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

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 %

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.

SPECIFIC GRAVITY TEST OF BITUMINOUS MATERIAL

The density of a bituminous binder is a fundamental property frequently used as an aid in classifying the binders for use in paving jobs. In most applications, the bitumen is weighted, but finally in use with aggregate system, the bitumen content is converted in volume basis. Thus an accurate density value is required for conversion of weight to volume. The specific gravity is greatly influenced by the chemical composition of the binder. Increased amounts of aromatic type compounds cause an increase in the specific gravity. The test procedure has been standardized by the ISI.