Based on the successful use of Stone Matrix Asphalt (SMA) in Europe, the Maryland State Highway Administration (MDSHA) began to specify SMA on its major highways in 1992. Since then, it has constructed more than 85 SMA projects. The SMA mixes have been placed on high-volume, high-speed highways where traffic counts exceed 20,000 average daily traffic (ADT).

MDSHA’s experience to date on SMA projects, including projects 10 years old, has shown little or no rutting or increase in roughness and little decrease in friction. Other benefits include reduced tire splash and reduced tire noise. Nationally, SMA mixes typically cost 20 to 50 percent more than conventional dense-graded asphalt mixes, but MDSHA’s experience has been more like 10 to 30 percent higher. The increase in pavement life due to reduced rutting and increased durability more than outweighs the increased cost.

SMA Mix Design

The principal features of Maryland’s SMA mixes are aggregate quality, mix gradation, binder properties and laboratory compaction. High-quality coarse aggregates are required for SMA mixes. Coarse aggregate is cubical with 100 percent crushed faces. The coarse aggregate must be sufficiently hard and durable to minimize particle breakage at the stone-to-stone points. Fine aggregate must be 100 percent crushed.

SMA aggregates are gap-graded to maximize stone-to-stone contact in the aggregate skeleton. (see Maryland SMA Gradations for a summary of current MDSHA specifications for SMA gradations. Note that 12.5 mm and 19 mm are the two most common nominal maximum aggregate sizes for SMA mixes in Maryland.)

Maryland SMA mixes typically have high asphalt binder contents. Asphalt binders are classified using the Superpave Performance Grading system. The high temperature Performance Grade based on project climate conditions is often increased by one or two grades for SMA mixes. Nearly all Maryland SMA projects have used either modified PG 70-22 or modified PG 76-22 binders, with the PG 70-22 used on relatively lower traffic roads and the modified PG 76-22 used on high-volume projects such as the Baltimore and Washington beltways.

Maryland uses mineral filler and binders to provide a satisfactory binder consistency and to prevent draindown of the binder during transport and paving.

SMA mixes can be compacted in the laboratory using either a Marshall hammer of the Superpave gyratory compactor. The design asphalt content is selected to achieve 4 percent air voids in the compacted mixture. Asphalt contents for SMA mixes are typically greater than those for conventional dense-graded asphalt mixes and usually exceed 6 percent. Voids in mineral aggregate (VMA) values for SMA mixes also are high, generally exceeding 17 percent. (see Maryland Specifications for SGC Compacted SMA Mixtures for a summary of Maryland specifications for Superpave compacted mixes.)

Production

Both batch and drum plants have been used with equal success for HMA production. Plant control must be tighter for SMAs than for conventional mixes. Aggregate gradation, mixing temperature, mineral filler content and distribution, and fiber content and distribution must be consistent.

Gradation is critical. Deviations from the Job Mix formula will affect all paving and testing parameters, as well as the performance of the pavement. In particular, it can affect the quality of the stone-on-stone contact that is the foundation of SMA performance.

The HMA plant must be run continuously in order to assure uninterrupted flow of stabilizer and mineral filler. Most plants in use today are not set up to handle large amounts of mineral filler or baghouse fines. Plant production rate for SMA is usually controlled by the limitation of the mineral filler system. When fibers are used in SMA, a separate system for feeding must be used to proportion the required amount into the mix to insure uniform distribution.

A consistent production temperature is important for achieving a smooth finished surface. The degree of compaction is directly related to the temperature at which compaction is initiated. Varying the temperature will result in uneven compaction depths, resulting in a rough finished surface.

Placement and

Compaction

Due to the high temperature sensitivity of SMA, quick placement and immediate compaction are essential. SMA mixes are generally produced at approximately 310F (155C) and must be placed and compacted completely before cooling to 265 to 285F (130 to 140C).

