May 2002

The Case for Base.
Aggregate industry research and compaction equipment improvements support economic construction of thicker, single-lift bases.

Aggregate industry research and compaction equipment improvements support economic construction of thicker, single-lift bases.

By Bob Drake

Improved equipment allows complete compaction of thicker layers of unbound aggregate base than at the time many state specifications were written. For example, Cat’s new 18-ton CP-663E padfoot vibratory soil compactor provides up to 332 kN centrifugal force in a high amplitude setting.

Road-construction markets in the United States have been in transition for more than a decade. As the nation’s interstate system inched toward completion, state transportation departments gradually increased their focus—and resources—on pavement and bridge maintenance and rehabilitation and highway lane additions. New construction of long, major highways, requiring large volumes of dense-graded aggregate base material, has been on the decline. At the same time, innovations in hot mix asphalt (HMA), such as Superpave, open-graded mixes and Stone Matrix Asphalt (SMA) have increased demand for cleaner aggregate that contains a lower percentage of fine-grained material.
As the aggregate industry responded to the changing product demands, decreased sales of base material and increased sales of clean stone set many aggregate processing plants out of balance with their markets. Producers faced more rapidly growing stockpiles of fine-grained material, which is—in most cases—an unavoidable product of blasting and crushing rock. With this backdrop, the aggregate industry initiated research in the mid-1990s to increase markets for products such as unbound aggregate base in order to help restore production balance, reduce waste and improve profitability.
One such effort is the promotion of thick, single-lift unbound aggregate bases (UAB). A survey of state transportation departments by the International Center for Aggregates Research (ICAR) revealed that out of the DOTs responding, 12 states limited lift thickness of UAB to 6 in. or less; one state allowed 7-in. lifts; 16 states allowed 8-in. lifts; one state allowed 9-in. lifts; one state allowed 10-in. lifts; and one state allowed 12-in. lifts.
“Studies have shown that restricting lifts to a maximum of 8 in. is too limiting,” said ICAR researchers Ken Stokoe, The University of Texas, and Joe Allen, Texas A&M. “We know that placing 10- to 16-in. lifts can produce an excellent product. Now we’re trying to show that using these thicker lifts in pavement construction is feasible and saves time and money.”
Time and money can be saved if one thick lift of UAB can be placed and effectively compacted more efficiently than two thinner lifts. In addition, a thicker base course provides greater structural support for the roadway and gives engineers greater flexibility in designing the total pavement/base structure.
Researchers and equipment manufacturers contend that with technology advances since the time most state specifications were adopted, thicker UAB layers can be compacted to the required 95 to 100 percent of maximum dry density. “Discussions with DOTs indicated that the current maximum lift thickness was most likely established in the early days of highway construction when only static rollers or limited vibratory rollers were available,” Charles Saunders, manager of technical services, Vulcan Materials Co., wrote in a 1998 paper. “Meetings with equipment manufacturers and contractors assured us that [a] current maximum single compacted lift thickness of 8 in. does not reflect the compactive capabilities of today’s vibratory rollers.”

Demonstrations help change specs
In an effort to show the viability of thicker UAB lifts, ICAR—in cooperation with aggregate producers, state DOTs, state aggregate associations and contractors—conducted several demonstration projects on both private and public roadways. Single-lift UAB thickness ranged from 12 to 21 in. on the various projects (Table 1). For a Georgia road-widening project, two thick-lift test strips and a target strip incorporating two thin lifts (a 7-in. layer followed by a 6-in. layer) were constructed for comparison. The thick-lift test strips comprised 20.5 in. of loose UAB placed with a mechanical spreader box and compacted to 13 in. using four passes with a vibratory sheepsfoot roller then four passes with a vibratory smooth-drum roller. Both rollers were set at high amplitude, high frequency and high centrifugal force to ensure full-depth compaction (Table 2).

Table 1. Thick-lift base demonstration projects
State	Lift Thickness	Base Placement	Base Compaction
Georgia	13 inches	Spreader box	45,000-55,000 lb. vibratory sheepsfoot and smooth drum rollers
Tennessee	12 and 14.5 inches	Wheel loader	28,000-55,000 lb. vibratory smooth drum rollers
Texas	12 and 21 inches	Road grader	50,000-55,000 lb. vibratory sheepsfoot and smooth drum rollers

Table 2. vibratory roller settings for georgia thick-lift project
Component	Sheepsfoot Roller	Smooth Drum Roller
Amplitude	1.63 mm	1.75 mm
Frequency	30 Hz	30 Hz
Centrifugal Force	249 kN	203 kN
Source: Douds, R.A., 1998, Georgia Highway 92 Deep Base Test, ICAR Symposium.

