The viscosity of bitumen determines its behavior at high temperatures.
Viscosity refers to the internal resistance of liquids that impedes their movement and flow. Fluid viscosity is measured in poise, which is the same as Pascal-seconds [Pa/s]. As both viscosity and penetration tests measure bitumen's stiffness, what is the significance of the viscosity test? Engineers have found that two bitumens that penetrate the same amount at 25°C behave differently at higher temperatures. To demonstrate these differences, they performed an experiment.
Significance of Viscosity Testing of Bitumen
Temperature changes have a direct effect on bitumen viscosity. A road surface made of bitumen can become unstable over time and experience rutting in hot weather and fatigue cracking in cold weather. During hot summers, asphalt swells and becomes billowy if its viscosity is too low. During extremely cold weather, a bitumen with very high viscosity causes cracks in the asphalt. We can predict the resistance of bitumen in different temperatures by measuring the viscosity of bitumen in a lab. Additionally, viscosity indicates how well bitumen compacts on roads. In order to determine the best temperature for mixing and compacting bitumen, we need to predict both viscosity of the bitumen at two different temperatures, 60° C and 135° C. Due to its higher viscosity, bitumen may be difficult to compact, leaving many air voids in the final asphalt, which significantly reduces its durability. By controlling the viscosity of bitumen, you can experience better resilience, reduced maintenance costs, less fatigue cracking, and reduced temperature sensitivity.
How We Measure the Viscosity of Bitumen?
The viscosity of bitumen is tested at two different temperatures. At first, the absolute viscosity of bitumen sample is measured in 60° C. To make sure that the bitumen viscosity is suitable for being mixed under a higher temperature, we do another test that measures the kinematic viscosity in 135° C.
How viscosity test is conducted?
Apparatus:-
Tar viscometer is used for determining industrial viscosity of bitumen
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Cup: It is also known as a 10-mm cup and has a specified orifice and valve. It is made up of hard brass tube. It has an external brass collar at the upper open end of the cylindrical cup. The collar helps in supporting the cup in the sleeve of the water bath. While a phosphor-bronze plate which is circular in shape is screwed into the cylinder and made conical. It has centrally-located extensions made up of same material and cylindrical in shape. The plate helps in the drainage of tar after use.
The extension is drilled and polished to form an orifice of 10 mm diameter. If the upper rim of the orifice is not perfectly circular, then the valve cannot be properly seated.
Internal Diameter of cup = 40.0 ± 0.5 mm
External Diameter of cup = 42.0 ± 0.5 mm
Diameter of orifice = 10.000 ± 0.025 mm
Length of jet = 5.000 ± 0.025 mm
Water bath: It is cylindrical in shape and is made of a copper sheet. Heating of the bath is done electrically. Ensure that local heating is avoided. The water bath is placed on three equidistant legs riveted to the wall of the bath.
Diameter of water bath = 160 mm
Depth of water bath = 105 mm
Valve: It is in the shape of a sphere and is made up of phosphor-bronze. It is attached to a metal rod provided with a levelling peg at upper part and hemisphere by which the valve is held. It helps in closing the orifice attached to the 10-mm cup.
Sleeve: It is in the form of a stout brass tube, which is bronzed into a central hole cut in the bottom of the water bath. It helps in receiving the cup so that the cup remains in position. It has an easing sliding fit.
Internal Diameter of sleeve = 45 mm
Height of sleeve = 105 mm
Stirrer: It has 4 vertical vanes. The upper and lower portions of the stirrer can be turned in opposite directions. The stirrer is mounted on a cylinder slipping on the sleeve through an easy sliding fit.
The cylinder is cut between the vanes so that heat can be transferred from water in the bath to the tar in the cup. To prevent the water from the water bath from entering the tar cup on raising the stirring system, vertical grooves are provided on the inner surface of the cylinder.
Curved shield: It is fixed at the upper edge of the cylinder. It is extended to a distance within 5 mm of the walls of the water bath. An insulating handle to facilitate rotation of stirrer, a swivelled support for the valve, and support for thermometer are present on this curved shield.
