EVALUATION OF THE EXPERIMENTAL BRIDGE DECKS AND CONCRETE
KTRP personnel inspected the bridge decks in December 1986 (5). The surface finish of both decks was not good. Pull cracks were detected along the north edge of the KY 152 bridge deck (Figure 2). That deck was observed to have irregular tyning varying from almost flush to approximately 1/4-inch deep. In areas where tyning was deep, aggregates were pulled from the concrete giving the deck a "pock-marked" appearance. Several small areas where plastic shrinkage cracking occurred had been sealed with epoxy.
The Gose Road deck also exhibited an irregular surface (Figures 3 and 4). Uneven tyning was noticeable, especially on the lower end of the bridge where the tyning depth was shallow. That portion of the deck surface apparently had been finished and tyned after the concrete had stiffened. Deep tyning had pulled loose or exposed aggregate at many points on the deck; however, pull cracking was not detected. Some surface voids were present where the concrete had not been completely consolidated during placement.
Surface finish problems were not indicative of the calcium-nitrite admixed concrete strength and durability. Concrete cylinders cast at the KY 152 bridge had average compressive strengths of 5,340 psi, 5,627 psi and 6,420 psi at 5, 7, and 28 days, based on two, four and five tests at the respective time intervals. The Gose Road bridge concrete cylinders had average compressive strengths of 6,110 psi and 7,260 psi at 7 and 28 days, based on two tests at each time interval. In part, the strength difference may be due to the use of a super water reducer in the Gose Road bridge concrete.
Freeze-thaw tests were performed on two concrete prisms cast from concrete batched for the Gose Road bridge. The prisms were tested in accordance with ASTM-666 Method B, Freezing in Air and Thawing in Water and ASTM 0215, Fundamental Transverse ... Frequencies of Concrete Specimens. The specimens were soaked 13 days prior to freeze-thaw testing that commenced on October 20, 1986. The concrete prisms met both ASTM and Kentucky Specifications (300 and 350 freeze-thaw cycles) prior to the dynamic (sonic) modulus reaching 60 percent of the initial modulus. The prisms were not monitored for expansion. Since there was no reduction in modulus after 360 cycles of rapid freezing and thawing, it may be safe to assume the concrete expanded less than the
0.                  050 percent (the maximum value recommended for concrete specimens by KYDOH Division of Materials).
Unfortunately, prisms of class AA concrete, which normally would have been specified for this project, were not cast for freeze-thaw comparison testing. As a result, a direct comparison of the freeze-thaw durability of class AA concrete containing calcium nitrite and conventional class AA concrete is not possible. However, a review of past freeze-thaw data at the Kentucky Department of Highways’ Division of Materials indicates that coarse aggregate used in the concrete mixture for the Gose Road bridge has been approved for size No. 78’s and No, 8’s only. Durability factors for these sizes are only slightly less than 100 percent.
CORROSION TESTING
At the onset of this work, KTRP and W. R. Grace personnel intended to install special stainless-steel reference electrodes in both decks. Those electrodes were to be custom made by W. R. Grace and installed by their personnel. In June 1986, work on the KY 152 Bridge proceeded faster than anticipated and electrodes could not be furnished for that bridge in time for installation. W. R. Grace personnel stated they could eventually perform polarization-resistance tests without the buried reference electrodes. That method will be used on future tests conducted over the next several years when more meaningful corrosion-rate data are expected.
In October 1986, W. R. Grace personnel installed four reference electrodes along a transverse reinforcing bar on the top mat of the Gose Road deck. The probes were placed: 1) 12 inches from the curb in the gutter line, 2) 8’-8" from the curb under the center of the northbound lane, 3) 12’-5" from the curb under the left wheel-track area in the northbound lane and 4) 16 feet from the curb in the center of the bridge. The probes were embedded when the deck was placed. Lead wires were run to a small junction box cast into the outer face of the east barrier. The wires were color coded for identification.
In December 1986, KTRP personnel performed half-cell (saturated copper- sulfate) corrosion-potential tests on the two decks. For each test, the probe lead wire was grounded to a gutter. Test results are shown on Figures 5 and 6. Active corrosion was not detected (i.e., corrosion potentials measured were less than 337 mV). However, some corrosion potentials were measured in the 300-mV range. Those high readings are typical of new bridges and are expected to decrease with time. A low oxygen concentration may occur in concrete containing calcium nitrite which may cause such high readings. It is too early in the service lives of the bridges to expect any significant corrosion (whether the calcium nitrite is beneficial or not).
On July 24, 1987, W. R. Grace and KTRP personnel visited the Gose Road bridge to perform corrosion measurements in conjunction with the reference electrodes embedded in the deck. A series of corrosion measurements were taken on the embedded corrosion cells (Figure 7). The ambient temperature was 90° F and the deck temperature was 95° F.
