Salvage Troublesome Soils With Premium Road Bases

Stable, premium bases can be simple or sophisticated; condition of existing materials, haul distances, and economics of reconstruction for in-place stabilization play a role.

by , Contributing Editor

As traffic loads and volumes rise, and as long-term pavement performance gets increased attention, what lies under the pavement is getting the same attention as what is on top.

Methods of building superior-performing road bases — either for reconstructed roads, or new roads on new alignment — are getting a close look by road agencies, designers, and builders.

Stabilized, premium bases tame troublesome soils, when they’re either saturated or subject to pavement-destroying, long-term expansion. And if water is the enemy of a pavement, premium bases can either waterproof a road structure to keep water out, or drain water right out of the pavement structure and into the roadsides, where it belongs.

One element impacting road bases will be the new American Association of State Highway and Transportation Officials pavement design manual — the 2002 Guide for the Design of New and Rehabilitated Pavement Structures — which will change how pavements are designed in a big way, incorporating local elements of a road into the design in ways that haven’t been done before.

Local parameters such as subgrade, existing pavement condition for rehabilitation, traffic, climate, and construction conditions will be included in proposing a trial design for a new pavement or rehab. Then, using new software, the trial design will be evaluated through prediction of key distresses and smoothness. If the trial does not meet the demanded performance criteria, the pavement design must be revised until it does. In anticipation, the National Lime Association has developed a guidebook for lime stabilization under the new AASHTO design guide (see the For More Information sidebar).

Also, a new generation of soil stabilizing machines/asphalt pavement recyclers is revolutionizing base stabilization in the United States. Both improvements in technology and improvements in the big reclaimers are driving the new interest in premium bases.

“Older persons will remember the pulvimixers, which were little more than oversized roto-tillers,” said John Huffman, P.E., long-time stabilization and recycling contractor, and new asphalt and soil materials engineer for Terex Roadbuilding (see related sidebar). “They didn’t have the power to do the depths that we can do today with the new machines. The new generation of soil stabilizers have revolutionized soil stabilization and recycling.”

The new generation incorporates an obvious increase in size and horsepower, which has improved productivity and permitted greater depths of treatment. They also incorporate high-tech systems for controlling liquid additives and water.

No trick to durable bases

There is no special trick to constructing durable, stable road bases, experts say. The key is to design the base predicated on existing materials and soils, anticipated traffic counts and loads, and final pavement, and then sticking to it. Owning road agencies should consider some of the high-performance, base stabilization methods that now are available to them, because they will provide enhanced durability and stability, although at a higher price that will be recovered over the extended life of the road.

The California DOT defines three separate layers beneath a pavement: the base course (just beneath the pavement); the subbase, which is structural section material between the basement soil and the base course; and the basement soil itself.

Subbase. “Subbase may be a treated or untreated granular material, or a layer of soil treated with a suitable admixture such as lime,” Caltrans says in its interim design guide. “It is usually a material with less stringent specification requirements than the base course for strength, plasticity, and gradation, yet is often of better quality than the basement soil.”

In addition to functioning as part of the structural section, the subbase prevents intrusion of fines from the basement soil into the base course, minimizes the damage of frost action, prevents the accumulation of free water within or below the structural section, and provides a working table for construction equipment.

Base course. The base course is the layer immediately below the pavement or surface course, Caltrans explains. “It may be located above the subbase, or if no subbase is placed, immediately above the basement soil,” Caltrans says. “Base may be an untreated or treated granular material ... [or]  treated with asphalt, cement-fly ash, lime, or portland cement. The specifications for base are more stringent than for subbase materials for strength, plasticity, and gradation.”

The major function of a base course is structural support, and in tandem, the base and subbase materials must be an economic and efficient way of distributing wheel loads.

