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Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements (2022)

Chapter: Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility

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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
×
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
×
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
×
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
×
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Suggested Citation:"Chapter 2 - Literature Review: Assessing and Mitigating Moisture Susceptibility." National Academies of Sciences, Engineering, and Medicine. 2022. Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26725.
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6 This chapter presents the background for the causes and mechanism of moisture damage in asphalt pavements. The factors that cause and contribute to moisture damage can be classified as either intrinsic material and/or mixture properties, or as external factors pertaining to the pavement itself. A literature review of the various means of identifying and characterizing mois- ture susceptibility of asphalt mixture or its components used in HMA, specifications and standard- ized test methods are reviewed. These include the two most widely used test methods, the Modified Lottman Test and the Hamburg Wheel-Track Test (HWTT), as specified in AASHTO T 283 and AASHTO T 324, respectively. In addition, a review of some emerging technologies used to assess moisture susceptibility that are being pursued by a few state DOTs where moisture-related damage is of major concern is also included. The last two sections in this chapter introduce common preventive measures taken by the state DOTs to mitigate or limit moisture-related damage to the pavements and the cost-benefit of using some of the mitigation measures. 2.1 Basics of Moisture Damage of Asphalt Pavements Stripping or moisture-induced damage can be defined as a loss of bond between asphalt binder and aggregate in the presence of moisture. It is a complex form of pavement distress that depends on many variables, and is often expressed in various physical manifestations, such as ravelling and cracking, thus making early identification difficult. Some of the mechanisms used to explain the moisture damage observed in asphalt pavements include detachment, displace- ment, spontaneous emulsification, and hydraulic scour accompanied with pore pressure (2, 3). These mechanisms attempt to explain the weakening of cohesive and adhesive bonds at the aggregate-asphalt interface leading to the eventual stripping observed in flexible pavements, as described as follows: • Detachment, as the word implies, is the peeling away of asphalt film from aggregate surface caused by the presence of water. This phenomenon is all the more likely to occur in cases where the bond between asphalt and aggregate is inherently weak (because of charge incom- patibility), even in the absence of water. Since both water and aggregate are more polar than the asphalt, water tends to bond readily with aggregate. This tendency is referred to as wet- tability. In a three-phase system containing water, aggregate, and asphalt, the preferential bonding of water molecules to the aggregate surface lowers the overall thermodynamics of the system and thus will be a dominating type of interaction. Loss of adhesion between the asphalt film and aggregate results in separation or detachment of these two mixture components. • In the displacement mechanism, the bond between asphalt and the aggregate surface is physi- cally broken because of chemical forces. Specifically, water molecules are thought to penetrate C H A P T E R   2 Literature Review: Assessing and Mitigating Moisture Susceptibility

Literature Review: Assessing and Mitigating Moisture Susceptibility 7   incompletely coated or ruptured asphalt film and displace the asphalt molecules (3). Sharp corners of aggregate serve as other entry sites, as the asphalt film is under increased tension at these locations. When water molecules are introduced at these entry sites, changes in the pH of water at the asphalt-aggregate interface can occur. This altered pH leads to changes in the types of polar groups adsorbed at the aggregate surface. Separation of the asphalt film from the aggregate surface is a result of the loss of adhesion. • Spontaneous emulsification was observed to occur when asphalt film is submerged in water (4), with water droplets forming the discontinuous phase and asphalt being the continuous phase. This inverted emulsion leads to the loss of adhesion at the interface, as the water penetrates the aggregate surface and weakens the bond. Fromm (4) also observed that this phenomenon was reversible with the evaporation of water from the emulsion. The degree and rate of emulsifica- tion depends on the chemical nature of asphalt and other additives present in it (2, 6). • Hydraulic scouring and pore pressure buildup are thought to occur when saturated asphalt pavements are subjected to traffic loads. The former is observed in surface layers because of the pumping action of water by passing vehicle tires. The presence of salts in the water leads to osmotic pressure buildup. Water and water vapor diffuse through the asphalt film, causing stripping of the asphalt film off the aggregate surface. Pore pressure buildup is likely to occur in open-graded mixtures and in mixtures with high air-void content that allow easy water movement. If undrained pavement conditions occur in these mixtures, water entrapped in the air voids can experience pressure fluctuations when subjected to passing traffic loads. Under repeated loading, microcracking of the asphalt mastic accompanied by cohesive and adhesive failure occur, leading to stripping. Susceptibility of HMA to moisture damage is a function of many factors, such as quality of aggre- gate, chemical composition of asphalt (source), volumetric properties, bond between aggregate and surrounding asphalt, construction/compaction practices, site conditions, and environment. These factors can be broadly separated into two categories: intrinsic and external. Factors intrinsic to the mixture or mixture components include • Aggregate type and surface properties; • Asphalt type, source, and viscosity; • Mixture volumetric properties; and • Types of mixture. Examples of external factors include pavement drainage conditions, climatic and traffic condi- tions, inadequate compaction, construction practices, and so forth (2, 7, 8, 9). All aggregate particles carry both positive and negative surface charges, with the dominant charge being governed by the mineral composition of the rock. Aggregate types with either type of predominant surface charge have a higher affinity for water adsorption than they do for asphalt binders. This leads to a tendency for the water molecules to weaken/displace and replace the weaker asphalt-aggregate bond (2). Examples of rocks with predominantly negative charge or “acidic” character include chert, quartzite, sandstone, and granite. The chemical and mineral composition of these rock types • Is siliceous in nature (> 63–65% silica); • Contains minerals such as quartz, feldspar, or biotite; • Is typically poor in calcium, magnesium, and iron; and • Is typically rich in alumina and alkalis. On the other hand, basalt, gabbro, limestone, and dolomite rocks have a positive surface charge and thus display a “basic” character. Their mineral composition can be broadly described as being lower in silica (≈ 45–53%) and as containing higher concentrations of calcium, magnesium, and iron (3).

