Ever see those concrete business cards online? The ones that seem impossibly slim that they could never be really practical?
I set out to find a way to make real slim concrete 'cards' that would still be robust enough to take some moderate handling. And I set myself the additional task of using my 3D Printer for all the non-concrete elements: printing the mould and even the reinforcements too!
This instructable documents that attempt at 3D Printed reinforced concrete business cards.
*Yes, I do realise that 'concrete' refers to the mixture of sand and gravel aggregates bound with cement. However this experiment aims to test out the principles of reinforced concrete at a much smaller scale. As such it is much clearer in lay-person speak to call these 'reinforced concrete' business cards rather than 'reinforced cement', which just sounds odd.
Step 1: Project Goals
To create a slim concrete business card with shallow embossed detail, that does not crumble or crack with moderate handling.
To experiment with casting concrete directly in 3D printed moulds.
To experiment with the efficacy of various 3D printed meshes as concrete reinforcement.
To see if there are any direct or indirect learnings that can be translated into a larger scale for building-scale RC work.
These photos show my final business card with a 'ucn' mark embossed in it.
Step 2: Experiment Design & 3D-Printing the Components
To load test to failure concrete business cards with various reinforcing mesh configurations.
The business card moulds were 3D printed on my FDM machine. Standard size 90x50mm x 2.5mm deep
The meshes were also 3D printed in PLA on my FDM machine, at 0.5mm height, 20% infill.
I used 0.25mm layer heights.
Each concrete business card mesh variation was load tested 3 times to destruction.
Step 3: Casting Concrete Prototypes
A digital scale was used to ensure a consistent ratio of 4:3 cement:water for all the prototypes
The moulds (formwork) were sprayed with Silicone mould-release spray, then filled with the concrete mix. The printed mesh was inserted, and the whole formwork was levelled with a metal spatula to ensure the mesh was covered and the surface was flush with the edges of the formwork.
These were left to cure overnight.
Step 4: De-moulding the Prototypes After 24 Hours
The prototypes were de-moulded after 24 hours, by carefully bending the PLA formwork away on all 4 edges before separating the cast from the formwork. The concrete stuck hard in some areas, despite the Silicone Lubricant.
The prototypes were left to cure for a further 24 hours before load testing, for a total of 48 hrs.
Step 5: Batch #1: Un-reinforced Concrete; 2.5mm
The un-reinforced concrete broke at the corners even before load testing. This was not unexpected as pure cement at such a thin size (2.5mm) is obviously fragile and crumbly.
Lesson learnt: Formwork cannot have straight (parallel) sides. The formwork binds to the cast object when bending it to try to release the cast. Next time I will introduce a slight draft-angle to the formwork design.
A load test on the most intact piece of non-reinforced concrete (cement) had a fail-weight of a mere 160g.
Step 6: Batch #2: Diagonal Mesh; 2.5mm
The prototype batch with diagonal meshes tended to have chipped corners. This is because the diamond shape mesh did not follow the edges and the corner of the concrete card.
The load test consisted of the prototype RC card laid across 2 supports 7cm apart. This was loaded with gravel in mini-trays until it 1) cracked and 2) gave way.
The diagonal-mesh cards had an average fail-weight of 595g. The failures started with a crack and bend in the card, followed by a catastrophic clean break or bend at a higher weight.
The mesh tended to fail at the intersection points of the 'diamonds', which I guess are 'weak points' caused by the rigidity of the PLA as it tries to stretch in tension.
Decent, but not good enough!
Step 7: Batch #3: Square Mesh; 2.5mm
The square mesh seemed to work better, firstly because there was little chipping at the corners with this mesh design.
Load tests on this batch gave an average fail-weight of 670g. The failures for this batch tended to be catastrophic, with no cracking first.
The mesh seemed to work better in this orientation as the longitudinal 'fibres' of PLA run the length of the card, similar to how steel re-bars run in a slab or beam configuration.
I could improve on performance with a thicker mesh layer that doesn't break under tension so easily.
Step 8: Batch #4: Frame+Mesh; 2.5mm
This was the strongest batch of all the 4, as it utilises a printed plastic frame to protect against bumps, and also provides a rigid plastic edge that also opposes bending forces.
The 2.5mm high PLA frame includes the 0.5mm mesh.
The cards had an average fail-weight of 1680g. The failures started with a crack and bend in the card, followed by a catastrophic clean break or bend at a higher weight.
Step 9: Comparing Results
The results were predictable. For the 2.5mm thick concrete cards, the strongest option was Batch #4 - to have a full PLA border plus PLA mesh on the interior of the concrete cards. However, this was not aesthetically pleasing as the PLA border looks 'cheap' and there were gaps where the concrete did not fill up the mould all the way to the PLA border.
Comparing the square and diamond mesh grids, the diamond mesh grid was weaker, while the square grid performed better under load testing. This could be because the square mesh grid's 'members' are parallel to the direction of the tensile forces.
And finally, of course, the un-reinforced concrete card is about as strong as a single biscuit.
Step 10: Further Development
While the plastic border was the strongest prototype, it lost the 'raw concrete' look and appears quite 'cheap' with the visible plastic frame.
As such, I chose to work with the square grid reinforcement, and make it into a prototype Reinforced Concrete Business Card. I increased the thickness to 3.0mm, and added a shallow motif of only 0.5mm depth. Thus the minimum thickness of the card is still 2.5mm at localised areas, while the rest of the card benefits from a thicker cross-sectional depth of 3.0mm.
Load tests showed that this could hold about 1000g before breaking.
Using PLA as small-scale concrete reinforcement seems useful, but the PLA itself is too brittle. ABS may work better, as would many other fine mesh products on the market that can just be cut to size. Mosquito netting would work well, I reckon. However for this instructable I wanted to limit myself to see what could be done solely with a 3D printer.
The other component to develop further would be the cement mix itself. I am curious to see the impact of mixing fibres directly into the cement (Similar in concept to GFRC: Glass Fibre Reinforced Concrete). Alternatively, would there be any flexible concrete composites that can work better as a business card? Perhaps using a mixture of silicone and cement, or some other flexible glue and cement? Such a flexible cement would create many many new possibilities in design and construction.
Thanks for reading. This is just the beginning of many more experiments in 3D printed concrete formwork and reinforcement. Many more to come!
Oh yes, this is also entered in the 3D Printing Contest. Do vote if you like this instructable please!