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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
E. PEREIRA | M. H. F. de MEDEIROS
Before starting up the bonding of metallic discs in the concrete
surface, the surfaces were prepared to obtain good adhesion.
This procedure is adopted mainly for removal of the cement
paste on the concrete surface and to make the aggregate ap-
parent. A caution was taken so that the place of bonding was
plain enough to ensure that the adhesive is present in the en-
tire contact surface between the disc and the concrete, ensur-
ing that the strength is applied uniformly throughout the area
of bonding. To regularize this site was used a sander with fine
sandpaper. The bonding was performed with a thin layer of ad-
hesive and the surplus concentrated around the disc was re-
moved still fresh, thus ensuring that the rupture surface had
the area of the disc. An epoxy resin was used as adhesive with
curing time of 24 hours.
4. Results and discussion
Initially, the test results obtained in the experimental program were
treated statistically in order to eliminate spurious values
,
which
could compromise the validity of analyzes.
For the rebound hammer tests nine readings on each specimen
were determined, being considered a valid reading when at least
five individual values did not differ from average by more than
10%. According to NBR 7584 [11], the values outside the range
were discarded and the average was recalculated only with valid
values
.
This procedure was performed on five specimens of the
same concrete. At the end of the preliminary treatment, the final
result was calculated as the simple average of the final values ob-
tained from valid samples.
To prismatic and cylindrical specimens tested in compression and
in ultrasound tests, it was considered the average of the six speci-
mens. The Pull Off test is not standardized in Brazil and therefore
in this study were used the recommendations of BS 1881 Part 207
[13]. Nine points in each specimen were tested, being considered
valid if at least six individual values did not differ from the mean ±
one standard deviation. All specimens were within this limit. The
Table 3 – Gathered data in non-destructive tests and compressive
strength test using cylindrical and cubic specimens
Concrete
Concrete
Cubic specimens (15 x 15 x 15cm)
Rebound hammer
Rebound hammer
Ultrasound test
Ultrasound test
Pull Off
Pull Off
Compressive strength
Compressive strength
(MPa)
(MPa)
(MPa)
(MPa)
(MPa)
(MPa)
(MPa)
(MPa)
Low cement content
31
4511.2
1.91
30.32
Intermediate cement content
38
4635.2
2.8
37.83
High cement content
40
4702.5
3.08
44.63
Cylindrical specimens (Ø15 x 30cm)
Low cement content
37
4570.5
1.91
30.00
Intermediate cement content
36
4613.8
2.8
38.87
High cement content
46
4641.5
3.08
45.73
pullout strength was obtained with the average of the valid values
after the initial treatment.
The average values of compressive strength, ultrasound test, Pull
Off test and rebound hammer after pretreatment are shown in
Table 3.
With the data in Table 3 were created graphs to demonstrate
the correlation between the non-destructive tests and the
compressive strength of cylindrical and cubic specimens for
the three concrete mixtures studied. Figure 9 refers to the
correlation between the compressive strength and the re-
bound number.
In Brazil, the rebound hammer is the most popular non-destructive
test, but some researchers question the effectiveness of the meth-
od to accurately estimate the concrete strength. Rebound hammer
data show a direct relationship between the compressive strength
results and the rebound number. Moreover, there is a weak cor-
Figure 9 – Correlation between rebound
hammer and compressive strength test
in cylindrical and cubic specimens