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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
E. PEREIRA | M. H. F. de MEDEIROS
3.3 Proportioning concrete
For proportioning the concrete used in the study, it was used
the IPT/EPUSP experimental method of concrete proportion-
ing, also called the four quadrants method. This method is
based on the laws of Lyse, Abrams and Molinari, adjusted
on resistance and workability curves depending on structural
requirements. The Slump was set to 100 ± 10 mm and the
mortar content of
a
= 52%. Data about proportioning and con-
sistency of the material are shown in Table 2. Mineral addition
and superplasticizer admixture were not used in the concrete
composition.
For each mix proportions of concrete was molded a prismatic slab
of 55 cm x 55 cm x 20 cm for the Pull Off test, 6 cylindrical speci-
mens of 15 cm x 30 cm and 6 prismatic specimens with 15 cm
x 15 cm x 15 cm for simple compression, rebound hammer and
ultrasound test.
To ensure homogeneity of the mixture, all the concrete used in this
study was made
by a concrete mixer with an inclined axis and a
capacity of 240 liters. The consolidation of concrete was executed
in a vibrating table and completion of the specimen in two layers of
concrete for both cylindrical and prismatic specimens. Each layer
was submitted to a cycle of 20 seconds for consolidation on the
vibrating table.
After molding, the specimens were cured in a humidity chamber
(95% relative humidity and 23 ± 2 ° C temperature) according to
NBR 5738 [29], for 112 days. The choice of a high age for the
testing of resistance is due to the fact that the structures tests are
designed to evaluate old structures with a high degree of hydra-
tion. Therefore, testing in high ages has greater correlation when
compared to practice.
3.4 Speed of propagation of ultrasound waves
After curing the specimens, ultrasound tests were performed on
all specimens according to NBR 8802 [12]. Initially, 24 hours be-
fore the test, the specimens were removed from the chamber to
surface drying in a laboratory environment. Before starting the
tests the surfaces of the specimens were verified to ensure these
were dry and flat.
The transducers were positioned on opposite faces of the speci-
mens, as observed in Figure 6. The contact between the trans-
ducer and the concrete surface was conducted with ultrasound gel
with the aim of producing a near perfect coupling. For the same
place, the highest pulse propagation velocity was regarded as the
Table 2 – Mix proportions of concretes
Cement:
aggregates
Cement :
fine aggregate :
coarse aggregate :
w/c
Slump (mm)
1: 3
1: 1.08: 1.92: 0.43
105
1: 4
1: 1.60: 2.40: 0.50
90
1: 5
1: 2.12: 2.88: 0.59
95
representative value of the test specimen, since this value is inter-
preted as the closest perfect coupling obtained in reading.
The end result of each test was derived from the average of the six
representative readings of specimens of each concrete studied.
3.5 Rebound hammer
After the ultrasound test, the specimens were tested with the re-
bound hammer. For implementation of the rebound hammer, the
equipment used was a model of CT-320AM Soiltest.
In the procedure followed in this research, the first step was
to test a specimen compression according to NBR 5739 [1].
This practice aimed to determine the concrete resistance to
calculate the load value that would be applied to the speci-
mens for the implementation of the rebound hammer test, as
the NBR 7584 [11] determines to be applied to the specimens
a load of approximately 15% of its rupture load to restrict
movements in specimens during testing. Thus, by knowing
approximately the concrete strength, the other five cylindrical
specimens intended for rebound hammer test and compres-
sive strength were previously loaded with 15% of their maxi-
mum resistance and the determination of the rebound number
was performed.
In each specimen, both prismatic and cylindrical, nine rebound
numbers were obtained. Figure 7 (a) shows the execution of a test
specimen. In the case of test cylinders, each specimen was la-
beled in three parts of 120 ° where in each one three readings were
performed, one in the center, one in the upper end and another
at the lower end of the specimen [Figure 7 (b)]. In the prismatic,
nine readings were performed on the same face as can be seen
in Figure 7 (c).
3.6 Compressive strength test
After execution of the rebound hammer and ultrasound tests,
the cylindrical and prismatic specimens were tested for simple
compression. For the capping of the specimens sulfur was used.
The concrete press machine was an EMIC with application load
speed control.
Figure 6 – Ultrasound test