Typical Layers Of A Flexible Pavement 2c5z1j

Bitumen is a black adhesive compound manufactured from crude oil. When hot, it is in liquid form and when cold, it solidifies but retains some viscous and elastic properties enabling it to act as a flexible binder. It comes in a range of grades denoted by the results from the penetration test. In this test, a standard needle is pressed down into the bitumen for 20 seconds. The further it penetrates (measured in tenths of a millimetre) the softer the material. Bitumen for roads falls in the range 10 to 190. Asphalt is the term used for all mixtures of aggregate and bitumen including Dense Bitumen Macadam and Hot Rolled Asphalt. In the USA and literature emanating from there, it is known as hot mix - the term ‘asphalt’ is used for bitumen alone. CBR - This is the abbreviation for California Bearing Ratio which is a test commonly used to indicate the strength of the material below an asphalt road, runway, car park etc. It is the ability of a material to resist an applied load compared to that of a standard granular material expressed as a percentage. The smaller the CBR, the lower the strength of the material. A typical soft clay may have a CBR value of 2%; a granular subbase may have a CBR of 200%. The CBR varies according to the nature of the material itself but may also be affected by the amount of water in the material. Pavement The pavement is the structure built on the subgrade using layers of material as shown below. This is not to be confused with the same word in common use that refers to a

footway or footpath. Typical Layers of a Flexible Pavement:

Seal Coat - is a thin surface treatment used to water-proof the surface and to provide skid resistance.

Tack coat - is a very light application of asphalt, usually asphalt emulsion diluted with water. It provides proper bonding between two layers of binder course and must be thin, uniformly cover the entire surface, and set very fast. Applied on top of stabilized base layers and between lifts in thick asphalt concrete surface layers to promote bonding of the layers. Prime coat - is an application of low viscous cutback bitumen to an absorbent surface like granular bases on which binder layer is placed. It provides bonding between two layers. Unlike tack coat, prime coat penetrates into the layer below, plugs the voids, and forms a water tight surface. Applied on the untreated aggregate base layer to minimize flow of asphalt cement from the asphalt concrete to the aggregate base and to promote a good interface bond. Prime coats are often used to stabilize the surface of the base to the paving construction activities above. Cutback asphalt (asphalt cement blended with a petroleum solvent) is typically used because of its greater depth penetration. Surface course also called the wearing course - is the layer directly in with traffic loads and generally contains superior quality materials. They are usually constructed with dense graded asphalt concrete (AC). Binder course also called the asphalt base course - This layer provides the bulk of the asphalt concrete structure. Its chief purpose is to distribute load to the base course. The binder course generally consists of aggregates having less asphalt and doesn't require quality as high as the surface course, so replacing a part of the surface course by the binder course results in more economical design. Base course - is the layer of material immediately beneath the surface of binder course and it provides additional load distribution and contributes to the sub-surface drainage. It may be composed of crushed stone, crushed slag, and other untreated or stabilized materials. sub-base course - is the layer of material beneath the base course and the primary functions are to provide structural , improve drainage, and reduce the intrusion of fines from the sub-grade in the pavement structure If the base course is open graded, then the sub-base course with more fines can serve as a filler between sub-grade and the base course A sub-base course is not always needed or used. For example, a pavement constructed over a high quality, stiff sub-grade may not need the additional features offered by a sub-base course. In such situations, sub-base course may not be provided. Top soil or sub-grade - is a layer of natural soil prepared to receive the stresses from the layers above. It is essential that at no time soil sub-grade is overstressed. It should be compacted to the desirable density, near the optimum moisture content. Components of a Pavement System The pavement structure is a combination of subbase, base course, and surface course placed on a subgrade to the traffic load and distribute it to the roadbed.

The subgrade is the top surface of a roadbed upon which the pavement structure and shoulders are constructed. The purpose of the subgrade is to provide a platform for construction of the pavement and to the pavement without undue deflection that would impact the pavement's performance. For pavements constructed on-grade or in cuts, the subgrade is the natural in-situ soil at the site. The upper layer of this natural soil may be compacted or stabilized to increase its strength, stiffness, and/or stability. The subbase is a layer or layers of specified or selected materials of designed thickness placed on a subgrade to a base course. The subbase layer is usually of somewhat lower quality than the base layer. In some cases, the subbase may be treated with Portland cement, asphalt, lime, flyash, or combinations of these ixtures to increase its strength and stiffness. A subbase layer is not always included, especially with rigid pavements. A subbase layer is typically included when the subgrade soils are of very poor quality and/or suitable material for the base layer is not available locally, and is, therefore, expensive. Inclusion of a subbase layer is primarily an economic issue, and alternative pavement sections with and without a subbase layer should be evaluated during the design process.

Alternate Types of Pavement

The most common way of categorizing pavements is by structural type: rigid, flexible, composite and unpaved. 

Rigid pavements in simplest are those with a surface course of Portland cement concrete (PCC). The Portland cement concrete slabs constitute the dominant load-carrying component in a rigid pavement system.

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Flexible pavements, in contrast, have an asphaltic surface layer, with no underlying Portland cement slabs. The asphaltic surface layer may consist of high quality, hot mix asphalt concrete, or it may be some type of lower strength and stiffness asphaltic surface treatment. In either case, flexible pavements rely heavily on the strength and stiffness of the underlying unbound layers to supplement the load carrying capacity of the asphaltic surface layer.

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Composite pavements combine elements of both flexible and rigid pavement systems, usually consisting of an asphaltic concrete surface placed over PCC or bound base.

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Unpaved roads or naturally surfaced roads simply are not paved, relying on granular layers and the subgrade to carry the load. Seal coats are sometimes applied to improve their resistance to environmental factors. Emulsified asphalt is a suspension of asphalt in water by using an emulsifying agent which imposes an electric charge on asphalt particles so that they will and cement together. Cutback asphalt is simply asphalt dissolved in petroleum. The purpose of adding emulsifying agent in water or petroleum is to reduce viscosity of asphalt in low temperatures.

Asphalt pavement refers to any paved road surfaced with asphalt. Hot Mix Asphalt (HMA) is a combination of approximately 95% stone, sand, or gravel bound together by asphalt cement, a product of crude oil. Asphalt cement is heated aggregate, combined, and mixed with the aggregate at an HMA facility.