SECTION 401
to produce a properly and uniformly mixed batch. If, in the course of mixing, two adjacent paddle tips become broken, immediate
repair will be called for. If the paddle tips become broken at widely separated points, repair may be delayed until the end of the
working day. The clearance of the tips from all fixed and moving parts shall not exceed 3/4”. Badly worn or defective tips shall
not be used in mixing operations. The mixer shall be covered to prevent loss of fine material. The discharge gate shall be so
designed that no uncoated material is retained at the gate opening during the mixing operation. Leakage from the pug mill gate
during operation will not be permitted.
3.4.9 Each plant shall be equipped with an accurate time lock to contro! the operations of a complete mixing cycle. A
mixing cycle shali consist of two periods, the dry mixing period and the wet mixing period. The dry mixing period shall be the
interval of time between the opening of the aggregate weigh hopper gate and the start of the application of asphalt binder. The
wet mixing period shall be the interval of time between the start of the application of asphalt binder and the opening of the mixer
gate. The time lock shall be capable of being set at intervals of five seconds or less throughout the mixing cycle and shall have
a suitable case equipped with an approved lock. The setting of time intervals shall be performed in the presence and under the
direction of the Engineer who may lock the case until such time as a change is to be made in timing periods. The time lock shail
lock the asphalt binder bucket throughout the dry mixing period and shall lock the mixer gate throu ghout the dry and wet mixing
period.
3.4.10 | The use of a fully automatic batching plant will be required and shall meet the following requirements:
(a) The automatic proportioning controls shall include equipment for accurately proportioning batches of the various
components of the mixture by weight in the specified sequence and for controlling and timing the mixing operation.
Interlocks shall be provided that delay, stop, or lock out the automatic batch cycling whenever the batched quantity
of any component weight or the total batch is not within the specified weight tolerance, or when there is a
malfunction in any portion of the control system.
(b) The automatic control for each batching scale system shall be equipped with a device for stopping the automatic
cycle in the underweight check position and in the overweight check position for each material so that the tolerance
setting may be checked.
(c) Each dial scale system shall be equipped with a removable dial puller that can be attached to the dial lever system
so that the dial can be moved smoothly and slowly through its range to check the settings of the automatic control
system. The plant operator shall perform this automatic control system checkout procedure periodically as
requested by the Engineer.
(d) The weigh batching controls shall meet the following tolerances for the various components weighed in each batch:
Percentage of
Component Weighed Total Batch Weight
Tare weight of aggregate weigh box +0.5
‘Tare weight of asphalt binder w
_Each aggregate component HS
Mineral filler +0.5
Asphalt +0.1
(e) The total weight of the batch shall not vary by more than +2.0 percent of the designated batch weight.
(f) Recording equipment shall be provided in all plants employing automatic proportioning. Each recorder shall
include an automatic printer system. The printer shall be positioned so that the scale reading and the printer can
be readily observed from one location by the plant inspector. The printer shall produce, in digital form, a weight
slip conforming to the requirements of 109.01 and 401.3.8.1.
(g) Ifat any time the automatic proportioning or recording system becomes inoperative, the plant will not be allowed
to operate.
3.4.11 The aggregate shall be dried and heated to a minimum temperature of 260° F. The asphalt binder shall be heated to a
temperature between 260° and 325° F. Each size of hot aggregate, the mineral filler if required, and the bituminous cement shall
be measured separately and accurately to the proportions in which they are to be mixed. The mixture shall be made by charging
the mixer with the hot aggregate, coarse sizes first, unless otherwise directed, which shall be dry mixed for 5 to 15 seconds. The
asphalt binder shall be added and the mixing continued until a uniform coating is obtained and all particles of the aggregate are
thoroughly coated. The total dry and wet cycle shall not be less than 35 seconds for base and binder mixtures and not less than
40 seconds for the wearing course. In no case shall the total mixing period exceed 75 seconds.
3.4.12 Ifthe aggregate in the hot bins contains sufficient moisture to cause foaming in the mixture, such aggregate shall be
removed from the bins, and production rate shall be reduced so as not to exceed the capacity of the dryer. Material having once
gone through the mixing plant shall not be returned to the stockpiles.
2016 NHDOT
Go To=) TOC Division 100 Division 200 Division 300 Division 400 Standard
Division 500 = Division 600 Division 700 Specifications
