By Craig Coppersmith, PE, C2 Consulting, LLC

Since the early 1980s, insulated concrete sandwich panels have been utilized as a design and construction technique for tilt-up. These types of panels are comprised of an inner and outer wythe of concrete, separated by a layer of rigid foam insulation. Connectors are used to fasten the two wythes of concrete together. To date, the majority of the insulated panels have been designed using non-composite action, meaning that only one wythe of concrete was considered for the panel’s structural integrity. The widespread acceptance and adoption of the International Energy Conservation Code (IECC) has reinforced the insulating benefits of sandwich panel systems and motivated many in the tilt-up industry to take a closer look at ways to maximize performance and constructability by creating a standard for composite behavior of insulated systems. To that end, the Tilt-Up Concrete Association (TCA) has collaborated with the University of Nebraska’s Durham School of Architectural Engineering and Construction to conduct what is believed to be the largest, full-scale field testing of this type. The study’s goal is to develop a slender wall design methodology that can predict composite behavior under combined axial and bending forces.

“This is an exciting time for the engineering community as we advance the understanding for partially composite tilt-up panel behavior in a cracked concrete member,” said Philip Kopf, PE, SE, FTCA, and cochair of the engineering task group on composite sandwich panels with Kopf Consulting Group, Inc. “While partially composite panels have been used in a prestressed, uncracked concrete member, we really do not understand the behavior in a cracked concrete member. This is the first significant full-scale testing of tilt-up panels since the early 1980s. I believe that the knowledge we gain from this partially composite testing program will be as equally significant as the full-scale solid panel tests performed back then.”

As of the writing of this article, the testing phase of the study is largely complete. Testing of the final two panels is expected to occur this spring, with analysis of data and a highly anticipated summary report expected later in 2021. “These are exciting times, indeed, for the advancement of design protocols using composite action of insulated concrete sandwich panels in tilt-up construction,” said Kopf.

STUDY LEADERSHIP

Test Agency/Supervision

Marc Maguire, PhD – Durham School of Architectural Engineering and Construction, University of Nebraska-Lincoln

Engineering Task Group on Composite Sandwich Panels

Andrew McPherson, FTCA – Seretta Construction, Inc.

Craig Olson, PE – C. E. Doyle, LLC

James R. Baty, FACI, FTCA – Tilt-Up Concrete Association

John Hart, FTCA, PE, SE – Peak Engineering, Inc.

Jeffrey R. Needham, PE, SE, FTCA – Needham DBS

Marc Maguire, PhD – University of Nebraska Lincoln

Mitch Bloomquist – Tilt-Up Concrete Association

Philip Kopf, PE, SE, FTCA – Kopf Consulting Group, Inc.

Scott Collins, PE – Leviat

Kimberly Waggle Kramer, PhD, PE, SE, FACI – Kansas State University

Joseph J. Steinbicker, PE, SE, FTCA – Steinbicker & Co., LLC

Jim Lintz, PE – LJB, Inc.

Contractor for Test Specimen and Erection

Steve Miers – Tilt-Up Concrete, Inc.

Participating Manufacturers Advisory Council 

Brad Nesset – Leviat

Matt Saguibo – HK Composites

Sean Hirka – Dayton Superior Corporation

Joel Foderberg – Iconx, LLC

Kim Blackburn – Innovative Structural Solutions

On-Site Project Management

Craig Coppersmith, PE – C2 Consulting, LLC

Director of Photography

Tim Lillethorup – Lillethorup Productions, Inc.

CONSTRUCTION 

Test panels were constructed between the dates of September 14, 2020 and October 29, 2020. The test panels were built on a prepped job site that was exposed to the elements (not an indoor controlled environment).

Construction of all Test Panels – Crews from Tilt-Up Concrete (of Lincoln, Nebraska and owned by Steve Miers) completed all construction activities required to fabricate, finish, and lift the test panels. During panel construction, concrete slump was checked, and concrete cylinders and flexural beams were made for compression and tension testing. Graduate Assistants, working for the University of Nebraska, managed and performed all material testing efforts throughout construction of the test panels to verify the concrete material properties. In addition, they installed and monitored the strain and deformation gauges.

Size and Configuration of the Insulated Sandwich Wall Panels – Three test panels were built for each specific insulated panel system. Each test panel was built with overall dimensions of 41’ long by 4’ wide. All insulated sandwich wall panels (except for the Innstruct panels that use a different system) were built using a 3-wythe configuration consisting of 4” of concrete, 2” of insulation, and 4” of concrete, for a total of 10” of thickness. Pin/tie configuration was as determined and specified by the appropriate insulated panel-system supplier. Reinforcing mats consisted of #4 rebar with 6 longitudinal bars and a 1¾” distance from edge of longitudinal bar to face of concrete. All test panels were constructed with appropriate steel half-pipe roller embeds on each end (along the 4’ dimension) to allow for proper test loads to be applied. 

It should be noted that double shear test specimens were also built for each connector/pin type to test and verify respective material performance.

