Two-course Bonded Concrete Bridge Deck Construction: Interim Report No. 1, An Evaluation of the Technique Employed

About the report:

A two-course bonded technique that has evolved from the continuing national and local interest in bridge deck durability was used in June 1974 to construct six bridge decks in Virginia. During the construction, which was the first phase of a five-year study on the construction, condition and performance of two-course decks, detailed observations were made of the activities used to construct the six two-course decks and two conventional single-lift decks, and data concerning the several concretes utilized in the construction of the decks were recorded. Based on comparisons of two-course and single-lift construction techniques the following conclusions are made. An overlay should be placed not sooner than two days after a base layer is placed. Light sandblasting of the base layer removes laitance that might adversely affect the bond between the base layer and overlay. A bonding layer of cement slurry should be broomed onto the base layer not further than 10 feet (3 meters) nor longer than 15 minutes ahead of the overlay placements. The same depth of clear concrete cover above the top reinforcing steel resulted from the two-course construction as from the single-lift technique. Using conventional equipment selected by the contractor, the construction activities proceeded in an orderly and satisfactory manner and coefficients of variation for the time intervals required to install the base layers and the overlays are comparable to values representing excellent control for single-lift construction. Although a 7-yd ³ (5.4 m³) truckload of concrete was screeded over 3 1/2 times as much surface area for an overlay as for a conventional single-lift deck, the average duration per truckload between the initial depositing and the completion of the screeding activity on the wearing surface was approximately the same in both the two-course and single-lift techniques. Not including texturing and curing activities, the average man-hours required to install concrete in the separate layers of the two-course decks was 33% greater than for conventional single-lift decks, but in terms of project days required for construction the two-course and single-lift techniques are equivalent. The total additional cost of the two-course technique is approximately 5% of the cost of the bridge superstructure. Three overlay concretes were selected for use as wearing courses on the two-course concrete decks on the basis of their protective qualities. A broad range of handling characteristics is represented by these concretes, which included a latex modified concrete, a high quality PCC, and a wire fiber reinforced concrete. The differences in these wearing course concretes did not significantly affect the placement activities, other than the batching, during their respective installations. In general the decks constructed by the two-course technique are equivalent to those resulting from conventional single-lift construction, however these successfully installed special wearing course concretes offer improved potential for deck performance.

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