SGC Tests for Influence of Material Composition on Compaction Characteristic of Asphalt Mixtures
Compaction characteristic of the surface layer asphalt mixture (13-type gradation mixture) was studied using Superpave gyratory compactor (SGC) simulative compaction tests. Based on analysis of densification curve of gyratory compaction, influence rules of the contents of mineral aggregates of all sizes and asphalt on compaction characteristic of asphalt mixtures were obtained. SGC Tests show that, for the mixture with a bigger content of asphalt, its density increases faster, that there is an optimal amount of fine aggregates for optimal compaction and that an appropriate amount of mineral powder will improve workability of mixtures, but overmuch mineral powder will make mixtures dry and hard. Conclusions based on SGC tests can provide basis for how to adjust material composition for improving compaction performance of asphalt mixtures, and for the designed asphalt mixture, its compaction performance can be predicted through these conclusions, which also contributes to the choice of compaction schemes.
Using discrete element models to track movement of coarse aggregates during compaction of asphalt mixture
Abstract The objective of this study is to analyze the movement characteristic of differently-shaped compositions in the Superpave Gyratory Compactor (SGC) test through tracking coarse aggregates and the numerical simulation of Discrete Element Method (DEM). First, the coarse aggregates were classified into five shape types (rounded, fractured, angular, elongated and flat) and scanned by a 3D scanner. Second, seven groups of asphalt mixtures with different combination types of hybrid, 100%fractured, 100%angular, 80%fractured + 20%rounded, 60%fractured + 40%rounded, 80%fractured + 20%elongated, and 80%fractured + 20%flat groups were simulated by Particle Flow Code (PFC) Version 5. Third, numerical simulations were conducted to analyze the SGC test process and the movement paths of differently-shaped coarse aggregates were obtained. Finally, statistical analysis on the results from the modeling test were used to study the movement characteristics of asphalt mixtures with differently-shaped aggregate compositions. Through this study, it was found that: (1) there were three main stages of particle movement in the SGC test; (2) the ratio of vertical displacement was obviously larger than that of horizontal displacement, while the variation of vertical rotation was obviously smaller than that of horizontal rotation in asphalt mixtures with differently-shaped compositions; (3) the rounded, elongated and flat coarse aggregates have greater influence on horizontal displacement compared with vertical displacement, but have adverse effects on particle rotations during the compaction process; (4) the effects of elongated coarse aggregates on particle movement were larger than those of flat coarse aggregates, but the flat coarse aggregates have more influence on particle rotation for the variations of horizontal and vertical rotational angles.
AGGREGATE DEGRADATION IN BITUMINOUS MIXTURES
A LABORATORY STUDY WAS PERFORMED USING A GYRATORY TESTING MACHINE TO DETERMINE THE FACTORS AFFECTING THE DEGRADATION OF AGGREGATE IN BITUMINOUS MIXTURES. THREE KINDS OF AGGREGATES WITH DIFFERENT LOS ANGELES VALUES WERE USED. THE AGGREGATES WERE BLENDED ACCORDING TO THREE DIFFERENT GRADATIONS RANGING FROM AN OPEN GRADATION TO A FULLER MAXIMUM DENSITY GRADATION. FOUR DIFFERENT ASPHALT CONTENTS WERE USED. THE RESULTS OF THE STUDY INDICATED THAT EACH FRACTION OF AGGREGATE DEGRADED IN SUCH A WAY THAT ITS SIEVE ANALYSIS CURVE WAS A SMOOTH CURVE APPROACHING A PARABOLA, WHICH IMPLIED THAT THE PATTERN OF DEGRADATION IS CONSTANT. THE MAGNITUDE OF DEGRADATION, AS MEASURED BY PERCENT INCREASE IN SURFACE AREA, WAS FOUND TO VARY AND TO DEPEND ON THE FOREGOING VARIABLES. GRADATION WAS FOUND TO BE THE MOST IMPORTANT FACTOR AFFECTING DEGRADATION, THE DENSER THE MIX THE LESSER THE DEGRADATION. DEGRADATION ALSO VARIED WITH TYPE OF AGGREGATE. IN GENERAL, AGGREGATES WITH HIGH LOS ANGELES VALUES RESULTED IN MORE DEGRADATION THAN THOSE WITH LOW LOS ANGELES VALUES.