Performance of a pavement is directly related to the density of the asphalt. You may have heard that the real estate agent’s mantra is “location, location, location.” In asphalt pavement construction, it’s “compaction, compaction, compaction.” In this edition of the RoadReady newsletter, we will focus on measuring the density of a pavement and therefore how well it was compacted, as well as examining the challenges around to obtaining proper compaction.
Compaction is how proper density is achieved. Compaction is typically expressed as “percent air voids.” In many mixes, a denser hot mix pavement leads to a higher quality pavement, so density values are often used in industry to pay or penalize paving contractors.
Poor compaction decreases pavement quality
Failures such as rutting, raveling and moisture damage are commonly attributed to poor compaction, and as a result, this is one of the main metrics measured when assessing quality. Low density pavements can impact project payment incentives and in the longer term could cause accelerated failures where poorly compacted pavements may need reconstruction.
Factors affecting compaction
Many different factors can affect compaction, but some of the most important are ground and air temperature, speed and timing of the rollers, haul distance, and roller type.
Temperature, speed and timing of the rollers, and haul distance are important because compaction of dense-graded hot mix below the cessation temperature – the temperature at which the asphalt binder becomes too stiff to allow further reduction in air voids, 175 degrees Fahrenheit – is not effective for compaction and may actually crush the aggregate. These three factors affect how quickly the mix gets to 175 degrees.
The cessation temperature mentioned in the previous paragraph varies from warm mix asphalt, which has a lower cessation temperature than hot mix. The image below is typical of hot mix asphalt.
Type of roller is important because of the compactive forces involved in each type of roller’s operation. For example, vibratory rollers can actually rearrange the aggregate so that the mix can be further compacted, beyond the compactive effort that a static roller can create.
Measuring and expressing density
You may have seen density expressed as “percentage of TMD (Theoretical Maximum Density)” or “percent Rice” – Rice specific gravity is named after James Rice, who developed the test procedure, and is also called theoretical maximum specific gravity. Multiplying the specific gravity by the unit weight of water gives TMD. Percent Rice, or Percent TMD is easy to convert to air voids, since any volume that is not aggregate or binder is air. For example, a density reported as 94% Rice means that there are 6% air voids. The desired density is generally specified by the road owner or contracting agency.
Coring to measure density
Coring is a destructive testing method, which means that the cores are cut and removed from the final product – the in-place asphalt mix. Then they are taken to a lab for density measurement using water displacement.
Coring is generally considered to be the most accurate way to measure density, but the drawback is that it also takes the longest. Results may take several days, making their use in real-time decision making difficult. The number of samples is also limited by the destructive nature of the test.
Portable measurement devices
Field density testing devices include nuclear and electric gauges, which are non-destructive, because they don’t leave a core hole. They provide results quickly.
Non-destructive testing devices can be used for both quick shots to check consistency as well as absolute measurements when calibrated. Fore example, non-destructive tests can be used to monitor density improvements after each pass of the roller to help determine the most efficient and effective compactive effort to achieve proper density. While these devices provide a quick way to measure and compare density, they must be properly calibrated in order to provide accurate results to determine pay factors.
Calibration and correlation factors
To take advantage of the accuracy of coring and the speed of a portable measurement device, one must establish a correlation factor. To do so, use the portable measuring device on a location, then take a core at that same location and measure the accurate density value in the lab. This gives a correlation factor that can now be used to confidently take accurate field measurements using the portable measurement device.
Putting it all together
With proper calibration to a known density value, an accurate and immediate measurement of density using a good quality portable device can be obtained in the field. This provides an advantage to both contractors and road owners in the immediacy of results though cores when measured properly in the lab provide more accurate results. Accurate results determined quickly can help with real-time decision making, allowing for process adjustments to be made, ultimately improving pavement quality which has clear benefits for contractors, road owners, and the traveling public.