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Adopting Appropriate Gradation in Design of Bituminous Concrete Mixes at Constant Combined Index determined Corresponding to Actual Aggregate Size

Mohamed Ilyas Anjum


The MoRTH in India specifies gradation tables for preparing bituminous mixes with desirable range of aggregates passing for each aggregate fraction. Conventionally, the mix is designed by adopting midpoint of the specified range of aggregates passing, anticipating a satisfactory mix and avoid several trials required to arrive at the best gradation within the specified range. In this approach, the aggregate shape factors expressed as combined index being the sum of flaky and elongation indices of aggregates in the mix, are determined using conventional instruments. The limitation is that the slot sizes available on instruments are not the same as the sieve sizes in gradation tables. In spite of this, the shape factors determined are still assumed to be approximately the same as those for the actual aggregates sizes, example available is 25–20 mm while size required is 26.5–19 mm. As such, an attempt was made to determine the shape factors for the actual aggregates sizes and ascertain if the midpoint gradation approach is still valid. Bituminous concrete mixes were prepared corresponding to the midpoint (MG) as well as the lower (LG) and higher (HG) limits besides the mid points of lower – mid point (LMG) and higher - mid point (HMG) gradations by using same aggregates. All the other materials and their properties were maintained uniform. The Marshall Test properties of bituminous concrete mix–grade 1, at these five gradations were compared. It was noted that the conventional midpoint gradation does not produce the most satisfactory mix.
Keywords: aggregates shape factors, aggregates gradation, bituminous concrete mix, midpoint gradation

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