One pour, forty minutes, and the joint that is not there
Most workshops build a terrazzo tub from flat panels bonded to a frame, because panels travel well and can be replaced one at a time. We cast the whole body instead. A 400-litre shell takes about 340 kilograms of mix and all of it goes into the mould inside forty minutes, which is the strictest rule in the Tabanan yard. If the first batch starts to set before the second arrives you get a cold joint: an invisible line on handover day that weeps five years later, usually at an inside corner where nobody can reach it.
The mix is white cement with marble aggregate graded 6-12 mm, one part cement to 2.4 parts stone, with the water-cement ratio held at 0.42. Wetter mixes are far easier to place and leave far more pores behind, and pores in a wall holding 4-degree water mean moisture travelling into the body every day of the year. A poker vibrator goes in every 30 centimetres for eight seconds and no longer, so the aggregate does not sink and the top surface ends up as dense as the floor of the tub.
Twenty-one days of curing, and what that does to a build programme
After the pour the body is wrapped in wet cloth and plastic for seven days, then left in shade until day 21 before any machine touches it. Terrazzo shrinks about 0.4 millimetres per metre during that time. Grind it on day five, as workshops chasing a delivery date sometimes do, and the remaining shrinkage happens after the surface is already flat — hairline cracks in the inside corners, which is the hardest place on the tub to repair convincingly. If you are mid-renovation the curing costs you nothing, because it runs while your own contractor is still working.
Mix1 part white cement : 2.4 parts marble aggregate
Water-cement ratio0.42
Aggregate6-12 mm, re-sieved in the workshop
Wet cure7 days wrapped, 21 days before grinding
Measured shrinkage0.4 mm per metre
Sealersilane, penetrates 3-4 mm, not a film
28-day compressive strength38 MPa on cylinder samples
Three grinding passes, and a rim finished by hand
- 60 grit, wet. The cement skin is cut back until the marble grain stands out evenly. This pass decides what the tub will look like for twenty years and removes the most material.
- 200 grit, wet, then a thin cement slurry is rubbed in to close the fine pores opened by the first pass, left 24 hours, and ground again.
- 800 grit until the surface returns light without looking lacquered, followed by a silane sealer that soaks 3-4 mm into the body rather than sitting on top of it as a film that can peel.
- The rim is sanded by hand to a 12 mm radius. A machine cannot hold one radius around a curved corner, and the rim is the only part of the tub that every person touches on the way in and on the way out.
Specifying terrazzo with an architect
About a third of our terrazzo tubs arrive through a designer rather than an owner, and the questions are always the same four. Filled weight is 640 kilograms over less than 1.5 square metres. The shell cannot be cut on site, so the opening it passes through is fixed at drawing stage, not on delivery day. Colour is matched from a physical sample, never a screen. And the plant space — pump, cartridge, chiller hose run — has to appear on the plan before the terrace is tiled. We send a DWG section, the point load and the service clearances free of charge; the process is described under bespoke design and fabrication.
| Mix | Aggregate | Cement base | How it reads on site |
|---|
| Tabanan grey | grey marble 6-10 mm | grey cement | neutral, the most-chosen mix |
| Tulungagung white | white marble 8-12 mm | white cement | bright, large grain clearly visible |
| Volcanic black | crushed basalt 6-8 mm | grey cement plus black oxide | dark, the water reads almost solid |
| Beach sand | cream marble plus quartz | white cement plus yellow oxide | warm, sits well next to timber |
Physical samples 15 by 15 centimetres go out free before production, because terrazzo looks like a different material wet and dry and a photograph is honest about neither. Outside these four mixes we will blend a colour for you: add Rp 1.8 million and three days to cast and cure the test piece. We will not mix a new colour straight into a shell worth tens of millions.
When terrazzo is the wrong answerThe empty body weighs 210 kilograms and cannot be taken apart. If the tub has to ride a passenger lift, pass an 80-centimetre door, or stand on an apartment balcony rated at 250 kg per square metre, choose the acrylic ice bath at 48 kilograms instead. If you are renting and may move within two years, terrazzo is also the wrong purchase — relocating one means a crane at both ends and a new concrete pad at the destination.
What it costs to buy, and what it costs to hold at 3 degrees
Terrazzo shells from 250 to 900 litres run Rp 22-38 million, which includes full insulation, the insulated lid, the circulation pump, a 20-micron cartridge and a calibrated probe thermometer. The chiller is quoted separately because it depends on where you live: for a covered 400-litre tub that is 1.5 PK in Jakarta and Denpasar, 2 PK in Surabaya, 1 PK in Batu. Full component pricing sits on the pricing page, and the chiller calculator sizes the unit for your own city in about ten seconds.
Running cost for a 400-litre covered tub on the domestic tariff is roughly 4.9 kWh a day, about Rp 214,000 a month. The 210 kilograms of stone also buy you something on a power cut: a covered terrazzo tub warms about 1.1 degrees an hour with the chiller off, while a thin-walled shell of the same volume warms more than twice as fast. That is the only thermal advantage stone has — the daily holding load is 516 watts either way, because load follows water surface area, not material.
How to inspect a terrazzo tub somebody else built
- Run a hand around every inside corner. A change in texture or a faint straight line means panels bonded to a frame rather than a single pour.
- Look at the rim in low, raking light. Machine-finished rims wander in radius at the corners; you can see it before you can feel it.
- Ask what the sealer is. A film sealer feels slightly plastic and will peel in patches; a penetrating silane feels like stone and cannot peel because it is inside the body.
- Ask where the insulation sits. It belongs outside the terrazzo, so the mass is on the cold side. Insulation applied inside the shell wastes the thermal mass entirely.
- Put a spirit level across the rim in two directions. A tub that is out by more than 3 mm per metre will never look full, because the concealed overflow only runs on one side.
Will terrazzo crack at 3 degrees?
Not at that temperature. Terrazzo is damaged when water freezes inside its pores, and our tubs never run below 2 degrees. What actually cracks a shell is an uneven base: 640 kilograms standing on paving that settles on one side. That is why we ask for a 100 mm reinforced pad and check it with a spirit level before the tub goes down.
Can it be sunk flush with the terrace?
Yes, and roughly a third of ours are installed that way. You need a pit with a concrete base slab, a drain at the lowest point and at least 40 centimetres of service space on one side for the pipework. If the ground is being excavated anyway, compare the numbers against a microcement cold plunge, which is cast in the hole itself.
Does the colour fade in full sun?
Cement and marble do not fade. Dark oxide pigments lose roughly 5 per cent of their depth over five years in direct tropical sun; the grey and white mixes are effectively unchanged. For an unshaded garden we suggest a light mix, or shade — the trade-offs are set out under outdoor cold plunge.
How often does it need resealing?
Three to four years indoors, two years outdoors. The material costs about Rp 450,000 and the job takes half a day. Our technician does it during a service visit if you hold a maintenance contract.
How long does a terrazzo tub take to build?
Ten working days for Bali because the yard is in Tabanan, 14 for Java, 20-35 for the other islands depending on shipping. The 21-day wet cure runs alongside chiller assembly and crating, so it adds nothing to the total.
