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3Problem Statement and Research Objectives Hot-mix asphalt (HMA) mixtures are complex materials composed of mineral aggregates and asphalt binder. Because about 95 percent by weight of the HMA mixture is aggregate, the coarse and fine aggregate properties influence pavement performance significantly. Studies have shown that HMA pavement rutting and stripping can be directly related to improper selection and use of aggregates (1). Tests and associated criteria used by highway agencies to select aggregate for HMA mixtures are empirical. Often, they have not been related to pavement performance directly. Aggregate tests that provide clearer relationships with per- formance will provide better means for evaluating and select- ing aggregates. The completed NCHRP Project 4-19, âAggregate Tests Related to Asphalt Concrete Performance in Pavements,â rec- ommended a set of performance-related aggregate tests for evaluating aggregates for use in HMA pavements. Pavement performance indicators assumed to be related to these labora- tory aggregate tests were permanent deformation because of traffic loading (both with and without stripping), fatigue crack- ing, and surface defects (e.g., raveling, popouts, and potholes). The performance relationships were developed based on labo- ratory tests with the Superpave Shear Tester (SST) and the Geor- gia Loaded Wheel Tester (GLWT); however, these relationships were not validated with prototype-scale traffic tests; the NCHRP 4-19 researchers recommended additional research as shown in Table 1. The objective of the current research was to use accel- erated pavement testing techniques to perform the rutting, fatigue, and moisture susceptibility validation experiments identified in NCHRP Project 4-19.Analysis was directed toward developing a descriptive ranking of each aggregate test indicat- ing how well it related to HMA performance. Also, appropriate tests for given combinations of climatic conditions, materials, and traffic loads have been suggested. Scope of Study Research was conducted to document the ability of aggregate tests identified in NCHRP Project 4-19 to predict in-service performance of HMA pavements.Relationships between aggre- gate properties and HMA pavement performance were evalu- ated in full-scale accelerated loading conditions. Individual aggregate tests, as well as combinations of tests that related to HMA performance, were identified. Recommendations for use in HMA aggregate selection and mixture design procedures have been provided. Specifically, a practical set of performance- related aggregate tests has been recommended for inclusion in HMA mixture design systems. Future research to determine the ruggedness, precision, and bias of the test methods has been suggested. Research Approach The research was performed in two phases. Phase I included review of NCHRP Report 405 and other relevant lit- erature and the development of a research plan. Phase II included execution of the research plan established in Phase I and preparation of the final project report. Relating results of the aggregate tests shown in Table 1 to the HMA distresses of rutting, moisture susceptibility, and fatigue served as the basis for the validation experiments. The research was conducted according to the plan, shown in Figure 1, which involved aggregate testing, identification of HMA mixture designs, and HMA mixture testing using accel- erated pavement tests. The accelerated testing was completed in three series, each relating to one of three HMA distresses noted above. Aggregate Testing Aggregates were characterized using tests listed in Table 2. These include the tests identified in NCHRP Project 4-19, C H A P T E R 1 Introduction and Research Approach
