Engineering Properties of Asphalt (Part 2)

Abstract

The deformation of asphalt during sustained loading is caused by creeping; the creep characteristics of asphalts are strongly viscosity dependent.1)In this paper, some convenient expressions for asphalt viscosity are proposed. One of their merits is that the expressions can be deduced at any temperature from such physical properties of asphalt as penetration, softening point and penetration index. It is a normal practice to measure viscosities (using a Saybolt Furol viscometer, a Cylindering Rotating Viscometer or a Micro-Viscometer) at high, medium and low temperatures. Because of the inconvenience of measuring techniques and facilities, especially at medium and low temperatures, the concept of asphalt viscosity has not been adopted as an engineering design parameter for analysis of asphalt structures. When the loading time is long, the viscosity of asphalt is given by the formula.[Formula Omitted] Fig. 1 shows viscosities calculated from “PONOS & POEL”2) for various asphalts with PI values between -3.0∼+1.0 plotted as a function of the temperature difference between the Softening Point (TR&B) and a range of temperatures. It can be seen that asphalts in the range of PI between -3.0∼+1.0 have the same viscosity, 1×105 poise at T-TR&B=-8.5°C. This is in good agreement with the results of Nakajima3)who proposed a range of equiviscosity of 1×105 to 1.3×105 poise for asphalts. The Williams Landel Ferry formula 5) has been applied in many cases to calculate the viscosity of asphalts4), 6), 7). The WLF equation normally takes the form of:-[Formula Omitted] Where aT: Shifting factor C1, C2: Constants T: Viscosity at arbitrary temperature T(°C) TS: Viscosity at specific temperature TS(°C) There is currently no established theory for physical meaning of “TS” and Williams et al., Dobson8), and Jongepier and Kuiliam9) have advanced different concepts for it in their papers. It is convenient to define the temperature(TR&B-8.5)°C as the “TS” which gives equiviscosity of 1×105poise. Using this figure, the constants C1 and C2 in Eq. (2) may be shown to be functions of PI of the following forms. [Formula Omitted] and the viscosity and shift factors for time-temperature superposition principle at any arbitrary temperature are given by the formulas [Formula Omitted] Thus, when rheological properties (stiffness or viscosity) are known at temperature, TS=TR&B-8.5°C, the properties at an arbitrary temperature, T, can be extrapolated using the shift factors derived from Eq. (7). The shift factors for some typical asphalts are given in Fig. 3. The results calculated using Eq. (6) were compared with the data originally obtained by Traxler11), Romberg12), and Pheiffer13). Fig. 4 shows an excellent agreement between calculated and experimental results tabulated in Tables 1, 2, 3. For asphalt viscosity measurements, theoretical considerations suggest that the difference in the shear rate can be deemed negligible when considering the engineering properties of asphalt in its practical context. Accordingly, viscosities calculated from Eq. (6) can be applied to creep phenomena. Viscosities of asphalts, their Frass breaking point temperatures, TF, are tabulated in Table 6. These results have been analyzed and it can be shown that they may be represented by the following relationship. [Formula Omitted] Although asphalts are applied at high temperatures, their performances at medium and low temperatures are of prime importance. Accordingly, a close temperature control during application is necessary to ensure satisfactory performance. Optimum asphalt viscosities of 200cSt and 20,000cSt have been recommended for mixing and for compaction phases of road construction7). To protect against fatigue cracking and thermal cracking7), the maximum stiffness 0°, 50c/s and -10°, 104sec has been recommended. On the other hand, a viscosity of 14,000±4,000 poise has been specified in asphalt paving manuals, to provide resistance to rutting at 60°. These recommendations and specifications are plotted in Fig. 5 using calculated results from Eq. (6). © 1980, The Japan Petroleum Institute. All rights reserved.