The difference between temporary and permanent pothole repairs illustrates how there are different kinds of asphalt. So why not fix potholes right the first time?
We tend to think of road pavement, most of which is asphalt (some is concrete), as generic. In fact it’s not. And that’s why most municipalities dealing with winter potholes use a type of temporary pothole filler. Come spring and summer, road crews return with “permanent asphalt” to do permanent fixes.
Which begs some obvious questions: Why fix something twice? Does that not double the work and double the materials used? And double the costs?
The answer is in the nature of asphalt formulations – there are many – and how winter weather makes road repairs difficult all around. That said, there are newer types of asphalt and methods that overcome the challenges of ice, snow and cold temperatures – fixing roads faster and probably reducing overall costs (there are several variables). This gives motorists and taxpayers something to cheer.
To understand why there is this pothole repair two-step, it helps to look at the different formulations for the binder – which in the trade is referred to as “asphalt cement,” the black gooey stuff that the holds the rocks and sand (“aggregate”) together. And even the aggregate itself can vary from region to region, affecting the binder. Most binder is hot-mix asphalt (HMAs), but there are also cold-mix asphalts (CMAs). The use of each roughly corresponds with the seasonal temperatures: HMA in summer, CMA in winter.
To achieve a more resilient, permanent asphalt road, the all-important physical task in this process is compaction. A mix laid down into a pothole, for example, will be more permanent if the air between the aggregate and binder is minimized (7% air voids or less). Rolling a heavy truck tire or rollers over newly laid asphalt, or by way of hand-held tampers, is the primary means for achieving that compaction.
But it’s not solely dependent on the brute force of pressing the asphalt together. With hot-mix asphalt, that compaction has to be done while it’s still hot. If the mix has cooled before compaction, as will happen in colder weather, there will be more air voids. Which is why permanent pothole repairs are generally done in warmer weather. It’s not only difficult to keep the asphalt mix hot after leaving the asphalt plant, in many markets where winter is long and cold the asphalt plants simply cease operations during winter.
This is why most potholes get a temporary, loose aggregate of cold mix asphalt. In some cases, the road maintenance crew fixes potholes with a simple “throw and go” treatment. There may be a small amount of compaction achieved with the back side of a shovel, but the first cars and trucks to run over relatively loose piles of asphalt filler are the primary method to tamp down the mix. But that same traffic is also what leads to further deterioration in a relatively short time. The net result is potholes return quite quickly – within days or weeks.
There are better quality cold mix asphalt formulations available. They typically cost more, but the savings come from a single, versus double, pothole repair. This is why one city might have better roads than another nearby.
Also confounding wintertime pothole repairs is the presence of ice or water in potholes. Even hot mix asphalt shouldn’t be laid in wet conditions. But some of the newer cold mix variations can be placed and compacted in water, a significant advantage in winter conditions.
So it is possible to permanently repair potholes with a single treatment in winter months. Unfortunately, that’s not yet how most cities and states currently do it. So temporary fixes in winter are still pretty much the rule, with permanent repairs usually done several months – and many flat tires – later.