Drainable bases may be constructed that permit easy egress of rain and meltwater from the base (also see Making Edge Drains Work, January, 2003, page 42). “Although usually there is only one base and one subbase material in the structural section, there may be an added base or subbase material designed to be a highly permeable drainage layer,” Caltrans notes. The alternative is to seal the pavement with asphalt or other bituminous surface treatment to waterproof the pavement structure in its entirety.

Premium bases work

That a premium base adds to long life was borne out by Nevada DOT materials engineer, Sohila Bemanian, P.E., at a workshop on pavement preservation in San Diego in 2002. There, Bemanian said Nevada had saved over $30 million since 1997 using cold-in-place recycling and overlay, versus full-depth reclamation. “Cold-in-place is recognized as a very cost-effective strategy,” she said. “But proper training and a knowledgeable contractor is essential for a successful project.”

Adequate depth is required, she said, and added lime improves long-term performance of the stabilized base. For CIR without lime, a 10- to 12-year life was expected; with lime, 12 to 15 years was anticipated. “Nevada has three reasons why we believe in CIR,” she said, “economics, economics, economics.”

Geosynthetic materials also will boost performance of bases and longevity of pavements, so long as they are properly designed into a structure. In road bases, geosynthetics provide pavement layer separation and can provide reinforcement, and serve as a fluid barrier. They include geotextiles, geogrids, and geomembranes; outside the base they serve as erosion-control blankets and mats, and geocomposite drainage materials.

The Colorado DOT says a pavement structure may consist of layers of material placed on the subgrade, and that premium materials should go higher in the structure, closer to the pavement.

“In establishing the thickness of each layer, it is necessary to provide proper thickness of overlying material so that the next lower layer is not stressed beyond its ability,” the Colorado DOT says. “Consequently, in a layer system, lower quality materials may be used in the bottom courses of the pavement structure if sufficient cover of higher-quality materials is provided.” Colorado recommends removal and replacement of any subgrade soils that are susceptible to the detrimental effects of frost or swelling.

“Not all cities and counties understand the importance of quality base stabilization,” said Bill O’Leary, president, Foundation for Pavement Preservation, and of Prime Materials & Supply Corporation, a manufacturer of asphalt emulsions. “But most of the road people today are well-informed enough to understand that bases are a very important road component.”

Soils, treatments vary

Soils vary considerably across the country, from clean, sandy materials, all the way up to very heavy clay soils, each with its own stabilization needs and procedures. “Soils have more variability than any other construction material we deal with,” Terex’s Huffman said.

Fortunately, due to legacy information, all road agencies and local contractors will have a good knowledge of the types of soils in their area. For them, building a premium road base will hinge on detecting variations in the existing soil types and fine-tuning their stabilization design to accommodate those variations. And that’s done through testing, testing, and more testing.

“They should know generally what kind of soils are in their areas,” Huffman told Better Roads. “However, they may not know specifically what they have on the road. There are areas in the Midwest with loessial soils, or wind-deposited, that may be fairly consistent but still varying to some degree. That is why soils must be considered more variable than most other construction materials and could vary considerably within a job.”

Therefore, thorough soil or base testing is indicated along the route of a project — either new construction on-grade, or base recycling — for guaranteed success. “The agency or contractor will have to do a complete project investigation, which would include analysis of the types of soils and their engineering properties,” Huffman said. “DOTs or other larger entities may have that capability in-house. But smaller entities may require use of a geotechnical consulting firm.”

Failure to do so will result in poor performance of the pavement, due to a less-than-optimum stabilized base. Volume changes, such as swellage or shrinkage, may distort and crack the pavement above. Early cracking or widespread alligator cracking in HMA pavements may result from low-strength bases. Frost heave is another condition that can be controlled by stabilization. Some soils absorb water more readily than others; stabilization can make road bases less likely to hold water and pavements less likely to frost heave or form potholes.

“It doesn’t take much swelling or heaving in the base to make the ride rough or unacceptable,” Huffman said. But properly stabilized bases will quell volume changes and increase strengths, with a concomitant decrease in total asphalt pavement thickness and increase in construction cost savings.

