Asphalt Wearing Course
The asphalt wearing course is the uppermost layer in the construction of roadways, airfields, and dockyards. It is also known as a wearing surface, friction course, or surface course. This layer is crucial as it directly interacts with vehicular traffic and environmental factors, thereby requiring materials that provide durability, smoothness, and adequate traction.
Composition
The asphalt wearing course typically consists of asphalt concrete, which is a composite material commonly used to pave roads, parking lots, and airports. Asphalt concrete is made from a mix of construction aggregate and a bituminous binder. The specific composition and properties of the asphalt used can vary based on the intended use and local environmental conditions.
Importance in Pavement Structure
The wearing course performs several critical functions:
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Protection: It protects the underlying base course and sub-base course by providing a waterproof layer that prevents moisture penetration.
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Load Distribution: It aids in the distribution of vehicular loads to the layers beneath, thereby reducing stress on the sub-structure.
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Friction and Texture: The wearing course provides the necessary friction and surface texture for vehicles, which is essential for safety, especially in wet conditions.
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Durability: It must withstand surface abrasion, weathering, and the high stress of traffic loads over time.
Types of Asphalt Wearing Courses
Various formulations and techniques are used to enhance the properties of the wearing course:
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Stone Mastic Asphalt (SMA): This type of asphalt provides a textured, durable, and rut-resistant surface. SMA's surface texture characteristics are similar to open-graded asphalt, which helps in noise reduction.
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Porphyry Aggregate: In some regions, highways use asphalt made from porphyry aggregate for the wearing course to withstand extreme wear from conditions such as spiked winter tires.
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Polymer-modified Asphalt: Sometimes, polymer concrete is used as a bonded wearing course to enhance durability and strength.
Challenges and Maintenance
Over time, the asphalt wearing course may experience issues such as cracking and plastic deformation. For instance, in the Suvarnabhumi Airport project, plastic deformation was observed near the takeoff position of the runway. Regular maintenance, including resurfacing and crack sealing, is crucial to extend the life of the wearing course.
The wearing course's performance can be influenced by various factors, including the quality of the bituminous binder, traffic volume, and environmental conditions, necessitating periodic evaluations and maintenance to ensure safety and functionality.