The Performance Improvement of Hot Mix Asphalt and Enhanced Applicability to Pavement Structure Thickness Design Using DAMA (Drain Asphalt Modified Additive) Jung-hoon, Jin* * University of Incheon ITS. R.C. Researcher 3. Laboratory Test Methods and Results 3.6 Layer coefficient(ai) of hot asphalt mixture materials The relative stiffness index of domestic asphalt mixing materials had been using the value suggested by 1961 AASHTO temporarily since 1960's. Then, AASHTO was revised in 1972, and Korea has been using the average value of the relative stiffness index of four states in the U.S.A. since 1988, for their similarity of weather and temperature to Korea. The layer coefficient(ai) of the asphalt layer used in Korea is shown in table 13. ¢Æ Table 13 Layer coefficient(ai) of HMA materials in Korea(Before 2000.8) When measuring the relative stiffness index of the asphalt surface layer, Korea uses two methods using the laboratory test results. (1) AASHTO's deriving equation for layer coefficient The measurement of the layer coefficient of the asphalt mixture according to 1986 AASHTO differentiates the surface layer material, pitch/tar supporting layer, and the estimated values for each layer are shown in a table. In order to derive the layer coefficient of asphalt mixture surface layer from resilient modulus, Ullidtz suggests the following Equation (3). Eq. (3) Here, MR(resilient modulus) : MPa, 0.079 ¡´ ai ¡´0.173 1 Mpa = 10.197 kg/§² ai = layer coefficient 1986 AASHTO guideline shows this design method in a figure as shown in figure 24. Figure 24 Layer coefficient of using resilient modulus (2) Domestic deriving equation for layer coefficient The following Equation (4) is derived from resilient modulus by Korea Highway Corporation. Eq. (4) Here, E1 : Resilient modulus ai : relative stiffness index