Bituminous mix design
Lecture Notes in Transportation Systems Engineering
Contents
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1 Overview
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.
2 Evolution of road surface
- Unsurfaced earthen roads, or cart-track
- Unsurfaced earthen roads upgrades with natural soil from borrow pits and attention to
drainage, and compaction is by traffic
- Dry aggregate and sand-clays mix, in which the the former act as wear resistant and the
latter as natural binder
- Water-bound macadam, the above constituents, mixed together (pre-mix or in-situ) with
water and compacted to improve the strength
- Oiled roads, introduced to reduce dust by bitumen stabilized soils
- Seal coat: the base course is protected from traffic and moisture by sealing the surface
with a thin film of bitumen aggregate mix, which is structurally strong surface for
pneumatic-tyred traffic. This is provided on firm and smooth base course after a tack coat
using cutback bitumen or bitumen emulsions with a penetration of 5 mm.
- Asphaltic concrete: Traffic and the axle configuration are increasing very much which
raises demand for the new type of pavement which can meet the above requirements. The
asphaltic concrete is one which is the high dense graded premix and it is termed as the
highest quality pavement surface course.
- Bitumen mix or asphalt mix overlays of minimum 20 - 40 mm to as high as 300 - 500 mm
or even more.
2.1 Objectives of mix design
The objective of the mix design is to produce a bituminous mix by proportionating various
components so as to have:
- sufficient bitumen to ensure a durable pavement,
- sufficient strength to resist shear deformation under traffic at higher temperature,
- 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
- sufficient flexibility at low temperature to prevent shrinkage cracks.
2.2 Constituents of a mix
- Coarse aggregates: Offer compressive and shear strength and shows good interlocking
properties. E.g. Granite
- Fine aggregates: Fills the voids in the coarse aggregate and stiffens the binder. E.g.
Sand, Rock dust
- Filler: Fills the voids, stiffens the binder and offers permeability. E.g. Rock dust, cement,
lime
- Binder: Fills the voids, cause particle adhesion and gluing and offers impermeability.
E.g. Bitumen, Asphalt, Tar
2.3 Types of mix
- Well-graded mix:- Dense mix, bituminous concrete has good proportion of all
constituents and are called dense bituminous macadam, offers good compressive strength
and some tensile strength
- Gap-graded mix:- Some large coarse aggregates are missing and has good fatigue and
tensile strength.
- Open-graded mix:- Fine aggregate and filler are missing, it is porous and offers good
friction, low strength and for high speed.
- Unbounded:- Binder is absent and behaves under loads as if its components were not
linked together, though good interlocking exists. Very low tensile strength and needs kerb
protection.
2.4 Different layers in a pavement
- Bituminous base course Consist of mineral aggregate such as stone, gravel, or sand
bonded together by a bituminous material and used as a foundation upon which to place
a binder or surface course.
- Bituminous binder course A bituminous-aggregate mixture used as an intermediate
coarse between the base and surface courses or as the first bituminous layer in a two-layer
bituminous resurfacing. It is sometimes called a leveling course.
- Asphaltic/Bituminous concrete Bituminous concrete consists of a mixture of
aggregates continuously graded from maximum size , typically less than 25 mm, through
fine filler that is smaller than 0.075 mm. Sufficient bitumen is added to the mix so that the
compacted mix is effectively impervious and will have acceptable dissipative and elastic
properties.
Table 1: Aggregate gradation for BC
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Sieve size | Passing (%)
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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 |
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3 Requirements of Bituminous mixes
3.1 Stability
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.
3.2 Durability
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.
3.3 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. 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.
3.4 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.
3.5 Workability
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.
3.6 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
4 Summary
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.
5 Problems
- Granite is an example for
- Coarse aggregate
- Fine aggregate
- Filler
- none of these
- Grooving is
- deterioration of pavements locally
- exposure of aggregate due to losing of bitumen
- longitudinal ridging due to channelization of traffic
- none of these
6 Solutions
- Granite is an example for
- Coarse aggregate√
- Fine aggregate
- Filler
- none of these
- Grooving is
- deterioration of pavements locally
- exposure of aggregate due to losing of bitumen
- longitudinal ridging due to channelization of traffic√
- none of these
Exercises
- Not Available
References
- Y. H Huang. Pavement analysis and Design. Prentice-Hall, Englewood Cliffs, New Jersey,
1993.
Acknowledgments
I wish to thank several of my students and staff of NPTEL for their contribution in this lecture. I
also appreciate your constructive feedback which may be sent to tvm@civil.iitb.ac.in
Prof. Tom V. Mathew
Department of Civil Engineering
Indian Institute of Technology Bombay, India
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Thu Jan 10 12:42:37 IST 2019