Mixing - Caltrans requires aggregate, supplemental fine aggregate and asphalt binder to be mixed in a batch mixer (pugmill), a continuous mixing pugmill mixer or a continuous mixing drier-drum mixer. In all cases the mixing shall continue until a homogeneous mixture of thoroughly and uniformly coated aggregates of unchanging appearance is produced.
Slat Conveyor – A slat conveyor transports completely mixed AC from ground level to the surge batcher at the top of the mix storage silo. Both batch and continuous mix AC plants can use slat conveyors. Continuous mixing AC plants don’t have a choice, all the AC produced from this type of plant must be placed in a silo before being loaded into a truck. A transfer conveyor must be used to transport the completed AC mix from the batch plant pug mill to the slat conveyor.
Reject Chute – All slat conveyors must be equipped with a reject chute. The reject chute is opened whenever the AC mixture is of questionable proportions. This situation may occur at the beginning or end of a plant start-up or shut down. This could also apply to any non-homogenous AC mixture or if the mix temperature is outside of specifications.
Storage - As the completed AC mixture exits the mixing chamber, a drag slat typically transfers the mix to a surge batcher. The surge batcher is mounted on top of the storage silo. Caltrans requires a surge batcher capture at least 1,800 kg of mix before depositing the material into the storage silo. The amount of asphalt concrete or asphalt concrete base in any one silo during mixing shall be a minimum of 18 tonnes, except for the period immediately following a plant shutdown in excess of 2 hours. When asphalt concrete of asphalt concrete base is stored, it shall be stored only in silos. Asphalt concrete or asphalt concrete base shall not be stockpiled. Silos are typically insulated and heated to maintain the mix temperature.
Weights and Measures - Each District employs a Weight’s and Measures Coordinator, responsible for calibration and testing all weighing devices and proportioning equipment. All material producing plants used by Caltrans, are required to be tested and approved prior to any use. If the plant passes the California Test 109 and a safety review, the Weights and Measures Coordinator affixes an HC-17 decal indicating the production tolerances and the date of calibration. The California Test 109 is valid for 6 months on continuous mixing type plants, and one year for batch type plants. If repairs are made that affects any proportioning device or plant controls, a new California Test 109 must be performed before the plant produced materials for Caltrans.
Mass Proportioning - Refers to the method used in determining amounts of each ingredient as measured by the plant controller, prior to being combined and mixed. Mass indicates all the ingredients are proportioned by weigh, such as pounds, kilograms or tons.
Volumetric Proportioning - Refers to the way each ingredient is measured by the plant controller before being mixed together. Volumetric indicates all the ingredients are