Civil Engineering Reference
In-Depth Information
Fig. 10-7. Mobile batcher measures materials by volume
and continuously mixes concrete as the dry ingredients,
water, and admixtures are fed into a mixing trough at the
rear of the vehicle. (54087)
Fig. 10-6. Truck-mixed concrete is mixed completely in a
truck mixer. (1153)
setting becomes a persistent problem, a retarder may be
used to control early hydration, especially in high-
cement-content mixes. Mixture adjustments at the jobsite
for air entrainment, and the addition of other admix-
tures, is permitted, followed by sufficient mixing.
Indiscriminate addition of water to make concrete
more fluid should not be allowed because this lowers the
quality of concrete. The later addition of water and
remixing to retemper the mixture can result in marked
strength reduction.
Mobile Batcher Mixed Concrete
(Continuous Mixer)
Mobile volumetric mixers are special trucks (Fig. 10-7)
that batch by volume and continuously mix concrete as
the dry concrete ingredients, water, and admixtures are
continuously fed into a mixing trough, typically an auger
system. The concrete must conform to ASTM C 685
(AASHTO M 241) specifications and is proportioned and
mixed at the jobsite in the quantities needed. The concrete
mixture is also easily adjusted for project placement and
weather conditions.
TRANSPORTING AND
HANDLING CONCRETE
Good advanced planning can help choose the appro-
priate handling method for an application. Consider the
following three occurrences that, should they occur
during handling and placing, could seriously affect the
quality of the finished work:
Delays.
The objective in planning any work schedule is
to produce the fastest work with the best labor force and
the proper equipment for the work at hand. Machines for
transporting and handling concrete are being improved
all the time. The greatest productivity will be achieved if
the work is planned to get the most out of personnel and
equipment and if the equipment is selected to reduce the
delay time during concrete placement.
Early Stiffening and Drying Out.
Concrete begins to
stiffen as soon as the cementitious materials and water are
mixed, but the degree of stiffening that occurs in the first
30 minutes is not usually a problem; concrete that is kept
agitated generally can be placed and compacted within 1
1
⁄
2
hours after mixing unless hot concrete temperatures or
Remixing Concrete
Fresh concrete that is left to agitate in the mixer drum
tends to stiffen before initial set develops. Such concrete
may be used if upon remixing it becomes sufficiently
plastic to be compacted in the forms. ASTM C 94
(AASHTO M 157) allows water to be added to remix the
concrete when the truck arrives on the jobsite and the
slump is less than specified providing the following con-
ditions are met: (1) maximum allowable water-cement
ratio is not exceeded as calculated including surface
water on aggregates as well as batch water and water
added on site; (2) maximum allowable slump is not
exceeded; (3) maximum allowable mixing and agitating
time (or drum revolutions) are not exceeded; and (4) con-
crete is remixed for a minimum of 30 revolutions at
mixing speed or until the uniformity of the concrete is
within the limits described in ASTM C 94 (AASHTO M
157). Water should not be added to a partial load. If early
