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Bituminous Mix Design

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.

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.

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.

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

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.

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.

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 (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

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

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

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:

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

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.

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

Solutions

Granite is an example for

Coarse aggregate$\surd$

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$\surd$

none of these

https://www.civil.iitb.ac.in/tvm/1100_LnTse/408_lnTse/plain/plain.html

 

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