Slab Formwork

What slab formwork actually does

A reinforced concrete slab does not exist when the concrete is wet. Wet concrete is a fluid with the rough density of a thick milkshake, pouring at about 2400 kg/m³, and it cannot support itself for a single minute. So before any concrete is poured, the entire planned slab is supported from below by a temporary engineered structure: a layer of plywood sheathing pressed flat against where the soffit will be, joists running in one direction beneath the plywood, primary beams crossing under the joists, and a forest of adjustable steel props standing on the floor below, each carrying its share of the wet-concrete load up to the column heads or back to a previously cured slab. This temporary structure is the slab formwork.

Once concrete is poured into the form and rebar mat, it cures over hours and days, gaining strength as cement hydration progresses. After 7 days at 20°C, ordinary Portland cement concrete reaches roughly 75% of design strength; after 28 days, 100%. Only then can the formwork be stripped: plywood pulled off the soffit, joists removed, props lowered. Until that point the form carries every newton of the slab's weight, plus the workers, the rebar, the wheelbarrows, the partial vibration shocks of the screed pass — every load the slab will eventually carry by itself.

Formwork is invisible in the finished building. Walk through any reinforced-concrete building's ground floor and you'll never know there used to be 200 props per floor underneath the slab above. But formwork is the largest single labour cost in concrete construction — typically 35 to 60% of the total — and the technology of slab formwork has evolved from hand-cut timber to fully reusable engineered systems that turn 100-floor towers around at 3 days per floor.

Anatomy, bottom up

Layer	Member	Function	Typical size
Foundation (the floor below)	Cured slab or ground	Carries prop point loads	—
Vertical supports	Adjustable steel props (acrow)	Carry vertical load from formwork to floor below	1.7 to 4.5 m, 30 to 50 kN capacity each
Primary beam	Steel H-beam or aluminum H-section	Transfer joist loads onto props	2 to 4 m long, 150 to 250 mm deep
Secondary beam (joist)	Timber LVL, H20 timber, aluminum joist	Carry sheathing load to primary beams	2 to 4 m, 200 mm deep
Sheathing	Plywood, film-faced ply, steel sheet	Form the slab soffit; resist concrete pressure	18 mm plywood typical
Edge formwork	Plywood or aluminum panel	Forms the perimeter of the slab	Slab thickness deep
Bracing	Diagonal struts, lateral ties	Resist lateral loads (wind, dumping, sway)	—

Prop spacing is a 1.2 to 1.8 m grid for ordinary slabs — closer for thicker slabs or higher live loads. The primary-beam, secondary-beam, sheathing hierarchy is necessary because each layer's deflection is limited; making one layer span too far makes the slab soffit visibly wavy after cure. Engineered systems publish span tables: e.g. an 18 mm film-faced plywood supports up to 3 m of joist span at L/360 with a 6 kN/m² total load.

Pressures, loads, and load combinations

Wet concrete is a Bingham fluid: it has a small yield stress but otherwise behaves as a viscous liquid until cement hydration starts to set the matrix (typically 1 to 4 hours after placement at normal temperatures). While it is fluid, it pushes outward on vertical formwork (walls, columns, beams) with full hydrostatic pressure ρgh — about 24 kPa per metre of depth.

For slabs, the formwork carries primarily vertical load — the slab's own weight plus live loads. Code-prescribed loads (ACI 347, BS 5975, DIN 18218) typically combine:

VERTICAL LOAD ON SLAB FORMWORK

  Concrete dead load:    w_c = γ_c × t_slab
                              = 24 kN/m³ × 0.20 m = 4.8 kN/m²
  Formwork self-weight:  w_f ≈ 0.5 kN/m²
  Construction live load:w_L ≈ 1.5 kN/m² (workers, tools)
  Impact / vibration:    w_v ≈ 10% of w_c during placement

  Design factored vertical load (per ACI 347):
    Q_d = 1.2 × (w_c + w_f) + 1.6 × w_L
        = 1.2 × 5.3 + 1.6 × 1.5
        = 6.4 + 2.4 = 8.8 kN/m²

LATERAL LOAD ON VERTICAL FORMS (walls, deep beams)

  Maximum hydrostatic pressure:
    P_max = γ_c × h  (with reductions per ACI 347 for set rate)
           = 24 kPa per metre of depth before set

  Lateral load on bracing from concrete dumping:
    H ≈ 0.025 × Q_d  (per ACI minimum)
       ≈ 0.22 kN/m² horizontal

Wall formwork sees much higher peak loads because of the full hydrostatic pressure. A 3 m wall pour is 72 kPa at the base — 3× the slab vertical load. That's why wall forms use heavy steel tie rods through the wall, anchored to walers on both sides, while slab formwork uses light steel props standing on the floor.

