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

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41   This chapter presents examples of four state DOT approaches to managing or mitigating damage resulting from moisture-susceptibility of hot-mix asphalt in different regions in the United States. The selection of these DOTs was based on a variety of factors, such as diversity of climatic and topographic conditions within the state, heavy rainfall, lack of good quality aggre- gate, or the presence of aggregate resulting in mixtures prone to moisture damage, different/ unique approaches to damage mitigation, use of non-standard mix design methodology, or a combination of these factors. 4.1 Florida Department of Transportation Although the state of Florida receives heavy annual rainfall and uses moisture-susceptible aggregate in their mixtures, the Florida Department of Transportation (FDOT) does not con- sider moisture damage as a major problem in their flexible pavements. This may be attributed to the following three factors: • The mandatory use of liquid anti-stripping agents in all mixes regardless of mix type and traffic level. • The rigorous evaluation procedure (implemented about 5 years ago) for anti-stripping prod- ucts before they are placed on their Approved Product List (APL). • The fact that all mixtures used in Florida, except for open-graded friction courses, are fine- graded Superpave mixtures which are inherently impermeable when adequate density is achieved. The predominant aggregates used in the construction of pavements are Florida limestone, Alabama limestone, Georgia granite, and Nova Scotia granite. Florida limestone aggregates tends to be more porous and hence, more absorptive, compared with limestone obtained from Alabama. Due to the low-friction characteristics of limestone from locations other than three sources from southeast Florida (which are approved for use in friction courses), the surface or friction mixtures are typically constructed using granites from neighboring Georgia or imported from Nova Scotia because of their superior frictional value. Although granites have superior frictional characteristics, they are siliceous in nature, making granite-based mixtures prone to stripping. As granites are used in the majority of the pavements (approximately 80% of friction courses) in Florida, the use of LAS is required in all mixes. Hydrated lime is also permitted; however, it is not the preferred choice among contractors because of its higher associated costs. A combination of liquid anti-stripping additives and hydrated lime is required in granite-based open-graded friction course mixtures in Florida. On an average, the state verifies or approves around 300 mixes per year, which puts an enor- mous demand on the agency’s testing and approval/acceptance process. The state revised its C H A P T E R   4 Case Examples

42 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements APL process for liquid anti-stripping agents to reduce the extent of mix design verification test- ing and to provide better flexibility with respect to using liquid anti-stripping additives. Under the revised program, liquid anti-stripping additives must be evaluated in eight standard mix types using the four predominant aggregate types. The evaluation is performed using the Modi- fied Lottman Test (AASHTO T 283). All eight mixes are required to satisfy a minimum Tensile Strength Ratio requirement of 0.80 before the liquid anti-stripping agents are placed on the APL. Mix designs containing less common aggregates, and thus not included in the APL-related evaluation process, are subject to moisture-susceptibility testing during the mix design phase. FDOT uses the Modified Lottman test procedure to assess mixture susceptibility to mois- ture damage. During a study conducted by the University of Florida in 2007 (71), researchers developed a new mixture-conditioning procedure that was essentially a modified version of the Moisture-induced Stress Tester. However, because of its complexity, the test procedure was never adopted by the state DOT. The Hamburg Wheel Tracking device, which is used in some states to assess the combined effects of traffic load, temperature, and moisture on pavement performance, is performed at 50°C in Florida. The Hamburg Wheel-Track Test rarely yields reliable results that can be correlated with field performance or TSR. This poor correlation may be partly attributed to selection of lower test temperature in a state that experiences rather high pavement temperatures. Using a test temperature of 60°C may yield better results, and may be considered by the agency in the future. Open-graded friction courses used in Florida are common as they facilitate quick drainage of the surface water and minimize splash and spray conditions. All FDOT OGFC mixtures con- tain 12.5-mm nominal maximum aggregate size (NMAS) aggregate gradation and a PG 76-22 polymer-modified binder (PMB with 2–3% load). SBS and, to a lesser extent, ground-tire rub- ber (GTR) and styrene-butadiene are the polymers used along with the base PG 67-22 binder. In spite of the potential for OGFCs to store water during severe rainstorms, the use of PMB along with other design practices mentioned above have ensured the long-lasting durability, good rut resistance, and resistance to stripping of Florida pavements. The OGFCs and DGAs placed in Florida interstates typically last 13–14 years. Dense graded friction courses typically last 18–19 years. Looking ahead, the increasing frequency of extreme weather events in recent times has raised the threat of coastal flooding and more severe flooding of low-lying inland areas. The state DOT is increasingly aware of the need to investigate the impact of sustained high levels of seawater on their HMA pavements. The agency is also exploring the use of epoxy modifiers in their OGFCs. The successful way in which FDOT was able to mitigate moisture-related damage issues in their asphalt pavements is attributable to the following key elements: (a) adequate lift thickness to attain target density, (b) good density measurement methods, (c) good permeability of the pave- ment, (d) good knowledge of the materials being used in the pavement, and (e) the use of LAS in all mixes. 4.2 Georgia Department of Transportation Georgia, like its neighbor Florida, it known to receive a significant amount of rainfall and has a warm and humid climate. The bedrock in large portions of the state is granite, with some limestone that can be found in the northwestern part of state. In spite of these two factors (aggre- gate type and weather/climatic conditions) that are conducive to creating moisture-induced pavement damage, currently, the state does not report significant stripping-related failure. The lack of significant stripping-related failures is mostly caused by changes made to the mix design process over the years. The majority of the state’s pavements use only granite, but contractors are given the option of using other types of aggregates, for example, limestone,

Case Examples 43   to achieve density requirements. However, pavements containing limestone have traffic limita- tions imposed, because of their tendency to fracture under heavy traffic loads. The base binder used in Georgia is PG 64-22, but a modified PG 76-22 and GTR-modified asphalt (∼10% load) are also allowed in their mix designs. Historically, the agency used the Georgia Loaded-Wheel Tester and the Modified Lottman Test to assess rut-resistance and moisture-resistance, respectively. Early observations from field cores recovered for rehabilitation purposes indicated severe delamination of asphalt film from the aggregate (stripping), which led to the introduction of an additional design requirement, namely, the addition of 1% hydrated lime to all design mixtures. These changes or modifications were done to facilitate better bond between their predominant, moisture-susceptible aggregate type (granite) and the asphalt. When the Superpave mix design system was introduced in the late 1990s, the agency did observe some stripping in their flexible pavements. Early Superpave subsurface mixes had higher permeability than the earlier Marshall mixes. While the switch from Marshall mix design meth- odology to Superpave methodology addressed the earlier rutting issues, it resulted in excessive permeability in their underlying mixes for Georgia conditions. Georgia Department of Trans- portation (GDOT) made changes to their gradation to lower the permeability to 3.6 ft/day for mixes ≥12.5-mm NMAS. These changes were based on further investigation into the perme- ability of the mixes used in the state, anecdotal reports, and experience gained by other states that had implemented the Superpave system. The state DOT started making incremental changes to their design specifications that allowed them to manage the problems associated with the presence of moisture. Initially, these changes involved the compulsory addition of hydrated lime, followed by modifications to the gradation, accompanied by a permeability requirement for acceptance, all of which helped address the issue. Additionally, the GDOT has also modified the AASHTO T 283, making it more specific to Georgia mixes. These modifications included specifying a lower loading rate and using a more rigorous conditioning regime involving 12 specimens divided into three subsets. The %HL requirement was reduced to 0.9% for mixes containing recycled-asphalt pavement (RAP), to account for any HL that might already be present in the RAP. Liquid anti-stripping additives were allowed in Georgia mixes, as a part of a specific pay item or per contract, typically on projects funded by Local Area/Assistance Roadway Projects. While both LAS and HL can be used to mitigate moisture sensitivity, LAS could be used only as a contract pay item or as an approved design requirement. When used, LAS is allowed as an asphalt additive, but LAS cannot be used to pretreat aggregate before mix production. However, hydrated lime is the preferred treatment