Uncompacted Void Research Articles

Comparison of Fine Aggregate Uncompacted Voids Content Measured by the ASTM C 1252 and the VTM-5 Devices

Abstract As part of the National Cooperative Highway Research Program Project 4–19(2), a series of uncompacted voids tests was performed using the ASTM Test Methods for Uncompacted Void Content of Fine Aggregates, C 1252, and the Virginia Department of Transportation VTM-5 test devices. Comparisons between the uncompacted voids measured by the two devices were made by using five different measurement methods on each of six different fine aggregates. The effects of test procedure and/or device, and their interactions with particle size on the measurement of uncompacted voids were also examined. As expected, the findings indicate that the graded sample of ASTM C 1252, Method A produces a significantly lower result for uncompacted void content than do the other four methods. However, the five test methods are significantly correlated. Furthermore, while there appears to be an interaction between test device and particle size, this effect is negated when the graded sample of ASTM C 1252, Method A is used.

Comparison and Evaluation of Tests for Coarse Aggregate Particle Shape, Angularity, and Surface Texture

Abstract Particle shape, angularity, and surface texture are critical properties in assessing aggregate usage for asphalt concrete. Fractured and flat and/or elongated particles are used in most specifications to assure quality. The particle index and uncompacted void tests indirectly measure particle shape, angularity, and surface texture and offer alternative options. Several methods were used to test crushed gravels and crushed stones. Test results were compared and evaluated. Fractured and flat and/or elongated particle tests are somewhat subjective for evaluating aggregate structural quality. The basis for determining percentages (mass or particle count) for fractured faces did not make any difference. However, the basis (mass or particle count) for computing percentages of particles with particular dimensional ratios (flat and/or elongated) should be clearly stated in specifications since the results are distinctly different. Procedures in ASTM D 4791 contain two methods for assessing particle shape, i.e., flat or elongated particles and flat and elongated particles. Percentages of flat and elongated are larger than flat or elongated. Particle index (ASTM D 3398) and uncompacted void (proposed) tests provide comparable measures of aggregate particle shape, angularity, and surface texture. They are somewhat more analytical and precise than the “fractured particle” and “flat and/or elongated” tests. Flat and/or elongated particles have a contrary influence on the assessment of aggregate quality with particle index and uncompacted voids. Flat and/or elongated particles are generally thought undesirable, but increase both particle index and uncompacted voids, which is generally thought desirable.

Uncompacted Voids and Particle Index Tests for Evaluating Coarse Aggregate

Coarse-aggregate angularity and surface texture are properties specified under the SuperpaveTM mix design system. To control specification requirements, the fractured-face count test is suggested. The use of a more objective test was investigated. Uncompacted voids and index of aggregate particle shape and texture tests were studied to measure coarse-aggregate angularity. The gradation of the sample has an obvious influence on both test results. Ways to separate the effect of gradation from aggregate angularity were studied. Viable options include testing a standard graded sample and computing a weighted average from tests of individual size fractions based on the gradation of the blend. When comparing aggregate sources, either option produces a comparable evaluation, but testing standard graded samples saves time. On the other hand, the weighted average is more appropriate for evaluating an aggregate blend. The index of aggregate particle shape and texture test and the uncompacted voids test provide comparable measures of coarse aggregate angularity and surface texture.

Imaging Indices for Quantification of Shape, Angularity, and Surface Texture of Aggregates

Three image indices are presented that characterize aggregates in a hierarchical scheme involving overall shape (or form), roundness of corners, and particle surface texture. The corresponding dimensionless image indices are called aspect ratio, angularity, and roughness, respectively. Calculating these indices requires that image-analysis techniques be used to find the following data for a sample of two-dimensional particle images: shortest and longest dimensions, perimeters, convex perimeters, and perimeters of ellipses with areas equivalent to those of the associated particles. To demonstrate the usefulness of these three dimensionless indices, selected sieve-size fractions for 25 fine-aggregate sources were subjected to image-analysis testing and to uncompacted void testing (ASTM C1252). Both uncompacted void contents and image-analysis indices could generally differentiate between natural sands and crushed fine aggregates. However, the image-analysis indices provided additional useful information. For example, the image-analysis results identified that two sources of crushed fine aggregates had relatively low uncompacted void contents because they had relatively smooth surface textures or relatively low aspect ratios. In addition, the image-analysis results were able to distinguish morphological differences between aggregate sources that were measured to have similar uncompacted void contents.

