The appropriate passage of material through the paver plays a key role in the proper spreading of a mixture. After starting in the hopper, the mixture is moved by slat conveyors (with flow gates in older equipment) to augers and then under a screed. During each of these stages the following significant parameters affect the final result:
• The hopper should be fitted with independently lifting wings, and its shape should eliminate places from which the mixture does not slide to the slat conveyor. Such “dead areas” or “cool corners” create accumulations of cool mixture and cause other problems (see Chapter 11). For the same reasons, the insulation of wings is desirable.
• Care should be taken so that the mixture does not adhere to the walls of the hopper where it cools off fairly quickly. These effects can be seen in various forms of segregation (see Chapter 11). It is perhaps worth dedicating one of the paving crew to systematically throw the cooled remains of mixture down in to the middle of the hopper, particularly when work is done on cool and windy days.
• Completely emptying the hopper of mixture should not be permitted. Newly delivered material should be added to the hopper when it is still filled to about 20% of capacity.
• Augers are intended to divide the mixture across the width of the screed plate; the quality of the layer’s surface depends, among other things, on appropriate adjustments to the augers. The amount of mixture supplied to the augers should be constant; it can be controlled by setting the slat conveyors’ speed and allowing an adequate opening of the flow gates (if applicable).
• The distribution of mixture at the middle of the paver screed plate has a significant influence on the segregation of an SMA mixture. In some machines, there is a feeding screw without a large chain transmission on the axis of the paver; rather the feeding screw is propelled from outside by hydraulic engines and intersecting axis gears.
• The screed plate must be fitted with a heating system. A number of solutions are available and include power supplies, heating fuels, and gas burners. A properly heated screed enables the appropriate travel of the mixture without dragging and pulling out particles.
• Screeds are fitted with vibration systems and rammers. The frequency and amplitude of vibrations and rammers’ strokes should be compatibly matched.
• To ensure quality of the placed layer, a constant paver speed should be maintained. Stoppages should be avoided.
