BACK TO BASICS SEGREGATION IN THE FIELD The most common cause of segregation related to the feed system is the conveyors and augers running at high and inconsistent speeds. High conveyor or auger speeds and on/off auger movement results in longitudinal stripes, patches, and chevrons in the mat. Setting the feed system to run consistently is nearly impossible if the sonic or paddle (mechanical) feed sensors are not positioned or aimed properly. Once feed sensors are properly positioned, the mix height dial on the screed can be used to fine-tune the head of material at onehalf auger height, and the conveyor ratio dials or flow gates can be adjusted to control auger speed in the range of 20 to 40 revolutions per minute (rpm). Set up the feed system as follows: 1. Position sonic or paddle feed sensors 18 inches from the last auger segment to “read” the live, moving mix. Improperly aimed sonic feed sensors are the number one reason for repeated starting and stopping of the augers. This leads to repeated patch segregation. 2. Adjust the mix height dial to get the mix in the auger chamber at one-half auger height. 3. Check auger speed and adjust conveyor ratio dials or flow gates until augers are turning at 20 to 40 rpm. For example, if the auger is moving too fast, turn the conveyor ratio dial up to bring more mix back from the hopper, thus slowing down the auger rpm’s. Use of manual override switches on the feed system should be kept to a minimum, as they cause the conveyors and augers to go full blast. When conveyors and augers run at high speed, rocks are thrown. This will show up as streaks in the mat in line with the end of the auger. Equipment Setup Always have auger and tunnel extensions to move the blended mix to the end gate. For front-mount screeds, have auger extensions and tunnel extensions within about 18 inches of the end gate. For rearmounted extension screeds, it is generally acceptable to have up to 36 inches between the last auger segment and the end gate. It is also important to always have tunnel extensions in front of any auger extensions. If centerline segregation or stripe is present, check that reversing (kick-back) augers are installed next to the gearbox or chain case for the augers. Reversing auger segments moves the mix underneath the chain case to eliminate centerline segregation in the mat. Raise or lower the auger height in small increments of one-quarter to one-half inch to improve mat texture and reduce or eliminate segregation based on visual inspection. Augers should be approximately two inches above the height of the loose mat being placed. Spills on the Grade The main causes of the mix spilled on the grade are: • Hopper flashing in poor condition, allowing the mix to spill in front of the paver • Raising the truck bed too fast and spilling mix on the grade • Trucks pulling away and banging tailgates to clean out in front of the paver On thinner lifts, high spots caused by mix spilling on the grade will cause aggregates to drag as the screed passes over it and give the appearance of a “rock pocket.” Material spilled on the grade will cool quickly and never properly compact, even after rolling. Remember that every time a mix is handled or moved, it is prone to segregating. Move mix in mass as much as possible and ensure the feed system is set up properly. Paying attention to these details around the paver will ensure a segregation-free mat that will last for years to come! VAASPHALT.ORG 25
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