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

MARSHALL STABILITY AND FLOW VALUES OF BITUMINOUS MIX SPECIMEN

The specimens to be tested are kept immersed under water in a thermostatically controlled water bath maintained at 60⁰ ± 1⁰C for 30 to 40 minutes. The specimens are taken out one by one, placed in the Marshall test head and the Marshall stability value (maximum load carried in kg. before failure) and the flow value (the deformation the specimen undergoes during loading up to the maximum load in 0.01 mm units) are noted. The corrected Marshall stability value of each specimen is determined by applying the appropriate correction factor, if the average height of the specimen is not exactly 63.5 mm. The correction factors are given in Table.

Correction Factors for Stability and Flow Value

Volume of Specimen  in Cubic Centimeters	Approximate Thickness of Specimen in mm	Correction Factors
457-470	57.1	1.19
471-482	58.7	1.14
483-495	60.3	1.09
496-508	61.9	1.04
509-522	63.5	1.00
523-535	65.1	0.96
536-546	66.7	0.93
547-559	68.3	0.89
560-573	69.9	0.86