ForMatter/Processes/formative/Concrete Casting (Formwork)
proc_concrete_casting

Concrete Casting (Formwork)

formative · concrete pouring, concrete molding, cast-in-place concrete, precast concrete, GFRC casting, concrete formwork
highlast updated 2026-04-28
Compatible materials
Portland Cement Concrete
Terrazzo (Cement-Bound Aggregate Composite)

Pouring wet concrete into a mold — wood, plywood, steel, fabric, foam, sometimes 3D-printed plastic — and letting it harden into the shape the mold described. The mold is the form. Everything else about the surface, the joinery, the texture, follows from how the formwork was built. The cast is true to the form because nothing about the concrete resists; the concrete is liquid until it isn't, and what was a soft pour is a hard block in 24 hours and a fully-cured structural element in 28 days.

Liquid concrete (typically Portland cement, water, sand, aggregate, ~10–20 percent paste by volume) placed in formwork and allowed to hydrate. Setting begins in 30–90 minutes; initial strength reaches in 24 hours; specified compressive strength is named at 28 days (typical structural mix is 25–40 MPa, high-performance concretes reach 80–150 MPa). Formwork is the dominant cost and craft variable: phenolic-faced plywood gives a smooth finish, board-formed plywood leaves a wood-grain texture, fabric formwork (a Mark West technique) gives organic curved shells, GFRC (glass-fiber-reinforced concrete) sprays into thin-shell molds. Reinforcement is rebar (typical) or fiber (steel, glass, polypropylene microfiber for GFRC) cast into the pour. Cure conditions matter — concrete poured below 5 °C needs heating blankets; above 30 °C needs evaporation control. Stripping the form too soon ruins the surface; too late fights the formwork release.

Scale & Tolerance

  • scale (mm)50 – 100000
  • tolerance (mm)5
  • skillbeginner can pour a stepping stone in an afternoon. Tilt-up panels and architectural exposed concrete demand a multi-trade crew (formwork carpenters, ironworkers, concrete finishers) and a foreman who has done it before.
  • min skillintermediate
  • wheredesktopschool shopprofessionalindustrial
  • costlow per cubic meter; high in formwork labor for one-off architectural work, low for repeated precast

Equipment

  • school_shopwheelbarrow or mixer (cement, sand, aggregate, water by recipe), formwork lumber + screws + form-release oil, vibrating poker (small) or hand tamping, rebar + tie wire + bolt cutters, screed board, trowels, curing blanket or burlap
  • professionalconcrete pump truck for high-rise pours, ready-mix delivery from a batching plant, motorized vibrators, helicopter trowel for slab finishing, custom modular formwork systems (Doka, Peri, Symons), tilt-up panel beds
  • industrialprecast plants with reusable steel molds, automated rebar placement, steam-cured fast-strip cycles, automated GFRC spray booths, 3D-printed concrete systems (Cobod, ICON) for site-printed walls

Environmental

  • energy_usehigh upstream (cement clinker production at 1450 °C dominates the embodied-carbon load — ~0.9 kg CO2 per kg cement); low at the pour itself
  • waste_streamwash-out water (high pH, regulated), formwork ply / lumber (often single-use for architectural finishes; reusable for utility pours), broken-out concrete (recyclable as crushed aggregate or road base)
  • consumablescement, aggregate, sand, water, form-release oil, rebar, tie wire, formwork ply (single- or multi-use)

Trade-offs

constraints · what is lost · what is gained
Portland Cement Concrete
  • constraints
    • gravity-fed pour requires accessible top opening and continuous fall (no enclosed cavities without removable formwork)
    • minimum cover over reinforcement (~25–50 mm) sets the panel-thickness floor
    • corner radii ≥ 6 mm to avoid cold-joint cracking at sharp inside corners
  • what is lost
    • formwork joint lines, tie-rod holes, and bug holes are part of the surface — Kahn's "wall that shows its making" reading or a problem to remediate
    • patina / staining over decades is intrinsic to outdoor concrete
  • what is gained
    • monolithic, in-place geometry impossible by any other route
    • compressive structural mass at the lowest material-cost-per-MPa of any structural material
    • on-site casting at any scale

Plain language. Neutral framing — perfection is contextual, defined by use. Cf. Winchester, The Perfectionists (HarperCollins, 2018).

