By: Rajib B. Mallick and Brian Marquis
For most of us living outside its boundaries, Maine is the picture-perfect vacationland. But it is also home to important industries such as logging and paper, and roads in Maine take quite a beating from trucks and the harsh environment with its long, cold and snowy winters.
A good base is the key to building a strong and long-lasting pavement. Realizing this early on, the Maine Department of Transportation (MDOT) embarked on a state-wide program of rehabilitation of its more than 20,000 mile network of low-medium volume hot-mix asphalt (HMA) pavements with cost-effective and environment-friendly methods of recycling those results in a new base course and surface layer.
The two most important things in pavement technology are to use advanced materials and methods for building roads, and to use sophisticated and appropriate technology to evaluate them effectively. MDOT keeps on working in both of these areas with a number of research and evaluation projects.
Since the early 1990s, MDOT has been recycling asphalt pavements. A typical candidate pavement for recycling consists of 3-4 in. of HMA layer over 12-20 in. of base/subbase gravel over a variety of subgrade soils. In most cases, the damage in the base layer precipitates destruction of the entire pavement structure—a fix by milling and overlaying the surface layer in such a pavement is just not the solution. Besides, it involves hauling and dumping existing materials, leading to a cycle of waste of natural resources and landfill space. A much better option, followed by MDOT, is to use the existing HMA layer to produce a better base and then use an overlay of HMA.
The recycling of the existing HMA layer can be done either in-place, with foamed asphalt (FA), or by using a plant-mixing process where the milled recycled asphalt pavement material (RAP) is mixed with emulsion and then put back with a paver as a base course.
In its current form, the process of producing foamed asphalt consists of combining hot liquid asphalt binder with cold atomized water under pressure. Generally, about 2 to 3% water is added to the asphalt binder. The process results in the formation of “foam” by the expansion of the asphalt-water mix, and hence provides a significantly increased surface area. This increased surface area and the considerable reduction of viscosity of the asphalt binder helps in improved coating of a large number of fine aggregates including mineral filler, and provides a uniform mix with stone-on-stone contact in coarse aggregates particles, as well as a significant amount of time during which the mix remains workable in the field.
The central cold mixing plant used for producing foamed asphalt treated RAP generally consists of a foamed asphalt spraying system with a thermostat-controlled heating unit, expansion chambers on spraybars and a pugmill mixer. Heated asphalt binder is supplied from an insulated tank, and pre-crushed and stockpiled RAP is added into pugmill for producing recycled mix.
The key to success in the recycling process is to design the mix and the materials as well as the pavement structure on the basis of sound engineering principles to get the best out of it. Just like bridges and buildings, pavements are engineered structures designed to carry traffic safely and effectively over its life. MDOT has started a number of projects to look at the strength of pavements with foamed asphalt and PMRAP bases. A series of testing evaluation were carried out by MDOT and researchers from Worcester Polytechnic Institute (WPI) to evaluate their strengths and identify potential problems and their solutions.
MDOT and WPI researchers worked side by side on a number of projects in 2004 and 2005 and arrived at several important conclusions. They found that both foamed asphalt and PMRAP improved the strength of the base and the entire pavement significantly; there is considerable variation in the properties of the materials; and that a more suitable grade of asphalt is needed to prevent thermal cracks in the surface HMA layer to therefore extend the life of the entire pavement structure.
In terms of engineering properties critical for designing pavements, foamed asphalt and PMRAP showed similar results. The key lies within the selection of the right technique for the specific road.
Both FA and PMRAP processes use existing materials—either in-place or stockpiled—and the mixes must be designed properly before construction. The design of FA mixes requires laboratory-scale foamed asphalt equipment, which therefore adds to the cost (one time cost).
During construction, the presence of large aggregate particles has a detrimental effect on both products by reducing the bonding properties of the binder. PMRAP material is screened prior to blending, whereas FA is, in most cases, produced in-place and is relatively more difficult to achieve particle size quality control in FA than PMRAP projects. Proper quality control measures are necessary for the FA projects to reduce the amount of large particles. At present, the use of ground penetrating radar (GPR) to detect unknown layers prior to in-place reclamation is being investigated.
The in-place FA process does not require transporting the material to a central mixing plant and is preferred when the project does not involve the need for much realignment or grade changes. On projects that require significant realignment or grade change, PMRAP is the material of choice. RAP material can be stockpiled until the project is ready for application then is blended, transported and placed. Costs are higher for this application as compared to FA due to transporting the RAP from the project to the plant then back to the project after processing.
Foamed asphalt is placed in one operation as thick as 200 mm, but MDOT has limited the depth to 150 mm due to the difficulty of compacting the mix below 150 mm. PMRAP is placed with a paver generally in lifts of 75 mm. For thicker layers of about 150 mm multiple lifts are necessary, which adds to the amount of time the project can be opened to traffic. After placement, FA layers can generally be overlaid with HMA within three days, whereas PMRAP needs five to seven days for curing prior to the application of HMA overlay.
Because of the inherent viabilities of the existing materials along the road, as well as across the sections, a fairly significant variability of the engineering properties was observed in roads reclaimed with both FA as well as PMRAP materials. While this is not a problem, it is critical to determine this variability as best as it is possible and consider it while designing pavement. The use of fast and reliable non-destructive technology can be a big help in this evaluation. Use of techniques such as ground penetrating radar and seismic techniques are being investigated by MDOT at this time for proper implementation in the near future.
While most of the projects are being constructed without major problems, and are performing well, some unique issues have resulted regarding the determination of the structural strength of the reclaimed layers. For example, the existence of thin HMA and, in many cases, unknown layers (such as penetration macadam or old HMA layers) inside the pavement makes it difficult, if not impossible, to obtain reliable results from traditional methods such as the analysis (backcalculation) of data from the falling weight deflectometer (FWD) test. Furthermore, it is not possible to take cores from the entire depth of the reclaimed layers. The bottom half or one third of these layers in most cases are not dense or cohesive enough to hold themselves as a core, therefore the laboratory tests cannot be conducted. For structural evaluation and design of these pavements with confidence, the most practical approach for the determination of strength of these layers seems to be the use of a fast and nondestructive method so that a sufficiently large amount of data can be gathered in a short period of time.
In view of this problem, MDOT, along with researchers from WPI and University of Texas, El Paso, has developed a method based on seismic testing—portable seismic pavement analyzer—to reliably estimate the stiffness of subsurface reclaimed layers. The process is fast and can be used to collect a large number of data—something that is very important for layers that exhibit a large variation in properties. The predicted stiffness can be used effectively in mechanistic empirical design of pavement structures.
Using results of field studies, the researchers modeled these recycled pavements with the newly developed Mechanistic Empirical Pavement Design Software developed by the National Cooperative Highway Research program, and distributed through the Federal Highway Administration to predict their performance in different parts of Maine, from its fairly warm southeastern part to very cold areas in the north and the west.
The major results were well supported by field investigations because asphalt binder with capabilities of performing well at the low temperatures is needed for making these pavements last through the winters, specifically in the northern parts of Maine. Based on these findings, MDOT is considering the use of a PG-34 grade binder in projects in northern Maine instead of the currently used PG-28 grade.
With more than a decade’s experience in recycling, and a continuous strive for adopting and implementing the latest technology, MDOT is making sure that its tax dollars are spent in the most effective way.
About The Author: Rajib B. Mallick works in the Civil and Environmental Engineering Department with the Worcester Polytechnic Institute and Brian Marquis works in the research division with the Maine Department of Transportation.
