By Rick Hinson, PE, ECS Mid-Atlantic
Many of us may have seen or remember the large plotters used to print drawings or posters. But drawings or posters are two dimensional. What if you could print a lot of pages and stack them in layers to get three dimensions? Printing in three dimensions has been done for years in plastics and metals. Now, there is a push to use concrete and print structures, specifically residential homes. Here in Virginia, these have popped up in places like Williamsburg, Accomack, Richmond and the Town of Pulaski among others. They are being printed across the nation and overseas.
The process is simple in concept. The footing and foundations are commonly slab on grade concrete slabs. From there a large house size gantry is erected to handle a motorized trolley which holds a nozzle and concrete feed hose. The feed hose is connected to a small batch plant and provides a supply of concrete that is plastic enough to be pumped and then extruded but not lose shape once placed. The nozzle trolley is moved around in a closed loop pattern controlled by a computer. The computer has the design and moves the nozzle trolley in two dimensions. With each pass, the third dimension brings the shape up to the design height. As the printing proceeds, openings for doors, windows, and utilities are provided by the computer which will also stop and start the nozzle operation. Lintels and related miscellaneous steel can be added as the printing proceeds.
Walls can be poured for the required thickness based on the design. Normally, the walls are printed as two vertical segments tied together by horizonal steel reinforcement which is manually laid in place at regular intervals. This reinforcing is very similar to ladder truss type reinforcing commonly used in masonry construction. Based on gravity and lateral load requirements, vertical reinforcement can also be provided from the slab as the walls are printed. Blown or pumped insulation is added to the space between the two printed segments and the wall is capped with a bond beam.
When complete, the wall has the appearance of stacked pancakes, with each printed layer forming a rounded edge a few inches thick. The final look can be left as printed or parged or furred out to hide the concrete wall. Some projects will print just the exterior walls, others will print both interior and exterior walls. Generally, the mechanical, electrical, and plumbing systems are conventional systems installed later to complete the building. After printing, the gantry and the concrete footings for the gantry are removed, the property landscaped, and the work completed.
The future is unclear as to the long-term viability of this technology in a market where the choices for wall construction include wood and steel framing, pre-engineered and pre-made wall frames in steel, wood, and concrete, insulated concrete form block, etc. In addition, certain challenges also need to be addressed in terms of water and air control at the building envelope. But the next time you see a huge printer extruding concrete, you can be pretty sure you have seen the next in 3-dimensional printing technology, a full-size structure, not just a model.
Rick Hinson, PE serves as facilities department manager with ECS Mid-Atlantic.