The minimum recommended pavement temperature for SMA placement is 50F (10C). SMA mixes should not be placed in cold or inclement weather.

Haul times should be kept as short as possible. SMA mix temperatures should not be elevated to compensate for cooling during transport because of possible drain-down of the asphalt binder.

MDSHA recommends that transport trucks be clean with no remnants of previous loads. If not, clumps of material may travel through the paver and show up as uncoated sections or tears in the mat. Cover the haul trucks to prevent air-flow over the mix during transport and to provide an insulating shield when standing still. MDSHA recommends that the paving contractor designate a specific truck clean-out area close to or on the project to allow drivers to clean the truck beds before the residual material hardens.

Continuous Paving

For optimum paving efficiently and surface smoothness, MDSHA says:

• Keep the paver moving continuously. The stiff nature of SMA and its tendency to cool quickly will cause the screed to ride up after stopping and then dive afterwards, creating a ridge that cannot be rolled out.

• Maintain a supply of mix in the hopper at all times. Use a material transfer vehicle (MTV) to keep the paving train moving and allow for breaks in truck arrivals. The MTV helps provide a consistent mix temperature, avoid segregation, and improve continuity of the overall paving operation.

• Compact the mat quickly before it cools and stiffens. Doing so allows the paving contractor to achieve target density — typically 94 percent maximum density. The stone-on-stone structure of SMA results in typical rolldown depth of one-eighth in. per inch of mat thickness, roughly half that of conventional HMA.

• Keep the breakdown roller or rollers right behind the paver. All rollers should stay within 500 ft. of the paver. There should always be enough rollers available to accommodate the production rate. SMA requires a smooth rolling operation. Stopping the roller on a hot mat will leave an impression in the surface that can be difficult or impossible to roll out.

Fat Spots

Fat spots are common defects in SMA pavements. They may be either cosmetic or functional. Cosmetic fat spots usually wear off with traffic, but functional fat spots can affect the full thickness of the mat and can lead to deformation or deterioration in a localized area. Fat spots can result from the following conditions:

• Insufficient mineral filler in the mix;

• Inconsistent mineral filler feed;

• Excess moisture in the mineral filler;

• Lack of or inconsistent distribution of fibers;

• Excess temperature, especially with modified binders;

• Excessive haul times or extended waiting periods before unloading; or

• Excess water in the truck bed from release agents.

QC/QA

MDSHA’s quality control and quality assurance (QC/QA) procedures for SMA mixes are similar to the procedures for conventional HMA mixes. Frequent monitoring of all aspects of production, paving and compaction ensures high-quality SMA construction. Production and paving properties should be modified frequently and continuously during production and paving. These properties include mixture gradation, binder content, air voids content, VMA, and in-place density.

A trial field section should be included in the project to confirm the contractor’s ability to:

• produce the job mix formula;

• meet the mixture volumetric requirements (binder content, air voids and VMA); and

• achieve the required density and any other specification properties.

Performance

Maryland has constructed more than 85 SMA projects since 1992, totaling more than 1,300 lane miles of paving.

The SMA mixes have been placed exclusively on high-volume, high-speed highway segments where traffic counts exceed 20,000 ADT. Maryland SMA mixes have included 9.5-, 12.5-, and 19-mm nominal maximum aggregate sizes. The 19-mm mixes have been the most widely used.

Values for cumulative rut depth for Maryland SMA projects, including projects with up to 10 years of performance data, averaged 0.14 in. for the 12.5-mm mixes and 0.13 in. for the 19-mm mixes. Cumulative International Roughness Index (IRI) values at the latest survey date averaged 75.7 in. per mi. for the 12.5-mm mixes and 97.1 per mi. for the 19-mm mixes.

In summary, the performance of SMA pavements in Maryland has been outstanding. They have shown themselves to be rut resistant and durable. Many of the Maryland SMA pavement sections look better today than the day they were opened to traffic and they continue to perform well.

(This article appears courtesy of “Asphalt” magazine.)