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8 CHAPTER 2 Findings The sections below present the specific findings of NCHRP VMA for these mixtures appeared to cover a range of air void Projects 9-25 and 9-31. This chapter is divided into eight sec- contents--from about 2% to 5%. A dramatic increase in rut- tions: Literature Review and Survey of Practice, Laboratory ting potential has been associated with in-place air void con- Testing, Analysis of Other Data Sets, Rut Resistance, Fatigue tents of around 2% or less (8). Thus, very low design air void Resistance, Permeability and Age Hardening, Apparent Film contents (say less than about 3%) should be avoided in wear- Thickness and HMA Performance, and Summary. These dis- ing and intermediate course mixtures because low design air cussions describe the most effective relationships between void contents should be expected to promote low in-place air performance-related properties and volumetrics as identified voids and increase the possibility of constructing a pavement and/or developed during this research and graphically illus- with poor rut resistance. Therefore, it appears reasonable to trate what these models predict in terms of property changes use a range for design air voids of from 3% to 5%. However, as as a function of VMA, design air voids, and related composi- discussed later in this report, engineers and technicians should tional factors. The practical implications of the findings pre- be aware that changing the value for design air void content sented here are discussed in Chapter 3. will significantly affect HMA performance. A variety of models were identified in the literature review for predicting performance-related properties from Literature Review and Survey HMA composition and other properties. The Hirsch model of Practice for predicting HMA modulus, as developed during the early A variety of research papers and engineering reports were phases of NCHRP Project 9-25, was found to be more suit- reviewed during the first few months of NCHRP Project 9-25 able for relating modulus to volumetric composition than and presented in the Interim Report for that project; an other existing models--Bonnaure's equation and Witczak's updated version of the Literature Review was included in the equation (7, 9, 10). Two models for predicting rut resistance NCHRP Project 9-31 Interim Report. were identified, both developed by Witczak and associates One of the most important issues in HMA mix design is (11, 12). In both cases, the model predicted the results of a how to define "optimum" asphalt content. In the current laboratory test for evaluating rut resistance and not field Superpave system, this is defined as the binder content that rutting. The models were similar, and both found that rut produces 4% air voids at the given compaction level. In order resistance increased with decreasing binder volume and air to evaluate the effectiveness of this practice, a range of Super- voids and increasing binder viscosity. A serious shortcom- pave mix designs, Marshall mix designs, and stone matrix ing of both models was the use of binder apparent viscosity asphalt (SMA) mix designs were reviewed, and the optimum values at 21.1 °C, rather than Superpave binder properties. binder content--defined in this case as the point at which A more useful model for predicting rut resistance was devel- minimum VMA is obtained--was determined. This is a more oped during NCHRP Projects 9-25 and 9-31: it predicts that fundamental definition of optimum asphalt content than that rut resistance increases with decreasing VMA relative to which is currently used in the Superpave system. It was found aggregate fineness and increasing binder viscosity (or com- that for these data, the optimum binder content based on min- plex modulus). imum VMA occurred at an average air void content of 3.4%, Existing models for predicting the fatigue resistance of but could also be defined as occurring at an average of 75.3% HMA have been empirically derived from laboratory flexural VFA. In fact, the optimum asphalt content based on minimum fatigue tests. Typically, such fatigue equations relate applied
9 stress or strain, initial complex modulus, and either VBE or Projects 9-25 and 9-31 and of field data generated in a variety VFA to cycles of failure. In all cases, better fatigue resistance of other projects indicated that any modeling of the relation- is predicted as a mixture becomes increasingly rich in asphalt ship between volumetric composition and performance must binder, either as indicated by VBE or VFA (1315). The account for relative compaction--the air void content of empirical nature of these relationships and their relatively either the laboratory specimen or in-place pavement relative poor accuracy when applied to fatigue of actual pavements to the air void content as designed. It is essential that this are serious shortcomings that lead to the development of a information be included in this report so that researchers practical continuum damage approach