Research on new concepts in asphalt mix design

Road & Transport Research, Jun 2000 by Hao, P, Zhang, D

Abstract

Systematic tests of the performance of asphalt mixtures of different gradations and asphalt content have been carried out. The following characteristics of the asphalt mixture were analysed: (1) hightemperature rutting, (2) resistance to cracking at low temperature, (3) moisture stability, (4) fatigue resistance, and (5) skid resistance. The results of these and previous studies have enabled the authors to establish a system of proportioning the asphalt mixture, which is suitable for application in the various climate areas of China. The recommended optimal proportions of asphalt mixture for each of the climate areas are also presented. Hightemperature rutting is taken as the control factor for climate areas where the yearly extreme low temperature is higher than -21.5 deg C and the average maximum temperature in July is higher than 30 deg C. Low-temperature cracking is considered as the main control factor for climate areas where the yearly extreme low temperature is lower than -21.5 deg C and the average maximum temperature in July is lower than 30 deg C. For the remaining climate areas, both low-temperature cracking and high-temperature rutting of asphalt mixture are taken as control factors.

INTRODUCTION

In most countries the Marshall method is still used to determine asphalt content, after asphalt and gradation type have been selected. However, results show that there is no close relationship between the Marshall stability and flow and the pavement performance; flow sometimes meets the criteria, but the pavement rutting is very deep at high temperature. The Marshall method can not fully reflect the pavement performance of asphalt mixtures, and this is one of reasons that early stresses such as fatigue cracking, permanent deformation and stripping appear on some asphalt pavements (Zhang 1993).

In order to remedy the defects of present specification systems, it was necessary to study the properties of asphalt mixtures and relate these properties to climate type, based on permanent deformation, fatigue cracking, low-temperature cracking and water damage. The relationship between the failure types of asphalt pavements and the asphalt mixtures should be considered in the mix design to ensure these failures will not recur in the asphalt pavement. In this paper, an analysis of the failure types of asphalt pavement, in relation to climate features in China is presented. Thus results offer a new design method for asphalt mixture.

MATERIALS

The properties of the asphalt used in this study are shown in Table 1.

The properties of the limestone aggregate selected am shown in Table 2.

Five gradations were selected from the main mixture types for the study of the asphalt pavement construction specification used in China. They include the upper (A) and middle (B) values of the gradation limits of AC-16-I type; the middle (C), and lower (D) values of AC-16-II type; and the middle values (E) of the gradation limits of the open graded friction course (OGFC), and are shown in Table 3 (Kautz and Sebaaly 1993).

SPECIMEN PREPARATION

The asphalt mixtures tested by the Marshall method to measure the structural parameters and the results are shown in Table 4. According to the determined asphalt content and aggregate gradation, slab specimens were prepared with the rolling wheel compactor. Five cm specimens were used in the rutting test, and 6 cm specimens were sawn into 250 mm prism specimens which were used in the creep test at low temperature and the fatigue test.

PAVEMENT PERFORMANCE OF ASPHALT MIXTURE

Skid resistance test of asphalt mixture

The results of the friction number test for asphalt mixtures are shown in Table 5. Table 5 shows that the friction number of asphalt mixtures increases with increasing air void, for a limited range of air void values.

Marshall retained stability test of asphalt mixtures

The results of the Marshall retained stability test for asphalt mixtures are shown in Table 6.

Table 6 shows that the Marshall retained stability of asphalt mixtures decreases with increasing air void, and reaches the minimum value at the air void of about 10%.

Rutting test of asphalt mixture

The results of the rutting test of asphalt mixtures are shown in Table 7. The dynamic stability of asphalt mixtures approaches the maximum value at an optimum air void of about 8%.

Creep test of asphalt mixtures at 0 deg C

The creep test results of asphalt mixtures in Table 8 show that the creep rate of the mixture increases with decreasing air voids. The higher the creep rate, the greater the deformation resistance, and the better the performance of the mixture at low temperature.

Fatigue test of asphalt mixture at 15 deg C

The fatigue test results of asphalt mixtures are shown in Figure 1.

Figure 1 shows that the life until fatigue gradually increase as the air void decreases; when air void becomes larger, there are many micro-cracks in asphalt mixture so that the time taken for the asphalt mixture to fatigue gets shorter.

NEW CONCEPT OF ASPHALT MIXTURE DESIGN