used in the state, and liquid anti-stripping agents are rarely used. About 5 years ago, Georgia stopped using their state-modified version of AASHTO T 283 (GDT-66) (72), and transitioned to using the Hamburg Wheel-Track Test to assess moisture sus- ceptibility during the mix design approval process and for evaluating cores from old roads slated for rehabilitation. Unlike the individual tests used earlier, the HWTT evaluates both rutting and stripping potential simultaneously and thus was considered to be more advantageous. The state DOT requires the test specimens from mixtures that fail the HWTT to be cut and exam- ined visually. Following the visual examination, GDT-66 must also be conducted on the failed mixtures to verify whether the failure was a result of stripping dynamics or caused by structural weakness under applied loading conditions. OGFCs and SMAs are also used extensively on Georgia interstates, but these types of mixtures are tested for stripping potential using the Boiling Water Test (GDT-56) (73). Although the test is highly subjective and does not relate directly to field performance, it is nevertheless specified

44 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements for all open-graded mixes and SMAs. OGFC mixes are not tested using the HWTT, as they do not hold up well under the loading conditions (invariably, they will rut and thus yield unreli- able data). Moreover, in pavement design, OGFCs are treated solely as an overlay to drain away surface water, and are not considered to be a part of pavement load-bearing structure. However, these mixes also require a minimum of 1% of HL. If the design mix fails to meet the specification requirements at 1% HL, an extra 0.5% lime should be added to satisfy the requirements. There is always the danger of adding too much hydrated lime to these mixes, which will have the reverse effect on stripping potential of HMA, hence the limiting value of 1.5% HL. Currently, GDOT reports having <1% failures in the HWTT method. Most failures typi- cally occur in field cores obtained from old mixtures that are being evaluated for rehabilita- tion purposes. Going forward, GDOT is considering adopting a Balanced Mix Design (BMD) methodology. To this end, the data from the Moisture-induced Stress Tester are being evaluated in comparison to data from HWTT. They envision that MiST data might be a needed input when (or if) the DOT adopts the BMD. 4.3 Nevada Department of Transportation A majority of interstates in Nevada are asphalt, with concrete pavements accounting for only about 2–3% of the network. However, it is the only state within the United States that still prac- tices the Hveem Mix Design for flexible pavements, while utilizing the Superpave Plus binder grading system. In general, the state has a good distribution of high-quality limestone and dolo- stone in the southern part of the state. The majority of northern and central parts of the state have varied but typically complex geologies that include volcanic rock types, such as basalt, rhyolite, and granite. However, the quality of the latter rock types is not consistently good because of the high levels of geologic activity that created a fair percentage of metamorphic and geothermally altered aggregate sources. The daily variations in the maximum and minimum temperature can be as high as 45°F, with the northern parts of the state having high elevations and experiencing heavy snowfall. During a year, the number of freeze-thaw days (when the tem- perature crosses the 0°C threshold) in the state can vary between 100–200 days, with overall low rainfall except for areas around Lake Tahoe and other high mountains. Historically, the agency used AC-20p asphalt in their mixes. The pavement temperatures in northern Nevada can range from 52°C to 64°C at the high end and −16° to −40°C at the low end. With the introduction of the Superpave Performance Binder Grading system in the late 1990s, evaluation of the asphalts used in the state indicated that a straight PG 64-28 graded binder led to several early pavement failures. However, the state DOT had a good track record of field per- formance of asphalt pavements, which led the state DOT to modify the original PG 64-28 speci- fication by adding two PG+ requirements to their asphalt specifications to make the Superpave system applicable to their state. These requirements included low-temperature ductility and toughness or tenacity. In Clark County, the PG 76-22NV requires a minimum of 3% polymer rather than the toughness/tenacity requirements. Currently, PG 76-22NV and PG 64-28NV that are SBS-modified are used in Clark County’s Hveem mixes. The state reports very little stripping-related failures, because of a combination of “good prac- tices” developed by Nevada Department of Transportation (NDOT). While the availability of good aggregate is variable throughout the state, the moisture-damage issue in pavements has been minimized through the use of polymer-modified binders and a mandatory 48-hour mari- nation process for all mixtures containing hydrated lime as an anti-stripping additive. These measures added an extra layer of safety against stripping failures. One percent hydrated lime is added to all coarse aggregate and 2% is added to fine aggregate, resulting in approximately

