Cindy Estakhri

Evaluation of durability tests for permeable friction course mixtures

Durability of porous or permeable friction courses (PFC) is an important aspect to address when designing this type of hot mix asphalt. At present, several agencies perform the mix design of PFC primarily by determining volumetric mixture properties. This approach ensures adequate mixture functionality, but it does not guarantee mixture durability. This paper evaluates the Cantabro loss test, the Hamburg Wheel-Tracking test (HWTT) and the Overlay test (OT) to determine the one most appropriate for mix design and laboratory performance evaluations. The Cantabro loss test, performed in both dry and wet conditions, is recommended for PFC mix design to corroborate the suitability of the optimum asphalt content defined based on volumetric determinations. The HWTT and the OT are not recommended, since the variability of the test results indicated that these tests may not be suitable for PFC mixtures.

Determination of Volumetric Properties for Permeable Friction Course Mixtures

Current hot mix asphalt (HMA) mix design procedures used to determine the optimum asphalt content (OAC) for permeable or porous friction course (PFC) mixtures are based on volumetric properties, primarily total air void (AV) content. This calculated volumetric parameter depends on the bulk specific gravity (Gmb) and the theoretical maximum specific gravity (Gmm) of the mixture, which are generally difficult to measure in a laboratory due to the high asphalt contents, high total AV contents, and the use of modified asphalts for PFC mixtures. This study evaluated two methodologies for determining Gmb (vacuum and dimensional analysis) and two methodologies for determining Gmm (measured and calculated) for use in calculations of total AV content. For the mixtures assessed in this study, originally designed with a total AV content of 20 %, the alternative methodologies studied led to total AV content values outside the design range (18 to 22 %), which implies the necessity of gradation modifications or changes in the fiber content to meet AV requirements and define an OAC. Dimensional analysis and a calculation procedure, based on values of Gmm measured in the laboratory at low asphalt contents, are recommended for determining Gmb and Gmm values, respectively. In addition, dimensional analysis is preliminarily recommended to compute the water-accessible AV content of PFC mixtures based on the assessment of two methods (vacuum and a methodology proposed for dimensional analysis) to compute this parameter. Water-accessible AV content is considered as an alternative parameter for mix design and evaluation.

Optimizing the Design of Permeable Friction Course Mixtures

Permeable friction course (PFC) mixtures, or new-generation open-graded friction course mixtures, are one of the safest and quietest available alternatives for surface paving. Advantages to the use of PFC mixtures are improvements in safety, reduction in environmental impact (e.g., cleaner runoff), and economy. These advantages are closely related to their high total air void (AV) content and aggregate gradation, which confers high permeability and noise reduction properties to PFC mixtures. Research was completed at Texas A&M University to improve PFC mix design. Findings included improved determination of the total AV content through the use of a calculation procedure to obtain the mixture theoretical maximum specific gravity, Gmm, and dimensional analysis to determine the mixture bulk specific gravity, Gmb. Water-accessible AV content was proposed as a surrogate of the total AV content for mix design and evaluation. In addition, the water flow value (outflow time) and the expected value of permeability (E[k]) were recommended, respectively, to assess field drainability and to estimate the permeability of both laboratory- and field-compacted (i.e., road cores) mixtures. The Cantabro loss test was suggested for assessing mixture durability. Furthermore, an improved criterion for verification of stone-on-stone contact for mix design and a field-density control during construction were recommended to guarantee adequate stability and durability. Recommendations to improve fabrication of Superpave® gyratory compactor specimens and sampling of field-compacted mixtures (by using road cores) were included. This set of recommendations was integrated in an improved mix design method for PFC mixtures.

Reducing Greenhouse Gas Emissions in Texas with High-Volume Fly Ash Concrete

The objective of this study was to determine the potential for reductions in carbon dioxide emissions in Texas by substituting high volumes of fly ash in concrete production and to identify the resulting benefits and challenges. Researchers reviewed the literature and determined that high-volume fly ash can improve the properties of both fresh and hardened concrete. It can improve workability, heat of hydration, strength, permeability, and resistance to chemical attack. Researchers compiled data from 18 power plants located throughout Texas and determined that 6.6 million tons of fly ash are produced annually in Texas and about 2.7 million tons (or 40%) are generally sold for use in concrete or other end products. Researchers estimated the production of concrete in Texas and determined that if 60% of the portland cement used in Texas concrete production were replaced with fly ash, carbon dioxide emissions could be reduced by 6.6 million tons annually by the year 2015. More education is needed for design engineers and for the concrete industry regarding the performance and environmental benefits that can be realized through increased use of fly ash in concrete.

Design and Performance Evaluation of Fine-Graded Permeable Friction Course

Permeable friction courses (PFCs) are popular in Texas, where the current specification for PFC (Item 342) has a maximum aggregate size of 1/2 in. and is typically placed in layer thicknesses of 1.5 to 2 in. In this study fine-graded PFCs composed of a single aggregate fraction are proposed for placement at a nominal thickness of 1 in. Initial laboratory testing found that the target air void content for volumetric design would be around 26% air voids, substantially higher than the current PFC designs, which are between 18% and 22% air voids. To minimize the likelihood of failure, extensive laboratory testing was performed to arrive at the proposed design. Tests included Hamburg wheel-track testing, overlay tester cracking, and Cantabro, draindown, and water flow tests. The proposed fine PFC mix was first placed on a test track in Pecos, Texas. Two designs were placed and subjected to limited traffic loadings, field water flow, noise, and skid measurements. These test sections performed well. The next section was placed on a Texas Department of Transportation project in May 2011 and subjected to extremely intense traffic loading conditions on an exit ramp on US-59 in Lufkin. This ramp has a high frequency of wet-weather accidents. In addition to extreme traffic loads, the surface experienced extreme heat (air temperatures approaching 105°F) and heavy localized rain (a 6-in. rain event within a 24-h period). After 3 months the fine PFC is holding up well.