Worked example: a 200 mm slab over a 5 × 6 m office bay

Reinforced-concrete slab 200 mm thick, supported on perimeter beams. Total slab area 5 × 6 = 30 m². Formwork: 18 mm plywood on H20 timber joists at 300 mm centres, joists supported by aluminum H-section primary beams at 1.8 m centres, primary beams on adjustable props.

SLAB FORMWORK DESIGN

Loads:
  Concrete self-weight (200 mm slab):  4.8 kN/m²
  Formwork:                            0.5 kN/m²
  Construction live load:              1.5 kN/m²
  Total service load:                  6.8 kN/m²
  ACI 347 design load:                 8.8 kN/m²

Sheathing — 18 mm film-faced plywood:
  Joist span L_j = 300 mm
  Plywood moment capacity: 0.9 kN·m/m  →  M_demand = w L²/10 = 8.8 × 0.3² / 10 = 0.079 kN·m/m  ✓
  Plywood deflection: Δ = 5 w L⁴ / (384 E I)
                       = 5 × 8.8 × 0.3⁴ / (384 × 11,000 × 4.86e-7) = 0.18 mm  (< L/360 = 0.8 mm)  ✓

Joists — H20 timber (200 mm deep, 80 mm wide):
  Joist span L_p = 1.8 m (between primary beams)
  Distributed load on joist: w = 8.8 × 0.3 = 2.64 kN/m
  Bending moment: M = w L²/8 = 2.64 × 1.8² / 8 = 1.07 kN·m
  H20 capacity: 5 kN·m  ✓
  Deflection: Δ = 5 w L⁴ / (384 E I) ≈ 2.1 mm  (< 1800/360 = 5 mm)  ✓

Primary beams — aluminum H section:
  Beam span ≈ 1.8 m (between props)
  Tributary load: 8.8 × 1.8 = 15.8 kN/m
  Bending moment: M = 6.4 kN·m
  Capacity of typical aluminum H: 10 to 15 kN·m  ✓

Props — adjustable steel:
  Prop spacing: 1.8 × 1.8 m grid
  Vertical load per prop: 8.8 × 1.8 × 1.8 = 28.5 kN
  Prop capacity (3 m extension): 30 to 50 kN  ✓ with margin

Total prop count for 30 m² bay: 5 × 6 / 1.8² ≈ 9 to 12 props.

Stripping schedule:
  Side forms: 24 to 48 hours
  Bottom forms: 14 days (slab attains 75% f'_c at 20°C ambient)
  Back-prop: maintain reduced props until 28 days
  Full removal: 28 days

Cross-section, drawn

SLAB FORMWORK ELEVATION (one bay)

  ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓   ← FRESH CONCRETE
  ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓        (200 mm thick)
  ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓
  =====================================================       ← Plywood (18 mm)
  ║   ║   ║   ║   ║   ║   ║   ║   ║   ║   ║   ║   ║   ║       ← Joists (H20 @ 300 c/c)
  ═══════════════════════════════════════════════════════     ← Primary beams (Al H)
       │              │              │              │
       │              │              │              │
       │              │              │              │         ← Adjustable steel props
       │              │              │              │            (1.8 m grid)
       │              │              │              │
       │              │              │              │
       │              │              │              │
   ━━━━╧━━━━━━━━━━━━━━╧━━━━━━━━━━━━━━╧━━━━━━━━━━━━━━╧━━━━     ← Floor slab below
                                                                  (already cured)

  Prop load = (slab + formwork + live) × (prop tributary area)
            = 8.8 kN/m² × 1.8 × 1.8 = 28.5 kN per prop
  Prop length = floor-to-floor − 200 mm slab thickness − formwork stack height
              = 3 m − 0.5 m ≈ 2.5 m

Modern formwork systems

Traditional stick-built. Lumber, plywood, and adjustable props assembled piece by piece on every floor. Cheapest material, highest labour. Common in low-rise residential and small commercial. Reusable 8 to 20 cycles before plywood degrades.

Aluminum table forms (Doka Dokaflex, Peri Skydeck). Pre-assembled 2 × 4 m to 6 × 12 m panels with integrated props, aluminum joists, and plywood face. After cure, the table is rolled out from under the slab on castors, lifted by tower crane through a temporary opening, and set on the floor above. Cycle time drops from 7 days to 3 to 4 days per floor for repetitive high-rise work. Capital cost is ~3× stick-built but the tables amortise over 100+ uses.

Flying forms. Even larger table-form units, sometimes the full bay between columns, flown by crane in one piece. Used on hotel and apartment towers with regular column layouts. Most efficient when every floor is identical.

Climbing wall forms (Doka SCP, Peri ACS). Vertical wall forms that lift themselves up the cured concrete below using hydraulic rams sliding on rails. Used for skyscraper cores: the central elevator-stair shaft is the first thing built, climbing 4 to 5 m per cycle ahead of the slab construction that follows behind. Burj Khalifa's core climbed at 5 m every 3 days using ACS forms.