Case Examples 45   1.25% HL overall load. The virgin aggregate is kept about 2–3% above SSD condition before the addition of HL via a pug mill, and the aggregate-lime system is marinated for a minimum of 48 hours (but no longer than 6 weeks), before being used in production. RAP stockpiles are not treated with HL. While the use of HL as an anti-stripping additive is more expensive (∼$2/ton) than the use of liquid additives, the state does not see the need to switch to LAS, given the work required to find compatible chemical agents to suit their aggregate and climatic conditions. To improve drainage and pavement friction, Nevada interstates also use OGFCs that merely serve as a sacrificial wearing course, but offer no structural support to the pavement. The OGFCs used in this state may be likened to small size SMAs, with 9.5-mm (⅜″) maximum aggregate size in the northern part of the state and 12.5-mm (½″) in the southern regions. These mixes are also treated with lime and marinated before placement, as per NDOT specifications. A customized version (Nev. T341D) (74) of the Modified Lottman test (AASHTO T 283) developed by the state October 21, 2005 is used to evaluate mixtures with respect to moisture susceptibility during the mix design approval and acceptance stages. Specimens are compacted to 7 ± 1% air voids. The required saturation of the conditioned set can range from 55%–80%, with no specified requirement as to the amount of partial pressure that should be applied to attain that level of saturation. In reality, the degree of saturation in the test protocol is higher than that specified in AASHTO T 283; hence, the samples are subjected to a more severe freeze- thaw treatment. The agency requires a minimum TSR of 0.70 for mix design acceptance, with a minimum dry strength requirement of 100 psi and 65 psi for PG 76-xx and all other binders, respectively. The failures typically observed in their pavements today are mostly associated with old mix- tures or pavements that exhibit low temperature cracking or some block cracking. To this end, the DOT is experimenting with test methods to better predict low temperature cracking, in collaboration with the research team at the University of Nevada at Reno. However, at the time of this writing, no research was being conducted into other methods to evaluate stripping resistance. 4.4 Washington State Department of Transportation The state of Washington is known for its diverse climatic conditions and geology. The Cascade Mountain Range separates the western, windward side of the state that is hilly, and has relatively mild temperatures, heavier rainfall and rare snowfalls, from the eastern, leeward side that is hotter and drier. The plains in the central portion of the state have volcanic soils and enjoy a combination of coastal and continental climate. Given the geologic history of the state, the state is fortunate to have an abundance of high-quality aggregate, especially in the western part of the state. The predominant rock types used in pavement construction are largely basalts, gravels, and some granites. However, the peninsula and the northeastern regions of the state suffer from some poor-quality aggregates. About 85% of the mainline highways are paved with HMA, with the remaining 15% being paved with portland cement concrete. The portland cement concrete pavements are used in high-elevation (mountain-range) roads. The state DOT does not consider moisture damage to be a major factor affecting the durability of their pavements. When tested for moisture sensitivity, only about 7% of HMA mixtures fail to meet the mix design approval criteria, of which very few (<2%) may be attributed to stripping failures in the HWTT. Basalt is ranked high in its resistance to moisture damage, so the use of anti-stripping agents is not mandatory in all mixes. The anti-stripping additives are, however, added in cases when the contractor determines that they are needed in order for the mixture to

46 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements meet the mix design HWTT requirement. Prior to the introduction of Superpave mix design methodology, the Washington State Department of Transportation (WSDOT) followed the Hveem design process. WSDOT adopted the AASHTO M 320 specification (75) in 1999, with no PG+ requirements for testing stiffer binders at higher temperatures. Prior to switching to the Hamburg Wheel- Tracking Test in 2014, a TSR of ≥0.80 was used as the acceptance criterion with respect to moisture susceptibility. Under these conditions, testing of design mixes using the AASHTO T 283 called for increasing the level of addition of anti-stripping additives to meet the minimum TSR requirement. However, this change resulted in very poor mixture-stripping