|You know asphalt pavements have the highest riding quality among all others|
The transportation engineers, know that the two basic types of pavements is rigid pavement or flexible pavement. Flexible pavement deforms slightly under the application of the dynamic load of traffic whereas rigid pavement is the one that stands firm and it bridges the voids and gaps in the underneath bearing soil.
We would not here go into the details of these but as far as flexible pavements are concerned these are one of the most common, high riding quality paved roads on earth. The flexible pavements have a wearing course (the top few cms of the highway which is expected to get wear and tear due to traffic movement) which is made up of high quality fine asphaltic concrete.
Asphalt is basically combination of bitumen and aggregate, the surface of aggregates are applied with bitumen and then bitumen acts as a binding agent that makes it a solid and firm material called asphaltic concrete.
Like we all know about traditional Portland cement concrete that its properties and strength is largely dependent on the amount and proportion in which the constituents are mixed, same is the case here in asphaltic concrete.
What is HMA or Hot Mix Asphalt
Sometimes there is a term written in some texts as HMA or Hot Mix Aspahlt it is basically the same thing like that of Bitumen as asphalt binder and aggregate.
HMA mix design is the process of determining what aggregate to use, what asphalt binder to use and what the optimum combination of these two ingredients should be.
Main objective is to determine an economical and workable mix which comprises of a suitable blend of aggregates and a corresponding asphalt content.
What is Job Mix Formula (JMF)The end result of a successful mix design is a recommended mixture of aggregate and asphalt binder. This recommended mixture, which also includes aggregate gradation and asphalt binder type is often referred to as the job mix formula (JMF)
Properties of Ideal Asphaltic Concrete MixThe main objective of the Job Mix Formula, Mix Design Method, Marshall Mix Design method is to determine an ideal and optimum asphalt mix that has following properties :-
1. Resistance to permanent deformation: The mix should not distort or be displaced when subjected to traffic loads. The resistance to permanent deformation is more important at high temperatures.
2. Fatigue resistance: the mix should not crack when subjected to repeated loads over a period of time.
3. Resistance to low temperature cracking. This mix property is important in cold regions.
4. Durability: the mix should contain sufficient asphalt cement to ensure an adequate film thickness around the aggregate particles. The compacted mix should not have very high air voids, which accelerates the aging process.
5. Resistance to moisture-induced damage.
6. Skid resistance.
7. Workability: the mix must be capable of being placed and compacted with reasonable effort.
8. Low noise and good drainage properties: If the mix is to be used for the surface (wearing) layer of the pavement structure.
Marshall Method of Mix DesignAccording to ASTM D1559 which uses Marshall mix design method for getting JMF as optimum Aspahlt Mix, there are 6 basic steps involved in it :-
- Aggregate Selection
- Asphalt Binder Selection
- Sample Preparation (including compaction)
- Density and voids calculations
- Stability determination using the Marshall Stability and Flow test
- Optimum asphalt binder content selection
There are two major features of the Marshall method of designing mixes namely,
- density – voids analysis
- Stability – flow tests.
Marshall Mix Design ProcedureThe first step in the Marshall Mix Design method is to prepare the samples as described below :-
The aggregate gradation is selected from the following table and then after sieving the desired quantity of each gradation approximately 1200 grams of aggregates are taken.
|Table no. 1 Aggregate Specification|
|Marshall Drop Hammer|
Theoretical Specific Gravity
Bulk Specific Gravity
Air Voids Percent Vv
Percent Volume of Bitumen Vb
Voids in Mineral Aggregate (VMA)
Voids Filled with Bitumen VFB or Voids Filled with Aspahlt (VFA)
Marshal Stability and Flow Test
|Marshall Sample Extractor|
Selection of Optimum Asphalt Binder Content
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Marshall Mix Design Procedure