(edited from an article by Dr. Shad Sargand, Ohio University, published in Ohio Asphalt)
Warm mix asphalt (WMA), first introduced in Europe in 1995, is drawing considerable attention in the pavement community. WMA offers several advantages over conventional asphalt mixtures including:
- Reduced energy consumption in mix preparation
- Reduced emissions and consequently reduced fumes and undesirable odors
- Reduced binder aging
- Extended construction seasons in temperate climates.
WMA requires the use of additives to allow the compaction of asphalt mixtures at lower temperatures than conventional hot mix asphalt (HMA). In the United States, only preliminary studies have been conducted to date to evaluate the significant properties of WMA. These studies have focused on evaluating the compactibility of mixes, the effect on stiffness properties, WMA’s rutting potential, and the need for curing time, and assessment of the increased potential for moisture damage.
Since WMA mixes require additives, higher unit material costs are expected. However, these costs may be offset by the reduction in energy expended for mix preparation. Even if WMA costs are higher, environmental factors may tilt a decision to use WMA—provided its engineering properties and performance are within expected levels.
Currently, four techniques are known to improve the workability of mixes to allow the preparation and compaction of WMA. These are:
- Aspha-min, which includes the addition of sodium aluminum silicate or zeolite
- Sasobit, which uses foam in the form of a paraffin-wax compound extracted from coal gasification
- Evotherm, which includes additives in the form of an emulsion to improve the coating and workability of WMA mixes
- WAM-Foam, which uses a soft binder and a hard, foamed binder added at different times during the mixing process.
The National Center for Asphalt Technology (NCAT) at Auburn University has completed laboratory studies of Aspha-min and Sasobit, and concluded that each method improved compactibility at temperatures as low as 190°F and that there was no effect on resilent modulus or rutting potential. Air voids were reduced and the warm asphalt mixture did not require any curing period. There is the potential for increased susceptibility to moisture damage, which in the case of Aspha-min could be mitigated by adding hydrated lime.
To address the lack of extensive field studies of warm asphalt, a detailed evaluation study to assess the performance and benefits of WMA mixes is being sponsored by the Ohio Department of Transportation (ODOT) and the Federal Highway Administration (FHWA). The study, done by the Ohio Research Institute for Transportation and the Environment (ORITE), will assess both field test sections and sections in the controlled environment of the Accelerated Pavement Load Facility (APLF).
The APLF at Ohio University’s Lancaster Campus is a state-of-the-art, indoor test facility for road pavements. It is large enough for a 45-foot long, 24-foot wide test road to be built inside the building, from the soil layer up.
The temperature in this environment is controlled from 10 to 130°F. Humidity is also controlled and moisture can be added to test its effect on sub-grade soil. A rolling tire load of 9,000 to 30,000 pounds can be applied to simulate a passing truck with standard single or dual tires or wide, single tires, up to 500 times per hour.
Testing WMA Perpetual Pavement
A warm mix asphalt Perpetual Pavement was constructed on September 6, 2006, at the APLF in Lancaster, Ohio, for testing WMA under a variety of loads and environmental conditions.
Three WMA surface courses are being tested at the APLF and compared to a conventional HMA surface course. The warm mixes are Aspha-min, Sasobit, and Evotherm, the same as were applied in the field. Each of the four mixes, including conventional HMA, was constructed as a 1.25-inch surface course on different thicknesses of Perpetual Pavement. These thicknesses are 16 inches, 15 inches, 14 inches and 13 inches, depending on the section, including a 4-inch fatigue layer. ODOT placed their standard 6- to 9-inch thick dense-graded aggregate base over the prepared sub-grade.
The goal of the project is to subject each test section and monitor their load responses under repeated loadings at high, medium and low temperatures. One particular objective is to examine the relationship between the thickness of the pavement and the tensile strain at the bottom of the Perpetual Pavement layer. The instrumentation includes single-layer deflectometers, pressure cells and longitudinal and transverse strain gages.
