CALCIUM NITRITE IN KENTUCKY BRIDGES
In the fall of 1986, the Kentucky Department of Highways (KYDOH) constructed two experimental bridges incorporating calcium nitrite. Plans were made for a third bridge using calcium nitrite in the pier caps, but that application was eventually cancelled. The two experimental bridges selected were the KY 152 bridge over Beech Fork Creek in Washington County (BRS 5129 - Item No. 4-154) and the Gose Road bridge over Clark’s Run in Boyle County (BRM 7104 - Item No. 7-076).
Calcium nitrite was used to protect uncoated reinforcing steel in diaphrams, slabs, and barriers of the Washington County bridge and only on the slab of the Boyle County structure. Black reinforcing steel was used in the members inhibited by the calcium nitrite. A KYDOH special note provided for the addition of W. R. Grace PCI calcium-nitrite corrosion inhibitor. The admixture was to be introduced as an aqueous solution consisting of 30 + 1 percent calcium nitrite at a solution weight of 10.6 +, 0.1 pounds per gallon. The admixture dosage was to be 4.0 gallons per cubic yard (Appendix I). For 6.6 bags of cement per cubic yard used in KYDOH AA concrete, that resulted in 2 percent calcium nitrite by weight of cement. That dosage should offset the corrosive action of 2 to 3 percent chlorides by weight of cement and provide chloride protection for the .functional life of the bridge deck.
KY 152 Bridge
The KY 152 bridge was constructed in late July, 1986. Calcium nitrite was used in concrete placed in three diaphrams. The concrete was batched at the Nally-Gibson Concrete plant at Springfield, Kentucky. The initial concrete mixture included calcium nitrite (W. R. Grace PCI), a retarder (W. R. Grace Paratard 17), and an air-entraining agent (Airlon). W. R. Grace representatives assisted in the batching operation at the plant. Initial attempts to batch satisfactory mixtures using Airlon on July 27 and 29, 1986 failed. Another air-entraining agent. Protex. was successfully substituted. Kentucky Transportation Research Program (KTRP) personnel monitored placement of the diaphrams on July 29, 1986 by the contractor, Judy Construction Company. The final mixture design is given in Table 1.
The bridge site was approximately six miles from the Nally-Gibson plant. When the diaphram concrete arrived at the site, it had a 3.0-inch slump and a 3.5-percent air content. After five gallons of water were added to the 8 cubic yard batch of concrete at the bridge site, the air content increased to 4.0 percent and the slump remained unchanged.
The deck concrete was placed on August 28, 1986. The first batch delivered to the site was too dry and was rejected. The second batch had a 4.0-inch slump and a 6.1-percent air content at the plant. When it arrived at the bridge, the slump was 2.0 inches and the air content was 5.1 percent. Five gallons of water were added to the mixture to provide workability. Subsequent batches used in the deck varied in (on-site) slump from 4.5 to 6.0 inches and in air content from 5.0 to 6.0 percent.
The concrete was placed by pumping and was finished with a single drum transverse screed transverse finishing machine. Placement proceeded smoothly; however, some problems were encountered in finishing the concrete.
The concrete had a sticky texture and was difficult to strike off with the screed. Material adhered to the screed drum leaving rough, pulled areas on the surface of the concrete after the first pass (Figure 1). The screed had to be run over the same areas again to smooth out the rough concrete surface. At times, a workman would spray water onto the concrete to enhance finishing. Later in the deck-placing operation, the cement content was increased from 6.6 to 7.0 bags per cubic yard to enhance workability. Except for finishing problems, concrete placement proceeded satisfactorily and the deck was completed by the end of the day.
KTRP personnel did not monitor any barrier concrete-placement operations.
Gose Road Bridge
The Gose Road Bridge incorporated calcium nitrite only in the deck concrete. The decision was made to use a super water reducer (Daracem 100 Type G) instead of a retarder for that bridge. The initial design mixture is given in Table 2, The material was batched a few miles from the bridge site at the Concrete Materials Inc. plant in Danville. The calcium nitrite solution was dosed at the plant by pumping.
The bridge was a short 45-degree skewed structure having a slight grade. The Jim Skaggs Construction Company placed the deck on October 7, 1986.
The specified AA Concrete slump was 3 +. 1/2 inches and the specified air content was 5.5 +_ 1.5 percent. The slump of the first batch was 2-3/4 inches and the air content was 4.8 percent. Additional water (10 gallons) was provided at the site to increase the slump to about 4-1/2 inches. The contractor was unwilling to use a greater slump (5 to 6 inches requested by W. R. Grace personnel) because he feared the concrete would run during placement due to inclination of the deck.
The first batch of concrete was not manually finished promptly. That was due to delays in adding water to achieve the desired slump and to the amount of time required to clear the obliquely mounted finishing machine for transverse manual finishing (bull floating and tyning) across the width of the deck. The concrete began to harden and was difficult to finish. The sticky texture of the concrete exacerbated the problem.
Placement and finishing improved toward the upper end of the bridge after W. R. Grace personnel convinced the contractor to allow the specified 5 to 6- inch slump. The mix design air content was specified to be 5.5% ± 1.5%. During placement it varied between 4.8 and 5.0 percent.