Full-Depth Precast Concrete Deck Panels
Full-depth precast concrete panels typically span the width of a bridge and are placed longitudinally adjacent to one another. The transverse joints between adjacent panels are often grouted, and post-tensioning may also be used to keep the transverse joint in compression. The deck panels are made composite with the steel girders through the use of shear studs. Block-outs in the panels are provided to allow for the placement of shear studs onto the top flange of the steel girder. Composite action between the deck and the girders is provided by shear studs that extend out of the girder and into the block-outs in the panels, once the block-outs are filled with grout. Figure 1 shows a bridge deck being constructed with full depth precast concrete deck panels, where the block-outs for the shear studs can be seen above the steel girders. Also shown in Figure 1 are block-outs for connecting the post-tensioning strands. The block-outs for the post-tensioning strands are at the panel joints, transversely across the width of the bridge.
After the precast deck panel is installed, it is set to the correct elevation with the use of leveling bolts that bear on the girder top flange, as shown in Figure 2. The transverse joints are then filled with grout. Once the grout reaches the required compressive strength the longitudinal post-tensioning strands are tightened (if post-tensioning is used in the design of the deck system). The shear studs are then welded to the girder top flange in the provided block-outs. The shear stud block-out, the haunch between the girder top flange and deck panel, and the post-tensioning ducts are filled with grout. A wearing surface or overlay is typically installed on top of the deck panel after all installation is complete, providing the necessary riding surface.
Two particular design issues require due consideration by the engineer specifying the use of full depth precast concrete deck panels. One issue is related to the grout used in the shear stud blockouts. Given that the block-outs are located in areas of negative moment in the transverse direction of the deck, the grouted block-outs are susceptible to cracking. One method to reduce the potential for cracking is to use block-outs with rounded corners to prevent stress concentrations. Additionally, Article 9.7.5.3 of the AASHTO LRFD (7th Edition, 2014) states that upon completion of the post-tensioning, the block-outs should be filled with grout having a minimum compressive strength of 5.0 ksi at 24 hours. The designer should check to ensure that specified grout is capable of resisting the tensile stress caused by negative moment in the transverse direction of the deck.
The transverse joints between adjacent panels must be able to transfer wheel load shear forces and deck axial forces from flexure of the bridge. The transverse joint must also prevent the flow of water and corrosion causing materials through the deck. In practice, non-grouted and grouted transverse joints have been used. Non-grouted joints are typically match-cast at a precast concrete plant, and use a male-to-female shear key type joint with a thin neoprene sheet in between, and sealed with a polyurethane sealant. In some cases it has been found that even though the panels are match-cast, it is difficult to achieve a prefect fit in the field due to construction tolerances and elevation adjustments of the panels (5). This detail is used in conjunction with post-tensioning, in order to close the joint.
However, most joints used in practice are female-to-female shear key type joints that are filled with grout prior to post-tensioning taking place. Unlike joints that have male-to-female shear keys, the female-to-female joints allow for some adjustment related construction tolerances in the field, as the joint is not interlocking. A sketch of a typical female-to-female joint is shown in
Figure 3. The full depth of the female-to-female joint should be filled with grout, as placing grout only in the top portion of the joint can cause poor performance of the joint because of the reduced bearing area (6). Grout used in the transverse joint should be of high quality and have a high early strength, high bond capability, and low shrinkage. In fact, Article 9.7.5.3 of the AASHTO LRFD (7th Edition, 2014) states that the transverse joints are to be filled with a nonshrink grout having a minimum compressive strength of 5.0 ksi at 24 hours. Once the joint is grouted, longitudinal post tensioning can be tightened. Per Article 9.7.5.3 of the AASHTO LRFD (7th Edition, 2014), the minimum average effective prestress from the post-tensioning should not be less than 250 psi. The effective prestress is typically interpreted as the net prestress under loading. The post-tensioning places the joint in compression, which can help to prevent cracking due to applied loads and shrinkage, help prevent leakage of corrosive materials through the joint, and helps to keep the panels in compression under loading. In continuous span designs, the designer may elect to increase the post-tensioning to achieve the 250 psi effective stress under all loading conditions.