Above, proprietary modified asphalt emulsion from Koch Pavement Solutions stabilizes road base in Morton, Illinois; right, the resulting base is compacted with a padfoot roller.

Types of stabilization

There are three types of soil and base stabilization.

Mechanical is the lowest-cost technique, improving base gradation and lowering plasticity through incorporation of coarser materials, either stone or reclaimed asphalt pavement.

With mechanical stabilization, a granular material is added to a soil or base and mixed in using a soil stabilizer or rotomill. This is the simplest way existing but failed asphalt pavements are recycled into base for a new road; the road is pulverized and incorporated into the underlying material. Or, in the case of a new road, new stone may be brought in and blended with the soil. The addition of coarser fractions improves the material gradation, and reduces its Index of Plasticity.

For premium mechanical stabilization, attention must be given to the base’s compaction and moisture content. “We really have to make sure we have optimum moisture and density,” Huffman said.

Chemical stabilization takes that a step further by introducing a chemical agent to the base, such as portland cement, hydrated lime (Ca(OH)2), quicklime (CaO), Class C or F fly ash, cement, or lime kiln dust, and to a lesser extent, calcium chloride.

Class C fly ash is self-cementing, while Class F requires an activator such as lime or cement. Quicklime must be hydrated to bind particles in the base, so often it is mixed with water on the site in a slaking tank, and then distributed on the base prior to stabilization, or pumped into a modern base stabilizer/recycler where it is mixed in with the base in a mixing chamber.

Products such as cement, lime, kiln dust, and fly ash traditionally have been spread dry on the base, and then wetted by a water truck prior to mixing, but this practice is diminishing due to environmental fugitive dust regulations.

Bituminous or asphaltic stabilization includes application of asphalt emulsion or foamed asphalt to the base. Historically, bituminous stabilization has been done with asphalt emulsions and cutback asphalts. Cutback asphalt is a petroleum distillate, the use of which has been greatly curtailed in recent years due to their inherent volatility (volatile organic compound, VOC). Higher costs for petroleum also has impacted cutback asphalt use.

“The market moved to asphalt emulsions, some enhanced with modifiers for improved performance, and also not foamed (expanded) asphalt for stabilization,” Huffman said. “The emulsions are generally the mixing grades of anionic slow-setting, cationic slow-setting, and high-float, the high-float emulsions containing petroleum distillate and so having extended workability.”

Which to use?

Selecting the right method to use depends on soil type and characteristics. “You will want to go through the site and look at the P.I. and gradation of the soils,” Huffman told Better Roads. “You also will want to look at its strength. There are some cases where cement can be used with some clay soils, or where two materials will be used together. Some soils may be pre-treated with lime, let sit several days to mellow or cure, during which the lime makes the stiff soil more friable and granular. Then you can come in and treat it with cement. The high P.I. material now starts to act more like a silty type, and is more constructable and workable.”

Lime is very commonly used with higher plasticity material such as clayey soils, Huffman said. “Hydrated lime added dry is really not done anymore, due to environmental constraints. The next step then was to go to pelletized quicklime, and then add sufficient water for the slaking or hydration to occur as it is mixed. Then the next step is use of lime slurry, because it’s easy to handle.”

Lime slurry is not indicated for very wet sites, such as in early spring. Slurry will make a muddy site wetter, perhaps so wet that your reclaimer or stabilizer won’t be able to get through the material.

Cement is used for a wide range of applications, but in particular soils with a lower plasticity, such as sandy or silty materials, Huffman said. Because foamed asphalt requires an adequate amount of fines in the material to work well, cement may be added to a project’s material ahead of the foaming process, based on advance testing.

