TRANSPORTATION RESEARCH RECORD 1615 Paper No. 98-0134 65

The performance of asphalt concrete mixtures is influenced by the prop-erties of the included aggregates, such as grading, shape (angularity andelongation), and texture (roughness). Complete and accurate quantifica-tion of aggregate properties is essential for understanding their influenceon asphalt concrete and for selecting aggregates to produce high-qualitypaving mixtures. Recent developments in the use of digital image analy-sis techniques for quantifying aggregate morphological characteristics inasphalt concrete are summarized. Image morphological characteristicswere used to quantify flatness and elongation of coarse aggregates, toestimate the proportion of natural sand in fine aggregates, and to corre-late aggregate characteristics with engineering properties of asphalt con-crete mixtures. Image analysis of sections also revealed informationabout the grading, shape, and orientation of coarse aggregates in a mix-ture. An overview is presented of the broad range of useful pavementengineering applications of this relatively new approach for evaluatingaggregate characteristics.

Approximately 85 percent of the total volume of asphalt concretemixtures consists of aggregate. It is not surprising that the perfor-mance of asphalt concrete mixtures is influenced by the propertiesof their aggregate blends, such as gradation, shape (angularity andelongation), and texture (roughness). In asphalt concrete, numerousstudies (1–5) have related the gradation, shape, and texture of aggre-gate to durability, workability, shear resistance, tensile strength,stiffness, fatigue response, rutting susceptibility, and optimum bin-der content of the mixtures. In recognition of the importance ofaggregate properties on pavement performance, limits on flat andelongated particles or the amount of natural sand typically are incor-porated into specifications. However, often there is a lack of con-sistency between the aggregate specifications and the ability tomeasure all the desired properties of aggregates (6). For example,the most common test methods for evaluating aggregate angularityand surface texture are indirect measures at best. Proper selectionand evaluation of aggregate properties will remain necessary to produce high-quality asphalt concrete mixtures, particularly as traf-fic and loads increase. Quantification of aggregate properties withrational, objective characterization methods is desirable.

Digital image analysis is a powerful computer-based method forgathering information and has been an important tool in manydiverse fields. Recently, it has been applied in a variety of newresearch and practice areas in civil engineering. With the aid of amodern image analysis system, numerous attributes (e.g., area,length, perimeter, orientation) of each individual feature (particle)in an image can be measured almost instantly, which makes digitalimage analysis excellent for evaluating aggregate properties. This

paper summarizes some encouraging results of using digital imageanalysis in evaluating aggregate properties for asphalt concretemixtures.

The work reported was performed at the U.S. Army EngineerWaterways Experiment Station (WES) by using a Princeton Gamma-Tech (PGT) image analysis system, although the measurementsdescribed can be measured easily with other systems.

MORPHOLOGICAL STUDY OF COARSE AGGREGATES

In general, cubical, not flat, thin, or elongated, coarse aggregate par-ticles are preferred. Flat and elongated particles tend to cause prob-lems with compaction, particle breakage, loss of strength, andsegregation. Construction specifications usually include limits onflat and elongated particles. The proportion of flat and elongated par-ticles in a coarse aggregate sample can be determined by followingASTM D4791, using a proportional caliper device. These manualmeasurements using the caliper device are tedious and are rarelyused on a daily basis for quality control of aggregates on construc-tion sites. Kuo et al. (7) suggested the use of image analysis tech-niques for characterizing the morphological characteristics of coarseaggregate particles. In the proposed method, aggregate particles areattached to adhesive clear plastic trays with two perpendicular facesso that the sample trays can be rotated 90 degrees to establish twoorthogonal planes of measurement (Figure 1). The aggregates (sam-ple trays) are placed on a light box, which illuminates the samplesand makes definite contrast between particles and background. Theparameters of length, width, and thickness are obtained by measur-ing the two orthogonal planes. These parameters provide a directmethod for determining the flatness and elongation of the particles(Figure 2). Kuo et al. (7) demonstrated that the image analysismethod could yield results comparable with those made by usingmanual measurements.

