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Developing a friction performance index for asphalt concrete mixtures

Description:

For the majority of asphalt mixtures produced in the U.S., their design is based solely on volumetric considerations. For a small but growing number of these mixtures, the design may also be based on structural performance, e.g., balanced mixture design (BMD). With the BMD concept and/or changes to the current design methodology, it is likely that mixtures will become finer, have an increased asphalt content, and potentially higher amounts of recycled materials. All of these changes will affect the functional characteristics; however, there does not currently exist a methodology to assess asphalt mixture designs to ensure that their functional performance characteristics. Worse, engineers lack a reliable, vetted procedure to consider functional performance at the mixture design stage. As such, state DOTs cannot actively engineer asphalt mixtures to exhibit sufficient functional and structural performance thresholds. Thus, they rely on ad hoc corrections when issues are identified.

This project will address this need by developing a procedure for state DOTs to evaluate their mixtures to either screen for those with poor functional performance or (ultimately) to design mixtures to avoid functional failures associated with material characteristics that do not balance functionality with structural durability. The experiences in developing balanced mixture design practices for rutting and fatigue demonstrate that failure to consider the consequences of potential test implementation (for example implementing a rutting test without including a cracking test) drives mixture behaviors in unexpected and unforeseen ways.

This research will address this problem by identifying the most effective, discriminating, and implementable test method or methods that state DOT’s can adopt and that other researchers can use to develop practical mixture design methodologies and also for quality assurance approaches. This project would be clearly aligned with the AASHTO Committee on Materials and Pavements, which is concerned with, among other things, asphalt materials testing, specification, and performance, as well as management. Given the connection between the surface condition of roadways and safety, this research also addresses the strategic goals for the Safety committee whose primary goal is to eliminate serious traffic crashes and save lives.

Objective:

The objectives of this research are to develop a methodology, applicable to both the mixture design/selection procedures and quality assurance/quality control process, to assess the functional performance of asphalt concrete mixtures with respect to friction and macrotexture.

Benefits:

There is an urgent need for this work to address the unknown impacts on safety (i.e. friction qualities) of asphalt mixtures as the balanced mix design method is implemented by agencies. Developing a means of evaluating functional performance properties that can be applied both during design and post-production will allow functional performance and safety to be designed into asphalt mixtures, and will provide a means of ensuring that this performance is maintained through the QC/QA process.

Related Research:

Many functional failures associated with asphalt mixture characteristics can be ultimately traced to insufficiencies in surface macro and microtexture. In fact, the AASHTO Guide for Pavement friction explicitly recognizes the role of texture on friction and ultimately the safety performance of roadways when it defines friction demand as “the level of friction needed to safely perform braking, steering, and acceleration maneuvers.” Owing to the tremendous safety implications from poor skid resistance, much of the research in this domain has focused on finding correlations between laboratory assessment of skid and polishing resistance and the actual skid resistance situation in the field (Hofko et al. 2019; Kogbara et al. 2016; Masad et al. 2009; Merritt et al. 2015). The main objectives of these studies have been to understand the impact of employed materials and field conditions on the evolution of in-service skid resistance and how to simulate this evolution in the laboratory. The main elements considered in such studies can be summarized as follows:

  1. Characterization of the polish resistance of the individual aggregate fractions, e.g., BS EN 1097-8:2020 or aggregate abrasion measured via AASHTO T 327 (ASTM D7428-15),

  2. Characterization of the aggregate shapes, mixture gradation, and surface mean texture depth (MTD) or mean profile depth (MPD), e.g., testing via the Circular Track Meter (ASTM E2157 -15 2015), laser scanners, the ASTM E965 sand patch method, and others.

  3. Characterization of the change in surface skid resistance under traffic. Currently, there are two laboratory devices that have been used in different studies; 1) the Three Wheel Polishing Device (TWPD) and 2) the Wehner Schulze Device (WZD), manufactured in Germany.

Some advances have been made to describe and standardize the methods for measuring the surface friction and texture characteristics. However, more research is needed because there is still not a formal procedure to address final surface skid resistance during material design nor is there a process to accommodate the varying levels of friction that are required across and along typical roadways. A similar lack of knowledge and tools exists for other functional characteristics as well. To develop a mix design procedure that incorporates asphalt mixture texture, some challenges must be addressed; for example, both the TWPD and the WZD use field or lab-compacted slabs while the current Superpave mix design protocol produces a specimen with geometry that is dictated by the Superpave Gyratory Compactor. In addition, multiple devices can be employed to measure friction and texture in the field; this has caused problems in developing a single and reliable transfer function linking laboratory measurements with field observations.

Tasks:

A draft set of tasks that will likely need to be performed to achieve this goal are:

· Review existing methodologies for determination of asphalt mixture texture and friction and ongoing research in this domain. The evaluation should consider the role of friction and texture in safety (as defined by AASHTO and in NCHRP Report 37) and availability of datasets such as the KY continuous friction measurement data, asphalt mix design, and crash data.

· Select the most promising experimental method(s) to evaluate asphalt mixture performance with respect to friction and texture that can be applied during design and after construction.

· Develop the methodology for performance assessment. This methodology should be capable of evaluating asphalt mixtures macrotexture and microtexture but could consist of a combination of component material and composite tests and models.

· Establish initial performance thresholds for evaluation at the mixture design stage and for process control and quality assurance, and refinement in later research.

· Develop a draft AASHTO specification(s) for review and submission as a provisional specification.

Implementation:

The results of this research will be used by state DOTs (specifically state DOT materials engineers), asphalt mixture designers, and asphalt paving contractors to ensure that asphalt mixtures are designed and produced with balanced structural and functional performance. Implementation activities may include:

· conference presentations (e.g. TRB Annual Meeting, RPUG, AASHTO), regional user-producer group presentations, BMD workshops, and webinars (e.g. TRB, AASHTO).

· supporting supplementary materials could include: a technical brief, short instructional videos, and whiteboard videos.

· a national workshop or webinar with invited participants for discussion on implementing the inclusion of functional assessment as part of mixture design and quality assurance/quality control processes.

· development of a follow-up effort to formulate guidelines to help state DOTs implement the functional assessment methodology into their mixture design and quality assurance processes.

· partnership with state DOTs currently participating in TPF-5(345/463) or similar studies.

Sponsoring Committee:AKM40, Asphalt Mixture Evaluation and Performance
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Shane Underwood, Boris Goenaga, Ilker Boz, Stacey Diefenderfer, Magdy Mikhail, Brian Schleppi
Source Info:
Date Posted:11/10/2021
Date Modified:01/05/2022
Index Terms:Friction factor, Index numbers, Asphalt concrete, Asphalt mixtures, Mix design,
 
Subjects    
Highways
Design
Pavements

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