Bitumen grade and compaction temperature

The grade or type of bitumen and its quantity in the mixture are the major factors affecting asphalt compaction. Each grade bitumen, at a specific temperature, has its respective hardness and viscosity. As a consequence, when hard bitumen is incorporated into the asphalt, the compaction at a given temperature is more difficult than the one of asphalt containing soft bitumen. 

To phase this problem, the asphalt produced with hard bitumen is compacted, and thus produced, at higher temperatures. The appropriate compaction temperature is determined by the viscosity/temperature relationship (see Section 3.9.3). 

As a general rule, compaction of asphalt with grade bitumen should never start when the mix temperature is less than 85 C to 90°C. In most cases, insufficient compaction is attributed to the low mix temperature starting the compaction. However, there may be the case that the temperature of the asphalt before compaction is high and the mix cannot sustain the weight of the roller. This situation is solved by simply waiting the temperature of the mix to drop, which only delays compaction procedure. 

Finally, the increase of bitumen content has, up to a point, a beneficial effect on the asphalt workability/compaction. This derives from the fact that the bitumen operates as a lubricant medium between aggregates and decreases aggregate-to-aggregate friction during compaction. However, asphalts rich in bitumen, if they also contain harder bitumen, are more difficult to compact. 

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The factors affecting compaction are (a) aggregate material, (b) bitumen grade and compaction temperature, (c) environmental conditions, (d) layer thickness, (e) compaction equipment and (f) compaction procedure. 

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