Iano's backfill

Ultra-High Performance Concrete

Iowa Bridge Gives Glimpse Into the Future (Ascent, Designing With Precast, Summer 2006) describes the use of extraordinarily high-strength concrete in precast beams for the Mars Hill highway bridge in Wapello County, Iowa.

The "bulb tee" beams  (photo left) were cast with a proprietary concrete mix from Lafarge North America capable of producing concrete with compressive and tensile strengths as high as 30,000 psi and 7000 psi, respectively. (For comparison, conventional concrete is typically specified with compressive strength in the range of 2500 - 5000 psi, and taller buildings are being designed with concrete with compressive strengths as high as 10,000 - 20,000 psi.) Among the reported benefits are:

• A lighter, thinner structural member: The bulb tee beams used in the Mars Hill Bridge have webs 4-1/2 inches wide, compared to 6-1/2 inches for a conventional precast bulb tee, bottom flanges  5-1/2 inches deep, compared to 7-1/2 inches for a conventional tee, and top flanges 2-3/4 inches deep, compared to 3-3/4 inches for a conventional tee.
• The more dense concrete results in a structural member that is more resistant to the effects of weather and road salts, and that should be longer-laster.
• Reduction in the need for conventional steel reinforcing: Aside from the prestressing strands, the only other steel reinforcing in the Mars Hill Bridge beams are U-shaped bars used to develop bond between the precast beams and the cast-in-place concrete bridge deck. There are no stirrups or other conventional reinforcing steel.

More Info
For more about Lafarge Company's proprietary Ductal concrete, see Iano's Backfill Flexible Concrete.

Emerging Technologies for Residential Construction

The Partnership for Advanced Technology in Housing (PATH) has announced its Top Ten Technologies for 2004, including:

• Frost Protected Shallow Foundations: Protect foundations against frost damage without the need for excavating below the frost line.
• Home Run Plumbing Systems: Feeds flexible piping directly to one fixture from the central manifold.
• Engineered Panelized Systems: Prefabricated panels form a structural envelope that reduces or eliminates the need for on-site framing and can be assembled swiftly.
• HVAC Optimization: To maximize efficiency and comfort, use properly sized equipment, and place HVAC equipment inside the conditioned space.
• Tankless Water Heaters: Provide hot water on demand, reducing or eliminating standby loss.
• Shared (Community) Waste Water treatment: A single drainfield/treatment area connected to each house's individual septic tank.
• Air Admittance Vents: Pressure-activated, one-way plumbing valves, eliminating the need for conventional pipe venting and roof penetrations.
• Low Impact Development: LID techniques can offer a cost-effective way to address stormwater management through site design and "Best Management Practices".
• Integrated Steel/Wood Combination Framing: Combinations of wood and steel framing within the overall building shell use the best attributes and cost benefits of each.
• Pre-cast Concrete Panels (Walls and Foundation): Made under quality-controlled factory conditions, pre-cast concrete panels are ready in a fraction of the time needed for a poured foundation.

Several of the Top 10 may be of particular interest to readers of Fundamentals of Building Construction. See the PATH page links below for additional technical background, information on regulatory acceptance, and links to additional resources and manufacturers related to these systems:

Engineered panelized systems discusses a variety of innovative insulated panel systems including composites of rigid foam/light gauge steel framing, rigid foam/concrete, and rigid foam/fiberglass/PVC. For a brief introduction to prefabricated panel systems, see also page 100 of the textbook.

Shallow frost-protected foundations are also discussed and illustrated on page 65 of the textbook. In these systems, rigid insulation installed below grade protects against soil freezing and allows shallow footings to be constructed above the normal frost line.

Combination steel/wood framing combines light gauge steel studs and joists with wood framing members such as sills, band joists, and plates. These systems offer the advantages of steel framing (light weight, recyclability, dimensional stability, resistance to moisture, etc.) while relying on traditional wood light framing connection methods and tools (nail guns, staples, etc.). Both loadbearing and nonloadbearing systems are feasible.

Pre-cast concrete foundation panels are insulated, loadbearing, reinforced concrete panels that according to PATH, allow erection of residential basements in less than one day. Panels come with concrete "studs" and metal or preservative-treated furring for attachment of finishes. [Author's note: Waterproofing of joints between panels appears to rely on conventional joint sealants, a potential long-term vulnerability.]

What is PATH?
PATH describes itself as

a voluntary partnership in which leaders of the homebuilding, product manufacturing, insurance, and financial industries join forces with representatives of Federal agencies concerned with housing. By working together, PATH partners improve the quality and affordability of today's new and existing homes, strengthen the technology infrastructure of the United States, and help create the next generation of American housing.

Its Top 10 Technologies are intended to represent "practical, easy-to-use building technologies that are on the edge of industry acceptance and are quickly gaining importance."

