There is an inherent brutalist beauty in these concrete tetrapods. An interlocking army in a never ending battle with elemental erosion. Manufactured to stop the advance of water that would eat more delicate structures such as harbors and beaches. The coastal line of Japan is dotted with Dolos (or “knuckle bones”) each individually placed, numbered and able to be traced by satellites. Due to their massive weight (around 20 tons / unit) they are constructed next to the placement site to minimize transportation. With a concrete lifespan around 70 years, allegedly every single Dolos is still functional and intact.
As a side project, I decided to create and cast a miniaturized army of Dolos. The process would entail, an accurate CAD model of a tetrapod / 3d printing of a Master model / making a DIY silicone mold (with said model) and casting a few concrete copies. Figuring the tetrapod geometry is the one and only crucial step for modeling a Dolos. The magic number is 109 1/2°, anything else will make things awkward. Interestingly enough the basic shape is constructed in relation to a cube.
Making a silicone mold is a test of patience, waiting for materials to arrive, molds to cure and cast parts to rest. Having a single mold provides much contemplation time. Cement is not a delicate material and seems to revolt the narrow confinement through a stubborn flow.
Mold making is also an unforgiving process, every defect, a minor flaw, hairline abnormality or process fault will be picked by the final part with extreme accuracy. The time you take to make sure your molds are spotless is time well spent. After a few copies, I find a routine that works quite well, a mixture of 3 parts cement 6 parts sand and just enough water for good rheology. Curing time 2 days. A miniature Dolos breakwater army in the making…