Concrete, GFRC, plaster and Jesmonite casting
Mix and bag counts for cast concrete and its relatives — volume from a countertop with a sink cut-out, a round or square planter, a sphere, a block or an STL — with mix designs by weight and volume for concrete, mortar, wet-cast countertop mix, GFRC face and backer coats, hypertufa, gypsum plasters by consistency, Jesmonite AC100 and Forton MG; water and water/cement ratio, US and UK bags, pigment loading, reinforcement, a strength curve, release agents for each mould material, sealers and a printable plan.
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How to use it
Casting concrete in a mould is mostly a sum and a recipe: how much will this countertop or planter take, and how much cement, sand, polymer, fibre and water is that? This works it out for the common mixes and prints a plan you can take into the workshop. The rubber mould comes from mould making or the printable mould maker; plaster for slip-casting moulds is also in the kiln calculator, and resin parts are in resin casting.
How to use it
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What you are casting: a slab or countertop (with a rectangular or round sink cut-out and tap holes), a round planter or square trough (outside size, wall and base), a solid sphere or a shell, a block, or an STL file read in the browser. Set how many pieces and a waste allowance.
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Mix:
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Concrete and mortar by volume (1:2:3, 1:1.5:3, 1:2:4; 1:3 and 1:4 mortar): the dry ingredients take more room than the concrete they make, so the volume is multiplied by a dry-volume factor (1.54 for concrete, 1.33 for mortar) and split by the parts. Weights use typical loose bulk densities. Bags: 94 lb or 25 kg cement, 50 lb or 25 kg sand and gravel.
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Bagged premix: counts of 80, 60 and 40 lb bags (about 0.60, 0.45 and 0.30 ft³ each) or 25 and 20 kg bags (about 11.5 and 9.2 litres).
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Weight recipes (wet-cast countertop mix, GFRC, Jesmonite, Forton MG) use the absolute-volume method: each ingredient’s weight divided by its specific gravity, plus a little air, gives the volume one batch makes, so the batch is scaled to the volume you need. For GFRC the water/cement ratio (0.30–0.35) counts the water in the acrylic polymer; the face coat (about 3 mm, sprayed or brushed, no fibre) and the backer with 3–5 % AR glass fibre are batched separately.
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Plasters — plaster of Paris, pottery plaster, Hydrocal, Ultracal 30, Hydrostone, alpha casting plaster — by consistency: parts of water per 100 of powder by weight (70 for plaster of Paris, about 32 for Hydrostone).
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Jesmonite AC100 is 1 part liquid to 2.5 parts powder by weight. For a laminated shell it uses the mould face area × (a 1.5 mm face coat + 1.5 mm per layer of Quadaxial fibre matting).
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Forton MG's proportions are a published starting point — check your supplier’s sheet.
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Pigment: as a percentage of the binder (cement or powder) weight. Iron oxides give full colour by about 5 %; over 10 % weakens the mix.
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Reinforcement: bar spacing and edge cover give the number and length of bars each way in a slab.
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The strength curve shows the usual gain for moist-cured concrete — about 70 % at 7 days — and the plan lists demould times, how long to keep it wet, the release agent for each mould material (silicone, urethane rubber, melamine, MDF, PLA / PETG prints, HDPE, plaster, metal) and sealers.
Every recipe is typical and labelled so; the bag or product sheet wins. Wet cement is alkaline and burns skin — gloves and eye protection, and a dust mask for dry powders.
Works in any current browser. Metric or imperial is shared with the other casting tools.
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