Precast Concrete-Open Web Steel Joists Hybrid Structure

New Hybrid Precast Concrete Building Provides Optimum Solution for Metal Recycler, PCI Journal, January-February 2004, describes a hybrid structural system combining precast concrete wall panels and long span double tees with open web steel joists and metal roof decking, used to construct an industrial metal recycling facility. In comparison to an alternative all-steel structural solution, the precast/steel hybrid was budgeted at an additional $2 per square foot, but with benefits including more open space with fewer interior columns, a more durable, low-maintenance interior, and a reduction of two months in the (fast track) construction schedule.

Structural components include:

• Eight-inch thick flat precast concrete loadbearing exterior wall panels
• Sixteen-inch thick x 72 inches wide interior precast loadbearing double-column bents
• Standard precast double tees, 10 feet wide x 34 inches deep, narrowed to 6 feet

Erection of precast elements was completed in 20 days. The use of precast panels rather than metal panel cladding reduced sitework costs by allowing the panels to act as retaining walls where excavation would otherwise have required expensive rock removal. The owner reports that the precast concrete wall panels offer superior sound absorption properties as well.

Carbon Fiber Concrete Reinforcing

Precast concrete technology replaces steel with carbon fiber to produce thinner panels, Building Design & Construction, 02-02, reports on the use of carbon fiber reinforcing in wall panels and double tees to produce precast units that are up to 66% lighter and one-third thinner than conventional units. The reduced weight and size of these units can contribute to cost savings in transportation, erection, and the building superstructure.

According to the article, primary reinforcing in the precast units is steel, while a 1 mm (0.04 inch) high-strength, resin-bonded carbon fiber grid is used for secondary and shear transfer reinforcing. The carbon fiber reinforcing requires only 1/4 inch of concrete cover, compared with 3/4 inch to 3 inches of cover required for conventional steel.

More Information
Altus Group, CarbonCast Brochure

Atlantic City NJ Casino Collapse

Missing Connection to Wall May Have Weakened Garage, ENR November 17, 2003, reports on the ongoing investigation into the October 30 collapse of multiple concrete floor decks of an Atlantic City New Jersey casino under construction. Missing or inadequate connections between the decks and an adjacent bearing wall may be a significant factor contributing to the incident. Other possible factors that have been cited included inadequate shoring and inadequate curing time between deck concrete pours. According to the article, some workers reported seeing bowed or bent shoring columns prior to the collapse. Four workers were killed in the collapse.

This incident has also brought attention to the innovative construction system being used on this project, the filigree wideslab concrete construction system. The filigree concrete system is a composite system using both precast concrete formwork and cast-in-place concrete. It is one of a variety of permanent form systems, which aim to achieve higher economy through reduction in forming costs.

In the filigree system, formwork for slabs and beams are precast at a fabrication plant. They may be either prestressed or conventionally reinforced. Additional reinforcing in the form of shallow-height steel bar joists are cast into the precast units with the upper portion of these joists projecting out of the upper surface of the precast. When concrete is later poured on top of these units, the bar joists create a bond between the precast and the site-poured concrete. After delivery to the construction site, the precast units are lifted into place and supported on temporary shoring. Next additional reinforcing is placed above the formwork. Foam plastic blocks may also be added to the assembly. Where these blocks are placed, voids will be formed in the final pour where excess concrete is not needed, thereby lessening the overall quantity of concrete and reducing the weight of the structure. Finally, concrete is poured on site to complete the system.

In the completed assembly, the precast and sitecast concrete work as a single composite structural unit, bonded through the shallow-height joists described above. Bottom steel is contained within the precast units, and the remainder of the required reinforcing resides within the cast-in-place portions of the system. Variations on this system include flat plate construction over structural steel beams and girders, all-concrete flat plate construction, and concrete beam and slab construction (as illustrated above). Additionally, both one-way and two-way systems are feasible.

More information:
See the text for more about innovative formwork systems (pages 536 - 537), prestressing concepts (beginning on page 493), and one-way and two-way slab systems (pages 490 - 491).
Mid State Filigree Systems describes the advantages of the filigree concrete system, and provides diagrams illustrating variations on the system.
BOCA Research Report No. 96-14 describes the filigree system in additional technical detail.

Architectural Precast Concrete Design

Designing with Architetural Precast, a continuing education supplement in Architecture Record 09.02, provides guidelines on economical design for this material, including discussion of :

• Molds and repetition
• Mold costs
• Other forming considerations
• Panel size
• Material costs
• Economies in comparison to other materials.

15 - Precast Concrete Framing Systems Links

This article contains external links to resources on the Web relevant to Chapter 15 Precast Concrete Framing Systems.

Architectural Precast Association

Information resources for architectural precast concrete.

Precast Designer's Knowledge Bank

By the Precast/Prestressed Concrete Institute, provides technical papers, design guides, articles, online presentations, etc.

Spancrete

Technical resources and good photo library of precast structural systems and elements.