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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 4
D.V. RIBEIRO | J.A. LABRINCHA
|
M.R. MORELLI
sis. The contact between the reference electrode and the speci-
men was aided with a damp sponge.
The corrosion test was started after 63 days, when the specimens
showed a constant mass (1.0 gram of variation in two consecu-
tive 24-hour readings) and when the measured corrosion poten-
tial indicated the formation of a passive film on the surface of the
steel bars (Ecor > -0.124 V). This reference value corresponds to
a lower than 10% possibility of corrosion occurrence, according to
the ASTM C 876/91 standard (for the saturated calomel electrode
used in this work).
In other works [13, 14], it was necessary to define a specific age, or
reference age, from which the procedures of the accelerated cor-
rosion tests started. The authors of those studies associated the
reference age to the stabilization of the cement hydration process
and defined the ages of 63 days [14] and 80 days [13] as sufficient
for the cement paste to acquire a relatively well developed physical
structure and a significantly high degree of hydration.
After reaching the “safe potential,” the specimens were subjected
to semi-cycles of partial immersion in 3 wt% NaCl solution for two
days and semi-cycles of drying in a ventilated oven at 50 °C for
five days. During the semi-cycle of partial immersion, the level of
immersion solution was kept at half height of the specimens. In
this condition, the chloride inflow occurs primarily by capillary ab-
sorption, since the specimens are first dried, and after the pores
become saturated, the inflow occurs by diffusion, which is accel-
erated due to water evaporation through the exposed concrete.
The corrosion potential (Ecorr) was measured at the end of each
semi-cycle.
According to MCCARTER apud SANTOS [13], in a porous mate-
rial, there is a relationship between the force of capillary suction
and the saturation degree. Thus, when there is a region exposed
and dried, the suction forces will be greater and will result in a
faster water movement within the concrete.
The concentration of 3 wt% NaCl was taken to be a concentration
close to that presented by seawater, in addition to be the critical con-
centration do rebar corrosion. The concentration of the dipping solu-
tion was measured before each half-cycle of wetting and adjusted
when necessary. Furthermore, this solution used was replaced by a
new solution with the same concentration after four cycles.
At the end of each half-cycle were measured the corrosion poten-
tial (E
cor
) and the mass of the specimens. The corrosion potential
was used as an indicative of the bars passivation.
The active state of corrosion (Ecorr < - 0.274 V) or passive state
(Ecorr > - 0.124 V) was analyzed based on the corrosion potential
(Ecorr), using the saturated calomel electrode (SCE) as reference.
The test was concluded when two consecutive and full cycles re-
sulted in corrosion potential values below the critical value (Ecorr
< - 0.274 V).
After concluding the test, the rebars were extracted from the
samples, cleaned according to the ASTM G-1/03 standard, and
weighed to determine weight loss and to compare them with the
initial value. Thus, the corrosion rate (CR) can be calculated ac-
cording to equation (1).
(1)
DTA
WK TC
. .
.
=
ian NBR 11578 standard, commercially available in São Carlos, Bra-
zil, was selected as reference in all the tests. The coarse aggregate
was dense, crushed granitic stone and the sand was supplied from a
river deposit commercially available in São Carlos, Brazil.
The red mud came from Poços de Caldas-MG and was supplied by
ALCOA Brazil. It is a mixture containing about 60% of solids, collect-
ed immediately after alumina recovery from the digestion process.
2.2 Methods
2.2.1 Materials characterization and Concrete Dosage
The materials characterization involved X-ray diffraction (Rigaku
Geirgeflex ME 210GF2 Diffractometer) and X-ray fluorescence
(Philips PW1480 X-ray Fluorescence Spectrometer) analyses,
while physical parameters such as the specific surface area (es-
timated by BET, using a Micrometrics Gemini 2370 V1.02 equip-
ment) and specific gravity (Helium Pycnometer Accupyc 1330
V2.01 from Micrometrics) were also determined. Similar determi-
nations were performed on sand and on the red mud.
The concrete formulation used as reference was prepared in a 1.0
: 1.5 : 1.3 : 0.5 (Portland cement + red mud : sand : coarse ag-
gregate : water) weight ratio. The mortar content was 65.8% and
the cement consumption was 540 kg/m
3
compared to a reference
mixture. Distinct concrete mixtures in which cement was partially
replaced by red mud (10, 20, and 30% in weight) were analyzed.
To corrosion potential measurements, two commercial steel rebars
(Gerdau, type CA-50) with a diameter of 6.3 mm were embedded
in each prismatic concrete block (50x70x90 mm
3
) with a concrete
topping of 2.25 cm. With this geometry, the distance between the
inner surfaces of the steel rebars was 2.5 cm and the exposed
embedded area of each electrode was 15.83 cm2. The electrical
resistivity of the concrete was tested using molded concrete blocks
(200x200x100 mm
3
) with sensors embedded in them. All the speci-
mens were unmolded 24 h after being cast and were cured for 28
days in a humid chamber (> 95% HR). A minimum of 4 samples
were tested in all determinations.
2.2.2 Corrosion Potential
The reinforcement steel bars were weighed on an analytical bal-
ance with an accuracy of 0.01 g. The area exposed to chloride
attack (about 15.83 cm²) was then delimited by electrical insulating
tape, as shown in Figure 1. The bars were positioned so that the
exposed area was located in the central region of the specimens,
as illustrated in Figure 1d.
The corrosion potential is basically verified from a chloride-acti-
vated accelerated corrosion test. The electrochemical cell used
for corrosion potential measurements was composed of a working
electrode, the reinforcement steel bars and the saturated calomel
electrode (SCE) used as reference electrode.
Before taking the measurements, the side of the specimens to be
measured was pre-moistened with a wet sponge for one minute.
A conductive solution containing 5 mL of neutral detergent to one
liter of water, according to the ASTM C-876/91 standard (
Standard
Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel
in Concrete
) and an electrical conductivity of 0.15 ± 0.02 mS/cm
were used. For the measurements, the reference electrode was
positioned approximately at mid-span over the rebar under analy-