The influence of vehicle speed changes at mechanistic performance of asphalt mixture

Asphalt mixture is a viscoelastic material. Several factors affect the behavior of a viscoelastic material. Temperature and loading time are the most critical of these parameters. Vehicle speed affects time loading, therefore it’s very important to know the influence of vehicle speed changes in mechanistic performance of bitumen and asphalt mixture with different aggregate gradation types, namely dense graded and gap graded. Based on data of bitumen penetration, bitumen softening point, volume of bitumen, volume of aggregate and volume of voids, mechanistic performance of bitumen and asphalt mixture influenced by vehicle speed changes can be predicted using the software BANDS 2.0. The variation of vehicle speed will be carried out to see its significance. The results of the BANDS 2.0 simulation show that vehicle speed changes influence bitumen stiffness, asphalt mix stiffness and fatigue life asphalt mix. The higher the vehicle speed is, the higher bitumen stiffness and asphalt mix stiffness will be, but fatigue life asphalt mix will be lower. At the same vehicle speed, asphalt mix stiffness of dense graded mixture will be higher than gap graded mixture. The other way for fatigue life asphalt mix.


Introduction
Asphalt mixture is a viscoelastic material. The behavior of this material combines two parts: elastic behavior and viscous behavior. Materials with elastic behavior return to their initial state after removal of the applied loads, whereas permanent deformations remain under applied loads for viscous behavior. Several factors affect the behavior of viscoelastic material. Temperature and loading time are the most critical of these parameters. A viscoelastic material has more elastic behavior at low temperatures and is more viscous at high temperatures. Asphalt mixture as a viscoelastic material behaves as a viscous liquid at long times of loading and behaves like an elastic solid at short times of loading [1][2][3][4][5][6][7]. Vehicle speed affect time loading, therefore it is very important to know the effect of vehicle velocity changes in mechanistic performance of bitumen (for example: bitumen stiffness) and hotmix asphalt (such as asphalt mix stiffness and fatigue life asphalt mix) with different aggregate gradation types, namely dense graded and gap graded.
The type of asphalt mixture frequently used for wearing course in Indonesia is Asphalt Concrete-Wearing Course (AC-WC) and Hot Rolled Sheet-Wearing Course (HRS-WC) [8]. Nevertheless, those two types of hot mix asphalt have different gradation, in which AC-WC is a dense graded mixture while HRS-WC is a gap graded mixture.
Gradation is the distribution of aggregate based on size. Particle size or grain size can be obtained through sieve analysis. Aggregate gradation can be classified into dense graded and poor graded. The first is an aggregate which particle size is evenly distributed in a range of particle sizes. The second is an aggregate which does not meet the requirements of good gradation, consisting of uniform graded, open graded and gap graded. Uniform graded is composed of aggregates which have the same or nearly the same size particle size. Open graded is aggregate that has a particular particle size distribution in which the pores are not well filled. Meanwhile, gap graded is an aggregate that has uneven particle size distribution, or there is a particle size that does not exist and if there are some, only a very small amount is present [9].
In Indonesia, the testing of asphalt mixture is performed through Marshall Test. It was initially developed by Bruce Marshall and continued by the US Corps Engineer. It was then standardized by ASTM and AASHTO through several modifications, namely ASTM D 1559-76 and AASHTO T-245-90. The test is frequently employed to determine the performance of the asphalt concrete mixtures, such as the test specimen volume, stability, flow, Marshall Quotient, and voids (VIM, VMA, VFWA) in asphalt concrete mixtures [9,21].
Mechanistic performance of bitumen (for instance: bitumen stiffness) and hotmix asphalt (such as asphalt mix stiffness and fatigue life asphalt mix) can be measured quickly and easily using a test. For examples, indirect tensile stiffness modulus test can be carried out (ITSM test) to determine bitumen stiffness and asphalt mix stiffness, and by using fatigue test one could find out the fatigue life asphalt mix. However, when testing is not feasible, then the stiffness of a particular mixture at any temperature and time of loading can be estimated by empirical methods [3]. In 1977, Shell produced a nomograph to predict the stiffness of an asphalt [22]. These nomographs can be simulated by the software BANDS 2.0. This study aimed to know the effect of vehicle speed changes in mechanistic performance of bitumen (for example: bitumen stiffness) and hotmix asphalt (such as asphalt mix stiffness and fatigue life asphalt mix) with different aggregate gradation types, namely dense graded and gap graded using the software BANDS 2.0 based on data of bitumen penetration, bitumen softening point, volume of bitumen, volume of aggregate and volume of voids.

