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18 OCTOBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 10 MATERIAL MATTERS A concrete test slab with a hotspot meeting M-82 specifications is ready for testing. Photo source: U.S. Bureau of Reclamation. Schematic of the electrical connections in the test slab when a galvanic anode is used in the repair. Image source: U.S. Bureau of Reclamation. Continued f rom page 17 anode, but small enough to be moved by a fork lift truck and stored on shelves. Five concrete sample slabs are used as control specimens, and f ive are used as test speci- mens that incorporate the various anodes being tested. Eight reinforcing steel bars are located near the top of each sample slab and welded-wire reinforcement (W W R) is placed near the bottom of each slab to act as the cathode. A portion of the sample slab does not contain rebar so concrete core samples can be taken during the test to determine chloride content. A 1-in (25- mm) concrete cover is placed over the rebar. To speed-up corrosion and mimic conditions that call for a patch repair in the f ield, the slabs have a section on the surface called a "hotspot," where the con- crete cover is reduced to 0.75 in (19 mm). The two steel bars underneath the hotspot are half-length to facilitate easy removal for a patch repair. The concrete specimens are moist cured for seven days and air dried for 21 days. When chloride-induced corrosion of rebar is present in concrete, macrocell corrosion with a large cathode and local anode often occurs and causes local cor- rosion attacks and cross-section reduc- tion of the steel rebar. 3 To create a macro- cell in the sample slabs, the individual steel reinforcing bars and W W R are con- nected to a junction box attached to the side of the slab through a 1-Ω resistor. By measuring the voltage drop across the resistor, the macrocell corrosion current between each bar and the W W R can be determined. The two half-length bars in the hotspot are connected individually so the corrosion current both inside and outside of the patch area can be verif ied. To initiate rebar corrosion, a 5% sodium chloride (NaCl) solution (~5 gal [19 L] per specimen) is poured over the slab at a depth of between 0.75 to 1.5 in (38 mm) and allowed to pond. After 14 days of ponding, the solution is drained and the specimen dries for an additional 14 days. A complete cycle is 28 days. Mac- rocell currents are measured at the end of the chloride ponding period before the solution is drained. After every three 28-day cycles, once the NaCl solution is removed, a corrosion potential contour map of the slab is created. Corrosion is considered to have initiated the f irst time a steel reinforcing bar has a potential reading more negative than –300 mV vs. a copper/copper sulfate (Cu/CuSO 4 ) refer- ence electrode (CSE) and the macrocell current is >0.03 m A, or when the macro- cell current is >0.03 m A for two cycles. At corrosion initiation, a core sample is taken from the slab for chloride analy- sis, another corrosion potential contour map is created, and then patch repairs are made to the hotspot. The samples with sacrif icial galvanic anodes are repaired the same way as the control samples, except that an anode is attached to the bar and wired to the junction box. "By running a separate wire from the anode to the junction box, we can actu- ally measure its current, so we are able to see how much current the anode sends to the bars around it," Berke explains. Ideally, the anode sends enough current to the surrounding bars so that they become cathodes, he adds. After the repairs are made, the pond- ing cycles and macrocell corrosion cell testing continue, as well as complete potential contour mapping. When 80% (four out of the f ive) of the control slabs have corrosion-induced concrete crack- ing over the reinforcing bars, the test period continues for two more 28-day cycles; and then the test is concluded. In its entirety, the test protocol can last any- where from one and a half to two years. During the test, key parameter values that should be noted for later evaluation of the test results include time to corro- sion initiation and chloride content at corrosion initiation; and time to repair, integrated current at repair, chloride at repair, and potential map at repair. Upon test completion, another con-