Research On Composition Design Parameters And Method Of Open-graded Large Stone Asphalt Mixes

Asphalt pavement with semi-rigid type base often produces serious early damage, and it strongly reduces the useful time of the road. Therefore, rigid base with the characteristics of high strength and scouring resistance has been paid more and more attention. However, asphalt pavement with rigid base produces reflective cracking usually, which restricts the application of it. In addition, water damage to asphalt pavement is widespread, which strongly reduces the life of it. Open-graded large stone asphalt mixes (OLSM) has the largest aggregate nominal diameter greater than26.5mm and the porosity of it is larger than15%. It is usually used as the lower surface layer of asphalt pavement, in order to delay reflection crack and discharge water. This paper analyzes the performance evaluation method of OLSM, and studies composition design parameters and design method of it. It is significance to further popularize and apply OLSM.The paper designs coarse and fine aggregate gradation of OLSM by numerical calculation method of varying I, and determins key sieve pores of aggregate gradation based on the uniform experimental design. The relations between OLSM performances and volume ratio of coarse and fine aggregate (Rcf) are studied though the tests. The results show that: with the value of Rcf increasing, the porosity of OLSM increases, the strength, water stability and high temperature stability of it decrease, and low-temperature crack resistance and reflection cracking resistance increase first and then decrease. Based on the technical requirements, the reasonable ranges of Rcf and aggregate gradation of OLSM are recommended.To pavement compaction condition for standard, the paper evaluates the applicability of different molding methods. Wheel rolling method, gyratory compaction method and Marshall Compaction method are applicable to OLSM molding, but static pressure method is not suitable for forming OLSM. According to the characteristics of OLSM, the reasonable performance evaluation methods of it are confirmed. In addition, the paper developed the test device of reflection cracking resistance, and proposed the evaluation method and index of it based on the test results. And, action mechanism of reflection cracking resistance is illustrated. In order to provide parameters for the composition design, the relations between OLSM performances and asphalt mortar are studied though the tests. The results show that:with the value of the ratio of filler to bitumen increasing, the porosity of OLSM increases, the strength, water stability, high temperature, low-temperature crack resistance and reflection cracking resistance of it increase first and then decrease. And with the thickness of mortar film increasing, the porosity of OLSM decreases, the strength, high temperature, reflection cracking resistance of it increase first and then decrease, and the water stability and low-temperature crack resistance of it increase. Based on the technical requirements, the reasonable ranges of the mortar film thickness and ratio of filler to bitumen are recommended.By analyzing various calculation models of mineral aggregate surface area, the paper confirms that geometric progression method canaccurately reflect the actual distribution of grain diameter, and establishes the calculation model of mineral aggregate surface area based on the geometric progression method. Through regression analysis, it is found that mechanics index and pavement performance has good correlation,and the determination method of OLSM’s optimum asphalt content is proposed based on the mechanics index method. Collecting the research findings in the paper, composition design system for OLSM based on mortar is put forward.