DETERMINATION OF MARSHALL STABILITY VALUE
1. Concept and Significance of Marshall Stability Value
The test procedure is used in designing and evaluating bituminous paving mixes, and is widely applied in routine test programmes for paving jobs. The major features of the Marshall method of designing mixes are to determine the properties of strength and flexibility.
“Marshall’s stability” is defined as the maximum load carried by a compacted specimen at a standard test temperature of 60°C. This condition represents the weakest condition for a bituminous pavement in use. The flexibility is measured in terms of “flow value”, which is measured by the change in diameter of the sample in the direction of load application between the start of loading and the time of maximum load. In this test, an attempt is made to obtain the optimum binder content for the aggregate mix type and traffic intensity.
2. Objectives
- To determine the density-void analysis for a given mixture.
- To determine the strength (Marshall’s Stability Value) and flexibility (flow value) for a given bituminous mixture.
- To determine the suitability of a bituminous mixture to meet specified criteria for surface courses.
3. Apparatus
- Specimen Mould Assembly: (10 cm diameter, 7.5 cm height, base plate, and extension collars)
- Specimen Extractor: For extracting compacted specimen (suitable bar to transfer load)
- Compaction Hammer: (4.5 kg flat circular tamping face, free fall of 45.0 cm)
- Compaction Pedestal: (20 × 20 × 45 cm wooden block, 30 × 30 × 2.5 cm MS plate cap)
- Breaking Head: (Upper and lower cylindrical segments)
- Loading Machine: (Provided with a gear system, precalibrated proving ring of 5 tonnes capacity fixed at the upper end, specimen contained between the base plate and proving ring)
- Flow Meter: (Guide sleeve and gauge)
- Others: Oven, Mixing Apparatus, Water Bath, Thermometers, Containers
4. Procedure
Preparation of Test Specimen
- 1200 gms of aggregate was taken and heated up to a mixing temperature.
- Bitumen was added at mixing temperature to provide a viscosity of 170 centistokes.
- The materials were mixed in a heated pan.
- The mixture was placed on a compacting pedestal and compacted with 50 blows of the hammer. The sample was inverted and again compacted with the same number of blows.
- After compaction, the mould was inverted and the base was removed. The sample was extracted by pushing.
- The sample was allowed to stand for a few hours to cool.
Test Procedure
- The specimen was heated to 60 ± 1°C in a water bath for 30–40 minutes.
- The specimen was removed from the water bath and placed in the lower segment of the breaking head.
- The upper bearing segment of the breaking head was made to touch the upper surface of the specimen, and the load was applied at a rate of 50 mm per minute until maximum loading was obtained.
- The maximum load reading in N was observed, and the flow recorded on the flow meter was also noted.
5. Observation and Calculation
Sample 1
Dial gauge reading of proving ring = 761
Observed initial thickness = 63 mm
Sample 2
Dial gauge reading of proving ring = 494
Observed initial thickness = 55 mm
| Material | % of Material | Bulk Sp. Gravity | App. Sp. Gravity | Water Absorption (%) |
|---|---|---|---|---|
| Coarse Aggregate | 41.75 | 2.682 | 2.712 | 0.428 |
| Intermediate Aggregate | 29.13 | 2.674 | 2.707 | 0.479 |
| Fine Aggregate | 26.21 | 2.642 | 2.728 | 1.23 |
| Filler | 2.91 | 2.665 | – | – |
| Bitumen | Σ = 100% | 1.018 | – | – |
| S.N. | % Bitumen | Weight in air (gm) | Weight in water (gm) | SSD Wt. in air (gm) | Flow (Divs.) | Stability (Divs.) | Thickness (mm) |
|---|---|---|---|---|---|---|---|
| 1. | 4 | 1235.2 | 714.8 | 1239.4 | 170 | 930 | 65.79 |
| 2. | 4.5 | 1252.8 | 731.6 | 1255.2 | 291 | 1060 | 65.51 |
| 3. | 5 | 1244.1 | 728.5 | 1248.3 | 320 | 950 | 64.73 |
| 4. | 5.5 | 1234.9 | 723.6 | 1237.7 | 365 | 880 | 64.31 |
| 5. | 6 | 1240.8 | 728.9 | 1241.9 | 400 | 770 | 63.30 |