Fly ash is a byproduct of coal combustion in power plants, and can be subject to regional shortages, depending on the type of coal used (Class C ashes are from low-sulfur lignite or sub-bituminous “western” coals, and contain up to 20% CaO; Class F ashes are from bituminous or anthracite “eastern” coals, and contain less than 10% CaO. “Typically, Class C ashes contain 1 to 3% free lime and are reactive with water,” according to the Federal Highway Administration publication, Fly Ash Facts for Highway Engineers (downloading information in For More Information). “Class F ashes generally contain no free lime.”

Most soils that can accommodate lime stabilization are too plastic — with too much clay — to work with an asphalt emulsion, Huffman said. “The gradation and P.I. of the material will be controlling factors in whether an asphaltic or chemical stabilizer will be used,” he said. “If I have a soil that has too much minus 200 and plasticity to it, the emulsion or asphalt foam will simply ball up and won’t work. In other words, use of one material over the other depends on the existing soil properties.”

Economics also will dictate which is used; for example, cement, asphalt emulsion, and foamed asphalt all might be appropriate for a particular soil, but will differ in cost. In some applications, use of too much cement or other chemical may result in shrinkage cracks in the base, where that problem does not happen with bituminous stabilization.

Waterproofing the foundation

Stabilized, premium bases can keep water out of the substructure, and these are obtained through mixing asphalt emulsions into the base using a base stabilizer.

“Asphalt emulsions add stability to the pavement structure,” FP2’s O’Leary told Better Roads. “An emulsion-stabilized base will waterproof the flexible base, making the intrusion of water less likely. Any desirable characteristics that you gain from densities and strength of aggregate will remain constant, because asphalt emulsions waterproof the flexible base.”

This flexible base no longer will change with varying moisture contents, he said. Instead, moisture on the pavement is shed into the drainage alongside the road. The asphalt emulsion does not contribute much to the compressive strength of the base, he said; instead it waterproofs it and allows it to maintain constant strength.

“Because an emulsion-stabilized base is still a flexible base — yet waterproof — if under load, it experiences some movement. Instead of being a brittle system and cracking, it will flex,” O’Leary told Better Roads. “If the stabilized base is rigid, and you do get some movement, it likely will shear or break. And that will reflect through to the asphalt surface.”

The emulsion is injected using a system within a base mixer/pulverizer/stabilizer, or placed on the grade ahead of the mixer. In this method, it’s blended into the surface layer of the base in a single pass. Then, the freshly mixed base is graded and shaped, and then compacted with a padfoot (or sheep’s foot) roller. Finish rolling usually is accomplished with a pneumatic roller.

“Asphalt emulsion stabilization depth varies, but usually no more than 10 inches,”O’Leary said. “In some cases, the compaction lifts should be limited to 4 inches, depending on the grade of the emulsion selected and the project conditions. But you have to allow for moisture to escape from the base while compacting, so a very safe approach is to limit compaction depth to 4-inch lifts.”

Iowa examines subgrades

Some states are attempting to harmonize subgrade designs among municipalities and counties within their borders. Because state funds often are distributed to lower governments for road programs, the state has a valid interest in seeing that its funds are spent wisely. One such state is Iowa, which has just begun an 18-month project to develop its own Design Guide for Improved Quality of Roadway Subgrades and Subbases.

Work began on the design guide last November and a final product is anticipated in April 2006. The principal investigator is Radhey Sharma, Iowa State University. The guide will be problem-oriented and useful anywhere in the upper Midwest where soil conditions similar to Iowa’s prevail. “It will identify common subgrade problems encountered in Iowa, and recommended solutions, so when they come across a problem on a project they will have a resource to which they can refer for a specific fix,” said Mark Dunn, P.E., operations research engineer, Iowa Department of Transportation. “The counties will be able to use it as well.”

Such problems include wet areas, in soft, unstable locations, expansive soils, or nonuniformity of material. It also will identify the types and conditions of subgrades, ranging from recycled materials, granular materials, and low-cost pavement subbases.

“Iowa has a statewide urban design and standards group which is developing uniform specifications for all the cities in Iowa,” Dunn told Better Roads. “They are not adopted by all governments, but can be used by all cities, and helps our state move away from each city having its own specs and design standards. We want to make the standards as uniform as possible.”

