The penetration test assesses the consistency and stability of pure bitumen, oxidized bitumen (blown bitumen), and the residue of emulsion bitumen. However, this test is not suitable for very soft materials with low viscosity, such as tars.
This procedure categorizes bitumen based on its hardness, employing a needle with a 100-gram load that vertically penetrates the bitumen for 5 seconds at 25 degrees Celsius. The penetration value, measured in tenths of a millimeter, indicates the hardness of the bitumen. A higher penetration value corresponds to a higher bitumen grade.
For oxidized bitumen, a softening point test is used in conjunction with the penetration test for classification. The softening point test involves preparing bitumen discs and determining the temperature at which these discs can no longer support a standard metal ball.
The penetration test is conducted under specified conditions of time, temperature, and load. Typically performed at 25°C with a 100g load for 5 seconds, the test may also be conducted at different temperatures for low-temperature bitumen. A 200 pen bitumen, for example, has a needle penetration of 200 dmm at 25°C under a 100g load for 5 seconds.
The advantages of penetration grade bitumen include its relevance to typical pavement temperatures, potential correlation with low-temperature asphalt binder properties, and its quick and cost-effective nature, making it suitable for field use.
However, the test has its limitations. It is empirical and does not measure fundamental engineering parameters like viscosity. The variable and high shear rate during the test may impact results, and the test does not provide information for establishing mixing and compaction temperatures.
Bitumen penetration tests are conducted using standard methods, such as ASTM D5, ASTM D946 & AASHTO M 20, EN 12591, EN 1426, DIN 52210, SABS 307, IP 49 50 & 167, and IS Code 1203.
The penetration test apparatus includes components such as a water bath, sample container, transfer dish, penetration needle, stopwatch, thermometer, and penetrometer. The test procedure involves heating the bitumen specimen to 90°C, stirring it to achieve consistency, and then cooling it to atmospheric temperature. The penetration test is conducted by releasing a 100g load needle onto the bitumen surface for 5 seconds, with at least three determinations for each sample.
Accuracy is crucial in the penetration test method to avoid errors. Common pitfalls include poor sampling, inadequate sample preparation, equipment maintenance issues, and inaccuracies in temperature and time control.