Civil Engineering Reference
In-Depth Information
crete is placed. Fogging the area during placing and fin-
ishing operations not only cools the contact surfaces and
surrounding air but also increases its relative humidity.
This reduces the temperature rise of the concrete and min-
imizes the rate of evaporation of water from the concrete
after placement. For slabs on ground, it is a good practice
to moisten the subgrade the evening before concreting.
There should be no standing water or puddles on forms or
subgrade at the time concrete is placed.
During extremely hot periods, improved results can
be obtained by restricting concrete placement to early
morning, evening, or nighttime hours, especially in arid
climates. This practice has resulted in less thermal
shrinkage and cracking of thick slabs and pavements.
cracks which appear mostly on horizontal surfaces can be
substantially eliminated if preventive measures are taken.
Plastic shrinkage cracking is usually associated with
hot-weather concreting; however, it can occur any time
ambient conditions produce rapid evaporation of mois-
ture from the concrete surface. These cracks occur when
water evaporates from the surface faster than it can travel
to the surface during the bleeding process. This creates
rapid drying shrinkage and tensile stresses in the surface
that often result in short, irregular cracks. The following
conditions, singly or collectively, increase evaporation of
surface moisture and increase the possibility of plastic
shrinkage cracking:
1. Low air temperature
2. High concrete temperature
3. Low humidity
4. High wind speed
The crack length is generally 50 to 1000 mm (a few inches
to 3 ft) in length and they are usually spaced in an irreg-
ular pattern from 50 to 700 mm (a few inches to 2 ft) apart.
Fig. 13-8 is useful for determining when precautionary
measures should be taken. There is no way to predict with
certainty when plastic shrinkage cracking will occur.
When the rate of evaporation exceeds 1 kg/m
2
(0.2
lb/ft
2
) per hour, precautionary measures such as wind-
screens are almost mandatory. With some concrete mix-
tures, such as those containing pozzolans, cracking is
possible if the rate of evaporation exceeds 0.5 kg/m
2
(0.1
lb/ft
2
) per hour. Concrete containing silica fume is partic-
ularly prone to plastic shrinkage because bleeding rates
are commonly only 0.25 kg/m
2
(0.05 lb/ft
2
) per hour.
Therefore, protection from premature drying is essential
at lower evaporation rates. At some point in the process
of setting, bleeding goes to zero and the surface begins to
dry at evaporation rates much lower than the typically
specified 1.0 kg/m
2
(0.2 lb/ft
2
) per hour; in such cases,
further protection becomes necessary regardless of the
type of concrete mixture.
TRANSPORTING, PLACING, FINISHING
Transporting and placing concrete should be done as
quickly as practical during hot weather. Delays contri-
bute to loss of slump and an increase in concrete temper-
ature. Sufficient labor and equipment must be available at
the jobsite to handle and place concrete immediately
upon delivery.
Prolonged mixing, even at agitating speed, should be
avoided. If delays occur, stopping the mixer and then agi-
tating intermittently can minimize the heat generated by
mixing. ASTM C 94 (AASHTO M 157) requires that dis-
charge of concrete be completed within 1
1
⁄
2
hours or before
the drum has revolved 300 times, whichever occurs first.
During hot weather the time limit can be reasonably re-
duced to 1 hour or even 45 minutes. If specific time limita-
tions on the completion of discharge of the concrete are
desired, they should be included in the project specifica-
tions. It is also reasonable to obtain test data from a trial
batch simulating the time, mixing, and anticipated con-
crete temperature to document, if necessary, a reduction in
the time limit.
Since the setting of concrete is more rapid in hot
weather, extra care must be taken with placement tech-
niques to avoid cold joints. For placement of walls, shal-
lower layers can be specified to assure enough time for
consolidation with the previous lift. Temporary sunshades
and windbreaks help to minimize cold joints.
Floating of slabs should be done promptly after the
water sheen disappears from the surface or when the con-
crete can support the weight of a finisher with no more
than a 5-mm (
1
⁄
4
-in.) indentation. Finishing on dry and
windy days requires extra care. Rapid drying of the con-
crete at the surface may cause plastic shrinkage cracking.
PLASTIC SHRINKAGE CRACKING
Plastic shrinkage cracks sometimes occur in the surface of
freshly mixed concrete soon after it has been placed, while
it is being finished or shortly thereafter (Fig. 13-7). These
Fig. 13-7. Typical plastic shrinkage cracks. (1311)
