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

DENSITY AND VOIDS ANALYSIS OF BITUMINOUS MIX SPECIMEN

Soon after the compacted bituminous mix specimens have cooled to room temperature, and then the weight, average thickness and diameter of the specimen are noted. The specimens are also weighed in air and then in water. The bulk density value G_b of the specimen is calculated from the weight and volume. The voids analyses are made as given below:

 

Where,

            V_v = air voids in the mix, %

                        V_b = volume of bitumen

                        VMA = voids in mineral aggregates, %

                        VFB = voids filled with 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.

MARSHALL METHOD OF BITUMINOUS MIXES

Bruce Marshall, formerly Bituminous Engineer with Mississippi State Highway Department, USA formulated Marshall Method for designing bituminous mixes. Marshall’s test procedure was later modified and improved upon by U.S. Corps of Engineer through their extensive research and correlation studies. ASTM and other agencies have standardized the test procedure. Generally, this stability test is applicable to hot-mix design of bitumen and aggregates with maximum size 2.5 cm. In Bangladesh, bituminous concrete mix is commonly designed by Marshall Method.

In this method, the resistance to plastic deformation of cylindrical specimen of bituminous mixer is measured when the same is loaded at the periphery at a rate of 5 cm per minute. The test procedure is used in the design and evaluation of bituminous paving mixes. The test is extensively used in routine test programmers for the paving jobs. There are two major features of the Marshall method of designing mixes namely,

i) Density-voids analysis
ii) Stability-flow test.

The stability of the mix is defined as a maximum load carried by a compacted specimen at a standard test temperature of 60⁰C. The flow is measured as the deformation in units of 0.25mm between no load and maximum load carried by the specimen during stability test. In this test an attempt is made to obtain optimum binder content for the aggregate mix type and traffic intensity.