Not quite, but full-depth recycling offers promise of extensive road repairs done faster
Until just the last few years, when a roadway had deteriorated to the point that it needed repairs, city, municipal or provincial transportation managers typically had two main options to choose from. They could either completely rebuild the road or spread a new overlay of asphalt, provided the road was structurally sound.
Each option has its drawbacks, and, often enough, neither repair approach was the ideal solution. A full rebuild, as one might expect, is expensive and relatively time-consuming, potentially keeping sections of road out of commission for lengthy periods. So why do it unless it's absolutely necessary? On the other hand, a fresh overlay might not hold up for long if the base of the road was developing structural flaws.
A third option, known as full-depth recycling, has been adopted with increasing frequency in Alberta since about 2001. The process can be seen as somewhere between the two traditional remedies. It's cheaper than a total rebuild but better at strengthening the road for the long haul than a perfunctory asphalt overlay.
Full-depth recycling is an in situ process that involves recycling the existing roadway and the addition of binders and other materials to produce a stabilized base course.
Full-depth recycling differs from other road rehab techniques like cold planing or mill and fill, cold-in-place recycling, and hot-in-place recycling partly because the rotor or cutting head penetrates completely through the existing asphalt layer and into the underlying base, sub-base or sub-grade layers prior to pulverizing and recycling them. According to the U.S.-based Asphalt Recycling and Reclaiming Association, potential full-depth recycling candidates include parking lots, low- volume, secondary roads, city streets, medium- volume roads, interstate highways, and private and regional airports.
"For the most part in rural areas, roads will just need an overlay," says Jim Gavin, an engineer and roadway construction standards specialist with Alberta Transportation. "But if a deep overlay seems to be needed, [full-depth recycling] can be the better choice, as it's likely to hold up better with a lower life cycle cost over 30 years than a thick overlay - that's anything more than about 120 millimetres. If the road has a lot of transverse and wheel-path cracks, with [full-depth recycling] you bust it up and start with a new base. But, with just an overlay, there is no new base and cracks can propagate upwards, so the road's surface life is not as long."
It was not until 2007 that provincial roads repair projects began using full-depth recycling frequently. In 2010, says Gavin, contractors for Alberta Transportation are doing six full-depth recycling projects, with ones near Olds, Swan Hills and Rocky Mountain House.
STRENGTH AND STABILITY
The objective of full-depth recycling is to strengthen and stabilize the base of the road by recycling the existing roadway without the expense of a total rebuild. It is a simplification, but it could be said that there are two key elements to the process. Once road testing and analysis of core samples and other preparations are completed, full-depth recycling operations typically begin with the recycling and mixing machine digging up all the asphalt and a portion of the underlying materials to a pre-determined depth. The old asphalt is then pulverized. The second key element, typically, is the addition of binder materials, which can be added and mixed in as soon the road materials are pulverized, or added, mixed and spread later.
A 30-plus tonne Terex pulverizer, which is used by West-Can Seal Coating Inc., one of a handful of companies in Alberta that do full-depth recycling work, operates like a sort of rototiller, dredging up the old asphalt and base materials. Using a specialized machine called a reclaimer, the dredged up asphalt and base materials are pulverized into chunks to a specific size, which typically approximates that of the original aggregate pieces.
Depending on the formulation requirements, water and various binder materials are then added and mixed in the pulverized material. Cement, lime, fly ash, asphalt emulsion or foamed, expanded asphalt are common binders that are mixed in the reclaimer to form a new road base. After shaping and grading, the mixture of pulverized materials, water and binders is rolled and compacted to produce a new, strong and durable road base, or stabilized base course, for either an asphalt or concrete surface.
Determining just what is the optimal mix design of binders and other additives for a specific roadway is an important part of the process in a successful full-depth recycling project. Analysis of core samples has a key role in determining mix design. "Good engineering and mix design are critical," says Matthew Arnill, a partner in West-Can Seal Coating.