performance, when tested using the HWTT. An internal study conducted by the agency to investigate these causes pointed to the possibility of incompatibility issues arising from a combination of aging, PPA-modified asphalts and amine-based liquid anti-stripping agents that are often used in the industry. PPA, when used as a liquid additive, is completely miscible with asphalt, improves binder stiffness and its high temperature grade, with no detrimental effect on the low tempera- ture properties of the binder. In 2012, WSDOT implemented an elastic recovery specification using provisions of AASHTO T 301 (76) to address binder modification products and processes that could negatively impact performance. Continued parallel testing by the agency using the HWTT and the Modified Lottman Test showed inconsistencies in the results predicted by the two methods. In addition, the agency also conducted comparison testing between AASHTO M 320 and AASHTO M 332 (77) grading systems. Results from these two internal studies and field obser- vations led the agency to institute changes to their binder and mixture specifications to address the issue of moisture susceptibility. One of the changes was the adoption of the Multiple- Stress Creep Recovery test protocol, including the percent recovery requirement for testing the binders. These changes to the binder specification essentially eliminated the use of PPA-modified binders in WSDOT mixes. The second change was to reduce the amount of natural, rounded fines in the design mixes, as it was observed that some mixes were showing poor rutting resis- tance in the HWTT. The state DOT currently uses PG 58H-22 and PG 64H-28 base asphalts in the eastern and western parts of the states, respectively. All these changes, which were imple- mented between 2012 and 2019, have resulted in limiting or minimizing the moisture damage in the mixes, as reported by WSDOT. The agency does not use HWTT for design acceptance, but in the field, the mixtures are checked every 1,000 tons for gradation and volumetric prop- erties. The in-place density is tested every 100 tons. Every 10,000 tons, production samples are obtained and tested for mix design confirmation, including HWTT and Indirect Tensile testing. Since HWTT data very often do not yield the classic two-slope curve, WSDOT sees the need for further research into reliable determination of the Stripping Inflection Point. Hydrated lime is rarely used as an anti-stripping agent in the state, although the agency recently introduced a specification in their manual that allows for its use, if design mixes fails the HWTT. Additives used to pretreat the aggregate for moisture susceptibility are no longer employed in WSDOT mixes. Prior to use of the HWTT, moisture-susceptibility was assessed using the Modified Lottman Test on six specimens: untreated and conditioned, untreated and unconditioned, and treated with increments of 0.25% liquid anti-stripping agent (by weight of binder) to maximum of 1% dosage. Based on experience and discrepancies between observed laboratory test results and field performance, the agency now requires that the anti-stripping agent be blended at the terminal by the asphalt supplier, not by the contractor. In regard to the modern test methods being considered, the state is currently (for about 2 years) collecting data on the IDEAL-CT (ASTM D8225, 78) and Cracking Tolerance Index after seeing promising research data on that topic. The state DOT imposes a 175-psi max- imum strength requirement, based on ASTM D6931 (79). However, since the Indirect Tensile

Case Examples 47   Cracking Test may provide false positives and since it does not specify the loading duration for the test, the agency is looking for better ways to evaluate their high RAP and fatigue-susceptible mixes. They envision implementing the Cracking Tolerance Index into their specifications, based on the results of continued research. In summary, the adoption of preemptive measures by state DOTs with respect to mitigating or preventing moisture damage in pavements that have aggregate prone to stripping was found to be very effective. These measures include the use of anti-stripping additives in all the mixtures at minimum prescribed levels or the total elimination of specific rock types. A good knowledge of the surface chemistry of the predominant rock types used in the pavements and their inter- action with asphalt binders is also essential for managing moisture-susceptibility issues in the pavements. These measures lower rehabilitation costs during the life of the pavement, in addi- tion to minimizing or eliminating stripping. As the state DOTs switched from the old Marshall- mix design methodology to the Superpave methodology, understanding the importance of pavement permeability and drainage in preventing optimal conditions for stripping led to changes in aggregate gradation to suit local conditions. In-place density and/or permeability requirements were written into state DOT specifications to address this issue.