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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
E. PEREIRA | M. H. F. de MEDEIROS
2.4 Advantages and disadvantages of the three tests
Among the advantages of the rebound hammer test is the ease
of operation, lightness, speed of execution and little damage to
the concrete structure inspected. As disadvantages of this method
may be mentioned that the measure performed refers to a surface
layer of concrete (about 30 mm). Furthermore the structures, after
three months of age, suffer the influence of concrete carbonation
which densifies the region carbonates and raises the values
of the
rebound number [25, 15].
Compared to other methods, the rebound number can be applied
in slender elements, and, for this situation, it is more advantageous
than other methods such as extraction of specimens. In these cas-
es, it is advised to use the rebound hammer used in conjunction
with other techniques to confirm the elements homogeneity [26].
The ultrasound test is also a possibility as a portable and fast ex-
ecution. Compared to rebound hammer this technique has the
advantage of providing results which represent the material as a
whole and the reading is executed directly. This minimizes the in-
fluence of the carbonated layer, often observed in the inspection.
Comparing costs, this method involves equipments about 4 times
more expensive than the rebound hammer.
An important disadvantage of the ultrasound test is the different
influence of humidity on the results in resistance to compression
tests on specimens extracted and squeezed. The higher humidity
content of concrete, the greater the speed of propagation of ultra-
sound and the lower the values
of compressive strength of extract-
ed specimens. This detail is important in developing correlation
curves of the speed of ultrasound versus compressive strength for
estimates in the whole building.
Another disadvantage is the influence of reinforcement, because
the speed of propagation of waves in the carbon steel is much
higher than in the concrete [27]. Therefore, the presence of rein-
forcement increases the value of ultrasound speed propagation,
concealing the results.
Comparing Pull Off test with rebound hammer or ultrasound, the
Pull Off increases the time for test because depends on bonding a
metallic disc on the surface of the concrete. The execution of the
hole in the concrete surface, if this practice were adopted, also
involves the disadvantage of consuming more time and the use of
other equipment (drill and saw cup) for inspection. However, this
detail can also be cited as an advantage, because it has the pos-
sibility of executing the hole through the carbonated layer, eliminat-
ing this factor that influenced the measurements. The cost of the
equipment is in the same order of ultrasound equipment, meaning
that it is more expensive than the rebound hammer.
As a disadvantage can be cited the need for local repairs where
the tests were executed since the concrete in the area suffers sur-
face fractures. It should be also considered the delay time for the
curing of the resin used for bonding the disc prior to application
of load. This time varies between 1.5 and 24 h, depending on the
adopted type of adhesive. The resin most frequently used is epoxy.
Another limiting factor for application of this test is the impossibility
of its execution in high-strength concrete due to load limitations
equipment availability. As an example, the equipment used in this
study (Dyna Z16E Proceq) with maximum traction of 16 kN, this
means, to a disc of 5.0 cm a maximum tensile stress of 8.15 MPa.
This means that for this equipment, it would be possible to esti-
mate the concrete compressive strength of up to about 80 MPa,
whereas the tensile strength is about 10% of the compressive
strength of concrete. However, as the conventional concrete today
have a compressive strength between 20 and 35 MPa, the method
can be used in most of the cases of practical application of Port-
land cement concrete.
The aim of this study is to compare the accuracy of the methods
discussed, as there is distrust about both nationally standardized
techniques (rebound hammer and ultrasound). As an example,
Evangelista [17] and Castro [28] reported the low accuracy of the
rebound hammer test when correlated with compressive strength.
Grullón et al. [21] reported a low correlation between speed of
wave propagation and compressive strength. Furthermore, Mach-
ado et al. [22], using the rebound hammer, penetration resistance
and ultrasound to determine the strength of concrete, concluded
that ultrasound had the worst results as the correlation to obtain
the resistance.
Thus, the focus of this work is to verify if the Pull Off test has better
Table 1 – Chemical compositions and physical properties of cement
(a) Chemical composition
SiO
2
Cement
(%)
Al O
2 3
(%)
Fe O
2 3
(%)
CaO
(%)
MgO
(%)
SO (%)
3
Ignition
loss (%)
Free lime
(%)
Insoluble
residue (%)
Total alkalies
as Na Oe*
2
18.35
CP II-F-32
4.07
2.54
59.64
5.19
3.07
5.35
1.2
1.47
0.63
* Na Oe = Na O + 0.658 K O
2
2
2
(b) Physical and mechanical properties
Blaine’s specific
Setting time
surface area
2
Blaine (cm /g)
Residue on
# 200 (%)
Residue on
# 325 (%)
Compressive strength of mortar
(MPa)
Final set
Inicial set
(h:min)
(h:min)
1 day
3 days
7 days
28 days
4:10
3:31
3.338
3.87
17.76
11.1
25.1
32.0
40.4