7.4.3 Vibration—Maintain a uniform duration of vibration
for the particular kind of concrete, vibrator, and specimen mold
involved. The duration of vibration required will depend upon
the workability of the concrete and the effectiveness of the
vibrator. Usually sufficient vibration has been applied as soon
as the surface of the concrete becomes relatively smooth and
large air bubbles cease to break through the top surface.
Continue vibration only long enough to achieve proper consolidation
of the concrete (see Note 15). Overvibration may
cause segregation. Fill the molds and vibrate in the required
number of approximately equal layers (Table 2). Place all the
concrete for each layer in the mold before starting vibration of
that layer. When placing the final layer, avoid overfilling by
more than 1⁄4 in. [6 mm]. When the finish is applied after
vibration, add only enough concrete with a trowel to overfill
the mold about 1⁄8 in. [3 mm], work it into the surface and then
strike it off.