8 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements Unmodified asphalts tend to have a negative surface charge, and hence they do not bond well with negatively-charged rocks. Asphalt compatibility with aggregate can be modified to some extent by the use polymer additives and anti-stripping agents. However, this approach only aids in the mitigation of moisture damage to some degree, as the effect of aggregate charge incompat- ibility predominates. The importance of surface charge compatibility, zeta potential, and other surface characteristics of the mixture components has been reported by various researchers (2, 3, 4, 5, 6). Aggregate properties, such as porosity and surface texture, influence the strength of the adhe- sive bond between aggregate and asphalt, and hence the stripping potential. In general, while aggregate with high porosity is desirable (for easier penetration by asphalt), the size of the pores is also important. However, if the pore volume was primarily composed of fine pores, there is an increased likelihood of entrapped air bubbles rather than penetration by the asphalt (1, 2). This can have a detrimental effect on the durability of the mixture, particularly in the presence of moisture. Therefore, it is essential to ensure proper drying of aggregate during the mixture design and production stages to remove any entrapped air that may lead to moisture-related damage. From a mechanical point of view, a smoother texture provides inferior interlocking or grain- to-grain contact in comparison to aggregate having a coarser texture, which has better inter- facial contact necessary for the strength to the HMA. Smoother texture is associated with low aggregate surface area and high moisture susceptibility (2, 11, 12). Regardless of the texture, the presence of dust coating around the aggregate can seriously limit the bond and contribute to moisture-induced damage in asphalt pavements. The effects of porosity and asphalt viscosity (temperature dependent) on stripping poten- tial of HMA are interrelated. Low-viscosity asphalt is able to coat the aggregate particle better and penetrate pores more easily compared with high viscosity asphalt. However, the adhesive strength at the asphalt-aggregate interface is stronger with high viscosity asphalt than it is with low-viscosity asphalt. Hence, mixtures with higher viscosity asphalt are more resistant to mois- ture damage than mixtures with low-viscosity asphalt (when all other design factors remain the same) (2). Climatic and pavement site conditions also influence the stripping potential of asphalt pave- ments. Stripping occurs more frequently in regions with predominantly hot and wet/humid conditions or with cold and dry conditions. Within the United States, hot/wet and cold/dry con- ditions are common in the southern and western regions, respectively (1, 7). High water table and poor pavement drainage lead to the development of waterlogged pores. When accompanied by temperature fluctuations, this can cause repeated freeze-thaw or wet-dry conditions in the pavement that weaken the bond between the aggregate and the binder. Stripping and ravelling are commonly observed under such conditions (1, 2). Another construction-related factor that negatively affects stripping potential is the pres- ence of excess air voids or low asphalt content in dense-graded mixtures. As mentioned earlier, poor pavement drainage in mixtures with high air voids can create waterlogged pores. When this condition is accompanied by the application of traffic loads and freeze-thaw condi tions, excess pore pressure buildup (1, 2, 3, 13) can occur, which, in turn, degrades the adhesion between the asphalt film and aggregate (3). Lower mixing and compaction temperatures can also nega- tively affect the resistance of HMA to moisture-induced damage, as was reported in an NCHRP study (14) on warm-mix asphalt (WMA). Reports by Bahia and Ahmad (9) and by Omar et al. (15) summarize earlier research on identifying some of the factors affecting moisture damage and other studies that attempted to provide a theoretical basis for or to explain the stripping phenomenon.

Literature Review: Assessing and Mitigating Moisture Susceptibility 9   Among mixture types, mixtures with high permeability or high air void content tend to be more susceptible to moisture damage. This is because the interconnected void space allows for easy water penetration and water accumulation within the mixture, if adequate pavement drainage is not provided. While both stone-matrix asphalt (SMA) mixtures and dense-graded asphalts (DGAs) are designed to the same target density, SMAs are premium, gap-graded mix- tures with higher dust content (of a high quality) than DGAs. The presence of high dust content in SMA designs as well as fillers and additives lend stiffness to the overall matrix and make SMA mixtures less permeable than open-graded mixtures. Additionally, in SMA mixtures, the thick- ness of asphalt films tends to be higher than in DGA mixtures, which provides a good barrier against water penetration. All of these features present in combination lead to the improved stripping resistance of SMA. In the case of mixtures with lower air void content, movement of water is restricted because of their low permeability. Penetration by water or diffusion into the pore spaces is also limited. In these cases (e.g., DGAs), moisture damage is primarily caused by the incompatibility of material/mixture components, poor compaction, and other pavement construction factors. A comprehensive synthesis of the existing theories on the causes and mechanisms of mois- ture damage, in terms of chemical interactions between asphalt, water and aggregate, boundary layer theories, mechanical theory, and thermodynamics, is presented by Hefer et al. (16). The authors stress the importance of factors such as surface free-energies of the asphalt and aggre- gate, asphalt-aggregate interactions, and the pH of interface water as important predictors of performance prediction. The proceedings of a national seminar on the moisture sensitivity of asphalt pavements (17) contain a comprehensive review of several factors that can cause and influence moisture damage in pavements, present the extent of a moisture damage problem in the United States, and list steps toward mitigation or minimization and provide other relevant information. 2.2 Tests to Characterize Moisture Susceptible Materials and Mixtures Over the years, a wide range of tests have been used to characterize the susceptibility of materials and mixtures to moisture damage (6). These tests range from simple boiling water tests to sophisticated analyses of the chemical compatibility of materials, such as surface free- energy characterization. According to Goetz (18), the earliest attempts to develop a quantitative method to study resistance to moisture damage may be attributed to Riis (19) and Neumann (20) in Europe and Krchma and Loomis (21) in the United States. In 1991, Hicks (1) summa- rized the main moisture-damage-related tests in use at that time, including their relative merits and shortcomings, and concluded that no test was superior to the other. Due to the complex nature of the bond between asphalt and aggregate and the multitude of factors influencing stripping potential, all tests in use by state DOTs today are empirical (11). In broad terms, the test methods focus either on the loss of adhesion between asphalt and aggre- gate (adhesive failure) or on the breakdown of the cohesion within the HMA matrix (cohesive failure), which are the two main theories used to explain moisture-induced damage in pavements (1, 22, 23). Initial efforts to assess the moisture susceptibility of asphalt-aggregate systems started around the 1920s, with the measurement of bond energies at the aggregate-asphalt interface along with contact angle data and surface tension between the system components. Compat- ibility between the charge carried by the molecules present in asphalt and surface charge on the aggregate was also investigated (1, 2, 3, 15, 24). An NCHRP research results digest by Little and Bhasin (25) presents an exhaustive summary of various methods used to measure surface energy components and recommendations for selecting the best measurement methods.

