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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
E. PEREIRA | M. H. F. de MEDEIROS
curves (rebound number versus strength of concrete provided
with the equipment) very reliable.
This is a very controversial issue, because the current legislation
in Brazil recommends that generic curves are not to be applied to
all kinds of concrete. On this issue, these authors favor the view
that the most reliable way to make use of the rebound hammer is
associating the compression tests on specimens extracted from
the site inspected. That is, to adopt the practice of using the cor-
relation curves is not exactly a general practice. So, correlation
curves must be developed for each structure inspected, with the
realization of rebound hammer at some points, associated with
the extraction of specimens and further rupture test. This allows
the construction of a correlation between the rebound number and
compression strength obtained in the same points, allowing the
construction of a correlation curve similar to that shown in Figure 2.
These curves are used to evaluate compression strength at points
where only rebound hammer was performed. This practice pre-
vents the extraction of excessive amounts of specimens, which
is a technique that causes more damages to parts and is more
expensive for inspection work in the structures.
The rebound hammer test has other applications besides the
quantitative measure of the concrete strength. Castro et al. [19]
points it out as useful for the evaluation of the mechanical strength
uniformity with very little damage to structural elements, allowing
comparison between different parts of the structure. It is also pos-
sible to estimate the evolution of concrete strength in precast struc-
tures, application of loads on new structures, and verification of
resistance to service loads on damaged structures [9].
2.2 Ultrasound test
The first studies based on the measurement of waves propaga-
tion speed, generated mechanically, date from the mid 40s. These
studies demonstrate that the speed of propagation is correlated
to the elastic properties and material density. The studies also
indicate that the propagation speed is almost independent of the
geometry element. The test has been developed up to the cur-
rent process of measuring the ultrasound propagation speed with
equipment consisting of circuits able to generate and register
waves within a frequency of 20-150 kHz [6].
At the present, the commercialized equipment consists of a central
unit, which has an electrical pulse generator, a pair of transducers,
transmitter, receiver, amplifier and an electronic device for mea-
suring the time between the peak of the pulse generated in the
transmitter transducer (maximum amplitude) and the arrival of the
peak to the receiving transducer.
Ultrasound wave propagation speed measurement is proabably
more applicable than any of the non-destructive tests. Its main ap-
plications are the determination of the concrete homogeneity, eval-
uation of existence and estimate of depth of cracks, the existence
of large voids or holes, estimation of the compressive strength and
determination of the elastic modulus.
The speed of the pulse of longitudinal waves produced on the test
depends on the elastic properties of the material - such as elastic
modulus and Poisson’s ratio - and its density, as quoted by Castro
et al. [19] and ACI 228.2R [20]. The dependence on material prop-
erties and the behavior of the waves that travel through the mate-
rial allows correlation that can be used, for example, to determine
the mechanical strength of the concrete.
is a spring, a plunger and a hammer. The plunger is put in con-
tact with the concrete surface so as to move the hammer inside
the cylindrical tube and the spring is extended. When the hammer
comes to the end of the tube, a device releases the equipment so
that, by action of the spring, it strikes the piston and rebounds for
a given extent. Because of the impact effect, the mass returns by
a certain magnitude generating an index pointed by a cursor that
moves along a graduated scale [Figure 1]. The rebound number is
proportional to the distance traveled by the mass on rebound and
resistance of concrete is directly proportional to the distance that
the mass is reflected inside the unit after the impact.
The readings by rebound hammer are quite sensitive to local varia-
tions in the concrete, especially aggregates, holes near the surface
and discontinuities near the tested area. According to Malhotra [14],
the most influencing factors in the results of the rebound hammer
test are the type of surface treatments aggregate, rebound hammer
inclination, carbonation of the outer layers of concrete, age of struc-
ture, humidity, type of cement and proportioning of the concrete.
According to the ACI 228.1R [15], the test provides an estimate of
the outer structural element surface hardness (about 2-3 cm from
the surface). Although a superficial assessment, in most cases the
standard reports as satisfactory the relationship between the re-
bound number and the compressive strength of concrete, which
justifies the usual application in engineering analysis.
According to Machado [16] and Evangelista [17], the estimate of
the compressive strength of concrete, in tests on specimens with
the rebound hammer, has a reliability of ± 20% and the average co-
efficient of variation is approximately 10%. However, for Malhotra
[14], these tests should be viewed as an additional technique, and
not as substitutes for destructive testing.
The equipment used in the tests provides correlation curves be-
tween the rebound value and concrete strength. The NBR 7584
[11] recommends the use of appropriate correlation curves ob-
tained through tests on materials in the region where the con-
crete was manufactured, when one wants to use the rebound
hammer to evaluate concrete resistance to compression , guar-
anteeing thus safer results. Second, the rebound hammer test is
established and widespread, for the DNER [18] there is already a
consolidated experience on the procedure, including correlation
Figure 2 – Example of correlation between
compressive strength in extracted
specimens and rebound number