Uncompacted Void Content of Fine Aggregate as Quality Indicator of Materials Used in Superpave Mixtures

The uncompacted void content of fine aggregate, or fine aggregate angularity (FAA), was introduced in the Superpave mixture design system to screen smooth or rounded fine aggregates that may result in mixtures with low rutting resistance. The assumption is that fine aggregates with lower FAA values have lower shear strength (internal friction) and lower resistance to rutting. Continued implementation and evaluation of the Superpave system has led to numerous questions regarding the validity of this assumption and of the use of the FAA test in general. Nine fine aggregates were used to evaluate the FAA test and to determine whether it was a reliable indicator of fine aggregate shear strength. FAA tests were performed using three gradations and the three standard test methods (A, B, and C). Microscopic analysis was conducted to obtain independent visual measures of angularity and texture. Direct shear tests were performed at four confining stresses to determine shear strength parameters. Results indicated that FAA values were related to visual measures of texture and angularity. However, although FAA value contributed to shear strength, it appeared that other factors such as toughness, gradation, and packing characteristics of the fine aggregate overshadow its effect. For the fine aggregates tested, the FAA test rejected aggregates with high shear strength and accepted aggregates with low shear strength. It was concluded that FAA values alone may not be adequate to assess shear resistance of fine aggregate. Direct shear strength may be a better parameter, but additional work is required to evaluate its validity and feasibility.

Tests for Evaluating Fine Aggregate Particle Shape, Angularity, and Surface Texture

Particle shape, angularity, and surface texture are critical properties in assessing fine aggregate quality for asphalt concrete. The index for aggregate particle shape and texture and uncompacted voids tests indirectly measure these properties. Several sources of fine aggregate from Alabama were evaluated by both tests. Modifications to current test procedures to include all particle sizes normally included in fine aggregate are suggested. Particle index and uncompacted voids are affected by gradation, and this effect should be eliminated in assessing fine aggregate quality. Testing of individual size fractions and computation of composite indexes or void contents or testing of samples with standard gradations are viable options for elimination of gradation effects. Particle indexes or uncompacted voids of blends of two aggregates may be assessed by linear proportioning of values for individual components.

Morphological Study of Coarse Aggregates Using Image Analysis

A newly developed digital image-analysis method was used to characterize the morphological characteristics of coarse aggregates. Regression analyses found that the imaging morphological characteristics of coarse aggregates correlated well with results of other indirect characterization tests such as the test method for index of particle shape and texture (ASTM D 3398); the test method for uncompacted void content of coarse aggregate as influenced by particle shape, surface texture, and grading (modified ASTM C 1252); and the test method for unit weight and voids in aggregate test (ASTM C 29). The imaging morphological characteristics of coarse aggregates also correlated well to the effects of coarse-aggregate properties on hot-mix asphalt concrete mixtures. This research offers a new approach for evaluating coarse-aggregate characteristics of asphalt-concrete mixtures and demonstrates use of an emerging technology for pavement engineering.

Influence of Aggregate Properties on Performance of Heavy-Duty Hot-Mix Asphalt Pavements

Because approximately 85 percent of the total volume of hot-mix asphalt (HMA) mixtures consists of aggregates, the performance of HMA mixtures is greatly affected and influenced by properties of the aggregate blend. The angularity, shape, and texture of the aggregate particles have a significant effect on the performance of HMA mixtures by controlling the mixture's strength and rutting resistance. Rough, angular aggregates have been proved to produce higher-quality HMA pavements than smooth, round aggregates. Current aggregate tests are primarily based on experience and empirical characterization tests. A study was conducted to evaluate test methods that could be used to characterize aggregate properties that are related to HMA rutting potential of heavy-duty pavements. Specifically, FAA aggregate properties and aircraft loading conditions were addressed. The aggregate particles were characterized with the particle index (ASTM D3398), uncompacted void content for fine aggregate (ASTM C1252), modified ASTM C1252 for coarse aggregate, and unit weight and voids in aggregate (ASTM C29). The HMA mixtures were evaluated for rutting potential using the confined repeated load deformation (dynamic creep) test. The laboratory investigation indicated that the tests for particle index, uncom-pacted void content for fine and coarse aggregates, and unit weight and voids in aggregate could be used to characterize the shape and texture of aggregate particles. The study also indicated that the confined creep test could differentiate between HMA mixtures with different aggregate properties in terms of their rutting potential.

Influence of Aggregate Properties on Performance of Heavy-Duty Hot-Mix Asphalt Pavements

Because approximately 85 percent of the total volume of hot-mix asphalt (HMA) mixtures consists of aggregates, the performance of HMA mixtures is greatly affected and influenced by properties of the aggregate blend. The angularity, shape, and texture of the aggregate particles have a significant effect on the performance of HMA mixtures by controlling the mixture's strength and rutting resistance. Rough, angular aggregates have been proved to produce higher-quality HMA pavements than smooth, round aggregates. Current aggregate tests are primarily based on experience and empirical characterization tests. A study was conducted to evaluate test methods that could be used to characterize aggregate properties that are related to HMA rutting potential of heavy-duty pavements. Specifically, FAA aggregate properties and aircraft loading conditions were addressed. The aggregate particles were characterized with the particle index (ASTM D3398), uncompacted void content for fine aggregate (ASTM C1252), modified ASTM...