Asphalt pavement slippage is an all too common occurrence on roads and bridges. Slippage typically resembles sequential set of half-moon cracks in the roadway (see photo).
The causes can range from continuous braking to turning at busy intersections to bus stops or a traffic light at the bottom of hill. The common element in the above scenarios is the pavement or “wearing course” is moving or slipping when it should be adhered in place to the substrate. The lack of a proper bond results in wearing course slippage.
A tack coat is applied to the substrate prior to installation of the wearing course. The purpose of a tack coat is to promote and ensure proper adhesion of the wearing course to its under layer. There are a number of different types of tack coats, but they basically fall into one of two categories: solvent based and waterborne. The criteria for the selection of one type over the other varies from project to project.
Related article: Bridge Deck Preparation
The key point to keep in mind is tack coat is specified to ensure the proper adhesion of the surface course to its under layer.
Specifications for a liquid-type waterproofing membranes for bridges, on long-span elevated structures and cable-stayed bridges have become a common component in the “belt and suspenders” approach to protecting bridge decks, especially steel decks. Liquid-type waterproofing membranes are applied to the steel substrate prior to the application of the tack coat. Because of their chemistry, waterproofing membranes cure to a very hard, dense and smooth finish. These properties are necessary to protect the steel or concrete substrate from moisture permeation and chloride contamination.
Why is the chemistry of liquid-type waterproofing membranes important? Because the properties cited above can contribute to wearing course slippage, especially on long-span suspension bridge decks.
Bridges have both deck and cross slope. The grade can start at 2% and increase depending on the structure of the bridge. A bridge specification calling for both a waterproofing membrane and a polymer-modified asphalt should take into consideration proper adhesion of the tack coat to the waterproofing membrane. The surface profile of the membrane is hard and smooth and can sometimes be a challenge to properly adhere to an asphalt pavement.
The application of a tack coat needs to be able to form both a chemical and physical bond to its underlaying surface. The receiving surface should not be smooth and hard. The surface should have a profile for the tack coat to adhere to, similar to surface preparation of structural steel receiving an abrasive blast cleaning – to impart a surface profile and remove corrosion – prior to the application of a prime coat. The same principle should apply to liquid-type bridge deck membranes.
The practical technique for improving adhesion is to broadcast aggregate (such as sand) into the liquid membrane before it cures. The result is a similar surface of anti-slip medium used to promote friction on walking surfaces. Broadcasted aggregate in the membrane will both impart a surface profile and promote physical adhesion of the tack coat to the membrane.
The addition of an anti-slip aggregate into the waterproofing membrane can eliminate potential downtime and eliminate repairs required during or after the completion of the project with the goal of ensuring long service life for the owner.
If you have any questions or require information about applying tack coat for your bridge deck pavement, please visit the Chase website at www.chasecorp.com/Rosphalt.