Next-generation precast insulated wall panels

by nithya_caleb | January 10, 2020 12:00 am

by Monica Schultes

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It is common knowledge buildings have traditionally been energy hogs. Globally, buildings and construction are responsible for 60 percent of electricity use, 12 percent of water consumption, and 40 percent of material resource use. The U.S. Department of Energy (DOE) believes next-generation building envelopes have considerable potential to reduce energy consumption in buildings. However, to make any serious progress toward that goal, technologies must be market-ready with minimal cost impact to facilitate widespread adoption.

One of the challenges in achieving this goal is quality control can be difficult to enforce on a construction site without additional inspection costs, which can compromise the energy efficiency and durability of the building envelope. Other challenges are controlling expenses and impact of weather on site construction. Prefabrication enables better quality control for an enhanced building envelope as it is easier to control environmental conditions in a factory setup than onsite.

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The Oak Ridge National Laboratory (ORNL) in Tennessee, DOE, a materials science company, the University of Tennessee (UT), the Institute for Advanced Composites Manufacturing Innovation (IACMI), and the Precast/Prestressed Concrete Institute (PCI) are collaborating on a research project to advance the building envelope using precast concrete insulated wall panels. ORNL commissioned this project through the DEO U.S.-China Clean Energy Research Center.

Rethinking the building envelope

Research is underway at ORNL and UT to advance precast concrete insulated wall panel technology by developing materials and design to double its thermal performance and reduce weight by half without increasing costs. The main goal of this collaborative project is to promote passive envelope technologies for improving energy efficiency in new construction.

The new and improved precast concrete insulated wall panel would be 50 percent lighter with a 200 percent increase in thermal performance, all in a cost-neutral design. To achieve this, the research team developed a multiprong approach encompassing several technologies.

The first advancement is a lighter-weight solution. To reduce the weight of typical precast concrete panel by half, the prototype wythes are a mere 38 mm (1½ in.) thick with a lower concrete density of 1602 kg/m³ (100 lb/cf).

Weight-loss goals

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The new and improved high-performance concrete (HPC) was developed by researchers and chemists at ORNL and UT (Chattanooga). The slimmer version of a precast concrete insulated panel weighs 1602 kg/m³ at an estimated $300 per cubic yard. The team continues to tweak the high-performance concrete mixture proportions.

“Precasters conducted trials with preliminary mixes, but we are trying to further optimize them,” says Diana Hun, sub-program manager for building envelopes at ORNL. “We reached the target concrete properties we were tasked to achieve, namely 4137 kPa (600-psi) flexural strength in 12 hours and the 45-kg (100-lb) density. Now, we are researching if we can further reduce costs.”

The assembly consists of two thin concrete wythes attached to insulation.

“While the optimum R-value of future panels depends on local building codes, we are using four inches of XPS [extruded polystyrene foam] for test purposes,” says Hun.

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“The development of a high-performance concrete mix to allow precasters to reduce piece thickness translates to lighter cranes, fewer trucks, less concrete, and reduced costs,” says Roger Becker, vice-president of technical services for PCI.

Lightweight concrete is nothing new, as it has been around in various forms for centuries. While normal-weight concrete typically weighs 2323 kg/m³ (145 lb/cf), lightweight concrete is around 1762 kg/m³ (110 lb/cf).

Lightweight and normal-weight concrete have a comparable 28-day compressive strengths. However, high-performance, lightweight concrete has higher early flexural strength, by almost 30 percent. This is significant because stripping, yarding, and shipping precast concrete impose more significant stresses than experienced on a building.

Steve Brock, senior vice-president of engineering at a precast concrete manufacturer, has been actively involved in field testing the mix.

“This mix reduces cracking without being prestressed. There is not enough concrete cover over any type of steel. We are testing carbon and glass fibers and stainless-steel wire reinforcement,” he says.

Another benefit is the ease of widespread adoption for the lightweight concrete. It was designed so as not to require major changes to precast concrete manufacturing facilities across the country.

Brock continues, “There really is nothing unusual about the lightweight mix, except for the learning curve regarding additional admixtures and how to handle fibers. A typical precast facility might install additional silos to handle the increase in additives. Right now, we are handling small amounts for testing, but I do not anticipate large changes to batch plants.”

Non-corroding composites

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With ultra-thin panel sections, it is imperative the researchers replace traditional steel accessories with non-corroding composite materials.

“The lifting inserts were our first target,” says Hun. “We have achieved our goal by developing a non-corroding composite edge lifter with an ultimate load capacity of more than 5443 kg (12,000 lb).”

[6]The new and improved inserts have broad appeal. Given recent steel tariffs and price escalations, the timing is excellent.

“The intention was to use composites because the wythes are so thin you do not have appropriate concrete cover,” says Brock. “Ultimately, we could use them in any type of product, not just insulated wall panels. The same thing for the lightweight mix—you can use the mix any time you want to lighten up a piece.”

Hun agrees. “The technology can be easily transferred. The inserts can be used in precast or any concrete application where corrosion is a potential problem. This will have a broad-reaching impact.”