The work is being funded by the Iowa Highway Research Board, which is supported jointly by the Iowa DOT, and Iowa cities and counties. Even though many cities and counties model their design specs after state DOT specs, Dunn said the guide still was needed.

“In terms of subgrade, it appears that the local municipalities spend a lot less time and effort on them,” Dunn said. “We find that they will build roadways just on what is there, rather than doing any kind of modification to it. They don’t spend the money on subgrades the way it’s done on the state level.”

Proprietary emulsion in Illinois

A proprietary, new-chemistry asphalt emulsion from KPS, Koch Performance Solutions, was used to in-place stabilize an existing road in Morton, Illinois, just south of Peoria. South Fourth Avenue is in a rural area from Broadway north to Queenswood Drive at the south end of Morton.

“The cross section was pretty bad, and the crown was exaggerated,” said Robert Wraight, P.E., director of public works, Morton, Illinois. “Structurally, it was not up to carrying the traffic it now carries. As the village grows, it gets more traffic. We are familiar with recycling and emulsions, and chose to use what was here and make it stronger.” An asphalt overlay was placed atop the stabilized base.

The base was stabilized using a reclaimer/recycler owned by asphalt paving contractor Dunn Company, Decatur, Illinois. Base stabilization is a major part of Dunn’s operations, and two-thirds of that is dry lime stabilization, with the remainder being cement and asphalt emulsion.

In Morton, the spec for the base required 100% of aggregate to be minus 1.75 inches, and the recycler was able to meet sizing specs and incorporate the emulsion in a single pass, eliminating the need for pre-pulverization with a preliminary pass. The pavement was stabilized with 3.5% emulsion added. Extensive coring by emulsion supplier Koch Pavement Solutions confirmed in-situ material characteristics.

“The emulsion being used is a new-chemistry product which will disperse better than other emulsions, will cure and set better, and, versus dry treatments or pulverization, will add flexibility to the road,” said Todd Thomas, P.E., product development engineer, Koch Pavement Solutions. “It will seal the road, add durability and flexibility.”

Expansive soils, contractors

Road bases in areas of expansive soils, such as alluvial soils (in the western and southwestern states), glacially deposited soils with high clay content (the northeast and upper Midwest), and silty soils associated with high water tables (the lower Mississippi Valley and Gulf Coast) are all subject to heaving of one kind or another. A premium base may be required to quell this heaving, with special additives and contractor knowledge.

For example, a relatively new Colorado contractor in the Denver region has carved out a niche for itself specializing in stabilization of subgrades in the booming Denver region. Since 1985, Asphalt Recycling & Stabilization, Inc., Littleton, Colorado has become the largest stabilization contractor in the state.

ARS uses dedicated base stabilization and road recycling equipment to lime- and cement-stabilize civil engineering projects, and has benefited from the expansive soils which fill the valleys in the mountainous west. These soils are tamed through a variety of processes, depending on soil type and construction application, including lime, fly ash, cement kiln dust, and cement-treated base. Projects which ARS has stabilized include some of the most famous recent construction projects in Colorado, including Denver International Airport, the E-470 Tollway, and the C-470 Beltway.

“The vast majority of the stabilizing work we do is taking the expansive, damaging soils with high swell potential, and treating with lime or other calcium-based product to eliminate the swell potential,” said ARS president Jim Anttonen. “We’ve also done a lot of soil-cement, in which a sandy, nonexpansive  material is  turned into a strong base,” he said. Such a super strong base can reduce the thicknesses of asphalt or concrete driving courses and save enormous sums of money.

Yet another market ARS is developing is ongoing residential construction as farmland is converted to tract housing. Highly expansive soils in these areas require stabilization of land so the pavers can do their work.