10 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements 2.2.1 Boiling Water Test A simple and rapid test to visually assess the loss of adhesion in asphalt mixtures is the Boiling Water Test, ASTM D3625 (26). Briefly, the test is conducted on loose mixture samples (with or without additives) preheated to 85°–99°C. The mixture is placed in a container of boiling dis- tilled water and boiled for 10 minutes. The mixture is cooled to room temperature and visually inspected for the degree of asphalt coating that is retained at the end of this treatment. Aggre- gate can be observed using light and low magnification visual aids; and those particles that have brownish, translucent or black coatings are still considered to be fully coated. The percentage of visible aggregate area that retains its asphalt coating after treatment is used to estimate the stripping potential of the mixture. This test merely serves as an initial screening procedure to identify mixtures that are likely to exhibit loss of adhesion, or to assess the effectiveness of using additives. Due to the highly sub- jective nature of the assessment and the small size of the representative sample, this test is not widely practiced in the United States and is not recommended for use as an effective acceptance/ rejection tool. Additionally, since the test is performed on loose mix samples, factors affecting the pavement drainage, such as air-void content, aggregate gradation, and permeability of the mix- ture, are not addressed (27). As a result, this test shows poor correlation with field observations. 2.2.2 Immersion-Compression Test This test procedure, developed by the Bureau of Public Roads and first published in 1959 (28), ranks among the first to be used to evaluate moisture susceptibility of asphalt mixtures. As discussed earlier, proper adhesion between aggregate and asphalt is essential for providing strength to asphalt mixtures. This test is used to evaluate the effect of long-term water immer- sion on the compressive strength of HMA specimens. It compares the average compressive strength of a set of wet conditioned specimens (100-mm diameter × 100-mm height) to that of dry unconditioned specimens with similar air content (6–7%). Six test specimens are prepared from loose mixture, by applying a static load of 20.7 MPa (3,000 psi) for 2 minutes, and cured at 60°C (140°F) for 24 hours. Following this, the specimens are divided into two subsets: uncondi- tioned (dry) and conditioned (wet). The conditioned subset is kept submerged in a water bath at 48.9°C (120°F) for a period of 4 days, before testing at 25°C (77°F). A ratio of 0.75 is considered desirable to provide resistance to moisture damage. Further developments and refinements to the original version of the test method led to the publication of its latest form, as AASHTO T 165-02 (29), which was ultimately withdrawn in 2019. In that latest version, the conditioned specimens were kept submerged in water for 24 hours at 60°C (140°F), followed by 2 hours of conditioning under water at 25°C (77°F). An alternative procedure for wet conditioning involved immersion for 4 days at 49°C followed by 2 hours at 25°C. The revised protocol also allowed for the use of compaction methods other than static load. Even before it was withdrawn, this test had not been widely used because of its poor accuracy (12) in predicting susceptibility to moisture damage, long testing time, and poor repeatability (1). 2.2.3 Static Immersion Test The Static Immersion Test, AASHTO T 182-84 (30), can be used to assess the moisture sus- ceptibility of aggregate coated with cutbacks, emulsions, asphalts, and tars. The original version of this test was developed as a part of the Road Research Board program by the Department of Scientific and Industrial Research in the United Kingdom, in 1962. It considers the effect of water immersion for an extended time period on aggregate coated with asphalt but without the added effect of moving vehicular traffic (8). In its current form, specified quantities of

