From Nozzle to Neighborhood

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Additive construction, also called 3D printing, is a rapidly advancing technology and an innovative method for the construction of wall systems and other structural and nonstructural elements, that departs from the traditional technique of formed-and-poured concrete used for cast-in-place and precast concrete. In the case of Wolf Ranch, a master-planned community located north of Austin, TX, in the vibrant, growing city of Georgetown, 100 3D-printed homes meant a departure from the traditional technique of stick-framed and concrete construction.
As a leader in construction-scale 3D printing robotics, software, materials, and construction technology, ICON partnered with nationwide homebuilder, Lennar, internationally renowned design architect, BIG-Bjarke Ingels Group, and Austin-based structural engineering firm, Fort Structures, to build the largest community of 3D-printed single-family residences to date. The 3D-printed homes, offered in eight floorplans ranging from about 1,600 to 2,100 square feet, blend contemporary Texas ranch style aesthetics and energy-efficient designs that highlight the benefits of resiliency and sustainability with the digital possibilities of additive construction. This article overviews the 3D-printed wall system used in the project.

3D printing utilizes a material batching and delivery system and a robotically controlled process of extruding mortar or concrete layer-by-layer along a designated print path.

Section R104.11 of the 2021 International Residential Code (IRC) allows for alternative materials and methods, such as 3D-printed structures. The Authority Having Jurisdiction (AHJ) reviews and approves the alternative if it’s acceptable and equivalent to systems and products already incorporated in the Code.

The acceptability and equivalency of the 3D-printed wall system was supported by a suite of tests conducted by accredited third-party laboratories. These tests assessed the material properties and the wall system’s structural capacity. The material testing consisted of measuring the compressive strength, interlayer tensile bond strength, flexural bond strength, shrinkage, and freeze-thaw resistance. The structural testing consisted of measuring the axial capacity, out-of-plane flexural capacity, and in-plane shear capacity of over two dozen full-scale wall specimens following the general guidance of ASTM E72, “Standard Test Methods of Conducting Strength Tests of Panels for Building Construction.”

Material and structural test data and reports consistent with the requirements of Section R104.11 of the 2021 IRC were made available, and the 3D printing and structural design processes were discussed prior to the AHJ’s plan review of the structural engineer’s signed and sealed structural drawings.

Furthermore, test results, design procedures, and equivalency checks as described in the ICC reference document AC509, “Acceptance Criteria for 3D Automated Construction Technology for 3D Concrete Walls” were formally summarized. Therefore, in addition to review by the AHJ, the 3D-printed wall system was reviewed by the International Code Council Evaluation Services (ICC-ES). Based on their review, ICC-ES issued ESR 4652, “ICON 3-Bead Wall System,” which describes the wall system in more detail. The ESR, grounded in material and full-scale structural tests, provides basic information about this new type of wall system for structural engineers and demonstrates to AHJs that this wall system meets or exceeds the performance of existing wall systems already specified in the IRC. Structural engineers can use this general information, as well as more detailed information provided by ICON, to develop project specific details and designs.

Regular collaboration and communication with the structural engineer and the AHJ combined with the extensive testing regime and third party evaluation and validation of the 3D-printed wall system enabled a smooth and timely plan review process, building permit issuance, and construction.

Each of the 100 homes consists of a metal roof and prefabricated wood roof trusses sitting atop the 3D-printed wall system. Each home is supported on a post-tensioned concrete slab-on-ground foundation. Though the 3D-printed wall system was innovative and new, the foundation was typical for central Texas in terms of slab thickness, grade beam depth and spacing, post-tensioning, and the 28-day design concrete compressive strength of 3,000 psi.

The 3D printed wall system utilized a three-bead wall system, as the gravity force- and lateral force-resisting systems. The wall system consists of several parts, namely, beads, shells, and cores. A bead refers to a single, nominally 3/4-inch tall by 2-1/2-inch-wide extrusion of 3D printing material, called Lavacrete, added layer-by-layer by a 3D printer, called Vulcan, following a designated print path. A shell refers to a stack of multiple layers of one or more beads, and a core refers to a bounded vertical space printed integrally with the shell that is later reinforced and filled with Lavacrete. This 3D-printed wall system contains both an exterior and interior shell and is named the three-bead wall system because it contains three beads at any given elevation—a single bead on the exterior shell and a double bead on the interior shell.

