TM 5-822-7/AFM 88-6, Chap. 8
slabs are comparatively small, the plant capacity
c. Concrete mixers. Mixers having a capacity of at
least 5 cubic yards of mixed concrete are required for
generally will not be influenced by any requirements
for maintaining the concrete in a plastic condition
airfield paving projects, but smaller mixers may be
during placement. The main considerations will be
permitted for small road projects and other small
miscellaneous construction. Contract specifications
the required placing schedule to meet the completion
for jobs of suitable size and location will include an
date for the construction or, when slipform pavers
are used, the required amount of concrete to maintain
option for the contractor to reduce mixing time during
a uniform forward movement of the paver of not less
progress of work from the mixing time required at
the start of the job on the basis of mixer performance
than 2.5 feet per minute. However, the placement
rate specified for pavements constructed during hot
tests in accordance with CRD-C 55, provided the
following requirements are met at all times:
weather should also be considered in determining
plant capacity requirements.
Requirement, expressed as maximum permissible
Parameters Tested For:
range in results of tests of samples taken from three
locations in the concrete batch
Weight per cubic foot of mortar calculated to an air-
3
1.0
free basis, lb/ft
1.0
Air content, volume percent of concrete
1.0
Slump, inches
Coarse aggregate content, portion by weight of each
6.0
sample retained on No. 4 sieve, percent
Average compressive strength at 7 days for each
sample based on average strength of all test speci-
10.0
mens, percent
Water content, portion by weight of each sample
1.0
passing No. 4 sieve, percent
the inadequacy of the mixing or transportation
In addition, no reduction in mixing time will be
equipment to mix and discharge the concrete.
allowed if it results in a reduction in the 7-day
flexural strength from the average strength of five
d. Approval of mixers. Before truck mixers or
stationary mixers are approved for use, careful
consecutive sets of specimens made from batches
consideration will be given to the proposed plant and
mixed the full time. The requirements for stationary
facilities for storage and handling of materials, and
mixers and truck mixers will be included unless it is
for batching, mixing, transporting, and handling of
determined that concrete complying with the specifi-
concrete at the jobsite to assure that adequate control
cations cannot be manufactured by these types of
of the concrete can be exercised. When truck mixers
mixers. Truck mixers often have difficulty in
are used with a long haul between the batching plant
discharging the comparatively low-slump concrete
mixtures required for airfield pavement construction,
and the project, adequate control of the concrete
may be difficult due to variations in slump and air
therefore, truck mixers should comply with ASTM C
content caused by differences in mixing time. In
94. The tendency of the concrete to hang up in
such cases, it will be necessary to require that mixing
discharge chutes may cause delays, which frequently
be done after the mixer trucks arrive on the job.
lead to requests by the contractor that the concrete
Truck mixers will not be permitted for mixing concrete
slump be increased. This difficulty also may occur
on slipform construction. Truck mixers shall be
with vehicles used for hauling concrete to forms
equipped with accurate revolution counters. In
when stationary mixers are used. The slump of
general, truck mixers should only be used on jobs
ready-mix concrete will not be increased because of
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