Literature Review: Assessing and Mitigating Moisture Susceptibility 11   single-sized aggregate and emulsion (or cutback) are mixed, cured and cooled to room tem- perature. The mixing time, curing time, and temperature vary for cutbacks, emulsions, asphalts, and tar. The cooled aggregate is then placed under water at 25°C and stored for 16–18 hours. After this conditioning period, any floating film on the surface of the water is removed without disturbing the aggregate. A light is used to illuminate the specimen and visual inspection of the aggregate particles is done to estimate the percentage of coated aggregate. A coated area of 95% is desirable, to provide moisture resistance. Like the Boiling Water Test, this method is also considered highly subjective and does not measure the strength of the mixture. This test is found to have poor correlation with field per- formance (31) and also lacks the sensitivity to distinguish between varieties of the same aggre- gate type (1). For these reasons, it is not recommended for use as a predictive tool for assessing moisture-related damage. 2.2.4 Modified Lottman Test The Lottman test was first developed during Phase I of an NCHRP study (32) in the late 1970s and later expanded and validated in Phase II of the same study. In the original procedure, speci- mens were compacted to a target air void content of 3%–5%, which was the expected air void content in the pavement toward the end of service life. Testing was conducted in indirect tension mode with a loading rate of 1.6 mm/min at 13°C. Test specimens were randomly assigned into two groups (conditioned and unconditioned), regardless of average air voids of two groups. The specimens were not subjected to short-term aging, and target saturation level was at 100%. Under these conditions, the test was effective in qualitatively delineating between the good mix- tures (i.e., stripping resistant) and the damage-prone mixtures. However, identifying borderline mixtures was not always accurate (32). Accelerated aging of laboratory-prepared samples was found to compare well with aged field samples obtained at periodic intervals, but the accelerated aging protocol was very lengthy for practical application. It involved freezing vacuum-saturated samples at 0–10°F for 15 h, followed by thawing in a warm water bath at 140°F for 24 h, and finally in a room-temperature water bath for 3–5 h, before testing. Concerns that the moisture conditioning protocol suggested in the original version of the test was too severe led to the work by Tunnicliff and Root (33), which ultimately resulted in a modi- fied Lottman procedure. The compacted air void of test specimens was increased from 3–5% to 6–8%, which is the typical pavement density during initial laydown. Strength results from partially-saturated specimens were found to be comparable to those obtained from fully-saturated specimens. This led to a modified target of 55%–80%. Testing conducted at a loading rate of 1.6 mm/min at 13°C and 50 mm/min at 25°C showed no differences in the ratio of average condi- tioned specimen strength to that of the control specimen strength. This led to the adoption of the increased loading rate to 50 mm/min, which was more in line with the prevalent, built-in loading rate used in testing equipment during that time. A modified-Lottman procedure, AASHTO T 283 (34), is now widely used nationally. The Modified Lottman test is, in principle, similar to the Immersion-Compression Test, and can be summarized as follows. The test calls for the use of a minimum of six specimens (100-mm diameter × 63.5 mm or 150-mm diameter × 95 mm) divided into two subsets with roughly the same percentage of air voids. One set is tested dry, without conditioning, and the other set is sub- jected to partial water saturation (70–80%) and a freeze-thaw cycle. Testing can be conducted on either field cores or on lab-prepared specimens. The latter are first aged for 16 ± 1 hour at 60°C after mixing and then kept at compaction temperature for 2 hours ± 10 minutes before being com- pacted. The 16-hour accelerated aging and the water saturation are done to simulate some of the time, temperature, and moisture effects associated with field exposure. This conditioning regime is specified in AASHTO T 283. Both subsets of specimens (dry and conditioned) are tested

12 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements in split-tension mode to determine the indirect tensile strength, and the results of each subset are averaged. Strength testing is done at room temperature, with a loading rate of 50 mm/min (Figure 1). The ratio of the average conditioned strength to the average dry strength is called the Tensile Strength Ratio (TSR). State DOTs that use this test to measure the susceptibility of HMA to moisture-induced damage specify a minimum TSR value that should be met during mix design and/or during production/acceptance stages. Typical values of required TSR can range from 0.70 to even 0.90 (the higher values applicable to some specialized mixes). Some state DOTs also have an additional requirement on minimum conditioned (wet) and/or uncondi- tioned (dry) strength. While this is a simple test requiring no specialized equipment, the conditioning regime is con- sidered extreme and not applicable to all climatic conditions occurring in the country. Indeed, some state agencies do not require a freeze-thaw cycle, or they adjust the conditioning time and temperature to meet their local climatic conditions (2). Although this test was found to be successful in identifying mixtures that are highly susceptible to moisture damage, it was not as effective with the marginal mixtures (25). In some cases, it was observed that the strength of wet, conditioned specimens was higher than that of the dry, unconditioned specimens, and that the field performance was not satisfactory even though the minimum TSR requirement was met (9). A complete test cycle takes about 3 to 4 days, which may be considered to be time-consuming in practical terms. 2.2.5 Hamburg Wheel-Track Test The Hamburg Wheel-Track Test was first developed in the 1970s in Hamburg, Germany, by Helmut-Wind, Inc., to assess rutting potential of mixtures that were required to withstand heavy cargo trucks near the port area (35). It was later adapted to study the effects of moisture-induced damage in flexible pavements. The methodology was imported into the United States in the 1990s, and used in its present form as specified in AASHTO T 324-17 (36). The test device consists of a loaded steel wheel that goes back and forth (reciprocating action) over the test samples Figure 1. Indirect tension test setup.

Literature Review: Assessing and Mitigating Moisture Susceptibility 13   submerged under water at test temperature. The reciprocating action of the wheel, along with its sinusoidally varying position along the track, causes the wheel to spend an equal amount of time in the front and back portions of the samples. Test samples can either be cylindrical cores (field or lab-compacted) or slabs cut to specific dimensions. The recommended air void content of lab-compacted samples is 7 ± 1%, while field cores are tested as received. Figure 2 shows an example of the Hamburg Wheel-Tracking device. Test specimens are conditioned for 45 minutes under water at test temperature (typically, 50°C), before being “trafficked.” The test can also be performed under dry heated conditions, with an air hood accessory to maintain temperature control. Testing is continued until 20,000 cycles or until a specified rut depth is attained, whichever occurs earlier. The slope of the deformation curve versus the number of wheel passes is monitored to mark the onset of plastic flow, from which other parameters, such as creep slope, stripping slope, and stripping inflection point (SIP), can be calculated at the end of testing. Work done by Mohammad et al. in 2016 (37) identified four major brands of devices in use at the time and proposed modifications to the standard HWTT protocol to ensure good repeatability, and testing accuracy, regardless of the specific brand used. As this test combines the effect of moving traffic conditions and exposure to moisture, it does not identify mixtures that are purely moisture susceptible. To isolate the damages resulting from rutting (weak mix) and stripping (moisture susceptibility), it is possible to run the test using a set of duplicate dry samples. Specimen preparation can be somewhat cumbersome for this test method, because of the need for precise cutting to fit the molds. Researchers have found a good correlation between test results and field performance. In general, the test was able to screen the good mixes from the poor performers (35, 38, 39). Other work verified that the HWTT was sensitive to the presence of anti-stripping additives. However, in some cases, the test provided false positives and false negative results (40). In many cases, the rut depth versus number of passes data do not yield a classic plot indicat- ing the initial steady-state portion, followed by the onset of plastic flow (see Figure 3). In such cases the users resort to either visual determination of SIP as the point of intersection of the two best-fit lines drawn to the steady-state portions, or determine SIP using software developed by the manufacturers (bottom plot of Figure 3), state DOTs, or other researchers (see Appendix B). This can result in different SIP values. While other wheel-tracking devices exist that operate on the same basis as that outlined for the HWTT, they were primarily developed to evaluate rutting resistance and later on modified to include the combined effect of water- and traffic-related distresses. Figure 2. Hamburg wheel-track device.