This image analysis method was more time-efficient than isASTM D4791 and it provided more information. For example, theuse of ASTM D4791 necessitates the summary of particle shapeinformation in terms of the proportion of particles that exceeds apredetermined ratio of either length to width or width to thickness.In contrast, the image analysis method accounts for the magnitudeof these ratios for each aggregate particle. Therefore, the imageanalysis method results for flatness and elongation can be summa-rized as a cumulative probability distribution, as shown in Figure 2.The image analysis method therefore would permit the developmentof specifications that control the location and uniformity of theseprobability distributions, similar to common specification criteriafor aggregate grading.

Image Analysis Evaluation of Aggregatesfor Asphalt Concrete Mixtures

CHUN-YI KUO AND REED B. FREEMAN

C.-Y. Kuo, Airfields and Pavements Division, Waterways Experiment Sta-tion, 3909 Halls Ferry Road, Vicksburg, MS 39180. R.B. Freeman, Depart-ment of Civil Engineering, Montana State University, 205 Cobleigh Hall,Bozeman, MT 59717.

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In addition to the information on flatness and elongation, the imageanalysis method provides other shape indices that can be related tothe effects of aggregates on the properties of asphalt concrete. As anexample, Kuo et al. (8) quantified the imaging morphological char-acteristics of several coarse aggregates that had been used in an inves-tigation at WES (9) concerning the permanent deformation of asphaltconcrete mixtures. In the study, coarse aggregates had been charac-terized with some indirect tests such as the particle index test (ASTM

D3398), modified ASTM C1252 flow test (adapted for coarse aggre-gates), and the unit weight and voids in aggregate test (ASTM C29).The influences of aggregate grading, particle shape, and texture onrutting potential of asphalt mixtures were analyzed by conductingMarshall stability and flow, indirect tensile strength, direct shear, andconfined repeated-load deformation tests on mixtures containing 18different aggregate blends.

Kuo et al. (8) determined the imaging morphological characteris-tics of the coarse aggregates used in the WES study, including elon-gation, flatness, sphericity, shape factor, form factor, and roughness(Table 1). Statistical parameters such as mean, standard deviation,and half width (error) were determined. The half width (error), halfof the confidence interval, indicates the precision of the measure-ment at 95 percent confidence level. The results of multiple variable,general linear regression analyses indicated that these imaging char-acteristics correlated well with the indirect aggregate characteriza-tion methods and with the asphalt concrete mixture properties.Figure 3 shows the results of linear regression analyses betweenvoid contents from the indirect characterization tests and the siximaging morphological indices. Recall that the premise for each ofthe indirect characterization techniques is that angular, elongated,and rough-textured particles are more difficult to compact and there-fore tend to produce higher void contents, compared with a similargrading of smooth and rounded particles.

In the original WES study (9), the confined repeated-load de-formation (CRLD) test showed promise in relating to field perfor-mance of asphalt concrete mixtures. Marshall-sized asphalt sampleswere confined in a triaxial cell with 276 kPa (40 lb/in.2) air pressureand were subjected to a dynamic axial deviator stress of 1380 kPa(200 lb/in.2). The deviator stress was applied at 1 Hz for 60 min andthe mixtures were compared for permanent axial strain. Figure 4shows the results of a linear regression analysis between perma-nent strain from the CRLD test and the aggregate morphologicalcharacteristics.

MORPHOLOGICAL STUDY OF FINE AGGREGATES

The properties of asphalt concrete also are affected by the charac-teristics of fine aggregates. For example, excessive proportions ofnatural rounded sand is a common cause of premature rutting. Tominimize this problem, the Corps of Engineers specifies limits onnatural sand contents in pavement construction specifications(10,11). The use of the term natural sand has fostered ambiguity andhas placed all natural sources of fine aggregate into a single classi-fication, although their morphological characteristics may be verydifferent. The use of the term natural sand in specifications also hasprevented any quality assurance testing because after aggregates areblended in an asphalt concrete mixture, the natural sand cannot beseparated reliably from the crushed fine aggregates.

Imaging indexes could be used to quantify the characteristics offine aggregates, eliminating the need to specify a maximum propor-tion of natural sand. For example, smooth, round natural sands willhave a form factor (imaging index) close to 1, and manufacturedsands with angular and rough edges will have smaller form factors.The imaging index form factor is defined as

4π × area filledPerimeter2

FIGURE 1 Image analysis measurements for morphologicalcharacteristics of coarse aggregates: sample tray (top), longestdimension (middle), intermediate and shortest dimensions(bottom).

