Bituminous Mix Design for Pavement – Types, Materials and Properties

A pre-planned bituminous mix design for pavement construction helps in producing a mix that is workable, durable, strong, and economical in nature. One of the widespread use of bitumen is in the pavement design and construction.

There are mainly two stages of mix design in bringing a bituminous pavement:

• Dry Mix Design
• Wet Mix Design

In the bitumen mix design, we will determine the:

• Proportion of bitumen
• The fine aggregates required for the pavement design and construction
• The coarse aggregates for the construction
• The filler materials required

Objective of Bituminous Mix Design

The main objective of the bituminous mix design is to proportion various components for pavement construction to achieve the following objectives:

• To obtain a durable pavement, sufficient amount of bitumen is required
• Adequate strength must be provided to obtain resistance against the shear deformation under higher temperatures.
• Additional voids have to be incorporated to facilitate the compaction performed by the traffic
• The placement must be performed with ease which will demand sufficient workability
• The premature cracking in the bitumen pavement can be avoided by providing sufficient flexibility for the bitumen.
• The flexibility must be attained at smaller temperatures so that the shrinkage cracks can be avoided.

Main Constituents of Bitumen Mix

The main constituents of a bitumen mix for pavement construction are:

• Coarse and Fine Aggregates
• Filler
• Binder

The coarse aggregates are known for their abrasion resistance and toughness. These aggregates offer compressive and shear strength to the mix. These also facilitates good interlocking properties between aggregates. Examples for this type of aggregates are granites.

The fine aggregates fill the voids in the mix created by the coarse aggregates and provide stiffening to the binder. Examples of fine aggregates used are sand and rock dust.

Fillers play the role of filling the voids, they help in stiffening the binder and offer higher permeability. Example for this type of fillers are cement, lime and rock dust.

Binders also play the role of filling the voids. These help to undergo particle adhesion and for gluing purpose. This hence increases the impermeability property of the pavement. Example for this type of the binder are bitumen, tar, and asphalt.

Types of Bitumen Mixes

There are different types of mix that can be prepared. They Involve:

• Open-graded bituminous mix
• Well graded bituminous mix
• Gap-graded bituminous mix
• Unbounded bituminous mix

In case of Open-Graded Mix, the filler and the finer aggregates are absent. These mixes are porous in nature and offer good frictional properties. This lower the strength if the pavement is constructed for a high-speed pavement construction.

Gap-Graded Bituminous Mix: As the name implies, large coarse aggregates are missing in these types of mix. This has good fatigue property as well as tensile strength.

Well – Graded Mix: A well-graded mix is a dense mix. These have all ranges of aggregates and are sufficiently packed. This facilitate the proper filling of voids in a systematic manner. These types of mix offer good compressive strength and tensile strength.

Layers in a Bituminous Pavement

The different layers that is involved in a bituminous pavement are:

• Base Course
• Binder Course
• Concrete layer of bitumen or asphalt

The layer of mineral aggregates like gravel, stones, and sand together form the bituminous base course layer. This layer is treated as the foundation for the laying the binder and the surface course.

The bituminous binder course is the second layer above the base course which act as the first layer of bitumen. This is done in a two-layer resurfacing. This is also called as a leveling course.

The third layer is the concrete form of bitumen or asphalt. The bituminous concrete is a combination of aggregates that is continuously graded from the maximum size which is mainly less than 25mm to the fine filler of size smaller than 0.075mm to make the mix sufficiently impervious. This provides a layer will have acceptable levels of dissipative and elastic properties.

Basic Requirements of a Bituminous Mix

The main and essential features a bitumen mix must possess are explained below:

• Good Stability
• Good Durability
• Workability
• Skid Resistance
• Flexibility

Stability of Bitumen Mix

The resistance of the pavement towards the deformation under action of vehicle or the dynamic load is the called as the stability of the bitumen mix. The main two examples of failure caused in the bitumen pavement due to lack of stability are:

• Shoving
• Grooving

The areas in the pavement under severe action of acceleration gain a rigid deformation in the traverse direction. This is called as shoving. The channelization of the traffic will result in the formation of a longitudinal ridging which is called as grooving.

The stability of the bitumen pavement is influenced by the following parameters:

• Interparticle friction between the aggregates
• The cohesion offered within the bitumen
• Bond within all the particles

Stability is determined by sufficient bonding between the particles. This require adequate amount of binder so that it coats the particles at the time when it offers friction. There are also chances to have a lower value of stability when the binder content is large and the particles of the mix are kept apart without any kind of bond.

Durability of the Bitumen Mix

The resistance of the mix against the external weathering and abrasive actions can be called as the durability property. The wheel loads with high abrasion action on the pavement result in the formation of tensile strains. Lack of durability of the pavement result in the following failures:

• Pot Holes
• Stripping

Pot holes is the local deterioration and stripping is the failure that expose all the aggregates. Higher binder content help in reducing the chances of disintegration to some extend.

Skid Resistance of Bituminous Pavement

The skid resistance is the resistance of the pavement against the skidding action of the vehicle. This is mainly related to the surface texture of the pavement. The bitumen content also influences this property to a large extend. In high-speed traffic conditions, this is an essential factor to be considered. An open-graded coarse surface will help in providing maximum skid resistance.

Workability of the bituminous mix

A bituminous mix have to be easily handled for its proper placement and compaction. This can be only performed if there is sufficient workability for the mix. This property will let the mix to take adequate shape and condition.

The workability of the mix depends on the following factors:

• The aggregate type – angular, flaky and elongates
• The content of bitumen
• The gradation of aggregates
• Shape and texture of the aggregates
• Type of bitumen

Flexibility of Bitumen Mix

The level of bending strength required to counteract the traffic load is called as the flexibility property. This is the one that will help in preventing the cracking surface of the pavement. The fracture is the cracks that is formed on the surface. The reason behind these fractures are the shrinkage and brittleness of the binder.

The repeated bending of the surface due to the traffic loads over it result in the brittleness. Flexibility is increased with higher bitumen content.

Some of the desirable properties required by a bitumen mix are:

• Proper binding of the particles present in the bitumen mix. This facilitate waterproofing properties.
• The traffic demand has to meet adequately by having sufficient stability
• The bitumen mix have to be economical
• The bitumen mix must have sufficient workability to help construction procedures
• Voids must me intentionally left so that compaction during traffic can be performed
• The mix must be flexible to face the changing temperature conditions