Concrete boom from last century has relied on available work force, as formwork building is the most time-consuming phase in in-situ casting concrete structures. Ever-continuing urbanization and economic development has pushed disadvantages of in-situ casting into broad daylight due challenges listed below:
- Lack of time and space
- Pollution and waste generation
- Costs to society and environment and immediate or indirect health risks
Challenges and problems of construction on the other hand have plead for developing efficient or modular methods. Resulting development industrial development could be considered to be either of the following:
- prefabrication of structures (read further)
- mechanical or automation aided concrete casting (read later)
Prefabrication of reinforced and (steel-) composite concrete structures
Precast concrete (PC) structures have been manufactured centrally and then sent directly or through depots to the jobsite, at moderate distance by trucks (between cities or provinces typically). Industrialized casting makes possible (warm) regulated indoors work environment, serialized production with some freedom to structural shape and efficient quality control, just to mention the most obvious. Prefabrication of composite sections using steel as partly structural, partly as concrete casting mold, enabled also fast construction. Moreover, BIM, FEM and academic engineering advances have made many types of composite structures available, resulting in a variety of efficient slab construction methods, combining wooded or steel beams or sheets with concrete. (Figure 3)
Figure 1. Precast composite slimfloor slab using precast hollow-core (HCS) planks, note large perimeter formworks, to create cantilever slabs, or shapes in slab along façade - last picture is timber-concrete slab with shear- plate composite connectors
Figure 2. Precast column lifting and connection with shoe is a popular precast connection tightening with concrete anchor bolts
Figure 3. Modular modelling for standardized steel-concrete composite structure production in BIM-software
Precast needs early planning and dedication to the design of including specialized frame elements for variety of actions (Article discussing 3 beliefs from precast frontier). A factory on the other hand needs planning and acquisition of complex and advanced inventory for forming, reinforcing, tilting, moving etc.:
- Forms, formwork systems, perhaps complicated formwork elements, or even tilting tables
- Reinforcement storing, cutting, bending and cage assembly areas
- Concrete pouring, vibration (possibly a table) and curing equipment
- Overhead beam crane or other possible hoisting equipment for in-factory or outward hoisting
Figure 4. Top: precast slab factory typical equipment line-up
Bottom: Massive/monolithic PC – note protruding reinforcement for “wet” (casted, stiff joint) connecting on site
L to R, top-down: lintels, wall, façade and balcony, staircase, half-cast beams, etc.
Precast can start to be massive for industrial or extremely large constructions or when spans get very long. The element thickness or height can be limited to lighten the element, so have sufficient lifting capacity can become an issue. Special cases of lifting can utilize multiple smaller vehicles and a combination of lifting points and equipment added to the hook.
Figure 5. Right: Long span or massive precast elements can be an alternative when lifting or
Left: PC beams constructed on wide transfer beam and two columns
Figure 6. Variety of lifting insert types for countless variety of PC shape – on right, turning a large panel with 2 cranes
Figure 7. PC concrete lifting equipment and conditions
Precast faces challenge, with varying design, block size and building conditions. With mature and sofisiticated product family and combinations of connections and inserts this can be achieved.
Referred material
Literature and handbooks
Diaphragms globaljournals.org/:Investigation_into_the_Floor_Diaphragms_Flexibility_in.pdf
fib Bulletin 43 Structural connections for precast concrete buildings
fib Bulletin 74 Planning and design handbook on precast building structures
fib Bulletin 73 Tall buildings
Cuerrier-Auclair, Design guide for timber-concrete Composite floors in Canada, 2020, FPInnovations
Construction technics and solutions, referred above
Overhead crane example, https://overheadcranesuppliers.com/overhead-beam-crane/
Green slimfloor https://www.peikko.com/products/deltabeam-slim-floor-structures/deltabeam-green/
Making bolted connections seismic, https://www.peikko.com/products/connections-for-seismic-design/technical-information/
