AIM
To determine the Marshall Stability and Marshall Flow of a compacted bituminous mix (asphalt concrete). This test is a fundamental part of the Marshall Mix Design method used for hot mix asphalt. The stability value indicates the mix's resistance to permanent deformation and rutting under traffic loads, while the flow value measures its flexibility and ability to deform without cracking. Together, these values are used to evaluate the mechanical properties and overall performance of the asphalt mix.
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THEORY
The Marshall Stability and Flow test provides a measure of the load-carrying capacity and deformation characteristics of a compacted asphalt specimen. It's a key component of the Marshall Mix Design procedure, which aims to determine the optimum binder content for a specific aggregate blend. The test is empirical but widely used due to its simplicity and ability to produce consistent results.
- Marshall Stability: This is the maximum load a cylindrical specimen can withstand before failure. A higher stability value indicates a more resistant mix to plastic flow and permanent deformation (rutting). It is measured in kilograms (kg) or kilonewtons (kN). The stability is influenced by the aggregate's interlocking, friction, and the binder's cohesion.
- Marshall Flow: This is the total vertical deformation of the specimen at the maximum load. It is measured in units of 0.25 mm. A low flow value indicates a brittle mix, which is prone to cracking. A high flow value suggests a mix that is too soft and susceptible to rutting. The flow value helps balance stability with the necessary flexibility for a durable pavement.
The test is performed on a cylindrical specimen (101.6 mm diameter, 63.5 mm height) compacted using a standard Marshall hammer. The specimen is conditioned to a specific temperature and then loaded at a constant deformation rate until failure.
APPARATUS
- Marshall Compaction Hammer: A standard hammer with a 4.536 kg (10 lb) ram, a 457 mm free fall, and a 98.4 mm face. It's used to compact the loose asphalt mix into a cylindrical mould.
- Marshall Compaction Moulds: Cylindrical steel moulds (101.6 mm diameter) with a base and collar.
- Marshall Stability Testing Machine: A loading frame with a compression testing head. It includes a load cell to measure the stability and a dial gauge to measure the flow. The machine must apply a constant vertical load at a rate of 50.8 mm/minute.
- Breaking Heads (or Proving Rings): A set of semi-cylindrical heads used to apply a compressive load to the specimen.
- Water Bath: A temperature-controlled bath capable of maintaining a temperature of 60 ± 1°C for conditioning the specimens.
- Oven: A laboratory oven to heat the aggregates and binder to the required mixing and compaction temperatures (typically 150-160°C).
- Sieves: A set of sieves to grade the aggregates.
- Balance: An accurate balance to weigh the aggregates and bitumen.
- Mixing Tools: A mechanical mixer, bowls, and spatulas for preparing the asphalt mix.
PROCEDURE
- Specimen Preparation:
- Sieve the aggregates and combine them to create the desired grading.
- Heat the aggregates and the bitumen to the specified mixing temperature (e.g., 155°C).
- Mix the aggregates and bitumen thoroughly in a mechanical mixer to ensure a uniform coating of the aggregates.
- Place the hot mix in a pre-heated Marshall mould and tamp it with a trowel.
- Compact the specimen by applying 75 blows with the Marshall hammer on both the top and bottom faces.
- Allow the compacted specimen to cool to room temperature, then extract it from the mould.
- Prepare at least three specimens for each binder content to ensure reliable results.
Conditioning:
- Place the compacted specimens in the water bath maintained at a constant temperature of 60 ± 1°C.
- Allow the specimens to condition for a minimum of 30 minutes and a maximum of 40 minutes. This brings the specimen to the standard test temperature.
Testing:
- Remove a specimen from the water bath and dry it with a towel.
- Place the specimen in the lower segment of the breaking head.
- Place the entire assembly, with the specimen, in the Marshall testing machine.
- Apply the compressive load at a constant rate of 50.8 mm/minute.
- Record the maximum load reached on the load cell. This is the Marshall Stability value.
- Simultaneously, as the load is applied, the dial gauge measures the vertical deformation. Note the maximum deformation at the point of failure. This is the Marshall Flow value.
- The entire test from removal from the water bath to completion should not take more than 30 seconds to avoid a significant drop in temperature.
Calculations:
- The recorded stability value may need to be corrected using a standard correction factor based on the specimen's height.
OBSERVATION/RESULTS
The stability is reported in kilonewtons (kN), and the flow is reported in units of 0.25 mm. The average of the three specimens is reported as the final result.
Example Observation:
PRECAUTIONS
- The temperature of the water bath and the mixing oven must be maintained with high precision.
- The compaction of specimens must be uniform, with the specified number of blows.
- The test must be conducted quickly after removing the specimen from the water bath to prevent temperature loss.
- The loading rate of the testing machine must be strictly controlled at 50.8 mm/minute.
- The specimens must be dimensionally accurate to ensure the correction factor is applied correctly.
CODES/IS STANDARDS
- ASTM D1559: Standard Test Method for Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus (This is an older standard, but widely referenced).
- ASTM D6927: Standard Test Method for Marshall Stability and Flow of Bituminous Mixtures.
- IRC: SP-53-2010: Guidelines on the use of Modified Bitumen in Hot Mix Asphalt.
- MORTH (Ministry of Road Transport and Highways) Specifications for Road and Bridge Works.
APPLICATIONS/USES
- Mix Design: The Marshall Test is the primary method for designing and proportioning hot mix asphalt. It helps determine the optimum binder content that provides a balance between stability and durability.
- Quality Control: It's a standard quality control test at asphalt plants to ensure that the produced mix meets the specified strength and flexibility requirements.
- Performance Evaluation: The results are used to predict the in-service performance of a pavement. A mix with low stability might be prone to rutting, while one with low flow might be susceptible to thermal cracking.
- Pavement Thickness Design: The stability of the mix is sometimes correlated with the pavement thickness required for a given traffic volume.
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