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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 6
Time for concrete casting: a new paradigm
sequent stage, they start forming amass, continuous subgroups of
mechanically connected grains. The appearance of the first con-
tinuous path going through the group constitutes the percolation
threshold, which is the first continuous path of grains mechanically
connected, joining one face of the volume to the opposing face
(Figure1) (SILVA, 2007).
When the setting time has ended and the hydration process and
formation of the first amass start, the remix of the concrete neces-
sarily includes the breaking of the first crystals derived from the
hydration process. Would this fact damage the mechanical proper-
ties? Could this material be used with a structural function?
Vieira (2008) investigated the influence that micro fissures
caused by early loading have on the mechanical properties of
concrete. Microscopy tests detected the recovery of micro fis-
sures verified in samples of concrete produced with CPV ARI
and submitted to pre-loads of 75% of their breaking load, at 7
days. The recovery of the micro fissures corroborates the the-
ory that there occurs autogenous healing, cracking of weaker
crystalsand hydration of remaining resistant compounds, pro-
posed by that study.
causes of concrete setting, even though, for several reasons,
the setting time of the concrete does not coincide with the
setting time of the cement used in the concrete. The phenom-
ena of stiffening, setting and hardening are physical manifes-
tations of the progressive hydration reactions of the cement
(SILVA, 2007).
In practical terms, the initial set time gives an idea of the time avail-
able to manipulate the concrete made with particular cement. How-
ever, the result is not very precise, since the test is done under
standardized conditions, while on the field, the concrete suffers the
influence of uncontrolled factors, such as temperature and relative
air humidity, as well as the addition of chemical additives, which
can alter the set time (SILVA, 2007).
The percolation theory by ACKER (1988) apud SILVA (2007) al-
lows the simulation of cement hydration and a description of the
setting phenomenon. The solid grains of cement, isolated in the
suspension phase, start to come into contact with each other as
hydration products start forming around those grains. The estab-
lishment of these mechanical links between the grains occurs in a
random and discontinuous manner in the volume, and at the sub-
Table 1 – Chemical characterization of the cements used
Evaluated
characteristics
(% of the mass)
Types of Portland cement
CP IV
(manufacturer)
NBR 5736/91
requirements
5736/91
CP V – ARI
(manufacturer)
NBR 5733/91
requirements
MgO
4.39
≤6.50
3.01
≤6.50
SO
3
2.12
≤4.00
4.26
≤4.50
Loss on ignition
3.74
≤4.50
3.90
≤4.50
Free CaO
0.80
-
-
-
Insoluble residue
3.05
-
-
-
Table 2 – Physical characterization of the cements used
Characteristics
evaluated
Types of Portland cement
CP IV
(manufacturer)
NBR 5736/91
requirements
CP V – ARI
(manufacturer)
NBR 5733/91
requirements
2.80
-
3.05
-
0.0
≤5.0
0.0
≤5.0
04:12
≥01:00
02:55
≥01:00
05:21
≤12:00
04:35
≤10:00
34.9
-
-
-
4306
-
4970
≥3000
1.1
≤8.0
0.16
≤6.0
3
Specific mass (cm /g)
Heat expansion (mm)
Set time (h:mm)
Initial
Final
Normal slump (%)
2
Blaine fineness (cm /g)
Sieve residue #3250mm (%)
Sieve residue #200mm (%)
5.1
-
1.44
-