Intelligent Compaction Measurement Value vs. Density Direct: What’s the Difference?

We’ve seen a lot of advancements in Intelligent Compaction (IC) over the past few years aimed to reduce the guesswork in the paving process. In fact, the Federal Highway Administration (FHWA) continues to work with many states and manufacturers to enhance IC systems that provide real-time information to the operator that’s critical to achieving target density. These factors include number of passes, mat temperature and Intelligent Compaction Measurement Value (ICMV) — each allowing operators to make informed decisions on the fly.

But not all IC systems solely measure ICMV. Volvo compactors that utilize Compact Assist with Density Direct™, for example, actually deliver real-time, in-place density calculations — taking ICMV a step further. Take a look at the difference. 

HOW INTELLIGENT COMPACTION WORKS

Intelligent Compaction systems feature an accelerometer-based measurement system, as well as GPS, infrared temperature sensors, and an on-board computer that provides real-time compaction information to the operator. The primary features of IC systems — common among all providers — are pass mapping, surface temperature mapping and data collection.

• Pass Mapping – IC systems provide real-time information to the operator, showing how many passes have been completed and ensuring the operator can gain better uniformity across 100% of the mat. Over-compaction is equally as detrimental to quality as under-compaction, and pass mapping can help ensure the operator isn’t making too many or too few passes.
• Temperature Mapping – IC systems display real-time pavement surface temperature information to the operator. This is incredibly important, as the temperature greatly impacts the material’s ability to be compacted. Armed with real-time temperature information, a skilled compactor operator will know to make adjustments to the paving process to compact the asphalt at peak temperatures and avoid the need for additional passes that can drive up project costs and extend timelines.
• Data collection – The FHWA recognized that IC data could provide significant value for quality control protocols, so one important factor is ensuring that data is collected and reported in a standard format among all manufacturers’ IC systems.

While these three features are more or less standard across all IC systems, what varies significantly between systems is how the asphalt ICMV is calculated. The majority of IC systems offer real-time asphalt ICMV calculations; however, each manufacturer has a slightly different methodology for how it’s done. Despite the variance in calculations between manufacturers, each has the same goal in mind — to equate ICMV to density.

WHAT IS THE INTELLIGENT COMPACTION MEASUREMENT VALUE?

Intelligent Compaction Measurement Value is a broad industry term for a measure of stiffness that’s calculated using accelerometer measurements on vibratory compactors. ICMV isn’t a measure of density itself — it’s a value used in the calculation of it.

In 2014, the FHWA released a report about a study that attempted to identify a clear correlation between ICMV calculations and in-place asphalt densities. The report found that ICMV values alone can’t reliably estimate core densities, stating, “The pass-by-pass ICMV correlate well with nuclear density gauge measurements during breakdown compaction. The final ICMV does not correlate well with core densities.”

There are numerous factors cited in the lack of correlation including measurement depths, footprints and a change in drum rebounds when asphalt temperatures drop below a certain threshold. The report does, however, insist that IC systems “can be used as an enhanced tool for quality control by monitoring ICMV in real time during construction to maximize the window of opportunity for compaction.”

HOW VOLVO DENSITY DIRECT PROVIDES OPERATORS WITH REAL-TIME DENSITY VALUES

Density Direct — a technology exclusive to Volvo compactors — is an alternative method to predicting density. As with other IC systems, Volvo Compact Assist with Density Direct offers surface temperature mapping, pass mapping and data collection capabilities via the Volvo Co-Pilot display. However, the Volvo system offers the added benefit of real-time density calculations. At the touch of a screen, the compactor operator can see the predicted density of the material being compacted, and unlike ICMV values, which are unitless, Density Direct data is expressed on a scale of percentage of air voids removed.

Within the Density Direct system is a screen where the user sets up the system for a calibration test strip. After starting calibration, the system automatically plots the area for the operator to cover with four vibratory passes. These passes are used to train the systems artificial neural network. When complete, the calibration is saved, and the Volvo Co-pilot will display predicted density as a percentage. For best accuracy, pavement core samples are taken from the calibration strip (after final compaction) and tested in a laboratory. These values can then be put into the calibration to increase accuracy of the predicted density. ICMV does not require calibration, but it also does not correlate easily to density results.

The user interface of Volvo Compact Assist with Density Direct depicts a density map, showing each square unit of rolled surface in a color representing predicted density. It also provides real-time numerical readings of predicted density that come from an average across the width of the drum, surface temperature and pass count displayed on one side of the screen. The user also can toggle between density mapping, surface temperature mapping and pass mapping. The pass mapping function captures each compactor pass and drum overlap with a different color so the operator can easily see gaps and work to maintain uniform coverage. Temperature mapping shows the operator the path of the compactor overlaid with the recorded surface temperature.

THE ADVANTAGES OF INTELLIGENT COMPACTION WITH DENSITY DIRECT

Road contractors who use Intelligent Compaction are in a better position to win DOT bids and are better able to make quality control decisions on the fly. Those who choose Density Direct have the added advantage of a proven correlation to core densities as compared to these other systems.

As the necessity for IC grows, contractors need to familiarize themselves with the technology to stay competitive. Multiple states have already adopted IC specifications and are mandating that road contractors use IC systems on certain projects. This number is only expected to grow. Road contractors who have demonstrated experience with IC systems will have a clear advantage working on DOT jobs with IC specifications.

BUILD LASTING ROADS WITH VOLVO DENSITY DIRECT

If you haven’t yet run an Intelligent Compaction system, we offer Compact Assist Start which can help you take the first step. It’s available on Volvo DD110C, DD120C, DD128C and DD140C asphalt compactors. This entry-level package is easy to use with no setup required and includes pass mapping and temperature mapping functionalities, giving your operators real-time data about how the job is progressing.

Read more about Volvo Compact Assist with Density Direct to see additional advantages this system provides. You can also get step-by-step instructions on how to activate and calibrate Compact Assist with Density Direct on Volvo asphalt compactors.

By Mark Eckert – Product Manager, Compaction, Volvo Construction Equipment

Mark currently serves as the product manager for compaction equipment for Volvo CE. He has 20 years of experience in the construction equipment industry and currently trains customers on paving and compaction best practices, construction equipment and material handling applications.