14 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements 2.2.6 Moisture-Induced Stress Tester As mentioned earlier (Section 2.1), one of the factors that contributes to the moisture damage problem is vehicular traffic over saturated pavement as it leads to the movement of water in the interconnected void spaces. It is theorized that this pumping action, in turn, leads to pore pressure build-up and scouring. None of the tests discussed above explicitly simulate this phe- nomenon during testing. The Moisture-induced Stress Tester (MiST), developed and marketed by InstroTek, Inc., is shown in Figure 4. It purports to duplicate the field conditions that lead to a loss of adhesion and cohesion, resulting in stripping and ravelling. One of the main advan- tages of this test method is that it allows for changes in cyclic pressure level, test temperature, and operating pressure to suit local traffic and environmental conditions. A typical test lasts 24 hours, which is less than that required for the widely-used Modified Lottman Test. Current equipment and training costs place it around $25,000. Figure 3. Rut depth versus number of passes with typical two-slope intercept yielding SIP (top) and without the classic two-slope intercept point (bottom) (Source: Cooper Technologies, used with permission).

Literature Review: Assessing and Mitigating Moisture Susceptibility 15   This test can be conducted on 4″- or 6″-diameter specimens, either lab-compacted or obtained from the field cores. Depending on the thickness of the specimens, two or three specimens can be tested concurrently. Specimens are stacked on a base plate in a self-contained chamber, with spacers between multiple specimens to prevent the weight of the upper specimens crushing the lower specimens. The chamber is filled with water until the specimens are completely sub- merged, and it is sealed with the top lid using six hand-bolts. The program menu also allows the user to change conditioning time and temperature, cyclic hydrostatic pressure level, number of cycles, and so forth based on agency modification or in accordance with AASHTO TP 140, Standard Method of Test for Moisture Sensitivity Using Hydrostatic Pore Pressure to Determine Cohesion and Adhesion Strength of Compacted Asphalt Mixture Specimens (41). Typically, the test is run for 3,500 cycles at 276 kPa at a temperature determined by the binder grade of the mixture. At the end of the test, the specimens are visually examined for moisture damage. In addition, the bulk specific gravity (Gmb) of the conditioned specimens is also determined and used to calculate the percent change in bulk specific gravity (or Gmb Swell) with respect to unconditioned specimens. Lastly, the average conditioned and unconditioned strengths are used to calculate the TSR. Two out of three acceptance criteria must be met for the mixture to the considered resistant to moisture damage, or as specified by agency requirements. In a study by DeCarlo et al. (42), the indirect tensile strength values and the TSR of samples conditioned using the Modified Lottman method and those conditioned using the MiST pro- tocol were found to be very similar. However, that study also found that the ratio of dynamic modulus of MiST-conditioned versus unconditioned specimens was a better parameter for dif- ferentiating moisture-susceptible mixes from non-stripping mixes. Similar TSR comparisons between Modified Lottman-conditioned and MiST-conditioned specimens were conducted by Figure 4. Moisture-induced Stress Tester.

16 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements researchers at North Carolina State University (43). Both test methodologies yielded similar results in delineating the moisture-susceptible mixtures from stripping-resistant mixtures, in most cases. For lab-produced mixtures, the indirect tensile strength of the MiST-conditioned specimens was found to be lower than that of Modified Lottman Test-conditioned specimens, in a study done by Cross et al. (44). However, the opposite was found to be true for plant-produced specimens. No statistical differences were found between the TSRs using these two methods. 2.2.7 Surface Free-Energy Analysis Due to the proven importance of surface charges of aggregate and their compatibility with surface charge on the asphalt in determining moisture susceptibility of asphalt mixtures, the surface free-energy (SFE) approach offers a more scientific basis than the largely empirical test methods that are currently being followed by state agencies around the country. Surface tension and surface free-energy are mathematically equivalent and expressed in units of mN/m or mJ/m2. Surface tension is the tensile force acting in all directions on the surface of a liquid, while SFE is the work required to create a new unit surface area of a specific material. Work done by several researchers (45, 46, 47, 48) has indicated the existence of a positive correlation between reduc- tion in free energy of asphalt-aggregate-water system, debonding at the asphalt-aggregate inter- face, and displacement of the asphalt by water molecules. Surface energies of aggregates can be measured using a Universal Sorption Device (USD). A mass of adsorbed vapor on the surface of aggregate particles, when exposed to a known solvent and corresponding vapor pressures, is measured using the USD and used to calculate the surface energy of aggregate. The Wilhelmy Plate Method can be used to measure SFE, surface tension, interfacial tension between two liquids, and contact angle between solids and liquids. The device used for this purpose, shown in Figure 5, is called a force tensiometer. For asphalt binder applications, a thin plate coated with asphalt binder is slowly inserted into three solvents of known properties and then retracted. Figure 5. Force tensiometers used for Wilhelmy plate method (Source: Krüss, used with permission).

