The limitation of minimum temperatures of SMA laydown has been commonly understood and adopted; however, it is quite a different matter when laying down the SMA mixture on an underlying layer that has an excessive temperature, as in the following examples:
• On a recently placed intermediate course that has not yet cooled
• While hot recycling in situ (or in place) with the simultaneous placement of a new wearing course, so-called hot remix plus
• During the execution of the so-called Kompaktasphalt—the laydown and compaction of two courses (intermediate and wearing) at the same time with one passage of a paver (see Section 10.5).
As a general rule (see Section 10.4), rollers compacting an SMA layer operate directly behind the paver. This practice is correct in typical circumstances since the mixture cools quickly. But when the underlying layer is warm, such a method may create some problems, with the most frequent one resulting in the binder bleeding under the rollers and creating fat spots.
The following comments and observations deserve mention:
• The rolling operation cannot be started until the spread mixture has reached the optimum start temperature related to the viscosity of the applied binder; therefore placing the mixture on an underlying course that is still warm— with temperature exceeding 50-70°C (e. g., on a course made several hours earlier)—should be accompanied by ongoing temperature control.
• Because of the warm underlying course, the SMA does not cool as fast, consequently giving more time for compacting and enabling a greater distance between the rollers and the paver.
• In cases of fat spots being squeezed out by the rollers, rolling should be stopped until the mixture temperature in the course drops to a point where rolling does not overcompact the mat; the time (or temperature) at which rolling can resume should be determined experimentally for specific binders.
Unfavorable air temperatures (too low) and an excessively warm underlying course cause various problems. Having an excessively warm underlying layer creates more favorable laydown conditions because this allows for more time to work with the mix.
