Combining architectural and structural precast concrete components can provide the entire building envelope. The system can take several forms, including precast columns and beams with panelized cladding or load-bearing precast walls and double-tee or hollowcore flooring. It provides a significant number of advantages, including:
Speed to Market:
A precast concrete structural system can save a project six to eight weeks compared to steel and even more when compared with cast-in-place concrete, according to a survey of developers. That savings can be critical in bringing a new building into a competitive market or to meet a tenant’s need for occupancy on a specific date. As permitting and decision-making processes slow, total precast system’s speed helps keep projects on track.
The scheduling advantages result from several factors that grow through the process:
- One-stop shopping secures much of the building’s shell in one efficient contractual relationship.
- Designers find precast systems easier to design than masonry or other building materials thanks to aid from the precaster’s engineering staff.
- The ability to begin fabrication while permitting and site work are completed allows precast concrete to begin erection as soon as foundations are complete.
- Precast components can be erected in winter conditions, maintaining tight schedules.
- With total precast systems, speedy erection allows the contractor to enclose the building quickly, giving interior trades faster access.
- Precast’s inherent fire resistance eliminates the messy and time-consuming work of fireproofing a steel structure and then repairing it as other trades finish.
Precast concrete panels require less maintenance than other materials, typically only occasional recaulking, usually every seven to 10 years. As sealants improve, that timetable will lengthen. Incorporating the architectural design into the structure enlarges panel sizes, minimizing the number of joints. This significantly reduces the chance for water penetration that can weaken a structure and cause unsightly staining and mold problems.
PCI-Certified precast concrete fabricators must undergo two annual, unannounced inspections that review more than 120 production and quality-assurance processes. The tight control ensures components are produced with uniform consistency, finish and size. This minimizes concerns over the building’s final appearance and reduces site work required to achieve the final designer and owner approvals.
Because of precast concrete’s tightly controlled and shorter production process, costs can be more accurately estimated earlier in the process. Parallel effort by precast engineering ensures estimates remain stable, assuring the contractor, owner and design team that the budget is sound.
Plant casting keeps the site cleaner and eliminates trades from the construction zone, improving logistics and enhancing worker safety. The ability to provide a clean site is particularly vital on existing campuses and in dense urban areas, where adjacent businesses can maintain near-normal activities.
Interior Design Flexibility.
Precast concrete systems help buildings adapt to changing client needs. Double-tee spans of 45 to 50 feet match typical composite-steel framing and minimize the need for interior columns required with cast-in-place systems. Precast spans can reach as much as 70 feet, providing unique opportunities for challenging interior requirements. Precast also provides high floor-loading capability with little added cost.
Precast concrete offers a number of environmental benefits. It requires fewer chemicals to keep it clean over its long lifetime, and it offers a high thermal mass. It can be produced locally and creates no job waste. Cement reducers such as fly ash and other admixtures also aid its environmental friendliness. And its high durability gives it a total service life that far outpaces designs using other building materials.
Tight Floor-To-Floor Heights
Precast concrete systems sometimes fit within alternative system depths but shouldn’t add more than approximately 8 inches to each floor level, creating an approximate 5-percent increase in exterior wall material. This slight addition is easily overcome by working with the precaster to make effective use of the overall floor-plan shape and using the benefits precast provides in repetition of component fabrication.
Total precast concrete systems allow the architectural panels to serve structural functions, limiting the need to incorporate multiple materials and trades. For instance, spandrel panels can support floor systems and windows while providing final exterior finishes. Or sill panels can be used as grade beams to retain soil, support windows and provide the architectural exterior finish, avoiding complex cladding systems. Combining exterior spandrels into the structural system provides deeper elements, limiting deflections and the complexities of designing for relative movements between frame and cladding system.
Lateral Design Flexibility
Combining architecture and structure provides efficiencies in many buildings’ lateral support systems. External elements, typically cladding on steel or concrete-framing systems, can become laterally stiff and resist wind and earthquake forces. Parallel efforts by the precast engineer can provide the structural engineer with specific input ahead of final construction document preparation, limiting shop-drawing review effort and redesign.
A total precast concrete system provides one-stop shopping for the building envelope. Having one source design and supply the entire system produces a better coordinated set of drawings that allows construction to proceed more efficiently. It eliminates the added effort needed to coordinate various trades when using mixed systems controlled by different suppliers.
This combination of benefits creates cost savings that begin with the design process and last throughout the life of the building. When all of these hidden and long-term costs are included, precast concrete offers the best value and the highest return on investment.
|Costs||Precast Concrete||Cast-in-Place Concrete||Steel|
|Structure and enclosure||Lowest||Highest||Higher|
|Concrete decks and topping||Higher||lowest||NA|
* Costs for fireproofing and caulking are the same for a total precast system and cast-in-place concrete.
For more information, contact a PCI Certified Manufacturer.