Bituminous mix design

Lecture notes in Transportation Systems Engineering

3 August 2009


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 will be discussed.

Evolution of road surface

Objectives of mix design

The objective of the mix design is to produce a bituminous mix by proportionating various components so as to have:
  1. sufficient bitumen to ensure a durable pavement,
  2. sufficient strength to resist shear deformation under traffic at higher temperature,
  3. sufficient air voids in the compacted bitumen to allow for additional compaction by traffic,
  4. sufficient workability to permit easy placement without segregation,
  5. sufficient flexibility to avoid premature cracking due to repeated bending by traffic,and
  6. sufficient flexibility at low temperature to prevent shrinkage cracks.

Constituents of a mix

Types of mix

Different layers in a pavement

Table: Aggregate gradation for BC
Sieve size Passing (%)  -  
26 .5 mm    - 100
19 .  mm 90  - 100
9 .5 mm 56  - 80
4 .75 mm 35  - 65
2 .36 mm 23  - 49
300 .  micron 5  - 19
75 .  micron 2  - 8

Requirements of Bituminous mixes


Stability is defined as the resistance of the paving mix to deformation under traffic load. Two examples of failure are (i) shoving - a transverse rigid deformation which occurs at areas subject to severe acceleration and (ii) 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. Sufficient binder must be available to coat all the particles at the same time should offer enough liquid friction. However, the stability decreases when the binder content is high and when the particles are kept apart.


Durability is defined as the resistance of the mix against weathering and abrasive actions. Weathering causes hardening due to loss of volatiles in the bitumen. Abrasion is due to wheel loads which causes tensile strains. Typical examples of failure are (i) pot-holes, - deterioration of pavements locally and (ii) stripping, lost of binder from the aggregates and aggregates are exposed. Disintegration is minimized by high binder content since they cause the mix to be air and waterproof and the bitumen film is more resistant to hardening.


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. Shrinkage cracks are due to volume change in the binder due to aging. Brittleness is due to repeated bending of the surface due to traffic loads. Higher bitumen content will give better flexibility and less fracture.

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. Normally, an open graded coarse surface texture is desirable.


Workability is the ease with which the mix can be laid and compacted, and formed to the required condition and shape. This depends on the gradation of aggregates, their shape and texture, bitumen content and its type. Angular, flaky, and elongated aggregates workability. On the other hand, rounded aggregates improve workability.

Desirable properties

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


Bituminous mixes should be stable, durable, flexible, workable and should offer sufficient skid resistance. The mix consists of coarse and fine aggregates, filler and binder. It may be well graded, open graded, gap graded or unbounded as per the requirements. As far as possible, it should be economical also.


  1. Granite is an example for
    1. Coarse aggregate
    2. Fine aggregate
    3. Filler
    4. none of these
  2. Grooving is
    1. deterioration of pavements locally
    2. exposure of aggregate due to losing of bitumen
    3. longitudinal ridging due to channelization of traffic
    4. none of these


  1. Granite is an example for
    1. Coarse aggregate$\surd$
    2. Fine aggregate
    3. Filler
    4. none of these
  2. Grooving is
    1. deterioration of pavements locally
    2. exposure of aggregate due to losing of bitumen
    3. longitudinal ridging due to channelization of traffic$\surd$
    4. none of these

No References!

Prof. Tom V. Mathew 2009-08-03