Composite column with 3D printed concrete mold filled with SCC

The most frequent method to produce bridge columns is to erect a form and fill it with concrete. The construction of formwork is time-consuming and despite possibilities to reuse the wooden or steel formwork a small or large number of times, respectively, there are always increased material costs. In this pilot research project, we have investigated the possibilities to instead create permanent or lost formwork by using 3D concrete printers. The formwork will then constitute a part of a composite column and contribute to both the load-carrying capacity and durability. If Self Compacting Concrete (SCC) is used for the interior concrete, two efficient technologies will be combined, 3D-printed (3DP) concrete and SCC. Two promising BSc thesis works have been conducted at KTH The Royal Institute of Technology in co-operation with Luleå Technical University (LTU) and the companies ConcretePrint and Betongindustri.

A literature review shows that the number of publications within this narrow research field is small. A couple of scientific journal papers deals with the aesthetical possibilities that the 3Dprinting procedures provide. In a journal paper from 2021, six Chinese researchers summarize tests on cylindrical 3DP concrete forms filled with concrete. Based on the research outcome, they show that such a composite column is able to develop as high load-carrying capacity as corresponding homogeneous concrete column.

The first BSc thesis work was carried out during the spring 2021 at KTH. It covered four, almost wall-shaped, specimens with rectangular cross-section and the height 1,08 m. Two of the 3DP forms were filled with conventional concrete C40/50 and two other ones with SCC. Both 3DP and casting worked well and the forms sustained the casting without any cracking or leakage. Cores were drilled out of the specimens and among other measurements a bond strength between form and interior concrete of almost 1 MPa was obtained. This is of the same magnitude that usually is obtained between concrete and concrete in the field.

The second BSc thesis work was carried out during the autumn 2021, this time with cylindrical columns. The main research question was if the 3DP concrete forms would withstand the formwork pressure from the interior SCC. The height was 2,4 m and nor at this occasion there were any damages, cracks or leakage due to the formwork pressure.

The main conclusion form the pilot study is that the technology seems promising and that it is urgent to continue with further tests. These tests ought to contain loading tests and experiments to investigate the ability of the 3DP concrete formwork to protect the reinforcement in the interior concrete from corrosion.