Hot, Cold and Recycled: Different Asphalts for Different Conditions
America has spent more than one trillion dollars building its infrastructure of streets, roads, highways and superhighways. Because that building program began decades ago – for cars and bicycles 100 years ago, and in earnest since the 1950s – much of that investment today is crumbling. Potholes are everywhere – but so too is the innovative drive to plug up those breaks in asphalt (most roads are built with asphalt, although some are made with concrete).
It’s an elusive search for the Holy Grail of pavement – the smartest, most economical, efficient and long-lasting repairs that can extend the life of roads. Note, repairs are almost always much more economical and green than the wholesale reconstruction of a road. And as it turns out, there is a range of methods employed in different places at different times of year that seek to avoid reconstruction wherever and however possible.
To most people who work outside of the pavement and surface transportation business, all asphalt looks and functions the same. It’s all small rocks (aggregate), mashed and held together by some kind of black, goopy, hardened emulsifier. It’s laid down hot and steaming. And when road crews fix the potholes, those fixes seem to be temporary at best.
But asphalt is not one thing with one set of physical features. Far from it, in fact.
For example, the little aggregate rocks that are mashed together vary somewhat from location to location. Crushed gravel has to be angular, not rounded, and within a specified range of size. But the bituminous emulsifiers, the black stuff that holds it all together, varies quite a bit by use and location.
This is because bituminous emulsions are more than just petroleum-derived goop. They are complex and specific concoctions, stabilized by intermolecular repulsive and attractive forces. Chemists and engineers who work with emulsifiers have to ensure consistency of the materials so that it is appropriate for the application – for example, the differences between filling a truck-route highway pothole in January and paving a light-traffic parking lot in July. Further, the emulsion selected has to be stored and transported in ways that enable operational efficiencies.
This is a tall order. And, how right those chemists, engineers and road construction or maintenance crews get it affects everyone. The challenge encompasses motorists who want neither bottleneck traffic nor damage to their vehicles’ undercarriage, regional commerce that depends on surface transportation efficiency, and passengers whose safety is in part a function of road quality. We may not have theories of colloid science in our head as we travel the highways and byways, but fortunately, other people have already worked this out for us.
Complicating things further, roads are built in all climates, from the equatorial regions of Africa and South America to the sub-arctic and even arctic ranges of Canada, the U.S., Europe and Asia. At all latitudes, there are challenges to the integrity of the asphalt, and consequently pavement deterioration and potholes can and do occur in all regions – not just the classic temperate climates, where freeze-thaw cycles do a good job of creating potholes.
Pothole repair: Different materials, different methods
The original construction of those roads can be planned, scheduled for the right times of year when temperature and other climactic conditions are ideal. However, the repair of roads, filling potholes so that traffic can keep moving, is perhaps more challenging than building those roads. (Mayors are voted out of office, sometimes, if those darn potholes don’t get fixed in time for an election.) This is why different types of asphalt, largely defined by their emulsifiers and how they are applied, are so very important. We outline several choices here.
Hot mix (warmer, drier weather) – For larger jobs, the economics of a full hot mix operation usually makes sense. The asphalt is mixed at 300 degrees (Fahrenheit, 148 degrees Celsius), then must be kept sufficiently hot while the application and compaction is complete. For the most part, it can only be applied when the ground itself is warm and dry. A cold base will cause the mix to cool before setting, which is the primary reason that winter potholes are filled with one or the other version of a cold patch material. The presence of excess moisture also prevents good adhesion between layers (lifts) of asphalt.
Hot box mix (warmer, drier air) – Stockpiled hot mix that has cooled can be rejuvenated and held at a work site, heated by propane or diesel heating systems. Typically, this is done by the ton (two or four tons at a load), so it is more applicable to larger repair work than one-off potholes.
Recycled hot mix (warmer, drier weather) – The Federal Highway Administration advocates for the recycling of pavement, wherever and whenever possible, for both environmental and cost-management reasons. Recycling can reduce overall costs over the lifecycle of a road, reduce landfill dumping of old pavement and reduce the need to source raw materials. It generally is used in the construction of whole roadways, but is sometimes used to repair sections of streets and highways where there is significant deterioration but a full replacement of the road would be cost-prohibitive. Laying down the recycled material is similar to hot mix.
