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Composite Concrete Panels -The Future for Tilt-Up Walls?

By Jeffrey R. Needham, P.E.

As I have watched the tilt-up concrete construction industry develop over many years, I have appreciated the way it has become a preferred method of construction for many building types. This growth has been based on a number of improvements in construction and design technology, such as larger mobile cranes, the recognition and adoption of slender wall design in building codes, and a growing appreciation for the flexibility and speed of tilt-up methods. The widespread adoption of the International Energy Conservation Code (IECC) embraces the tilt-up industry, which already provides a fully code-compliant panel. The industry has the potential to make these panels more structurally efficient through the use of composite panels. This is a huge opportunity many designers and contractors may be missing.

The Composite Wall: What is it? Why do it?

In a composite wall, two wythes of concrete, separated by a continuous plane of insulation, are forced to structurally act as one single concrete structural element. The composite, or single element, behavior is developed by providing connections between the wythes that allow shear transfer. For engineers this behavior is often called “plane sections remain plane” (see Figure 1). For the non-engineer, it means the two concrete wythes cannot slide past one another. There is a slight misuse of the term “composite” here, though, that can affect the design of these panels.

Figure 1

In actual practice the two wythes almost always slide past each other some amount in this application. This means composite concrete panels are not fully composite. The reference in this article to composite panels is merely for convenience as the panels truly only have degrees of composite action. All of this can be cumbersome, so I suggest simply remembering that composite panels are not fully composite. This partial composite behavior does have the benefit of reducing the risk of thermal bow problems associated with fully composite behavior. Thermal bow results from the outer wythe being heated (or cooled) to much higher temperatures than the inner wythe, thereby causing the outer wythe to expand (or contract) at a different rate than the more stable inner wythe.

Composite construction is nothing new and has been widely used in steel and concrete floor systems (e.g. headed studs), bridge floor decks, and concrete-filled steel tubes. Even composite concrete wall systems have been around for decades. However, composite tilt-up panels are more complex in design and construction than non-composite insulated tilt-up panels and have unique issues designers and builders need to account for, some of which are listed below.

  • Thermal bow
  • Proprietary connector design
  • Significant thermal short circuits that undermine nominal
  • R-values
  • Difficulty in lifting and bracing due to thin wythes
  • The expense of the labor to install connectors
  • Lack of an accurate understanding of structural performance

With all this complexity, the big question is: Why should a tilt-up designer and contractor get involved? In other words, why mess with it? As with many advancements in construction, there are several answers:

  • It can significantly reduce the amount of concrete required in a
    wall panel.
  • The panels are often much stiffer than comparable non-
    composite, insulated designs.
  • It can significantly reduce seismic forces due to lower weight.
  • It can provide a much lighter panel which can reduce costs of
  • It can provide a fully code-compliant, edge-to-edge insulation.

Figure 2

Wrap these benefits together and the tilt-up contractor may have a less costly panel that performs better than a single wythe design. Figure 2 shows typical designs for a 32-foot panel with a typical 90 mph wind, exposure C. In other words, a common warehouse panel.

Figure 3

Composite Wall Performance

The easiest way to understand the benefits of composite wall performance is to compare a typical design. Using the examples from figure three, the design for each panel can be summarized in the table that follows.

Thirty-two-foot Panel Comparison

Using the solid panel as the base reference, the non-composite, insulated panel uses 35 percent more concrete and an equal amount of reinforcement, not including mesh or fiber in the exterior wythe. The non-composite, insulated panel provides code-compliant insulation while the solid concrete panel provides no insulation. Of course, the single wythe design can use an inside adhered insulation, but this is usually considered a sub-standard approach and is not preferred by the marketplace.

Now, comparing the composite panel to the solid panel, the composite design uses 17 percent less concrete than solid. When compared to the non-composite, insulated panel, it uses 49 percent less concrete and 15 percent less steel, not including the mesh or fiber in the non-composite design. Clearly there are structural advantages from a composite design.

The benefits of composite design become more evident as the panel length increases. Relatively short panels, under 24 feet, tend to perform more as two individual panel wythes just sharing a load. That is, there is less shear flow between the wythes. As panel lengths get longer, shear flow becomes significant and the key to successful structural design of composite panels.

Shear Flow, the Key Issue

Shear flow is the design criteria that truly defines composite version. The traditional non-composite pin systems do not have the strength to transfer significant shear flow.  Ideally, they function as tension-compression members only. Composite connectors are designed to function both for shear and tension-compression. This is a major structural difference that provides the necessary load-path to support composite, or semi-composite, action.