Literature Review: Assessing and Mitigating Moisture Susceptibility 17   The force used to insert the coated plate up to a certain depth is measured and used to calculate the contact angle between the binder and the solvents. Figure 6 shows the schematic of the Wilhelmy plate along with the force components acting on the film. These force components can be used to calculate the surface tension and surface free-energy components using the Wilhelmy equation and Young-Dupré equation, respectively. Additionally, adhesive and cohesive bond energies can be then determined from SFE values. In a two-component system like asphalt and aggregate, if the bond energies of both materials are positive, then their resulting bond strength will be relatively higher and therefore they will be less likely to exhibit moisture damage. Since surface energy measurements are sensitive to the changes in aggregate source and binder type, this method may be more accurate in predicting moisture-susceptibility of HMA. While this approach showed promising results in identifying moisture-susceptible mixtures, it has not yet gained acceptance in the industry, because of the additional investment needed in new test equipment and relative lack of data tied to field performance. The science behind this method is well established, and current equipment costs can range from $40,000 to $70,000, depending on the accessories needed for the asphalt pavement’s application. Limited laboratory testing of six asphalt mixtures from Texas and Ohio was performed and the results were compared with field performance (46). Data indicated that the ratio of adhe- sive bond energies under wet and dry conditions was a good parameter to identify susceptible aggregate-binder combinations. Similar findings were reported by Masad et al. (49) when mix- ture performance was evaluated using the Hamburg Wheel-Tracking device and compared with surface energy parameters. Bhasin et al. (47) studied the moisture susceptibility of 12 mixtures involving three rock types and four binder types and compared their lab performance with surface energy parameters. The researchers concluded that surface energy parameters could be used as an effective screening tool to eliminate poor aggregate-asphalt combinations in HMA. However, they still stressed the need for a comprehensive evaluation of the mixtures fabricated using the screened aggregate-asphalt combinations. In summary, it can be seen that the test methods used to assess the susceptibility of asphalt mixtures and/or their components to moisture damage can range from simple, empirical, subjec- tive practices to more complex, theory-based approaches. Empirical tests can be relatively simple to implement, and can show good reliability in delineating the stripping-resistant mixtures from Figure 6. Schematic of Wilhelmy plate and the force components acting on the liquid (Source: DataPhysics Instruments USA Corp., used with permission).

18 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements those prone to stripping. But these methods often fail to correctly evaluate the susceptibility of mar- ginal mixtures and show poor correlation with field performance. The theory-based approaches that attempt to relate laboratory-evaluated performance of the mixtures with specific properties of the components appear to show more promise. However, these methods have not yet been fully validated through controlled field studies. 2.3 Methods to Limit Moisture Damage The main corrective measures employed to mitigate moisture-susceptibility in HMA are primarily material-based; that is, they involve the selection of aggregate resistant to moisture damage and the use of additives (50). This section presents the common types of anti-stripping additives used in mitigation measures by state DOTs. The two main types of anti-stripping additives are surface-active chemicals and hydrated lime (1, 2). Surface-active chemicals are also called surfactants because of their ability to reduce surface tension between asphalt and aggregate and thus improve adhesion between the asphalt film and aggregate surface. In addition to the material-based mitigation methods, other measures related to structural design of the pavement, mixture design and construction-related practices are often used in parallel to aid in lessening the exposure time to water, whenever possible. Some states that have had moisture damage issues in the past or where the predominant aggregate type is prone to stripping take a more proactive approach and require the use of liquid anti-stripping additives and/or hydrated lime (HL) in all their mixtures. As mentioned earlier, the predominant surface charge of the aggregate and its interaction with the asphalt film are the leading factors influencing the stripping potential of HMA. The chemical composition of the aggregate type determines its surface charge and “hydrophilic” character. Studies rank basalt, limestone, slag, and dolomite as materials that create only slight to moderate problems with respect to moisture damage, while chert, granite, and quartzite are reported to be the worst performers in this aspect (1, 3, 12). However, many states successfully use granite in their flexible pavements by incorporating such additives as liquid anti-stripping agents (surfactants) and hydrated lime. 2.3.1 Lime Both calcium-rich and magnesium-rich (or dolomitic) limestone are used to produce lime. Calcination of limestone (heating in the absence of air) yields quicklime which can react with water to produce hydrated lime that is used in commercial applications. The use of hydrated lime or portland cement to improve the adhesion of asphalt film to aggregate was pioneered in the 1930s. It required the aggregate to be pre-moistened before being mixed with the lime. Calcium present in lime-derived anti-stripping agents enables its strong adhesion with the sur- face of silica-rich aggregate without negatively affecting the aggregate porosity. In these earlier works (51), asphalts with strong organic acid components (e.g., phenolics) were observed to work well in such applications. Results from research done by Stroup-Gardiner and Epps (52) revealed that both dolomitic lime and dry hydrated lime were equally effective in improving resistance to water damage, but unhydrated quick lime had the opposite effect. These authors used resilient modulus test data and tensile strength from the Lottman test method to evaluate the performance of mixtures with respect to stripping potential. Dry hydrated lime can be added to moist aggregate in either the dry form or as a wet slurry (with or without marination), typically about 1% by weight of total mix. When dry hydrated lime is added directly to moist aggregate, the coarse and fine aggregates are typically at 2–3% or 5–6%

