Description
A rebound hammer provides
a method for assessing the in-situ compressive strength of concrete. In this
test, a calibrated hammer impact is applied to the surface of the concrete. The
amount of rebound of the hammer is measured and correlated with the
manufacturer's data to estimate the strength of the concrete. The method has
also been used to evaluate the strength of masonry.
Equipment
A calibrated rebound
hammer is a single piece of equipment that is hand operated
Execution
The person operating the
equipment places the impact plunger of the hammer against the concrete and then
presses the hammer until the hammer releases. The operator then records the
value on the scale of the hammer. Typically, three or more tests are conducted
at a location. If the values from the tests are consistent, record the average
value. If the values vary significantly, additional readings should be taken
until a consistent pattern of results is obtained.
Since the test is relatively
rapid, a number of test locations can be chosen for each wall. The values from
the tests are converted into compressive strength using tables prepared by the
manufacturer of the rebound hammer.
Personal Qualification
A technician with minimal
training can operate the rebound hammer. An engineer experienced with the rebound
hammer data should be available to supervise to verify that any anomalous
values can be explained.
Reporting Requirements
The personnel conducting
the tests should provide sketches of the wall, indicating the location of the
tests and the findings. The sketch should include the following information:
·
Mark the location of the test marked on
either a floor plan or wall elevation.
·
Record the number of tests conducted at a
given location.
· Report either the average of actual
readings or the average values converted into compressive strength along with
the method used to convert the values into compressive strength.
·
Report the type of rebound hammer used
along with the date of last calibration.
·
Record the date of the test.
· List the responsible engineer overseeing
the test and the name of the company conducting the test.
Limitations
The rebound hammer does
not give a precise value of compressive strength, but rather an estimate of
strength that can be used for comparison. Frequent calibration of the unit is
required (ACI, 1994). Although manufacturers’ tables can be used to estimate
the concrete strength, better estimates can be obtained by removing core
samples at selected locations where the rebound testing has been performed. The
core samples are then subjected to compression tests. The rebound values from
other areas can be compared with the rebound values that correspond to the
measured core compressive strength.
The results of the
rebound hammer tests are sensitive to the quality of the concrete on the outer
several inches of the wall. More reproducible results can be obtained from
formed surfaces rather than from finished surfaces. Surface moisture and
roughness can also affect the readings. The impact from the rebound hammer can
produce a slight dimple in the surface of the wall. Do not take more than one
reading at the same spot, since the first impact can affect the surface, and
thus affect the results of a subsequent test.
When using the rebound
hammer on masonry, the hammer should be placed at the center of the masonry
unit. The values of the tests on masonry reflect the strength of the masonry
unit and the mortar. This method is only useful in assessing the strength of
the outer wythe of a multi-wythe wall.
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