---
title: "Viscosity test of bitumen"
subject: "Transportation Engineering I"
chapter: 6
key: "semester_6/transportation_engineering_i:viscosity-test"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/viscosity-test
site: BCT Engineering
---

# Viscosity test of bitumen

Transportation Engineering I · Chapter 6: Highway Materials

- Asked in: 2080 Chaitra, 6 marks; 2079 Jestha (asked 2 times)
- Video: [Viscosity Test On Bituminious Material](https://www.youtube.com/watch?v=_0OUwEDorYg) — Aditya P. Nagarale
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/viscosity-test
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-6

## Question

Explain the method of conducting viscosity test (on bituminous binders) in the laboratory, with a neat sketch.

## Short answer

A brass cup with a 10 mm orifice is filled with binder and heated in a water jacket to the test temperature. The time in seconds for 50 ml of binder to flow from the 25 ml to 75 ml mark on the receiver is recorded. This STV value indicates viscosity, used for grading and determining mixing and compaction temperatures.

## Answer

**Viscosity** is the property of a fluid that resists flow. For bituminous binders it determines the temperatures at which the material is suitable for spraying, mixing and compaction, so it is a measure of the flow property, or consistency.

#### Methods
1. **Orifice (efflux) viscometers** - *Saybolt Furol* and the *Standard Tar Viscometer (STV)*, which measure the time taken for a fixed volume of binder to pass through a standard orifice (used for cutbacks, tars and emulsions).
2. **Capillary viscometers** - kinematic viscosity at 135 degrees C and absolute viscosity at 60 degrees C (vacuum capillary viscometer, used for VG grading in IS:73).
3. **Rotational viscometer (Brookfield)** - for modified binders.

#### Standard Tar Viscometer (road tar viscometer) test (IS:1206)
**Apparatus:** A brass cup (about 100 mm in diameter) with an orifice of **10 mm** (4 mm for the more fluid grades) at the bottom, closed by a ball valve; the cup sits in a water jacket with a heater, thermometer, stirrer, a graduated receiver (100 ml) and a stopwatch.

```text
     thermometer
        |   __ __ 
     |  v  |    | cup (binder)
     |_____|    |
     | water jacket (heated) |
        \ _ /   <- orifice 10 mm
          |
        [receiver, 25-75 ml marks]
```

**Procedure:**
1. Select the orifice size and the test temperature (for example 25, 40 or 60 degrees C for cutbacks and tars).
2. Heat the water bath to the test temperature and bring the binder in the cup to that temperature (stirring for uniform temperature), with the orifice closed by the valve.
3. Fill the cup to the mark with the binder.
4. Place the receiver under the orifice and lift the valve so that the binder flows.
5. Start the stopwatch when the binder in the receiver reaches the **25 ml mark** and stop it when it reaches the **75 ml mark**.
6. The time in **seconds** for this 50 ml to flow is the viscosity value (STV seconds; Saybolt Furol seconds in the Saybolt test). Repeat the test and take the average.

#### Significance
- Gives the proper grade of binder (for example, the VG-10 to VG-40 classification by viscosity) and shows whether it is suitable for the climate.
- Gives the *mixing and compaction temperatures* (for example, the temperatures at which the viscosity is 170 +/- 20 cSt for mixing and 280 +/- 30 cSt for compaction).
- Shows the suitability of a cutback or emulsion for spraying, and is used for quality control.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does viscosity of a binder describe?
   A. Its resistance to flow, which controls suitable spraying, mixing and compaction temperature
   B. Its hardness measured by needle depth
   C. Its stretching length before breaking
   D. Its tendency to evaporate
   Answer: A. Viscosity measures flow and consistency of the binder. (Key point: Meaning of viscosity)

2. What is measured in the Standard Tar Viscometer test?
   A. Distance the binder stretches
   B. Time taken for 50 ml of binder to flow through the orifice at the test temperature
   C. Depth of needle penetration
   D. Temperature at which a ball falls
   Answer: B. Viscosity is the time in seconds for 50 ml to pass through the orifice. (Key point: Standard Tar Viscometer principle)

3. Why is the binder held in a water jacket during the test?
   A. To make it harden before testing
   B. To dilute the binder
   C. To keep it at the specified uniform temperature, since viscosity changes with temperature
   D. To clean the orifice
   Answer: C. Viscosity depends strongly on temperature, so the temperature is controlled. (Key point: Test temperature control)

## Related in Transportation Engineering I

- [Tests on bituminous binders](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Abitumen-tests-list) — Transportation Engineering I, chapter 6, asked 8×
- [Types of bitumen and bituminous materials](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Abitumen-types) — Transportation Engineering I, chapter 6, asked 6×
- [Penetration test of bitumen](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Apenetration-test) — Transportation Engineering I, chapter 6, asked 6×
- [Ductility test of bitumen](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aductility-test) — Transportation Engineering I, chapter 6, asked 7×
- [Softening point test of bitumen](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Asoftening-point) — Transportation Engineering I, chapter 6, asked 4×
- [Define bitumen premixes](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Adefine-bitumen-premixes) — Transportation Engineering I, chapter 6, asked 5×