I. Research Problem Statement
Thermal cracking is considered to be one of the most critical distresses in pavements built in regions where significant and rapid changes in temperature occur. Thus, the selection of asphalt materials with minimal susceptibility to thermal cracking is desirable.
In conventional low temperature cracking tests, linear viscoelastic concepts are used to infer mechanical response of material to thermal loading while making assumptions about coefficients of expansion/contraction (CTE) and ignoring glass transition change. These assumptions are believed to cause serious errors in estimating thermal stresses.
Recent research by Bonaquist (i.e., Characterization of Wisconsin Mixture Low Temperature Properties for the AASHTO Mechanistic-Empirical Pavement Design Guide, 2011) has indicated that the coefficient of thermal expansion/contraction (CTE) of the asphalt binder can be as important as the Creep Compliance for the estimation of the critical cracking temperature. Further, a Pooled Fund Study on Low Temperature Cracking in Asphalt Pavements by Marasteanu et al. (2007) indicated the importance of the non-linearity of the CTE of both asphalt binders and mixtures in the overall thermal cracking performance of the pavement.
Most of the thermal cracking models/tools (e.g., MEPDG) available to transportation agencies and contractors consider the Creep Compliance and/or Relaxation Modulus as the most important property driven thermal cracking performance and consistently assume simple linear CTE for asphalt materials. However, CTE of asphalt materials are non-linear function of temperature and its impact on low temperature performance can be as important as the Relaxation Modulus\Creep Compliance. The coefficient of thermal expansion/contraction (CTE) is a property which describes the potential for thermal deformation of the asphalt material. Therefore, an easy to use procedure that can be implemented into practice for proper estimation of CTE for both asphalt binders and mixtures is required. Furthermore, an analysis framework for thermal cracking estimation that includes the non-linearity aspect of the CTE is also desired.
II. Research Objectives
The objectives of this research are to
· Conduct literature review on current test procedures to measure the coefficient of thermal expansion/contraction as function of temperature for asphalt binders and mixtures. Summarize available models/tools used for the prediction of thermal cracking (e.g., MEDPG TC Model).
· Develop a simple test procedure to measure CTE and Glass Transition Temperature (Tg) for both asphalt binders and mixtures as function of temperature.
· Develop and complete an experimental matrix to evaluate the sensitivity of the propose test procedure to different physical properties of the asphalt binder and aggregate.
· Establish which properties of the mix (e.g., gradation, volumetric fraction, stiffness ratio) affect CTE the most.
· Develop an updated thermal cracking analysis framework that includes glass transition behavior and non-linearity of the CTE.
· Establish the relation between CTE of asphalt mixtures and its thermal cracking performance.
To accomplish the research objectives, the following tasks need to be conducted:
Task 1: Review thermal cracking mechanisms of asphalt mixtures and the associated theoretical work, devices and test setups used for material characterization.
Task 2: Develop a simple device, including instrumentation, data acquisition, and analysis software, to estimate the CTE of asphalt binders and mixtures.
Task 3: Validate the accuracy of the test method with a comprehensive experimental matrix which includes both laboratory and field prepared samples. Field samples with known performance are used to determine the value of the proposed procedure.
Task 4: Conduct ruggedness testing on the proposed system to assess and correct sources of variability and error.
Task 5: Update current thermal cracking models to account for glass transition and non-linearity of the CTE. Verify the validity of the analysis method and test procedure with available field performance.
III. Estimate of Problem Funding and Research Period
The estimated proposed funding is $300,000 over 2 years.
IV. Urgency, Payoff Potential, and Implementation
The US spends billions of dollars on constructing, maintaining, and operating its 2.6 million of miles of paved roads and highways (Bureau of Transportation Statistics 2009). The majority of its road network is built with asphalt materials. Therefore, the selection of thermal cracking-resistant materials is crucial on reducing the costs of maintaining the US highway system.
The proposed research has the potential of significantly reducing the maintaining costs for transportation agencies by improving the current methodologies used for the selection of asphalt materials that are less susceptible to thermal cracking.