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

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.

Daniel Pearce

Director - Pearce Geotech

The ZFG 3000 is an excellent tool and should be included in any professional contractors equipment list of assets. This tool should have a similar impact on the construction industry as the laser level. The ability to accurately and reliably measure this aspect of ground condition will improve the quality of the work and remove doubt as to the progress of the project.

I bought my ZFG 3000 and on the same day less than two hours later, it had paid for itself. Using this instrument, I was able to demonstrate scientifically, repeatably and independently the quality of the work that we had completed for our client. Ending the disagreement and creating an improved working relationship. Most jobs are not as confrontational as this was however, most jobs are built to a standard. The contractor can now measure as required, to know on site, at the time of testing if this standard is being achieved.

I strongly recommend the ZFG 3000 Deflectometer to any earthmoving, civil construction contractor. Good luck with your projects.

Jim Bassett

Managing Director, Bassearth Pty Ltd

Our Plate Load equipment is getting a lot of use. We’re very happy with it. 🙂

Mary Flux

Director & Engineering Geologist, WANT Geotechnics

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