Chapter 6: Fog Seals

4.0 Fog Seal Construction

4.1 Site Conditions

To be effective, fog seals need to break quickly (revert to solid asphalt) and cure completely (lose water to form a cohesive film).  This should be at a rate that allows traffic to be accommodated without the binder being picked up by vehicle tires.  To achieve this behavior, the film forming properties of the binder must be adequate (i.e., the binder must be able to coalesce into a continuous film prior to allowing traffic on the new seal).  Asphalt films do not form well at low temperatures in the absence of low viscosity diluents.  Thus, warm conditions with little to no chance of rain are necessary to ensure successful applications.  Fog seals should not to be applied when the atmospheric temperature is below 10°C (50°F), and pavement temperature below 15°C (59°F).

If unexpected rain occurs, prior to the emulsion breaking, the emulsion may wash out of the pores of the pavement and break on the surface of the pavement creating a slippery surface.

4.2 Surface Preparation

Immediately before applying a fog seal, the pavement surface must be cleaned with a road sweeper, power broom, or flushed with a water pump-unit to remove dust, dirt, and debris.  The pavement surface must be clean and dry before applying the fog seal.  If flushing is required, it should be completed 24 hours prior to the application of the fog seal to allow for adequate drying.

4.3 Materials Preparation

Although asphalt emulsions (original emulsions) contain up to 43% water, they must be diluted further before use.  This additional dilution reduces viscosity (see Figure 8) and allows the application of small amounts of residual binder to be adequately controlled.  Generally, the supplier will dilute the original emulsion, in the field or at the plant.  A dilution rate of 50% (1:1) (equal parts water to equal parts emulsion) is recommended.  Dilution water must be potable and free from detectable solids or incompatible soluble salts (hard water).

Water can be checked for compatibility with the emulsion by mixing a small amount of the emulsion in a can (approximately 1 liter).  The materials are mixed for 2 to 3 minutes with a stirrer and the resulting mixture is poured through a pre-wetted 150 mm sieve.  If more than 1% by weight of material is retained on the sieve, the water is not compatible and clogging in spray jets may result.  This test is illustrated in Figure 9.

Incompatible water may be treated with 0.5 to 1.0% of a compatible emulsifier solution (the emulsion manufacturer can provide advice regarding compatible solutions).  The emulsifier solution should be added to the water tanker and circulated for 10 to 15 minutes via pump before adding to the emulsion.  If a water treatment is used, the compatibility test should be repeated using the treated water to ensure compatibility.

The emulsion should be diluted no more than 24 hours before its intended use to avoid settlement of the diluted emulsion. (6) Water is always added to the emulsion and not the other way around.  The emulsion may be circulated using a centrifugal or other suitable pump to ensure uniformity (6). 

4.4 Application Rates and Spraying

Properly calibrated distributor trucks should be used to apply the emulsion (see Figure 2).  Spray nozzles with 4 to 5 mm (1/8” to 3/16”) openings are recommended (6).  The emulsion may be heated to 50°C (122 °F) maximum, although, generally the emulsion is sprayed at ambient temperature (6).  The emulsion is sprayed at a rate determined by the surface conditions (see Table 1).  A test section representative of the entire surface should be chosen to approximate application rates (see Section 4.5).  Typical application rates for diluted emulsion (1:1) range from 0.15 to 1.0 l/m² (0.03 to 0.22 gal/yd²) depending on the surface conditions (5).  A 1:1 diluted emulsion is an original emulsion that has been subsequently diluted with equal parts water.

Ideally, half the application should be sprayed in each direction to prevent build-up on one side of the stones (this is particularly important in the case of chip seals) and rough surfaces.  Build-up on one side can result in a slippery surface and is inadequate binder to fully enrich the surface or hold the stone.

4.5 Estimating Application Rates

To estimate the application rate, take a one-liter can of diluted emulsion (usually 1:1 dilution rate) and pour evenly over an area of 1 m².  This represents a diluted application rate of 1 l/m².  If the emulsion is not absorbed into the surface after 2-3 minutes, decrease the application rate of the emulsion and apply to a new 1 m² area and repeat until the approximate application rate is found.  If, after the first test, the surface looks like it can absorb more emulsion, increase the application rate of the emulsion and spread it over a new 1 m² area.  Repeat until the approximate application rate is found.  This same procedure can be followed using gallons and square yards to determine application rate.

4.6 Traffic Control

Place traffic control before work forces and equipment enters the roadway or work zone.  Traffic control is required both for the safety of the traveling public and the personnel performing the work.  Traffic control includes construction signs, construction cones and/or barricades, flag personnel, and pilot cars to direct traffic clear of the construction operation.  Traffic control details should conform to agency requirements.

Traffic control is also required to protect the integrity of the application.  The curing time for the fog seal material will vary depending on the pavement surface conditions and the weather conditions at the time of application.  Under ideal conditions, including increasing air and surface temperatures, it is suggested that traffic be kept off the fog seal material for at least two hours and until acceptable skid test values are achieved.

4.7 Safety (Personal Protective Equipment–PPE)

All employees should wear and use the safety gear required for a fog seal operation.  This includes, but is not limited to, items such as hard hats, approved shirts, safety vests, earplugs, gloves, and safety glasses (8, 9).

4.8 Quality Control

Quality control and workmanship are critical to the performance and life of a fog seal treatment.  There must be a cooperative effort between the Agency’s representative and the contractor’s representative to conduct inspections of all project equipment before and during the project.  The primary piece of equipment for a fog seal operation is the asphalt distributor.  It is critical that it is functioning as required by the project specifications.  The spray bar must be set to the appropriate height (distance) from the pavement surface and the nozzles must be set at the proper angle to assure a uniform application of material (1).  The material temperatures should also be measured for quality control purposes.

The emulsion must be certified to specification according to established sampling and testing procedures (6).  Excess emulsion can create slick pavements.

It is recommended that project inspections be conducted so that any deficiencies in workmanship or materials are addressed and corrected.  This process will also assist the department in identifying the performance of various fog seal materials; how they are performing on various surface conditions and how they are performing in various climatic zones.

4.9 Post Treatment

Sand blotters may be used at approximately 1 kg/m² (1.8 lb/yd²) to allow early opening to traffic.  Sweeping may be required.  Agency personnel will assess this after application and opening to traffic.  Even with sand cover, traffic control may be required to keep speeds down.

Skid resistance (a coefficient of friction) can be measured using ASTM E 274.  It is recommended that this be done after the application has cured to ensure the proper value is measured.  The final surface shall yield a coefficient of friction not less than 0.30 as determined by ASTM E 274.  A treated pavement shall not be opened to traffic until an acceptable value is recorded.  If a treated pavement does not produce an acceptable coefficient of friction, see Table 3 for corrective action.  Permeability may be monitored by Agency procedures to ensure that an effective seal has been achieved.  This should be done at the discretion of the Resident Engineer.