Chapter 6: Fog Seals

1.0 Introduction

Fog seals are a method of adding asphalt to an existing pavement surface to improve sealing or waterproofing, prevent further stone loss by holding aggregate in place, or simply improve the surface appearance.  However, inappropriate use can result in slick pavements and tracking of excess material.

The Asphalt Emulsion Manufacturers Association (AEMA) defines a fog seal as “a light spray application of dilute asphalt emulsion used primarily to seal an existing asphalt surface to reduce raveling and enrich dry and weathered surfaces” (1).  Others refer to fog seals as enrichment treatments since they add fresh asphalt to an aged surface and lengthen the pavement surface life (2).  Fog seals are also useful in chip seal applications to hold chips in place in fresh seal coats.  These are referred to as flush coats.  This can help prevent vehicle damage arising from flying chips.  The Asphalt Institute also adds that fog seals can seal small cracks (3).

1.1 Function of a Fog Seal

A fog seal is designed to coat, protect, and/or rejuvenate the existing asphalt binder.  The addition of asphalt will also improve the waterproofing of the surface and reduce its aging susceptibility by lowering permeability to water and air.  To achieve this, the fog seal material (emulsion) must fill the voids in the surface of the pavement.  Therefore, during its application it must have sufficiently low viscosity so as to not break before it penetrates the surface voids of the pavement.  This is accomplished by using a slow setting emulsion that is diluted with water.  Emulsions that are not adequately diluted with water may not properly penetrate the surface voids resulting in excess asphalt on the surface of the pavement after the emulsion breaks, which can result in a slippery surface.  Figure 1 conceptually shows a fog seal application.

During application, the emulsion wets the surface of the aggregate and the existing binder film.  Cationic (positively charged) emulsions can displace water from the surface of an aggregate or aged

asphalt film.  The emulsion then breaks by loss of water and chemical action, forming a film of new binder on the aggregate and existing binder film.  The rate at which the emulsion breaks is dependent on several factors with weather conditions (e.g., wind, rain, temperature, etc.) being dominant factors.  For anionic (negatively charged) emulsions, there is no surface specific interaction with most aggregates.  The emulsion breaks due to water loss by evaporation and absorption of water by the aggregates and surface voids of the pavement.

1.2 Function of a Rejuvenating Seal

Rejuvenating emulsions (e.g. Reclamite (oil emulsion), PASS (asphalt, oil and additives) and Topien C (asphalt, oil and additives)) have oils that soften the existing binder, thus reducing its viscosity.  These also improve the flexibility of the binder, which reduces the likelihood of cohesive failure.  This may be beneficial in situations where the surface has an open texture and the existing binder is brittle from age.  As with conventional emulsions, if these types of emulsion do not penetrate the surface, they may create a slippery surface after they break.