U.S. Army Corp. of Engineers' researchers, leading a team that recently completed the 3D printing of 9.5-ft-tall reinforced concrete walls for a 32-ft x 16-ft  barracks, are setting their sights on a future project--3D printing of concrete roof beams—even before they put the precast concrete lid on the printed walls.

The current research is considered the first to "definitively" demonstrate through full-scale prototype testing that a 3D-printed concrete barracks with a precast roof can be engineered to be structurally safe for occupancy.

"To our knowledge, before us, no one has done the structural testing to show they are safe," which is the main reason why there are no 3D-printed concrete barracks in use, says Michael Case, the program manager for the project, called Automated Construction of Expeditionary Structures (ACES), which is within the U.S. Army Engineer Research and Development Center.

ACES is funded by about $250,000 from the U.S. Marine Corps, with support from Caterpillar Inc. and NASA's Marshall Space Flight Center and Kennedy Space Center. Materials for the barracks, expected to be completed next month, cost $6,000.

Structures on Demand

ACES is developing a technology capable of 3D-printing custom-designed expeditionary structures on demand, in the field, using concrete sourced from local materials. With continuous printing executed in two or three shifts, construction time is potentially one day instead of five for a wood-framed barracks.

Once all kinks are ironed out, a trained crew of three per shift will be needed to build the "B-hut," instead of eight for a conventional barracks, according to ERDC. The technology also reduces the resources needed and logistics associated with material shipments for wood-framed barracks.

The primary goal of the 3-D printed concrete barracks project, ERDC's second, is to determine "what it would take for a structural engineer to sign off on the B-hut for permitting," says Case.

Construction of the walls is complete. The precast concrete roof should be done by the end of September. A report, with design guidelines, will follow in a few months. 

For the engineering, ERDC engaged the Chicago office of Skidmore, Owings & Merrill LLC. SOM signed on because it had designed a 3D-printed project for the Dept. of Energy, using a carbon-fiber-reinforced acrylonitrile butadiene styrene thermoplastic material.

"3D-printed structures require a different mindset about how the building is constructed," says Megan Kreiger, ACES project manager. "We were looking for an engineer that had the fundamentals down. SOM had the awareness and just needed to go from the polymer to concrete."

The research, conducted at the Corps' Construction Engineering Research Laboratory in Champaign, Ill., has several other goals. One is to build on the ERDC 3D-printed concrete barracks research, completed last year on a hut of the same size but with a different design. Marines are the builders of both prototypes.

"In 2017, it was all about whether we could print a building that size," says Kreiger. This project is about, "what can we do instead of can we do it?" she adds.

The 2017 project, funded by the Army, took 21.5 hours of print time over more than five weeks of clock time. The ACES team wanted to investigate what it would take to do continuous printing. The project explored the climate, maintenance and labor issues related to continuous production through different extremes of temperature during the day and night.  

"We learned we can potentially build a building in 48 hours of clock time," says Kreiger.

In the test, crews mixed 1/3-cu-yd batches in a small mixer--not the usual 10-cu-yd batches in a readymix truck--to align with the slow pace of the printer. "We were making artisanal concrete," says Case.

During the test, it took about four or five hours to establish the proper mix. "Some 'chefs' are better than others, even given the same ingredients," Kreiger explains.

A Few Kinks To Iron Out

Snag-free continuous printing is still a way off. After 40 printing hours and about 80% of the walls complete, work came to a halt. The biggest problem, aside from exhausted crews, involved maintenance. Operator errors and printer malfunctions had had a cascading effect, causing the material, which sat longer in the mixer, pump and hose, to begin to cure and build up on the sides of the equipment. The equipment had to be cleaned.

The printer, programmed to travel using robotics, set horizontal layers, like piled ropes, around the footprint, one layer at a time. In addition to its technology, the machine consists of a gantry crane with a hose, attached to the mixer, and a nozzle.

The team learned that for continuous printing, two or three dedicated crews--pretrained and working in shifts--are critical.

For SOM, the two biggest lessons are that concrete material performance is critical and stopping the printing process must be avoided, says Benton Johnson, an SOM associate director.

"We need to develop a field test for printed concretes similar to a slump test for normal concrete or spread test for self-consolidating concretes, so that we can verify the mix is correct before each print," he says.

The structure consists of printed concrete walls, made from about 25 cu yd of concrete, with intermittent vertical reinforcing within grouted cells, which are like concrete blocks. Rebar anchors were set in the foundation slab before printing. The anchors accept 18-in. long rebar dowels, installed after the first lift of the printed wall.

The printed walls transition from a chevron pattern at the base to a straight wall at the top, forming "undulating" walls. The design placed more materials at the base, where they are more efficient at resisting overturning moments, says Johnson. That also reduced the potential for shrinkage cracking near the foundation.

3D printed concrete shrinkage rates seem higher than rates of cast concrete, adds Johnson. Shrinkage cracks occur when tensile stresses accumulate, which happens over long segments of straight walls. The chevron wall design changes direction every 2 ft. That alleviates some of the accumulated shrinkage strains and reduces cracking, he says.

ACES went with the undulating-wall design, though it uses 15% more concrete, because it provides 2.5 times the strength in the wall's out-of-plane direction, is self-supporting during construction and allows field changes of the building shape without a total re-engineering.

"When Ben first proposed the idea of the undulating wall, most of the team was hesitant," says Case. That is because of the risk of collapse during printing due to the slight overhang of one layer of rope above the layer below it. 

Three Wall Types Tested

SOM got the green light for the chevron wall after full-scale mock-up prints of a straight wall, a wall with a pilaster and the chevron wall to verify printability and strength--to failure. All specimens were designed for wind and seismic loading, according to the International Building Code. Testing confirmed that the chevron walls are stronger than the code requirements, says Johnson.

The ACES printer can be transported on the Army's palletized load system and C-130 aircraft. ERDC is working with Caterpillar to explore commercialization of the technology.

For construction, at least three personnel are needed--one driving the concrete mixer, one supervising the pump and the third operating the mixer. The current construction process is "tough on Marines," says Matthew D. Friedell, 3D printing officer for the U.S. Marine Corps.

Crews had to shovel gravel and sand into barrels, which were then lifted using a forklift into a mixer before the material was pumped into the printer. Friedell looks forward to the day when the material handling is automated, not just the printing.

"Finding a batch plant is easier than finding a lumber yard," he says.

ACES has plans to train crews in the construction of the 3D-printed concrete barracks. "Hands-on training is most beneficial," says Kreiger.

During one early training program, the first six trainees then trained another six. It takes one or two days for soldiers or marines with some knowledge of construction to learn to print on their own, adds Kreiger.

The Marines are interested in 3D printing for concrete barriers, culverts, walls around neighborhoods to secure them, bunkers and more--not barracks.

"A big use is for humanitarian aid and disaster relief missions," says Friedell.

Temporary Housing

Building temporary housing in disaster areas can take five to 10 days with conventional construction. "Bringing in an inexpensive and simple printer to build structures in a day, and possibly leaving it there, would be a huge benefit to communities," adds Friedell.

During 2019, ACES expects to build four or five machines, put them in the field with Marine Corps units and get feedback during a year of their operation. Based on the data gathered, a decision will be made about whether to "push this permanently," says Case.

As far as a 3D-printed roof, without more research on strong 3D-printed concrete beams, "we don't have the confidence we could print a safe roof for that span," says Case.

Despite the growing pains, Friedell is excited about the potential of the technology. He sees the 3D printers more routinely constructing houses and buildings--in five or 10 years.