The Superpave mix design method differs from the Marshall Mix design methods by using performance-based and performance-related criteria to design the proper asphalt mix. This allows a direct relationship to be drawn between the lab and field performance of the asphalt mix.
Summary. Marshal stability test is the performance prediction measure conducted on the bituminous nix. The procedure consists of determination of properties of mix, Marshal stability and flow analysis and finally determination of optimum bitumen content.
Marshall Asphalt An Asphalt Concrete designed to achieve specified stability, flow, voids and density characteristics. Particle Size Fraction That portion of aggregate which passes one sieve but is retained on the adjacent smaller sized sieve in the sequence of sieves used to specify that grading. Page 11.
The flow value refers to the vertical deformation when the maximum load is reached. Marshall stability is related to the resistance of bituminous materials to distortion, displacement, rutting and shearing stresses.
Which bitumen does not need heating? Explanation: The cutback bitumen which is the most economical type may or may not require heating, whereas the remaining required to be heated.
The Marshall Mix Design method was originally developed by Bruce Marshall of the Mississippi Highway Department in 1939. The main idea of the Marshall Mix Design method involves the selection of the asphalt binder content with a suitable density which satisfies minimum stability and range of flow values.
The principle of designing the optimum amount of binder content is to include sufficient amount of binder so that the. aggregates are fully coated with bitumen and the voids within the bituminous material are sealed up. As such, the durability of the bituminous pavement can be enhanced by the impermeability achieved.
VFB is the voids in the mineral aggregate frame work filled with bitumen binder. It can also be described as the percent of the volume of the VMA that is filled with bitumen. VFB is inversely related to air voids and hence as air voids decreases, the VFB increases.
The flow is measured as the deformation in units of 0.25 mm between no load and maximum load carried by the specimen during stability test (flow value may also be measured by deformation units of 0.1 mm). This test attempts to get the optimum binder content for the aggregate mix type and traffic intensity.
Percent air voids is calculated by comparing a test specimen's bulk specific gravity (Gmb) with its theoretical maximum specific gravity (Gmm) and assuming the difference is due to air. Once Gmm is known, portable non-destructive devices can be used to measure HMA density in-place.
Concrete, Asphalt weighs 2.243 gram per cubic centimeter or 2 243 kilogram per cubic meter, i.e. density of concrete, Asphalt is equal to 2 243 kg/m³. In Imperial or US customary measurement system, the density is equal to 140 pound per cubic foot [lb/ft³], or 1.297 ounce per cubic inch [oz/inch³] .
Test Methods and Equipment
| Laboratory Test Equipment | ASTM/AASHTO Test Method | Asphalt Preparation & Compaction |
|---|
| Rice Test | D2041 / T 209 | |
| Bulk Density | D2726 D1188 D6752 / T 166 T 331 | |
| Marshall Compaction | D6926 / T 245 | X |
| Stability & Flow | D6927 D5581 / T 245 | X |
Let's look at VMA, which stands for “Voids in Mineral Aggregate.” VMA is the space between the rocks that can be filled with asphalt. The space filled with asphalt is known as VFA (Voids Filled with Asphalt). The remaining space is air voids.
This specification deals with the basic outline for the design, construction and controls needed while laying bituminous concrete (earlier termed asphaltic concrete in IRC: 29-[968) mix as wearing course on highway pavements.
Marshall quotient, also known as the rigidity of asphalt mix, is a ratio of stability to flow of the asphalt mixture.
Determine optimum bitumen content
- Binder content corresponding to maximum stability.
- Binder content corresponding to maximum bulk specific gravity ( )
- Binder content corresponding to the median of designed limits of percent air voids ( ) in the total mix (i.e. 4%)
Asphalt binder content is calculated as the difference between the initial mass of the HMA and the mass of the residual aggregate, correction factor, and moisture content. The asphalt binder content is expressed as percent of moisture-free mix mass.
Bitumen Extraction Test is used to determine the percentage of bitumen content present in the asphaltic pavement by cold solvent extraction.
The bituminous mix design aims to determine the proportion of bitumen, filler, fine aggregates, and coarse aggregates to produce a mix which is workable, strong, durable and economical.
This process can be subdivided as follows:
- Make several trial mixes with different asphalt binder contents.
- Compact these trial mixes in the laboratory.
- Run several laboratory tests to determine key sample characteristics.
- Pick the asphalt binder content that best satisfies the mix design objectives.
The optimum binder content should be determined by observing the excessive mixture draindown, or filling of un-compacted air voids through the pyrex dish. The technician must be careful not to allow the mixture to slide, or move while observing the uncompacted mixture. 5.1.
