Partial-Depth Precast Concrete Deck Panels

Partial-depth precast concrete panels span between girders and act as stay-in-place forms for cast-in-place concrete that is placed on top of the panels and act as the bottom reinforcing for the deck, creating a composite full-depth deck. Since partial-depth panels span between the girders, the overhangs must still be constructed with conventional cast-in-place concrete. The partialdepth panels are typically prestressed with strands located at the mid-depth, oriented in the bridge’s transverse direction, which is in the direction of the deck design span. The panels are placed adjacent to each other along the length of the bridge, and the prestressing serves as the bottom layer of reinforcing steel in the deck. The panels are not connected to one another at the transverse joints above the girder, as shown in Figure 4. The top layer of steel in the completed deck is placed after the panels are placed, and before the cast-in-place concrete is placed on top of the panels. Shear studs are placed on the girder flanges, and composite action is obtained by placing the cast-in-place concrete in this region, as shown in Figure 4. Typically, the precast deck panels are 3.5 inches thick, and the cast-in-place concrete layer is 4.5 inches thick.

Partial-depth precast concrete deck panels are typically considered as concrete stay-in-place formwork. Per Article 9.7.4.3 of the AASHTO LRFD (7th Edition, 2014), the depth of the stayin- place concrete should neither exceed 55 percent of the depth of the finished deck slab nor be less than 3.5 inches. For the cast-in-place slab placed above the precast deck panel, Article 9.7.4.3.2 allows the bottom distribution reinforcement, when used, to be placed directly on top of the precast panel. Splices in the top primary reinforcement in the cast-in-place portion of the deck slab are not to be located at the panel joints. Furthermore, the concrete cover below the prestressing strands should not be less than 0.75 inches.

The partial-depth precast concrete panels are typically fabricated in one of two ways. One long continuous panel is fabricated, and then cut into shorter panels so that the ends of the prestressing strands are flush with the edges of the panels. Alternatively, panels are fabricated by creating one longer panel but with forms between adjacent deck panels, and once the prestressing strands are cut, small lengths of prestressing strand extend beyond the edge of the panel. The effectiveness of prestressing strand extensions and the performance of partial-depth precast panels is an area that requires further research. Article 9.7.4.3.2 of the AASHTO LRFD (7th Edition, 2014) states that prestressing strands and/or reinforcing bars in the precast panels need not extend into the cast-in-place concrete above the beams. However, the commentary to this article notes that the lack of extended reinforcement may affect the transverse load distribution because there is a lack of continuity over the beams, which could result in cracking where the panels rest on the girders.

The bearing of the partial-depth panels on the supporting girders is an issue that must be properly addressed. A solid, uniform bearing between the partial-depth panels and the girders should be provided. Using soft bearing materials, such as fiber board, has caused problems as the panel lacks a uniform bearing area, causing cracks to develop in the cast-in-place concrete. Other problems have also been noted: the bridge deck may behave like simple spans between the girders instead of continuous spans over the girders; the ends of panels may delaminate from the cast-in-place concrete near the joints, forcing the cast-in-place concrete to carry the entire live load shear; and cracking may occur over the joints (7). In accordance with Article 9.7.4.3.4, current designs typically require that the panels be firmly bedded in grout on the supporting girder, such that the grout completely fills the voids between the panels and the girder flanges and is allowed to reach the required strength before placement of the cast-in-place concrete.

In accordance with Article 9.7.4.3.3, the upper surface of the precast partial-depth panels is to be intentionally roughened in such a manner to ensure composite action with the cast-in-place concrete. The partial-depth panels and the cast-in-place concrete must be capable of developing sufficient composite action, as the panel and the cast-in-place concrete act together to create the total thickness of the slab, and the panel’s reinforcing steel resists the positive flexural moment in the transverse direction of the deck.