Foamed asphalt stabilizes

Whenever premium road bases are discussed, foamed or expanded asphalt ought to be considered. Foamed asphalt is created by carefully injecting a predetermined amount of cold water into hot penetration-grade asphalt in the mixing chamber of a reclaimed asphalt pavement remixing unit.

When a carefully metered amount of cold water is introduced into hot asphalt, a foam is formed, increasing its volume and surface energy. This enables stiff road-grade asphalt to be mixed together with cold, moist aggregate without having to resort to the added cost of cutting back the binder with a solvent or emulsifying it. A benefit is that foaming reduces the viscosity of the binder, permitting easier dispersion through the aggregate.

“From coast to coast, road agencies and road builders in North America are waking up to the tremendous benefits base recycling with foamed asphalt can bring to their taxpaying customers,” said Stu Murray, president, Wirtgen America Incorporated. “Little wonder that base recycling using foamed asphalt is so desirable, because it reuses existing aggregates resources that already have been acquired, permitted, shot, loaded, crushed, screened, stockpiled, reloaded, and hauled. The existing investment in processed aggregate is optimized, because the material is just lying there in the roadbed waiting to be reused.”

“An advantage of foamed asphalt is that it contains very little water, and so more asphalt can be hauled per tanker load,” Huffman said. “Foamed asphalt typically has about 2% water by weight of asphalt, but an emulsion from 30% to 40% by weight of asphalt. Also, foamed asphalt can develop higher early strength and can cure more quickly than an emulsion. “However, asphalt emulsions and foamed asphalt both have their place,” Huffman told Better Roads. “Their selection must be based on material gradation and laboratory testing. Gradation is very critical to foamed asphalt as sufficient fines are required as a carrier for the asphalt, and bonding of coarser particles. As a result, cement addition may be necessary for foamed asphalt bases.”

Maine explores foamed asphalt

In late summer 2001, the Maine Department of Transportation research undertook a research project involving full-depth base reconstruction and recycling using foamed asphalt as a stabilizing agent on Maine State Route 8 near Belgrade Lakes, a popular recreation area north of Augusta. The project demonstrated that foamed asphalt could be used to build road bases in harsh northern climates, as well as more temperate southern and southwestern regions.

“Post-construction test results of the Belgrade project were so encouraging that the Maine DOT dramatically increased the number of foamed asphalt projects in 2003-2004,” a state research newsletter reported last summer. Other foamed asphalt projects followed, and in 2005, the Maine DOT was planning to foam-base recycle nearly a mile of Whitten Road in Kennebec County at the Hallowell town line.

For a successful foamed asphalt base, a pre-project mix design is essential. This can be done using a portable lab, which lets contractors, engineers, or government agencies pre-test materials and determine, for example, how much, if any, cement may be required to get optimum foaming in advance of a job or preparing a proposal.

On Belgrade Road, large boulders in the road’s gravel base were found at varying depths from the surface, ranging from 6- to 8-inches deep. Because an average foam-recycled layer of 8 inches is required to meet the design criteria, an additional 2 inches of crusher dust was placed on the road surface prior to recycling.

The final mix design included:

	The additional 2 inches of crusher dust.
	Addition of 1.5% portland cement by mass.
	Addition of foamed asphalt to the pulverized material to 2.5% by mass.
	Recycling of the pulverized pavement with added crusher dust to a depth of 8 inches.

The construction process began with cleaning out and re-cutting of roadside drainage ditches, followed by recycling of the existing pavement in place, and placement of the 2 inches of locally sourced crusher dust onto the reclaimed existing pavement. The entire section then was pulverized again, shaped, and compacted to accommodate existing traffic.

Then the portland cement was spread and the entire travel lane width was foam recycled in three passes with a base reclaimer capable of foaming reclaimed asphalt pavement, and compacted with a vibratory padfoot soil compactor in a minimum of three passes. The material was shaped to the cross-slope and grade and compacted with a vibratory steel drum roller to a minimum density of 98% of the target density as determined by a control section, the Maine DOT said in a subsequent report, Technical Report 02-2: Using Foamed Asphalt as a Stabilizing Agent in Full Depth Reclamation of Route 8 in Belgrade. After compaction, the roadway surface was treated with a light application of water and rolled with pneumatic-tired rollers to create a close-knit texture.

