For more than 20 years, dense-graded, high-float emulsion asphalt surface treatments (ASTs) have been often selected by the Alaska Department of Transportation and Public Facilities (DOT&PF) as the preferred low-cost alternative for primary and maintenance paving in Alaska.
These so called “high-float pavements” are simply a special form of AST as opposed to the more common single-shot or double-shot “chip job.” High-float pavements provide roughly the same service life and function as a double-shot AST.
Typically an AST consists of a thin layer of asphalt concrete formed by the application of emulsified asphalt or emulsified asphalt plus aggregate to protect or restore an existing roadway surface. An AST provides a nonstructural but durable and highly functional pavement surface when constructed properly with good materials. Within certain limitations, ASTs offer lower life-cycle costs than competing paving options. They give a long service life where traffic intensities are low and thicker pavements are inappropriate. ASTs are typically less than 1 in. (25 mm) thick.
High-float ASTs are constructed on a smooth base course surface. They consist of a single, heavily applied layer of high-float emulsified asphalt, followed by a single layer of dense-graded, crushed cover aggregate. The type of high-float emulsion used in Alaska is anionic HFMS-2s. The dense-graded, crushed cover aggregate is similar to standard minus 3?4 in. (minus 19 mm) or minus 1 in. (minus 25 mm) base courses commonly used in Alaska. Application rates of 0.75 gal/sq yd of high-float emulsion and 75 lb/sq yd of aggregate became the standard mix design “recipe” for most projects. The resulting pavement is usually 3?4 to 1 in. (19 to 25 mm) thick. The high-float cover aggregate is rolled, and after one to two weeks, broomed.
Alaska DOT&PF always aims at improving constructability and long-term performance of high-float pavements. Under the best conditions, high-float pavements are relatively inexpensive, easy to construct and provide good service for roads with a low volume of traffic.
Like other forms of AST, the normally positive economics of high-float pavements can be spoiled during or after construction. Problems are usually associated with: materials quality, construction technique, weather or application rates of asphalt and aggregate materials.
The following discusses some of the methods and research that are helping produce better high-float AST pavements in Alaska. More details can be found in the Alaska Surface Treatment Guide at www.dot.state.ak.us/stwddes/research/assets/pdf/fhwa_ak_rd_01_03.pdf
In addition to methods of standard AST practice that are found in The Asphalt Institute’s Emulsion Manual, MS 19, Alaska DOT&PF strongly advocates its maintenance crews and contractors to observe the following construction requirements.
Careful surface preparation is required. High-float pavements are much too thin to repair any problem of roughness, grade or crown. Also, the thin high-float pavement cannot compensate for poor quality or insufficient compaction of underlying layers.
Place cover aggregate for high-float ASTs within 200 ft of distributor. Do not move the high-float operation too fast or you can create a large wave of aggregate and emulsion in front of the cover aggregate spreader.
Control oil application to minimize overlap along longitudinal joints.
With too much overlap, you will produce a substantial ridge along the longitudinal joint. High-float emulsion is too viscous to produce an even thickness by surface flow if sprayed unevenly.
Applying emulsified asphalt
Apply the AST within 72 hours after surface preparation. The surface should not be completely dry. If dry, dampen the surface with a fine spray of water. Do not allow the spread of emulsion to be in excess of that which the aggregate spreader can immediately cover. Do not allow equipment or vehicles on sprayed asphalt before cover aggregate application. Do not proceed in a manner that allows the emulsion to chill, set up, dry or otherwise impair retention of the cover aggregate. Apply HFMS-2s at temperatures between 150 and 180?F.
Applying cover aggregate
Use cover aggregate that is at a temperature no lower than 40?F and that has a moisture content of 2% to 4% (by dry weight) at the time of application.
Apply cover aggregate within about 11?2 minutes after application of the emulsified asphalt. This time period should be kept as constant as possible.
