This paper addresses the first phase in developing a numerical model for polarization distribution prediction of the rebar-anode system in reinforced concrete slabs with residual chloride contamination, where sacrificial point anodes were embedded in the concrete patch repair area. The modeling approach was developed to characterize the performance of this system under two application scenarios: one with part of the rebar assembly in the active condition and the other with an all-passive assembly. The model results and experimental findings in > 2 year old concrete slabs are compared. Results could be used to determine the anode operating potentials that may be achieved with specific anode placement spacing for highway applications. Experimental polarization curves provide additional information for future modeling improvement.
Small anodes are commercially available to be cast in patch repairs of concrete, to prevent the initiation of "halo effect" corrosion on the rebar in areas around the patch with residual high chloride content. The anodes have a zinc alloy piece embedded in a mortar disk with connecting wires. The mortar around the zinc alloy has admixtures that promote high pH or otherwise activate the zinc, and also may contain humectants 1-4. In the present ongoing investigation two types of anodes were evaluated in reinforced concrete slabs configurations to permit determination of protective current distribution over a uniform reinforcing bar array. The results were used to formulate a model of the combined polarization performance of the anodes and rebar toward establishing predictive methods for quantifying throwing power in situations representative of highway applications. This work reviews also the data associated with anode polarization performance in laboratory controlled humidity chambers 5. At the end comparisons between laboratory and slab system installations are presented.
Six test reinforced concrete slabs with dimensions 4 ft long, 1.5 ft wide and ½ ft thick (1.2 m x 0.45 m x 0.15 m) were constructed. Each slab contains 12 embedded segments of #7 bars (2.2 cm nominal diameter) of plain steel rebar placed ladder-wise at equal intervals, 4 in. (10 cm) apart, spanning the length of each slab. The schematic of the specimen is shown in Figure 1. The slabs were built using Ordinary Repair Concrete (Table 1), except that the shaded portion near the center contained admixed NaCl to obtain 6 Kg/m3 chloride ion simulating a conventional patch repair case. Four bars were located near the center of the slab, in contact with chloride contaminated concrete, resulting in an active - net anode condition. The remaining rebars were passive. Galvanic, zinc alloy point anodes were placed between bars No. 4 and No. 5 and connected to the rest of the rebar assembly in each test slab. Two anode types were used and designated as follows: Type C (triplicate slabs numbered 1, 3 and 5) or Type W (triplicate slabs 2, 4 and 6). Both Type C and W anodes were made of zinc alloy anode embedded in pellet of mortar with zinc alloy mass of ~ 30 g and mortar pellet external diameter (or side dimension if approximately square) ~70 mm and thickness ~30 mm. For type C, the anode was embedded in highly alkaline mortar and for type W, the anode was embedded in mortar with humectants and proprietary zinc activators. The slabs were cast and kept curing in the molds for 7 days, then the slabs were demolded and placed horizontally, elevated 1 ft above ground, in an outdoor test yard ~ 20 km inland from Tampa Bay in Florida. The main anode was kept provisionally wired t