| 6. | 6.5 | 1225.9 | 719.0 | 1227.4 | 379 | 670 | 63.39 |
L.C. for flow measuring dial gauge = 0.01 mm
| Load | 620 | 740 | 870 | 990 | 1100 | 1240 | 1360 | 1490 |
|---|---|---|---|---|---|---|---|---|
| Dial Gauge Reading | 500 | 600 | 700 | 800 | 900 | 1000 | 1100 (1236) | 1200 (1242) |
| Multiplying factor | 1.24 | 1.233 | 1.243 | 1.238 | 1.222 | 1.24 | – | – |
Hence, calibration factor for proving ring = mean multiplying factor = 1.237
Calculations
Sample 1
Uncorrected Stability = 1.237 × 761 × 9.81 = 9234.71 N
Correction factor for 63 mm thickness = 1.013
Corrected stability = 9234.71 × 1.013 = 9354.76 N
Sample 2
Uncorrected stability = 1.237 × 494 × 9.81 = 5994.68 N
Correction factor for 55 mm thickness = 1.199
Corrected stability = 5994.68 × 1.199 = 7187.62 N
| S.N. | % Bitumen | Dial Gauge | Flow (in mm) |
|---|---|---|---|
| 1. | 4 | 170 | 1.70 |
| 2. | 4.5 | 291 | 2.91 |
| 3. | 5 | 320 | 3.20 |
| 4. | 5.5 | 365 | 3.65 |
| 5. | 6 | 400 | 4.00 |
| 6. | 6.5 | 379 | 3.79 |
| S.N. | % Bitumen | Dial Gauge Reading | Loading/Stability (N) | Thickness (mm) | Correction Factor | Corrected Stability (N) |
|---|---|---|---|---|---|---|
| 1. | 4 | 930 | 11285.52 | 65.79 | 0.947 | 10687.39 |
| 2. | 4.5 | 1060 | 12863.07 | 65.51 | 0.952 | 12245.64 |
| 3. | 5 | 950 | 11528.22 | 64.73 | 0.969 | 11170.85 |
| 4. | 5.5 | 880 | 10678.77 | 64.31 | 0.980 | 10465.19 |
| 5. | 6 | 770 | 8494.48 | 63.30 | 1.005 | 8536.96 |
| 6. | 6.5 | 670 | 8130.43 | 63.39 | 1.003 | 8154.82 |
3) Unit Weight (Gmb / γd)
Where:
WA = weight of mix in air
WA‘ = weight of SSD mix in air
WW = weight of mix in water
| S.N. | % Bitumen | WA (gm) | WW (gm) | WA‘ (gm) | γd (Gmb) |
|---|---|---|---|---|---|
| 1. | 4 | 1235.2 | 714.8 | 1239.4 | 2.355 |
| 2. | 4.5 | 1252.8 | 731.6 | 1255.2 | 2.393 |
| 3. | 5 | 1244.1 | 728.5 | 1248.3 | 2.393 |
| 4. | 5.5 | 1234.9 | 723.6 | 1237.7 | 2.399 |
| 5. | 6 | 1240.8 | 728.9 | 1241.9 | 2.419 |
| 6. | 6.5 | 1225.9 | 719.0 | 1227.4 | 2.411 |
Sample Calculation:
At % bitumen = 4% by weight:
4) Air Voids (Vv) / Void in Total Mixture (VTM)
Where:
Gmb = Bulk specific gravity of paving mixture
Gmm = Maximum theoretical specific gravity of paving mixture
Where:
Wb = % by weight of bitumen content
Gb = Specific gravity of bitumen
Ga = Average specific gravity of aggregates
From observed data for dense bituminous macadam:
| S.N. | % Bitumen | Gmm | Vv (%) | Vb (%) | Va (%) | Sum |
|---|---|---|---|---|---|---|
| 1. | 4 | 2.507 | 6.06 | 9.25 | 84.67 | 99.98 |
| 2. | 4.5 | 2.488 | 3.82 | 10.58 | 85.59 | 99.99 |
| 3. | 5 | 2.470 | 3.12 | 11.75 | 85.14 | 100.0 |
| 4. | 5.5 | 2.451 | 2.12 | 12.96 | 84.91 | 99.99 |
| 5. | 6 | 2.433 | 0.58 | 14.25 | 85.16 | 99.99 |
| 6. | 6.5 | 2.415 | 0.17 | 15.39 | 84.43 | 99.99 |
5) Voids in Mineral Aggregate (VMA)
| S.N. | % Bitumen | VMA | Voids filled with bitumen (VFB) % |
|---|---|---|---|
| 1. | 4 | 15.33 | 60.34% |
| 2. | 4.5 | 14.41 | 73.42% |
| 3. | 5 | 14.86 | 79.07% |
| 4. | 5.5 | 15.09 | 85.88% |
| 5. | 6 | 14.84 | 96.02% |
| 6. | 6.5 | 15.57 | 98.89% |
Graphical Plots
The following graphs represent the relationships between varying percentage bitumen content and different Marshall mix design properties derived from the calculations above.
6. Results
From the graphs and calculations:
- % bitumen content at maximum unit weight (B1) = 6.0%
- % bitumen content at maximum stability (B2) = 4.5%
- % bitumen content at 4% air voids in total mix (B3) = 4.5%
7. Discussion and Conclusion
The optimum asphalt binder content is selected based on the combined results of Marshall stability and flow, density analysis, and void analysis. Each of the values from the plots is compared against specification values, and if within specification, the preceding binder content is satisfactory. Otherwise, if any of these properties are outside the specification range, the mixture should be redesigned. In this test, the Optimum Bitumen Content was found to be 5%.