Precast has been used for architectural purposes since the early 1900s and came into wide use in the 1960s. The exterior surface of a precast can be designed as per the suitability, an exposed aggregate finish can be used for ornamental purposes while form shaped face can be designed for cast in place. Precast can be used almost everywhere in a structure from the panelling of columns with precast panels to precast walls or claddings with window openings that extend to several floors in height. These precast components are selected based on several factors such as performance criteria, and weather conditions. For smaller structures, standard-sized precast components are used whereas in large-scale constructions such as bridges or flyovers, custom precast components are prepared by the manufacturer for specific loads, connections, and erection loads, that can perform adequately and withstand respective weather conditions along with being durable enough.
Precast concrete is produced by casting concrete in a reusable mold which is then cured in a controlled environment to gain the predicted strength. It is then transported to the construction site and lifted with the help of heavy machines into respective places. As the precast is cured in a controlled environment, it has the opportunity to cure properly and to be closely monitored by precast plant employees. A variety of precast systems are available in different shapes, colours, textures and finishes to be chosen from as per the requirement. These precast varieties are assessed via chosen sample at the manufacturing factory itself prior to full-fledged precast panel production. This assessment is an addon to the standard quality control measures and field testing which is necessary during the production phase. Precast concrete offers many advantages over onsite casting such as there is greater control over material quality and workmanship in a precast plant, the forms used in precast plants can be used umpteen times before they have to be replaced which makes the entire casting process cheaper in the long run in terms of cost per unit of formwork.
Generally, every precast component is independently supported to the structure with the help of bolts and anchors or in short, an assemblage of metal components and the joints in between and around all precast components are filled with a sealant. They can also be grouted or concreted together with help of loops of steel reinforcements left protruding out of the precast concrete members. Precast components are typically used for structural requirements such as beams, columns, floors, staircases, wall panels, pipes, tunnels.
Precast Concrete Structural Elements and Precast Building Systems
A precast building structure is constructed by assembling and connecting a variety of prefabricated elements at proper alignments and angles. In lieu of precast elements, precast building systems can also be used as per the load-bearing structure requirements. The need for precast elements or building systems can be decided based upon the type of structure, the distance of the manufacturing factory from the construction site, feasibility, etc. Both the precast elements can be sub-categorized into a number of different elements and systems respectively which are as follows:
1. Precast Concrete Elements
Precast concrete slabs or beams are versatile components that can be inserted in a variety of ways into structures and foundations for small and large-scale construction projects. They have an advantage over traditional slabs because they have been poured in a controlled environment. Precast slabs constitute either precast concrete arched panels, hollow mini-slabs or planks that can be placed between smaller, more frequently spaced precast beams or joists spanning between walls or columns. A thin reinforced concrete layer is poured on top of it afterward. These slabs eliminate the need for extensive formworks and are more flexible for projects that need to be phased much like the improvement of structures in informal settlements.
b. Precast Concrete Columns
Precast concrete columns are precast elements that are single or double storied and one of circular, square or rectangular shapes. These can be designed and manufactured as per specifications by incorporating add-on features and fittings. Precast columns are usually considered for use on in-situ flat concrete slab projects where there is high demand due to tower crane hook time and tight construction programs. The connection of columns with the foundation can be done via a base plate connected to the columns or by grouting of reinforcing bars projecting from the column end into the sleeves. The column can also be placed in a preformed hole in the foundation block and grouted into position. Column to column connections can either be made by threaded rods joined with an appropriate connecter with concrete around the column dimensions resulting in deep stitch or by using grouting sleeves resulting in thin stitch.
c. Precast Concrete Walls
Precast walls are constructed by casting concrete in a reusable wall formwork and cured in a controlled environment which is then transported to the construction site. It is placed in the same manner as the precast columns and it needs 8- 12 inches of compacted gravel as its base. They can be used in the kitchen, bathroom or to form a partitioned living space or extra storage room. Precast is equally common in the construction of commercial buildings as well.
d. Precast Concrete Foundation
Precast foundations are prepared by pouring concrete mix into reusable molds that have been set up as per the design specifications and then cured in controlled environments and finally transported to construction sites. The concrete used for making precast foundations are high strength quality and made of natural materials. The precast foundation components include a concrete footer that rests on a sub-footer.
2. Precast Building Systems
a. Large- Panel Systems
Large panel systems using precast components are typically used for the construction of commercial buildings such as apartments and hotels. It is composed of precast floor concrete panels and large precast walls connected in both the vertical and horizontal directions.
The large panels are of the height of a single-storey of building. The gravity loads are borne by both the vertical walls and horizontal floor panels simultaneously. The large panel system using precast components can be installed in three arrangement types based on wall layouts that are cross-wall system, longitudinal- wall system and two-way system. Large panel systems are suitable for buildings that need close partitioning of space. These panels can be easily painted on and can be insulated for thermal comfort. When installed with proper joints, the large panel system can achieve adequate strength and stiffness to resist lateral loads.
b. Frame System
The frame system consists of linear precast elements or spatial beam-columns sub-assembled together. Precast beam-columns assemblies are beneficial in its construction because the connecting faces between the sub-assemblages can be placed away from the critical frame regions, but linear elements are usually preferred due to the difficulties associated with forming, handling and erecting spatial precast elements. The connecting beams can be seated on corbels at the columns, for ease of construction and assist the shear transfer from beam to columns which results in hinged joints. The space frame is constructed using two main modular elements, a cruciform and a linear beam. In frame systems, there is a possibility of columns being installed and thus free space and visual obstruction due to walls are negligible in an open layout. Also when using frame systems, there is freedom of choice for the selection of cladding elements for the facade.
c. Slab-Columns System with Shear Walls
In this precast arrangement, the vertical or gravity loads are supported by the roof or slab column structure whereas the lateral loads are resisted by the shear walls. They are mainly of two types, lift slab systems with walls where the load-bearing system consists of precast reinforced concrete columns and slabs assembled with special joints and a pre-stressed slab-column system where two pre-stressed columns are placed in orthogonal directions to achieve continuity. In the lift-slab system precast concrete floor slabs are lifted from the ground up to the final heights by lifting cranes and the columns used are 2 stories high.
The precast concrete columns used in prestressed slab-columns systems are usually 1- 3 stories high, and after erecting the slabs and columns of a storey, the columns and floors are prestressed by means of prestressing tendons that are passing through ducts in the columns at the floor level and along the gaps left between adjacent slabs. This kind of precast system cannot be used in regions of high seismicity.
Using precast concrete in the construction of buildings can be beneficial in numerous ways from strength criteria to affordability. Some of the advantages of using precast concrete components have been given below:
Disadvantages of Precast Concrete
There are a few disadvantages related to precast usage which are:
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