Slip forms. A continuous version of climbing form: the form is jacked upward at a slow steady rate (typically 200 to 300 mm/hour) while concrete is continuously placed at the top and cures by the time it emerges at the bottom. Used for grain silos, chimneys, and some bridge pylons. Requires continuous round-the-clock concrete placement.

Permanent formwork. Steel decking (composite deck), precast concrete planks, or insulating concrete forms (ICF) that stay in place after the concrete cures. Eliminates stripping labour entirely; deck becomes part of the structural system or finishes.

Stripping, back-propping, and ongoing construction

Stripping cannot happen until the slab can carry its own weight. Stripping prematurely turns the slab into a freely deflecting cantilever or simply supported member with no time to gain strength — the slab cracks or collapses. Standard practice:

Element	Strip time (20°C, OPC)	Strength at strip
Side forms (walls, beams)	24 to 48 hours	10 to 15 MPa
Beam soffit, slab bottom (small spans)	7 days	~50% f'c
Beam soffit, slab bottom (long spans)	14 days	~75% f'c
Full removal of back-props	28 days	100% f'c

"Back-propping" is the trick that lets construction proceed at speed: after the bottom formwork is stripped at 7 to 14 days, a reduced grid of props is reinstalled under the slab so that — when the floor above is being cast and the wet-concrete load is added — the still-curing slab beneath is sharing load with the slab below it (which has more cure time). Modern high-rise construction maintains 3 to 5 floors of back-props simultaneously, with each floor at a different cure age and the load distributed accordingly.

Real-world formwork

• Burj Khalifa core (Dubai, 2010). 828 m tall building, cores cast with Doka SCP self-climbing formwork at 5 m per 3-day cycle for two years. Pumping concrete to 600 m above ground required specialised mix designs and 200 MPa pumps.
• Empire State Building (New York, 1931). Built in 410 days using timber formwork and steel framework — formwork was a small contributor; the speed came from the steel-and-concrete hybrid system. Modern equivalents would use aluminum tables and reach similar speeds.
• Hoover Dam (USA, 1936). The concrete was placed in 5 ft cubic blocks with cooled forms, because solid mass-concrete pours that large would have generated cracking from heat of hydration. Each block was a separate small formwork operation.
• Shanghai Tower (China, 2015). 632 m core built with Doka self-climbing formwork. The twisting exterior used custom climbing forms that rotated 1° per floor following the building's helical geometry.
• Modern hotel and apartment towers (worldwide). Aluminum table-form systems with 3-day floor cycles dominate this market — a 30-storey tower can top out in 4 months of concrete construction.

Major formwork failures

• Skyline Plaza, Virginia, 1973. 23rd floor slab partial collapse during construction killed 14 workers when props for the 23rd floor were removed before the 22nd floor below had cured enough to carry the additional load. Triggered ACI 347 revisions on back-propping.
• Willow Island Cooling Tower, West Virginia, 1978. Slip-formed cooling tower collapsed killing 51 workers when the previous lift had not cured enough to support the next pour. Worst construction accident in US history at the time.
• 2003 Bigge Crane / Tropicana Garage, Atlantic City, 2003. Parking garage under construction collapsed when post-tensioning was applied to slabs whose props had been removed too soon; 4 workers killed.
• 2024 LIRR Atlantic Av station shoring incident. Reminder that even temporary works on transit projects are governed by formwork engineering — a slab shoring failure could close a station for months.

Common pitfalls

• Stripping props too early. Concrete strength gain depends on temperature. Cold weather can push 7-day strength to 14-day equivalent. Always verify by cylinder break, not by calendar.
• Inadequate back-propping. Removing all props and recasting the floor above puts the full new wet-concrete load on a slab that hasn't reached design strength. Back-propping shares the load until enough cure has occurred.
• Props on un-cured slab below. A prop on a 3-day slab can punch through under high point load. Spread the load with timber pads or steel base plates.
• Lateral instability. Slab formwork without lateral bracing can sway under wind, concrete dumping, or human contact. Diagonal struts and ties are mandatory for any formwork over 3 m tall.
• Edge-form failure. The perimeter edge form sees full hydrostatic pressure of the wet slab depth; if it's only nail-tacked to the joists, it blows out and concrete pours over the edge. Use proper edge clamps and ties.
• Ignoring concrete temperature. Hot concrete (over 32°C) sets fast — peak pressure stays lower but later strength is reduced. Cold concrete (under 5°C) cures very slowly — strip times must be extended.
• Failing to inspect for damage between uses. Plywood with delaminations or cracked joists can fail catastrophically. Each reuse cycle requires inspection and replacement of damaged members.