4Experiment Validation Experiment Performance 1 Uncompacted Void Content of Coarse Aggregate and Flat or Elongated Particles (2:1 ratio) in Coarse Aggregate Rutting and Fatigue 2 Uncompacted Void Content of Fine Aggregate Rutting and Fatigue 3 Methylene Blue Test of Fine Aggregate Moisture Susceptibility 4 Particle Size Analysis and Methylene Blue of p0.075 Material Rutting 5 Micro-Deval and Magnesium Sulfate Soundness Tests Durability/ Toughness Table 1. Validation experiments recommended by Kandhal and Parker [1]. Step 3: Accelerated Pavement Tests Step 1: Selection and Collection of Materials Step 2: Aggregate Tests and HMA Mixture Designs Step 3a: Rutting Tests Step 3b: Moisture Susceptibility Tests Step 3c: Fatigue Tests Step 4: Aggregate Test Evaluations Step 5: Analysis and Report Preparation Coarse Aggregate Tests: Uncompacted Void Content, Flat or Elongated Particles, Micro-Deval, LA Abrasion, Percent Fractured, Soundness, Deleterious Material Fine Aggregate Tests: Fine Aggregate Angularity, Methylene Blue, D60, D10, Sand Equivalent, Micro-Deval, Soundness HMA Mixture Designs: Ndesign = 100, Nmax = 160 Coarse Aggregates: Dolomite, Limestone, Gravel, Granite, Traprock Fine Aggregates: Two Natural Sands, Crushed Gravel Sand, Granite, Traprock Rutting performance evaluated by INDOT/Purdue APT, dry condition, 50ËC test temperature Moisture susceptibility performance evaluated by INDOT/Purdue APT, wet condition, 46ËC test temperature Fatigue performance evaluated by INDOT/Purdue APT, dry condition, 10ËC HMA plant stockpile aggregates evaluated for gradation, flat or elongated, uncompacted void content of fine aggregate, methylene blue value, D60, D10 Aggregate from plant mixtures and test section cores evaluated for gradation, flat or elongated, uncompacted void content of fine aggregate Statistical analysis, data interpretation, conclusions and recommendations Figure 1. Research work flowchart.
the aggregate tests specified by Superpave criteria, Uncom- pacted Void Content of Coarse Aggregate, Method B (AASHTO TP 56), and Virginia Test Method for Determin- ing Percent Voids in Fine Aggregates (VTM5) tests. Accelerated Testing Experiments Rutting Experiment The rutting experiment design is shown in Table 3. Coarse aggregate, fine aggregate, and particles smaller than the 0.075- mm sieve (p0.075) were evaluated for their effect on HMA rutting performance. For coarse-graded mixtures, a natural sand was used in combination with various coarse aggregates. For fine-graded mixtures, an uncrushed gravel was used as the coarse aggregate in combination with various fine aggregates. The effects of coarse and fine aggregate on HMA mixture performance were studied by constructing and testing 11 test sections in the Accelerated Pavement Tester (APT). Five of the test sections were coarse-graded HMA mixtures with grada- tions plotting below the maximum density line (MDL). The other six sections were fine-graded HMA mixtures with gra- dations plotting along or above the MDL. Moisture Susceptibility Experiment Five fine-graded HMA mixtures were used to investigate rela- tionships between moisture susceptibility and fine aggregate properties as shown in Table 4.Performance as affected by mois- ture damage was assessed by the amount of rutting observed in the HMA mixtures. The AASHTO T 283 test was also per- formed on cores extracted from the APT test lanes before accel- erated pavement testing. Stripping after traffic was also noted 5 Test Method Recommended by Kandhal and Parker Superpave Requirement Additional Test Sieve Analysis for Determining Gradation and Size (AASHTO T11 & T27) X X Uncompacted Void Content of Coarse Aggregate, Method A (AASHTO TP56) X Uncompacted Void Content of Coarse Aggregate, Method B (AASHTO TP56) X Flat or Elongated Particles in Coarse Aggregate (ASTM D4791) X (2:1) X (5:1) X (3:1) Flat and Elongated Particles in Coarse Aggregate (ASTM D4791) X (3:1, 5:1) Uncompacted Void Content of Fine Aggregate, Method A (ASTM C1252) X X Uncompacted Void Content of Fine Aggregate, Method B (ASTM C1252) X Virginia Test Method for Determining Percent Voids in Fine Aggregates (VTM5) X Methylene Blue Test for Fine Aggregate (AASHTO TP57) X Particle Size Analysis of p0.075 Materials for Determining D60, D30, and D10 Sizes X Methylene Blue Test for p0.075 Material (AASHTO TP57) X Micro-Deval Test (AASHTO TP58) X Magnesium Sulfate Soundness Test (AASHTO T104) X Clay Content by Sand Equivalent (AASHTO T176) X Clay Lumps and Friable Particle (AASHTO T112) X Percent Fractured Particles in Coarse Aggregate (ASTM D5821) X Los Angeles Abrasion Test (ASTM C96) X Specific Gravity and Absorption of Aggregate (AASHTO T84 and T85) X Table 2. Aggregate characterization tests.