The exterior shell protects the structural wall from impacts and transfers the out-of-plane loads to the interior shell via the cross ties. The double-bead interior shell is the structural portion which spans horizontally between the cores. The cores span between the foundation and the top of wall as simply supported elements and provide the resistance for axial, bending, and shear.
The exterior and interior shells are separated by a cavity which is filled with insulation for an overall nominal wall thickness of 12 inches. These two shells are connected via C- or Z-shaped cross ties consisting of 3/16-inch diameter plain wire spaced 18 inches on-center in both directions. At least one cross tie is included within each core. Each shell contains horizontal reinforcement consisting of #2 or #3 bars located in the center of the shells and spaced either 8 or 12 inches on center vertically. The horizontal reinforcement is designed to resist volume changes from temperature and shrinkage and, in the case of the interior shell, to span between cores. Due to the additive construction process, both the cross ties and horizontal reinforcement are placed incrementally during the print process.

The vertical cores are the main structural elements, resisting gravity, out-of-plane, and in-plane loads. Each core consists of a grouted element vertically reinforced with one #5 reinforcing bar. The vertical reinforcement in the cores is placed at the conclusion of the print process, and the cores are subsequently infilled with Lavacrete. Vertical cores are located at the edge of each opening, at locations of high gravity load, and not more than 6 feet on center.

At the top of the walls, a wood top plate and wood rim beam are bolted to the 3D-printed wall with a 3/4-inch diameter threaded rod that is coupled to the #5 vertical reinforcing bar. The continuous wood beam spans over windows and between the cores. The design team decided to use a wood rim beam to eliminate the need to shore and grout bond beams at all the openings which resulted in quicker construction. The roof trusses bear on the wood rim beam atop the exterior side walls and are connected to the top plate with hold down hardware and fasteners.

The structural engineer specified the requirements for inspections and testing for the construction of the 3D-printed wall system. During printing, the Lavacrete was routinely sampled from the nozzle to measure its fresh properties, including temperature, slump, density, and air content. In addition to measuring the fresh properties, sets of 3-inch by 6-inch cylinders were molded and submitted to a third party laboratory for compressive strength testing. At a minimum, fresh material samples were obtained and cylinders were molded once after the first 2,500 linear feet of printing and every 10,000 linear feet thereafter. For a single home, this sampling frequency typically yielded approximately 15-20 quality control samples and compressive strength tests. The Lavacrete average 28-day compressive strength for the project was greater than 4,000 psi, well more than the specified design strength of 2,000 psi.

While the structural engineer performed periodic observations, the construction documents required ICON to submit material test data, photographic evidence documenting the placement of reinforcement, print progress updates in terms of layers per day, and evidence of the application of a bond agent if interlayer print times exceeded 120 minutes. Reports were submitted to the structural engineer using traditional construction management software.
In addition to the project-specific requirement, the ESR for the three-bead wall system was contingent on ICC-ES reviewing, approving, and inspecting quality documentation and procedures. ICON is subject to semi-annual follow-up inspections from ICC-ES.

Construction-scale 3D printing is the latest, state-of-the-art building technology producing attractive, resilient, and energy-efficient homes. This method is one solution to the growing skilled labor shortage and the increasing costs of construction. U.S.’s first community of 3D-printed homes offers a glimpse into a promising path toward delivering resilient, beautiful, technology-driven homes that meet rising demand. ■

About the Authors

David P. Langefeld, PE, has a background in structural modeling and analysis, condition assessments, and testing supporting complex forensic engineering investigations. At ICON, he leads the evaluation and ongoing compliance of ICON’s structural building systems at the local, state, and federal levels, and he sits on multiple consensus-based committees for 3D printed concrete that are working to develop standards, tests, and guidelines for the construction industry.

Sam Covey, PE, is the founding principal of Fort Structures, overseeing multiple 3D-printed concrete projects as the Engineer of Record. At Fort Structures, Covey leads the construction technology division, specializing in structural engineering for innovative building systems along with engineering a wide range of specialty commercial structures and high-end custom residences.