Moisture Sensitivity of Asphalt Pavements (2003)

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3TOPIC 1 Introduction and Seminar Objectives R. GARY HICKS MACTEC LARRY SANTUCCI University of California, Berkeley TIM ASCHENBRENER Colorado Department of Transportation Moisture sensitivity in asphalt pavements is a national issue. This was demonstrated through a recent survey of state highway agencies throughout the United States. This national seminar is designed to address moisture-related distress in asphalt pavements through a series of focused papers followed by working breakout sessions. An introduction to the issues and the national seminar is given in this paper. The following items are covered: • The extent of the problem, • The purpose and scope of the national seminar, • Definition of moisture sensitivity in asphalt pavements, • Identification of moisture sensitivity problems, • Causes of moisture sensitivity problems, • Potential solutions to the problems, and • Expected deliverables for the seminar. In this paper, the stage is set for what is to occur over the ensuing 21⁄2 days. BACKGROUND National Problem Moisture sensitivity in hot-mix asphalt (HMA) is a national issue. In a recent survey (dated August 4, 2002) of 55 agencies conducted by the Colorado Department of Transportation that included 50 state departments of transportation, 3 FHWA Federal Land offices, the District of Columbia, and 1 Canadian province, it was determined that 82% of the agencies require some sort of antistrip treatment. Of those that treat, 56% treat with liquids, 15% with liquid or lime, and 29% with lime (see Figure 1). Eighty-seven percent of the agencies test for moisture sensitivity (Figure 2). Of those that test,

4 Moisture Sensitivity of Asphalt Pavements: A National Seminar • 82% use a tensile test (AASHTO T283, ASTM D4867, or similar), • 10% use a compressive test (AASHTO T115 or similar), • 4% use a retained stability test, and • 4% use wheel-tracking tests and tensile tests. When testing is specified, 62% test for mix design only, and 38% test for mix design and field acceptance (Figure 3). Details of the results of the survey are given in Appendix A of this paper. The impact of moisture sensitivity problems on pavement performance or pavement costs is not clearly defined, but 20% of the agencies continue to fund research to • Understand the fundamental chemical nature of the problem, • Refine an existing test procedure or develop an improved procedure, and • Identify the ability of the test procedure to correlate to field performance. Though moisture sensitivity is a national issue, the various states have used different strategies to mitigate the detrimental effects of moisture in pavements. These strategies as well as others will be discussed in this 21⁄2-day seminar. FIGURE 1 States that treat HMA for moisture damage.

Hicks, Santucci, and Aschenbrener 5 FIGURE 2 Tests used for moisture susceptibility. FIGURE 3 Time of testing for moisture sensitivity.

6 Moisture Sensitivity of Asphalt Pavements: A National Seminar Purpose and Scope of Seminar The goals of the national seminar are twofold: first, to provide a forum for technology transfer of the latest information relative to moisture sensitivity by leading experts in the field; and second, to develop a road map to help solve this problem. Specific topics to be covered include • Identification of the problem, which includes distinguishing between materials- and construction-related factors; • Fundamental concepts for understanding the interaction between the binder and aggregate; • Test methods along with the advantages and disadvantages of existing laboratory and field test procedures; • Remediation strategies, including the use of additives and improved design and construction practices; • Field performance with case studies showing what works and what does not; • Specifications and shortcomings of existing practices and how they might be improved; and • A road map for the future that includes an implementation package for eliminating the problem. The seminar will consist of a series of focused lectures followed by breakout sessions to identify • Best practices, • Gaps in knowledge, and • Research needs. The breakout sessions will focus on tasks to solve the problem and generate a road map to success for agencies to follow in minimizing the adverse effects of moisture on asphalt pavements. MOISTURE SENSITIVITY: DEFINITIONS AND DISTRESS MANIFESTATIONS Definition of Moisture Sensitivity Moisture-related problems are due to or are accelerated by • Adhesive failure—stripping of the asphalt film from the aggregate surface, or • Cohesion failure—loss of mixture stiffness. These mechanisms can be associated with the aggregate, the binder, or the interaction between the two ingredients. Moisture-related distresses are also accelerated by mix design or construction issues, including those given in Table 1. These factors will be the topics of the papers that follow and focused discussions in the breakout sessions later in the seminar.

