Bituminous Mix Design by Marshall Method

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Bituminous Mix Design by Marshall Method

Theory and Scope:

Bituminous mixes are used in the surface course of road and airfield pavements. The desirable bituminous mix properties include stability, density, durability, flexibility, resistance to skidding and workability during construction. Stability is defined as resistance of the paving mix to deformation under load and is thus a stress level which causes strain depending upon anticipated field conditions. Stability is function of friction and cohesion. Durability is defined as the resistance of the mix against weathering which causes hardening and this depends upon loss of volatiles and oxidation.

In this method the resistance to plastic deformation of cylindrical specimen of bituminous mixture is measured when the same is loaded at the periphery at 5 cm per minute. This test procedure is used in designing and evaluating bituminous paving mixes. ASTM vide designation D 1559-62 T has standardized the test procedure.

Aim:

To determine optimum binder content of given bituminous mix by Marshall method of Mix Design.

Apparatus:

Mould Assembly, Sample Extractor, Compaction Pedestal and Hammer, Breaking Head, Loading Machine, flow meter, thermometers, water bath and oven.

Procedure:

The coarse aggregates, fine aggregates and mineral filler material should be proportioned and mixed in such a way that final mix after blending has the gradation within the specified range.
Approximately 1200 grams of aggregates and filler are taken and heated to a temperature of 175° to 190° C.
The compaction mould assembly and rammer are cleaned and kept pre-heated to a temperature of 100°C to 145°C. The bitumen is heated to temperature of 121 ° to 138° C and the required quantity of first trail percentage of bitumen is added to the heated aggregate and thoroughly mixed using a mechanical mixer or by hand mixing with trowel.
Then the mix is heated and a temperature of 150° to 160°C is maintained and then the mix is
transferred into the pre-heated mould and compacted by giving seventy five blows on each side.
The specific gravity values of different aggregates, filler and bitumen used are determined first.
The theoretical specific gravity of the mix is determined.
Soon after the compacted bituminous mix specimens have cooled to room temperature, the weight, average thickness and diameter of the specimen are noted. The specimens are weighed in air and then in water.
The bulk density value of the specimen if calculated from weight and volume.
Then the specimen to be tested are kept immersed under water in a thermostatically controlled water bath maintained at 60° ± 1 ° C for 30 to 40 minutes.
The specimens are taken out one by one, placed in the Marshall test head and the Marshall stability value and flow value 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.
Five graphs are plotted with values of bitumen content against the values of density, Marshall Stability, Voids in total mix, Flow value, Voids filled by Bitumen.
Let the bitumen contents corresponding to maximum density be B 1, corresponding to maximum stability be B2 and that corresponding to the specified voids content (at 4.0%) be B3. Then the optimum bitumen content for mix design is given by: Bo= (B1+ B2+ B3) / 3