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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
application in structural elements of short section. One exposure
face of the element is enough to testing. Another important point is
the fact that the equipment used in the implementation of Pull Off is
the same used in adhesion assays in mortar, standardized in Brazil
for some years, which may facilitate its use.
The Pull Off test is based on the concept that the traction force
required to pull a metallic disc bonded to a surface layer of con-
crete is associated with the compressive strength of the material
[19]. In the test, the traction is axially transmitted to the disc previ-
ously bonded to the concrete. After a period of time enough to
cure the resin (adhesive), a traction force is applied to the disc
using a mechanical portable system [Figure 3 (a) and 3 (b)]. The
gradual increase of force (Megapascal (MPa)) can be observed in
equipments that have an integrated digital manometer. The tensile
force that causes breakage associated with the calibration curves
makes possible to estimate the compressive strength [5].
There are many factors that may influence the results obtained and
are responsible for variability in this test. In addition to the compo-
sition and concrete properties, also have influence the variation on
the rupture surface, the orientation and position of the aggregate
onto the disc, the disc material (steel or aluminum), the disc diam-
eter and thickness (ratio e/d), the backpressure system (ring or
tripod) and the speed of load application [7].
In Brazil, the Pull Off test is traditionally used to evaluate the adhe-
sion strength of mortar. A difference of this test applied to concrete
and mortar is the presence of coarse aggregate which becomes
an influencing factor. It is thought that the shape and position of
the aggregate over the disc surface influence the results, but this
is an issue that requires further studies to determine the conditions
of this variable.
Another factor of interference in the results is the presence of re-
inforcement in the area where the metallic disc is bonded to the
test performance. The lack of control of this variable could cause
distortions in the results damaging the efficiency of the inspection.
Therefore, it is recommended that before bonding the discs, a lo-
cator is used to ensure that the disc is not bonded in areas where
the presence of reinforcement occurs.
Grullón et al. [21] in their paper for evaluating the strength of con-
crete using ultrasound for different mix proportion of concrete and
humidity condition got a low sensitivity for this evaluation. For
strength of specimens ranging between 41 and 64 MPa, the speed
of ultrasound propagation ranged from 4425 - 4721 m/s, with
very
close values, susceptible to experimental errors.
Machado et al. [22], using rebound hammer tests, penetration
resistance and ultrasound to determine concrete strength, con-
cluded that ultrasound showed the lowest correlation for obtaining
the resistance. Popovics et al. [23] mentions that the estimate of
resistance cannot be obtained with high accuracy using just the
value of the ultrasound test. The author reports a lack of theoretical
relationship between the magnitudes listed, although in other tests
that also happens.
The propagation speed can be influenced by several factors being
the most important: the run length, size of the structure to be tested,
the presence of reinforcement, as well as temperature and humidity
condition of the concrete. Results with high accuracy are influenced
by preparation of surface to be tested, binding of the transducers
and the method of selection and placement of transducers.
2.3 Pull Off
The Pull Off test was developed in England in the 70s, with the
aim of determining the strength of concrete in situ, due to problems
with concrete produced with high-alumina cement [5]. The test has
been used successfully since then to evaluate the resistance of
concrete. This test can also be used to verify the adhesion strength
of the concrete repairing material [24].
The Pull Off test is not standardized in Brazil, but countries like Eng-
land and USA recognize this method as a possibility in studies to es-
timate the compressive strength in the field [13]. This approach has
revealed consistent results; in addition, the unsatisfactory results are
visible after the test by observing the rupture surface.
Due to its simplicity, the test may be performed in the structure,
including without planning prior to concreting. The technique is ef-
fective for use in beams and slabs and also shows appropriate
Figure 3 – Pull Off test: (a) Test in progress and (b) equipment