Posted by Scott Hackett on Thu, Aug 02, 2012 @ 8:04 AM
Pavement has several key purposes:
- to support loads without excessive cracking or deforming
- to provide a smooth surface for vehicles to improve comfort and efficiency
- to eliminate drainage problems such as mud and ponding
Pavement types – flexible, rigid and composite
Pavements typically consist of a number of layers, placed over the in situ material, which work together to withstand traffic and environmental conditions. The surface layer may be made of concrete, asphalt, aggregate, geocells, grids or blocks. Concrete provides a rigid pavement structure while almost all other pavements are flexible. Composite pavements, often the result of pavement rehabilitation, also exist comprising both flexible and rigid elements.
Load dispersion with depth
Rigid and flexible pavements distribute traffic load differently to the layers below, necessitating careful attention to design and construction of the layers and the thickness of the surface layer. Protection of the base layer, the structural layer nearest to the surface, and the sub-base layer below this, is essential for the longevity of the road.
Because the rigid surface structure does not bend, a point load is spread rapidly over a wide bearing area.
By comparison a flexible structure, for example asphalt, flexes to accommodate traffic load. A flexible surface laid over an aggregate base spreads the traffic load gradually to the layers beneath. In this case a thicker pavement structure is required to protect these layers to the same degree.
Wheel loads and global standards
Total traffic load on a pavement comprises the magnitude of individual load events such as pedestrians, cars and trucks, and the frequency of events over time. This range of loads is expressed in terms of a common unit of measurement, the standard reference vehicle. Any vehicle can be related to the reference vehicle by its equivalent wheel load (EWL) or equivalent single- axle load (ESAL).
The deterioration of pavement over time is directly related to traffic load expressed as ESAL. When a pavement is designed, it’s predicted ESAL is taken into account and its lifetime in years is calculated. Once this time arrives, the pavement is assumed to require some rehabilitation.
When calculating pavement loads – it is essential that the statutory load factors are applied. These are essentially additional load calculations that provide the necessary margin required for variations in environmental, mechanical and material factors. The chart below shows the difference between some of the major National Wheel Loads globally.
Chart and graph showing national wheel loads.
- Is the product as strong as the manufacturer claims, and can they produce laboratory test results verifying the ultimate strength of the product?
- Has a material safety factor been applied to the product (down-rating the ultimate strength of the product by a 20-25% safety factor)?
- Have the statutory load factors been applied to the wheel loads? (engineers have these tables)
- Is the Factored (down-rated) strength of the product, greater than the Factored Wheel Load?
Refer to the diagram (right) for a sample illustration of wheel load calculations.