Table 4.5Comments to works presented in Table 4.4.
Comment
number Comment
1) Very high w/c used.
2) Description of experiments is insufficient to fully comprehend reported re-sults.
3) Specimens were unloaded during testing, likely allowing crack width recov-ery, which was not measured.
4) Electrochemical results are suspect as the authors reported the ‘cathode’
rebar corroded in some specimens.
5)
Conclusion in papers that corrosion is “...not influenced by the widths of cracks or the existence of the crack itself” disagrees with the presented results.
6) Information on exposure time is not provided in papers, making results unclear and conclusions vague.
through a hole in the 30 mm long hollowed section as shown in Figure 4.15. Epoxy was then injected into the hollowed section to protect the wires, electrical connections and sensors. To provide adequate length of the instrumented rebar, two end parts of solid rebar were attached using threaded connections. Teflon tape was used at all threaded connections of the instrumented rebar.
4.2.2 Materials and specimen preparation
Details on the materials used and mixture procedure are found in Section 2.2.1. Only Mixture I.D. 1, from Table 2.1, was used for corrosion testing.
The fresh concrete was placed and vibrated into 100x100x600 mm3molds and covered by plastic for 24 hours at ambient conditions (i.e., 18◦C±2◦C). Upon demolding the speci-mens were sealed with multiple layers of plastic followed by aluminum foil and packaging tape and were stored at 20◦C ±1◦C until testing. As shown in Figure 4.16, the concrete beam specimens were cast with an instrumented rebar, a standard rebar, and a ruthe-nium/iridium mixed metal oxide activated titanium (MMO-Ti) mesh. The sensors in the instrumented rebar were cast to face the tension surface of the beam, while the MMO-Ti mesh was cast 10 mm±3 mm from the compression surface, opposite the instrumented rebar. Additional information on the use of MMO-Ti mesh as a counter electrode in concrete can be found in the literature [Nygaard, 2008;K¨uter, 2009].
4.2.3 Mechanical loading and environmental exposure
Steel cracking frames, shown in Figure 4.16 in gray, were used to load the beams to 2, 3, 4, and 5 mm mid-span deflection (MSD) to induce cracking (Table 4.6). Bolts were finger-tightened then the defined MSD was applied by turning the bolts a required num-ber of revolutions, using a wrench.
Figure 4.15Schematic of the instrumented reinforcement used for corrosion investigations (Note: Deformed reinforcement was used, ribs not shown in drawing).
Figure 4.16Beam specimen design including instrumented rebar, MMO-Ti mesh, and reservoir for chloride solution. Cracking frame is shown in gray.
Table 4.6Loading conditions, average crack width as measured from the tension surface of the concrete, and exposure durations for all beams.
Beam I.D. Mid-Span Avg. Crack Exposure Deflection Width Duration
— mm mm days
0∗ 2 0.4 —
1 2 0.4 62
2 2 0.3 35
3 3 0.6 14
4 3 0.7 62
5 4 0.9 35
6 5 1.2 62
∗Beam 0 used for fluorescent epoxy impregnation
Prior to application of mechanical load, the sides of the specimens (i.e., not the compres-sion and tencompres-sion surfaces) were sealed using silicon caulk. This was completed prior to loading as to not seal the induced crack(s). The hardened silicon did not rupture during loading of the beam specimens. After loading, a plastic ponding dike (40x80 mm2) was placed over the crack and the tensile surface outside the ponding area was sealed with silicon caulk. In cases where multiple cracks occurred only the crack directly over the steel fulcrum (Figure 4.16) was ponded. The compression surface was left unsealed. The ponding dike was then filled with a 10% chloride solution by weight (using NaCl). The ponding dike was refilled as necessary during testing.
4.2.4 Assessment of cracking behavior
To compare the cracking behavior of the concrete surrounding the instrumented rebar, epoxy impregnated plane and serial sections were investigated using Beam 0, a 2 mm MSD beam. Beam 0 was used as part of an trial investigation presented in [Hansen, 2009]. The corrosion data from Beam 0 is not presented here due to an error, which was corrected through the trial investigation. The impregnated beam was kept loaded in a cracking frame during the impregnation process described below.
To impregnate the cracked beam the crack was rinsed with deionized water multiple times to minimize crystallization of NaCl, which may impede the flow of epoxy. The specimen was allowed to dry for 7 days at 20◦C±1◦C and 50%±2% relative humidity.
