PreCast Concrete Construction
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Background & Introduction
The concept of precast or “prefabricated” construction includes those buildings where the majority of structural components are standardized and produced in plants in a location away from the building, and then transported to the site for assembly. These components are manufactured by industrial methods based on mass production in order to build a large number of buildings in a short time at low cost. The main features of this construction process are as follows:
- The division and specialization of the human workforce
- The use of tools, machinery, and other equipment, usually automated, in the production of standard, interchangeable parts and products
In general, precast building systems are more economical when compared to conventional multifamily residential construction (apartment buildings) in many countries.
Categories of PreCast Building Systems
Depending on the load-bearing structure, precast systems can be divided into the following categories: # Large-panel systems
- Frame systems
- Slab-column systems with walls
- Mixed systems
Large-Panel Systems
The designation “large-panel system” refers to multistory structures composed of large wall and floor concrete panels connected in the vertical and horizontal directions so that the wall panels enclose appropriate spaces for the rooms within a building. These panels form a box-like structure (see Fig 1). Both vertical and horizontal panels resist gravity load. Wall panels are usually one story high. Horizontal floor and roof panels span either as one-way or two-way slabs. When properly joined together, these horizontal elements act as diaphragms that transfer the lateral loads to the walls.
Depending on the wall layout, there are three basic configurations of large-panel buildings:
- Cross-wall system. The main walls that resist gravity and lateral loads are placed in the short direction of the building.
- Longitudinal-wall system. The walls resisting gravity and lateral loads are placed in the longitudinal direction; usually, there is only one longitudinal wall, except for the system with two longitudinal walls developed in Kazakhstan.
- Two-way system. The walls are placed in both directions.
Thickness of wall panels ranges from 120 mm for interior walls to 300 mm for exterior walls. Floor panel thickness is 60 mm. Wall panel length is equal to the room length, typically on the order of 2.7m to 3.6 m. In some cases, there are no exterior wall panels and the facade walls are made of lightweight concrete. A typical interior wall panel is shown in fig 2.
Panel connections represent the key structural components in these systems. Based on their location within a building, these connections can be classified into vertical and horizontal joints. Vertical joints connect the vertical faces of adjoining wall panels and primarily resist vertical seismic shear forces. Horizontal joints connect the horizontal faces of the adjoining wall and floor panels and resist both gravity and seismic loads. Depending on the construction method, these joints can be classified as wet and dry.
Wet joints are constructed with cast-in-place concrete poured between the precast panels. To ensure structural continuity, protruding reinforcing bars from the panels (dowels) are welded, looped, or otherwise connected in the joint region before the concrete is placed.
Dry joints are constructed by bolting or welding together steel plates or other steel inserts cast into the ends of the precast panels for this purpose. Wet joints more closely approximate cast-in-place construction, whereas the force transfer in structures with dry joints is accomplished at discrete points.
Fig shows a plan of a large-panel building from Kazakhstan with the connection details. In this system, vertical wall panel connections are accomplished by means of groove joints, which consist of a continuous void between the panels with lapping horizontal steel and vertical tie-bars. Horizontal joint reinforcement consists of dowels projected from the panels and the hairpin hooks site-welded to the dowels; the welded length of the lapped bars depends on the bar diameter and the steel grade. Vertical tie bars are designed for tension forces developed at the panel intersections.
Lateral stability of a large-panel building system typical is provided by the columns tied to the wall panels. Boundary elements (called “bulbs”) are used instead of the columns as “stiffening elements” (Stiffeners)at the exterior, as shown in Fig. The unity of wall panels is achieved by means of splice bars welded to the transverse reinforcement of adjacent panels in the vertical joints. Longitudinal dowel bars placed in vertical and horizontal joints provide an increase in bearing area for the transfer of tension across the connections. Wall-to-floor connection is similar to that shown in Figure 3.
Frame Systems
Precast frames can be constructed using either linear elements or spatial beam-column sub-assemblages. Precast beam-column sub-assemblages have the advantage that the connecting faces between the sub-assemblages can be placed away from the critical frame regions; however, linear elements are generally preferred because of the difficulties associated with forming, handling, and erecting spatial elements. The use of linear elements generally means placing the connecting faces at the beam-column junctions. The beams can be seated on corbels at the columns, for ease of construction and to aid the shear transfer from the beam to the column. The beam-column joints accomplished in this way are hinged. However, rigid beam-column connections are used in some cases, when the continuity of longitudinal reinforcement through the beam-column joint needs to be ensured. The components of a precast reinforced concrete frame are shown in Fig.
Precast reinforced concrete frame with cruciform and linear beam elements is an example of a frame system with precast beam-column sub-assemblages. The system was developed in Kyrgyzstan in 1975. The load-bearing structure consists of a precast reinforced concrete space frame and precast floor slabs. The space frame is constructed using two main modular elements: a cruciform element and a linear beam elementFig. The cruciform element consists of the transverse frame joint with half of the adjacent beam and column lengths. The longitudinal frames are constructed by installing the precast beam elements in between the transverse frame joints. The precast elements are joined by welding the projected reinforcement bars (dowels) and casting the concrete in place.
