Fabric molds could bring material-saving concrete beams within reach.
MIT engineers cast a 3-meter concrete beam in sewn fabric molds, using about one-third as much concrete and half as much steel as a conventional beam designed for the same load.
Engineers at MIT reported that a highly material-efficient concrete beam can be cast in sewn fabric rather than in rigid molds or with specialized machinery. The work appeared in the Journal of Building Engineering. Computer design methods such as topology optimization can produce beams that use less concrete and steel, but the resulting shapes are often too complex or costly to pour into conventional molds.
The team built a 3-meter (about 10-foot) test beam in fabric molds made through an automated pattern-making process. Compared with a conventional beam designed to carry the same weight, the optimized beam used about one-third as much concrete and half as much steel reinforcement. In a load test, its measured response was within about 2 percent of the computer model's prediction.
To design the beam, the researchers combined graphic statics, which generates structural designs almost instantly, with topology optimization, which is slower but can place material freely. The result was a beam with a top section, narrow angled legs and a thin bottom section for the steel. Software translated the 3D design into flat fabric pieces that account for how the fabric deforms under wet concrete. A simple timber frame held the mold.
The team cast three molds of the same design, made from 36, 56 and 76 fabric pieces. More seams gave finer control over details but did not necessarily make the overall shape more accurate, since small errors can accumulate. The 56-piece mold gave the best balance. For the load test, the researchers stacked 36 water-filled buckets on one beam, up to about 30 percent of its design load.
A cost comparison for a single beam showed that location matters. In the Global South, where materials cost more relative to labor, the fabric versions were about 10 to 20 percent cheaper. In the Global North, higher sewing labor costs made them about 16 to 55 percent more expensive. The researchers say the approach suits one-off or small-batch custom components, while conventional reusable molds remain more economical for repetitive rectangular beams where labor is costly.
The researchers call the beam a proof of concept. Senior author Josephine Carstensen said a larger beam would need a new design because its weight and the pressure on the mold change, and the next step is "to test multiple beams to failure." The work was partially supported by the U.S. National Science Foundation.
At a Glance
- The test beam was 3 meters long and used about one-third as much concrete and half as much steel as a conventional equivalent.
- Measured deflection at the beam's center was within about 2 percent of the computer model's prediction.
- A mold made from 56 fabric pieces gave the best balance, compared with molds of 36 and 76 pieces.
- Fabric versions cost about 10 to 20 percent less in the Global South and 16 to 55 percent more in the Global North.
- The load test reached about 30 percent of the beam's design load using 36 water-filled buckets.
Sources
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