Materials and method
The materials are bitumen penetration 60/70 (penetration = 60,5 (in 0,1 mm), softening point = 51,5°C), coarse aggregate and fine aggregate based on the Bina Marga specification (2010). Subsequently, the dense graded asphalt concrete-wearing course (AC-WC) and gap graded hot rolled sheet-wearing course (HRS-WC) are also prepared. The AC-WC and HRS-WC specifications also refer to Bina Marga [23]. Specifically, the gradations of AC-WC and HRS-WC are presented in Table 1. These hotmix asphalts are tested using Marshall test to find the optimum bitumen content and Marshall properties. The optimum bitumen content for AC-WC was 5.80% and for HRS-WC it was 6

Marshall properties
Based on the optimum bitumen content of several AC-WC and HRS-WC asphalt mixtures, specimens are made to reveal the Marshall properties. The properties are presented in Table  2.  Table 3 and Table 4. Based on the tables above, the effect of vehicle speed changes at mechanistic performance of asphalt mixture are presented in Figure 1, Figure 2 and Figure 3 .  The mechanistic properties of bitumen and hotmix asphalt influenced by vehicle velocity speed simulated by software BANDS 2.0 are that the higher the vehicle speed is the the higher bitumen stiffness (Sbit) will be. While mechanistic performance of hotmix asphalt at variation of vehicle speed with different gradation asphalt mixtures, namely dense graded and gap graded are as follows:  Asphalt mix stiffness (Smix). Asphalt mix stiffness is used to assess the resistance asphalt mix to deformation. The higher the vehicle speed is the higher the asphalt mix stiffness (Smix) will be. At the same vehicle speed, AC-WC mixture has a higher Smix than HRS-WC mixture. Dense graded has a higher Smix than gap graded, therefore dense graded is more rigid than gap graded and more stable to support traffic load.  Fatigue life asphalt mix (NFAT). Fatigue in bituminous pavement is the phenomenon of cracking that is caused by tensile strains generated in the pavement by traffic loading, temperature variations and construction practices. Fatigue life is the number of load applications to initiate a fatigue crack. The higher the vehicle speed is the lower the fatigue life asphalt mix (NFAT) will be. At the same temperature, AC-WC mixture has lower NFAT than HRS-WC mixture. Dense graded has lower NFAT than gap graded, so dense graded has fatigue resistance lower than gap graded. AC-WC is a dense graded mixture, HRS-WC is a gap grade mixture. Dense graded has lower NFAT than gap graded, so AC-WC has a lower fatigue resistance than HRS-WC. The higher speed (less loading time) will be faster to initiate cracks because asphalt mixtures with less loading time has a high stiffness mixture (Smix), which means it is more rigid. Rigid tends to initiate cracks faster than flexible.

Conclusions
Mechanistic performance of bitumen and hotmix asphalt are influenced by vehicle speed changes sensitively. The higher the vehicle speed is the higher the bitumen stiffness (Sbit) will be. The higher the vehicle speed is the higher the asphalt mix stiffness (Smix) will be, while fatigue life asphalt mix (NFAT) will be lower. At the same temperature, dense graded mixture has a higher value of bitumen stiffness (Sbit) and a lower value of fatigue life asphalt mix (NFAT).