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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 4
D.V. RIBEIRO | J.A. LABRINCHA
|
M.R. MORELLI
Therefore, no clear correlation was found between rebar depassiv-
ation time and corrosion rate, i.e., the rebar corrosion process may
begin sooner, but after this moment, the corrosion process may oc-
cur at a lower rate. However, this may have occurred simply due to
our reliance on the parameters defined in the ASTM 876 standard
to determine the onset of corrosion and the variation in the half-cell
potential measured by our technique with the location of the anodic
areas on the rebar surface.
The specialized literature offers controversial explanations about
phenomena involving corrosion potentials. However, it seems
there is a consensus that this technique is insufficient and should
always be accompanied by some other technique to quantitatively
determine the corrosion kinetics of rebars [15, 16].
3.3 Electrical Resistivity
Electrical resistivity is a property widely used to monitor concrete
structures because it is a nondestructive method and allows for ex-
ternal monitoring by means of embedded electrodes. This property
is fundamentally related to fluid permeability and to ion diffusivity
through concrete pores.
Several authors [13, 17, 18, 19, 20] have shown that electrical re-
sistivity is related to the microstructural characteristics of the ce-
ment matrix, such as porosity, pore size distribution, pore connec-
tivity, and the conductivity of the aqueous solution in the matrix.
In this study, three specimens were prepared for each amount of
red mud content, providing a total of six results (each specimen
yielded two different measures because of the two electrodes em-
bedded at different measuring depths).
Figure 7 shows the average results of the electrical resistivity of
the reference specimens (0%) and the specimens containing red
mud additions (10%, 20% and 30%). The specimens were kept in
a moist chamber up to the age of 28 days, and the dotted lines in
the figure represent the corrosion risk levels: high (<10 KΩ.cm),
moderate (10 – 50 KΩ.cm), low (50 – 100 KΩ.cm), and insignificant
(> 100 KΩ.cm), according to the COST 509.
All the samples showed increased electrical resistivity due to in-
creased paste hydration and to the reduction of fluid concentration
in concrete pores as the specimens became increasingly dry, mak-
ing them less conductive. According to ANDRADE [21] and SAN-
TOS [13], the conduction of electrical current through concrete oc-
curs through continuous pores and microcracks that are present in
the matrix and filled with water.
The behavior of the specimens differed significantly according to
different moisture contents. Among the specimens that were kept
in the moist chamber (up to 28 days), the samples containing red
mud were more resistive than the reference samples (0%). This
effect continued to be visible in the first days after the specimens’
removal from the moist chamber.
After drying the specimens, the reference samples showed a high in-
crease in resistivity which exceeded that of the 40 to 80-day-old sam-
ples containing red mud. This effect can be explained by the high ionic
concentration of red mud, which becomes more pronounced and ac-
tive as the moisture content decreases when compared to reference
samples. A similar behavior was observed byWHITING and NAGI [18].
The equivalent electrical conductivities of aqueous ions typically
found in concrete pores were determined by ADAMSON apud SHI
[16] and these values are presented in Table 3. As can be seen, the
Na
+
, OH
-
, Ca
2+
and K
+
ions in red mud are highly conductive, con-
tributing to lower the resistivity of concrete when it loses moisture.
Another factor to be considered is the higher porosity of concrete speci-
mens containing red mud, which contributes decisively in reducing re-
sistivity. Although they showed lower resistivity values than the refer-
ence samples, the samples of concrete containing red mud showed
values well above the limit considered as low corrosion probability (> 50
KΩ.cm). Hence, even if the presence of red mud does not prevent the
occurrence of corrosion, it also cannot be considered harmful.
Another positive analysis factor is that the specimens containing
red mud showed a higher resistivity in a humid environment, which
is more conducive to corrosion. Unfortunately no measurements
were taken of specimens kept moist throughout the experiment in
order to verify if this behavior would be maintained.
4. Conclusions
This research led to the following conclusions:
n
Electrical resistivity is a good indicator of the possible occur-
rence of chloride ion penetration. Thus, the higher the con-
crete’s resistivity, the lower the penetration of chloride ions,
and hence, the lower the probability of corrosion;
n
The degree of saturation (humidity) of the concrete samples
containing red mud appears to exert a considerable influence
on the concrete’s resistivity;
n
The concrete specimens containing red mud presented higher
resistivity in a humid environment, which is more favorable for
corrosion.
n
Evaluating the evolution of the corrosion process by corrosion
potential tests is not possible; moreover, this technique only in-
dicates the possibility of corrosion occurring, and should there-
fore be used as a complementary technique;
n
The difference between the corrosion potential measures in
wet and dry states is more pronounced in the reference sam-
ples (0%) which, due to their larger network of capillary pores,
have a higher capacity to absorb NaCl solutions (capillary suc-
tion) and greater difficulty in losing this moisture (lower poros-
ity) than samples containing red mud.
n
The depassivation process of reinforcing bars is retarded by
the presence of red mud.
Table 3 – Equivalent conductivity
of aqueous ions in infinite concentrations at 25 °C
(ADAMSON apud SHI [16])
( )
0
l
Ion
+
Na
+
K
2+
Ca
2-
So
4
-
OH
-
Cl
-1 -1
l
(m
Ω
)
0
0.00501
0.00735
0.00595
0.00798
0.0198
0.00763