Precast Concrete Combines Factory Quality with Site Speed
Precast concrete components are manufactured in a controlled factory environment and transported to the site for erection. This approach offers several advantages over cast-in-place concrete: better quality control (factory conditions, controlled curing), faster erection on site (components arrive ready to install), and the ability to finish or texture surfaces in ways difficult to achieve in the field. The ARE PPD exam tests precast concrete in the context of structural system selection, understanding component types, and recognizing the connection challenges inherent in precast systems.
Common Precast Structural Components
Double Tees
Double tees (DTs) are the most common precast horizontal spanning element. The cross-section resembles two parallel T-beams side by side - two legs (stems) connected at the top by a thin flange. Double tees are efficient for spans of 40–80 feet and are the typical choice for parking structures, warehouse floors, and roof systems in commercial buildings. They are prestressed (pre-tensioned at the plant) to control deflection over long spans. Standard widths range from 8 to 15 feet; depths from 12 to 32 inches.
Hollow Core Planks
Hollow core planks are flat, prestressed slabs with longitudinal voids running through the cross-section to reduce weight while maintaining bending strength. Typical depths are 6, 8, 10, or 12 inches; standard widths of 4 feet. Hollow core can span 20–40 feet. They are commonly used in multifamily residential construction, parking structures, and office buildings where a flat floor and ceiling are desired. The voids can also be used to route electrical conduit.
Precast Columns, Beams, and Wall Panels
Precast columns are used in parking structures and industrial buildings where standardized repetition makes factory production efficient. Precast beams (inverted tees or rectangular) support double tees or hollow core planks. Precast architectural panels serve as cladding and may also serve as load-bearing elements in certain building types (tilt-up construction and bearing-wall precast systems).
Connections in Precast Construction
Connections between precast elements are critical and differ fundamentally from cast-in-place concrete construction. Because joints between precast elements cannot be made continuous in the field (unlike a cast-in-place joint), precast connections rely on: embedded steel plates and angles welded in the field, mechanical connectors (bolted plate connections), grouted connections where small amounts of concrete fill gaps, and post-tensioning to tie elements together. These connections must transfer both gravity and lateral forces but are inherently more complex to make ductile and moment-resistant than cast-in-place construction.
Precast vs. Cast-in-Place
| Factor | Precast | Cast-in-Place |
|---|---|---|
| Quality control | Factory-controlled | Field-controlled |
| Speed of erection | Fast (prefab components) | Slower (forming, pouring, curing) |
| Connection complexity | High | Low (poured monolithic) |
| Repetitive forms | Excellent | Formwork re-use required |
| On-site crane required | Yes (heavy lifting) | Yes (forms, rebar, concrete) |
Key Exam Points
- Double tees: 40–80 ft spans; parking structures and roofs; prestressed.
- Hollow core planks: 20–40 ft spans; flat floors; voids reduce weight.
- Precast connections: embedded plates, welded connections, grouted joints - more complex than cast-in-place.
- Precast excels in repetitive, factory-efficient applications.
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