If it seems like winter potholes and summer road repair are on an endless cycle, you’re right. For most pavement repairs, the temperature matters.
Asphalt is a little like zucchini. And asphalt plants are like a zucchini garden. Each has a season and they are most productive when the weather is warm. You could even argue there’s an oversupply of both zucchini and road repair work in the hottest seasons.
This may be news to people who don’t pay attention to roads, road building and pothole remediation. But it’s true: The trillion dollars worth of asphalt connecting America from coast to coast can divide time into two parts, when the damage is done (winters) and when it gets fixed (summers).
That’s because potholes largely form from the freeze-thaw cycles of late fall through winter and early spring while the repair work – with hot asphalt – mostly happens in drier, warmer seasons. This varies by latitude, where winter is shorter or longer. But critical to the repair cycle is that hot asphalt from the plant be kept above a certain temperature until it’s installed.
Side note: In some regions the cause of potholes is simply moisture (Honolulu, Los Angeles, Houston and Miami get potholes too).
And to be clear, some potholes get fixed in the dead of winter where there are safety-critical issues in play. This is why a few asphalt plants are kept open in major metropolitan areas all year round. Those repairs are ultimately more expensive and harder to do, often with specialized equipment. For routine maintenance on most roadways, it’s not possible to keep the material sufficiently hot while repair crews search for potholes, then clear, dry and pack them in time.
Otherwise, temporary “cold patch” asphalt is used to reduce hazards for a few weeks or months until hot asphalt is available and can be used (to adhere to fixed pavement it has to be dry and free of debris, which is unlikely in winter conditions).
There is also higher-quality, higher-performing polymer-modified cold patch material that can be used as a permanent fix in winter. The material is more expensive, however, but it arguably saves money because it only requires a one-time, even-in-winter application.
As with most zucchini farms, asphalt plants are largely out of view in industrial districts or rural areas. We just expect smooth roads, just as we like a nice side of roasted zucchini on our plates without thinking about where it was farmed, picked, processed, shipped and sold.
The business of asphalt production has some variation, which is to say not all asphalt is created equal. A batch asphalt heater plant, used for shorter, specialized runs, sends aggregate (angular crushed rock) through hoppers into a heater drum, after which it is sorted by size and then mixed with bitumen, the sticky, black and viscous liquid derived from petroleum (also called tar).
This compares to a continuous asphalt plant, where the aggregate is fed at a continuous rate into the heater drum. There, it is coated with the bitumen. In both plants, the combined material needs to be kept hot for applying to potholes, else it gums up and hardens. Timing is critical – and if outdoor temperatures are cool, the process has to move more quickly.
Modern asphalt plants are highly technological. They use a combination of programmable logic controllers and industrialized computer controls to manage the process. But the nature of asphalt production and its application still depends on the temperatures and moisture that nature provides.
So next time you’re in a backup of summertime traffic caused by road repair operations, take heart: it’s necessary to ensure a smoother ride all year long. You might also find a bushel of zucchinis when you get home, left by a friend whose garden is also in seasonal hyperdrive.