Size and Configuration of the Control Test Panel – Noninsulated control panels were also constructed (for comparative purposes) and were built with overall dimensions of 41’ long by 4’ wide. Panel thickness of the control panels was 8”. Reinforcing mats consisted of #4 rebar with 6 longitudinal bars and a 1” distance from edge of longitudinal bar to face of concrete. All control panels were constructed with appropriate steel half-pipe roller embeds on each end (along the 4’ dimension) to allow for proper test loads to be applied.

Summary of the panels constructed:  

Panel System	# Panels
HK Composites	3
Thermomass	3
IconXusa	3
Dayton Superior	3
Innstruct	3
Control Panel	3
TOTAL	18

Concrete Mix Design – Concrete was acquired from the ready-mix companies Ready Mixed Concrete Company and Husker Ready Mixed Concrete, which are owned by NEBCO, Inc. Both ready-mix companies had access to the mix design approved by the TCA task force. In most instances, Glenium was used to improve flow and workability of the mix. Dr. Maguire and the University of Nebraska team will provide detailed data in the final report on the specifics of mix design and the admixtures that were used. 

Curing Compound – All test panels were cured using Silcoseal Select Cure and Bondbreaker, manufactured by Nox-Crete Products Group, Inc. 

Internal Sensors – Each test panel was cast with two vibrating strain gages and one relative humidity (RH) gage strategically positioned in each wythe of all panels. Thermistor sensors and strain transducers monitor longitudinal internal temperature and strains throughout the test panel’s life. All internally placed sensors are (and will be) continuously monitored. 

TESTING

Testing of panels was completed on the prepped job site where the panels were constructed. Testing occurred between the dates of December 7, 2020 and January 23, 2021. Two panels remain to be tested, as of the writing of this article. 

Testing consisted of applying axial dead loads to the end of the panel; lateral forces were applied using distributed pressure from an air bag/bladder. Test apparatus for application of these loads was put through a verification protocol to ensure accurate performance and data collection. “During the load testing of each panel, the 44 sensors resulted in approximately 1,000,000 total data points [that were] collected and analyzed for each of 19 panels tested to date,” said Marc Maguire, PhD, Durham School of Architectural Engineering and Construction, University of Nebraska-Lincoln.

For purposes of testing, the orientation of the panel included 41’ of the panel length (height) running horizontally and 4’ of the panel width running vertically (i.e., with the panel positioned sideways. During testing, deflections at quarter and halfway points, and differential wythe displacements, were measured. Loads at each ram location and from air bladder/bag and axial dead loads were monitored as well.

“The data collected showed all composite connector systems exceeding panel design factored loading,” said Dr. Maguire. “A representative from each manufacturer was present to observe panel construction and testing. This data is currently being used to develop analytical models for engineers to design partially composite walls in the post-cracking range—a requirement for slender wall design.”

The double shear specimens were tested to verify connector material performance. “Double shear testing was performed for all connectors involved in the study,” said Dr. Maguire. “This test is the manner in which a connector’s unit strength and stiffness can be determined for design.” An example of a double shear test is presented 0n page 39. 

RESULTS, CONCLUSIONS, AND NEXT STEPS

“At present, there are two initiatives taking place within the American Concrete Institute with value to the positioning and progress of this research,” said James R. Baty II, FACI, FTCA, and manager for regulatory and technical affairs for TCA. “ACI 551 – Tilt-Up Concrete is moving forward with the next version of the Design Guide (551.2R). This document reports on the appropriate method for designing tilt-up panels and gives multiple examples of how to apply ACI 318 to common panel types. The committee is intent on expanding the present guide to offer examples of designing for insulated tilt-up panels, particularly with regards to the semi-composite behavior. There are numerous TCA members present on this committee with voting status who will be challenged by the incorporation of timely research.” Additionally, Baty is a voting member of ACI 319 – Precast Structural Concrete Code. This committee was initially created as a joint effort between ACI and the Precast/Prestressed Concrete Institute. It has broadened to incorporate the ACI 318 perspective of what constitutes precast concrete by definition, which includes site cast tilt-up or tilt wall building elements. “As this research sets the new standard for behavioral understanding of semi-composite insulated panels, it will be the interest of TCA to assist in the development of specific sections for this new code,” said Baty.

Dr. Maguire will present results, conclusions, and next steps at the 2021 Tilt-Up Convention and Expo in St. Louis, Missouri. A general session overview will be provided on September 17 and an in-depth analysis will be offered on September 18.

“The work of this testing program will provide engineers with solid design procedures to create more economical, tall, well-insulated concrete panels for the next generation of tilt-up construction,” said Jeffrey R. Needham, PE, SE, FTCA with Needham DBS. “It also validates the current design procedures that have not been verified since the early 1980s, despite the major increase in panel size and complexity. Finally, early test results indicate that semi-composite tilt-up panels show significant strength and deflection performance that are similar to old ‘Green Book’ projections.”