Apply compaction using rubber-tired rollers for the full width of the aggregate immediately after placement of cover aggregate. Continue compaction for at least six complete coverages. The rolling operation should be accomplished within 500 ft of the cover aggregate application. The high-float application operation must be slowed if rolling cannot be completed within this distance. Do not allow the rubber-tired roller to exceed 5 mph.
Sweeping to remove excess cover aggregate is required. Sweeping should occur after the material is sufficiently cured to prevent damage, usually between one and two weeks following application of cover aggregate. Redistribute ridges of loose aggregate created by traffic during the period before final sweeping.
Typical problems include: Bad weather or paving too early or too late in the season combine to produce most high-float pavement construction failures in Alaska; poor pavement structure, including poor drainage; non-specification materials; and problems along joints.
Cold, damp or windy weather (i.e., any conditions that retard the fastest possible curing) may reduce short-term aggregate retention. The response to this has too often been to increase the oil application rate. Records from several AST failures indicated that the emulsified asphalt application rate was raised more than 50% above the mix design amount.
The moisture content of high-float cover aggregate stockpiles is critical. With a moisture content of near 0%, the material will not absorb the high-float emulsified asphalt (overly dry material will likely never be a problem under Alaskan conditions). When the moisture content is too high, the aggregate may not spread uniformly and may form clumps. High moisture also greatly increases curing time. On a typical project, there may not be enough time to dry materials that are too wet at the source; the contractor should think about this when choosing an aggregate source and scheduling a project. Always avoid any conditions that cause an increase in curing time.
The normal practice during construction has been to adjust the standard recipe in the field for emulsion/aggregate applications by visual inspection. The goal was to achieve the right combination of materials so that the emulsion barely reached the aggregate surface. This was judged from the surface appearance immediately after compaction, where emulsion would (ideally) be visible at many random locations across the compacted surface. With proper adjustment, “shows” of emulsion were a meter or so apart and the surface would take on a somewhat mottled appearance. Adjustment to too much emulsion caused it to become visible over much or all of the surface after compaction. Performance-wise, the long-term results of such field adjustments were hit and miss. A rational mix design method was needed.
Using a standard mix design recipe to define the relative amounts of aggregate and high-float emulsion simply does not work with some materials. Many areas of high-float pavement apparently failed simply because they were constructed using too much or too little asphalt. Excess asphalt can migrate to the pavement surface (a condition known as “flushing” or “bleeding”) during warm weather. Insufficient asphalt cement causes loss of paving aggregate leading to raveling, potholing or, in some cases, large areas of total pavement loss.
The Alaska DOT&PF is developing a new mix design method for high-float pavements. The objective is to define combinations of emulsion and aggregate materials that are volumetrically compatible. Volumetric compatibility means that an application of high-float emulsion will completely fill the voids in the design aggregate thickness. The emulsion and aggregate combination also must bond compatibly, compact well during the construction process and resist post-construction compaction. Alaska’s proposed mix design method defines emulsion content using simple volumetric calculations and accounts for the moisture content of the aggregate prior to addition of the emulsion.
Importantly, the method defines application rates for emulsion and aggregate that will minimize unintended aggregate loss. Unintended aggregate loss means loss of voids that are supposed to contain asphalt. If enough voids are lost, the remaining asphalt cement has no place to go except up to the road surface. The result is bleeding.
How does the mix design method discussed here prevent unintended aggregate loss?
It defines the application rate of aggregate so that the compacted aggregate thickness is about equivalent to the largest particle size. This is the design aggregate thickness.
It defines an emulsion application rate such that essentially all the available voids in the design aggregate thickness are filled.
A layer of aggregate no thicker than the largest particle size is not readily compactable. Therefore 15% additional aggregate (above mix design amount) is added to facilitate compaction and curing. Most of this “sacrificial” aggregate will be lost to traffic action.
As part of the mix design research effort, high-float pavement samples were cut from locations within the DOT&PF Northern Region. Aggregate losses were found to be as high as 50%.