Civil Engineering Reference
In-Depth Information
layer rodded 25 times for 150-mm (6-in.) diameter cylinders;
beam specimens up to 200 mm (8 in.) deep should be filled in
two equal layers with each layer rodded once with a 16-mm
(
5
⁄
8
-in.) rod for each 1400 mm
2
(2 in.
2
) of the specimen's top
surface area. If the rodding leaves holes, the sides of the mold
should be lightly tapped with a mallet or open hand. Cylin-
ders to be vibrated should be filled in two layers with one in-
sertion per layer for 100-mm (4-in.) diameter cylinders and
two insertions per layer for 150-mm (6-in.) cylinders.
Beams over 200 mm (8 in.) deep and cylinders 300 to
450 mm (12 to 18 in.) deep to be vibrated (slump of 75mm
[3 in.] or less) should be filled in two layers; beams 150 to
200 mm (6 to 8 in.) deep to be vibrated can be filled in one
layer. Internal vibrators should have a maximum width of
no more than
1
⁄
3
the width of beams or
1
⁄
4
the diameter of
cylinders. Immediately after casting, the tops of the speci-
mens should be (1) covered with an oiled glass or steel
plate, (2) sealed with a plastic bag, or (3) sealed with a
plastic cap.
The strength of a test specimen can be greatly affected
by jostling, changes in temperature, and exposure to
drying, particularly within the first 24 hours after casting.
Thus, test specimens should be cast in locations where sub-
sequent movement is unnecessary and where protection is
possible. Cylinders and test beams should be protected
from rough handling at all ages. Remember to identify
specimens on the exterior of the mold to prevent confusion
and errors in reporting. Do not etch identification numbers
into the surface of fresh concrete test specimens. Use tape
or identification tags that do not damage the sample.
Standard testing procedures require that specimens
be cured under controlled conditions, either in the labora-
tory (Fig. 16-9) or in the field. Controlled laboratory curing
in a moist room
or in a limewater
storage tank gives
an accurate in-
dication of the
quality of the con-
crete as delivered.
Limewater must
be saturated with
hydrated lime,
not agricultural
lime, in accor-
dance with ASTM
C 511 (AASHTO
M 201) to prevent
leaching of lime
from concrete spe-
cimens.
Specimens
cured in the field
in the same man-
ner as the struc-
ture more closely
represent the ac-
tual strength of concrete in the structure at the time of
testing; however, they give little indication of whether a
deficiency is due to the quality of the concrete as delivered
or to improper handling and curing. On some projects,
field-cured specimens are made in addition to those des-
tined for controlled laboratory curing; these are especially
useful when the weather is unfavorable, to determine
when forms can be removed, or when the structure can be
put into use. For more information see “
Strength Tests of
Hardened Concrete”
in this chapter and ASTM (2000).
In-place concrete strength development can also be
evaluated by maturity testing (
ACI Committee 306
and
ASTM C 1074), which was discussed in Chapter 14.
Time of Setting
Test method ASTM C 403 (AASHTO T 197) is used to
determine the time of setting of concrete by means of pene-
tration resistance measurements made at regular time
intervals on mortar sieved from the concrete mixture (Fig.
5
35
Final setting
28
4
21
3
14
2
7
1
Initial setting
0
0
Fig. 16-9. Controlled moist curing in the
laboratory for standard test specimens
at a relative humidity of 95% to 100%
and temperature of 23±2°C (73±3°F)
(ASTM C 511 or AASHTO M 201). (8974)
180
210
240
270
300
330
360
390
420
Elapsed time, min.
Fig. 16-10. (top) Time of setting equipment. (bottom) Plot of
test results. (69788)