Referring to a project on Highway 766, he notes that existing materials can change significantly over a 10 km stretch of road, so that two or more samples are taken from each kilometre of road. Samples are analyzed at an independent geo-technical lab. "Ownership of roads has sometimes changed over the years between the province, county or even the town, so different systems and materials may have been used at different times and sections of the road," Arnill says.
Factors to be considered when designing the mix include not only the materials comprising the existing roadway, but also, on occasion, the age of its asphalt. Foamed asphalt is probably the most widely used in Alberta for full-depth recycling projects, but in instances where the asphalt on the existing road is old and brittle, an asphalt emulsion can be the better choice.
"It won't dry as fast as a foamed asphalt mixture, but emulsion has solvents that soften the old asphalt," says Dave Rose, sales manager for West-Can Seal Coating. "So where the old asphalts are relatively harder, emulsion is a better product at the end. It's better at achieving a more tightly bound base mixture."
Older second-tier roads, he says, often have significant amounts of clay in their composition. The binder mix for these will typically include Portland cement, fly ash and perhaps some lime. But if the asphalt content is high, then an asphalt binder is used.
Besides eliminating the potential of reflective cracks propagating up from below the new overlay, full-depth recycling has other advantages:
- The roadway is easily widened and the cross-slope and crown easily restored.
- It conserves energy and materials as existing ones are re-used in situ, not hauled away, which reducesing material handling and trucking.
- The shorter construction time cuts costs and reduces traffic delays.
- In Alberta, improved technologies and its lower cost, compared to a total rebuild, appear to have had a role in spurring increased acceptance and use of full-depth recycling.
"[Full-depth recycling] has become more prevalent partly because of economics," confirms says Roman Stefaniw, president of J.R. Paine & Associates Ltd.
"In the last 10 years, technology has improved with better emulsions and foamed asphalt stabilization. Also, the types of equipment for pulverizing and application have become more high-tech. There are different types of equipment and different systems, but all working toward the same successful result."
The geo-technical and construction materials engineering firm's services include materials testing, consulting services and mix designs for clients with full-depth recycling projects. Once a client has provided roadway samples, the firm first evaluates whether the project is a suitable candidate for the process. From there, a process of sampling and analysis leads to the development of a precisely calibrated design mix of water, binders and additives for the pulverized materials.
Although a single- pass operation is sometimes feasible, pre-pulverizing is often done a few days before mixing the binder/additive cocktail with pulverized materials and laying the resulting new stabilized base course.
Although the exact design mix differs between each project, most full-depth recycling projects in Alberta have been using foamed asphalt as the main binding component. When cold, atomized water is added to hot asphalt (165 degrees Celsius), the asphalt foams and within seconds expands 20 times or more in volume. The asphalt's viscosity is sharply reduced and it becomes stickier.
"This enables it to coat small particles and to form a mastic to hold larger particles in place," says Hugh Donovan, a construction services engineer responsible for pavement design with the City of Edmonton's transportation department.
Besides benefiting from better delivery technologies, the process, which was patented in 1956, began attracting more interest after its patent expired in 1996, he says. The equipment, he says, does not come cheap. Although licence fees for the process are now behind them, contractors can expect to shell out almost a million dollars for some of the latest foamed asphalt-capable road-reclaiming machines.
Once the new stabilized base course is laid, a three-stage rolling process takes place in preparation for the overlay. Donovan says about 99 per cent of the city's full-depth recycling projects are done with foamed asphalt. In the last eight years, the city has applied the process to about two million square metres of roadway.
It doesn't take long to cover a lot of ground - or roadway, when using full-depth recycling. Dave Fath, a co-owner of O'Hanlon Paving, points to a project near Rocky Mountain House, where a 330,00 sq. m section of Highway 22 are is getting the treatment. It's a two-month project, he says.
He says that another advantage of the foamed asphalt process is that the surface is ready for capping with the overlay within a couple of days, while emulsion can take five 5 to 10 days or more before it is ready.