Current Composite Wall Insulation Methods

With strong demand for improved insulation products, the choices for types of board insulation are growing. Recent rough costs for an R-10 value vary from $0.50 per square foot for expanded polystyrene (white board) to about $1.30 for 1-inch graphite polystyrene (grey board). Extruded polystyrene has been a very common insulation product in tilt-up walls (blue or pink board) and provides an R-value of about 6.1 per inch. Polyisocyanurate has also been used, and a new product on the market is phenolic foam insulation. This product provides a phenomenal R-value of 8 per inch, but is sold only in metric thicknesses.

The advantage of a composite wall is the insulation thicknesses becoming a structural design parameter, not just a thermal performance factor, which greatly affects the strength and deflection performance of the panel. In taller panels, a thicker, but less costly insulation, may yield a stronger and stiffer panel for a given wythe thickness. But in shorter panels where the composite performance is less pronounced, a thinner, but more expensive insulation may be a better choice. The industry should expect pricing for insulation products to improve as the volume used increases.

The Tilt-Up Industry and Insulated Panels

The use of insulated tilt-up panels and the energy code mandate are really nothing new.  However, most current, successful insulated systems are non-composite in nature. Pin systems have been the preferred insulation approach on many projects and are likely to continue to have widespread use in the future. While composite systems in tilt-up have been around for more than a decade and have been used with great success by some contractors, the majority of the industry is yet to embrace them.

From Past to Present

Despite readily available technology and all of the advantages mentioned in this article, the tilt-up industry has been slow to embrace the approach. The plant precast wall producers have actively been advancing connector technology and delivering composite wall panels for the last 10 to 20 years, though they have not always been ideal. Early connectors were simple metal-truss systems, various steel-tie connectors, and even solid-edge concrete between wythes. While these approaches worked structurally, they completely failed at proving to be effective insulated panels since the huge thermal short circuits destroyed efficiency. They often reduced the nominal R-values by 40 percent or more and did not perform to the levels required by code.

Both tilt-up and precast industries now use a variety of proprietary systems to eliminate the thermal-short problem and deliver competitive products. All of the quality systems on the market use some form of non-conductive, advanced material such as glass fiber resin or carbon fiber. While the production numbers for these types of products are not published, I am aware the precast industry is experiencing rapid growth in code-complaint wall panels. It is no surprise this is particularly strong in the Northeast, Midwest and Canada.

So Why the Lackluster Implementation in the Tilt-up Industry?

Joel Foderberg of IconXUSA suggests some of the hesitation in the tilt-up industry may be due, in part, to design responsibility. “The IconX family of connectors has seen limited use in tilt-up,” says Foderberg. “I found much more resistance to change in the tilt-up world with this system than with the precasters. I think the design responsibility issue is a major barrier for tilt-up builders that the precast companies don’t face.”

While the precast industry has several decades of experience in producing insulated, prestressed panels requiring a high percentage of composite behavior, much of the tilt-up wall construction has been uninsulated, single-wythe concrete. The slender wall design method in the code assumes this type of wall construction, and the lifting and bracing engineering easily deals with this as well. The mindset of many high-volume tilt-up contractors has either been that insulation is a different trade’s issue or that it is not necessary at all. They have missed the valuable added design opportunity presented by using composite construction.

Current efforts are underway to define systemology and behavioral conditions. These include white papers being produced through committees for the American Concrete Institute (ACI) coinciding with research on connector types and shear capacities.

Recent Developments

The recent adoption of the 2015 International Energy Conservation Code by the State of Texas should be a very real indicator of change. While the full impact of this development may not be known for some time, it is very significant that a major Sun Belt state has adopted this code. This means the widespread use of uninsulated panels is going to be in decline. Even without this state regulation, more and more owners are at least asking for pricing on insulated panels.

Now, Where Does the Industry Go?

The adoption of wall insulation requirements in the energy code presents a real challenge to the future of all construction methods. Like all challenges, it also presents opportunities. For the composite concrete wall industry, the proprietary nature of the connector systems leads to a “black box” or a secretive approach to design that inhibits agreement on design principles. Without good design principles, the products may be misused, causing actual panel performance to be inconsistent, thereby degrading the quality of the panel. Greater understanding of each system and better awareness of the design and construction process would go a long way.

The real question for the industry is how to embrace composite construction and provide a high-quality wall panel that maintains the current advantages of tilt-up construction and provides a composite panel that clearly meets or exceeds the performance of other systems.

The 2017 International Tilt-Up Convention and Expo in Miami, Florida from September 28-30 will be a great opportunity to dive deeper into this topic. Several talks related to this issue have been planned. For details, check out the full schedule online at

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TILT-UP TODAY, a publication of the Tilt-Up Concrete Association, is THE source for Tilt-Up industry news, market intelligence, business strategies, technical solutions, product information, and other resources for professionals in the Tilt-Up industry. A subscription to TILT-UP TODAY is included in a TCA membership. Subscriptions for potential TCA members are also available. If you would like to receive a complimentary subscription to the publication, please contact the TCA.