blades shall be turned a minimum of 30 additional revolutions at mixing
speed. Water shall not be added to the batch at any later time.]
1.12.2.3
Mixing Time and Uniformity
**************************************************************************
NOTE: Retain bracketed subparagraph if truck mixers
are permitted. Otherwise, delete.
**************************************************************************
a. Stationary Mixers: For stationary mixers, before uniformity data
are available, the mixing time for each batch after all solid materials
are in the mixer, provided that all of the mixing water is introduced
before one-fourth of the mixing time has elapsed, shall be 1 minute for
mixers having a capacity of 0.75 cubic meter 1 cubic yard. For mixers
of greater capacity, this minimum time shall be increased 20 seconds
for each additional cubic meter 1.33 cubic yard or fraction thereof.
After results of uniformity tests are available, the mixing time may be
reduced to the minimum time required to meet uniformity requirements;
but if uniformity requirements are not being met, the mixing time shall
be increased as directed. The mixing time for full batch production
shall be a minimum of 75 seconds. Mixer performance tests at new
mixing times shall be performed immediately after any change in mixing
time. When regular testing is performed, the concrete shall meet the
limits of any five of the six uniformity requirements listed in Table 1
below. When abbreviated testing is performed, the concrete shall meet
only those requirements listed for abbreviated testing. The concrete
proportions used for uniformity tests shall be as used on the project.
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 the approval of the Contracting Officer. All mixer
performance (uniformity) testing shall be performed in accordance with
COE CRD-C 55 and with paragraph titled TESTING AND INSPECTION FOR
CONTRACTOR QUALITY CONTROL.
TABLE 1
UNIFORMITY REQUIREMENTS--STATIONARY MIXERS
Regular Tests
Abbreviated Tests
Allowable
Allowable
Maximum Range for
Maximum Range
Parameter
Average of 3 Batches
for 1 Batch
___________
______________________
____________________
Unit weight of air-free mortar
32 kg/cubic m
32 kg/cubic m
(Unit weight of air-free mortar
2.0 lbs/cubic ft
2.0 lbs/cubic ft)
Air content
1.0 percent
--
Slump
25 mm
25 mm
(Slump
1.0 inch
1.0 inch)
Coarse aggregate
6.0 percent
6.0 percent
SECTION 02753
Page 28
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