visually.After construction and before testing,water was pooled on the test lanes for 2 days with the pavement heating system turned on. During APT testing, the test sections were kept wet by adding water to the pavement surface. Fatigue Experiment Relationships between fatigue cracking and coarse and fine aggregate properties were evaluated through the construction and testing of six APT sections as indicated in Table 5. These mixtures were selected from the 11 mixtures used in the rutting experiment based on performance. Fatigue performance was characterized by percent fatigue cracking in the wheel path. Analysis Methods Experiments were designed to test the hypotheses that there are relationships between aggregate tests (properties) and HMA performance when full-scale accelerated loading is applied. Analysis of variance (ANOVA) was used to develop a descriptive ranking indicating how well each test related to performance. Subsequently, multivariable regression analyses were conducted to investigate whether a single test or a com- bination of aggregate tests best predicted HMA performance. This process provided a rational basis for recommending aggregate tests related to HMA performance. The relative effect of traffic was determined through analysis of the APT test results. For example,APT traffic repetitions and rutting at different levels of one or more material characteristics (e.g., fine aggregate angularity) were plotted. An example of these relationships is shown in Figure 2. Alternatively, a level of distress could be selected (e.g., 10-mm rut depth) at which to compare the effect of the aggregate characteristic on perform- ance. Figure 3 shows a relationship for FAA and a 10-mm rut depth distress level. This type of analysis was used to determine the aggregate characteristicâs sensitivity to traffic level and also provide a method of developing traffic-related input for regres- sion analysis targeted at determining the performance of HMA pavements at different traffic levels with the individual or mul- tiple aggregate properties as input factors. 6 AggregateAggregate Performance Test Category Mix Coarse Fine CA-1 Dolomite (IN) CA-2 Limestone (IN) CA-3 Granite (NC) CA-4 Gravel (IN) Coarse Aggregate Test Methods Evaluation (Coarse-graded Mixtures) CA-5 Traprock (VA) Natural Sand A (IN) FA-1 Dolomite (IN) FA-2 Granite (NC) FA-3 Traprock (VA) FA-4 Crushed Gravel Sand (IN) FA-5 Natural Sand A (IN) Fine Aggregate Test Methods Evaluation (Fine-graded Mixtures) FA-6 Gravel (IN) Natural Sand B (OH) Table 3. Rutting experiment design. Aggregate Aggregate Performance Test Category Mix Coarse Fine FAM-1 Granite (NC) FAM-2 Traprock (VA) FAM-3 Crushed Gravel Sand (IN) FAM-4 Natural Sand A (IN) Fine Aggregate Test Methods Evaluation (Fine-graded Mixtures) FAM-5 Dolomite (IN) Natural Sand B (OH) Table 4. Moisture susceptibility experiment design. AggregateAggregate Performance Test Category Coarse Fine CA-2 (Limestone) CA-3 (Uncrushed Gravel) Coarse Aggregate Test Methods Evaluation (Coarse-graded Mixtures) CA-4 (Granite) Natural Sand A (IN) FA-1 (Natural Sand A) FA-3 (Natural Sand B) Fine Aggregate Test Methods Evaluation (Fine-graded Mixtures) Uncrushed Gravel (IN) FA-4 (Granite) Table 5. Fatigue experiment design.
70 5 10 15 20 25 0 1000 2000 3000 4000 5000 6000 Load Repetitions (N) R u t D ep th (m m ) FAA = 39, N10 = 1000 FAA = 43, N10 = 2450 FAA = 47, N10 = 5000 Figure 2. Effect of FAA on rut depth. 0 1000 2000 3000 4000 5000 6000 35 40 45 50 FAA Lo ad R ep et iti on s to 1 0m m R ut D ep th Figure 3. Effect of UVA on traffic.