Hicks, Santucci, and Aschenbrener 7 Moisture-Related Distress Manifestations Moisture-related distress is similar in many ways to distress caused by other factors (materials, design, construction). Moisture tends to accelerate the presence of the distress types. The types of distress that can be related to moisture, or the other factors, are described below: • Bleeding, cracking, and rutting: These distresses are caused by a partial or complete loss of the adhesion bond between the aggregate surface and the asphalt cement. This may be caused by the presence of water in the mix due to poor compaction, inadequately dried or dirty aggregate, poor drainage, and poor aggregate–asphalt chemistry. It is aggravated by the presence of traffic and freeze–thaw cycles and can lead to early bleeding, rutting, or fatigue cracking. Figures 4 and 5 show some of the various manifestations of this type of moisture-related distress. • Raveling: Progressive loss of surface material by weathering or traffic abrasion, or both, is another manifestation of moisture-related distress. It may be caused by poor compaction, inferior aggregates, low asphalt content, high fines content, or moisture-related damage, and it is aggravated by traffic. Figure 6 shows different stages of this type of moisture- related distress. TABLE 1 Factors That Can Contribute to Moisture-Related Distress MIX DESIGN • Binder and aggregate chemistry • Binder content • Air voids • Additives PRODUCTION • Percent aggregate coating and quality of passing the No. 200 sieve • Temperature at plant • Excess aggregate moisture content • Presence of clay CONSTRUCTION • Compaction—high in-place air voids • Permeability—high values • Mix segregation • Changes from mix design to field production (field variability) CLIMATE • High-rainfall areas • Freeze–thaw cycles • Desert issues (steam stripping) OTHER FACTORS • Surface drainage • Subsurface drainage • Rehab strategies—chip seals over marginal HMA materials • High truck ADTs.

8 Moisture Sensitivity of Asphalt Pavements: A National Seminar (a) (b) FIGURE 4 Pavement damage: (a) bleeding; (b) rutting. 1 (a) (b) FIGURE 5 Pavement distress—cracking: (a) early stages; (b) advanced stages.

Hicks, Santucci, and Aschenbrener 9 (a) (b) FIGURE 6 Pavement distress—raveling: (a) early stages; (b) advanced stages. FIGURE 7 Pavement distress—localized failures. • Localized failures: This type of distress can be the end result of either of the types discussed above. It is progressive and can be due to the loss of adhesion between the binder and the aggregate or the cohesive strength in the mix itself. Figure 7 shows this type of distress. • Structural strength reduction: This is a result of a cohesive failure causing a loss in stiffness in the mixture. Figure 8 illustrates this type of moisture damage, and Figure 9 shows the effects of moisture on stiffness.

10 Moisture Sensitivity of Asphalt Pavements: A National Seminar Summary A major product of this seminar needs to include methods for identifying what pavement distress is moisture related. In the next section, a first attempt is made to provide a framework for addressing this problem. (a) (b) FIGURE 8 Moisture-related distress—effect on fatigue: (a) σ versus Nf; (b) ε versus Nf.

Hicks, Santucci, and Aschenbrener 11 FIGURE 9 Moisture-related distress—effect on asphalt mix stiffness. IDENTIFICATION OF MOISTURE SENSITIVITY PROBLEMS How can engineers distinguish between moisture-related problems and problems associated with poor construction practices? This is a difficult question, since the distress types associated with stripping (rutting, bleeding, early fatigue cracking, localized potholes) and raveling (rock loss) can also be caused by inadequate design or construction factors such as the following: • Mix design—too much or too little asphalt, • Low compaction—high voids and permeability, and • Poor mix gradation. The larger question is whether the distress is associated with poor materials, design, or construction, or an adhesion or cohesion problem associated with moisture in the asphalt mix. Sampling and testing the in-place hot mix are often required to isolate the cause of the observed distress. Figures 10 and 11 are examples of simple flowcharts that could be used to assist engineers in determining whether the distress is caused by moisture. Certain pavement distresses are often attributed to moisture and as a result require the use of additives. In some cases, the distress can be directly related to poor construction practices. This will be addressed in more detail during the course of the seminar. Moisture-sensitive mixes need to be identified during the course of the mix design process. Numerous laboratory tests have been used to identify moisture-related problems. These include tests on (a) loose mix to determine coating during water immersion or in boiling water and (b) compacted mix to evaluate the retained strength or stiffness and the amount of rutting or disintegration during wheel-tracking tests. Both types of tests measure the effects of moisture on

12 Moisture Sensitivity of Asphalt Pavements: A National Seminar adhesion, cohesion, or some combination of the two (see Figure 12). However, many of the tests have shortcomings, including the following: • They are not performance related. • They exhibit poor reproducibility between laboratories. • They do not provide a good indication of the effects of traffic or climate. New tests continue to be developed and evaluated. A considerable amount of field evaluation will be required to determine whether they will do a better job of predicting moisture- related distress. FIGURE 10 Generalized flowchart for identifying moisture-related distress.