Literature Review: Assessing and Mitigating Moisture Susceptibility 19   above saturated surface-dry (SSD) condition, respectively. Some states (California, Nevada, and Utah) practice mix marination, where the moist aggregate is premixed with the dry hydrated lime and stored in stockpiles for a minimum of 48 hours, but no more than 45 days, before use. The benefits of marination and other added advantages of using hydrated lime in HMA, such as a reduced degree of oxidation and age hardening, improved crack and rut resistance, and so forth, were published by the National Lime Association in a document authored by Little and Epps in 2001 (53) and updated by Sebaaly et al. in 2006 (54). The report also summarizes some of the significant findings from work done on the use of hydrated lime in HMA in the United States and Europe, at the time. 2.3.2 Liquid Anti-stripping Additives Chemical anti-stripping agents (also known as liquid anti-strips or LAS) work to reduce the surface tension of an asphalt binder system or to alter its surface charge, thereby enhancing its affinity for binding with aggregate. Some anti-stripping agents are designed specifically for use in HMA, while others are more effective in emulsions and cutbacks. As mentioned earlier, these products are essentially surfactants which can be either cationic or anionic in nature. The cationic or anionic nature of these products depends on their chemical composition, which in turn, determines their application. Cationic surfactants were observed to adhere to the aggregate better (2, 55). The most commonly used cationic anti-stripping agents are composed of amines, which are ammonia-based fatty acids attached to a hydrocarbon chain with polar and non- polar ends. They occur both in liquid and solid form and can either be introduced directly to the asphalt binder before being mixed in with aggregate or can be premixed with the aggregate for maximum effectiveness. While the latter method is more effective because of the direct availability of the surfactant at the surface of the aggregate, it is the more expensive option of the two modes of application, as it requires more product (3). When premixed with hot asphalt and subsequently used to coat the aggregate, the reactive components of the anti-stripping additives migrate to the aggregate- asphalt interface and create a lipophilic layer which has a higher affinity for oil than for water. As a result, water, if present, is displaced from the asphalt-aggregate interface (1, 10, 56, 57). The use of anti-stripping agents was also found to soften the asphalt, as they tend to change the molecular interactions among the polar components of the asphalt (58). The dosage rates and modes of application of LAS usually follow manufacturers’ recommen- dations. The typical amounts of LAS used vary between 0.5 and 1.5% by weight of asphalt binder (59). The simplest, more practical, and economical method of application is blending directly with the asphalt binder. Blending can be performed by the asphalt supplier at the terminal or through in-line blending at the HMA plant. Alternatively, LAS can be added to the asphalt stored in tanks at the plant site. All these methods ensure the uniform distribution of the LAS within the mixture as the additives are already a part of the asphalt binder film that coats the aggregate particles during HMA production. Polyphosphoric acid (PPA) is used as an asphalt modifier to increase its high temperature performance grade and to extend the overall useful temperature range of performance-graded binders. It has been reported that PPA-modified asphalt binders, when used with certain liquid anti-stripping additives, tend to have a detrimental effect on the mixture properties (60, 61, 62). This was found to largely depend on the type of aggregate and LAS used. PPA-modified asphalts used in mixtures with stripping-prone aggregate and phosphate ester class of LAS were only minimally impacted or not at all. However, mixtures containing PPA-modified asphalt binders and amine-based anti-stripping additives did exhibit stripping when tested using the Hamburg

20 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements Wheel-Track Test. In this case, the improved high temperature stiffness resulting from the addi- tion of >1% PPA was apparently “neutralized” by the addition of amine-based LAS. However, no premature aging and no binder brittleness were found at low temperatures. Loss of binder stiffness that occurs under certain conditions can be rectified by using the cor- rect formulation of LAS + PPA in the asphalt (63). There are still uncertainties in understanding the exact nature of the reaction between PPA and amine-based anti-stripping additives. As a result, many state DOTs that use PPA-modified asphalts either avoid their use in combination with LAS or evaluate their compatibility by means of moisture-susceptibility tests in the labora- tory, before construction. The addition of PPA to mixtures treated with hydrated lime was not found to be detrimental with respect to its stripping resistance. The 1990 report by Curtis (64) summarized research work performed on the use of LAS and HL as a part of a Strategic Highway Research Program study. According to this report, ideal liquid anti-stripping agents must • Be heat stable; • Be able to improve the bond between aggregate and asphalt; • Have no detrimental effect on asphalt and mixture properties, health, and the environment; and • Be easy to use and economical. Heat stability during mixing and compaction is an important and necessary requirement for all chemical surfactants to allow them to maintain efficacy during their performance life. This prop- erty is manifested by the chemical surfactants’ ability to remain unreactive when mixed with asphalt and exposed to high working temperatures that are typical in the asphalt industry. Using the correct dosage of anti-stripping agents is very critical with respect to maximizing their effec- tiveness, as the use of excessive amounts of these materials can oversaturate the aggregate sur- face charge and weaken the bond (2, 51, 56). A recent report by Gu et al. (65) described the effects of HL and four types of LAS on selected properties of open-graded friction courses (OGFCs) containing granite from two sources and a Styrene-Butadiene-Styrene (SBS)-modified asphalt, typically used in Florida. In addition to the mixture performance testing, the performance grade (PG) of the blended binders with the additives was also determined. The test results confirmed that an addition of up to 0.5% of LAS additive did not significantly influence the low- and high-temperature grades of the polymer- modified binders (PMBs) used in this study. Other polymeric anti-stripping agents (e.g., organosilanes) are of limited use currently but are gaining popularity in the industry as additives that can be used to mitigate moisture damage to flexible pavements. The reported data (66) indicate that these materials generally do not have detrimental impact on the low- and high-temperature properties of the asphalt, with the excep- tion of a slight decrease in penetration grade and viscosity. These additives have the added advantage of lowering mixing and compaction temperatures. Polymeric anti-stripping agents have been reported to be very effective, when used with sili- ceous aggregate (67), as they work by forming covalent bonds between inorganic and organic compounds and improve adhesion between aggregate and binder. The silicon functional group of these polymeric materials bonds to the asphalt, and their organo-functional group bonds to the aggregate. These materials can be used effectively with polymer-modified asphalts and crumb- rubber modified binders (68). When these materials are used as anti-stripping agents, their application rates range between 0.03% and 0.1% (by weight of binder). These same materials can also be used as a compaction aid in warm-mix asphalt. In such applications, the addition rates are in the range of 0.05%–0.1%.

