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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
Time for concrete casting: a new paradigm
procedure adopted was to determine the slump of the mix. After
a checking of the slump, the procedure was to re-establish the
slump so that it was equal or higher than 120 mm. This was done
by incorporating a superplasticizer into the mix.
Once the intended slump was attained, for each mix produced
three cylindrical test samples were cast, measuring 10 cm in di-
ameter by 20 cm in height, as prescribed by NBR 5738/03 – con-
crete – procedure for casting and curing test samples to carry out
compressive strength tests.
In view of the characteristics of the mix, test samples were cast
at zero minutes, immediately after the homogenization of the mix,
and at 120, 180, 240, 300 and 360 minutes, i.e., every hour, start-
ing at two hours.
After they were cast, the cylindrical test samples were covered with
humid burlap bags and exposed to ambient temperature and hu-
midity during 24 hours, thereafter they were demolded and taken
to a tank filled with lime-saturated water, where they were sub-
merged at a temperature of 23±2°C, until one day prior to the test-
ing date. On that day, the test samples were coated and taken to
the compression test.
The compressive strength tests were carried out as prescribed by
NBR 5739 – concrete –compression test of cylindrical test sam-
ples. They were executed using the servo controller machine Shi-
madzu 2.000 kN at a tension application speed of 0.45 MPa/s,
maintained constant throughout the test. The test samples were
coated with sulphur.
3. Results and discussion
The test samples were broken at 7 and 28 days after casting. The
average of the results are shown on figures 2 to 5. Figure 2 presents
the average values obtained from 3 test samples, for the concretes
produced with CP V ARI. In general, the average results obtained
for the three mixes, at 7 days, were constant for the intermediary
mix throughout the different casting times. However, for the bad mix,
as well for the good mix, an increase in average resistance was
verified over time, from the mixing until the casting. This fact can be
attributed to the loss of water from the mixture to the environment,
which decreases the actual water/cement ratio of the mixture, thus
increasing resistance. Another possible explanation is the breaking
of the first hydration products, which are larger and more fragile, and
are subsequently replaced by smaller, more resistant crystals.
Figure 3 presents the average resistance of test samples pro-
duced with CPV-ARI and broken 28 days after casting. Gener-
ally, at this age, as shown in Figure 2, resistance increases as
the time interval between mixing and casting increases. At 28
days, a significant increase occurs for the richer mix proportion,
but the intermediary and the poorer mix also have their resis-
tances increased.
Figure 4
presents the average resistance of the test samples cast
with CPIV and broken 7 days after casting. For the CPIV cement,
which presents a weaker resistance gain in the early ages, compared
with CPV, the behaviour of the resistance, particularly for the richer
mix proportion, was the maintenance of the resistance over the time
elapsed until casting. However, for the poorer mix proportion the aver-
ages increase as the time elapsed until casting increases.
Figure 5 presents the average resistance for test samples cast with
CPIV and broken at 28 days. A small increase in resistance is veri-
fied through the time interval between mixing and casting. Similarly
to what is observed in Figure 4, the increase in resistance for the
poor mix proportion is greater than for the others.
A variance analysis (ANOVA) was made using the individual val-
ues to check whether the analysed values are significant. Table
1 summarizes the parameters of the analysis of the experimental
data collected in this study.
As expected, the variables
type of cement
,
breaking age
and
mix
proportion
are significant, and their behaviour is widely consolidated
in the current literature. The variable
casting time
, which is the time
elapsed from the mixing of the materials until the casting, and which
constitutes the object of this study, has also proven to be signifi-
cant. The behaviour observed in figures 2 to 5, however, show that
compressive strength increases as the time interval until the casting
increases. This observation is evidently only valid for the particular
conditions through which the slump was maintained constant and
which were proposed in the methodology of this study.
As previously mentioned, the observation of the increase in the
average compression strength of concretes cast over a longer
time may be explained by the loss of water to the environment
and, consequently, a reduction in the actual water-cement ratio
of the mix, as well as a possible breaking of the first hydration
products formed, which are larger and more fragile. It is impor-
tant to highlight that this increase can vary depending on the
characteristics of the equipment used and the volume produced.
Hence, it is necessary to investigate whether the same occurs
Figure 2 – Compressive strength of the concrete
made with CPV-ARI, at seven days
Figure 3 – Compressive strength at twenty
eight days, made with CPV