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47 General Conclusions The literature review conducted in this study revealed that the shape properties of coarse and fine aggregates used in hot- mix asphalt, hydraulic cement concrete, and unbound base and subbase layers influence the performance of the pavement system in which they are used. Aggregate characteristics can be decomposed to three independent scales: shape, angularity, and texture. Methods currently used for measuring these char- acteristics have several limitations: they are laborious, subjec- tive, lack direct relation with performance parameters, and have limited ability to separate the influence of angularity from that of texture. A number of research studies have shown that aggregates, especially coarse aggregates that exhibit high texture, do not necessarily have high angularity. Consequently, it is important to develop methods that are capable of quantifying each of the aggregate characteristics rather than a manifestation of their interactions. Test methods used for measuring aggregate shape proper- ties were evaluated in this study. The evaluation considered accuracy, repeatability, reproducibility, cost, ease of use, ease of interpretation of the results, readiness of the test for imple- mentation, portability, and applicability for the analysis of different sizes and types of aggregates. Thirteen different coarse aggregate types and five different fine aggregate types were used in this evaluation. Analyses of repeatability and reproducibility results were conducted under the guidelines of ASTM standards E 177, C 802, and C 670. Accuracy of the analysis methods used in the imaging systems was assessed by analyzing some particle projections that have been used by geologists for visual eval- uation of particlesâ shape. Also, all analysis methods were used to analyze images of aggregate particles in order to identify the ability of these methods to accurately rank aggregates and cap- ture unique characteristics of aggregates. The analysis results revealed that some of the available analysis methods are influ- enced by both angularity and shape changes and, therefore, are not suitable to distinguish between these two characteristics. Also, some of the analysis methods do not distinguish between changes in texture and angularity. The following analysis methods are recommended: â¢ Texture: Wavelet analysis of gray images of particle surface (Implemented in AIMS software). â¢ Angularity: The gradient method (implemented in AIMS software) and the changes in the slope of a particle outline (implemented in the UIAIA software). â¢ Shape: Sphericity or the proportions of the three particle dimensions (implemented in MRA, AIMS, and UIAIA). Accuracy of test methods was assessed through statistical analysis of the correlations between the results from test methods and measurements of shape using a digital caliper and visual rankings of surface irregularity and texture by experienced individuals. The Analytical Hierarchy Process (AHP) was implemented in a program to rank the test methods. This process provided flexibility to examine the influence of changes in the importance of the characteristics on the ranking of test methods, and also provided extensive information on the relationship between test methods and desirable characteristics. The Aggregate Imaging System (AIMS) is recommended for measuring the characteristics of both coarse and fine aggregates. It employs methods based on sound scientific concepts for the analysis of shape, angularity, and texture and provides the distribution of each of the characteristics in an aggregate sample. It has very good control of lighting and provides repeatable and reproducible results. The Uni- versity of Illinois Aggregate Image Analyzer (UIAIA) can also be used for measuring the shape, angularity, and texture of coarse aggregates. For measuring the coarse aggregate shape only, the Multiple Ratio Shape Analysis method (MRA) is most desirable, and is much cheaper than all the other test methods. The MRA provides the distribution of shape in an C H A P T E R 4 Conclusions and Suggested Research
aggregate sample, but it cannot be used for measuring angu- larity or texture. A methodology that uses direct measurements of shape (three dimensions), angularity, and texture was developed to classify aggregates based on the distribution of their charac- teristics. It unifies the methods used to measure the charac- teristics of fine and coarse aggregates. The analysis methods are simple, and the results have physical meanings that can be interpreted easily. The classification ranges were found to be similar for the different aggregate sizes. This finding simplified the methodology, as one set of ranges is needed irrespective of aggregate sizes. This classification methodology is valuable for the interpretation of the results and in order to facilitate implementation. The classification results are presented in terms of the dis- tribution of shape properties within an aggregate sample. This feature gives capabilities to (1) explore the influence of differ- ent processes such as crushing and blending on aggregate shape, (2) conduct quality control activities to detect changes in the distribution of any of the aggregate characteristics, (3) relate the distribution of different characteristics to performance, and (4) develop specifications based on the distribution of aggregate characteristics rather than average indices. Applicability and Suggested Research This study provides the pavement community with practical, reliable, and accurate methods for rapidly measuring aggregate characteristics. The recommended methods can be used in the design of pavement layers, in Quality Control (QC) and Qual- ity Assurance (QA) programs, and for problem diagnosis based on understanding of the effects of aggregates on performance of pavement structures. In addition, these methods will help the industry set criteria for providing aggregates with the desired characteristics. The recommended methods can be implemented in the following aspects of pavement engineering: â¢ QA and QC procedures during aggregate production: statistical parameters based on the distribution of aggregate characteristics can be used to detect changes in these charac- teristics and make appropriate adjustments. This will ensure the supply of aggregates with the desired characteristics, thus leading to good performance and cost savings. â¢ Evaluation of crushing methods: crushing methods can be evaluated by measuring aggregate shape characteristics produced using these methods. This evaluation will help identify crushing methods that could produce aggregates with the desirable characteristics. â¢ Evaluation of changes in aggregate texture: the proposed imaging systems can be used to measure the aggregate tex- ture and its change due to polishing which can be indicative of change in frictional characteristics. The following research projects are recommended to enhance the study findings and their implementation: 1. Research to further evaluate the ruggedness of the recom- mended test methods. This work is essential to evaluate the ability of each test method to provide repeatable and reproducible results. 2. Research to develop methods for incorporating aggregate physical properties in the design of pavement materials. Generally, each of the paving mixtures (unbound aggregate layers and layers bound and/or stabilized with asphalt, hydraulic cement or other stabilizing material) does not consider aggregate characteristics, making it difficult to relate aggregate characteristics on performance. Although several studies have evaluated the relationship between physical characteristics and mixture performance, these studies have not developed adaptable specifications that accommodate variations in aggregate characteristics. 3. Research to develop methods for optimizing the design of paving mixtures based on the physical properties of aggre- gates. This study did not specify performance-based limits for shape, angularity, and texture that should be used in the mixtures of the different pavement layers. Such research will provide means to allow highway agencies and the industry to efficiently utilize the available sources of aggregates and reduce construction costs. 48