Chapter 2. EXPERIMENTAL METHOD

Test Specimens

Table 5 describes the test slabs employed in this study. The ECR specimens from the 96-week SE test that remained for extended outdoor exposure were coated with six different powder coating products, which were manufactured by Akzo, O'Brian, 3M, and Ajikawa (Japanese). Individual coatings were identified as Akzo-Grey ^TM, Akzo-Green^TM, Akzo-Olive^TM, O'Brian Gold^TM, 3M 213^TM, and Ajikawa^TM. They are randomly designated as coatings A through F in this report. This nomenclature is identical to that in Report No. FHWA-RD-98-153.^[4] As summarized in table 5, the slabs were classified into four configuration groups. These were (1) slabs containing ECR in the top mat and black bar in the bottom mat (ECR top-black bottom, 19 slabs); (2) slabs containing ECRs in both mats (ECR top-ECR bottom, 6 slabs); (3) control slabs containing black bars in both mats (black top-black bottom, 3 slabs); and (4) slabs containing straight stainless steel bars coupled with either black or stainless steel bottom bars (stainless steel, 3 slabs). Among the 19 ECR top-black bottom slabs, 6 had two 180-degree bent ECRs in the top mat, and the concrete cover of another 7 slabs was precracked over the straight bars. One specimen of each black top-black bottom and stainless top-black bottom slabs contained 180-degree bent bars in the top mat. In addition, one black and stainless slab had precracks over the top mat bars. None of the ECR top-ECR bottom slabs contained bent bars or precracks.

Every ECR bar was intentionally damaged by drilling holes through the coating to represent either 0.004 or 0.5 percent artificial coating defect using two different drill bit sizes. In every ECR top-ECR bottom slab, the two top mat ECRs contained different defect sizes, and a top mat ECR was paired with two bottom mat ECRs containing the same size of defect.

Data Collection

Corrosion progress of the top bar was monitored by both short-circuit potential (SCP) and macrocell current. SCP was measured when top and bottom bars were electrically connected. On the other hand, macrocell current was measured as the voltage drop across a 10-Wresistor connected between the top and bottom bar mats. The current data then were converted into macrocell current density according to Ohm's law and a known surface area of 145.7 cm² (22.6 in.²) for the anode (top bar). The same area was used for both coated and uncoated steel. Test data was collected periodically. During the last data measurements, additional data were collected, including the open-circuit potential (OCP) of top mat bars after the top and bottom mat bars were disconnected, the AC resistance between the top and bottom mats, and the impedance modulus (|Z|) at 0.1 Hertz (Hz) of top mat bars using Electrochemical Impedance Spectroscopy (EIS). Before autopsy, researchers documented the exterior condition of the test slabs using a digital camera. Photographs of the test slabs are provided in appendix A.

Autopsy

Autopsies were conducted in groups of four or five test slabs. A detailed autopsy of each slab was performed according to the following procedures:

Autopsy procedure for ECR

1. Rebar identification tags were attached to each test bar.
2. Digital photographs were taken to record final physical condition of the test slabs before demolition.
3. The slabs were broken open using a gas-powered saw with a diamond blade, and the bars were extracted carefully.
4. The bar surface facing the top side of the slab was marked with an arrow at one end of the bar.
5. The concrete/rebar interface was examined for corrosion products and photographed.
6. Coating defects were identified visually and with an holiday detector.
7. Using a magnifier, researchers identified coating defects, characterized these by type of defect (bare area, mashed area, crack, or holiday), and marked them with a permanent marker. Photographs were taken of both sides of each ECR to document the defects.
8. Deteriorated coating was removed with a utility knife. A knife adhesion test also was performed at six spots of undamaged coating areas to determine qualitative adhesion strength.
9. Photographs were taken of both sides of each ECR after loose (disbonded) coating was removed to document its extent of coating disbondment and assess the condition of the steel substrate beneath the removed coating.
10. The extent of removed coating was measured quantitatively.
11. Coating thickness was measured using a digital coating thickness gage.

The elapsed time between steps 6 and 11 did not exceed 1 week.

Autopsy procedure for black bars and stainless steel bars

1. Rebar identification tags were attached to each bar.
2. Photographs were taken to record final physical condition of the test slabs before demolition.
3. The slabs were broken open using a gas-powered saw with a diamond blade, and the bars were extracted carefully.
4. The bar surface facing the top of the slabs was marked with an arrow at one end of the bars.
5. The concrete/rebar interface was examined for corrosion products and photographed.
6. Photographs were taken of both sides of each bar to document the condition of the bar.
7. The bars were cleaned using chemicals and a wire brush to remove corrosion products, and corrosion damage was assessed.
8. Photographs were taken of both sides of the cleaned bars.

The elapsed time between steps 6 and 8 did not exceed 1 week.

Chloride Analysis

Concrete powder samples were taken by drilling with a 9.5-mm (3/8-inch) diameter masonry drill bit along the reinforcing steel imprints in the top mat of every slab. In addition, 14 powder samples were also collected from the selected bar imprints in the bottom mat. These powder samples were analyzed for water-soluble chloride concentration according to ASTM C1218 to determine the chloride content readily available at the bar depth. Based on the water-soluble chloride concentration analysis results, 23 samples (15 from top mat and 8 from bottom mat) were further selected and analyzed for acid-soluble chloride concentration according to ASTM C1152 to determine a relationship of water-soluble versus acid-soluble chloride concentrations.