Effects of Aggregate Gradation on Resilient Modulus and CBR in Unbound Granular Materials

Document Type: Research Paper


Assistant Professor, Faculty of Transportation, University of Isfahan, Isfahan, Iran


Resilient modulus and California Bearing Ratio (CBR) in unbound granular materials are the key technical characteristics of layers in a flexible pavement design. Among the factors affecting these two parameters, the aggregate gradation is the most important. Using particle size distribution curve developed by AASHTO, together with other considerations mentioned in the related regulations have yielded desirable results in many cases. However, many roads loaded by heavy vehicles, for which all technical instructions of standard regulations were observed, have undergone deformations caused by subsidence of layers. According to the related technical documents, one hypothesis could be the proximity of aggregate gradation to the boundary areas. Therefore, the aim of this study was to determine the effect of changes in the scope of aggregation in the border areas on strength parameters. For this purpose, effects of aggregate grading variation on two types of aggregates, i.e. limestone and quartzite (as determined by AASHTO) were investigated using specific gravity, CBR, and resilient modulus tests. The results showed that, in the gradation boundaries determined by AASHTO, the difference between specific gravity values was insignificant. In the CBR and resilient modulus tests, however, there was a significant difference between test results in the upper and lower limits of gradation. In addition, gradation variation had a lower impact on resistance parameters in quartzite aggregate compared to limestone aggregate. Therefore, under special utilization conditions, materials with highest values of technical specifications should be used, since even materials whose technical specifications are in the standard range may not behave as expected in real world situations. 


-AASHTO (1990) "Construction Manual for Highway Construction", Washington, AASHTO

-AASHTO® (1993)" AASHTO Guide for Design of Pavement Structures", Washington, AASHTO

-AASHTO (1993) "Guide Specification for Highway Construction", USA Washington, D.C.

-AASHTO (2003) "Determining the resilient modulus of soils and aggregate materials", 20th ed., Washington, AASHTO

-AASHTO (2014) "Standard Specifications for Transportation Materials and Methods of Sampling and Testing", 34th Edition and AASHTO Provisional Standards, 2014 Edition 4-Guide Specification for Highway Construction, Washington, AASHTO

-Barksdale R. (1991) "The aggregate handbook", USA Washington, D.C.

-Bilodeau, J. P., Dore, G. and Pierre, P. (2008) "Gradation influence on frost susceptibility of base granular materials", International Journal of Pavement Engineering, Vol. 9, No. 6, pp. 397-411.

-Bilodeau, J. P.; Plamondon C. O. and Dore G. (2016) "Estimation of resilient modulus of unbound granular materials used as pavement base: combined effect of grainsize distribution and aggregate source frictional properties", Materials and Structures, Vol. 49, No. 10, PP. 1-11.

-Cunninghama, C. N., Evansb, T. M. and Tayebalib, A. A. (2013) "Gradation effects on the mechanical response of crushed stone aggregate", International Journal of Pavement Engineering, Vol. 14, No. 3, pp. 231–241.

-Ekblad J. (2008) "Statistical evaluation of resilient models characterizing coarse granular materials", Materials and Structures, Vol. 41, No. 3, pp. 509–525.

-Fuller, W. and Thompson, S. (1907) "The laws of proportioning concrete", Trans ASCE, Vol. 59, pp. 67–143.

-Ghabchi, R., Zaman, M., Khoury, N., Kazmee, H. and Solanki, P. (2013) "Effect of gradation and source properties on stability and drainability of aggregate bases: a laboratory and field study", International Journal of Pavement Engineering, Vol. 14, No. 3, pp. 274-290.

-Golalipour, A., Jamshidi, E., Niazi, Y., Afsharikia, Z. and Khadem, M. (2012) "Effect of aggregate gradation on rutting of asphalt pavements", Procedia - Social and Behavioral Sciences, Vol. 53, pp. 440-449.

-Gu, F., Sahin, H., Luo, X., Luo, R. and Lytton, R. (2014) "Estimation of resilient modulus of unbound aggregates using performance-related base course properties", Journal of Materials in Civil Engineering, Vol. 27, No.6.

-Hamidi, A., Azini, E. and Masoudi, B. (2012) "Impact of gradation on the shear strength-dilation behavior of well graded sand-gravel mixtures", Scientia Iranica Transactions A: Civil Engineering, Vol. 19 No. 3, pp.. 393-402.

-Huang, Y. (2004) "Pavement analysis and design", second Ed. ed., U.S.A: Pearson Education Inc.

-Jiang, Y., Wong, L. N. Y. and Ren, J. (2015) “A numerical test method of California bearing ratio on graded crushed rocks using particle flow modeling”,  Journal of Traffic and Transportation Engineering (English Edition), Vol. 2, No. 2, pp. 107–115

-Kim, D. and Kim, J. R. (2007) "Resilient behavior of compacted subgrade soils under the repeated triaxial test", Construction and Building Materials, Vol. 21, No. 7, pp. 1470-1479.

-Lavin, P. (2003) "A practical guide to design, production and maintenance for engineers and architects", London and New York: Spon Press.

-Lekarp F. and Isacsson U.  (2001) "The effects of grading scale on repeated load triaxial test results", International Journal of Pavement Engineering, Vol. 2, No. 2, pp. 85-101.

-Pan, T. and Tutumluer, E. (2005) "Imaging based evaluation of coarse aggregate size and shape properties affecting pavement performance", In Geotechnical Special Publication, Austin, TX; United States.

-Sang and Kooh Company (2015) "Sang and Kooh Company’s technical report of projects of south of Iran".

-TRB (2013) "NCHRP Synthesis 445, Practices for Unbound Aggregate Pavement Layers", USA Washington, D.C.

-Uthus L. (2007) "Deformation properties of unbound granular materials", Doctoral dissertation, Norwegian University of Science and Technology.

-Xiao, Y. and Tutumluer, E. (2016) "Gradation and packing characteristics affecting stability of granular materials: Aggregate Imaging-Based Discrete Element Modeling Approach", International Journal of Geomechanics, Vol. 17, No. 3, http://dx.doi.org/10.1061/(ASCE) GM.1943-5622.0000735.

-Xiao, Y., Tutumluer, E., Qian, Y. and  Siekmeier, J. (2012) "Gradation effects influencing mechanical properties of aggregate base-granular subbase materials in Minnesota", Transportation Research Record: Journal of the Transportation Research Board, No. 2267, PP. 14-26.

-Yildirim, B. and Gunaydin, O. (2011) “Estimation of California bearing ratio by using soft computing systems”, Expert Systems with Applications, Vol. 38, N0. 5, pp. 6381-6391.