Temporary cold patch (cool, drier weather) – This is used for pothole patching and other repairs in colder months. Because it can be applied at colder temperatures, road crews can move from site to site without concern for the viscosity of the mix as the day wears on (hot mix would harden before application). Or, the mix might be kept on a general maintenance truck for one-off pothole repairs while crews are tending to a variety of roadway repair matters. It is not durable and consequently is considered temporary, providing just enough patch to last to the warmer months when a more permanent hot mix might be used. Also, moisture at the repair site needs to be dried out (with a version of a hot air blower) in order to achieve some tack between the material and the surrounding intact pavement.
Advanced polymer-modified cold patch (cool and wet weather) – Although more expensive by the pound, a higher-quality cold asphalt mix is formulated to create a permanent fix. Because it can be applied in cold temperatures and in wet conditions, it operationally is very useful for road crews making repairs in the winter (sparing them from making a return repair trip three to six months after application of a temporary cold mix). As with the lower-quality temporary cold patch, small quantities can be kept on hand on all maintenance vehicles, ready for repairs where and when the occasion calls for it. Repairing pavement once instead of two or three times might well offset the higher expense of the base polymer-modified material.
Polyurethane-base systems (all temperature, dried surfaces) – This is a material sold for highway, commercial and residential applications, and is specifically recommended for pothole repair. It sets in about ten minutes, requires no primer, can be used to block moisture in concrete and asphalt fissures, and with the addition of a catalyst, can be applied in cooler temperatures. The surface area where it applied – remember that most potholes are in low, moisture-prone areas – must be dried before application of the material. A treated sand is included with the product, mixed onsite immediately before application.
Polyphalt polymer-modified bitumen technology (all temperatures, wet and dry surfaces) – Engineered specifically for the colder climate of Canada and other countries with similar climates – stemming from research and development at the University of Toronto – this employs a proprietary process that stabilizes plastics in asphalt and the devulcanization of crumb rubber from waste car tires.
Mobile infrared repair-recycle vehicles (all temperature, dried surfaces) – Using heat technology administered by a mobile infrared unit, existing asphalt is recycled in place. Technicians use the unit to warm the area to be repaired, softening the intact pavement around the pothole up to five inches into the asphalt. Once softened, the unit moves aside and the old asphalt is raked and reshaped to fill the pothole (in some cases, oil pitting will require the disposal of old asphalt). A rejuvenating emulsion is then used on the recycled material, usually with some new asphalt added, which is laid down and compacted by a separate compactor while still hot. Different grades of new asphalt are used for summer and winter applications.
“Pothole killer” truck-mounted units (all temperature, dried surfaces) – In use by several municipalities, including Utica, New York, this is a single-person operation that is recognized for taking work crews out of harm’s way. A robotic arm handles the entire operation, hovering several feet over the hole (and in front of the operator, sitting in the truck cab) starting with a blast of air pressure to clean the pothole. This is followed as the arm delivers an emulsion coagulent (tack coat), then a spray of emulsion and stone (tar and rock), filling the hole. A dry coat of stone is applied on top – slightly different procedures and formulations are applied in winter and summer.
Ultra-tough, low-viscosity nanomolecular resin (all temperature, in development) – Researchers at UCLA Engineering (University of California at Los Angeles) are working on a pothole-repair technology that employs a nanomolecular resin that infiltrates pavement cracks, then cures and hardens to form a continuous network of mechanical “cages” that mechanically lock in asphalt aggregates. The product takes “a radically different approach, infiltrating the compacted asphalt-aggregate mixture with an ultra-high-toughness, low-viscosity nanomolecular resin,” says a co-principal investigator of the research program, J.W. Ju, UCLA professor of civil and environmental engineering.
There are other technologies developed by a handful of companies that are proprietary and difficult to examine other than by their benefits. It’s clear that all companies are driving toward providing a more permanent pothole repair, materials and processes that are operationally easy to implement and which are affordable by municipalities, commercial enterprises and homeowners.
Road building and repair methods have evolved considerably in recent years in response to the aging nature of U.S. roads and highways. Much stands to be gained by further research and development of colloid science in bituminous emulsions used by the paving and road maintenance industries.
For now, the smart application of a variety of materials at the right time and in the right places already ensures better economics and road performance than most people know.