This was followed by a 3.2-inch asphalt wearing course. As a further test of the structural capabilities of the foamed asphalt treated-base, a half-mile long section of road received only 1.6 inches of wearing surface.

Louisiana fights heaving

Cement and lime stabilization will create a premium base, but may not be indicated in certain regions where native soils contravene their use.

That’s what public works officials in Louisiana found. Soil-cement base courses were failing within a year after construction. Typically, Louisiana has high plasticity soils with high silt contents. Because the state averages 55 inches of rain each year, with little relief, a very high water table results, which impacts base stability. Compounding the base situation, no naturally occurring aggregates remain in Louisiana; instead, crushed stone must be barged in.

“For base course work, the most economical method has been a soil-cement stabilized road base, because graded aggregate base was too expensive,” said James M. Winford, P.E., James Corporation, Opelousas, Incorporated.

To get needed compressive strengths for road base, cement contents in excess of 10% by volume are required. “That meant we are spending $3 per square yard for portland cement material only, exclusive of processing,” Winford said. “We would end up with a very, very rigid base with high cement contents,” he said. “And when we place our typical 3.5 inches of dense-graded hot-mix asphalt in two lifts, we’re able to achieve initially smooth pavements. But once the cement hydrated we’d experience severe reflective cracking.”

A severe, three-year drought in the late 1990s led to cracks as wide as 3 inches. “We had roads that had structurally failed within a year after they were built,” he said. Compounding this was the fact that water easily enters those cracks, eroding the subgrade below the cement-treated base.

In 2001 the state authorized a base stabilization project on Bolden and Dry Bayou roads in St. Landry Parish in the central part of that state, hoping that foamed asphalt base courses will provide an alternate to problematic soil-cement bases. “We looked for a base that would provide a good support value, yet be flexible and not develop reflective cracking,” Winford said.

During the initial survey, all cross drains were located, making sure that at least a foot of cover would lie over them. Hydrated lime was spread at 3% by weight of soil (112 pounds per cubic foot) and was mixed in conventionally to an 8-inch depth by a road reclaimer, compacted with a padfoot roller, graded, rolled again with a pneumatic roller, and finally compacted with a nonvibratory, double-drum steel roller.

The lime base was watered and the next morning, foamed asphalt was applied. A performance-graded PG 64-22 paving asphalt was used and the foaming application moved forward at about 50 feet per minute.

Some 2,700 lineal feet in two sections of Dry Bayou Road were foam stabilized. On Bolden Road, some 9,100 lineal feet were processed. “All sections were processed 21-feet wide, and were to receive a 20-foot-wide hot-mix asphalt section surfacing,” Winford said. A bituminous surface treatment would be placed on another section for comparison.

After the pass with the foamed asphalt, once full cross-sectional coverage was achieved, the road was compacted with the padfoot roller, followed by initial grading, finish compaction with a pneumatic roller, and finally with a smooth drum roller with no vibration.

“We also watered under final grading to seal the surface, and did some grading the next morning and applied an SS-1 curing membrane on top of that,” Winford said.

For More Information

Consider visiting these sites for much more detail on some of the topics discussed in this article:

The indispensable manual, Fly Ash Facts for Engineers, was first published by the FHWA in 1986, but an updated edition was published in 2003. Download it in .pdf format at www.fhwa.dot.gov/ pavement/fafacts.pdf; an html version can be found at www.fhwa.dot.gov/pavement/fatoc.htm.

Information on asphalt emulsions for stabilization is available at the Asphalt Emulsion Manufacturers Association, www.aema.org. Its Basic Asphalt Emulsion Manual is now available in CD-ROM format at a low price, in addition to a hard copy; ordering info is at www.aema.org/CDOrder.htm.

