How To Test For Compaction Of Gravel Bases Before Laying Asphalt On Top

Sep 24, 2017 | Asphalt Pavement Investigation, Compaction Control

Fulton Hogan Asphalt Placement

One of the challenges our asphalting contractor clients and their consultants have been struggling with is how to test the gravel base before laying asphalt on top to make sure you have compaction.

They have been aware of the problem of non uniform compaction and soft spots but have not had a repeatable fast method of testing.


A case study is their work with councils on suburban streets. They are asked to mill off the top surface and then re-asphalt. Due to changes in the councils asphalt mix requirements, it’s harder to achieve the asphalt density specification (voids). Failure to meet specification can result in significant fines (e.g. $100K+) which can be more than the contractor will make as profit on the job.

Light Weight Deflectometer (LWD) base compaction control

Solving this problem has led them to the Light Weight Deflectometer (LWD), a portable device that provides immediate data on the level of compaction of the gravel base across the pavement. Now, they regard the Light Weight Deflectometer as insurance that the base is compacted properly before they lay the asphalt on top. Fast results are very important as roads are typically closed from e.g. 8pm to 4am and all works need to be completed within this limited time window.

Having this insight has enabled the contractor to have an articulate data based discussion with their client (in this case the council) if non uniformity and compaction below specification is discovered. There is potential for the contractor to perform rectification works, for example.

Armed with this new insight, the contractors are now engaging with Australian Asphalt Pavement Association (AAPA) as the organisation to bring the councils, road authorities and contractors together to work through the issues of responsibility for a poorly compacted base and how to resolve it.

Leading this industry change has been Fulton Hogan in Queensland. This fits well with their Right First Time strategy and drive to use innovative technology to solve practical problems.

FH is now routinely using the LWD with very good feedback and useful engineering data.
Laszlo Petho, Pavements Manager, Fulton Hogan

Having seen the insights the equipment brings immediately at the site, their peers are also now investing in the Light Weight Deflectometer so they can test the gravel base in a fast repeatable way to make sure they have compaction before laying asphalt on top.

The ZFG 3000 Light Weight Deflectometer (LWD) with GPS from Zorn Instruments is an easy to use instrument used to determine the stiffness of a granular base. Stiffness of a base is essentially a measure of a material’s ability to withstand deformation under load.

Fulton Hogan Asphalt Laying

The Light Weight Deflectometer (LWD) was first used in 1998 by the Minnesota Department of Transportation (Mn/DOT) at the Minnesota Road Research Project. Starting in 2005 the LWD has been used by Mn/DOT as an acceptance tool for the compaction of roadbed and miscellaneous embankment and trench construction, culvert treatments and other tapered construction.

Minnesota Department of Transportation (Mn/DOT)

We have found the PANDA to be an invaluable acquisition to our company, using it on projects ranging from Remote Community Housing in Central Australia to a NASA Rocket Launch Facility in Arnhem Land, NT.

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I would hate to go back to doing the Plate Load Test using the old method using dial gauges. It would be so time consuming, not to mention manually recording and then processing all the results.

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