The beam compression surface was coated using a thick layer (∼5 mm) of silicon to seal the specimen (needed to draw a vacuum). A 5 mm thick cylindrical acrylic vacuum chamber with an inner diameter of 85 mm was placed over the crack. Channel-shaped rubber gaskets coated with vacuum grease placed on each end of the acrylic chamber provided an adequate seal. The chamber was then evacuated to a pressure of 10 mBar and fluorescent epoxy was impregnated as described in accordance with the procedures in [DSF 423.39, 1998; Laugesen, 2005]. After 24 hours the epoxy hardened and the specimen was removed from the cracking frame and sectioned using a wet saw as shown in Figure 4.17. The transverse and parallel sections were used to compare the cracking behavior of the concrete immediately surrounding the instrumented and standard rebars.
The location of transverse and parallel sections discussed in Section 4.3.1 are indicated in Figure 4.17. Additional details on the epoxy impregnation process can be found in Section 2.2.4 of this thesis and in [Laugesen, 2005].
4.2.5 Corrosion testing
The OCP of each sensor was measured versus a standard calomel electrode (SCE) placed in the pond and versus the embedded MMO-Ti mesh. OCP of the MMO-Ti mesh was also measured against the SCE. OCP measurements were adjusted to the standard hydrogen electron (SHE) using an offset of 244 mV (values range from 240 mV to 245 mV in the literature [Elsener et al., 2003;Bardal, 2004;Myrdal, 2007;K¨uter, 2009]).
Potential measurements were recorded every 2 hours by a LabVIEW controlled corrosion measurement system which is described in [K¨uter, 2009;K¨uter et al., 2010]. The system
Figure 4.17Locations of the parallel and transverse epoxy impregnated sections. 150 mm was removed from each end of the beam prior to cutting sections.
(a) (b)
Figure 4.18(a) Schematic of the printed circuit board which consists of two switch cards, Cards 19 and 20, with connections being made to the indicated electrodes.
(b) The wiring diagram for Cards 19 and 20.
utilizes specially designed printed circuit boards to cluster up to 8 electrodes and connect them to a to a reference electrode simultaneously. This minimizes the required number of data acquisition channels. Figure 4.18(a) illustrates the printed circuit board, which consisted of two ‘switch cards.’ Cards 19 and 20 are shown in the figure. Lead wires from the various electrodes (sensor pins in the instrumented rebar and MMO-Ti mesh) are connected to the switch card. It should be noted that connections 2 and 12 (Figure 4.18), used to measure corrosion current, were not used in this initial investigation. Fig-ure 4.18(b) shows the corresponding wiring diagram for switch cards 19 and 20. FigFig-ure 4.19(a) shows wiring diagrams for all cards used for measurements and as seen a total of 6 cards were required to record data from three beams at the same time. Figure 4.19(b) identifies the location of each sensors in relation to the crack.
OCP measurements are conducted in sweeps wherein an electrical connection is initially established between a switch card and a reference electrode. After establishing the elec-trical connection a delay period is set in the program (2 minutes) at which time the corrosion potential of the sensors is measured and recorded. The program automatically switches to the next card, establishes an electrical connection between the card and the reference electrode material, measures and records the corrosion potential of the sensors.
Additional information on the measurement system and modifications made to the sys-tem for this testing is available in [K¨uter, 2009;K¨uter et al., 2010] and [Hansen, 2009], respectively.
At the conclusion of corrosion measurements the concrete covering the standard and instrumented rebar was removed from the side of the beam for inspection of the concrete and rebars. To facilitate this, 25 mm deep notches were cut above and below the rebars (standard and instrumented) and the concrete was removed by chisel and hammer. Silver nitrate, AgNO3, and Rainbow Indicator were sprayed on opposite exposed surfaces to assess the ingress behavior of chloride ions and change in pH, respectively. Rainbow Indicator is an aqueous pH indicator that changes color based on the pH range of concrete pore solution [Campbell et al., 1991;Instruments]. As shown in Figure 4.20 pH ranges include 5 to 7, 7 to 9, 9 to 11, and 11 to 13. Inspection of the rebars included removing the steel pins from selected sensors to assess the corrosion behavior and assessment of corrosion of the standard rebar.