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39 maximizing profits (and therefore staying in business), the ifications, or in specifications for other HMA types, should competitive marketplace demands that they adjust their com- consider in some way the way in which all changes in a speci- paction methods to optimize their profits, based on the cost fication act together to affect performance. This includes not of performing compaction and the penalties and/or bonuses only changes in volumetric composition and compaction, but that result from different levels of compaction. Lowering also changes in materials specifications; especially important Ndesign will help improve field compaction, but unless this is are how changes in aggregate specifications might affect per- combined with a payment schedule adjusted to provide addi- formance. The models and analyses presented in this report tional incentive for thorough field compaction, in the long were largely developed on HMA mixtures made with aggre- run it will not likely result in significant lowering of in-place gates that either meet or come close to meeting current Super- air voids. pave specifications. The models may or may not be accurate for aggregates that do not meet these requirements. An example of how specification changes can negatively Linking Aggregate NMAS affect performance is useful to illustrate the importance of and Minimum VMA considering how these changes can work together. Consider Traditionally, HMA design has linked aggregate NMAS again the mixtures listed in Table 12. Imagine a third alterna- with requirements for minimum VMA--as NMAS decreases, tive modification, Approach 3, in which design air voids are minimum VMA increases. There are two reasons for this link- lowered to 3% to improve fatigue resistance. At the same time, age: (1) smaller NMAS is usually associated with higher aggre- Ndesign is reduced to 50 in order to try to improve compaction, gate specific surface so that too maintain a more or less but no effort is made to make specifications for field com- constant apparent film thickness, more binder is needed, and paction more stringent so that in-place air voids remain at VMA should therefore increase; (2) smaller NMAS is in gen- 8.0%. It might at first seem that these changes would be ben- eral associated with higher VMA--that is, all else being equal, eficial to fatigue resistance and durability, but the analysis aggregate gradations with smaller NMAS will tend to yield suggests otherwise--rut resistance in this case is reduced by higher VMA. However, neither of these trends is extremely 30%, fatigue resistance is reduced by 40%, and permeability strong; aggregates with large NMAS may have a large amount remains nearly constant at 440 × 10-5 cm/s. The proposed of fines, leading to a high specific surface and requiring higher changes have significantly decreased fatigue resistance, done VMA to maintain a reasonable apparent film thickness. Simi- nothing beneficial for permeability, and decreased rut resist- larly, aggregates with small NMAS may have inherently low ance. If these changes were to be simultaneously implemented VMA, making it difficult in the mix design process to achieve with reduced standards for aggregate angularity, the results the higher VMA values required for aggregates with small could go from simply being bad to being disastrous. It is sug- NMAS. It can be questioned whether aggregate NMAS and gested that agencies considering both changes in materials minimum VMA should be linked. A theoretically more sound specification and changes in specifications for volumetric approach might be to establish aggregate NMAS on the basis composition and compaction should implement and evalu- of lift thickness and to set minimum VMA on the basis of ate such changes separately, to avoid unanticipated negative desired fatigue resistance and durability. However, this would impacts on pavement performance. be a monumental change in the way people think about HMA and design mixes. It would be very difficult to implement and Implementation would probably lead to much confusion among engineers and technicians. Within the current approach, it is still possible to Because of the execution of NCHRP Project 9-33, a com- provide HMA mixtures with a reasonable range in fatigue plete and formal implementation of the results of NCHRP resistance and durability. Maintaining the current system will Projects 9-25 and 9-31 would be redundant. Most of the help ease implementation of the more critical findings of this findings and recommendations of this research are being research, while still providing engineers with an adequate slate evaluated and refined as appropriate for possible incorpora- of mixtures to address most paving applications. tion into the Mix Design Manual being developed under NCHRP Project 9-33. It is therefore suggested that imple- mentation of the results of this project be kept simple and Effect of Multiple Changes informal. The initial phases of implementation have already in HMA Specifications taken place through publication of several papers dealing A number of the analyses presented previously involve mul- with the various models developed during this research. It is tiple, simultaneous changes in specifications and illustrates expected that one or two additional publications will be sub- how these changes work together to affect performance. Any mitted summarizing the final results of NCHRP Projects 9-25 engineer or agency contemplating changes in Superpave spec- and 9-31.