Inside a Precast Concrete Manufacturing Plant

Precast concrete panels are cast and cured in manufacturing plants, then transported to the job site to be lifted into place. This presentation provides a walk-through of a precast concrete manufacturing plant and explains the process used to make panels and double tees. Manufacturers follow complex and precise specifications for each project. Each concrete member is essentially a puzzle piece designed to connect to or support other members within the structure.

The manufacturing process for factory-cast panels must account for the transportation, handling, and lifting required for the panels to be placed in the structure at the job site. Understanding this process helps inspectors differentiate between factory-cast and site-cast panels and provides the background knowledge to better assess concrete members for physical deficiencies or workmanship issues.

The video reviews the following aspects of the precast panel manufacturing process:

• Production setup: Panels are cast on large steel tables. The tables are cleaned and customized based on the engineering and architectural specifications for the specific project, including panel dimensions and detailing. For example, relief strips can be added to the face of the table to create recessed lines that mimic joint lines.
• Openings and voids: Wood and foam blockouts are secured to the panel table before the concrete is poured to create openings for doors, windows, and loading docks. Foam is also used to form complex detailed shapes or serve as void fills inside the panel.
• Reinforcement: Steel cables are used to strengthen the concrete. They’re threaded through headers and footers and tensioned, a process known as prestressing, to help prevent flexure failure. Additional steel reinforcement is incorporated into portions of a panel designed to carry concentrated structural loads.
• Finishing and curing: After the concrete is poured, panels can be removed from the table within a few hours and moved to a “yard” for final finishing and curing. Finishing can include surface treatments, such as sandblasting, to expose the aggregate and create a uniform finish, or another technique based on the architectural plans and desired appearance. Most manufacturers use a 28-day curing period.

The double-tee manufacturing process follows a similar approach, but double tees are created on specialized tables with slots for the stems. The process incorporates tensioned cables in the flange and stems, as well as static steel cages held up by chairs to ensure a monolithic structure. After the form is cleaned with a blower and vacuum, concrete is poured and vibrated to remove air pockets, just as with the panels, before the double tees are moved to the yard for final finishing and curing.

Factory-Precast vs. Site Precast

Factory-precast concrete is commonly used for parking structures, warehouses, industrial facilities, and other commercial or institutional buildings. It’s often selected because manufacturing in a controlled environment allows for consistent quality and the use of specialized equipment for forming, prestressing, and finishing. It also allows members to be manufactured while other construction activities are underway at the job site.

Precast units arrive ready to assemble, cutting down on crane and erection time. Site-precast tilt-up panels offer distinct advantages, including lower shipping costs, fewer size limitations, and the flexibility to adapt quickly if design changes occur during construction. During an inspection, panels that exceed 15 feet in width are typically site-cast due to transportation limitations on public roadways. Oversized factory-precast panels require special permits, escort vehicles, or other accommodations that further increase shipping costs.