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26 Aging Ratio (5 yr/Ini.) 8.0 any healing from occurring at the pavement surface during 9% in-place voids hot summer weather. 6.0 7% in-place voids · In general, the effect of increasing air voids by 2% on age 5% in-place voids hardening is comparable with the effect of decreasing 4.0 FM300 by 5%. More careful control of aggregate specific sur- 2.0 face should help maintain good resistance to age harden- ing in HMA. 0.0 10 20 30 40 50 60 FM300 Apparent Film Thickness and HMA Performance Figure 19. Predicted Mixture Age-Hardening Ratio at 25 C and 10 Hz as a Function of In- One of the objectives of this project was to evaluate the Place Air Void Content and FM300 for a MAAT relationship between film thickness and HMA performance. of 15.6 C. Since the 1950s, some pavement engineers have proposed that film thickness is an important characteristic in determining the durability and fatigue resistance (44­48). Film thickness practical findings can be made based upon the results of is generally estimated by dividing the effective volume of this analysis: binder in a mix (in units of m3/kg aggregate) by the specific surface of the aggregate (m2/kg). In the late 1950s Campen · Mixture age hardening as indicated by complex modulus and his associates proposed that HMA mixes should be increases with increasing air voids and decreasing aggre- designed with film thickness values between 6 and 8 m (44, gate specific surface. This effect is not extremely large-- 45). Much later, Kandhal and Chakraborty suggested that film typically, age-hardening ratios decrease 2% to 7% for each thickness values between 9 and 10 m should be used with 1% increase in FM300 and increase 5% to 14% for each 1% mixes designed according to the Superpave system in order to increase in field air voids at a MAAT of 15.6 °C. However, prevent premature aging (48). the combined effect of high air voids and low aggregate Despite the many proponents of film thickness, its use to specific surface can increase age hardening by 50% or design or specify HMA mixes remains controversial. The more. The amount of age hardening that occurs in a mix- Superpave system does not include any requirements or ture not only is dependent upon the air voids and aggregate guidelines for film thickness. Many pavement engineers object fineness, but also is strongly dependent upon the specific to the term "film thickness" on the grounds that individual binder used and the MAAT. asphalt films do not exist in an HMA mix and that instead · Age hardening as indicated by binder viscosity values can asphalt is a continuous phase in what is in reality a particulate be extremely high--often greater than 100. The effect of composite. Although this latter view is technically correct, the increasing air voids by 2% is to increase age hardening by fact remains that film thickness values can be calculated for about a factor of 2 at a MAAT of 15.6 °C and by a factor of HMA mixes and these values relate two important character- about 3 at a MAAT of 23.9 °C. The very high binder vis- istics of HMA mixes--asphalt binder content and aggregate cosities that can potentially exist in aged pavements could specific surface. To address the objection that asphalt films do contribute significantly to surface cracking by preventing not really exist in asphalt mixes, the term "apparent film thick- ness" (AFT) is in general used throughout this report. 10,000 In general, the research performed during this project does 9% in-place voids Aging Ratio (5 yr/Ini.) 7% in-place voids not support the direct use of AFT values in the design and 1,000 5% in-place voids specification of HMA mixes. At the same time, it should be 100 pointed out that this research has demonstrated that AFT in many cases will indirectly relate to HMA performance. The 10 strongest of such relationships is that between rut resistance and AFT. Resistivity is proportional to the square of aggregate 1 specific surface and is inversely proportional to the cube of 10 20 30 40 50 60 VMA, and because most Superpave mixes are designed at FM300 or very close to 4.0% air voids, there is a direct relationship Figure 20. Predicted Binder Age-Hardening between VMA and effective binder content. Therefore, there Ratio at 40 C as a Function of In-Place Air Void should be a very good relationship between AFT and resistiv- Content and FM300 for a MAAT of 15.6 C. ity and between AFT and rut resistance. To evaluate this rela-