Literature Review: Assessing and Mitigating Moisture Susceptibility 21   Some polymers can also form a waterproof barrier around the aggregate particles, and thus improve bonding between the asphalt and aggregate (50). The presence of this waterproof coat- ing around porous aggregate, for example, results in a slight decrease in the required asphalt content, thereby saving overall construction costs. However, polymer additives do interact with asphalt and care must be taken to ensure their compatibility with the asphalt used. 2.4 Impacts of Moisture Damage on Service Life of Asphalt Pavements Only a limited amount of cost-benefit analysis data is available in the literature on the effec- tiveness of the moisture damage control methods with respect to extending the service life of the pavements. As of 2001, lime was used extensively in at least 15 states within the United States to treat moisture sensitivity in HMA (51). The cost per ton of mixture with lime additive can vary, depending on the mixing method and whether or not the mixture marination process is employed, if specified by the state agency or at the discretion of the contractor. It is also impor- tant to realize that the cost-benefit estimates are highly dependent on both the assumptions made when using the models employed and the methods (probabilistic versus deterministic) used to arrive at those estimates. The 2003 summary report by Hicks and Scholz (69) presents data obtained by contacting 10 states with the request for an estimate of the cost per ton of mix with anti-stripping additives. Depending on the method of mixing adopted, the range was between $1 and $4.50 per ton of the mix. Life-cycle cost analysis software developed by the Federal Highway Administration (FHWA) was used to evaluate cost effectiveness in using alternate designs, that is, with and without anti-stripping additives. In the deterministic approach, based on the selected scenarios described in the report, the use of hydrated lime was the most cost-effective option with respect to the utilization of the anti-stripping additives, resulting in savings between $0.29 and $8.21/yd2 of interstate lines. The probabilistic approach, given the same assumption and conditions, put the cost-savings estimate between $0.84 and $5.24/yd2. The authors of the report concluded that the overall savings achieved when using lime to mitigate moisture sensitivity issues in HMA was in the range of $2 to $3/yd2. Gu et al. (57) performed cost-benefit analysis on the use of anti-stripping additives in two Florida friction courses. Two types of granite were used in this study, sourced from Nova Scotia and Junction City. The authors estimated that the use of 1% HL in the Nova Scotia mixture gave a service life of about 8 years. By adding an extra 0.5% HL or 0.5% LAS, the service life of the same mixture type was extended by an additional 2.4 years. However, when used in combination (0.5% HL + 0.5% LAS), the service life was doubled (from 8 to 16 years), based on estimates using Cantabro mass loss results. The associated expense of adding 0.5% HL cost the contractor about $0.5/ton ± 0.2 (standard deviation), and $0.6/ton ± 0.1 in the case of LAS. In case of the Junction City mixtures, the use of 1% HL added about 3 years to the service life. However, the addition of the extra 0.5% of HL or 0.5% LAS did not improve the service life, nor did the addition of the HL + LAS in combination yield a low cost-benefit ratio. The Pennsylvania Department of Transportation (PennDOT) evaluated the cost benefit asso- ciated with using anti-stripping agents in its HMA by considering two scenarios, each with different traffic growth rates and two saturation levels (low and as recommended by specifica- tion) for the Modified Lottman test (70). The optimistic performance scenario assumed that the mixtures would perform better than their current estimate, which was represented by the realistic performance scenario. The analysis was confined to HMA overlays over existing asphalt pavements, which represent a majority of the work done by the agency. The study found that

22 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements mixtures with low saturation levels did not provide useful information with respect to identify- ing mixtures with low or poor resistance to stripping. At the higher saturation levels (specifica- tion recommended), the benefit/cost ratio for both the mandatory use of LAS in all mixtures and usage based on testing was greater than 1. Due to the added costs associated with the latter case (testing-based LAS addition), the benefit/cost ratio for the mandatory usage of LAS in all mixtures was much higher than the testing-based option. In the optimistic performance sce- nario, the use of anti-stripping agents reduced the life-cycle costs for mixtures that were highly susceptible to moisture damage, but had minimal benefit on moderately susceptible mixtures. Overall, the agency recommended the mandatory use of anti-stripping agents in all susceptible mixes to reduce the maintenance costs and prolong pavement life. The use of anti-stripping additives as a preemptive measure to improve resistance to moisture- related damage introduces additional construction costs. However, studies indicate that these costs can be offset by lower rehabilitation costs and by the increased service-life of the pavement (57, 62). An exact estimate of cost-savings in using these additives is highly dependent on the modelling software used, model assumptions, types of additives used, and so forth.

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Incompatibility between aggregate type and asphalt binder, presence of standing water or water under pressure in the pavement layers, and improper construction practices are some of the many factors that influence moisture susceptibility of flexible pavements.

The TRB National Cooperative Highway Research Program's NCHRP Synthesis 595: Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements documents practices used by state departments of transportation to prevent or to minimize moisture damage in hot-mix asphalt pavements.

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