Bituminous Surface Treatments (BST) play a crucial role in maintaining road surfaces, particularly in areas with lower traffic volumes. These treatments involve the application of a liquid asphalt coating with a thin layer of aggregate, creating what is commonly known as a chip seal or seal coat. Let’s delve into the types, materials, and application processes associated with BST.
In this method, bitumen is applied to a prepared surface in a single layer, followed by immediate rolling, typically using a pneumatic-tired roller.
Multiple layers of bitumen, each with decreasing aggregate sizes, are successively applied and rolled. This approach enhances the durability of the surface treatment.
For single surface treatments, the aggregate size should ideally range from 1⁄2 to 1⁄4 inch (13 to 6 mm). In multiple surface treatments, the bottom layer’s aggregate size ranges from 1 to 1⁄2 inch (25 to 13 mm), with successive layers using approximately 1⁄2-inch aggregates.
Cubical-shaped aggregates are preferred, as flat or elongated aggregates can lead to alignment issues, while rounded aggregates may have poor retention.
Aggregate cleanliness is crucial; dusty or coated coarse aggregates can hinder bitumen adhesion. It is recommended that aggregates passing the No. 200 mesh screen do not exceed 1% by weight.
Three types of bitumen are used:
Homogeneous, water-free asphalt cement conforming to specified requirements.
Non-foaming cutback asphalt, meeting specific criteria for rapid or medium curing.
Homogeneous emulsified asphalt meeting requirements after thorough mixing.