PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 284

FIGURE 7.34 Rebound hammer for nondestructive eval-
uation of hardened concrete.

rebound to strength. The test can also be used to check uniformity of the
concrete surface.
The test is very simple to run and is standardized by ASTM C805. To
perform the test, the hammer must be perpendicular to a clean, smooth
concrete surface. In some cases, it would be hard to satisfy this condition.
Therefore, correlations, usually provided by the manufacturer, can be
used to relate the strength to the amount of rebound at different angles.
Rebound hammer results are also affected by several other factors, such as
local vibrations, the existence of coarse aggregate particles at the surface,
and the existence of voids near the surface. To reduce the effect of these
factors, it is desirable to average 10 to 12 readings from different points in
the test area.

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7.5.5 Penetration Resistance Test

The penetration resistance test, also known as the Windsor Probe test, is
standardized by ASTM C803. The instrument (Figure 7.35) is a gunlike de-
vice that shoots probes into the concrete surface in order to determine its
strength. The amount of penetration of the probe in the concrete is inversely
related to the strength of concrete. The test is almost nondestructive since it
creates small holes in the concrete surface.
The device is equipped with a special template with three holes, which
is placed on the concrete surface. The test is performed in each of the holes.
The average of the penetrations of the three probes through these holes is de-
termined, using a scale and a special plate. Care should be exercised in han-
dling the device to avoid injury. As a way of improving safety, the device
cannot be operated without pushing hard on the concrete surface to prevent
accidental shooting. The penetration resistance test is expected to provide
better strength estimation than the rebound hammer, since the penetration
resistance measurement is made not just at the surface but also in the depth
of the sample.