Enad Mahmoud

Discrete Element Analysis of the Influences of Aggregate Properties and Internal Structure on Fracture in Asphalt Mixtures

Increased loads on hot-mix asphalt (HMA) pavements have necessitated the development of new-generation asphalt mixture designs that rely on stone-on-stone contact among coarse aggregates in resisting and distributing applied loads. These stone-on-stone contacts, however, promote localized internal stresses that could cause aggregate fracture. Therefore, it is imperative that methods are developed to analyze the contribution of the coarse aggregate structure and properties to HMA performance during mixture design. This paper introduces an approach that combines the discrete element method (DEM) and image processing techniques to analyze the combined effects of aggregate gradation, shape, stiffness, and strength on HMA resistance to fracture. The DEM input parameters were determined based on measuring aggregate and HMA properties. Consequently, the model was used to quantify the internal forces in asphalt mixtures and determine their relationship to aggregate fracture which cannot be accomplished by conventional experimental methods. The model was successful to large extent in representing the influence of variability in aggregate properties and blending two sources of aggregates on mixture strength. The results showed that the required aggregate strength and optimum blending limits to achieve certain mixture strength are dependent on mixture design.

Stone-on-Stone Contact of Permeable Friction Course Mixtures

Stone-on-stone contact of the coarse-aggregate fraction is one of the main characteristics of permeable friction course (PFC) asphalt mixtures that is required to provide adequate resistance to both raveling and permanent deformation. Currently, stone-on-stone contact is determined by comparing the air voids content in the coarse aggregate (VCA), assessed in both the dry-rodded condition (VCA DRC ) and the compacted PFC mixture (VCA mix ). The underlying assumption is that the coarse aggregate of a compacted PFC mixture with VCA mix equal to VCA DRC would develop a stone-on-stone contact condition equivalent to that existing in the dry-rodded aggregate. This study focused on proposing enhancements for the quantitative determination of stone-on-stone contact of PFC mixtures. The assessment supported on both laboratory testing and application of the discrete element method and image analysis techniques, led to recommendation of a criterion to determine the breaking-sieve size. In addition, verification of stone-on-stone contact using a maximum VCA ratio of 0.9 was recommended to ensure the design and construction of PFC mixtures with fully developed stone-on-stone contact.

Comprehensive Evaluation of AIMS Texture, Angularity, and Dimension Measurements

Aggregates are the most widely used construction materials in the world in structures built from both asphaltic and portland cement concrete composites. The performance of these composites is affected by aggregate shape characteristics (e.g., angularity, texture, and dimensions). The aggregate imaging system (AIMS) is a computer automated system that was developed to measure aggregate shape characteristics using digital camera images of aggregates. This paper addresses four issues concerning AIMS measurements: (1) enhanced ways of handling and classifying the large data sets typically generated; (2) enhanced automation in processing fine aggregate images that appear to contain touching particles; (3) an improved consideration of measurement variability or repeatability between different operators, different AIMS units, and for aggregate placement and orientation; and (4) comparison of AIMS dimensional measurements with true three-dimensional measurements using X-ray computed tomography on a large coarse a...

Relationship of Aggregate Microtexture to Asphalt Pavement Skid Resistance

Aggregate properties are one of the important factors that influence the asphalt pavement skid resistance. This paper presents a detailed analysis of aggregate texture and its relationship to pavement skid resistance. A new method is developed for the evaluation of aggregate resistance to polishing. This method relies on the Micro-Deval test as the mechanism for polishing aggregates and the Aggregate Imaging System (AIMS) for quantifying the change in texture due to polishing. The results show that the Micro-Deval test is an effective method for polishing aggregates within a short time. Also, the AIMS texture analysis is able to rapidly and accurately quantify the influence of polishing on texture. The verification of the new method was achieved through measuring the skid resistance of pavements constructed using three different aggregate sources and three different aggregate gradations. The skid resistance was found to be related not only to average aggregate texture, but also to the texture distribution within an aggregate sample. The developed method can be used in models for predicting the change in asphalt pavement skid resistance as a function of aggregate texture, mixture properties, and environmental conditions.

A Probabilistic Model for Predicting the Resistance of Aggregates in Asphalt Mixes to Fracture

ABSTRACT Increased loads on asphalt pavements have lead to the development of asphalt mixes that rely on stone-on-stone contacts in resisting applied loads. These mixtures tend to experience high forces at the contact points that could cause aggregate fracture and make the asphalt pavements more prone to distresses such as cracking and permanent deformation. This paper introduces an approach to express the probability of aggregate fracture in asphalt mixes as a function of the contact bond strength and internal force distribution at the contact points. The internal force distribution was obtained using a discrete element model of the internal structure of asphalt mixtures subjected to loading. The contact bond strength was determined using statistical analysis of the results of a single aggregate crushing test. The developed approach can be used to determine the asphalt mixture design and aggregate properties that minimize fracture and achieve desirable mixture performance.