Mixers shall be stationary mixers [or truck mixers]. Each mixer shall
combine the materials into a uniform mixture and discharge this mixture
without segregation. Mixers shall not be charged in excess of the capacity
recommended by the manufacturer on the nameplate. Excessive over-mixing
requiring introduction of additional water will not be permitted. The
mixers shall be maintained in satisfactory operating condition, and mixer
drums shall be kept free of hardened concrete. Mixer blades or paddles
shall be replaced when worn down more than 10 percent of their depth when
compared with the manufacturer's dimension for new blades. Should any
mixer at any time produce unsatisfactory results, its use shall be promptly
discontinued until it is repaired or replaced.
3.1.3.1
Stationary Mixer Uniformity Requirements
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NOTE: The option for the government to perform the
initial mixer evaluation may be invoked.
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The size of the batch, the mixing time, the charging sequence, and other
factors identified by the contractor shall be adjusted to provide concrete
that meets the uniformity limits specified herein. All testing shall be
performed in accordance with COE CRD-C 55. When regular testing is
performed, the concrete shall meet the limits of any five of the six
uniformity requirements. When abbreviated testing is performed, the
concrete shall meet only those requirements listed for abbreviated testing.
The initial mixer evaluation test shall be a regular test and shall be
performed prior to the start of concrete placement. The concrete
proportions used for the evaluation shall contain the largest size
aggregate on the project and shall be as directed. Regular testing shall
consist of performing all six tests on three batches of concrete. The
range for regular testing shall be the average of the ranges of the three
batches. Abbreviated testing shall consist of performing the three
required tests on a single batch of concrete. The range for abbreviated
testing shall be the range for one batch. If more than one mixer is used
and all are identical in terms of make, type, capacity, condition, speed of
rotation, etc., the results of tests on one of the mixers shall apply to
the others, subject to approval. Mixer evaluations shall be performed by
the Contractor in accordance with paragraph MIXER UNIFORMITY. [However,
the initial evaluation will be performed by the Government. The Contractor
shall provide labor and equipment as directed to assist the Government in
performing any evaluation made by the Government.]
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NOTE: The following table to be used with SI units.
**************************************************************************
ABBREVIATED
REGULAR TESTS
TESTS
ALLOWABLE
ALLOWABLE
MAXIMUM RANGE FOR
MAXIMUM RANGE
PARAMETER
AVERAGE OF 3 BATCHES
FOR 1 BATCH
Unit weight of air-free
mortar, kg/m3
32
32
Air content, percent
1.0
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SECTION 03700
Page 51
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