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.

MIX DESIGN METHOD OF BITUMINOUS MIXES

Various methods of designing the mix proportions of dense bitumen concrete have been evolved. The most widely used methods are the following:

i) The Marshall method.
ii) The Hubbard-field method.
iii) The Hveem method.
iv) The Smith triaxial method