- Asphalt mixing mix drums can be grouped into two categories:
The main difference of the convective drums over the 'lift and drop' ones is the shearing or cutting action imparted to the mixture during the mixing process. Convective mixers impart much shear or cut and tend to be used for cohesive materials (with RAP).
The 'Lift and drop' drums exert a very low shear, so they are used when the materials do not agglomerate or compress.
The selection of the type of drum mixer is given according to the properties of the asphalt mixes that are desired to be handled.
Shearing effects in the asphalt mixture.
- 'Lift and drop': Mixer drums by means of paddles.
- Convective: Double barrel type drums and mixer drums equipped with 'Pugmill'.
Convective mixing drums
- Non-cohesive (free-flowing) mixtures do not cause interactions between their particles, so whether or not there is a shear effect, has little or no effect on the final mixture. However, in the case of cohesive mixtures (with RAP) the presence of shear can have a significant impact on the homogeneity of the final mixture.
- 'Double Barrel' drum mixer: From the manufacturer Astec©, it has the action of a simple shaft paddle mixer.
- Drum mixers equipped with 'Pugmill': From the manufacturers Triaso, Amman, Ciber, etc., have the action of double synchronized shaft. It is also called the Forberg type.
- Below is the comparison of the mixing times between a simple shaft vane mixer type 'Double Barrel', and a synchronized double shaft mixer 'Pugmill' type Forberg.
- The synchronized double shaft mixer 'Pugmill' type Forberg achieves an acceptable mix at 27 seconds.
- The Double Barrel type single shaft paddle mixer needs 170 seconds to achieve the same.
A mixing time of 170 seconds causes a lot of oxidation to the asphalt mixture.
This is the reason why in the United States, many Double Barrel drums that work with RAP are being equipped with synchronized double shaft mixers 'Pugmill' type Forberg.