to characterizing attempting to validate the results of this research will under- fatigue phenomena in HMA as part of NCHRP Projects 9-25 stand the importance of accounting for the effects of relative and 9-31 (16). compaction. However, to place the findings of NCHRP Proj- A number of researchers in the past attempted to relate ects 9-25 and 9-31 in proper perspective, research found in mixture volumetrics (most often VMA and or asphalt the literature concerning the effect of in-place air voids on binder film thickness) to durability. This work mostly performance must also be discussed. Two significant such involved conjecture without substantial supporting data studies are NCHRP Project 20-50(14) and the research of and so was inconclusive. The concept of binder film thick- Linden et al. (23, 24). In NCHRP Project 20-50(14), Seeds et ness remains controversial. In general, there is agreement in al. analyzed data from the Long-Term Pavement Performance this early research that a certain amount of asphalt binder is (LTPP) program. They found that the data could not be used needed in a mixture to ensure adequate durability and that to develop performance models relating in-place air voids to the optimal binder content will depend to some extent on either fatigue or permanent deformation (23). In 1988, Lin- the properties of the aggregate used, including NMAS and den et al. published results of a study conducted by Washing- specific surface (1719). Most researchers have found a ton State evaluating the relationship between pavement decrease in permeability and age hardening with decreased performance and in-place air voids (24). They reported that air void content, although such relationships usually exhibit as a "rule of thumb," every 1% increase in in-place voids a large amount of variability (1921). Recent research on the results in about a 10% reduction in performance. This figure permeability of Superpave mixtures in Florida has demon- was a very rough, typical value, based on the results of several strated that unlike the relatively fine, dense-graded HMA studies: (1) three analytical studies relating fatigue life to in- used in the past, coarse-graded Superpave mixtures can place voids; (2) a survey involving 28 state highway agencies; exhibit relatively high levels of permeability unless thor- (3) an unpublished study of flexible pavements in Washing- oughly compacted (3). The substantial data set published by ton State; and (4) observed fatigue cracking in three pave- the Florida researchers has been analyzed to generate a use- ments placed in Washington with high air void contents. The ful equation for estimating mixture permeability from air analytical studies cited by Linden and Mahoney reported a void content and aggregate fineness, which is discussed later 10% to 30% reduction in fatigue life for every 1% increase in in this report. in-place voids (2527). No analytical studies on the effect of Only one method for predicting age hardening was located in-place air voids on rut resistance were cited in this study. The in the literature--Mirza and Witczak's global aging system results of these studies should be considered inconclusive-- (22). This model predicts age hardening of asphalt binder in NCHRP Project 20-50(14) was unable to develop any useful, pavements based upon mean annual air temperature reliable relationships between in-place air voids and per- (MAAT), binder viscosity, depth in the pavement, and air void formance; the study by Linden et al. was limited in scope, and content. This model has several shortcomings, the most even the authors admitted their conclusions represented only important being a reliance on binder apparent viscosity val- "a rule of thumb." ues estimated from obsolete empirical measures of binder As part of NCHRP Projects 9-25 and 9-31, in late 2001 and consistency and the prediction of age hardening only in terms early 2002, a survey was conducted of the manner in which of a change in apparent viscosity, rather than in terms of state highway agencies are implementing Superpave specifi- changes in the overall flow characteristics of the binder. A cations for volumetric composition. Many states have slightly modification of the global aging system was developed that modified the requirements for Superpave mixture composi- addresses some of these problems while maintaining consis- tion as given in AASHTO M323 and R35. Most commonly, tency with the original model. This model is used later in this the design air voids content is expanded to a range of 3% to report to estimate the effect of changes in mixture composi- 5% and a maximum VMA is established at 1.5% to 2% above tion on typical age hardening of asphalt mixtures and the established minimum values. A number of states have also binders. slightly increased minimum VMA values, providing for Although not specifically listed as one of the project objec- somewhat richer mixtures than produced by the current tives, analysis of laboratory data generated during NCHRP version of Superpave.