Civil Engineering Reference
In-Depth Information
plunger after it has struck a smooth concrete surface. The
rebound number reading gives an indication of the com-
pressive strength and stiffness of the concrete. Two dif-
ferent concrete mixtures having the same strength but
different stiffnesses will yield different readings. In view
of this, an understanding of the factors influencing the
accuracy of the test is required.
The results of a Schmidt rebound hammer test (ASTM
C 805) are affected by surface smoothness, size, shape, and
rigidity of the specimen; age and moisture condition of the
concrete; type of coarse aggregate; and degree of carbona-
tion of the concrete surface. When these limitations are
recognized; and the hammer is calibrated for the partic-
ular materials used in the concrete (Fig. 16-21) by compar-
ison with cores or cast specimens, then this instrument can
be useful for determining the relative compressive
strength and uniformity of concrete in the structure.
Penetration Tests.
The Windsor probe (ASTM C 803),
like the rebound hammer, is basically a hardness tester
that provides a quick means of determining the relative
strength of the concrete. The equipment consists of a
powder-actuated gun that drives a hardened alloy probe
into the concrete (Fig. 16-22). The exposed length of the
probe is measured and related by a calibration table to the
compressive strength of the concrete.
The results of the Windsor-probe test will be influ-
enced by surface smoothness of the concrete and the type
and hardness of aggregate used. Therefore, to improve
accuracy, a calibration table or curve for the particular
concrete to be tested should be made, usually from cores
or cast specimens.
Both the rebound hammer and the probe damage the
concrete surface to some extent. The rebound hammer
leaves a small indentation on the surface; the probe leaves
a small hole and may cause minor cracking and small
craters similar to popouts.
Fig. 16-22. The Windsor-probe
technique for determining the
relative compressive strength
of concrete.
(top) Powder-actuated gun
drives hardened alloy probe
into concrete. (69783)
(left) Exposed length of probe
is measured and relative com-
pressive strength of the
concrete then determined from
a calibration table. (69784)
Maturity Tests.
The maturity principle is that strength
gain is a function of time and temperature. ASTM C 1074
generates a maturity index that is based on temperature
and time factors. The estimated strength depends on
properly determining the strength-maturity function for a
particular concrete mixture. The device uses thermocou-
ples or thermistors placed in the concrete and connected
to strip-chart recorders or digital data-loggers that record
concrete temperature as a function of time. The tempera-
ture in relation to time data is correlated to compression
tests performed on cylindrical specimens to generate a
temperature-time versus strength curve that is used to
estimate in-place concrete strength.
Pullout Tests.
A pullout test (ASTM C 900) involves
casting the enlarged end of a steel rod in the concrete to be
tested and then measuring the force required to pull it out
(Fig. 16-23). The test measures the direct shear strength of
the concrete. This in turn is correlated with the compres-
sive strength; thus a measurement of the in-place
compressive strength is made.
Break-Off Tests.
The break-off test (ASTM C 1150) deter-
mines the in-place strength of the concrete by breaking off
an in situ cylindrical concrete specimen at a failure plane
parallel to the finished surface of the concrete element. A
break-off number is generated and assessed in relation to
the strength of the concrete. Similar to pullout tests, the
relationship between break-off test numbers and compres-
40
Horizontal position
Concrete: saturated surface dry
Limestone aggregate
5000
30
4000
3000
20
2000
10
1000
0
0
10
15
20
25
30
35
40
Rebound reading
Fig. 16-21. Example of a calibration chart for an impact
(rebound) test hammer.
