DETERMINATION OF OPTIMUM BITUMEN CONTENT

Five graphs are plotted with values of bitumen content against the value of

                        i)          Density G_b g/cm3

                        ii)         Marshall stability, S kg

                        iii)         Voids in total mix, V_v %

                        iv)        Flow value, F (0.01 mm units)

                        v)         Voids filled with bitumen, VFB%.

Let the bitumen contents corresponding to maximum density be B₁, corresponding to maximum stability be B₂ and that corresponding to the specified void content V_v (4.0%in the case of dense AC mix) be B₃. Then the optimum bitumen content for mix design is given by:

B₀ = (B₁ + B₂ + B₃) / 3

The values of flow and VFB are found from the graphs, corresponding to bitumen content B₀. All the design values of Marshall Stability, flow, voids and VFB are checked at the optimum bitumen content B₀, with the specified design requirements of the mix.

RELATED FIGURES/GRAPHS ARE GIVEN IN NEXT ARTICLE : LABORATORY TEST RESULT OF MODIFIED BITUMEN

DESIGN REQUIREMENTS OF THE BITUMINOUS MIX

As per IRC: 29-1968, when the specimens are compacted with 50 blows on either face, the design AC mix should fulfill the following requirements of the mix.

i)          Marshall stability value kg (minimum) = 340

ii)         Marshall flow value, 0.25 mm units = 8 to 16

iii)         Voids in total mix, V_v% =3 to 5

iv)        Voids in mineral aggregates filled with bitumen, VFB% =75 to 85

The highest possible Marshall Stability values in the mix should be aimed at consistent with the other three requirements mentioned above. In case the mix designed does not fulfill any and or more of the design requirements, the gradation of the aggregates or filler content or bitumen content or combination of these are altered and the design tests are repeated till all the requirements are simultaneously fulfilled. IRC: 29-1968

DESIGN PROCEDURE OF BITUMINOUS MIXES

In the design procedure of a bituminous mix, following steps are involved.

i) Selection of aggregate to be employed in the mix.

ii) Selection of aggregate grading.

iii) Determination of the proportion of each aggregate required to develop the design grading.

iv) Determination of the specific-gravity of the aggregate blend.

v) Determination S.G. of the bitumen.

vi) Make up trial specimens with varying bitumen contents.

vii) Determination of the specific-gravity (S.G) of each compacted specimen.

viii) Make stability test on the specimen.

ix) Determine the percentage of voids in each paving specimen and if the design method in use requires it, calculate the VMA and percent voids filled with bitumen.

x) Selection of the optimum bitumen content from the obtained data.