Warm Mix History
1920–2008
All of the "warm mix" technologies currently being promoted include the introduction of water into the asphalt cement in order to cause foaming. Foaming aids in distribution of the AC across the aggregate surfaces to obtain maximum coating and temporarily lowers the AC viscosity. Some of the technologies employ additional chemicals to improve the AC adhesion and extend the foaming reaction and/or prolong the reduction of viscosity, but this is not necessarily new technology. Patents dating back into the 1920s disclose drum mixers that spray soap solutions on the aggregates prior to AC coating. These patents disclose aggregate temperatures of about 250 degrees Farenheight in the mixer section – very much in line with today’s targets.
In 1965, K.E. McConnaughay filed for a drum mixing process known as the "turbulent mass mixer," which came into commercial use in the early 1970s. This patent described a counter-flow drum mixer where the process introduced water, additives, and asphalt simultaneously that created a thoroughly coated mix at low temperatures. The patent described in detail the reaction as "turbulent mass" and "cloud", what we now more commonly call "foamed." This process was first commercialized in 1970, and by the end of 1971 had produced over 170,000 tons.
In 1974, R. Kenyon Construction had several of these systems that produced "warm mix" rather than hot mix. Mixing temperatures were normally in the range of 150 to 170 degrees Farenheight for base and binder courses and surface course temperatures were between 190 to 230 degrees Farenheight. The additives that were used are similar to those being marketed today to improve adhesion. At that time, there was a considerable advantage in mixing at lower temperatures and still being able to produce quality mixes while meeting all the requirements and specifications of the highway department. Fuel savings was reported as a distinct advantage even in the days of low fuel costs. Millions of tons and hundreds of miles of Iowa roads were produced with turbulent mass mixers and with very positive results. Other reported advantages were less rolling and compaction and improved smoothness. In fact, one particular Iowa Contractor with a "turbulent mass" mixing plant won an award for smooth pavement.
In 1973, the Boeing Construction Equipment Company introduced drum mixers based on technology patented by H.N. Shearer that used a similar low temperature process but in a parallel flow drum. Boeing quickly dominated the market with this improved technology, selling more plants than all other manufacturers. The early asphalt injection caused a foaming reaction between the hot asphalt oil and the water vapor released in the drum boiling zone, yielding superior aggregate coating at all mix temperatures. The low mix temperatures and early asphalt injection also yielded complete aggregate coating and proper residual moisture for superior compaction.
With the Shearer process, the product was still easy to place and compact since the asphalt viscosity was low. This low viscosity was achievable due to moisture which was purposely left in the "dried but coated aggregate." This internal moisture would continue to be expelled as steam from the mix and in doing so, kept the asphalt film foamed. The secret to the success of this process was to leave enough moisture in the aggregates that this continuous foaming would occur, but not so much that it would cause the asphalt to strip from the aggregate surface.
Gencor Industries, the successor to Boeing, purchased BCE in the 1980s. Today, Gencor continues on its strong heritage of producing innovative "Green" technologies to the industry that are simple in design, durable in construction, and cost effective to operate for contractors.