Some thirty years ago Mr. Carll Hunt discovered that there was enough water in the concrete mix to allow the hydration reaction to occur in an uninterrupted way, if the water could be prevented from evaporating. His idea was to spray a liquid compound that would from an impervious film once the carrier, in most cases a solvent, evaporated. The film had to be harmless to the concrete, free of pinholes, capable of adhering strongly to the surface, and able to prevent the concrete mixing water from evaporating, thus providing sufficient water readily available for the hydration reaction of the concrete to occur within the mass itself in a continuous manner. This concept marked the birth of the liquid membrane curing compounds which are widely used in the United States and rapidly gaining acceptance throughout the rest of the world. The most appropriate time for the application of liquid membrane curing compound is after the concrete has received its final finish and the water sheen has disappeared for the surface. It is important not to apply the compound when standing water is present on the concrete surface. Standing water will prevent the formation of a continuous film free of pinholes and voids, thus allowing the evaporation of the water from the concrete mix. This will result in weak, soft, porous, improperly cured concrete. All concrete must be cured, regardless of whether it is on a horizontal or on a vertical surface, in a driveway, a residential slab or a major concrete structure. The type of liquid curing compound used will vary somewhat with the type of construction. On engineering projects, such as highways, the white pigmented liquid type is most widely used. Gray pigmented compounds are used mostly on Bureau of Reclamation projects. The requirements for such construction projects as high rise buildings are different from those for engineering projects due to the need to apply paint and tile adhesives to the concrete surface. The most commonly used are clear curing compounds. The black curing compound has been somewhat neglected because of its color, but it can be used to good advantage in such places as backfilled with dirt and on slabs that will receive asphalt.
