Civil Engineering Reference
In-Depth Information
5900
40
5400
35
4900
4400
30
3900
25
3400
2900
20
Soak
48 hr
Soak
7 days
Air dry
7 day
In bags
1 day
In bags
7 day
Fig. 16-14. Effect of core conditioning on strength of drilled
cores (
Fiorato, Burg and Gaynor 2000
).
Fig. 16-15. Testing hardened concrete specimens: (left) cyl-
inder, (right) beam (44178, 69684).
when obtaining compression test specimens. A cover-
meter (electromagnetic device) or a surveyor's magnetic
locator can be used to locate reinforcing steel. Cores taken
from structures should be tested in a moisture condition
as near as that of the in-place concrete as possible.
Conditioning options for preparing specimens are
described in ASTM C 42 (AASHTO T 24) and
ACI 318
.
Fig. 16-14 shows the effects of core conditioning on
the strength of drilled core samples. Forty-eight hour
water immersion of the specimens prior to testing yields
significantly lower test results than air-drying specimens
for seven days prior to testing. Measured strengths varied
by up to 25%, depending upon the time and type of condi-
tioning prior to testing.
Flexure test specimens that are saw-cut from in-place
concrete are always immersed in lime-saturated water at
23.0°C ± 2.0°C (73.5°F ± 3.5°F) for at least 40 hours imme-
diately prior to testing.
Test results are greatly influenced by the condition of
the ends of cylinders and cores. For compression testing,
specimens should be ground or capped in accordance
with the requirements of ASTM C 617 (AASHTO T 231) or
ASTM C 1231. Various commercially available materials
can be used to cap compressive test specimens. ASTM C
617 (AASHTO T 231) outlines methods for using sulfur
mortar capping. Caps must be allowed to harden at least
two hours before the specimens are tested. Unbonded
neoprene caps can be used to test molded cylinders if
quick results are needed. Sulfur mortar caps should be
made as thin as is practical to avoid a cap failure that
might reduce test results.
ASTM C 1231 describes the use of unbonded neoprene
caps that are not adhered or bonded to the ends of the spec-
imen. This method of capping uses a disk-shaped 13 ± 2-
mm (
1
⁄
2
±
1
⁄
16
-in.) thick neoprene pad that is approximately
the diameter of the specimen. The pad is placed in a cylin-
drical steel retainer with a cavity approximately 25 mm
(1 in.) deep and slightly smaller than the diameter of the
pad. A cap is placed on one or both ends of the cylinder; the
specimen is then tested in accordance with ASTM C 39
(AASHTO T 22) with the added step to stop the test at 10%
of the anticipated load to check that the axis of the cylinder
is vertical within a tolerance of 0.5°. If either the perpendic-
ularity of the cylinder end, or the vertical alignment during
loading are not met, the load applied to the cylinder may be
concentrated on one side of the specimen. This can cause a
short shear fracture in which the failure plane intersects the
end of the cylinder. This type of fracture usually indicates
the cylinder failed prematurely, yielding results lower than
the actual strength of the concrete. If perpendicularity
requirements are not met, the cylinder can be saw-cut,
ground, or capped with a sulfur mortar compound in
accordance with ASTM C 617 (AASHTO T 231).
Short shear fractures can also be reduced by: dusting
the pad and end of cylinder with corn starch or talcum
powder, preventing excess water from cylinders or burlap
from draining into the retainer and below the pad, and
checking bearing surfaces of retainers for planeness and
indentations. In addition, annually clean and lubricate the
spherically seated block and adjacent socket on the
compression machine.
Testing of specimens (Fig. 16-15) should be done in
accordance with (1) ASTM C 39 (AASHTO T 22) for
compressive strength, (2) ASTM C 78 (AASHTO T 97) for
flexural strength using third-point loading, (3) ASTM C 293
(AASHTO T 177) for flexural strength using center-point
loading, and (4) ASTM C 496 (AASHTO T 198) for splitting
tensile strength. Fig. 16-16 shows the correlation between
compressive strength and flexural strength test results.
For both pavement thickness design and pavement
mixture proportioning, the modulus of rupture (flexural
strength) should be determined by the third-point loading
test (ASTM C 78 or AASHTO T 97). However, modulus of
rupture by center-point loading (ASTM C 293 or AASHTO
T 177) or cantilever loading can be used for job control if
empirical relationships to third-point loading test results
are determined before construction starts.
