The Marshall stability test and flow test on bitumen were developed by the Mississippi State Highway Department. This test is used to design hot mix bituminous concrete for paving jobs using aggregates with a maximum size of 2.5 cm. The Marshall Method is commonly used to design the bituminous concrete mix.
The test is performed by compacting a specimen at a standard temperature of 60°C and measuring the maximum load that the specimen can carry. The stability of the mix is defined by this maximum load. The flow of the specimen is also measured by determining the deformation in units of 0.25 mm or 0.1 mm between no load and maximum load.
The objective of the test is to determine the optimum binder content for the aggregate mix type and traffic intensity. By drawing the Marshall Stability vs. % bitumen graph, engineers can determine the most suitable binder content for a particular mix type and traffic intensity. The Marshall stability test and flow test on bitumen are extensively used in routine test programmes for paving jobs.
Test Procedure of Marshall Stability Test – Flow Test on Bitumen
The Marshall Stability test apparatus is composed of several components. The first is the specimen mould assembly, which includes mould cylinders measuring 10.16 cm in diameter and 6.35 cm in height, as well as a base plate and extension collars. The second component is the specimen extractor, which is used to extract the compacted specimen from the mould. A bar is needed to transfer the load from the extension collar to the upper proving ring attachment during this process.
The third component is the compaction hammer, which has a flat circular tamping face and a 4.5 kg sliding weight designed to provide a free fall of 45 cm. The fourth component is the compaction pedestal, which consists of a wooden block measuring 20x20x45 cm and capped with a 30x30x2.5 cm MS plate. The mould holder is a spring tension device that keeps the compaction mould in place on the compaction pedestal during compaction.
The breaking head is the fifth component, consisting of upper and lower cylindrical segments or test heads with an inside radius curvature of 5 cm. The longer segment is mounted on a base with two perpendicular guide rods to facilitate insertion in the holes of the upper test segment. The sixth component is the loading machine, which has a gear system that lifts the upward direction. A pre-calibrated proving ring with a 5-tonne capacity is fixed on the upper end of the machine. The load jack produces a uniform vertical moment of 5 cm per minute and is capable of reversing its moment downward to allow for adequate space for placing the test head system after one specimen has been tested.
The seventh and final component is the flow meter, which consists of a guide, sieve, and gauge. The activating pin of the gauge slides inside the guide sleeve with a slight amount of frictional resistance, and the 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 the value at maximum load. The dial gauge of the flow meter should be able to accurately measure the total vertical moment upward.
In addition to these components, the test also requires general equipment such as an oven or hot plate, water bath, thermometers with a range of up to 200°C and a sensitivity of 2.5°C, and miscellaneous equipment like containers, mixing and handling tools.
Preparation of Test Specimen
To prepare asphalt mixtures, 1200 grams of aggregates are blended in the desired proportions and then heated in an oven to the mixing temperature. At this temperature, bitumen is added to achieve a viscosity of 170 ± centi-stokes at different percentages. The materials are mixed in a heated pan using heated mixing tools.
Next, the mixture is returned to the oven and reheated to the compacting temperature, which produces a viscosity of 280 ± 30 centi-stokes. Once the desired viscosity is achieved, the mixture is placed in a heated Marshall mould with a collar and base. A filter paper is put under and on top of the sample, which is then spaded around the sides of the mould.
After that, the mould is put on the Marshall compaction pedestal, and the material is compacted with 50 blows of the hammer (or as specified) in one face. The sample is then inverted and compacted on the other face with the same number of blows. Once the compaction is complete, the mould is inverted, with the collar on the bottom, and the base is removed to extract the sample by pushing it out of the extractor.
The extracted sample is allowed to cool for a few hours, and then its mass is measured in air and when submerged to calculate the density of the specimen. This measurement allows the void properties of the asphalt mixture to be calculated.
| Sample no | Weight in air(g) | Weight in water(g) | Specific gravity |
| 1 | |||
| 2 | |||
| 3 |
| Sample | Softening point(0C) |
| 1 | |
| 2 |
Test Procedure
The procedure for testing specimens involves heating them to a temperature of 60 ±1oC. This can be done either in a water bath for a period of 30-40 minutes or in an oven for a minimum of 2 hours.
Once heated, the specimens are removed from the water bath or oven and placed in the lower segment of the breaking head. The upper segment of the breaking head is then positioned and the complete assembly is placed in position on the testing machine.
To begin the testing process, the flow meter is placed over one of the posts and adjusted to read zero. Load is then applied at a rate of 50 mm per minute until the maximum load reading is obtained.
During this process, the maximum load reading in Newton is observed. At the same instant, the flow is recorded on the flow meter in units of mm. These measurements are used to determine the properties of the specimens being tested.