Composite Structure That Combines 3D Printer Mortar with Slim-Crete

We have developed a composite structure that combines special 3D printing mortar with ultra-high-strength fiber-reinforced concrete in order to use cement materials with 3D printers to manufacture one of the largest structures of its kind in Japan.

August 29, 2019

Obayashi Corporation has developed a structure that unifies special 3D printing mortar and the ultra-high-strength fiber-reinforced concrete Slim-Crete in order to use cement materials with 3D printers to manufacture one of the largest structures of its kind in Japan (shell-shaped bench: 7,000 mm wide x 5,000 mm deep x 2,500 mm tall).

In 2017, Obayashi Corporation developed a 3D printer that discharges special 3D printing mortar from a nozzle attached to a robot arm to achieve additive manufacturing. In general, concrete and many other cement materials are mainly used to bear the compressive force of structures. Therefore, when using cement materials for structures, it is necessary to employ a composite structure that combines cement with rebar and other steel materials used to bear the tensile force, a common example of which is a reinforced concrete structure. In terms of making 3D printers that use cement materials practical as well, one key issue was to develop a method for bearing this tensile force.

Obayashi Corporation therefore developed a composite structure that involves filling an external shape manufactured using special 3D printing mortar with Slim-Crete capable of bearing the tensile force. As proof of this concept, we also used cement materials with 3D printers to manufacture a shell-shaped bench, the largest scale structure of its kind in Japan.
The shell-shaped bench was first manufactured as 12 separate pieces, and then the completed members were set up at the installation location. We are currently conducting an outdoor exposure test to evaluate the bench's durability and other characteristics.

The features of our recently developed technology are described below.

1. Composite structure that includes the ultra-high-strength fiber-reinforced concrete Slim-Crete

   Slim-Crete is a cement material that boasts high tensile strength, can be used even on its own as a structure, and hardens at room temperature. In addition, because this material is self-compacting, it can easily be used to fill external shapes manufactured using special 3D printing mortar, and—compared to manually arranging rebar inside structures—this material greatly reduces the amount of work.

2. Development of a 3D printer that uses proprietary control technology of a pump and a large robot arm

   With conventional approaches, the discharge of special 3D printing mortar could not be interrupted, and there was no choice other than to handle the entire layer path in one stroke. However, by linking the robot arm to a pump and controlling the system, our 3D printer achieves additive manufacturing for which it is possible to freely start and stop discharging, thereby eliminating the need to handle the entire layer path in one stroke. The layer path is automatically generated from 3D design data. In addition, by introducing a large robot arm with a length of around 3.0 m, we have enabled the manufacturing of large structural members.

3. Achieving complex designs and pursuing structural rationality through topology optimization

   For the shell-shaped bench design, we incorporated both curved surfaces and hollow parts and took advantage of the ability of our 3D printer to manufacture complexly shaped members without using forms.
   In addition, for the internal structure design, we pursued structural rationality by using topology-optimization technology, which is capable of deriving a lightweight, durable form similar to a frame. As a result of using this technology to decide on the hollow parts of the internal structure, we achieved approximately 50% lighter weight (compared to a dense internal structure) without undermining the structural performance.

   

   Considering an internal structure using topology optimization