Theory of Marshall Mix design, Apparatus and Uses
The test procedure is used in designing and evaluating bituminous paving mixes, and is widely applied in routine test programs for the paving jobs. The major features of Marshall method of designing mixes are to determine the two important properties of strength and flexibility.
Strength is measured in terms of the ‘Marshall’s Stability’ of the mix which is defined as the maximum load carried by a compacted specimen at a standard test temperature of 60 oC. This is a temperature that is the weakest condition for a bituminous pavement in use. The flexibility is measured in terms of the ‘flow value’ which is measured by the change in diameter of the sample in the direction of load application in the interval of the start of loading and the time of maximum load. In this test we can mad attempt that is to obtain optimum binder content for the aggregate mix type and traffic intensity.
Objective of Marshall Mix Design
- To determine the density – voids analysis for the given bituminous mixture;
- To determine the strength (Marshall’s stability value) and flexibility (flow value) for the given bituminous mixture;
- To determine the suitability of the bituminous mixture to meet the specified criteria for the surface course.
Apparatus of Marshall Mix design
Following are the apparatus used for the Marshall stability test:
- Specimen mould Assembly comprising mould cylinders 10 cm diameter by 7.5 cm height, base plate and extension collars. They are designed to be interchanged with either end of cylindrical mould. Three mould cylinders are recommended.
- Specimen extractor for extracting the compacted specimen from the mould. A suitable bar is required to transfer load from the extension collar to upper proving ring attachment while extracting the specimen.
- Compaction hammer having a flat circular tamping face 4.5 kg sliding weight constructed to provide a free fall of 45 cm. two compaction hammers are recommended.
- Compaction Pedestal consisting of a 20 x 20 X 45 cm wooden block capped with a 30 x 30 x 2.5 cm MS plate to hold the mould assembly in position during compaction. There is use of a Mould holder consisting of spring tension device designed to hold compaction mould in place on the compaction pedestal.
- Breaking head. The breaking head consist of upper and lower cylindrical segments or test heads having an inside radius curvature of 5 cm. the lower segment is mounted on a base having two perpendicular guide rods which facilitate insertion in the holes of upper test segment.
- Loading Machine. Loading Machine is provided with a gear system to lift the upward direction. Precalibrated proving ring of 5 tonne capacity is fixed on the upper end of the machine, specimen contained in the test head is placed in between the base and the proving ring. The loading jack produces a uniform vertical movement of 5 cm per minute. Machine is capable of reversing its movement downward also. This facilities adequate space for placing test head system after one specimen has been tested.
- Flow meter. The flow meter consists of guide sieve and a gauge. The activating pin of the gauge is arranged in such a way that it can slide inside the guide sleeve with a slight amount of frictional resistance.
Least count of 0.025 mm is adequate. The flow value refers to the total vertical upward movement from the initial position at zero load to value at maximum load. The dial guage at the flow meter should be able to measure a accurately the total vertical movement upward.
In addition to the above the following general equipments are also required:
- Ovens or hot plates
- Mixing apparatus
- Water bath
- Thermometers of range upto 200 oC with sensitivity of 2.5 oC and
- Miscellaneous equipment like containers, mixing and handling tools etc.
Preparation of test specimen
- Measure out 1200 g of aggregates blended in the desired proportions. Heat the aggregates in the oven the mixing temperature.
- Add bitumen at the mixing temperature to produce viscosity of 170± centistokes at various percentage both above and below the expected optimum content.
- Mix the materials in a heated pan with heated mixing tools.
- Return the mixture to the oven and reheat it to the compacting temperature ( to produce viscosity of 280 ± 30 centistokes).
- Place the mixture in a heated Marshall mould with a collar and base. Spade the mixture around the sides of the mould. Place filter papers under the sample and on top of the sample.
- Place the mould in the Marshall compaction pedestal.
- Compact the material with 50 blows of the hammer (or as specified), invert the sample, and compact the other face with the same number of blows.
- After compaction, invert the mould. With the collar on the bottom, remove the base and extract the sample by pushing it out the extractor.
- Allow the sample to stand for a few hours to cool.
- Obtain the sample’s mass in air and submerged, to measure density of specimen, so as to allow, calculation of the voids properties.
- At least three specimen for each combination of aggregates and bitumen should be prepared.
- For surface courses with 12mm aggregate, the expected optimum bitumen content may be about 6.5%. Therefore specimen should be made at 5.5%, 6.0%, 7.0% bitumen content.
- Specimens are heated to 60◦ ±1◦ (37.8◦ ± 1◦ for specimens in which tar has been used in place of bitumen), either in a water bath for 30-40 minutes or in an oven for a minimum of two hours.
- Remove the specimen from water bath (or oven) and place in the lower segment of the breaking head. Then place the upper segment of the breaking head on the specimen and place the complete assembly in position on the testing machine.
- Place the flow meter over one of the post and adjust it to read zero.
- Alpply a load at a rate of 50 mm per minute until the maximum load reading is obtained.
- Record the maximum load reading in Newtons (N). At the same instant obtain the flow as recorded on the flow meter in units of mm.
Record of observation
Mass of aggregates in mixing pan = 1200g
Mass of bitumen added = g
Bitumen content = %
Mixing temperature-aggregate = ◦C
Mixing temperature – bitumen = ◦C
Compacting temperature = ◦C
Number of blows with hammer per face =
Mass of specimen in air ‘Wa’ = g
Mass submerged ‘Ww’ = g
Volume of specimen ‘V’ = cm3
Computation of Results
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