Concrete speed can be a requirement in acute need, such as public service buildings or economic reasons. Bringing too much raw materials to the site and building everything from scratch can waste both material and time. Hiding part of the construction process inside factories keeps sites neat and clean and fitting to the milieu, as the concrete pouring phase is offset. Can sites be neat and construction fast even if concrete is poured on site? The answer is naturally yes, and here are a few points to back this up.
Mechanical form or precast for long and high structures
Precast technology has helped keep the messy industry and cosy urban milieu apart. There are, however, other ways to tidy up the building process; especially, block-building can be difficult to lift conventionally when elements are large or complicated by shape.
Symmetricity and continuous cross-section can make bridge road and tower design and construction efficient. Regular shapes in building also help by distributing loads more evenly, thereby, avoiding complex solutions such as bracing or stiffening structures or joints locally. Mechanised construction can be applied locally or partially in more complicated architectural designs or extremely long constructions, such as highway bridges for efficiency, easier process, and time savings.
Figure 1: Pylon slipform, precast bridge beam and shell casting
Especially tall (high-rise) buildings are subject to high sideways wind or seismic load, as they are many times as high as wide. This causes high flexure and shear over the height of the building, which is solved typically by building stiff ‘cores’ and wall-like shear elements, leaving the column-beam frame to perform a supporting role by transferring a portion of the vertical loads to the foundation.
If the vertical elements’ lateral stability is taken care of by monolithic or bracing structures, more flexibility is allowed in joints and connections. Continuous and high structures casting has attained reasonable speed using mechanical casting forms that climb in various ways (Figure 2). A comparison of different core forming options can be seen in Figure 3.
Figure 2: Climbing or sliding formwork allow limited discontinuity, but can inventively allow even casting to shape; Hydraulic auto climbing systems can adapt to changing shape also (leftmost)
Figure 3: Main types of climbing formwork in comparison table (fib 73 bulletin – Tall buildings, p.49)
The horizontal frame member needs connection parts and recesses that can pose a challenge of continuous core casting. Post-installed hidden corbels and threaded anchor bars allow undisrupted core casting and fast connection to slabs, either in-situ or precast (Figure 4).
Figure 4: Hidden corbels and recess-board anchors enable uninterrupted (smooth) core casting with beam or slab connections
Flat slabs optimise building space, and benefit installations and consumption during usage. Apart from precast slim floors, it remains a practical challenge to lighten the floors. However, this has been addressed by installation techniques such as tensioned bubble deck, utilising various types of sophisticated design, formwork, and reinforcement simultaneously (Figure 5).
Figure 5: Bubble-deck eliminates ineffective concrete from deadweight, its capacity can be enhanced with post-tensioning and shear studs (on fast reinforcing rails) at critical zones, such as column tops, and lastly, formwork systems make construction fast
Precast and cast-in situ techniques can work well together to accomplish optimised and beautiful modern constructions. The challenge is to manage various solutions and fit them together. This requires designers and constructors to familiarise themselves with solutions and understand their principles in advance.
References:
Handbook: fib Bulletin 73 Tall buildings
Construction techniques referred:
