Everything you want to know about asphalt
Question: Why can’t you asphalt pave at night or during the winter months?
Fall weather, with the cooler temperatures it brings, poses a challenge for hot mix asphalt paving on the Grand Avenue bridge project. The hot asphalt mix pavement we drive on (also simply called asphalt) is about 95 percent crushed stone, gravels and sand all bound together with about 5 percent asphalt cement, also known as oil.
The oil that is used to bind these aggregates together so nicely is a by-product of processing crude oil. Heated up and mixed together at a hot mix paving plant, the engineered mixture is hauled to the site and typically dumped into the hopper of a paving machine. The paving machine operator adjusts the machine to place the asphalt at the specified width and thickness, often requiring multiple layers, or “lifts.” A 6-inch thick pavement is regularly placed in three lifts.
Gregg Rippy with Grand River Construction is our asphalt paving contractor on the Grand Avenue bridge project. Gregg has been in the asphalt business for more than 50 years, and he knows quality. There are always challenges to ensuring the highest durability for asphalt pavement, and one of those challenges is the weather.
In Colorado, temperatures begin to drop this time of year. CDOT’s paving specifications do not allow for asphalt placement in most situations until minimum air and surface temperatures are 50 degrees and rising. This is because it becomes more difficult to compact the pavement to the required density. Freezing temperatures at night this time of year result in less available production time as daylight diminishes.
Both the new Eighth Street connection roadway (between Defiance Street and Midland Avenue) and Exit 114 eastbound off-ramp will be asphalt paved in the upcoming weeks.
Rippy explained that the two factors that weigh into asphalt durability are temperature and compaction. Compaction is the biggest factor in longevity of the asphalt, but temperature is the most unpredictable. In order to get good compaction, the weather along with the oil mix must be just the right conditions. Rippy consults with a Denver-based road base tester to ensure the mixture and compaction meets CDOT specifications.
There are also asphalt pit restrictions in place, meaning asphalt plants are not open in the evening due to the proximity to residential communities and noise ordinances. For comparison, in Utah asphalt plants are open 24 hours a day, allowing for night paving.
When the asphalt mix is placed at the paver, it can often be near 320 degrees to 350 degrees Fahrenheit. Optimal compaction occurs within a range of temperatures, and there are different grades of asphalt depending on the type of pavement design, but in general, it needs to be compacted before it cools too much and becomes almost brittle under the rollers.
On the Grand Avenue bridge project, the specifications call for a higher grade of polymer for next year’s Midland Avenue overlay project. For quality compaction at the higher grade of polymer mixture, some specifications call for the temperature to be at 60 degrees and rising. The specifications for the Midland overlay are the same durability and specifications used on CDOT Interstate 70.
Upcoming paving overlay projects include the Midland Avenue asphalt overlay and the West Sixth Street asphalt overlay project. Midland Avenue from Eighth Street west to Exit 114 will receive an asphalt overlay prior to the 2017 traffic bridge detour.
• Eighth Street connection project will begin work on the intersection of Eighth and Midland Avenue this week. A new right turn lane will be constructed as well as a wide turn radius for trucks turning onto Eighth.
• Utility project is almost wrapped up as the final stages of the north and south utility vault are underway.
• Reminder: Eighth Street bridge closure is scheduled for Oct. 8-18.
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