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Canada’s “Rock
to Road” Magazine
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Hot mix segregation - causes and prevention Material segregation is the separation of aggregate particles by size. Hot mix asphalt’s strength is based on the uniform population of the desired aggregate size, liquid asphalt and specified air voids. Any deviation from the targeted specification can lead to poor product stability, lack of density, inappropriate air voids and poor performing hot mix. Material segregation can occur in several areas including the quarry/pit, the hot mix asphalt facility and the paving operation. The following information takes a closer look at these areas.
Proper loading of trucks can reduce material segregation. Quarry/pit Hot mix asphalt facility Hot mix facility areas of concentration: 1. Cold feed blending system. The use of properly designed self-relieving bins and closed loop automated blending systems control product gradation going to the drum or drier. 2. Closing the slat conveyor chop gate at the appropriate time. Also, many chop gates allow for finer material to be carried over the gate opening. 3. Loading and controlled opening of silo batcher. 4. Material level of the silo. Maintaining the material level so that product is being pulled out of the centre of the silo is a good rule of thumb. Silos that consistently run too full or out of the cone will lead to segregation. 5. Truck loading practices that reduce the risk of truck roll. Both longitudinal, along the side of the box, or transverse in the front and rear. Similar to poor stockpiling practices, anytime material is piled, potential for segregation in-creases. Paving operation Four most common types of segregation that occur in the mat: Truck End Segregation: Normally occurs longitudinally and on each side of the lane being paved. Usually caused by improper truck loading, silo segregation, improper truck unloading or running the hopper empty between loads. Centre Line Segregation: Occurs in the centre of the lane. Caused by coarse aggregate discharged from the conveyor system that rolls underneath the auger chain drive area and concentrates in the mat centre. Outboard Auger Bearing Support Segregation: Occurs at outboard auger bearing support of paver. Caused by augers not being run at sufficient speeds. Random Segregation: Occurs with improper mixing either at the batch plant or drum mixer but can occur in other places also. Segregation of all types leads to premature ravelling, bleeding, lack of smoothness, lack of specified density, poor longitudinal and transverse joints, poor visual appearance and unnecessary rehabilitation costs. Quality paving practices minimize segregation: 1. Controlled hopper wing cycling. 2. Correct head of material full width of screed and extensions. 3. Correct lead crown setting. 4. Proper strike-off adjustment. 5. Correct length of auger to convey material to screed end plate.
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