Assessing a concrete commercial building requires a clear understanding of construction types. This article focuses on concrete panels precast in a factory and summarizes how they are manufactured, transported, and assembled. That background helps identify the type of concrete construction during an inspection and better assess its condition in the field.

History of Precast Concrete

In 1905, British engineer John Alexander Brodie pioneered the use of precast concrete panels as a repeatable system for prefabricating structures. It was not until the 1930s and 1940s, with advancements in prestressing technology, that precast panels became lighter, stronger, and more cost-effective to produce at scale. When these improved methods met the urgent need for post-World War II reconstruction, adoption took off.

In North America, precast concrete found important early applications, including the Walnut Lane Memorial Bridge in Philadelphia, widely regarded as the start of the modern U.S. precast industry. Use of precast concrete panels for buildings also developed through the 1950s. By 1954, the industry had grown enough to establish national manufacturing standards, design guidelines, and quality control with the founding of the Precast/Prestressed Concrete Institute (PCI).

Today, the precast concrete industry includes underground, conventionally reinforced elements (such as utility vaults, manholes, and drainage structures) and above-ground prestressed elements for architectural and structural applications like buildings, parking structures, and bridges.

What Are Precast Concrete Panels?

In contrast to concrete formed, cast, and cured in its final position, precast concrete panels are cast elsewhere on site or in a factory. Site-cast panels are formed, cast, and cured at the job site before being lifted into place. Factory-cast panels are manufactured in a controlled factory environment, then transported to the construction site.

How Are Precast Concrete Panels Made?

Precast factories manufacture both structural and architectural concrete. Structural concrete is designed to carry and transfer loads, whereas architectural concrete is designed for the finished appearance of exposed concrete. A precast concrete wall panel can be both architectural and structural. Manufacturing processes for both are similar and begin with preparing the panel table where the panel is sized and reinforcement, connection points, and finishes are incorporated based on engineered plans.

Panel Table and Size Limits

Precast panels are constructed on a table using forms to create their size and shape. Tables typically range from about 20 to 70 feet long and can accommodate one or multiple panels at a time. Panel dimensions are influenced by transportation limits because panels travel from the factory to the site by truck over public roadways. Oversized panels can require special permits and escort vehicles, which increase cost and travel time. That is a primary reason many fabrication tables and precast panels are limited to a width of about 14 feet 6 inches.

With that width limitation in mind, panels wider than about 15 feet can often be assumed to be site cast rather than factory cast.

Openings, Reinforcement, and Connections

Forms are used to block out openings for doors, windows, loading docks, vents, or other design elements. Wood is often used as a blockout for openings, while foam is often used for complex shapes or voids. Openings generally cannot be added to a precast panel after this phase.

While width is limited, a precast panel can be up to about 50 feet long. These panels are heavy and can flex when lifted, so reinforcement is added to prevent cracking or breaking. The most common reinforcement approach is prestressing: cables are arranged on the panel table, stretched tight, and concrete is poured around them. After curing, the cables are released, producing a panel stable enough for transportation and erection. Those tensioned cables should never be cut, which is why factory precast panels cannot be cut or manipulated after construction without structural consequences.

If panels are structural components that support loads from the roof, beams, or other building elements, additional steel reinforcement may be concentrated where those loads will be carried. Anchor points are embedded before the pour so panels can later be welded or bolted together on site. Lifting points are also added so panels can be moved repeatedly from form table to yard to trailer to building site.

Finishing, Curing, and Assembly

Surface finishing may include sandblasting or texturizing after the panel has been moved to the storage yard. Panels typically cure in the yard for about 28 days before transport. In some cases concrete is tinted before it is poured, which can reduce long-term painting and maintenance needs.

Once on the building site, each panel is hoisted into place by crane and secured according to the design until the building perimeter is complete.

Understanding this process helps explain why modification of a factory precast panel is a serious concern. If an inspector is aware of cutting or other modification to a factory precast panel, the client should engage an engineer or other qualified person for further review.

Source: Certified Commercial Property Inspectors Association (CCPIA). PHW Inspections is a CCPIA member. Original article: https://ccpia.org/how-precast-concrete-panels-are-made/

How Precast Concrete Panels Are Made