(4) Field-Molded Sealants . The following types of materials, as 

 listed in Table 20, are currently used as field-molded sealants. 



(a) Mastics . Mastics are composed of viscous liquid rendered 

 immobile by the addition of fibers and fillers. They do not usually 

 harden, set, or cure after application, but instead form a skin on the 

 surface exposed to the atmosphere. The vehicle in mastics may include 

 drying or nondrying oils (including oleoresinous compounds), polybutenes, 

 polyisobutylenes, low -melting point asphalts, or combinations of these 

 materials. With any of these, a wide variety of fillers is used, including 

 asbestos fiber, fibrous talc, or finely divided calcareous or siliceous 

 materials. The functional extension-compression range for these materials 

 is approximately ±3 percent. 



(b) Thermoplastics, Hot-Applied . These are materials which 

 soften on heating and harden on cooling usually without chemical change. 

 They are generally black and include asphalts, rubber asphalts, pitches, 

 coal tars, and rubber tars. They are usable over an extension-compression 

 range of ±5 percent. This limit is directly influenced by service tempera- 

 tures and aging characteristics of specific materials. Though initially 

 cheaper than some of the other sealants, their effective life is, in 

 practice, shorter. They tend to lose elasticity and plasticity with age, 

 to accept rather than reject foreign materials, and extrude from joints 

 that close tightly or that have been overfilled. Overheating during the 

 melting process adversely affects the properties of those compounds con- 

 taining rubber. Those with an asphaltic base are softened by hydrocarbons, 

 such as oil, gasoline, or jet fuel spillage. Tar-based materials are fuel 

 and oil resistant, and these are preferred for service stations, refueling 

 and vehicle parking areas, airfield aprons, and holding pads. 



Use of this class of sealants is restricted to horizontal joints since 

 they would run out of vertical joints when installed hot or subsequently in 

 warm weather. They have been widely used in pavement joints, but they tend 

 to be superseded by chemically-curing thermosetting field-molded sealants 

 or compression seals. They are also used in building roof decks and con- 

 tainers. 



(c) Thermoplastics, Cold-Applied (solvent or emulsion type) . 

 These materials are set either by the release of solvents or the breaking 

 of emulsion on exposure to air. They are sometimes heated to a temperature 

 not exceeding 49 Celsius (120° Farenheit) to facilitate application but 

 are usually handled at ambient temperature. Release of solvent or water 

 can cause shrinkage and increased hardness with a resulting reduction in 

 the joint movement permissible and in serviceability. Products in this 

 category include acrylic, vinyl and modified butyl types which are available 

 in a variety of colors. Their maximum extension-compression range is ±7 

 percent. Heat softening and cold hardening may, however, reduce this 

 figure. These materials are restricted in use to joints with small move- 

 ments. Acrylics and vinyls are used in buildings, mainly for calking and 

 glazing. Rubber asphalts are used in canal linings, tanks, and fillers for 

 cracks . 



(d) Thermosetting, Chemically Curing Sealants . Sealants in 

 this class are either one- or two-component systems which cure by chemical 



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