Flexible Pavement Essay Example

Flexible Pavements

Flexible Pavements for Freeways

Flexible pavements for freeways in Australia are typically made of three layers of material, which consist of a surface layer, base course, and a sub-base course. The surface course is made of selected aggregates bound together with asphalt cement to provide a smooth and skid-resistance surface with minimum wear on tires, and resistance to wheel load stresses. The asphalt layer is laid on the base and sub-base courses, which are made of granular material. These layers receive loads from the surface layer and spread them out, making the pavement structure to flex, or deflect under loading, hence, the term “flexible pavement”.

Materials used in making Flexible Pavements for freeways in Australia

  1. Surface course material

Asphalt, more specifically, asphalt concrete is widely used in making the surface layer of flexible pavements because of its nature of load distribution. It is mixed with selected aggregates and bitumen binder to gives it a plastic deformation-resistance properties to prevent failure or fatigue from loading. Asphalt is categorized as cold mix, warm mix, or hot mix, depending on the temperature at which the material is applied. For freeway pavements, hot mix asphalt (HMA) with temperatures of over 150oC is usually applied. Before asphalt is used, adequate tests have to be undertaken, including density, binder volume, and modulus, to assess variability and select a suitable design modulus that will give a confidence level of 90%. Controls are put in place in the production and construction process of the material to ensure that the desired properties are achieved. Typically, a modulus of 800 MPa is suitable for surface course pavements for all design speeds (Standard Specification for Urban Infrastructure Works, 2010).

Advantages of using asphalt in flexible pavements include relatively low cost compared to other paving methods, low traffic noise, and easy to repair. Its disadvantages include less tensile strength compared to concrete, and the tendency to become soft and slick during hot weather. The type of binder used with asphalt affects its fatigue life.

  1. Base course material

The base layer is laid directly below the surface layer and acts as the strengthening and load bearing component of a flexible pavement, and also providing drainage and reducing the load stress applied to the bottom sub-base layer. Granular materials for base layer are obtained from recycled building materials, gravels and sand, or hard and durable crushed stone aggregates. The materials have to be free of organic matter, clay lumps and objectionable amount of deleterious substances. Recycled building materials can be used as a base material for construction of flexible pavement provided that the material is substantially crushed concrete or recycled pavement material. However, the material should meet the requirements of the type of material for the applicable traffic classification. The composition of contaminates is limited to provisions given in Roads and Traffic Authority (RTA T276) as shown in table 1 below (Standard Specification for Urban Infrastructure Works).

Table 1: Limit of contaminates in granular material

Contaminates

Maximum percentage by mass

Non-compressible high density substances such as glass, mortar, asphalt, metal, slag and ceramics.

Crushable or low-density materials such as plaster, brick, clay lumps, plastic films, and other friable substances.

Combustible material such as wood, plastic lumps, rubber, or vegetable matter.

  1. Sub-base course material

The sub-base is the lowest layer of a flexible pavement structure and serves as the main foundation of the pavement profile. For this layer, unbound granular materials are obtained from suitable natural gravels, crushed slag, crushed hard stone, or recycled building materials. The materials have to be free from organic substances, clay lamps and deleterious substances. In some cases, lean concrete is laid as a sub-base layer. This is because of its proven ability to resist erosion and provide uniform support.

The use of granular materials typically requires development of standards based on the experience with the materials, i.e. its historic performance, handling and construction requirements, future performance expectations, etc. Laboratory testing methods such as CBR and triaxial test, are used to predict their performance over a given range of moisture content. The gradation size specifications of granular materials vary with the type of layer (whether it is base or sub-base) in which the materials are to be applied. The granular base and sub-base are usually dense-graded, with a limited quantity of fines to allow for drainage. However, the granular materials in the sub-grade tend to be more course compared to the granular base. Given that the sub-base and the base layers make the largest thickness of the structure of a flexible pavement, providing drainage and bearing strength to the structure, proper selection of size, shape, grading, durability and resistance to abrasion are very important properties of granular materials for the performance of the structure.

Reference

Standard Specification for Urban Infrastructure Works, 2010. SECTION 4: Flexible Pavement Construction. Available at:
[Online] https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwj-xo6m2MvOAhWItI8KHTYlCnUQFggeMAA&url=http%3A%2F%2Fwww.tccs.act.gov.au%2F__data%2Fassets%2Fpdf_file%2F0003%2F397119%2FSS04_Flexible_Pavement_01_00_Final.pdf&usg=AFQj
[Accessed 18 August 2016].