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During this period, the asphalt cement is exposed to the air, and some hardening takes place. Usually several minutes are required to obtain complete coating of the aggregate. The laboratory mixing process is accomplished by hand or by machine, with the time necessary to blend the asphalt cement and aggregate depending on the efficiency of the mixing process.
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The extent of this expected variability needs to be considered during the laboratory mix design process. The milling and processing to reclaim material may add a significant amount of fines to the mix. In most instances, however, the aggregate gradation and asphalt content of the reclaimed asphalt pavement actually incorporated into the mix may vary from the values obtained from the representative sample. In the laboratory, the reclaimed asphalt pavement used in the mix design process may be a representative sample of the materials to be recovered from the paving project. However, blending of the reclaimed asphalt pavement with the new aggregate in the laboratory is not subject to the same variability and is subject to the type of plant that will be used to manufacture the hot mix asphalt mix. Blending of reclaimed asphalt pavement with the new aggregate differs for a batch plant, a parallel-flow drum-mix plant, and a counter-flow drum-mix plant. However, the mix plant, the degree of mixing and the transfer of heat from the new aggregate to the reclaimed material are functions of many variables, such as: (i) the amount of reclaimed asphalt pavement in the mix, (ii) the point of introduction of the reclaimed asphalt pavement, (iii) the temperature of the new aggregate, and (iv) the mixing time. If reclaimed asphalt pavement is incorporated into the mix, it is usually mixed in the laboratory until thoroughly heated and blended with the new aggregate. Speight PhD, DSC, in Asphalt Materials Science and Technology, 2016 10.7.3 Mixing Process