- This method is used to find the optimum binder content for aggregate mix type and traffic intensity.
- In India, a bituminous concrete mix is commonly designed by Marshal Method.
- The Stability of the mix is defined as a maximum load carried by a compacted specimen at std. test temp. of 60 degrees.
- The flow is measured as the deformation in units of 0.25mm between no-load and maximum load carried by the specimen during the stability test.
- A compaction pedestal and hammer are used to compact a specimen by 4.54 kg weight with 45.7 cm height of fall.
- Approximately 1200 gm of the mixed aggregate and the filler are taken and heated to a temp. of 175 to 190 degrees. The bitumen is heated to a temp. of 120 to 145 degree and the required quantity of bitumen (3.5-4% by weight of material aggregate) is added to the heated aggregates
- The mix is placed in a mold and compacted by a rammer with 50 blows on either side.
- Bulk density, Stability, Flow, % air voids, VMA, and VFB is calculated and graphs are plotted with the bitumen content on the x-axis.
The optimum bitumen content for mix design is found by taking the average value of the following three bitumen contents found from the graphs
- Bitumen content corresponding to maximum stability
- Bitumen content corresponding to maximum unit weight
- Bitumen content corresponding to the designed limit of percent air voids
- Check for design
- Marshall stability (kg) 340 minimum
- Flow value 0.255mm units vary between 8-16
- Air voids in total mix 3-5%
- VFB% 75-85
- Check for design
Important terms of Marshall Mix Design
a) Theoretical Specific Gravity of Mix
Where W1, W2, W3 are the weight of aggregates, and Bitumen and G1, G2 and G3 are their respective Sp. Gravities.
b) Bulk Specific Gravity of Mix
c) Air voids percent
d) Volume of bitumen percent
Where Wb is the weight of bitumen and Gb is the apparent specific gravity of bitumen
e) Voids in Mineral Aggregate (VMA)
Voids in Mineral aggregate is the volume of voids in the aggregate and it is the sum of the volume of air voids (Va) and volume of bitumen (Vb)
VMA= Va+ Vb
f) Voids filled with bitumen (VFB)
Voids filled with bitumen is the voids filled by bitumen in mineral aggregate whereas Vb is the percent bitumen content in a mix.