Cast-in-Place and Precast Construction Methods
As mentioned previously, filled and partially filled grid decks, and grid decks composite with a reinforced concrete slab can have the concrete slab portion installed with cast-in-place or precast construction methods. With cast-in-place construction, the unfilled steel grid deck is placed on the supporting components, such as the top flanges of the girders. Since the grid deck is divided into multiple panels longitudinally and transversely, adjacent panels are typically spliced together with mechanical fasteners, shear reinforcement (transverse only), or other methods. The unfilled grid deck is set to the required elevation using leveling bolts that are built into the grid deck panels. Shear stud connectors are placed through the grid deck and welded to the top flange of the supporting component. Haunches are formed at the supporting components, usually with galvanized thin gage steel sheets, structural angles, or timber. Mild steel reinforcement, if not already incorporated into the grid deck, is then installed, and concrete is then placed in a conventional fashion, filling the haunches full depth and the specified depth and overfill of the grid deck.
Alternatively, the concrete for the grid deck panels can be precast off site, and the panels with concrete installed delivered to the work site. When the concrete is precast, forms or block-outs are used to prevent the placement of concrete in panel areas that will eventually be located over supporting components in the final structural configuration, in deck panel connection areas, and in areas where cast-in-place barriers will be placed. However, when precast methods are used, the panels must be properly handled and stored to reduce the possibility of concrete cracking. In general, the installation of the precast panels, shear stud connectors, and haunch formwork are similar to cast-in-place construction. Once all components are installed, rapid setting concrete is used to make the full depth closure pours in the haunch and deck panel connection areas. Rapid setting concrete provides the high early strength that is typically required, while also limiting the potential for shrinkage cracking.
In both methods of construction, a maximum coarse aggregate size of 0.375 in. is typically specified, and pencil type vibrators are recommended as the concrete is placed. Also, after concrete placement is complete, and any grinding or finishing has occurred, a sealant or overlay is typically installed. Overlays are more commonly used with precast concrete decks. The sealant or overlay should be compatible with conventional reinforced concrete deck slabs.