Civil Engineering Reference
In-Depth Information
1. Central-mixed concrete is mixed completely in a sta-
tionary mixer (Fig. 10-3) and is delivered either in a
truck agitator (Fig. 10-4 bottom), a truck mixer oper-
ating at agitating speed (Fig. 10-3), or a nonagitating
truck (Fig. 10-4 top). Fig. 10-5 illustrates a central mix
ready mix plant.
2. Shrink-mixed concrete is mixed partially in a sta-
tionary mixer and completed in a truck mixer.
3. Truck-mixed concrete is mixed completely in a truck
mixer (Fig. 10-6).
ASTM C 94 (AASHTO M 157) notes that when a truck
mixer is used for complete mixing, 70 to 100 revolutions of
the drum or blades at the rate of rotation designated by
the manufacturer as
mixing speed
are usually required to
produce the specified uniformity of concrete. All revolu-
tions after 100 should be at a rate of rotation designated by
the manufacturer as
agitating speed
. Agitating speed is usu-
ally about 2 to 6 rpm, and mixing speed is generally about
6 to 18 rpm. Mixing at high speeds for long periods of
time, about 1 or more hours, can result in concrete
strength loss, temperature rise, excessive loss of entrained
air, and accelerated slump loss.
When truck mixers are used, ASTM C 94 (AASHTO M
157) also limits the time between batching and complete
discharge of the concrete at the job site; this time is 1
1
⁄
2
hours or before the drum has revolved 300 times after
introduction of water to the cement and aggregates or the
cement to the aggregates. Mixers and agitators should
always be operated within the limits for volume and speed
of rotation designated by the equipment manufacturer.
Fig 10-4. (top) Nonagitating trucks are used with central-
mix batch plants where short hauls and quick concrete
discharge allows the rapid placement of large volumes of
concrete. (bottom) Truck agitators are also used with
central-mix batch plants. Agitation mixing capabilities allow
truck agitators to supply concrete to projects with slow
rates of concrete placement and at distances greater than
nonagitating trucks. (69897, 69898)
Fig. 10-3. Central mixing in a stationary mixer of the tilting
drum type with delivery by a truck mixer operating at
agitating speed. (69926)