Optimum moisture content of the UAB, an important requirement for achieving maximum compaction, was attempted two ways. On the first Georgia test strip, the in-place UAB was soaked using a water truck and the material was mixed using a road grader blade and scarifier. On the second test strip, UAB was delivered to the construction site at the desired moisture content.
Adding water on site and mixing with a grader took four times the effort typically put into constructing a graded aggregate base, according to Rick Douds, Office of Materials and Research, Georgia DOT. Material delivered from the quarry with moisture added before stockpiling was placed and compacted with less effort than typical two-layer bases. “Controlling the moisture content of the crushed stone base was crucial to facilitating compaction and achieving maximum density,” he said.
Based on the Georgia study, ICAR researchers recommended that new specifications be developed to allow for placement of thicker-lift bases. “Those specifications should include provisions for controlling the moisture content through the mixing and spreading methods, compaction equipment and minimum density requirements,” they said.
As a result of the Georgia study and demonstration projects in other states, Georgia, North Carolina and Virginia have changed specifications to allow thicker lifts of UAB, according to Charles Pryor, vice president of engineering, National Stone, Sand and Gravel Association (NSSGA). In addition, two districts in Texas have put down thick lift sections and more are planned, he said.
“The states that changed increased their specs by 2 to 4 in.,” said Pryor. “While ICAR research demonstrated that much thicker lifts could be placed and compacted to specified densities, these initial increases (25 to 33 percent) represent a significant change in a very conservative construction industry.”

Does more base mean less asphalt?
Despite the moderate changes in specifications that currently affect only three states, the asphalt paving industry is understandably concerned about a move to thicker UAB lifts. While a thicker base provides greater structural support to an overlying asphalt pavement, it can also be used, according to the American Association of State Highway and Transportation Officials (AASHTO) design guide, to replace some asphalt in the pavement profile.
“If you use the structural numbers provided by AASHTO, about 3 in. of crushed aggregate base equals the strength of 1 in. of HMA,” explained Larry Quinlivan, vice president of marketing for NSSGA. “Therefore a roadway could consist of 9 in. of HMA, or 3 in. of HMA and 18 in. of crushed aggregate base. A 21-in. pavement cross section sells 2.3 times more [aggregate] material than 9 in. [of HMA]. The amount of base material goes from none to 18 in., which helps balance [aggregate] production and reduce any excess fines problem.
“The asphalt industry is pushing full-depth asphalt and Perpetual Pavements, which maximize the use of HMA products,” Quinlivan said. “Our efforts with full-depth base maximizes the use of crushed aggregates in the pavement cross section. Both, when properly designed and installed, will provide a cost-effective, durable roadway for the taxpayer. Because of the different cost structures, either side may be more economical on any single project.”
With tight state budgets and the possibility of reduced federal funding for highways, economics could play a larger role in determining the relative amounts of asphalt and UAB used in pavement profiles. “It is during these periods everyone is looking for new ways to build and repair roads that allow more miles to be built for the same or lesser amounts of money,” said Quinlivan.
Vulcan’s Saunders concluded that an increase in UAB single-lift thickness provides pavement design engineers another option to consider in choosing the most economical design. It also allows the contractor to expedite placement and compaction by eliminating unnecessary splits in the aggregate base lift. Saunders provided an example of three possible pavement designs, with varying amounts of asphalt and UAB, all meeting a required structural number (Table 3).
Pryor points out, however, that thicker UAB lifts is a construction technique, not a design issue. Of perhaps greater importance to the asphalt-vs.-aggregate question is the structural value assigned to UAB by the AASHTO design guide. Coefficients in the current guide, developed 40 years ago in the AASHTO road test, penalize the true technical advantage of thick UAB layers, according to Pryor.

Table 3. Alternate pavement designs with 4.70 required structureal number
	Layer Thickness (in.)	Layer Type	Structural Number
	2.5	Asphalt surface	1.10
	3.5	Asphalt Binder	1.54
	7.0	Asphalt base	2.10
Total	13.0 in. Asphalt; 0 in. Aggregate		4.74
	2.5	Asphalt surface	1.10
	3.0	Asphalt binder	1.32
	4.0	Asphalt base	1.20
	8.0	Aggregate base	1.12
Total	9.5 in. Asphalt, 8.0 in. Aggregate		4.74
	2.5	Asphalt surface	1.10
	3.0	Asphalt binder	1.32
	3.0	Asphalt base	0.90
	10.0	Aggregate base	1.40
Total	8.5 in. Asphalt; 10.0 in. Aggregate		4.72
Source: Saunders, C.H., 1998, Increasing the Single Lift Thickness for Aggregate Base, National Stone Association Fines Seminar.

“The new AASHTO design guide is still in development, but we anticipate that it will move toward a design methodology that will better reflect unbound aggregate’s technical advantages in the pavement structures,” he said. “A structural model that accurately reflects the contributions of UAB combined with the construction techniques that allow for thicker lifts to be placed to specified densities will give the [highway] designer and owner an economic design alternative that they have not had in the past.”
“The highway community is a conservative industry,” said Pryor. “People do not like to take risks with the taxpayers’ money. Proven techniques that provide the economic incentive to the contractor, designer and owner will encourage wide use of this technique.”

Bob Drake is editor-at-large for AggMan.