Proceedings of the International rilem conference Materials, Systems and Structures in Civil Engineering 2016
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98 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
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.
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
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
101 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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