Civil Engineering Reference
In-Depth Information
Segregation from striking reinforcing steel without ade-
quate vibration may also contribute to streaking.
Cold joints are a discontinuity resulting from a delay
in placement that allowed one layer to harden before the
adjacent concrete was placed. The discontinuity can
reduce the structural integrity of a concrete member if the
successive lifts did not properly bond together. The con-
crete can be kept alive by revibrating it every 15 minutes
or less depending on job conditions. However, once the
time of initial setting approaches, vibration should be dis-
continued and the surface should be suitably prepared for
the additional concrete.
Placement lines or “pour” lines are dark lines be-
tween adjacent placements of concrete batches. They may
occur if, while vibrating the overlying layer, the vibrator
did not penetrate the underlying layer enough to knit the
layers together.
Subsidence cracking may occur at or near the initial
setting time as concrete settles over reinforcing steel in rel-
atively deep elements that have not been adequately
vibrated. Revibration at the latest time that the vibrator
will sink into the concrete under its own weight may elim-
inate these cracks.
Defects from overvibration include: (1) segregation as
vibration and gravity causes heavier aggregates to settle
while lighter aggregates rise; (2) sand streaks; (3) loss of
entrained air in air-entrained concrete; (4) excessive form
deflections or form damage; and (5) form failure caused
by excessive pressure from vibrating the same location too
long and/or placing concrete more quickly than the de-
signed rate of pour.
Undervibration is more often a problem than over-
vibration.
Fig. 11-14. A laser level stimulating the sensors on this
screed guides the operator as he strikes off the concrete.
Screed poles and chairs are not needed and fewer workers
are required to place concrete. Laser screeds interfaced
with total station surveying equipment can also strike off
sloped concrete surfaces. (69939)
should not be operated after the concrete has been ade-
quately consolidated.
Because surface vibration of concrete slabs is least
effective along the edges, a spud or poker-type vibrator
should be used along the edge forms immediately before
the vibratory screed is applied.
Vibratory screeds are used for consolidating slabs up
to 250 mm (10 in.) thick, provided such slabs are nonrein-
forced or only lightly reinforced (welded-wire fabric).
Internal vibration or a combination of internal and surface
vibration is recommended for reinforced slabs. More
detailed information regarding internal and external
vibration of concrete can be obtained from
ACI
Committee 309
.
Consequences of Improper Vibration.
Following are
some of the worst defects caused by undervibration:
(1) honeycomb; (2) excessive amount of entrapped air
voids, often called bugholes; (3) sand streaks; (4) cold
joints; (5) placement lines; and (6) subsidence cracking.
Honeycomb results when the spaces between coarse
aggregate particles do not become filled with mortar.
Faulty equipment, improper placement procedures, a con-
crete mix containing too much coarse aggregate, or con-
gested reinforcement can cause honeycomb.
Excessive entrapped air voids are similar to, but not
as severe as honeycomb. Vibratory equipment and oper-
ating procedures are the primary causes of excessive
entrapped air voids, but the other causes of honeycomb
apply too.
Sand streaks results when heavy bleeding washes
mortar out from along the form. A wet, harsh mixture that
lacks workability because of an insufficient amount of
mortar or fine aggregate may cause sand streaking.
CONCRETE SLABS
Concrete slabs can be finished in many ways, depending
on the intended service use. Various colors and textures,
such as exposed-aggregate or a pattern-stamped surface,
may be called for. Some surfaces may require only
strikeoff and screeding to proper contour and elevation,
while for other surfaces a broomed, floated, or troweled
finish may be specified. Details are given in
ACI Commit-
tee 302
,
Kosmatka (1991)
,
Panarese (1995)
,
PCA (1980a)
,
and
Farny (2001)
.
The mixing, transporting, and handling of concrete for
slabs should be carefully coordinated with the finishing
operations. Concrete should not be placed on the subgrade
or into forms more rapidly than it can be spread, struck off,
consolidated, and bullfloated or darbied. In fact, concrete
should not be spread over too large an area before strikeoff,
nor should a large area be struck off and bleed water
allowed to accumulate before bullfloating or darbying.
Finishing crews should be large enough to correctly
place, finish, and cure concrete slabs with due regard for
the effects of concrete temperature and atmospheric con-