Louis I. Kahn (dead — channeled)

I asked the concrete what it wanted to be, and the concrete said: I want to be a wall that shows my making. So I gave the concrete its formwork and its tie-rod holes and its joints between the panels, and I did not try to hide any of them, and the wall is honest because nothing about how it became a wall is missing from how it stands as one.

Channeled within the philosophy of Louis I. Kahn, *Louis I. Kahn: Writings, Lectures, Interviews*, ed. Alessandra Latour (Rizzoli, 1991); compare the formwork-and-tie-hole expression at the Salk Institute (1965) and the Kimbell Art Museum (1972).
Adrian Forty (living — quote)

The absolute and decisive effect of formwork upon exposed concrete means that it is how this is built that largely determines the appearance of the result.

Forty, *Concrete and Culture: A Material History* (Reaktion Books, 2012), Chapter 1, 'Mud and Modernity'.
Adrian Forty (living — quote)

Formwork carpentry has been the Achilles heel of concrete's claim to need no skill. From concrete's nineteenth-century origins, formwork was the one stage of concrete production where it was impossible to dispense with skilled labour — and since this compromised the claims that concrete represented an 'alternative' mode of construction, advocates of concrete generally made no reference to this element of the work.

Forty, *Concrete and Culture: A Material History* (Reaktion Books, 2012), Chapter 4, 'A Question of Skill,' on the contradiction at the heart of the concrete-needs-no-skill myth: in Britain in the mid-1960s, twenty per cent of all carpenters and joiners were long-term specialist shuttering fabricators. The formwork-determines-appearance reading from Ch.1 (above) describes the rule; this Ch.4 passage names the labour cost the rule depends on. Adrian Forty (UCL Bartlett emeritus) verified living 2026-04-28.
Thomas Schröpfer (living — quote)

On an architectural scale, concrete is the only material available without inherent form, making it capable of taking on the modulations imposed upon it. While metals and plastics can be cast as well, this process is often too expensive to play out in architecture. Metals, glass, plastics, stone, and wood are all available as products with industrially produced form, in largely orthogonal geometries that makes products inexpensive to the building trade. These preformed orthogonal geometries can be manipulated to create modulated forms just as fluid in appearance as concrete.

Schröpfer, *Material Design: Informing Architecture by Materiality* (Birkhäuser, 2011), Chapter 5, 'Modulation: Transformation by Shaping and Texturing', section 'Modulating the Shaped — Curving', p. 94. Schröpfer is living (SUTD Singapore); verbatim only. Anchors why concrete sits at the center of any architectural-modulation discussion — it is the one material with no orthogonal industrial form, and so the form must come entirely from the formwork. Pairs with Forty above on formwork-determines-appearance and the Kahn channeled voice on the wall-that-shows-its-making.
Compatible materials
Portland Cement Concrete
Terrazzo (Cement-Bound Aggregate Composite)

Second life

reversibilityzero — cured concrete is committed; demolition-and-recycle as aggregate is the end-of-life path.
output recyclabilityyes
waste streams
  • formwork material (reused 5–50 times before disposal)
  • wash-out water from mixers (alkaline, regulated discharge)
  • curing-membrane and bond-breaker chemicals
  • CO₂ emissions from cement manufacture (the dominant lifecycle impact)
repair compatible withproc_concrete_casting

Forty *Concrete and Culture* (Reaktion 2012) on cement carbon-intensity; American Concrete Institute (ACI) sustainability literature.

Citations

Further reading