MIT researchers have 3D-printed and load-tested a 2.3-meter concrete bridge using a computational framework that bakes a printer’s physical limitations directly into the design process, and the results revealed a surprise: today’s printing hardware, not the strength of concrete, determines how efficient a structure can be.
The team, from MIT’s Department of Civil and Environmental Engineering, developed the framework to close a stubborn gap in the field. Engineers use topology optimization to find the strongest structure that uses the least material, but those mathematically ideal designs don’t account for what large-scale concrete printers can actually do — their thick nozzles, limited turning radius, and requirement to print in a single continuous motion. The new approach folds all three constraints directly into the math, generating fully printable designs in about two minutes on a laptop. When the team needed to slightly reduce the bridge’s size on the day of printing, they reran the optimization and had an updated design five to 10 minutes later.
The bridge itself took about 30 minutes to print using off-the-shelf mortar. During testing, the roughly 900-pound structure held more than 2,000 pounds of concrete blocks spread across its top without measurably bending, closely matching the team’s simulations. But the test exposed how over-engineered the result was. “From zero to 200,000 pounds, your design is entirely driven by these ‘can I build it or not’ constraints. And then, after 200,000 pounds, you can start to think about the physics,” said co-first author Hajin Kim-Tackowiak, a postdoc in MIT’s CEE department.
The framework uses mixed-integer optimization, a mathematical approach long considered too computationally expensive to be practical. “You go back five, 10 years ago, the solver we used, even three years ago, could not solve these problems,” said co-first author Zane Schemmer, a PhD student in CEE. Because the method finds a global optimum rather than just a good solution, the researchers could also quantify precisely what each hardware constraint costs in material. The single biggest factor was bead width. The bridge used a 4-centimeter bead; a machine capable of laying a 1-centimeter bead could cut material use by as much as 76 percent, according to senior author Josephine Carstensen, the Gilbert W. Winslow (1937) Career Development Professor in Civil Engineering. “I thought the continuous path would be the problem, the one that had the highest effect,” Carstensen said. “But it wasn’t. It was the bead width.”
The bridge is built entirely in compression, which is concrete’s strength. Every element is being pushed rather than pulled. That design principle revealed itself dramatically after testing: the structure had held more than 2,000 pounds without budging, but when a worker lifted one corner a few inches to sweep beneath it, it broke immediately. “It’s optimal in one way, but it’s definitely not optimal in every way,” Kim-Tackowiak said.
The team’s next step is reinforced concrete. “We know a pure concrete structure is not necessarily going to be the most optimal thing, so we’re moving it more into the world we live in today, which is reinforced concrete,” Kim-Tackowiak said, though she added that working out how to feed rebar into a printed concrete structure “is proving its own challenge.” The work was funded by the National Science Foundation and supported by the MIT Center for Advanced Production Technologies, with co-authors Pittipat Wongsittikan, a PhD student in MIT’s Building Technology Architecture program, and Jackson Jewett MEng ’18, PhD ’25.
Source: news.mit.edu