Hicks, Santucci, and Aschenbrener 13 FIGURE 11 Example flowchart for determining moisture-related distress for bleeding and rutting.

14 Moisture Sensitivity of Asphalt Pavements: A National Seminar (a) (b) FIGURE 12 Typical appearance of stripped mixes: (a) loose mix; (b) compacted mix.

Hicks, Santucci, and Aschenbrener 15 CAUSES OF MOISTURE-RELATED DISTRESS Numerous factors can contribute to moisture sensitivity problems in asphalt pavements (see Table 1). Some of these factors are discussed briefly in this section. They are addressed further in the technical papers that follow and in the subsequent breakout sessions. Moisture-Sensitive Aggregates Aggregates can greatly influence whether a mixture will be moisture sensitive or not. The aggregate surface chemistry and the presence of clay fines are important factors affecting the adhesion between the aggregate and the asphalt binder. Common methods of combating these factors are through the use of antistrip agents such as liquids or lime and by the elimination of detrimental clay fines through proper processing or specification. These issues are discussed in more detail in the paper dealing with chemical and mechanical processes. Asphalt Binder Sensitivity The asphalt binder can influence both the adhesion between it and the aggregate and the cohesion of the mastic. Adhesion is influenced by the chemistry of the asphalt as well as by the stiffness of the binder. The cohesive strength of the asphalt matrix in the presence of moisture is also influenced by the chemical nature of the binder and processing techniques. The fundamentals of binder sensitivity to water are also treated in the paper on chemical and mechanical processes. Presence of Water and Traffic Moisture-related problems do not occur without the presence of water and traffic, which provides energy to break the adhesive bonds and cause cohesive failures. Repeated freeze–thaw cycles can also accelerate the distress in the pavement. Moisture comes from rain infiltration or from beneath the surface. Once the moisture is in the pavement, it can affect either the adhesive bond or cohesive strength. Test methods, which have historically been used to evaluate mixes for moisture sensitivity, have generally examined the effect of moisture on the mix strength or the coating on the aggregate. They have not included the effect of traffic on accelerating the moisture-related distress. In the paper on test methods the strengths and weaknesses of present tests and their relationship to pavement performance are addressed, and new methods and future directions are discussed. Pavement Design Considerations Pavements may have fundamental design flaws that trap water or moisture within the structural layers. There must be good drainage design, both surface and subsurface, since water causes moisture-related distress. The application of surface seals to a moisture-sensitive mix can also be a factor in accelerating moisture damage. The paper on design and production issues addresses these factors and others in more detail. Material Production Issues The paper on design and production issues also addresses material production issues that can affect the moisture sensitivity of asphalt mix. Some of the issues to be discussed are as follows:

16 Moisture Sensitivity of Asphalt Pavements: A National Seminar • The influence on moisture damage of the method used to refine the binder, particularly the effects of acids and bases; • Aggregate production issues including cleanliness, moisture content, and hardness; and • Mix handling, including the use of storage silos. Construction Issues A number of construction issues can affect the moisture sensitivity of the mix. Weather conditions are important in that they can affect mix compaction or trap mix moisture. Mix handling techniques (e.g., windrows truck loading) can influence segregation and affect the permeability of the mix. Joint construction techniques can also affect compaction and permeability. The amount of compaction achieved (relative density) has a major effect on the air void content, the permeability of the finished pavement, and the mix sensitivity to moisture damage. Control (or lack thereof) of required additives can influence the long-term performance of the mix. These factors are discussed in the paper on construction issues. POTENTIAL SOLUTIONS The factors discussed in the prior sections contribute to moisture-related distress. How can industry prevent these problems? Potential solutions are discussed in the remaining papers. Treatments In the paper on treatments, the various types of treatments used and their effectiveness are discussed. The various methods to add lime to asphalt concrete are discussed, and documented evidence on the effectiveness of the various methods is provided. Similarly, the merits of adding different types of liquids and the effectiveness of each method are discussed. Finally, the cost- effectiveness of the various treatments is presented. All of the treatments work under some conditions. Guidelines need to be developed to ensure that the treatment selected is the best and most cost-effective for the application. Field Experiences Documentation concerning what has worked and what has not is given in the paper on field experiences. The paper includes a discussion of selected case histories from throughout the United States. For each agency, the following items are discussed: • History of problems with moisture sensitivity, • Solutions to moisture sensitivity problems, • Performance relations or forensic tools, • Specifications to control moisture sensitivity, and • Ongoing research on moisture sensitivity. Specifications The factors that need to be specified to minimize moisture-related problems, including the material properties that must be monitored, were discussed in a presentation, which is not included in this proceedings. According to the presentation, the material properties could include the compatibility between the binder and the aggregate, the quality and condition of the aggregate, the need for additives as determined by the mix design tests, and other items. The