FIGURE 2 Cumulative distribution of flatness and elongationfor crushed Alabama limestone (19 mm to 12.7 mm).

TABLE 1 Results from Image Aggregate Morphological Analysis

FIGURE 3 Correlation of void contents with imagingmorphological indexes.

FIGURE 4 Correlation of permanent strain with aggregatemorphological characteristics.

Kuo and Freeman Paper No. 98-0134 67

where area filled is the area of particle with holes (if any) filled.Figure 5 shows the cumulative distributions of form factor forcrushed Alabama limestone and natural concrete sand. For crushedAlabama limestone, about 50 percent of the particles have a formfactor value smaller than 0.8 and few have a form factor valuelarger than 0.9. In contrast, for natural concrete sand, about 50 per-

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cent of the particles have a form factor value larger than 0.9 andfew have a form factor value smaller than 0.8. Similar to the cumu-lative probability distribution for flatness and elongation, thecumulative probability distribution for form factor offers a newdimension for specification criteria. Instead of specifying fineaggregate angularity with a single-valued representation, such asthat which would be provided by uncompacted voids (ASTMC1252), angularity criteria can address the location and uniformityof a cumulative distribution plot. Again, this is similar to criteriacommonly applied to aggregate grading.

In a preliminary exercise using the imaging index form factor,excellent linear relationships were obtained between the averageform factor and the percent of natural sand in aggregate mixes (Fig-ure 6). Such relationships could be used to estimate the natural sandcontent in aggregates extracted from asphalt concrete during pave-ment construction. By measuring the average form factor of the

extracted fine aggregates, the sand content for pertinent sieve sizescan be estimated by using the regressions established before con-struction. This information can be combined with the particle sizedistribution to estimate the natural sand content of the asphalt con-crete, as shown in Table 2.

Figure 7 shows the comparison of target and estimated sandcontents for eight aggregate blends with natural sand contentsranging from 0 percent to 30 percent. In general, the image analy-sis method provides estimates within 3 percent error for high sandcontent (≥15 percent) asphalt concrete, which are those of primaryconcern for pavement engineers. The image analysis method isquick and simple enough to be used as a quality assurance tool andis appropriate for forensic investigations when high sand contentsare suspected.

GRADING, SHAPE, AND ORIENTATION OF COARSE AGGREGATES IN ASPHALTCONCRETE MIXTURES

Digital image analysis has been used to study the structure ofasphalt concrete specimens quantitatively (12–14). Yue et al. (12)showed that the grading, shape, and orientation of coarse aggre-gates (≥2 mm) in asphalt concrete can be measured by using imageanalysis. However, the efficiency of digital image analyses by Yueet al. suffered from the need to manually mark the boundaries ofcoarse aggregate on the digitized images, because of insufficientcontrast between aggregates and asphalt matrix (background). Thisseparation process was reported to require about 30 min per crosssection. Motivated by the interesting results of Yue et al., a studywas conducted at WES. The objectives of this study included thefollowing:

1. Increase the efficiency of digital image analysis by improvingcontrasts between aggregates and asphalt matrix, thus eliminatingthe need to outline aggregates manually.

2. Examine the appropriateness of using imaging gradation toapproximate the gradation of coarse aggregates in asphalt concrete.

3. Determine the appropriate sample size of cross sections to beexamined to characterize an asphalt concrete mixture.

It was found that the contrast between coarse aggregates andasphalt matrix could be enhanced greatly by painting the dark coarseaggregates on the cross sections white before image analysis. Thisprocess required only a few minutes of labor for each section.

Asphalt concrete specimens (65 mm tall, 102 mm diameter) wereproduced with the U.S. Army Corps of Engineers’ gyratory testmachine. The specimens were cut into either three vertical sectionsor three horizontal sections with exterior faces removed. Both sidesof each section were used for image analysis. It was found that withtwo specimens (one cut horizontally and the other cut vertically) and12 images examined, the gradation of coarse aggregates could beestimated, as shown in Figure 8. The area equivalent diameter usedin the figure is the diameter of a fictitious circle that has the samearea as the aggregate. Area equivalent diameter is defined as

As expected, results from horizontal and vertical sections were dif-ferent. A comparison of the image gradation indicates that coarse

4 × area filledπ

FIGURE 5 Cumulative distribution of form factor for crushedAlabama limestone and natural concrete sand smaller than 0.6 mm (No. 30) sieve.

