Scrub seals were very crude in the early years. As depicted below, you can see one of the first scrub seal setups. A horse and carriage pulling a small homemade broom and scrubbing asphalt into the roadway. In the late 1980's, California and Missouri were some of the first state DOTs to use scrub seals. These scrub seals were comprised of an asphalt emulsion that was scrubbed into the roadway and then covered with sand. Some referred to this treatment as a sand seal.
In the late 1990's, a polymer modified rejuvenating emulsion (PMRE) was developed by a company called Western Emulsions and this technology combining an asphalt emulsion, a polymer and a rejuvenator took the scrub seal to a new level. The rejuvenator replenishes the lost oils in the oxidized pavement and the polymer modified emulsion provides a surface that can hold stone as opposed to sand.
The broom has also transformed from a series of crossed 2'x4's with broom heads to now metal frames with specialized broom heads that allow the emulsion to be scrubbed into cracks. These newer brooms have wireless operation, variable widths, and adjustability to apply more or less broom pressure.