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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
Pull Off test to evaluate the compressive strength of concrete: an alternative to Brazilian standard techniques
relation (R ² ≈ 0.60) when the analysis is conducted with cylindrical
specimens, as standardized in Brazil.
For the rebound hammer test conducted in prismatic specimens
the correlation increases significantly (R² = 0.92). A possible expla-
nation for this difference may be the fact that in cylindrical speci-
mens the impact of the rebound hammer is applied to a surface
that is not flat, unlike the case of prismatic. It is possible that this
detail may adversely affect the results, but studies have yet to
be made to better understand this influence, as the uncertainties
about the efficiency of the rebound hammer remain.
In Brazil, a complementary method to rebound hammer is the ultra-
sound test [12]. Thus, Figure 10 shows results of this kind of test.
Results of ultrasound test using two types of specimen (cubic and
cylindrical) showed a very good correlation with the compressive
strength (R ²> 0.98). The ultrasound test performed on cylindrical
specimens showed correlations very close to the tests performed
on prismatic. This confirms the assumption discussed above about
the test being performed on curved surfaces that could adversely
affect the results. In ultrasound test the propagation velocity of
sound waves is measured on the flat faces of cylindrical specimens
and possibly for this reason the results obtained in both types of
specimens (cubic and prismatic) show similar correlations. None-
theless, the propagation velocity has a strong influence from the
shape of the specimens, since to the same concrete mix proportion
there is a large difference in response between the cylindrical and
cubic specimens.
Albeit the higher accuracy obtained with the ultrasound test, the
required equipment is more costly than the rebound hammer, for
example. Alternatively to ultrasound and rebound hammer test,
there is the Pull Off test, which although it is also expensive, uses
the same equipment used to bond strength measurement between
mortar coating and masonry substrate. Figure 11 shows the corre-
lation between the results of Pull Off test obtained by pulling on the
surface of concrete and compressive strength measured in speci-
mens molded with the same concrete of prismatic plates.
Analyzing the values obtained in the experimental program and the
graphs, it can be concluded that the Pull Off test (in situ test) shows
consistent results for comparison with the concrete strength results
obtained in laboratory being a suitable tool to evaluate the strength
of concrete in situ.
This check is based on the observation of correlation with com-
pressive strength tests (both with R² above 0.93), which, under
the technical point of view, would enable the test analyzed in this
work to be used for analysis of concrete strength. In this case, it is
important to say that the estimative of the compressive strength by
Pull Off test leads to very similar results for both cubic and cylin-
drical specimens, which did not occur with rebound hammer and
ultrasound tests.
The execution of the test was simple, which indicates that com-
pared to other non-destructive tests, the Pull Off test does not have
very complex details and can be performed by an employee with
a simple training, as in the case of the rebound hammer and ultra-
sound tests. The fact that the equipment is simple to operate and
being the same used for testing the bond strength of mortar coat-
ings, the availability of equipment and staff to run the test becomes
easier.
5. Conclusions
Non-destructive tests are an appropriate and useful tool in monitor-
ing concrete structures and estimating their strength in-place. The
rational use of the various available methods, as well as a possible
combination of more than one technique can be particularly inter-
esting from the viewpoint of the results validity.
The rebound hammer method showed distinct correlation level
with the compressive strength when measured in cylindrical or
prismatic concrete specimens. A possible cause of distortion is
the measurement in curved surfaces of cylindrical specimens. In
general, it appears that studies on the efficiency and reliability of
the rebound number results should also be better achieved. It is
recommended that the rebound hammer is used as a complement
to other in-place tests or in a preliminary stage of inspection.
With regard to the ultrasound test, it was found that it has excellent
correlation (R ²> 0.95) with the compression strength of concrete
for both cylindrical and cubic specimens. However, the behavior of
the results appeared to be influenced by the shape of the speci-
mens, which goes against the method theory.
The results obtained with the Pull Off test showed that it is fully
feasible to be employed to estimate the strength of concrete in situ.
Figure 10 – Correlation between ultrasound test and
compressive strength test in cylindrical
and cubic specimens
Figure 11 – Correlation between Pull Off test
and compressive strength test in
cylindrical and cubic specimens