FIGURE 6 Correlation of natural sand content with averageform factor.

Kuo and Freeman Paper No. 98-0134 69

aggregates on horizontal sections tended to have a larger area (size)than those on vertical sections. The aggregate particles tended to ori-ent horizontally during gyratory compaction. Nevertheless, the aver-age of horizontal and vertical results shows good agreement with thesieve analysis. When fewer sections were examined, the results werefound to be less accurate. Several aggregate blends were included in this study and the changes in coarse aggregate grading in asphaltconcrete mixtures were distinguished successfully.

During standard feature analyses, the PGT image analysis sys-tem obtains 12 directed diameters for each particle. The directeddiameters are the projections of the particle on 12 lines that are eachseparated by 15 degrees. The orientation of the particle is deter-mined by taking the ratio of each directed diameter to its perpen-dicular directed diameter. The position of the largest ratio gives the orientation. Figure 9 illustrates that the orientation of coarseaggregates on the horizontal cross sections were distributed rela-

TABLE 2 Natural Sand Content of Asphalt Concrete Estimated from Form Factor

FIGURE 7 Comparison of target and estimated natural sand contents.

FIGURE 8 Comparison of image gradations and sieve analysisof coarse aggregates.

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flatness or elongation (or both) that is greater than a specifiedvalue. Moreover, the image analysis method also provides someother shape indexes, which can be used to quantify aggregate characteristics.

The cumulative distribution of form factor provides a promisingalternative for the characterization of shape and texture of fine aggre-gates. In contrast to the single-valued results of the particle index(ASTM D3398) or uncompacted void content (ASTM C1252), thecumulative distribution of form factor can be used to develop speci-fication criteria for location and uniformity in a manner similar to thatused for aggregate grading.

These preliminary results are encouraging. They indicate thatimaging technologies have major potential for evaluating someaggregate properties for engineering analysis, and these technologiesmerit further work and consideration.

ACKNOWLEDGMENTS

The support of the U.S. Army Corps of Engineers WaterwaysExperiment Station is sincerely acknowledged. The valuable com-ments of Dr. Larry N. Lynch and William P. Grogan of WaterwaysExperiment Station are appreciated.

REFERENCES

1. Benson, F. J. Effects of Aggregate Size, Shape, and Surface Texture onthe Properties of Bituminous Mixtures—A Literature Survey. In SpecialReport 109,HRB, National Research Council, Washington, D.C., 1970,pp. 12–22.

2. Brown, E. R., J. L. McRae, and A. B. Crawley. Effect of Aggregates onPerformance of Bituminous Concrete. Implication of Aggregates in theDesign, Construction, and Performance of Flexible Pavement.ASTMSTP 1016 (H. G. Schrvevders and C. R. Marek, eds.). American Societyfor Testing and Materials, Philadelphia, Pa., 1989, pp. 34–63.

3. Hargett, E. R. Effects of Size, Surface Texture, and Shape of Aggre-gate Particles on the Properties of Bituminous Mixtures. In SpecialReport 109,HRB, National Research Council, Washington, D.C.,1970, pp. 25–26.

4. Karakouzian, M., M. R. Dunning, R. L. Dunning, and J. D. Stegeman.Performance of Hot Mix Asphalt Using Coarse and Skip Graded Aggre-gates. Journal of Materials in Civil Engineering,Vol. 8, No. 2, May,1996, pp. 101–107.

5. Sanders, C. A., and E. L. Dukatz. Evaluation of Percent Fracture of HotMix Asphalt Gravels in Indiana. Effects of Aggregates and Mineral Fillerson Asphalt Mixture Performance.ASTM STP 1147 (R. C. Meininger,ed.). American Society for Testing and Materials, Philadelphia, Pa., 1992,pp. 90–103.

6. Rollings, M. P., and R. S. Rollings. Geotechnical Materials in Con-struction.McGraw-Hill Companies, Inc. New York, 1996.

