Non Nuclear Density Gauge for Asphalt: PQI and Uses

A non nuclear density gauge gives a quick density reading on fresh asphalt without a radioactive source. It sits on the mat, sends a weak electrical field into the material and reads how the asphalt responds. Crews use it to set rolling patterns and keep an eye on density as they pave.

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How it works

Asphalt is a mix of stone, binder and air. Air has a very different dielectric constant from stone and binder, so as compaction squeezes air out, the material's dielectric response changes. The gauge reads that response through a sensing plate and converts it to density using a calibration for the mix.

Common devices include the Pavement Quality Indicator (PQI) and other electromagnetic gauges from several makers. AASHTO T 343 and ASTM D7113 cover their use on compacted asphalt.

Non nuclear vs nuclear gauges

	Non nuclear	Nuclear
---	---	---
Source	Electromagnetic field	Radioactive source
Licensing	None	License, storage, transport rules, trained operators
Reading time	A few seconds	About a minute or more
Moisture sensitivity	Higher	Lower
Needs core correlation	Yes	Yes, for best accuracy

Calibrating to cores

Both kinds of gauges read best when matched to cores from the same mix.

Pave a test section.
Take gauge readings at several marked spots.
Cut cores at those spots and test them for bulk specific gravity.
Calculate the average difference between core and gauge values.
Enter that offset in the gauge for the rest of the mix.

Repeat when the mix, aggregate source or lift thickness changes.

Getting good readings

Dry the surface: water from rollers or rain changes the dielectric reading.
Seat the plate fully: coarse, open surfaces with gaps under the plate read low.
Take several readings: rotate the gauge and average.
Mind the temperature: follow the maker's limits.
Check lift thickness: gauges measure a certain depth, so very thin lifts read part of the layer below.

Using it to set a rolling pattern

During breakdown rolling, take readings after each pass on a test strip. When density stops climbing, the pass count is set. The gauge is also handy at longitudinal joints, where density is often lowest; see notched wedge joint.

From density to air voids

Percent compaction is density divided by the mix's maximum density, which comes from theoretical maximum specific gravity. Air voids are 100 minus percent compaction. Example (made-up numbers): a calibrated reading of 145.2 lb per cubic foot against a maximum density of 155.0 gives 93.7 percent compaction, or 6.3 percent air voids.

For tonnage on your project, use the asphalt tonnage calculator.

Questions people ask

How does a non nuclear density gauge work?

It sends a weak electromagnetic field into the asphalt and measures the material's dielectric response, which changes with density. Software converts that to a density reading.

Is a non nuclear gauge as accurate as a nuclear gauge?

Both need correlation to cores for best accuracy. Non nuclear gauges can track density changes well once calibrated, but moisture and surface conditions affect them more.

Do non nuclear gauges need a license?

No radioactive source is involved, so they avoid the licensing, storage, transport and operator certification rules that apply to nuclear gauges.

What standard covers non nuclear asphalt density?

AASHTO T 343 and ASTM D7113 cover the use of electromagnetic devices to measure the density of compacted asphalt.

Can non nuclear gauges be used on soil?

Asphalt gauges are designed for asphalt. Soil density uses different non nuclear devices and methods, or the nuclear gauge and sand cone test.

When should cores still be taken?

To set the gauge offset at the start of a job or mix change, and for acceptance testing when the specification requires cores.

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