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.

OBJECTS OF BITUMINOUS PAVING MIX DESIGN

According to Asphalt institute design manual, the over all objective for the design of the bituminous paving mixes, to determine an economical blend and grading of aggregates and a corresponding bitumen content which would yield a mix having the following characteristics.

• Mix should exhibit sufficient stability to satisfy the service requirements of the pavement and the traffic conditions, without undue displacements.

• It should have sufficient bitumen content to ensure a durable pavement by coating the aggregates and bonding them together and also by water-proofing the mix.

• Bitumen mix should have sufficient voids in the total compacted mix as to provide a reservoir space for a slight amount of additional compaction due to traffic to avoid flushing, bleeding and loss of stability.

• It should have sufficient flexibility even in the coldest season to prevent cracking due to repeated application of traffic loads.

• The mix should have sufficient workability while placing and compacting the mix. 

• The mix should be the most economical one that would produce a stable, durable and skid resistant pavement.

BITUMEN-RUBBER

An experimental pavement bound with a bitumen-rubber mixture was laid in Holland in 1929. The first use of this binder in the United States was made in 1947 when a section was laid Akren, Ohio. The pavement was conventional, except that finely divided rubber amounting to 5 – 7.5 % of the bitumen by weight was included since that time experimental roads have been laid by (among others) the state highway departments of Virginia, Ohio, Texas, Massachusetts, California, Colorado, and Utah the cities of New York and Baltimore and the great Britain. In addition, bituminous binders modified with rubber have used for seal coats.

Certain advantages are attributed to rubber additives. For example, skid tests in Virginia, reported in 1950, showed very little improvement in coefficients of friction on newly laid pavements but considerable advantage after six months early test by the Bureau of public roads indicated both favorable and unfavorable results. Rubber added in powdered form brought unfavorable consequence, when preblended with the asphalt, it improved the stability of some but not all laboratory specimens. Recently, greater elasticity reduces temperature susceptibility and brittleness and longer life in the pavement have been claimed. In sum however the conclusions of a 1954 analysis by the Bureau of public roads still appears to be valid. It stands than an appraisal of the real economic value of the addition of rubber to asphalt must wait on further observation of the behavior of experimental pavements under the influence of age, weather and traffic.