7. Kuo, C.-Y., J. D. Frost, J. S. Lai, and L. B. Wang. Three-DimensionalImage Analysis of Aggregate Particles From Orthogonal Projections. InTransportation Research Record 1526,TRB, National Research Council,Washington, D.C., 1996, pp. 98–103.

8. Kuo, C.-Y., R. S. Rollings, and L. N. Lynch. Morphological Study ofCoarse Aggregates Using Image Analysis. Journal of Materials in CivilEngineering,1998 (in press).

9. Ahlrich, R. C. Influence of Aggregate Gradation and Particle Shape/Texture on Permanent Deformation of Hot Mix Asphalt Pavements.Technical Report GL-96-1, U.S. Army Engineer Waterways Experi-ment Station, Vicksburg, Miss., 1996.

10. U. S. Army Corps of Engineers. Bituminous Paving for Roads, Streets,and Open Storage Areas.Guide Specification for Military Construction(CEGS) Section 02551, Department of the Army, 1989.

11. U. S. Army Corps of Engineers. Asphaltic Bituminous Heavy-DutyPavement (Central-Plant Hot Mix).Guide Specification for

tively uniformly across all possible orientations. The orientationsof coarse aggregates on vertical cross sections, however, showstronger anisotropy with most of the aggregates oriented in therange of (0°, 15°) and (150°, 165°) (i.e., in the nearly horizontaldirection). For a complete random orientation of aggregates, thearea percentage would be the same for all the 12 orientation rangeswith a percentage of 8.33. The coefficient of variation (CV) of areapercentages, using 8.33 percent as the mean, provides a measure ofthe degree of anisotropy. In this respect, aggregates on vertical sections exhibited approximately twice the anisotropy of those onhorizontal sections, with CVs of 0.60 and 0.29 respectively. Thetendency for the aggregates to orient horizontally appears to be acharacteristics imposed during gyratory compaction.

CONCLUSIONS

The image analysis techniques described in this study are simple andhave practical value. As demonstrated, modern image analysis tech-niques can be used successfully to quantify aggregate morphologi-cal characteristics. These characteristics can be quantified in arational, objective manner that could lead to a better understandingof their influence on asphalt concrete mixtures and could improveaggregate selection for pavement construction.

Image analysis techniques can be used to determine the amountof flat and elongated particles in coarse aggregates or to estimate thenatural sand content in fine aggregates. These parameters are impor-tant for quality control and quality assurance. The characterizationof an asphalt concrete structure (i.e., the grading and orientation ofcoarse aggregates) is important for understanding the effects ofcompaction methods and related phenomena. As illustrated, aggre-gate orientation distribution leads to a measure of the degree ofanisotropy, which allows for comparisons between field compactionand lab compaction techniques.

The proposed image analysis methods provide more informa-tion on aggregate morphological properties than the available test methods. For example, the complete cumulative distributionof flatness and elongation of coarse aggregates can be obtained,but ASTM D4791 yields only the proportion of particles that have

FIGURE 9 Comparison of orientations of coarse aggregates onhorizontal and vertical sections.

Kuo and Freeman Paper No. 98-0134 71

Military Construction (CEGS) Section 02556, Department of theArmy, 1991.

12. Yue, Z. Q., W. Bekking, and I. Morin. Application of Digital Image Pro-cessing to Quantitative Study of Asphalt Concrete Microstructure. InTransportation Research Record 1492,TRB, National Research Council,Washington, D.C., 1995, pp. 53–60.

13. Harvey, J., K. Eriksen, J. Sousa, and C. L. Monismith. Effects of Labora-tory Specimen Preparation on Aggregate-Asphalt Structure, Air-voidContent Measurement, and Repetitive Simple Shear Test Results. InTransportation Research Record 1454,TRB, National Research Council,Washington, D.C., 1995, pp. 113–122

14. Eriksen, K., and V. Wegan. Optical Methods for the Evaluation ofAsphalt Concrete and Polymer-Modified Bituminous Binders.Note 244.Danish Road Institute, 1993.

The material presented herein represents the views of the authors and doesnot necessarily reflect any official view or policy. This article is publishedwith the permission of the Chief of Engineers.

Publication of this paper sponsored by Committee on Applications of Emerg-ing Technology.