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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 1
V. CECCONELLO | B. TUTIKIAN
3.2 Mixture of concrete and molding
of the specimens
At this stage, 62 cylindrical specimens were molded for trials to
compression and 10 traction trials samples for a total of 72 sam-
ples metal molded in shapes with dimensions of 10 cm of diameter
and 20 cm in height, two for each age, for six curing temperatures.
The mixture was conducted in a concrete mixer with vertical shaft,
with maximum capacity of 56 liters and rotational speed of 29 revo-
lutions per minute (RPM). The time for adding all materials and
homogenization of the mixture for all moldings was 18 ± 2 min. The
order of the material in the mixer was the following:
a) Course aggregate;
b) 80% of water;
c) Cement
d) Fine Aggregates;
e) 20% of water;
After the stage of mixing the materials, it was measured the con-
sistency of concrete through testing the concrete Abram’s cone,
according to ABNT NBR NM 67:1996 [16]. It was established the
a/c ratio, the curing time and the consistency of the concrete, only
for evaluating the influence of the curing temperatures on the me-
chanical properties of materials. The time for conducting the test
was 8 ± 2 minutes. The results of the slumps and a/c ratio are
presented in Table 3. The concretes analyzed are similar, with all
properties fixed, in order to analyze only the influence of the curing
temperature in the final properties of the material.
Afterwards, the samples were molded in a period of 25 ± 2 min-
utes. Thus, the total time between the contact of the cement with
water and the storage of specimens in the cure environment with
controlled temperature at 51 ± 6 minutes.
3.3 Conditions of curing
The condition of cure of the samples was carried out in an environ-
ment of low controlled temperatures, simulating the behavior of the
concrete when subjected to this incidence, as shows Table 4, and
relative humidity of 50%. The equipment used was a sealed box
with digital temperature control, which allows setting the tempera-
ture without affecting the relative humidity of the air for a predeter-
mined time, in the case study, for 7 days.
In this condition, by keeping the samples at various curing tem-
peratures in the first seven days, the behavior of the first reactions
which occur at early ages can be analyzed.
2. Objective
The overall goal of this study was to compare the resistances to
compression at 1, 7, 28 and 91 days and 28 days to traction for the
cured concrete at temperatures ranging from zero to 25 °C, with
intervals of 5 °C, thus analyzing the influence external temperature
on the development of the mechanical properties of concrete.
3. Experimental Programme
For the accomplishment of the trial program it was aimed the repro-
duction of the effect of the external temperature on the concrete,
by controlling the curing temperature of the specimens. Thus, the
samples were conditioned to various cures in the first seven days,
for testing the resistance to compression and traction.
It was given a trace to obtain the resistance to compression
common to project, around 25 MPa at 28 days. For this pur-
pose, it was defined the trace, in mass of 1: 2: 2.5 (cement: fine
aggregate: coarse aggregate) with consistency, performed by
the procedure specified in ABNT NBR NM 67:2003 [16], set at
90 +/- 10 mm.
3.1 Physical and chemical characterization
of the materials
The characteristics of the materials used in the concrete are di-
rectly related to the performance and properties of the mixture. For
the characterization of materials, tests were performed according
the ABNT norms.
3.1.1 Portland Cement
The Portland cement which was used was CP II E 32. It was speci-
fied because it is a cement with a lower rate of additions, among
the commercially ones available, and for not being a cement with
high initial resistance. The specific mass used for the Portland ce-
ment was
ρ
= 3000 kg / m³, determined according to the ABNT
norms NBR NM 23:1998 [17], while the mechanical and chemical
properties are listed in Table 1.
3.1.2 Aggregates
The fine aggregate used was extracted from the river sand that
presented a unitary mass
ρ
= 1419 kg/m³, determined accord-
ing to recommendation by the ABNT NBR NM 45:2006 [18], and
density of
ρ
= 2618 kg/m³, determined according to ABNT NBR
9776:1987 [19]. The coarse aggregate consists of crushed basal-
tic rock, with unit mass
ρ
= 1197 kg/m³, performed according to
ABNT NBR NM 45:2006 [18]. The density was determined at
ρ
= 2590 kg/m³, by the method of hydrostatic balance.
The test for sieve analysis of aggregates was performed according
to ABNT NBR NM 248:2003 [20], which foresees the determina-
tion of particle size distribution of coarse and fine aggregates for
concrete. The results are shown in Table 2.
3.1.3 Water
Water coming from the public water supply was used for the execu-
tion of concrete.
Table 3 – Water/cement ratio
and consistency of molding
Moldings
a/c ratio
Consistency (mm)
1ª
25° C
0,58
80
2ª
20° C
0,56
90
3ª
15° C
0,57
100
4ª
10° C
0,56
100
5ª
5° C 0,55
90
6ª
0° C 0,57
100