In addition to the non-corrosive aspect, the new edge lifters can eliminate thermal bridging. The composite materials have low heat transmittance. Research is still being conducted on the remainder of connectors used in precast concrete wall panels. Ultimately, the research will examine all types of lifters as well as gravity and tie-back connectors made out of composite material.

“While ORNL is focused on insulated panels, this could also benefit architectural panels that are insulated in the field. Metallic connectors from panel to building would be eliminated,” says Becker. “Composite connectors are of benefit to non-insulated panels because if we are not using steel then we are no longer creating a thermal bridge through the field-applied insulation.”

Thermal performance

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The target to double the thermal performance of a typical precast concrete panel is achieved by adding insulation without increasing the panel thickness. The prototype uses 102 mm (4 in.) of insulation, twice the amount typically used.

To establish a baseline panel (50 x 50 x 76 mm [2 x 2 x 3 in.]), ORNL performed hot box tests as per ASTM C1363, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus, to measure the performance of current industry standards.

“When we reach final panel configuration with composite lifters and thinner wythes, then they can do final hot box tests to compare [performance],” explains Becker.

Improved thermal performance is at the heart of this research.

The original test examined a jointed configuration (Figure 1). The results reiterated that two lines of backer rod and caulk proved effective and heat loss was minimal.

Instead of tackling the joint itself, the researchers focused on the more practical goal of improving the sealant and subsequently the air- and water-tightness of joints. Industry feedback has helped direct their efforts. The solution is a caulk that does not require a primer and uses self-healing polymers to make the whole system more durable.

“Here at the lab [ORNL] we collaborate with researchers from various fields, so I reached out to the polymer chemists. We had to raise the bar because there are so many sealants available in the market,” explains Hun.

Cooperative agreement

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PCI and ORNL have a cooperative research and development agreement (CRADA) enabling this opportunity for government, industry, and academia to jointly pursue common goals. The CRADA has made facilities and expertise available to collaborate and to develop technological knowledge into useful products.

“Ultimately, PCI has first option for licensing any new technology developed as a result of the research. PCI is allowed to restrict use of that new technology to its members,” explains Becker. “The end result is that members who make insulated panels will have sufficient information to demonstrate to the marketplace this system reduces operating energy as compared to conventional cladding materials.”

It is important to note this technology has not yet been licensed, and is unavailable in the marketplace at the time of writing this article.

Most of the research has focused on technologies for new construction. How could this research apply to the significant stock of commercial buildings constructed before energy codes?

The research team has begun studying retrofits and how to improve energy efficiency in older buildings. Retrofit data would demonstrate to owners the energy savings from envelope renovations. Precast concrete has been too heavy for recladding projects in the past. With the new, lighter panels, precast concrete can be considered as a possible solution.

The precast concrete envelope has not changed much in the past few decades, except for gaining a bit of weight. “Not only can this new panel be advantageous in new construction, but also be considered for the re- and over-clad market as well,” anticipates Brock. “If there was an old dorm with poor energy performance and the college cannot afford to move students out for renovation, lightweight precast panels could be a viable solution.”

Spin-offs

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The multiprong investigation has already developed and spun off profitable ideas. One of the standalone concepts generated from this research is 3D printing of molds. ORNL researchers are encouraged to cross-pollinate ideas. Tasked with reducing production time, the team introduced an idea from a different proposal. Several precasters have already turned to 3D printing to decrease the amount of time to create complex molds.

The industry is also exploring how to improve business opportunities and provide precast concrete fabricators with the capability to run two cycles of casting every 24 hours. Nanotechnology is a promising research field that may significantly improve the mixture proportions, performance, and production of concrete, so it makes sense to increase production capabilities. Colloidal nano-silica added to a concrete mixture improves the time of strength development significantly. ORNL wanted to add that challenge as an extension of their research project. They are working with UT (Knoxville) to create mixture proportions that attain 3447 kPa (500-psi) flexural strength and 24,132 kPa (3500-psi) compressive strength in six hours.

Moving ahead

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“All of this research can help energize the industry and prevent stagnation of market share. If we are able to match some or most of the goals that were established, it is great example of how a national lab can help the construction industry,” describes Hun.

As a result of this research project, there will be additional methods, materials, and resources to help designers, contractors, and building owners create an energy-efficient and high-performance structures.

“If all we wanted to do was increase energy efficiency, we could simply add more insulation. But the caveat with this research is that the solution be cost-neutral,” says Brock. The research team is closing in on that ambition.

Ultra-high performance concrete (UHPC) offers superior flexural and compressive strength, but it is significantly more expensive. The HPC discussed in this article is targeting some specific properties that are lower than UHPC, but sufficient
to create an energy-efficient building envelope, and also more economical.

Everyone benefits from improvements to thermal performance in building envelopes. When fully developed, these new insulated sandwich wall panels would be lighter, less costly, and easier to erect.

Monica Schultes is president of MM Schultes Consulting and is managing editor of the Precast/Prestressed Institute’s Ascent magazine, where this article originally appeared. She is also the executive director of the Pennsylvania Precast Association.

Source URL: https://www.constructionspecifier.com/next-generation-precast-insulated-wall-panels/

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