Hicks, Santucci, and Aschenbrener 17 importance of field quality control/quality assurance testing on loose mix, field cores, and the actual in-place pavement was also discussed. Defining appropriate acceptance criteria to mitigate moisture-related distress and validating the quality of field-produced mixes are essential. Implementation In the last paper on implementation, the group is challenged to document best practices, identify barriers or gaps in knowledge, and outline future research needs. It is expected that at the end of the breakout sessions, the following will have been identified: • Best practices, • Gaps in knowledge and barriers to progress, and • Research needs and options to find funding to complete the research. EXPECTED DELIVERABLES FROM THE SEMINAR Deliverables The expected deliverables from this seminar are focused in the following areas: • Identify best practices: A number of agencies have studied and evaluated moisture problems in pavements and have developed procedures to deal with them. This seminar will result in the identification of these procedures to mitigate the problem so others can take advantage of their experiences. • Identify gaps in knowledge and barriers to progress: An important part of the seminar will be to identify which procedures and processes have been successful and which have not. In addition, it will be important to identify the knowledge gaps that result in a lack of understanding of the causes of the problem and the barriers that need to be addressed. • Identify research needs: Once the gaps in knowledge and other barriers are identified, it will be possible to develop a set of research needs and prepare preliminary research problem statements for each issue. • Road map for the future: A road map presents options for FHWA and the state highway administrations to deal with short- and long-term solutions to the problem. The products from this seminar will be the papers, a summary of the breakout sessions, and a road map with options for solving this national problem. Specific Questions to Be Addressed Specific questions that should be addressed during the breakout sessions include but are not limited to the following: • Session 1: Fundamentals − What is moisture-related distress? − How can moisture-related distress be distinguished from distress resulting from construction-related problems? − What are the mechanisms causing moisture-related distress? − Are processes available for identifying moisture-sensitive aggregates and asphalts?

18 Moisture Sensitivity of Asphalt Pavements: A National Seminar − What are the major gaps in knowledge? − What fundamental issues still need to be addressed? • Session 2: Testing and Treatments − What test methods are best for identifying moisture-related problems? Which relate to field performance? − Are improvements to existing test methods still needed? − How effective are the various additives in minimizing the effects of moisture? − Is there documented evidence on how they affect pavement life? If not, why not? − What issues still need to be addressed? • Session 3: Design and Specifications − What mix design procedures are most effective in controlling moisture-related problems? − What items in the specifications should be controlled to minimize problems? − Are all the major factors in design and specifications being considered? If not, what additional factors need to be considered to minimize the effects of water on the asphalt pavement? • Session 4: Construction and Field Performance − What construction issues need to be controlled to reduce moisture problems? − What has worked and what has not worked? − What information is needed to make better decisions when it comes to preventing moisture-related distress? Summary Pavement problems related to moisture are being addressed through use of additives, improved mix design, construction practices, and better specifications. This seminar will result in the documentation of best practices to control the problem and produce a road map with options for improving the ability to deal with the problem. RESOURCES Anderson, D. A., E. L. Dukatz, and J. C. Petersen. The Effect of Antistrip Additives on the Properties of Asphalt Cement. Proc., Association of Asphalt Paving Technologists, Vol. 51, 1982, p. 298. Button, J. W. Maximizing the Beneficial Effect of Lime in Asphalt Paving Mixtures. In ASTM STP 899 (B. E. Ruth, ed.), American Society for Testing and Materials, Philadelphia, Pa., 1985, pp. 134–146. Coplantz, J. S., and D. E. Newcomb. Water Sensitivity Test Methods for Asphalt Concrete Mixtures: A Laboratory Comparison. In Transportation Research Record 1171, TRB, National Research Council, Washington, D.C., 1988, pp. 44–50. Fromm, H. J. The Mechanisms of Asphalt Stripping from Aggregate Surfaces. Proc., Association of Asphalt Paving Technologists, Vol. 43, 1974, pp. 191–223. Graf, P. E. Factors Affecting Moisture Susceptibility of Asphalt Concrete Mixes. Proc., Association of Asphalt Paving Technologists, Vol. 55, 1986, pp. 175–212. Hicks, R. G. NCHRP Synthesis of Highway Practice 175: Moisture Damage in Asphalt Concrete. TRB, National Research Council, Washington, D.C., 1991.

