Overview: Moisture, whether it is in the form of vapor or water, contributes to the premature failure of asphalt pavements through deterioration of the asphalt mixture. There are three mechanisms in which moisture may degrade an asphalt matrix: (1) loss of cohesion (structure); (2) failure of the adhesion (bond) between aggregate and asphalt (which is referred to as stripping); and (3) degradation of the aggregate.
Hot-mix asphalt is a mixture that consists of asphalt cement (petroleum product), aggregate, and additives (e.g. anti-stripping agents) and/or modifiers. A particular mixture design is tailored to a project depending on a number of factors including environment and traffic patterns. The mixture properties and, therefore, the mixture's susceptibility to moisture are dependent on material properties, quantities and combinations as well as external factors.
An asphalt mixture is deemed 'moisture sensitive' if laboratory specimens fail a moisture sensitivity test. A moisture sensitivity test consists of a conditioning phase and then an evaluation phase. There are numerous tests that have been developed to evaluate the moisture sensitivity of asphalt pavements with the goal of identifying mixtures at risk to water damage. The majority of these tests is strength-based and, unfortunately, is not able to predict performance (i.e. estimate the life of the pavement) and even further is not applicable to a wide-range of materials or conditions.
In the past decade, there have been tests developed to remedy this situation such as the Environmental Conditioning System (ECS) and the use of ultrasonic technology. These tests have been met with controversy and have not been widely implemented due to implementation issues such as the lack of sufficient repeatability, correlation with field results, and/or cost concerns.
It is evident from past research and practice that one test cannot fully and accurately predict the performance of asphalt pavements with regard to moisture damage. Thus, we arrive at an imperative question: Can we develop a methodology to quantify the risk of moisture damage for a particular asphalt mixture design?
Project Goal and Objectives: The broad goal of the proposed research is to develop a risk assessment methodology that will evaluate the potential for moisture susceptibility in asphalt mixtures and aid in the prediction of the performance of asphalt mixtures.
There are five key components of a general risk assessment. They include: i) identification of the hazard(s), ii) consideration of how the system may fail and whom it will affect, iii) evaluation of the risk and existing precautions and determination of additional safety measures, iv) an accurate and comprehensive record of all findings, and v) review and possible revisions. The specific research objectives that are included in this study are: (a) identify and rank in terms of importance the factors (i.e. 'hazards') that contribute to an asphalt pavement mixture being considered moisture sensitive; (b) correctly assess failure mechanisms and significant failure modes for the asphalt system and its components and external factors; (c) develop a quantitative risk computational methodology; (d) implement and validate a tool to quantify the risk to moisture sensitivity of a given mixture under specified conditions; and (d) demonstrate the capabilities of the proposed risk analysis methodology that will aid in decision-making for mixture and pavement design. The deliverables of this study is a detailed report that lists the important factors that contribute to moisture sensitivity, evaluates significant failure paths, and details the creation and use of the risk assessment methodology for hot-mix asphalt pavements that can be utilized by the federal highway administration and state transportation agencies.
Significance of Proposed Research: The proposed research seeks to identify moisture sensitive asphalt mixtures by quantifying each mixture's risk dependent on a number of factors. The risk assessment methodology will impact the design and service life prediction of hot-mix asphalt pavements. The methodology will provide a quantitative safety evaluation for each potential mix design and provide a proficient and reliable evaluation of a key component of our nation's transportation infrastructure: roads and highways. The tool will provide reliable information on the performance of asphalt pavements so that efficient maintenance and repair plans may be developed.
The proposed research will serve as a tool for proper material selection and optimization based on limited resources. For example, as part of the material selection and mixture design process, the benefits of including anti-stripping agents can be evaluated quantitatively and in terms of costs. Furthermore, the research will identify the crucial material properties (e.g. gradation) and tests that should be considered when evaluating the moisture sensitivity of an asphalt mixture. The proposed research will also assimilate multitudes of available data and provide conclusive information regarding moisture sensitivity based on the data.
The primary beneficiary of this knowledge is the Federal Highway Administration (FHWA) and state transportation agencies. This early identification and assessment tool may also be utilized by the hot-mix asphalt industry, pavement/highway designers, other government entities (e.g. European Union), asphalt associations and researchers in the area of bituminous and asphalt mixtures.
In addition to the asphalt pavement industry, there are other disciplines that may benefit from the proposed research. One example is the containment of hazardous wastes. The quantitative risk assessment model may be applied to asphalt mixtures that are used to contain hazardous materials (Rofer, Martinez et al. 1999). Moisture can accumulate under the asphalt cover potentially degrading the asphalt mixture and reducing the integrity of the containment structure. Hazardous waste containers, especially in the case of radioactive wastes, are subject to extensive risk assessment that will predict their performance for hundreds of years. The proposed risk assessment model for asphalt mixtures will contribute to an overall accurate reliability rating of hazardous waste containers that utilize asphalt.
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