PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 256

7.2 Mixing, Placing, and Handling

Fresh Concrete

The proper batching, mixing, and handling of fresh concrete are important
prerequisites for strong and durable concrete structures. Next we will dis-
cuss the basic steps and precautions to be followed in mixing and handling
fresh concrete (Mehta and Monteiro 1993; American Concrete Institute
1982; American Concrete Institute 1983).
Batching is measuring and introducing the concrete ingredients into the
mixer. Batching by weight is more accurate than batching by volume, since
weight batching avoids the problem created by bulking of damp sand. Water
and liquid admixtures, however, can be measured accurately either by
weight or volume. On the other hand, batching by volume is commonly used
with continuous mixers and when hand mixing.
Concrete should be mixed thoroughly, either in a mixer or by hand,
until it becomes uniform in appearance. Hand mixing is usually limited to
small jobs or situations in which mechanical mixers are not available. Me-
chanical mixers include on-site mixers and central mixers in ready-mix
plants. The capacity of these mixers varies from to ( to
). Mixers also vary in type, such as tilting, nontilting, and pan-type
mixers. Most of the mixers are batch mixers, although some mixers are
continuous.
Mixing time and number of revolutions vary with the size and type of
the mixer. Specifications usually require a minimum of 1 minute of mixing
for stationary mixers of up to of capacity, with an increase
of 15 seconds for each additional of capacity. Mixers are usually
charged with 10% of the water, followed by uniform additions of solids and
80% of the water. Finally, the remainder of the water is added to the mixer.

1.5 m 3 9 m 3 2 yd 3

12 yd 3

0.75 m 3 1 yd 3 1 2

0.75 m 3

■ 7.2.1 Ready-Mixed Concrete

Ready-mixed concrete is mixed in a central plant, and delivered to the job
site in mixing trucks ready for placing (Figure 7.4). Three mixing methods
can be used for ready mixed concrete:

1. Central-mixed concrete is mixed completely in a stationary mixer and
delivered in an agitator truck (2 rpm to 6 rpm).
2. Shrink-mixed concrete is partially mixed in a stationary mixer and
completed in a mixer truck (4 rpm to 16 rpm).
3. Truck-mixed concrete is mixed completely in a mixer truck (4 rpm to
16 rpm).

Truck manufacturers usually specify the speed of rotation for their
equipment. Also, specifications limit the number of revolutions in a truck