Civil Engineering Reference
In-Depth Information
Strength Tests of Hardened Concrete
Strength tests of hardened concrete can be performed on
the following: (1) cured specimens molded in accordance
with ASTM C 31 or C 192 (AASHTO T 23 and T 126) from
samples of freshly mixed concrete; (2) in-situ specimens
cored or sawed from hardened concrete in accordance with
ASTM C 42 (AASHTO T 24); or (3) specimens made from
cast-in-place cylinder molds, ASTM C 873 (Fig. 16-13).
Cast-in-place cylinders can be used in concrete that is
125 to 300 mm (5 to 12 in.) in depth. The mold is filled in
the normal course of concrete placement. The specimen is
then cured in place and in the same manner as the rest of
the concrete section. The specimen is removed from the
concrete and mold immediately prior to testing to deter-
mine the in-place concrete strength. This method is par-
ticularly applicable in cold-weather concreting, post-
tensioning work, slabs, or any concrete work where a
minimum in-place strength must be achieved before con-
struction can continue.
For all methods, cylindrical samples should have a
diameter at least three times the maximum size of coarse
aggregate in the concrete and a length as close to twice the
diameter as possible. Correction factors are available in
ASTM C 42 (AASHTO T 24) for samples with lengths of 1
to 2 times the diameter. Cores and cylinders with a height
of less than 95% of the diameter before or after capping
should not be tested. Use of a minimum core diameter of
95 mm (3.75 in.) is suggested where a length to diameter
(L/D) ratio greater than one is possible.
Drilled cores should not be taken until the concrete
can be sampled without disturbing the bond between the
mortar and the coarse aggregate. For horizontal surfaces,
cores should be taken vertically and not near formed joints
or edges. For vertical or sloped faces, cores should be
taken perpendicular to the central portion of the concrete
element. Although diamond-studded coring bits can cut
through reinforcing steel, it should be avoided if possible
Fig. 16-11. Fly ash particles retained on a 45
µ
m sieve after
washing, as viewed through a microscope at 200X. (58949)
Fig. 16-12. ASTM C 232 (AASHTO T 158) test for bleeding of
concrete; Method A without vibration. The container has an
inside diameter of about 255 mm (10 in.) and height of
about 280 mm (11 in.). The container is filled to a height of
about 255 mm and covered to prevent evaporation of the
bleed water. (69780)
mL/cm
2
or 0.1% to 2.5% of mix water. The bleeding test is
rarely used in the field (Fig. 16-12). Bleeding was also
discussed in Chapter 1.
TESTING HARDENED CONCRETE
Premolded specimens described in the previous section
“
Strength Specimens
” (ASTM C 31 [AASHTO T 23], ASTM
C 192 [AASHTO T 126], or ASTM C 873), or samples of
hardened concrete obtained from construction or labora-
tory work (ASTM C 42 [AASHTO T 24], ASTM C 823, or
ASTM C 873) can be used in tests on hardened concrete.
Separate specimens should be obtained for each test
performed because specimen preconditioning for certain
tests can make the specimen unusable for other tests.
Fig. 16-13. Concrete cylinders cast in place in cylindrical
molds provide a means for determining the in-place com-
pressive strength of concrete. (69781)
