Specific Gravity of Bitumen: Pycnometer Test and Formula

The specific gravity of bitumen tells you how heavy the binder is compared with water. It sounds like a minor lab number, but it is the bridge between the tons a supplier invoices and the gallons a plant measures in its tanks. It also feeds the volume math used in every asphalt mix design. This guide covers the standard pycnometer test, the formula, a worked example and the practical conversions it makes possible.

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Quick facts

Standards: ASTM D70 (pycnometer method), AASHTO T 228; IS 1202 in India.
Test temperature: 25.0 degrees C, held within 0.1 degree in a water bath.
Equipment: a 24 to 30 mL glass pycnometer with a stopper, a balance reading to 0.001 g and a constant-temperature bath.
Result: a ratio reported to three decimals. Paving binders land just above 1.000.
Symbol in mix design: Gb.

Specific gravity vs density

Specific gravity (also called relative density) has no units. It compares the weight of a volume of bitumen to the weight of the same volume of water at the same temperature. Density is mass per volume, in g/cm³ or kg/m³. To turn one into the other at 25 degrees C, multiply the specific gravity by the density of water at that temperature, 0.9970 g/cm³. A binder with a specific gravity of 1.030 therefore has a density of about 1.027 g/cm³.

How the pycnometer test works

Calibrate the bottle. Weigh the clean, dry pycnometer and stopper (A). Fill it with freshly boiled and cooled distilled water, let it sit in the 25 degree bath for at least 30 minutes, dry the outside and weigh again (B).
Add the sample. Warm the bitumen just enough to pour, stirring so no spot overheats, and fill the empty dry bottle about three-quarters full. Let it cool, then weigh it with the stopper (C).
Top off with water. Fill the rest of the bottle with water, seat the stopper, return it to the bath for at least 30 minutes, then dry and weigh (D).
Calculate the specific gravity with the formula below and report it to the nearest 0.001.

Air bubbles are the usual source of error. The sample has to cool without trapping air, and the water added in step 3 must fill every gap around the binder.

The formula

Specific gravity = (C − A) ÷ [(B − A) − (D − C)]

The top line is the mass of bitumen in the bottle. The bottom line is the mass of water that the bitumen pushed out: the full-of-water mass minus the water that still fit around the sample.

Worked example (made-up readings)

A, empty pycnometer: 29.500 g
B, pycnometer + water: 55.300 g, so the bottle holds 25.800 g of water
C, pycnometer + bitumen: 48.700 g, so the sample is 19.200 g
D, pycnometer + bitumen + water: 55.900 g, so 7.200 g of water fit around the sample

Water displaced by the bitumen = 25.800 − 7.200 = 18.600 g. Specific gravity = 19.200 ÷ 18.600 = 1.032. Density at 25 degrees C = 1.032 × 0.9970 ≈ 1.029 g/cm³.

Turning tons into gallons

Binder is sold by the ton and stored by the gallon, so plants convert all day. An Iowa DOT worked example uses 8.67 lb per gallon at 60 degrees F for binder. At that weight, one ton is 2,000 ÷ 8.67 ≈ 231 gallons. Because hot binder expands, a reading taken from a hot tank first gets a temperature volume correction back to 60 degrees F before that conversion is applied. Skip the correction and the binder count will run high.

Where Gb shows up in mix design

Gb is one of the inputs used to calculate how much binder the aggregate soaks up, how much effective binder is left to coat the stone, and the voids filled with asphalt. It also appears in the back-calculation of the aggregate's effective specific gravity from the theoretical maximum specific gravity. Small errors in Gb carry into those figures, which is why labs use the measured value from the binder supplier rather than a rounded 1.00.

Does a homeowner need this?

Not directly. If you are pricing a driveway, the numbers that matter are area, thickness and the in-place weight of the finished mix, which our driveway asphalt calculator handles. Binder specific gravity matters to plants, labs and anyone buying liquid binder in bulk, such as sealcoat or chip seal crews.

Questions people ask

What is the specific gravity of bitumen?

It is the ratio of the mass of a volume of bitumen to the mass of the same volume of water at the same temperature, usually 25 degrees C. Paving binders sit a little above 1.0, so a gallon of binder weighs slightly more than a gallon of water. Use the number on your supplier's certificate rather than a guess.

How is specific gravity of bitumen tested?

With the pycnometer method in ASTM D70 (AASHTO T 228). A small calibrated glass bottle is weighed empty, full of water, about three-quarters full of bitumen, and then with the bitumen plus water topping it off. All weighings that involve water happen at 25 degrees C in a controlled bath.

What is the formula?

Specific gravity = (C - A) / [(B - A) - (D - C)], where A is the empty pycnometer, B is the pycnometer full of water, C is the pycnometer with the bitumen, and D is the pycnometer with bitumen and water. Multiply by 0.9970 to get density in grams per cubic centimeter at 25 degrees C.

Why does specific gravity matter on a paving job?

Binder is bought by weight but measured in tanks by volume. Specific gravity links the two. It is also an input to mix design math for effective binder content, absorbed binder and voids in the mix.

Does temperature change the result?

Yes. Bitumen expands when heated, so the same tonnage takes up more gallons hot than cold. That is why results are tied to a reference temperature and why plants apply a volume correction factor to hot tank readings before converting gallons to tons.

How many gallons of asphalt binder are in a ton?

It depends on the binder's weight per gallon. Using 8.67 pounds per gallon at 60 degrees F, the figure in an Iowa DOT worked example, one ton (2,000 lb) is about 231 gallons. A heavier or lighter binder shifts that number a little.

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