Even the most skilled compactor operators have to do a great deal of guesswork throughout the process.
This article is the last of a five-part series on paving provided by Volvo Construction Equipment.
The lifespan of an asphalt road and its need for future repairs is largely dependent upon the final density of the finished pavement — so much so that density is one of the primary measures of job quality for DOTs. But even the most skilled compactor operators have to do a great deal of guesswork throughout the process, and they must rely on time-consuming nuclear gauge testing and destructive core sampling after the asphalt has cooled down to ambient temperature in order to get in-place density measurements. The results of these tests sometimes require the contractor to go back and correct areas of imperfection, which can drive up costs and reduce the likelihood of receiving a bonus. But this is all changing.
Technology advancements over the last decade — deemed Intelligent Compaction (IC) — offer opportunities to dramatically reduce the guesswork in the paving process. The Federal Highway Administration (FHWA) has been working hand-in-hand with a slew of states and manufacturers to develop and test IC systems that can help provide real-time information to the operator that is 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. However, up until now, no system has been able to deliver real-time, in-place density calculations.
In fall 2015, Volvo Construction Equipment is releasing Volvo Intelligent Compaction with Density Direct — the industry’s first real-time density mapping technology. The quest for this technology has been a long one, largely fueled by the industry’s need for better construction practices to extend road life in the face of funding uncertainties.
A Half-Century of Advances
IC technology was first introduced in Europe in the late 1960s and was used strictly for soil compaction applications. In the mid-2000s, the FHWA championed an effort to apply similar technology to asphalt compaction, and by 2006, the initiative had received funding under the Transportation Pooled Fund (TPF) Program. The TPF involved states that agreed to test and explore the benefits of IC systems for major highway projects — a program that lasted until 2011. During this time, multiple suppliers (including OEMs and aftermarket technology providers) entered the IC market. Dramatic improvements were made to hardware in the forty years since its development, yet the principles of intelligent compaction remained largely unchanged.
How IC Works
IC 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 percent 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 two few passes. In general, there should be a prescribed number of passes for a job (rolling pattern), but that number of passes only achieves target density if compacting at the correct asphalt temperature and at the correct amplitude settings.
• 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 distance behind the paver in order 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. Of critical importance, however, was ensuring that data was collected and reported in a standard format among all manufacturers’ IC systems. Thus, the FHWA recommended that all systems export data in a standard format for use in a program called Veta — a map-based software originally funded by both FHWA and Minnesota DOT.
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. This correlation (or lack thereof) between ICMV calculations and real-world asphalt density values has been subject to a great deal of scrutiny in the past several years.
Comparing ICMV and Density
In 2014, FHWA released a report about a study that attempted to identify a clear correlation between ICMV calculations and in-place asphalt densities. The study involved IC testing and evaluation on asphalt paving projects throughout Utah, Florida, Ohio, Maine, California, Idaho, Maryland, Kentucky and Washington. The FHWA evaluated the ICMV calculations of IC systems from manufacturers including Hamm, BOMAG, Caterpillar and Sakai. These calculations were then compared to actual asphalt density values from core samples. After two years of testing and evaluation, the FHWA released a 321-page report in December 2014 outlining their findings.
ICMV: Not Correlated to Density
The report found that ICMV values alone cannot reliably predict 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.”
Numerous factors are 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 in order to maximize the window of opportunity for compaction.”
Entering a New Age
In 2008, the FHWA devoted $200,000 under the Highways for LIFE Technology Partnerships program with the University of Oklahoma (owner of multiple patents for this technology) for studying an alternative method of determining density. Originally termed Intelligent Asphalt Compaction Analyzer (IACA), this technology is what eventually came to be named Volvo Density Direct.
Density Direct was used on various full-depth and overlay asphalt pavement projects and later evaluated at independent sites throughout the country. The results were overseen by University of Oklahoma researchers and showed that Density Direct calculations were proven to be within 1.5 percent of core samples every time at 180 test locations.
This technology has been licensed to Volvo by the University of Oklahoma and integrated into the newly available Volvo IC system. As with other IC systems, Volvo IC with Density Direct offers surface temperature mapping, pass mapping and data collection capabilities. However, the Volvo system offers the added benefit of real-time density calculations. At the touch of a screen, the compactor operator can see real-time density information across 100 percent of the mat, and unlike ICMV calculations, the Density Direct data has a proven correlation to actual core densities within 1.5 percent accuracy.
On the Job With Density Direct
Within the Density Direct system is a calibration screen, where the user sets the range of densities that are based on the mix design sheet for the lift being constructed. Calibrations also are saved, so operators can assign calibrations to certain elements of a job, and quickly switch back and forth between calibrations to meet the demands of specific job elements.
The user interface of Volvo IC with Density Direct depicts a density map, showing each square unit of rolled surface in a color representing density, and also provides real-time numerical readings of density, 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 that 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.
Density Direct has the advantage of a proven correlation to core densities as compared to other systems, but regardless of which IC system a road contractor chooses, one thing is clear: road contractors that use IC are in a better position to win DOT bids and are better able to make quality control decisions on the fly.
Get on Board or Miss the Boat
Road contractors aren’t always immediately receptive to new technologies that come with new expenses. However, the necessity for IC is growing, and contractors need to familiarize themselves with the technology in order to stay competitive. Currently, 20 states have adopted IC standards and are mandating that road contractors use IC systems on certain projects. This number is only expected to grow, and as such, road contractors who have demonstrated experience with IC systems will be the ones most apt to win the bids.
As a road contractor, you should be asking yourself which costs more — an IC system, or lost bids and reduced pavement quality.
Series List:
Part 1: Precision Paving Begins With Proper Equipment Setup
Part 2: Precision Asphalt Paving 101: Laying a Quality Mat
Part 3: Precision Asphalt Paving: Best Practices for Compaction
Part 4: Precision Asphalt Paving — Rolling Pattern, Safety
Part 5: Past, Present and Future of Intelligent Compaction
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