Bitumen Viscosity Grades VG-10, VG-20, VG-30 and VG-40 (IS 73:2013)

Paving bitumen in India is classified by viscosity at 60 °C under IS 73:2013 into four grades: VG-10, VG-20, VG-30 and VG-40. The minimum viscosity in poise is simply the VG number × 80 (VG-30 → 2400 poise), and a higher VG number means a stiffer binder: VG-10 for spraying and very cold regions, VG-30 for most highways, and VG-40 for hot climates and heavily loaded, slow-moving traffic.

Why India moved from penetration grade to viscosity grade

Earlier, bitumen was sold as penetration grades such as 80/100, 60/70 and 30/40, based on how far a standard needle sinks into the bitumen at 25 °C. The problem: a pavement in India often reaches 60 °C or more in summer, and bitumen is mixed at about 150 °C. Two binders with the same penetration at 25 °C can behave very differently at those temperatures, so penetration grading did not control rutting or workability well.

Viscosity grading (introduced in IS 73:2006 and revised in IS 73:2013) fixes the binder's behaviour at the temperatures that matter:

• Absolute viscosity at 60 °C – close to the maximum pavement temperature, so it controls resistance to rutting.
• Kinematic viscosity at 135 °C – close to mixing and compaction temperature, so it controls workability.
• Penetration at 25 °C is still tested, as a check at normal service temperature.

VG grade vs old penetration grade

The equivalence is approximate – the grading systems measure different properties, so a VG-30 is not identical to an old 60/70.

IS 73:2013 requirements (key values)

What each test tells you

Penetration test (IS 1203)

A standard needle loaded to 100 g is allowed to sink into the bitumen at 25 °C for 5 seconds. Penetration is reported in units of 0.1 mm. Higher penetration = softer bitumen. A VG-40 must show at least 35 (3.5 mm).

Softening point – ring and ball (IS 1205)

A steel ball rests on a bitumen disc in a brass ring, and the bath is heated at a controlled rate. The temperature at which the softened bitumen, carrying the ball, sags through 25 mm to touch the bottom plate is the softening point. Higher softening point = better resistance to flow in hot weather.

Ductility (IS 1208)

A briquette of bitumen is pulled apart at 25 °C at 50 mm/min; the distance it stretches before breaking is the ductility in cm. In IS 73:2013 ductility is measured on the residue after thin-film oven ageing, which shows how the binder will behave after mixing.

Viscosity (IS 1206)

Absolute viscosity at 60 °C is measured with a vacuum capillary viscometer and kinematic viscosity at 135 °C with a capillary viscometer. These two decide the grade.

Other tests

Flash point (IS 1209) is a safety check for heating in the plant; solubility (IS 1216) confirms purity; the thin film oven test (IS 9382) simulates the hardening that happens during hot mixing.

Worked example 1: which grade does this sample meet?

A lab reports the following for a bitumen sample:

• Absolute viscosity at 60 °C = 2650 poise
• Kinematic viscosity at 135 °C = 380 cSt
• Penetration = 52 (0.1 mm)
• Softening point = 49 °C

Result: the sample satisfies these VG-30 limits but not VG-40. It can be accepted as VG-30 only if the remaining tests (flash point, solubility, TFOT residue) also pass.

Worked example 2: bitumen quantity for a BC layer

Data (assumed for illustration): 1 km road, 7.0 m carriageway, 40 mm compacted bituminous concrete (BC), binder content 5.5% by weight of total mix, compacted density 2.35 t/m³.

• Compacted volume = 1000 × 7.0 × 0.040 = 280 m³
• Mass of mix = 280 × 2.35 = 658 t
• Bitumen = 658 × 0.055 = 36.2 t of VG-40 or VG-30 (as specified)

Use the binder content and density from your approved job mix formula, not these assumed values. Our bituminous mix design calculator works out Marshall parameters and optimum binder content.

Choosing the grade

• Hot plains, heavy truck traffic, slow or standing traffic: VG-40 in the wearing course (and often DBM) resists rutting.
• Normal highways and city roads in most of India: VG-30.
• Cold, high-altitude regions: VG-20 or VG-10, so the binder does not crack in winter.
• Surface dressing / spraying: VG-10.
• Always follow the grade named in the contract or the MoRTH/IRC specification for the project.

Site tips and common mistakes

• Overheating. Keeping bitumen too hot for too long hardens it (oxidation) and can make a VG-30 behave like a harder grade. Follow the specified heating and mixing temperature ranges.
• Mixing grades in one tank. Topping up a VG-30 tank with VG-40 gives an unknown grade. Empty or label tanks properly.
• Accepting on the supplier's certificate alone. Test samples from each lot at an independent lab, at least for viscosity, penetration and softening point.
• Confusing grade with mix. VG-40 does not fix a bad gradation or low compaction; mix design and rolling matter as much as the binder.

Preparing for AE/JE exams? Bitumen grades and tests are frequent questions – try our free mock tests.