Receiver: It is a measuring cylinder with graduations at 20, 25, and 75 ml capacities. Its total capacity is 100 ml.
Internal Diameter ≯ 29 mm
Thermometer: Two standard thermometers are required- one is placed in the water bath and another in the cup. The range of both the thermometers should be from 0 to 44 °C. The least count is 0.2 °C.
- Other accessory apparatus for viscosity of bitumen test include:
Timing device like stopwatch or stop clock- It should be able to measure up to 0.5 seconds.
- Bitumen sample to be tested
- Non-corroding Solvent, e.g. Phenol-free light tar oil
Following precautions should be taken while performing the viscosity of bitumen test to obtain accurate results:
- Rotate the stirrer when the sample is heated in tar cup
- The temperature should be strictly adhered to during the entire test
- Test temperature should not be lower than 20 °C and it should be in the multiples of 5 °C
- Care should be taken while using the tar cup during its cleaning- it should be cleaned gently
- Non-corroding solvents should be used such as light tar oils free from phenols
- Do not use duster for cleaning as it may lead to abrasion of the metal
- The orifice at the top of the tar cup should be checked for its diameter frequently with a gauge
- Calibration of thermometer should be done periodically
- Adjust the tar viscometer in such a way that the top of the tar cup is levelled.
- Heat the water bath to the specified temperature of the test and maintain it at the same temperature throughout the test duration. Tolerance of ± 0.1 °C is allowed. [The specified test temperatures are 35°C, 45°C, 55°C, and 65°C.]
- Rotate the stirrer gently at frequent intervals. Continuous rotation is preferred.
- Clean the orifice of the tar cup of viscometer with a suitable solvent and allow it to dry completely.
- Heat the bitumen sample to a temperature of about 20°C above the specified test temperature and allow it to cool. Meanwhile, rotate the stirrer continuously.
- Allow the sample to cool up to a temperature slightly above than the specified test temperature.
- Pour the tar into the tar cup then after till the levelling peg on the valve is just immersed. During this, the valve rod should be kept vertical.
- Take 20 ml of mineral oil or 1 % by weight solution of soft soap and pour it into the graduated receiver.
- Place the graduated receiver under the orifice of tar cup.
- Place the second thermometer in the tar.
- Continue stirring the tar until the temperature falls within ± 0.1 °C of the specified test temperature.
- Then, suspend the thermometer co-axially to the cup with its bulb at the geometric centre of the tar approximately.
- Allow the assembly of the apparatus to stand for 5 minutes. During this time, the thermometer reading should remain within 0.5 °C of the specified test temperature.
- Remove the thermometer.
- Also, remove the excess of tar, if any quickly. This ensures that the final level is on the centre line of the levelling peg when the valve is vertical.
- Suspend the valve on the valve support by lifting it.
- Start the stopwatch or stop-clock or any other time recording device when the reading on the cylinder is 25 ml.
- Note the time in seconds when the reading on the cylinder is 75 ml by stopping the stopwatch.
- Repeat the test for a few times with the same material and note down the results.
The time at which the reading of the cylinder reaches 75 ml from 25 ml is to be noted down for all the three samples along with the specified test temperature.
Whether the sample is tested as received or dried before testing is also reported. If drying is done, then the method of drying is also noted. If the sample is tested as received, then water is present in it. And the presence of water has a notable effect on viscosity.
Result:The time taken by 50 ml of tar to flow out is recorded to the nearest whole number as the viscosity of the tar sample.
The time taken by the three tar samples should lie within ±4 % of the mean value of the three readings.
While using CUTBACK BITUMEN, the tolerance in the difference in readings for industrial viscosity test of bitumen is tabulated below:
| Viscosity | Repeatability |
|---|---|
| < 20 sec | 2 sec |
| 20 - 40 sec | 2 sec |
| > 40 sec | 5 % of the mean |