AIM
To determine the penetration value of given Bitumen.
THEORY
The consistency of bituminous material varies depending up on several factors such as constituents, temperature, etc. Determination of absolute viscosity of bituminous material is not so easy. Therefore, the consistencies of these materials are determined by indirect method. Penetration value is an indirect method of measurement of hardness or consistency of bituminous material. It is the vertical distance traversed or penetrated by the point of a standard needle into the bituminous material under specific conditions of load, time and temperature. This distance is measured in one tenth of a millimeter. Softer the bitumen greater will be the penetration. The penetration test is widely used for classifying the bitumen in to different grades. The BIS has standardised the penetration test equipment and the test procedure. It is not regarded as suitable for use in connection with the testing of road tar because of the high surface tension exhibited by these materials and the fact that they contain relatively large amount of free carbon.
APPARATUS
a) Container - A metal or glass cylindrical, flat bottom container of essentially the following dimensions shall be used:
For penetrations below 225:
Diameter, mm - 55
Internal depth, mm - 35
For penetrations between 225 and 350:
Diameter, mm - 70
Internal depth, mm - 45
b) Needle- A straight highly polished cylindrical stainless steel rod. The needle is provided with a shank approximately 3 mm in diameter into which it is immovably fixed. The taper shall be symmetrical and the point shall be 'blunted' by grinding to a truncated cone.
c) Water Bath - A water bath preferably with a thermostat maintained at 25.0 ± 0.1 °C (containing not less than 10 litres of water. The sample being
immersed to a depth of not less than 100 mm from the top and supported on a perforated shelf not less than 50 mm from the bottom of the bath.
d) Transfer Dish - A small dish or tray, provided with some means which ensure a firm bearing and prevent the rocking of the container, and of such capacity as will ensure complete immersion of the container during the test.
e) Thermometer – a thermometer of range 0 to 44 °C.
f) Time device- For hand operated penetrometers, a timer device or stop watch, accurate to 0.1 second.
PROCEDURE
1. The bitumen is heated to a pouring consistency, about 75 to 100°C above the temperature at which bitumen softens. The sample is thoroughly stirred to make it homogeneous and free from air bubbles and water.
2. The sample is poured into the container of 35 mm depth (to a depth at least 10 mm more than the expected penetration).
3. The sample containers are placed on the transfer tray and cooled in atmosphere at temperature between 15 to 30 °C for 60 to 90 minutes.
4. Transfer the tray with containers is placed in the thermostatically controlled water bath maintained at a temperature of 25.0±0.1 °C for a period of 60 to 90 minutes (for testing bitumen with penetration value between 225 and 350, the container of depth 45 mm and diameter 70mm is used and the cooling period in atmosphere and in water bath is 90 to 120 minutes each.)
5. The transfer tray with sample container and water is removed from the water bath and placed under the needle of penetrometer.
6. Using adjusting screw, the needle assembly is lowered and the tip of the needle is made to just touch the top surface of bitumen sample and the needle assembly is clamped in this position.
7. The contact of the tipoff the needle is checked using the mirror placed on the rear of the needle. The initial reading of penetrometer dial is either adjusted to zero or the initial reading is taken before releasing the needle.
8. The needle is released exactly for a period of5.0 seconds by pressing the knob and the final reading is taken on the dial.
9. The needle assembly is then raised and the penetration needle is removed and replaced by a clean, dry needle.
10. At least three such measurements are made on this sample at testing distance of not less than 10 mm apart. After each test the needle is designated and cleaned with benzene and carefully dried. The test is repeated on the sample in the other containers, after keeping in the water bath maintained at a temperature of 25°C.
OBSERVATION AND CALCULATIONS
Period of curing in atmosphere =
Period of curing in water bath =
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Penetrometer Dial Reading |
Test 1 |
Test 2 |
Test 3 |
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Initial |
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Final |
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Penetration Value |
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Mean penetration value =
RESULT
Penetration value of bitumen =
INFERENCE FROM TEST
Penetration test is most commonly adopted test in bitumen to determine the grade of material in terms of its hardness because of its simplicity. The penetration grades of bitumen are generally denoted as 80/100, 60/70, or 30/40 grade bitumen. 80/100 bitumen denote that the penetration value of the binder ranges between 80 and 100. The penetration value of various type of bitumen used in pavement construction in this country ranges between 20 to 225. The appropriate grade of bitumen binder to be used for road construction work is decided depending upon the climatic condition of the project site, the type of construction / specification and the pavement layer (base course binder / course surface / course). Harder grade of bitumen binder with lower penetration value is generally used in warm region. In cooler region bitumen with higher penetration value is generally used. However, binders which become very stiff at low temperature are not preferred for road construction in region with cold weather.
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