expected breaking point. Then it is fastened in the retaining claws (2) of the test device. Then the bending device is hung in the inner test tube (5) of the cooling device. You now begin to cool down. The cooling is achieved by first filling the outer test tube (4) up to a height of 100 millimeters with ethanol . This must have the same temperature as the test panel. The cooling is now brought about by adding small pieces of dry ice . The pieces are introduced into the apparatus through the bore (9). The first three minutes of the test are used to achieve the correct rate of temperature drop. Afterwards, the cooling from 1 ° C may only take place within 60 ± 5 seconds .
Bending the test sheet begins at a temperature of 10 ° C above the expected breaking point. The sheet is bent back several times by turning the crank evenly until it stops and then stretched again. When the first cracks appear in the bitumen, the temperature is read to an accuracy of 0.5 ° C.
If the lighting conditions are poor, a lamp with a maximum of 15 watts may be used for brightening . This may only be brought up to ten centimeters from the experimental apparatus.
A maximum of four hours may elapse between the production of the test panels and the end of the test.
evaluation
The Fraass breaking point is the arithmetic mean of two tests. The results must not differ by more than 3 ° C. If the two are more than 3 ° C apart, a third value must be determined. The arithmetic mean is then determined from the two values that are closest to one another. These should also not be more than 3 ° C apart. If the mean value lies exactly between the other two, it is the break point.
If the three values do not meet the stated conditions, the attempt must be rejected and a new test carried out.
Norms and standards
- EN 12593 - Bitumen and bituminous binders - Determination of the Fraass breaking point