Polarization-resistance measurements were made with a computer-controlled potentiostat, the AUTOSTAT, made by Thompson Electrochem LTD (Figure 8). Reference voltages were first measured between each of the buried 316 stainless-steel reference electrodes and the reinforcing steel. The AUTOSTAT then slowly varied the direct-current potential between the reinforcing steel and a reference electrode at a rate of 2 mV per minute from -20 to +20 mV. The computer measured both the instantaneous voltage and current. The polorization resistance, Rp, was measured as the slope of the instantaneous voltage versus the instantaneous current curve at Ip--0. That process was repeated for each reference electrode. Half-cell potential measurements were taken with both copper-sulfate and calomel half-cell electrodes (the calomel cell produced lower corrosion-potential values). The test results are shown in Table 3.
The test results indicate that little, if any, corrosion was occurring (6). The reinforcing-steel potentials were passive in respect to the stainless-steel reference electrodes. That indicated the reinforcing steel was more corrosion resistant than the stainless steel at the time of the readings. The half-cell corrosion-potential readings revealed the presence of a galvanic current when attached to the zinc-coated guardrail. The distance over which that weak current was detected indicated that the concrete was relatively permeable and that the concrete resistivity was low. W. R. Grace personnel estimated that an area of about 10,000 cm² was polarized across the deck. Considering a polarization resistance of about 52 ohms, the product of the polarization resistance and steel surface area over which it acts is 500 kohms-em² which is high for steel, indicating that no corrosion is occurring (7).
Comparative cost data were obtained for the calcium nitrite and epoxy coating corrosion-protection methods. The KY 152 and Gose Road bridges used 273.4 and 31.6 yard of class AA concrete, respectively. Mr. Jim Render of W. R. Grace Company provided a unit cost of $24 per yard for PCI. Daracem 100 (super water reducer), and the additional air-entraining agent. If that mix design had been used in both bridge decks, the additional cost would have been $6,561 and $758, respectively.
The KY 152 and Gose Road bridges used 57,579 and 6,763 lbs of reinforcing steel, respectively, that would normally employ epoxy coating. The cost of epoxy coating is $0,104 per pound based on 1988 Kentucky Transportation Cabinet data. If epoxy coating had been used on both bridges, the additional cost would have been $5,998 and $703, respectively.
Costs of the two corrosion-protection methods are similar. The calcium nitrite method is slightly higher. However, that difference is negligible when compared to the price of the structures.
CONCLUSIONS
Corrosion tests performed on the two experimental ealciuxn-nitrite impregnated bridge decks revealed no active corrosion. However, the tests were too premature to predict future performance. FHWA research (op. cit. 4) indicates that for the calcium-nitrite dosages provided for the two bridges, they should provide corrosion protection similar to that provided by epoxy- coated reinforcing steel (in the top mat).
It is likely that a long time will be necessary for the two bridges to experience significant corrosion. That belief is due not only to the protection afforded by the calcium nitrite, but also to the low amount of deck salting that is anticipated for the two decks based on their locations. Portions of the two bridge decks could be deliberately salted to promote corrosion. It would be more desirable to use calcium nitrite on bridges in northern areas of the state that are subject to more applications of deicing salts.
Finishing was the main construction problem with concrete containing calcium nitrite. That problem was not evident when placing barriers or diaphrams, but only in finishing deck surfaces. Calcium nitrite renders concrete sticky similar to microsilica. Occasionally, concrete sticks to the surface of the spinning drum of a finishing machine. That creates rough areas in the finished surface of the deck.
When an experimental microsilica concrete overlay was placed on a deck at Seebree, the finishing problem was eliminated by substituting a vibrating- screed for the more common spinning-drum, Bidwell type screeds. Kentucky Department of Highways personnel have expressed concern with using a vibrating screed during construction of new decks. It would be possible to modify spinning-drum screeds to provide a greater stroke, thereby eliminating some excess working of the concrete surface. It should be noted that both decks were finished with single-drum screeds. The newer double-drum screeds would probably perform better.
An improved surface finish could be obtained by grooving a cured deck by sawing rather than by tyning a deck when the concrete was in a plastic state. That might further reduce finishing problems presently encountered during tyning.
For any new bridges incorporating calcium nitrite, close interaction should be promoted between the calcium-nitrite suppliers, concrete suppliers, and contractors to prevent problems similar to those experienced on the previous experimental decks. Some preliminary experimental work with finishing procedures and concrete mixtures would be desirable to optimize the concrete properties and improve concrete finishing of further experimental decks.
Use of super water reducers in concrete having 5-to 7-inch slumps may promote better workability of calcium nitrite-modified concrete. Super water reducers can be used to achieve low water/cement ratios and superior freeze- thaw resistance (though some problems have recently been encountered in this area with a specific super water reducer). Super water reducers used in conjunction with calcium nitrite also will yield stronger concrete than the conventional class AA mix. Super water reducers should also be studied separately since they may provide significant improvement of bridge-deck concrete used with epoxy-coated reinforcing steel.
Calcium nitrite also may be used with microsilica and super water reducers to produce very strong concretes that are impermeable to chloride penetration, resistant to reinforcing-steel corrosion, resistant to creep, resistant to abrasion, and resistant to freeze-thaw damage (8). Concrete containing those additives would be expensive and might only be justified in critical applications such as long prestressed beams or bridge decks carrying high traffic volumes. Additional developmental work is needed for those applications.
It would be desirable for the Department of Highways to continue developmental work with calcium nitrite. That consideration was noted in a previous departmental memorandum (9).