Civil Engineering Reference
In-Depth Information
FIGURE 7.31
Scanning electron image showing
the interface between a sand grain (lower left corner)
and the paste.
low-capacity testing machine, and less space needed for curing and storage.
Because of the strength variability of small specimens, more specimens
should be tested for smaller specimens than are tested for standard-sized
specimens. In some cases, five 0.10-m by 0.20-m replicate specimens are
used instead of the three replicates commonly used for the standard-sized
specimens. Also, when small-sized specimens are used, the engineer should
understand the limitations of the test and consider these limitations in in-
terpreting the results.
The interface between the hardened cement paste and aggregate particles
is typically the weakest location within the concrete material. When concrete
is stressed beyond the elastic range, microcracks develop at the cement
paste-aggregate interface and continuously grow until failure. Figure 7.31
shows a scanning electron microscope micrograph of the fractured surface of
a hardened cement mortar cylinder at 500x. The figure shows that the cleav-
age fracture surfaces where sand particles were dislodged during loading. The
figure also shows the microcracks around some sand particles developed
during loading.
■
7.5.2
Split-Tension Test
The split-tension test (ASTM C496) measures the tensile strength of concrete.
In this test a 0.15-m by 0.30-m (6-in. by 12-in.) concrete cylinder is subjected
to a compressive load at a constant rate along the vertical diameter until fail-
ure, as shown in Figure 7.32. Failure of the specimen occurs along its verti-
cal diameter, due to tension developed in the transverse direction. The split
tensile (indirect tensile) strength is computed as
2P
pLd
T
=
(7.4)
where
T
=
tensile
strength, MPa (psi),
at failure, N (psi),
of specimen, mm (in.), and
of specimen, mm (in.).
P
=
load
L
=
length
d
=
diameter
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