Lots of information on lime stabilization of subgrades is available on the Web site of the National Lime Association, www.lime.org. Don’t miss its technical digest on lime for wet or unstable soils at www.lime.org/LTStechbrief4pdf.pdf.

NLA also provides a construction manual for lime-treated soils, new in January 2004. Download it at http://www.lime.org/. And, its June 2004 guide to lime stabilized-soils and the new AASHTO mechanistic-empirical design guide can be downloaded at www.lime.org/MechEmpPavement.pdf.

More information about Maine’s project investigating foamed asphalt for base stabilization is available from the Recycled Materials Resource Center of the University of New Hampshire, at www.rmrc.unh.edu/Research/Rprojects/Project26/reports/P26summary.pdf.

The report on Belgrade Road by the Maine DOT, Technical Report 02-2: Using Foamed Asphalt as a Stabilizing Agent in Full-Depth Reclamation of Route 8 in Belgrade, February 2002, may be downloaded at http://mainegov-mages.informe.org/mdot/transportation-research/pdf/report 0202c.pdf.

A report on the Louisiana application in which foamed asphalt from a portable plant was used in place of a lime-treated base under portland cement concrete pavement will be found at the Federal Highway Administration’s Web site, at www.fhwa.dot.gov/pavement/foamrap.htm.

A great resource of information on stabilization products — albeit vendor-oriented — will be found on the Web site of KPS, Koch Pavement Solutions, www.kochpavementsolutions.com/.

Modulus testing of soils: A November 2004 report from the University of Mississippi, for the Mississippi DOT, Prediction of Resilient Modulus for Soil Index Properties, Final Report, by K.P.George, is available; download it at www.mdot.state.ms. us/research/pdf/ResMod.pdf.

Information on the new 2002 AASHTO Pavement Design Guide is available at a number of places. The official download page for the design guide, software, and climatic data is www.trb.org/mepdg/home.htm. A review guide is available for inspection and review only in an online .pdf version; see it at www.trb.org/mepdg/guide.htm. A review copy of the software may be downloaded at www.trb.org/mepdg/software.htm.

The current products available for review are described in a four-page flyer, NCHRP Research Results Digest 290: Recommended Mechanistic-Empirical Pavement Design Guide and Software (Phase II) Available for Evaluation. Review it at http://gulliver.trb.org/publications/nchrp/nchrp_rrd_290.pdf.

AASHTO’s June 23, 2004 memo on the same subject is available at www.trb.org/mepdg/AASHTO_memorandum.pdf. Washington State DOT’s tutorial is available at http://hotmix.ce.washington.edu/wsdot_web/.

John Huffman Joins Terex Roadbuilding

Long-time stabilization and recycling expert John Huffman, P.E., has joined Terex Roadbuilding as a consulting engineer in asphalt and soil materials.

Working within Terex Roadbuilding’s new Asphalt Mobile Equipment Group, provides assistance with field support, preparing brochures, technical manuals and in presenting training courses for company personnel, contractors, governmental agencies, consulting engineers, and testing laboratories.

Huffman has over 40 years of experience in asphalt pavement technology and soil stabilization. He served as bituminous and soils engineer, Kansas Highway Department; district/senior staff engineer, Asphalt Institute; vice president, operations and technical services, Riffe Petroleum, Tulsa; and director, asphalt technology, U.S. Oil and Refining Company, Tacoma. Most recently, John was an asphalt recycling and soil stabilization contractor for Brown & Brown, Salina, Kansas, following his 12-year tenure there as vice president of engineering.

Huffman’s professional activities include participation in the Association of Asphalt Paving Technologists , American Society of Civil Engineers, and the Transportation Research Board.  He also has served on the Board of Directors and as Chairman of the Committee on Recycling Education for the Asphalt Recycling and Reclaiming Association.

Reprinted from Better Roads Magazine
May2005

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