Hicks, Santucci, and Aschenbrener 19 Kandhal, P. S. Moisture Sensitivity of HMA Mixes—Identification of Problems and Recommended Solutions. QIP No. 119. National Center for Asphalt Technology. Kandhal, P. S., C. W. Lubold, and F. L. Roberts. Water Damage to Asphalt Concrete Overlays: Case Histories. Proc., Association of Asphalt Paving Technologists, Vol. 58, 1989, pp. 40–76. Kim, O. K., C. A. Bell, and R. G. Hicks. The Effect of Moisture on the Performance of Asphalt Mixtures, Water Damage of Asphalt Pavements: Its Effect and Prevention. In ASTM STP 899 (B. E. Ruth, ed.), American Society for Testing and Materials, Philadelphia, Pa., 1985, pp. 51–72. Lottman, R. P. NCHRP Report 192: Predicting Moisture-Induced Damage to Asphaltic Concrete. TRB, National Research Council, Washington, D.C., 1978. Lottman, R. P. NCHRP Report 246: Predicting Moisture-Induced Damage to Asphaltic Concrete: Field Evaluation. TRB, National Research Council, Washington, D.C., 1982. Mack, C. Physio-Chemical Aspects of Asphalt Pavements: Energy Relations at Interface Between Asphalt and Mineral Aggregate and Their Measurement. Industrial and Engineering Chemistry, 1935, pp. 1500–1505. Mathews, D. J. Adhesion in Bituminous Road Materials: A Survey of Present Knowledge. Journal of the Institute of Petroleum, Vol. 44, No. 420, 1958, pp. 423–432. Maupin, G. W. Final Report: Laboratory Investigation of Hydrated Lime as an Antistripping Additive. Report FHWA/VA-84/14. Federal Highway Administration, Washington, D.C., 1983. Maupin, G. W., Jr. Assessment of Stripped Asphalt Pavement. Report FHWA/VA-89/14. Virginia Transportation Research Council, Charlottesville, 1989, pp. 4–5. McCann, M., P. E. Sebaaly, and J. A. Epps. Lime in Hot Mix Asphalt Pavements: A Synthesis of Information. Pavements/Materials Program Report 1358-1. Department of Civil Engineering, University of Nevada, Reno, 2000. Nicholson, V. Adhesion Tension in Asphalt Pavements, Its Significance and Methods Applicable in Its Determination. Proc., Association of Asphalt Paving Technologists, Vol. 3, 1932, pp. 29–49. Petersen, J. C., H. Plancher, E. K. Ensley, R. L. Venable, and G. Miyake. Chemistry of Asphalt- Aggregate Interaction: Relationship with Pavement Moisture-Damage Prediction Test. In Transportation Research Record 843, TRB, National Research Council, Washington, D.C., 1982, pp. 95–104. Scherocman, J. A., K. A. Mesch, and J. J. Proctor. The Effect of Multiple Freeze-Thaw Cycle Conditioning on the Moisture Damage in Asphalt Concrete Mixtures. Proc., Association of Asphalt Paving Technologists, Vol. 55, 1986, pp. 213–236. Schmidt, R. J., and P. E. Graf. The Effect of Water on the Resilient Modulus of Asphalt Treated Mixes. Proc., Association of Asphalt Paving Technologists, Vol. 41, 1972, pp. 118–162. Scott, J. A. N. Adhesion and Disbonding Mechanisms of Asphalt Used in Highway Construction and Maintenance. Proc., Association of Asphalt Paving Technologists, Vol. 47, 1978, pp. 19–48. Stuart, K. D. Moisture Damage in Asphalt Mixtures: A State of the Art Report. Report FHWA- RD-90-019. Federal Highway Administration, Washington, D.C., March 1990. Taylor, M. A., and N. P. Khosla. Stripping of Asphalt Pavements: State of the Art. In Transportation Research Record 911, TRB, National Research Council, Washington, D.C., 1983, pp. 150–158.

20 Moisture Sensitivity of Asphalt Pavements: A National Seminar Terrel, R. L., and J. W. Shute. Summary Report on Water Sensitivity. SHRP-A/IR-89-003. Strategic Highway Research Program, National Research Council, Washington, D.C., 1989. Tunnicliff, D. G., and R. E. Root. NCHRP Report 274: Use of Antistripping Additives in Asphaltic Concrete Mixtures. TRB, National Research Council, Washington, D.C., 1984. Tunnicliff, D. G., and R. E. Root. Introduction of Lime into Asphalt Concrete Mixtures. Report FHWA/RD-86/071. Federal Highway Administration, Washington, D.C., 1986, pp. 1–97. Tunnicliff, D. G., and R. E. Root. NCHRP Report 373: Use of Antistripping Additives in Asphaltic Concrete Mixtures: Field Evaluation. TRB, National Research Council, Washington, D.C., 1995.