What makes precast concrete different to other forms of concrete construction? After all, the concrete does not know it is precast, whether statically reinforced or pretensioned (prestressed). It is only when we consider the role that this concrete will play in developing structural characteristics that its precast background becomes significant. The most obvious definition for precast concrete is that it is concrete which has been prepared for casting, cast and cured in a location which is not its final destination. The distance travelled from the casting site may only be a few metres, where on-site precasting methods are used to avoid expensive haulage (or VAT in some countries), or maybe thousands of kilometres, in the case of high-value added products where manufacturing and haulage costs are low. The grit blasted architectural precast concrete in Figure 1.1 was manufactured 600 km from the site, whereas the precast concrete shown in Figure 1.2 travelled less than 60 m, having been cast adjacent to the final building. What really distinguishes precast concrete from cast in situ is its stress and strain response to external (load induced) and internal (autogenous volumetric changes) effects. A precast concrete element is, by definition, of Figure 1.1: Architectural-structural precast concrete structure a finite size and must therefore be (courtesy Trent Concrete, UK). joined to other elements to form a
complete structure. A simple bearing ledge will suffice, as shown in Figure 1.3. But when shrinkage, thermal, or load induced strains cause volumetric changes (and shortening or lengthening) the two precast elements will try to move apart (Figure 1.4a). Interface friction at the mating surface prevents movement, but in doing so creates a force F R
Figure 1.3: Simple bearing nib.
Free shrinkage etc.
F = µR
F = µR
R Splitting cracks
lb
Figure 1.4a: Unrestrained movement between two precast concrete elements.
Figure 1.4b: Restrained movement but without tensile stress prevention.
which is capable of splitting both elements unless the section is suitably reinforced (Figure 1.4b). Flexural rotations of the suspended element (the beam) reduces the mating length lb (bearing length) creating a stress concentration until local crushing at the top of the pillar (the column) occurs, unless a bearing pad is used to prevent the stress concentration forming (Figure 1.4c). If the bearing is narrow, dispersal of stress from the interior to the exterior of the pillar
Bearing pad
Flexural rotation Possible spalling
R lb
R
Large shift in position of R
Small shift in position of R
Figure 1.4c: Reduced bearing length and stress concentrations due to flexural rotation.