The environmental parameters being monitored include pavement layer temperature and the temperature, moisture and groundwater table in the base and subgrade. Moisture and temperature are being monitored according to the SHRP protocol. Load response displacement, strain and pressure will be measured, and seasonal response in terms of displacement and pressure will also be recorded. Falling weight deflectometer (FWD) tests are also being conducted.
Field Test Sections on S.R. 541
Warm mix asphalt field test sections selected by ODOT included the monitoring and testing of four non-instrumented sections constructed by Shelly and Sands as an overlay on the existing S.R. 541 in Guernsey County, west of I-77.
Each section consisted of a 0.75-inch layer of conventional HMA with a 1.25-inch top layer containing one of three types of WMA—Aspha-min, Sasobit and Evotherm. The 2.7-mile Evotherm section was paved in September of 2006. The Aspha-min and Sasobit sections, 2.7 miles and 3.07 miles respectively, as well as the 3.03-mile HMA control section, were also placed in September 2006.
A forensic assessment of the pre-existing surface layer, base and sub-grade conditions was conducted prior to paving. The assessment included:
- Falling weight deflectometer measurements by ODOT
- A surface profile and dynamic cone penetrometer tests by ORITE to identify weak areas of the pavement prior to construction.
The FWD data are being used to back-calculate pavement layer stiffnesses. The test results were also used to identify weak areas, which were repaired prior to applying the overlay.
Additional non-destructive testing of pavement sections by the FWD was conducted by ODOT prior to opening the test road to traffic and at subsequent time intervals. Periodic visual condition surveys are also being conducted to observe and document the development of distress, as the number of loads applied to the pavement increases.
The careful monitoring of the construction process for the WMA sections will help determine if the modification of asphalt cement with additives, to allow lower-temperature compactions, adversely affects the physical and engineering properties of the asphalt.
An infrared camera was used during construction of the field test sections to document WMA temperature and cooling throughout the placement and compaction process, and to compare it with any variability in the density of the finished section.
Transverse Surface Profiles
Transverse surface profile surveys are also being obtained by ORITE with a profilometer to monitor rutting development as the loading of pavement sections progresses. The performance of WMA sections is being compared with conventional HMA sections, to determine whether or not the use of additives to the asphalt cement negatively affect the engineering properties and performance of WMA pavements.
Energy Consumption and Cost Assessment
WMA mixes are being compared to traditional HMA mixes with respect to reduced energy consumption in mix preparation, reduced emissions and consequently reduced fumes and undesirable odors. WMA is also being looked at for reduced binder aging and extended construction seasons.
Site sampling included monitoring fumes and emissions at the paver and adjacent to the road. In addition, the comparison of each type of WMA pavement treatments will include a complete cost comparison to quantify the lifetime savings of WMA compared to conventional HMA.
Laboratory testing included the preparation of specimens using the three selected WMA techniques plus a conventional hot mix asphalt. Specimens were collected from the APLF and the field site in Guernsey County at the time of construction. Additional samples are being periodically collected at the field site over the first two years of service.
The specimens are being tested to determine their engineering properties including resilient modulus at a minimum of three temperatures and three frequencies, fatigue and rutting characteristics, long-term durability and aging, low-temperature cracking resistance, moisture resistance and basic physical properties.
The effects of lowered production temperatures on asphalt binder aging are also being investigated. For this, one or more sets of asphalt binders have been prepared with modified RTFO/PAV aging conditions reflecting reduced production temperature.
Comprehensive tests are being made of cores taken from all field and APLF tests sections. Some of the testing being conducted by ORITE includes: density testing; assessment of reduced aging during construction and evaluation of aging of binder as a function of time; indirect tensile strength and low temperature cracking susceptibility; Hamburg tests; and beam fatigue tests.
Other testing will be performed by NCAT at Auburn University, including the moisture content in the truck at time of application, gyratory compaction, volumetric properties, rutting potential, maximum specific gravity, tensile strength ratio test and anticipated in-place field density.