Figure 8-17.-Basic drum-mix asphalt plant Mixing Time Total    mixing    time    begins    when    all the   combined   mineral   aggregates   are   in   the mixer    and    ends    when    the    mixer    discharge gate     is     opened.     Dry     mixing     time,     when specified,     begins     when     all     the     combined mineral    aggregates    are    in    the    mixer    and ends    with    the    introduction    of    the    asphalt. Wet    mixing    time    begins    with    the    start    of the    asphalt    application    and    ends    with    the opening    of    the    mixer    discharge    gate. The     asphalt     film     on     aggregate     is hardened     by     exposure     to     air     and     heat; therefore, mixing time should be the shortest    time    required    to    obtain    a    uniform distribution of aggregate sizes and a uniform coating of asphalt on all aggregate particles. The speed of the mixer   shafts   and   the   arrangement   and   pitch of    the    paddles    are    factors    governing    the efficiency    of    the    mixing. To    aid    further    the    operation    of    a continuous     mix     plant,     you     can     add     or extend     several     automatic     controls. These    include    the    following: 1.    Automatic    burner    controls 2.    Automatic    mix    discharge 3.   Automatic   mixer   and   gradation cutoffs in case of hot-bin shortage or improper     feed 4. Electric interlocks that shut down    the    complete    plant    in    case    of    a failure    anywhere    in    the    electric    system The operator’s manual for the particular     plant     being     used     gives     details on the setup and adjustments of the automatic equipment included with the plant. NOTE: Normally all automatic systems    have    manual    override.    You    should know   where   it   is   located   and   how   to   use it. See the manufacturer’s manual for specific details. DRUM-MIX ASPHALT PLANT The     drum-mix     plant     is     shown     in figure   8-3.   The   mixing   drum   for   which   the plant is named is very similar in appearance   to   the   batch   plant   dryer   drum. The    difference    between    the    two    is    that    in a   drum-mix   plant   the   aggregate   is   not   only dried    and    heated    within    the    drum,    but    it is    also    mixed    with    the    asphalt    cement.    In a    drum-mix    plant,    there    are    no    gradation screens, hot bins, weigh-hoppers or pugmills. Aggregate gradation is controlled    at    the    cold    feed. The    basic    plant    consists    of    a    cold- feed     system,     a     rotating     drum     dryer,     an asphalt proportioning and dispensing system,   and   a   surge   silo   (fig.   8-   17).   The ease    of    setup    and    operation    of    the    drum- mix    plant    makes    it    the    ideal    machine    for operations. Aggregate Storage and Feed Aggregate gradation and uniformity are entirely dependent on the cold-feed system.    Proper    care    must    be    exercised    not only    in    producing    the    aggregate    but    also in    storage.    Aggregates    used    for    drum-mix plants must be received, handled, and stored    to    ensure    there    is    no    danger    of contamination or intermingling. Stockpiles     must     be     properly     graded and split into different sized fractions to control the gradation of the mix properly. Uncorrected segregated stockpiles will result in mix gradation difficulties. The plant supervisor should establish     and     maintain     stockpiles     in     the most     economical     manner     and     correct     any deficiencies in uniformity before the aggregate    is    fed    into    the    mixing    plant. Since the typical drum-mix plant does not have a gradation unit, the aggregate must be proportioned before entering the mixing drum. This is accomplished     with     a     multiple-bin     cold-feed system equipped with precision belt feeders for control of each aggregate. 8-16