Effect of Aggregate Gradation on Design Mix Using Modified Bitumen

Abstract

This study evaluates the influence of aggregate gradation and modified binders on the performance characteristics of Dense Bituminous Macadam (DBM) mixes, employing the Marshall mix design method in accordance with ASTM D1559. The investigation compares two aggregate gradation types—dense-graded and open-graded—and two binder types: conventional bitumen (VG-40) and crumb rubber modified bitumen (CRMB-60). A comprehensive suite of laboratory tests was performed, including Marshall Stability, flow value, bulk density, and volumetric properties such as Voids in Mineral Aggregate (VMA), Voids Filled with Bitumen (VFB), and air voids. Additional evaluations included Indirect Tensile Strength (ITS) and the Texas Boiling Test to assess moisture susceptibility. The findings indicate that open-graded mixes exhibit enhanced strength and stability performance relative to dense-graded counterparts. Moreover, CRMB-60 outperformed VG-40 in terms of Marshall Stability, tensile strength, and resistance to moisture-induced stripping. The optimum binder content (OBC) was identified based on an optimal balance of maximum stability, bulk density, 4% air voids, and 80% VFB.Overall, the synergy of an open-graded aggregate structure with CRMB binder resulted in the most durable and high-performing bituminous mix. These results underscore the critical role of aggregate gradation and binder modification in improving pavement performance, suggesting their suitability for high-traffic roadways and long-term infrastructure sustainability

Introduction

Bituminous pavements, especially Dense Bituminous Macadam (DBM), are widely used due to cost-effectiveness, flexibility, and ease of maintenance. Their performance depends mainly on two factors: aggregate gradation and binder type. Aggregate gradation—the distribution of particle sizes—affects structural strength, load distribution, internal friction, and void content, influencing rutting resistance, fatigue life, and durability. Dense gradation ensures good interlocking, while open gradation enhances drainage but increases binder absorption.

Binder type also critically impacts pavement quality. Conventional binders like VG-40 are common but have limitations under extreme conditions. Modified binders such as Crumb Rubber Modified Bitumen (CRMB) improve elasticity, moisture resistance, and durability while supporting sustainable practices. Studies show CRMB enhances Marshall Stability, tensile strength, and reduces moisture damage compared to conventional binders.

The interaction between aggregate gradation and binder influences mixture properties; open-graded mixes often require higher-viscosity or modified binders to prevent binder drain-down and stripping. Performance tests, including the Marshall mix design method, Indirect Tensile Strength, and Texas Boiling Test, help determine optimum binder content and evaluate mechanical and moisture resistance properties.

Experimental results indicate that open-graded mixes with CRMB show superior stability and mechanical performance over dense-graded mixes with conventional binders. Proper aggregate gradation combined with advanced binders like CRMB significantly enhances DBM pavement performance, addressing increasing traffic loads and environmental challenges.

Conclusion

The research highlights that both aggregate gradation and binder type play a crucial role in determining the performance of Dense Bituminous Macadam mixtures. Dense-graded mixtures exhibited superior stability and tensile strength compared to their Open-graded counterparts. Additionally, the incorporation of Crumb Rubber Modified Bitumen (CRMB) significantly improved the mixture\'s strength, moisture resistance, and overall durability when compared to the standard VG-40 binder. The synergy between CRMB and optimized aggregate gradation was found to be the most effective combination for producing high-performance bituminous pavements.

References

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Copyright

Copyright © 2025 Biswajit Das, Krushna Chandra Sethi, Jyoti Prakash Giri. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.