This document is available to the public through the National Technical Information Service, Springfield, Virginia 22161.

Coal tar weathers differently than asphalts. Asphalts weather through the effects of oxidation and sunlight, whereas coal tar appears to weather through the evaporation of oils (26). Coal tar emulsions also differ from asphalt emulsions in that coal tar emulsions cure by water evaporation. Thus, curing time is influenced by humidity and similar environmental factors.

Major forms of distress associated with the use of coal tar emulsion seal coats included cracking, loss of adhesion or wear, and low friction values. Some of the factors involved in distress are listed below.

Cracking

1. Incompatibility between certain asphalts and coal tar
2. Shrinkage of the underlying pavement
3. Shrinkage and brittleness of the coating
4. Sand content
5. Thickness of application
6. Lack of prime coat

Poor Adhesion

1. Poorly cleaned surface of pavement before sealing
2. Mix proportions not correct
3. Poor mixing and placement (construction)
4. Type of latex additive

Low Friction Values

1. Sand loading
2. Type of sand
3. Type and application rate of top coat
4. Use of silicone

Poor Fuel and Water Resistance

1. Cracking
2. Application rate
3. Sand loading

In addition to the problem areas already discussed, Shook (34) reported that sand gradation, sand loading (quantity), and latex particle size are issues about which some suppliers of coal tar emulsion disagree. He also concluded that there appears to be major disagreements among industry suppliers on the desirable formulations of coal tar emulsion seal coats. There also appears to be concern about the adequacy of the Federal Aviation Administration (FAA) specification P-625* to insure that a product will perform adequately in the field. Some suppliers believe that coarse sand in the mixture has a tendency to become dislodged from the sealer when subjected to traffic and tends to damage spray bar nozzles. On the other hand, other suppliers suggest that the coarse sand gives the seal coat added flexibility, adhesion and fuel-resistant characteristics.

Another area of disagreement is the sand loading or quantity in the coal tar mixture. One supplier recommends use of up to 16 to 18 lbs. of sand per gallon of coal tar emulsion, while other suppliers believe that 8 to 10 lbs. of sand per gallon of coal tar emulsion is sufficient. They feel the lower sand loadings should be used to insure adhesion between sand particles and the coal tar. This issue is also addressed by Hergenrother (20).

The issue of latex particle size is another concern of the suppliers. The Federal Aviation Administration (FAA) specification P-625* allows an average latex particle size of 300 to 1500 angstroms. One supplier is concerned that a particle size over 1000 angstroms causes the coal tar emulsion to "conglomerate", while latex particle less then 1000 angstroms allow the seal coat to support more sand.

SUMMARY OF FINDINGS AND RECOMMENDATIONS

Results of the state-of-the-art review indicated that problems with scuffing, cracking, poor friction characteristics, poor adhesion, and poor construction control are often encountered in actual practice.

Most coal tar sealers used on airport pavements are sold as proprietary products. Formulations, type of additive, and sand loadings vary, and optimum formulations are not agreed upon by all suppliers.

The survey indicated that there was a need to develop test procedures that could be used to design seal coat formulations and that could be used for construction quality assurance purposes.

The laboratory study revealed that incompatibilities exist between coal tar and latex additives, which must be taken into consideration.

*Per US DOT/FAA bulletin dated 1/17/91, based on the report "Criteria for Coal Tar Seal Coats" the P-625 spec was withdrawn.

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