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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
A. B. ROHDEN | D. C. C. DAL MOLIN | G. L. VIEIRA
cific mass of 2.90 g/cm³, as determined by NBR NM 53 (2003),
maximum dimension equal to 25 mm and shape index equal to
2.66,as determined by NBR NM 248 (2003)and NBR 7809 (1983),
respectively.
2.1.4 Water
The water used was potable water, available in the water supply
network of the city of Porto Alegre, RS.
2.1.5 Superplasticizer
In order to maintain workability through time, a third generation
polycarboxylate additive with specific mass of 1.087 g/cm³ and 40
% solids content was used.
2.2 Methodology
Four independent variables (factors) were used. They are: type
of cement, cracking age, unit mix proportion, compacting time. An
experimental dosage was used with the natural aggregates, us-
ing the IPT/EPUSP (HELENE and TERZIAN, 1992) method. First,
the mortar content of the concrete mix proportion was set at 53%.
Based on the mortar content, three concrete mixes were made at
the proportions of 1:2.5, 1:4 e 1:5.5 (cement: aggregates) for each
cement, i.e., six mix proportions. The mix proportions were made
using a 120 ± 20 mm slump as standard. Initially, the mixes were
made without additives, hence the slump was adjusted by adding
water. The mixtures and the water/cement ratios necessary to at-
tain the slump are presented in table 3.
The concrete was mixed in a tumble mixer truck, with capacity for
250 L. Each mix was executed only once, with the volume of mate-
rial determined proportionally for100 kg of coarse aggregate. The
volume of dry material was maintained constant in all the mixes.
The materials were added to the mixer in the following order:
coarse aggregate, approximately 1/3 of the water in the recipe,
cement, second third of the water, sand, and the water necessary
to achieve a 120 ± 20 mm slump. The cement to water addition
time was designated asinitial time or zero. From the initial time, a
standard protocol was adopted:20-minute cycles, where the mixer
does not rotate/rests for 15 minutes, and then it is turned on for
the 5 remaining minutes. These cycles were carried out from 20
minutes up to 360 minutes or 6 hours. During the resting periods, a
humid cloth was placed over the mixer’s mouth in order to minimize
the evaporation of water from the mix.
Following the rest and tumble cycles of the mixture inside the mixer
at the time intervals of 120, 180, 240, 300 and 360 minutes, the
1.1 Justification
In view of the stiffening mechanism of the cement and concrete
paste and the capacity of recovery of its structure even when sub-
mitted to loads already in the hardened state, this study aims to
investigate the viability of using the concrete after the cement set
time. Hence, this initial study sought to characterize the compres-
sive strength of concrete mixed over a period of six hours.
Given the importance of the slump of the mix for the processes of
concrete casting and finishing, in this study, the slump was main-
tained at 100±20 mm over time, by using a superplasticizer.
2. Materials and experimental program
2.1 Materials used
2.1.1 Cement
With the aim of contemplating different test situations, two types of
cement with very different chemical compositions and initial resis-
tance were used. One was a high initial strength Portland cement
(CPV – ARI), with specific mass of 3.10 kg/dm³, which develops
high strength in its first days. CPV – ARI has minimal addition in its
composition (NBR 5733). The other cement used was pozzolanic
Portland cement (CPIV), with specific mass of 2.90 kg/dm³, with
the addition of large amounts of pozzolan, developing lower initial
strength (NBR 5736).
Table1 presents the chemical characterization of the cements
used, the data provided by the manufacturer and the Brazilian
standards’ requirements.
Table 2 presents the physical characterization of the cements
used, the data provided by the manufacturer and the require-
ments of the Brazilian standards. It was observed that the values
of specific mass previously cited, and which were determined in
a laboratory, are, as expected, similar to those informed by the
manufacturer.
2.1.2 Fine aggregate
The fine aggregate used was quartz sand, with specific mass of
2.62 g/cm³, as determined by NBR NM 52, maximum characteristic
dimension equal to 4.8 mm and fineness modulus of 2.4, according
to NBR NM248.
2.1.3 Coarse aggregate
The coarse aggregate used was crushed basalt stones, with spe-
Table 3 – Composition of the mixes and water/cement ratios
Mix
proportion
Unit mix proportion
water/cement ratio
Cement
Fine aggregate
Coarse aggregate
1:2.5
1
0.86
1.65
1:4
1
1.65
2.35
1:5.5
1
2.45
3.06
CPIV
CPV
0.39
0.31
0.45 0.43
0.52
0.50