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International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
Figure 3. Electrochemical setup
2.4
Observation of corrosion products, of steel/concrete interface and estimation of the
crack patterns
Visual observations
After the accelerated corrosion test, the specimens were observed and a special focus was
made on the top and the front sides because of their lower concrete cover (30mm). Cracks
opening were measured with a crack measuring magnifier (resolution 0.1mm) on these two
surfaces. Then the specimens were sliced (125x100x20mm
3
), considering corroded areas as
illustrated in [Figure 4]. After 24h drying in an oven at 45°C, the slices were photographied
and examined in order to characterize the crack pattern (angle and length) [Figure 5]. To
determine the angular position, the methodology developed by Sanz-Merino [8] was adopted
[Figure 5-a]. To estimate the length of the cracks in each cross-sections, the photographs of
the cross-section and a circle graduated every centimeter were superimposed as shown in
[Figure 5-b]. Moreover, the corrosion products embedding the rebar and filling the concrete
cracks were analyzed. Then from both results an attempt was made to correlate what was seen
on the concrete surface in 2Dimensions (by the bridge's owner) and what happened inside the
concrete with an aim of 3Dimensions.
Scanning Electron Microscopy observations
The slices were then impregnated with an epoxy resin under vaccum (to prevent decohesion
between concrete and steel when the corrosion damage was severe) and then cut into samples
which dimensions (2x4.5x4.5cm
3
) fitted in the Scanning Electron Microscopy (SEM)
observation room. A first goal is to determine the corrosion product thickness and their length
circling the perimeter of the rebar. A second objective is to observe the crack pattern and the
transfer of the oxides through the cracks.
99
International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
Figure 4. Specimen sawing design (25 slices named Tn° according to the axis. Corrosion test
zone is between x=10 and x= 40 cm
Figure 5. Determination of the crack angular position (a) and the crack length (b) after the
accelerated corrosion test on the two sections of a slice
X
Y
PVC tank length = 27.5 cm
T10
T13
T16
(a)
(b)
100
International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
3.
Experimental results
3.1
Voltage evolution during the accelerated corrosion test
Figure 6. Voltage evolution during the accelerated corrosion test
[Figure 6] shows the voltage evolution for each RC specimen subjected to the accelerated
corrosion test versus time. Before the test (time=0), the voltage reflects the electrical
resistance of the specimens which are in a close range from 2.5 to 3 V except for P004-7d
specimen (3.3V). During the test, three stages are observed. In the first stage, the voltage
increase (about 1V) may be explained by the formation of resistive iron oxides (passive layer)
[9], [10] around the rebar and also by the diffusion into the concrete and the filling up of the
concrete pores by the oxides in the vicinity of the steel. In the second step, the drastic voltage
decrease (50% loss) can arise from the concrete cracking and the steel / concrete debonding.
The last stage with a constant voltage (1.4V) appears for longer durations, 28d and 35d and is
likely representative of a constant impedance of the corrosion layer. These preliminary
observations need to be discussed considering the migration of the ionic species under current
and particularly the penetration of the chloride ions. Tests are under progress.
3.2
Corrosion rate and crack patterns
[Figure 7] shows the corrosion signs and the cracks observed for the top side and the front
side of the two specimens after an accelerated corrosion test of 21 days.
Regarding the top sides, specimen P-007 shows some corrosion product stains and a visible
crack roughly along the steel rebar whereas specimen P-008 is not damaged. Regarding the
front sides, the opposite behaviour is observed: specimen P-007 only exhibits a single spot of
corrosion product while specimen P-008 shows significant corrosion products along the crack
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International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
that follows the rebar. An assumption to explain this difference could be that the aggregate as
well as the rebar's ribs are not homogeneously distributed and promote this heterogeneity.
Top side P-007-21d
Top side P-008-21d
Front side P-007-21d
Front side P-008-21d
Figure 7. Qualitative results of the specimens after an accelerated corrosion of 21 days
[Figure 8] reveals that all corroded specimens for over 14 days have a J
corr
equal to 10 μA/cm²
approximately except the specimen P-004-7d corroded for 7 days. The proposed assumption
is that at 7 days chloride ions have not reached the steel/concrete interface yet and
consequently, the corrosion is still passive. To give an answer to this question, chloride
measurement ingress with AgNO
3
will be achieved on each slice. Regarding the crack width,
the behaviour of corroded specimens is different. As previously mentioned, all specimens are
not cracked on the same side. As already suggested, this difference could be ascribed to the
heterogeneity of the concrete (the random distribution of the aggregates into the cement
paste). Besides, the crack on the top side is wider than the one on the front side. This
observation might be attributed to the close localization of the counter electrode on the top
side of the specimen (by comparison with the steel rebar) that permits chloride ions to quickly
reach the rebar (modification of the physico-chemical conditions that locally enhances the
corrosion process).
[Figure 9] shows the evaluation of the crack patterns (angular position (a), length (b) and
width (c)) for each of the 10 cross-sections of the five slices of specimen P-008-21d. The
cracks propagate from the steel/concrete interface to the concrete surface.
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