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Variability in Pavement Materials and Construction


Variability is inherent in all pavement materials and construction processes. Understanding the magnitude of variability is important to pavement construction industry stakeholders. State Highway Agencies (SHAs) need to quantify the typical variability of pavement materials and construction processes in order to establish rational specification limits. Material producers and contractors need to identify and measure sources of variability in order to manage quality.

While quality has traditionally been defined as “fitness for use” or “degree of excellence,” a modern definition of quality states “quality is inversely proportional to variability.” Relating pavement material properties to variability has traditionally relied on quality characteristics such as binder content, aggregate gradation, mix volumetrics, in-place density and smoothness for asphalt pavement, and compressive strength, air content, thickness, permeability, and smoothness for concrete pavement. NCHRP Synthesis of Highway Practice 232, Variability in Highway Pavement Construction (1996) published typical variabilities for many of these quality characteristics. However, in the past twenty-five years, improvements in mix design practices, test equipment and methods, material processing and construction practices, increased use of recycled materials and changes in SHA specifications including improved quality measures such as percent-within-limits and alternative project delivery methods (which typically include advanced quality management program components) are likely to have impacted variability. In addition, new performance tests such as performance-graded binder tests, and asphalt mixture cracking and rutting tests and concrete tests such as the Super Air Meter and surface resistivity are being introduced, most of which have not been evaluated for typical variability values.

As SHAs move from pavement acceptance based on traditional quality characteristics to performance tests (some of which rely on test methods that cannot be performed as frequently as traditional tests due to testing complexity) it is critical that the variability of pavement materials and construction processes be better understood so that pavements consistently meet the long-term expectations of SHAs. Also, introduction of nondestructive test methods and technologies, with greater coverage than traditional methods, have the potential to provide a great deal of useful data regarding pavement construction that can be leveraged to improve quality if their variability is documented. It is also important to know if quality assurance (QA) specifications instituted by many SHAs since 1996 have resulted in decreased variability, and therefore have resulted in improved pavement quality.


Prepare a report quantifying the typical variability of pavement construction materials and processes and provide guidance for SHAs to evaluate the variability of newly-implemented performance tests


Understanding variability is critically important to setting rational specification limits for pavement materials. Specification limits that are either too wide or too restrictive can result in incorrect acceptance or rejection decisions or incorrect pay adjustment for pavements. Quality improvement is dependent on reducing variability; therefore, it is necessary to determine if variability has been reduced in order to evaluate whether or nor QA specifications have achieved their intended goal of improving quality. Incorrect pavement acceptance decisions could result in millions of dollars of over/under payment for pavement construction, as well as impact performance predictions for newly-constructed pavements.

Related Research:

1.Hughes, C.S., Variability in Highway Pavement Construction, NCHRP Synthesis of Highway Practice No. 232, Washington, DC: National Cooperative Highway Research Program, 1996.

  1. Hughes, Charles S., “State Construction Quality Assurance Programs”, NCHRP 346 Transportation Research Board, Washington, D.C., 2005.

Task 1 – Literature review to identify the major fundamental sources of variability for mixture production, transportation, and placement for both concrete and asphalt pavements. Compile a prioritized list of data and metadata that could be useful for the analysis, with consideration of what is possible to obtain from agencies, producers, and contractors.

Task 2 – Conduct a survey to identify SHAs that have pavement quality characteristic test data stored in digital format that spans a decade or more to evaluate in the range of variability over time.

Task 3 – Collect test data and metadata from agencies, producers, and contractors on various pavement quality characteristics using the list compiled in Task 1.

Task 4 – Identify the main components of variability that comprise overall material variability. Conduct statistical analysis to determine typical variability for various quality characteristics and the relative impacts of key materials and/or construction processes that impact the variability of these quality characteristics.

Task 5 – Quantify the changes in the variability of quality characteristics that has occurred since publication of Synthesis of Highway Practice 232.

Task 6 - Develop guidelines for SHAs to quantify variability of new test methods for pavement materials and processes, including mixture performance tests and nondestructive tests of various construction quality characteristics.

Task 7 – Publish a report of the research work and findings.


Results of the report will be directly applicable to agencies for evaluation of existing pavement QA specifications and for future implementation of performance specifications. As SHAs implement performance specifications, they will need accurate measures of material and construction variability in order to reliably predict long-term pavement performance from performance test data. The AASHTO committee on Materials and Pavements and Committee on Construction will be important stakeholders for implementation of the research findings. Industry groups including the National Asphalt Pavement Association, the American Concrete Page 3 of 3 Pavement Association and the National Concrete Consortium will benefit from the information provided by this project.

Potential challenges to implementation include resistance to change by agencies and industry, and perceived cost impacts if the data indicates that specification ranges can be reduced due to decreased variability.

Sponsoring Committee:AKC30, Quality Assurance Management
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Bradbury, Hoegh, Hand, Rao
Source Info:Rick Bradbury, Maine Department of Transportation
Tel: (207) 624-3482

Kyle Hoegh, P.E., Ph.D., Minnesota Department of Transportation
Tel: (506) 398-2669

Adam Hand, Ph.D., University of Nevada Reno
Tel: (775) 784-1439

Shreenath Rao, Ph.D., P.E., Applied Research Associates
Tel: (720) 328-7265
Date Posted:07/28/2021
Date Modified:08/11/2021
Index Terms:Paving materials, Quality assurance, Paving, Performance tests,
Cosponsoring Committees: 

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