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When houses are printed with concrete cement what replaces the steel rebar for reinforcement?

Here's a link referencing printing concrete: https://www.aniwaa.com/house-3d-printer-construction/

House 3D printers use extrusion technology. Some construction 3D printers look like super-sized desktop FFF/FDM 3D printers (gantry style), whereas others consist of a rotating mechanical arm.

In both cases, paste-type components such as concrete are used as filament. The material is pushed out of a special nozzle to form layers. To put it (very) simply, paste extrusion is similar using a piping bag to spread frosting on a cake.

The printer creates the foundations and walls of the house or building, layer by layer. The ground is literally the printer’s build plate. Some concrete 3D printers, however, are used to 3D print brick molds. When molded, the bricks are then piled atop each other manually (or with a robotic arm).

Like most of the people here my experience is with a printer (RepRap) that can use PLA or ABS. With all the materials normally put into concrete, using an extrusion printer to print concrete is puzzling.

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  • $\begingroup$ Welcome to 3dPrinting.SE! Do you have a link or something to exactly what you're talking about? Never heard before that foundations are 3d printed and would be HIGHLY interested in reading about it :o) $\endgroup$
    – Pᴀᴜʟsᴛᴇʀ2
    Apr 6, 2019 at 19:12
  • $\begingroup$ concrete can't be printed, it is cast. $\endgroup$
    – Trish
    Apr 6, 2019 at 20:33
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    $\begingroup$ @Trish Concrete can be sprayed, so "printing" should be possible. Walls can already be printed. $\endgroup$
    – 0scar ♦
    Apr 6, 2019 at 23:00
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    $\begingroup$ In this video they show the reinforcement being rebar. It looks as though the print "material" is concrete, but there's nothing stating that in the video ... it just looks like it. $\endgroup$
    – Pᴀᴜʟsᴛᴇʀ2
    Apr 7, 2019 at 4:49
  • $\begingroup$ They use Rebar and Glass Fiber (GFRC). $\endgroup$
    – user77232
    Apr 7, 2019 at 14:19

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There is no reinforcement. You can use this ‘printing’ solution for walls but by default not for floors. There are experiments that use the solution to span a gap but they use relatively thick plates.

For example, see this paper, Design of a 3D printed concrete bridge by testing.

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  • $\begingroup$ It looks like on bridges they are essentially using cable instead of rebar. Tightening the cables keeps the concrete under compression, so that some consider it superior to rebar. Concrete printing could easily replace using cinder blocks in buildings. Spanning with cinder blocks has a similar issue. $\endgroup$
    – Perry Webb
    Apr 9, 2019 at 23:34
  • $\begingroup$ @PerryWebb Cables are use to pre-tension the concrete; this effectively puts the concrete under a compression stress, exactly where the concrete excels in. Now when loading the concrete, these compression stresses need to be overcome first, if it can that is. $\endgroup$
    – 0scar ♦
    Apr 10, 2019 at 6:41
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To answer your question, they use steel or fiber glass.

Why does concrete need reinforcement? Concrete is very useful to absorb compression loads, but breaks and cracks very easily when subjected to tension loads. Rebar, or steel constructions are added to strengthen the concrete to bear the tension loads. My house has walls that are littered with steel wires/fibers of about 10 cm (about 4 inches) and about 1.5 mm in diameter close to the surface (drilling holes into the wall frequently means hitting such a fiber which comes out and leaves a scar on the wall...). My walls are constructed in a factory flat on the ground, those fibers are added to prevent shrink cracks in the first layer of the wall (the side you always look at); after that, rebar is added and the rest of the wall is cast and after drying the wall is put upright and transported to the build site.

Injection of fibers is not new to 3D printing; it is already possible to print short fibers embedded in filament, or continuous where filament and thermoplastic material join together in the nozzle. The company I work for creates molds and aerospace parts this way.

You can imagine that it would be possible to print concrete with fibers/wires or continuously with wires. Technically this should be feasible, but I guess that larger diameter wire (rebar) is actually needed instead of relatively small diameter wires. Also, fibers can only be laid down in the direction of travel of the nozzle, not perpendicular to. Rebar often shows strengthening in multiple (three) dimensions, that is not possible with 3D printing.

There is some reported success on printing reinforced concrete:

According to BAM Infra:

“They have succeeded in developing a process to also print the steel reinforcement at the same time. When laying a strip of concrete the concrete printer adds a steel cable so that the slab is ‘prestressed’ so that no tensile stress can occur in the concrete.”

Concrete bridge element with rebar for pretension

But this means that the print is a part printed at another location, not on location.

There is also some success with using horizontal fiberglass reinforcements within the walls during the printing process.

See e.g. this reference, this reference for more information on reinforced concrete printing.

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