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.

DURABILITY OR RESISTANCE TO WEATHERING OF BITUMINOUS MATERIAL

To perform its function satisfactorily as binder, paving bitumen must remain plastic. But when bitumen is exposed to weather in a thin film it loses quite a lot of its plasticity and becomes brittle. This happens because of certain chemical and physical changes. This phenomenon of natural deterioration is called weathering. Progressive weathering may ultimately result in the formation of fine cracks which go on widening with time. Engineer in charge should adopt suitable ways and means to minimize the effect of weathering. Rate of curing of the liquid bitumen (cut back) is affected by following factors:

1. Evaporation rate of the solvent or volatility.
2. Amount of solvent in the cut-back. 
3. Penetration of the bitumen used as base.