3.2.3
Batching Equipment
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NOTE: Retain the bracketed sentence concerning
truck mixers unless it is desired to prohibit truck
mixers. Always retain bracketed item about silica
fume when its use is allowed, otherwise delete.
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The batching controls shall be semiautomatic or automatic, as defined in
NRMCA CPMB 100. A semiautomatic batching system shall be provided with
interlocks such that the discharge device cannot be actuated until the
indicated material is within the applicable tolerance. The batching system
shall be equipped with accurate recorder or recorders that meet the
requirements of NRMCA CPMB 100. The weight of water and admixtures shall
be recorded if batched by weight. Separate bins or compartments shall be
provided for each size group of aggregate and type of cementitious
material, to prevent intermingling at any time. Aggregates shall be
weighed either in separate weigh batchers with individual scales or,
provided the smallest size is batched first, cumulatively in one weigh
batcher on one scale. Aggregate shall not be weighed in the same batcher
with cementitious material. If both portland cement and other cementitious
material are used, they may be batched cumulatively, provided that the
portland cement is batched first, [except that silica fume shall always be
batched separately]. Water may be measured by weight or volume. Water
shall not be weighed or measured cumulatively with another ingredient.
Filling and discharging valves for the water metering or batching system
shall be so interlocked that the discharge valve cannot be opened before
the filling valve is fully closed. Piping for water and for admixtures
shall be free from leaks and shall be properly valved to prevent backflow
or siphoning. Admixtures shall be furnished as a liquid of suitable
concentration for easy control of dispensing. An adjustable, accurate,
mechanical device for measuring and dispensing each admixture shall be
provided. Each admixture dispenser shall be interlocked with the batching
and discharging operation of the water so that each admixture is separately
batched and individually discharged automatically in a manner to obtain
uniform distribution throughout the water as it is added to the batch in
the specified mixing period. [When use of truck mixers makes this
requirement impractical, the admixture dispensers shall be interlocked with
the sand batchers]. Different admixtures shall not be combined prior to
introduction in water and shall not be allowed to intermingle until in
contact with the cement. Admixture dispensers shall have suitable devices
to detect and indicate flow during dispensing or have a means for visual
observation. The plant shall be arranged so as to facilitate the
inspection of all operations at all times. Suitable facilities shall be
provided for obtaining representative samples of aggregates from each bin
or compartment, and for sampling and calibrating the dispensing of
cementitious material, water, and admixtures. Filling ports for
cementitious materials bins or silos shall be clearly marked with a
permanent sign stating the contents.
3.2.4
Scales
The weighing equipment shall conform to the applicable requirements of CPMB
Concrete Plant Standard, and of NIST HB 44, except that the accuracy shall
be plus or minus 0.2 percent of scale capacity. The Contractor shall
provide standard test weights and any other auxiliary equipment required
for checking the operating performance of each scale or other measuring
SECTION 03300A
Page 36
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