Precast & Prestressed Concrete

Precast concrete combines the strength and durability of concrete with the precision and efficiency of factory manufacturing. Structural elements are cast in reusable steel forms in a controlled plant environment, transported to the site, and erected by crane. The result is a structure with excellent quality, fast erection speed, and inherent fire resistance. Precast construction dominates the parking garage market and is increasingly used for office buildings, schools, data centers, and multi-family residential buildings.

Most precast structural members are prestressed — steel strands or tendons are tensioned to place the concrete in compression before service loads are applied. This precompression counteracts the tensile stresses that would otherwise crack the concrete, allowing longer spans, shallower members, and reduced deflection.

Pretensioning: The most common method for precast members. Strands are stretched (tensioned) between abutments in the casting bed before the concrete is placed. After the concrete hardens and bonds to the strands, the strands are cut (released), and the strand force is transferred to the concrete through bond. The concrete is now in compression. Pretensioned members are manufactured in a long-line casting bed (300–600 feet long) and cut to length after strand release.

Post-tensioning: Strands are placed in ducts within the concrete and tensioned after the concrete hardens, using hydraulic jacks at the member ends. Post-tensioning is used for precast members that are too large for long-line pretensioning (large bridge girders, segmental bridge segments) and for cast-in-place concrete (covered in Lesson 6.3).

Double Tees

The double tee is the workhorse of the precast industry. It consists of two stems (legs) connected by a thin flange (deck) — creating a cross-section resembling two adjacent T-shapes. Double tees are pretensioned and can span 40–80+ feet. They are the primary floor and roof member for parking garages, warehouses, and office buildings. Standard depths range from 12" to 34" (plus the 2"–4" flange).

Precast double tees for parking garage floors typically have a 2" concrete topping poured after erection to create a smooth, level driving surface and to connect adjacent tees into a diaphragm.

Hollow-Core Planks

Hollow-core planks are pretensioned planks with continuous voids running through their length (reducing weight and material). Standard widths are 4 feet or 8 feet; depths range from 6" to 16". Hollow-core planks are used for floors and roofs where shorter spans (20–45 feet) and lower loads are appropriate. They are lighter and shallower than double tees and provide a flat soffit (underside), which is desirable for exposed ceilings.

Spandrel Beams and Panels

Spandrel beams span between columns at the building perimeter, supporting the edges of the floor system. They are often architectural — cast with a decorative finish, reveals, or thin-brick insets that serve as the building's exterior facade. Spandrel panels may also be non-structural cladding attached to the building frame.

Columns and Beams

Precast columns are typically square or rectangular (12"×12" to 36"×36"), pretensioned or conventionally reinforced. Precast beams (rectangular, L-shaped, or inverted tee) span between columns and support floor members.

Wall Panels

Precast wall panels serve as the building envelope — providing structure, insulation (for insulated sandwich panels), and architectural finish in a single element. Insulated wall panels consist of two layers of concrete with rigid foam insulation between them, connected by non-thermal-bridging ties. These panels provide structural support, insulation (R-15 to R-30+), and an architectural exterior finish in one factory-produced element.

Precast erection follows a planned sequence, typically starting at one corner or end of the building and working systematically:

1. Crane setup: Mobile cranes (typically 100–500 ton capacity) are positioned per the erection plan. Crane reach, capacity, and ground bearing must be verified.
2. Column erection: Columns are set on base plates or in grouted pocket foundations. Temporary bracing holds columns plumb until permanent connections are made.
3. Beam erection: Beams are lifted and set on column corbels (shelf-like projections) or bearing pads. Connections are made with field welding or grouted mechanical connections.
4. Floor/roof member erection: Double tees or hollow-core planks are lifted and set on the beams. Members must be properly seated and aligned before releasing the crane.
5. Connection completion: Welded connections, grouted connections, and closure pours (concrete placed between members to create continuity) are completed.
6. Topping slab (if specified): A thin concrete topping is placed over the precast floor members to create a level surface and connect the members into a structural diaphragm.

Connections between precast members are critical — they transfer gravity loads, lateral loads, and volume change forces (from temperature and shrinkage). Common connection types:

• Bearing connections: One member simply sits on another (beam on column corbel, plank on beam ledge). A bearing pad (elastomeric or fiber) distributes the load and accommodates minor movements.
• Welded plate connections: Steel plates embedded in the concrete (cast-in plates or weld plates) are field-welded together. Common for beam-to-column and member-to-member connections.
• Grouted connections: Reinforcing bars project from one member into a pocket or sleeve in the adjacent member, and the pocket is filled with non-shrink grout. Creates a strong, durable connection.
• Mechanical connections: Proprietary couplers, bolted connections, or post-installed anchors.

Precast construction requires tight tolerances in both manufacturing and erection:

• Manufacturing tolerances: Length ±1/4" to ±3/8"; width ±1/8" to ±1/4"; squareness and straightness within 1/8" per 10 feet.
• Erection tolerances: Plan location ±1/2" to ±1"; elevation ±1/2"; plumb ±1/4" per 10 feet.
• Joint widths: Precast buildings have visible joints between panels that must be sealed with caulk (sealant). Joint widths must accommodate manufacturing and erection tolerances as well as thermal movement.