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be seen from Table D2, on the horizontal cross section, thepercent variation ranges between 434% and 1188%; and on thevertical cross section, it varies between 255% and 1298%.From a statistical point of view, this wide variation greatlyincreases the value of the standard deviation. It could be oflarge positive value when considering large aggregates, and oflarge negative value if small aggregates are considered. It is

this writers belief that limits should be established for an ac-ceptable degree of variation beyond which the method ofanalysis should be altered to accommodate all of the parame-ters that may impact the outcome. The writer realizes that theseeffects are less pronounced in other types of higher qualitymixtures (e.g., HL4 and HL8); however, caution should befollowed if these test results are to be generalized.

Digital image processing for aggregate

orientation in asphalt concrete mixtures:1 Reply

Zhong Qi Yue and Isabelle Morin

The authors thank Professor Ghaly for his contribution to thediscussion of this paper and appreciate the kind remarks madeby him.

We chose the 2 mm dimension as the minimum size of ag-gregates for the aggregate orientation analysis based on thefollowing facts: The 2 mm dimension is the dividing size between sand and

gravel (or coarse aggregates) in particle size distributionanalysis using the classical sieve analysis method.

For the asphalt concrete (AC) mixtures analyzed in the pa-per, sieve analysis results indicated that aggregates withsizes greater than 2 mm occupy about 60%, 66%, 75%, and76% of the total aggregate dry weights in HL4, HL8, SMA,

and LSM, respectively (see Table 2b of the paper). Thecoarse aggregates dominate the microstructure of the aggre-gate distribution in these mixtures.We would also like to point out that the 2 mm dimension

is not the minimum particle size that can be identified usingthe digital image processing procedure described in the paper.For surfacing AC mixtures with finer aggregates such as HL1,the criterion dimension would be less than 2 mm and can bedefined using the results of sieve analysis in mix design.

The multiple vertical or horizontal plane cross sections ofAC samples were formed by cutting. This cutting process wasnon-recoverable and destructive. An AC laboratory sample orfield core was only used to form plane cross sections in onedirection (i.e., either vertical or horizontal direction). In other

words, there was no one single aggregate that could be cut inboth horizontal and vertical AC cross sections simultaneously.The aggregates used in the analysis satisfied the 2 mm size

criterion in both the vertical and horizontal AC cross sections.It should be kept in mind that the aggregates on the horizontalAC cross sections were not the same aggregates on the verticalAC cross sections. The validation of the approach adopted inthe paper is due to the following arguments: For AC samples or cores formed using the same mix design

formula and the same compaction methods, their propertiesand microstructure should be very similar even though theyare not exactly the same.

Thousands of aggregate particles on either the horizontal orthe vertical AC cross sections satisfied the size criterion andwere used in the statistical analysis of aggregate orientationin AC mixtures.

Three image dimensions, i.e., minor length, Feret diameter,and major length, were used in the statistical analysis andcompared well with the sieve analysis results.The authors agree with Professor Ghalys observations that

the standard deviations of aggregate particle size distributionare very high and that few large aggregates can have a highpercentage of the total area occupied by aggregates on ACcross sections. These observations were the basis that we com-pared the results of image area gradation on the horizontal ACcross sections with those on the vertical cross sections inFig. 6 of the paper. For instance, the comparison of the imagearea gradation for LSM indicated that aggregate particle sizeson the horizontal AC cross sections were greater than those onthe vertical AC cross sections. Such image area gradation takes

into account the influence of large aggregates into the statisti-cal analysis. The average area per aggregate is one of the sta-tistical parameters used as indexes to summarize the aggregateparticle size differences on the vertical and horizontal ACcross sections. Results of the average area per aggregate indexwere consistent with those of the image area gradationcomparisons.

Received November 6, 1996.Manuscript accepted November 15, 1996.

Z.Q. Yue. Halcrow Asia Partnership Ltd., 3201 Central Plaza,18 Harbour Road, Wanchai, Hong Kong.I. Morin. Canadian Hydraulics Centre, National ResearchCouncil Canada, Ottawa, ON K1A 0R6, Canada.

1 Discussion by Ashraf M. Ghaly. 1997. Canadian Journal ofCivil Engineering, 24(2): 333334.

Can. J. Civ. Eng. 24: 334 (1997)

Can. J. Civ. Eng. Vol. 24, 1997334

1997 NRC Canada

http://www.nrc.ca/cisti/journals/cjce/cjce24/civico97.pdfhttp://www.nrc.ca/cisti/journals/cjce/cjce24/civico97.pdf