Concrete Handbook for Artists: Sample Images from the Book
Three potential forces that can be exerted on concrete... Concrete, like stone, is very strong in compressive strength (resistance to a squeezing force), but weak in tensile strength (resistance to being pulled apart). Concrete's low tensile strength means we have to add strength in tension. Steel is the common solution because it has high tensile strength, the same expansion and contraction rate as concrete with temperature change and is inexpensive and easy to obtain in a variety of forms.
You can also use a bowl form as a one-part mould, and apply the concrete mix to the outside by hand, as was done in the series shown above. A flexible bowl is chosen and inverted on the work surface (1). You might want to cover it with plastic wrap or petroleum jelly as a release agent. A fibre-reinforced mix (glass fibres work well here) is applied starting at the bottom and building it up about 1 cm (1/2') thick, overlapping sections as you go (2). See the MESH chapter as this technique is similar to the layer building in that process. When the bowl form is completely covered in concrete (3), cover with plastic and let it set 24 hours. Uncover, make sure the first layer is not dry and add another layer of mix (4). You can shape feet or a rim at this stage.
The leaf pendant shown here was made using concrete as an inlay material. The silver structure was built using 1 mm X 5 mm silver. The inside of the walls had grooves ground into them for more of a mechanical bond. Holes were drilled in the leaf's vein to accept steel reinforcing wire that was buried in the concrete mix. The concrete mix was then packed into the 'cell.'
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