PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 261

FIGURE 7.10 Pumping concrete in a retaining wall.

on the application. Internal vibrators are the most common type used on
construction projects (see Figure 7.11). These consist of an eccentric weight
housed in a spud. The weight is rotated at high speed to produce vibration.
The spud is slowly lowered into and through the entire layer of concrete,
penetrating into the underlying layer if it is still plastic. The spud is left in
place for 5 seconds to 2 minutes, depending on the type of vibrator and the
consistency of the concrete. The operator judges the total vibration time
required. Over-vibration causes segregation as the mortar migrates to the
surface.
Several specialty types of vibrators are used in the production of precast
concrete. These include external vibrators, vibrating tables, surface vibra-
tors, electric hammers, and vibratory rollers (Neville 1981).

■ 7.2.6 Pitfalls and Precautions for Mixing Water

Since the water-cementitious materials ratio plays an important role in con-
crete quality, the water content must be carefully controlled in the field.
Water should not be added to the concrete during transportation. Crews
frequently want to increase the amount of water in order to improve work-
ability. If water is added, the hardened concrete will suffer serious loss in
quality and strength. The engineer in the field must prevent any attempt to