Bituminous Mix Design Dry And Marshall Mix Design

This Article Covers Bituminous Mix Design Their Requirements And Dry, Marshall Mix Design One By One.

Bituminous Mix Design

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.

The requirements of the mix design and the two major stages of the mix design, i.e dry mix design
and wet mix design.

Objectives of mix design

Sufficient bitumen to ensure a durable pavement,
Sufficient strength to resist shear deformation under traffic at higher temperature,
Then Sufficient air voids in the compacted bitumen to allow for additional compaction by traffic,
Sufficient workability to permit easy placement without segregation,
Sufficient flexibility to avoid premature cracking due to repeated bending by traffic,and
And Sufficient flexibility at low temperature to prevent shrinkage cracks.

Constituents of a mix

Coarse aggregates
Fine aggregates
Filler
Binder

Types of mix

Well-graded mix
Gap-graded mix
Open-graded mix
Unbounded

Different layers in a pavement

Bituminous base course
Bituminous binder course
Asphaltic/Bituminous concrete

Requirements of Bituminous mixes

Stability

Stability is defined as the resistance of the paving mix to deformation under traffic load. Two examples of failure are

Shoving – a transverse rigid deformation which occurs at areas subject to severe acceleration
Grooving – longitudinal ridging due to channelization of traffic. Stability depend on the inter-particle friction, primarily of the aggregates and the cohesion offered by the bitumen.

Durability

Durability is defined as the resistance of the mix against weathering and abrasive actions.

Pot-holes, – deterioration of pavements locally and
Stripping, lost of binder from the aggregates and aggregates are exposed.

Flexibility

Flexibility is a measure of the level of bending strength needed to counteract traffic load and prevent cracking of surface. Fracture is the cracks formed on the surface (hairline-cracks, alligator cracks), main reasons are shrinkage and brittleness of the binder.

Skid resistance

It is the resistance of the finished pavement against skidding which depends on the surface texture and bitumen content. It is an important factor in high speed traffic.

Workability

Workability is the ease with which the mix can be laid and compacted, and formed to the required condition and shape.

Desirable properties

From the above discussion, the desirable properties of a bituminous mix can be summarized as follows:

Stability to meet traffic demand
Bitumen content to ensure proper binding and water proofing
Voids to accommodate compaction due to traffic
Flexibility to meet traffic loads, esp. in cold season
Sufficient workability for construction
Economical mix

Dry Mix Design

The objective of dry mix design is to determine the amount of various sizes of mineral aggregates to use to get a mix of maximum density. The dry mix design involves three important steps, viz. selection of aggregates, aggregates gradation, and proportion of aggregates, which are

Selection of aggregates

The desirable qualities of a bituminous paving mixture are dependent to a considerable degree on the nature of the aggregates used. Aggregates are classified as coarse, fine, and filler.

The function of the coarse aggregates in contributing to the stability of a bituminous paving mixture is largely due to interlocking and frictional resistance of adjacent particles.

Aggregate gradation

The properties of the bituminous mix including the density and stability are very much dependent on the aggregates and their grain size distribution.

However, some minimum amount of void space is necessary to :-

Provide adequate volume for the binder to occupy,
Promote rapid drainage, and
Provide resistance to frost action for base and sub base courses.

Proportioning of aggregates

After selecting the aggregates and their gradation, proportioning of aggregates has to be done and following are the common methods of proportioning of aggregates:

Trial and error procedure
Graphical Methods
Analytical Method

Marshall Mix Design

The mix design (wetmix) determines the optimum bitumen content. This is preceded by the dry mix design.

Marshall mix design

The Marshall stability and flow test provides the performance prediction measure for the Marshall mix design method. The stability portion of the test measures the maximum load supported by the test specimen at a loading rate of 50.8 mm/minute.

Specimen preparation

Determine the properties of the mix

Theoretical specific gravity of the mix Gt

Bulk specific gravity of mix Gm

Air voids percent Vv

Voids in mineral aggregate VMA

Voids filled with bitumen VFB

Determine Marshall stability and flow

Marshall stability of a test specimen is the maximum load required to produce failure when the specimen is preheated to a prescribed temperature placed in a special test head and the load is applied at a constant strain (5 cm per minute).

Apply stability correction

Prepare graphical plots

The average value of the above properties are determined for each mix with different bitumen content and the following graphical plots are prepared:

Binder content versus corrected Marshall stability
Binder content versus Marshall flow
And Binder content versus percentage of void in the total mix v
Binder content versus voids filled with bitumen (V F B) 5. Binder content versus unit weight or bulk specific gravity

Determine optimum bitumen content

Determine the optimum binder content for the mix design by taking average value of the following three bitumen contents found form the graphs obtained in the previous step.

Binder content corresponding to maximum stability
Binder content corresponding to maximum bulk specific gravity
And Binder content corresponding to the median of designed limits of percent air voids in the total mix v (i.e. 4%)

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Ketan Pattekar

Author & Editor Of This Blog.

Founder Of “KPSTRUCTURES”