Moisture Sensitivity of Asphalt Pavements (2003)

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307 SUMMARY REPORT: BREAKOUT SESSION 3 Design and Specifications JOHN D’ANGELO Cochair Federal Highway Administration MICHAEL COOK Cochair California Department of Transportation LORINA POPESCU Note Keeper University of California, Berkeley The list of participants follows this report. • Jimmy Brumfield pointed out aspects of Mississippi Department of Transportation experience related to using lime-based additives. − Mississippi has one local source of aggregates; the rest are imported from outside the state. − In the early 1990s, projects were using 1% to 1.5% lime, and they encountered some stripping problems. − Since 1992, lime is added to 100% of projects regardless of material, and no stripping problems were observed. − In Mississippi state projects, no marination was used. − Lime is incorporated on damp aggregates on the cold feed. − Mississippi often adds liquid antistrip along with lime. − Mixing same grade asphalts from different sources will not give a final asphalt with the same grade that the initial components had; therefore, a modified Lottman test is applied on the final product. − Allow for lime in voids in mineral aggregate/mix design. − Use of boil test to track compatibility problems. • Rita Leahy emphasized that when talking about specifications, it is important to distinguish among − Materials specifications, − Construction specifications, and − Design specifications. BEST PRACTICES The following best practices were identified by members in attendance.

308 Moisture Sensitivity of Asphalt Pavements: A National Seminar Materials Specifications Aggregates • Nominal maximum aggregate size (NMAS): Use the current AASHTO Superpave® recommendations. • Coarse versus fine aggregates: The effects of mix type in relation to compaction and permeability must be considered when selecting aggregate. • Clean aggregates: Tony Limas (Granite Construction) mentioned the cleanliness value as a way to account for coarse aggregate cleanliness. • Mineral filler is important. All fillers supplied as an independent product should come with a manufacturer certification of compliance. • There are tests currently used and considered to be the best practice: sand equivalent test as a standard specification test to evaluate the amount of clay. In addition, there are two other tests. They are the plasticity index (currently used by the Nevada Department of Transportation) and the methylene blue test used by Texas that can assist in identifying how sensitive the material is to water. • Perform “washed” sieve analysis. • Limit natural sands. Texas uses a 15% limit on natural sand content, and Mississippi uses 10%. • The shape of aggregates is important. It is desirable to use well-crushed aggregates (angular aggregates) that have a good effect on mix performance overall. Coarse aggregate angularity and fine aggregate angularity are available tests. Binder and Additives • Modified binder: Does it improve moisture performance or have no effect? Pros and cons were presented. Texas results from a large forensic study show that sections with modified binder had no stripping problems, whereas the nonmodified binder sections did. Nevada experience shows the contrary: modified binder does not improve stripping properties. Worth noting is that Nevada has important problems with getting the aggregates clean of clay. • Asphalt rubber: Does it improve moisture performance or not? From practice, some participants emphasized that stripping problems were encountered with asphalt rubber. • Modified binders: They improve the resistance to moisture damage because of an increased asphalt film thickness. • Additives: They improve the resistance to moisture damage. Nevada combines the polymer and the lime to mitigate stripping problems. Utah uses modifiers to meet PG specifications. Lime is then added on all mixes to address stripping. According to Utah’s experience, the combination of lime and polymer or even rubber improves resistance to moisture damage. Nebraska uses liquid antistrip combined with the binder and points out that when mixing binder with additives, there are interactions that change the overall properties of the mix. A test of the combined mix (binder and additive) needs to be performed to check whether the PG grade requirements are still met. What was observed is that the combination binder and liquid antistrip reduces the PG grade.

D’Angelo, Cook, and Popescu 309 Dick Root gave a short presentation on “Mix Design Issues with Lime” and emphasized the importance of not specifying a restrictive gradation about the maximum density line and of specifying a minimum voids in mineral aggregate (VMA) to allow room for lime and asphalt. Mix Design Specifications • For a moisture test to be performed on the final mix (binder + aggregates + additives) is considered important. • VMA specifications for mix design are considered critical. It was considered important to restate the factors affecting VMA: gradation, shape, and texture. • Dick Root’s presentation brought out that is important not to use overly restrictive gradation bands. • Account for baghouse dust in the design. • Conduct field (plant) mix verification for volumetric and moisture testing. Structural Design Specifications • The surface layer should be designed with smaller NMAS to reduce permeability and better protect from moisture damage. • From Mississippi’s experience, layer permeability is important. It is good practice to ensure that the highest permeability is in the surface layer and that each succeeding layer is less permeable than the underlying one, except when open-graded friction courses (OGFCs) are placed on the surface, and the bottom layer should have low permeability. This observation led to consideration of the following topics in the structural design: − Ensure there is a good drainage at the subgrade level before sealing (for rehab and overlay jobs, redo the cross slope, if necessary). If good drainage conditions are met, seal the subgrade with a prime coat or cut back before placing a treated permeable base, for example, ATPB, or before placing aggregate base (AB). Also seal the top of AB. Sealing the top of ATPB is not required (see Figure 1). FIGURE 1 Layer permeability.

310 Moisture Sensitivity of Asphalt Pavements: A National Seminar − For regions with a wet environment, consider using internal drainage (e.g., ATPB). − Do not overlay OGFC. This should be removed before any overlays are placed. It is important to have a permeability test. − Higher traffic areas justify stricter adherence to the recommendations as given. − Follow National Asphalt Pavement Association–FHWA publication mix type selection guide recommendations regarding lift thickness versus NMAS. Construction Specifications • Construction specifications should include mix volumetric and compaction based on air voids. • Use percentage of the maximum theoretical density varied according to the mix type; coarser mixes have higher density requirements. • Use material transfer device to eliminate temperature segregation problems. • Joints proved to be weak points where moisture sensitivity problems initiate (joints are usually more permeable than mainline pavement). It was considered important to have joint density specifications (e.g., minimum 91% of maximum theoretical density or within a relative compaction of 3% of the main line). • Joint seal type materials and heaters can be used to reduce permeability of joints. • If additives are added in the field (e.g., lime, liquid antistrip), it is good practice to test the final product at the plant (in production) to make sure it meets the specifications (both PG grade and TSR). • Adopt and reference in the specifications the Hot-Mix Asphalt Paving Handbook as best practices. • The optimum quantity of liquid antistrip should not be specified, but should be determined through laboratory testing. GAPS IN KNOWLEDGE • Tests that correlate to field performance, • Gap in defining what are the failure mechanisms, • Gap in documenting and sharing the information on successful projects, and • No diagnostic tools for moisture damage. RESEARCH NEEDED • Look at the long-term effect of additives. • Consider the aging effects of additives on the mix in relation to moisture damage performance. • Standardize terms defining and related to moisture damage. • Develop a data format to be used by everybody involved in the study of moisture damage and maybe a Web-based database that will make it easy to document and share information for research purposes (forensic procedure).

D’Angelo, Cook, and Popescu 311 • Publish a synthesis on existing test procedures, mechanisms they address, their shortcomings, and so forth. • Look at the side effects of additives (lime, LS, LAS) on fatigue, rutting, and so forth.