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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 4
Effect of red mud addition on the corrosion parameters of reinforced concrete evaluated by
electrochemical methods
of the specimen in contact with the electrode (cm
2
); and L is the
distance between the electrodes (cm). The European CE Bulletin
– COST 509 (Corrosion and Protection of Metals in Contact with
Concrete) was used as the evaluation parameter since it is more
stringent in relation to values than those suggested by the CEB-
192 standard (Table 1).
3. Results and discussion
3.1 Materials Characterization
The Portland cement used here had a specific surface area of 0.93
m
2
/g and a specific gravity of 3.11 Kg/dm
3
. The sand had a specific
surface area of 0.68 m
2
/g and a specific gravity of 2.70 Kg/dm
3
,
classified by the Brazilian NBR 7211 standard as fine sand. The
gravel had a specific gravity of 2.74 kg/dm
3
and a maximum dimen-
sion of 19 mm.
The red mud was received in the form of paste containing about
40% free water. In the present study, the material was dried and
crushed, and then used as a powdered additive. Ideally, to dem-
onstrate its potential as a concrete constituent, red mud should be
tested in the as-received condition; hence, the free water present
in the mud should be considered a concrete mix component.
The red mud had a specific surface area of 20.27 m
2
/g, its specific
gravity was 2.90 kg/dm
3
and its pH was very high (12.95), exceed-
ing the limit (12.5) established by the Brazilian NBR 10004 stan-
dard for non-hazardous wastes.
Table 2 presents the chemical composition of the waste, while
Figure 3 shows the corresponding XRD pattern. As expected, the
predominant crystalline components were aluminium hydroxide
(Al(OH)
3
), calcium carbonate (CaCO
3
), and iron oxide (Fe
2
O
3
), but
relative amounts of SiO
2
, muscovite and FeO(OH) were also rel-
evant. Some of those oxides were also detected by XRD, in addi-
tion to aluminium hydroxide and a complex Na
5
Al
3
CSi
3
O
15
phase.
3.2 Corrosion Potential
Based on the corrosion potential measurements an analysis was
where K = constant (for CR(mm/year), K = 8.76 x 10
7
; for CR(g/
m
2
.year), K = 8.76 x 10
7
.D); W = weight loss (grams); A = exposed
rebar area (cm
2
); T = exposure time (hours); D = steel rebar density
(for CA-50 steel, D = 7.85 g/cm
3
). In this study, A = 15.83 cm
2
and
T = 4320 hours (180 days).
2.2.3 Electrical Resistivity
The electrical resistivity of concrete was calculated from the electri-
cal current (I) passing through the specimens. The system, which is
shown schematically in Figure 2, consists of two cylindrical probes,
each with two electrodes (different measurement levels) made of
stainless steel (rings/washers) and spaced at different layer depths.
The probes used in this study were supplied by the Institute of Cor-
rosion (ICorr, Portugal), which specializes in corrosion studies. The
two probes of the system should be placed 10 cm apart.
Through this monitoring system, the ionic resistivity of concrete at
each depth can be determined by the paired-electrode technique.
An alternating current is applied between the electrodes and the
resistivity is determined by measuring the resistance (DE/DI,
Ohm’s law) and by a parameter that depends on the geometry of
the electrodes and on the distance between them (A/L). Thus, the
resistivity (r) is calculated according to equation (2):
(2)
LI
AV
×
×
=r
For circular electrodes, equation (2) is equivalent to:
(3)
I
LV
× ×
=
p
r
2
where
ρ
is the electrical resistivity of concrete (Ω.cm); V is the ap-
plied voltage; I is the current intensity; A is the area of the side
Figure 2 – (A) Schematic diagram of the measurement of concrete electrical resistance,
(B) electrical probe used, and (C) electrical current measure
A
B
C