Why this calc exists
Formwork design hinges on the lateral pressure that fresh concrete exerts on the shutters. Two extremes bound the problem: zero pressure (if concrete behaved like a solid) and full hydrostatic head (γ·h, if concrete behaved like water). Reality is between, governed by placement rate, temperature, and chemistry.
ACI 347R-14 Eq 2.2 captures this with an empirical formula that gives p_max = Cw·Cc·[7.2 + 785·R/(T+17.8)] kPa, where R = placement rate (m/h), T = concrete T (°C). The 30 kPa floor and full hydrostatic cap (γ·h) bracket the result. The formwork pressure calculator applies the SI form with the validity bounds.
Code references (with provenance)
- ACI 347R-14 — Guide to Formwork for Concrete (Eq 2.2 SI form). The primary reference.
- CIRIA C544 — Concrete pressure on formwork (UK alternative; gives similar but slightly different values).
- IS 14687:1999 — Falsework and formwork for concrete structures — code of practice (Indian context).
Worked: column, 3 m tall, R = 2 m/h, T = 30 °C
Eq 2.2: p_max = 1.0·1.0·[7.2 + 785·2/(30+17.8)] = 7.2 + 1570/47.8 = 7.2 + 32.8 = 40.0 kPa.
Hydrostatic cap: γ·h = 24 × 3 = 72 kPa.
30 kPa floor: 30 < 40, so the Eq value stands.
Design pressure: 40 kPa (lower of Eq and hydrostatic, since 40 < 72).
Field notes (what trips people up)
- Outside validity bounds, use hydrostatic. ACI 347R's Eq is calibrated for slump ≤ 175 mm, internal vibration, R ≤ 2.1 m/h (columns), height ≤ 4.2 m. SCC mixes with slump 600+ mm behave differently; tall walls (> 4 m) often reach hydrostatic; very high placement rates push toward the hydrostatic cap.
- 30 kPa floor is conservative. For low walls (≤ 1 m), the equation can predict < 30 kPa; the code imposes 30 kPa minimum to account for construction vibration, workers on top, and impact loads.
- CIRIA vs ACI. CIRIA C544 gives slightly higher pressures for SCC and lower for low-heat mixes. ACI is more conservative for normal mixes; CIRIA for SCC. Site practice: pick the more conservative of the two.
- Formwork ties and walers must resist this pressure over the full panel area. Under-design of ties is the #1 formwork failure mode.
Worked example
Site: 4 m lift of shear wall, R = 1.5 m/h, T = 28 °C, OPC no retarder, internal vibration.
Eq: 7.2 + 785·1.5/(28+17.8) = 7.2 + 25.7 = 32.9 kPa.
Hydrostatic: 24 × 4 = 96 kPa.
Design: 32.9 kPa (Eq value), with 30 kPa floor → 32.9.
FAQ
Why does placement rate matter?
Faster placement means more fresh concrete on top of partially-set concrete — the lower layers behave more like liquid. Slow placement lets the bottom set and not contribute to lateral pressure.
What about superplasticizer / SCC?
SCC behaves much closer to liquid; the ACI equation underestimates. For SCC, use CIRIA C544 or a hydrostatic assumption (γ·h).
Why is the formula metric-unfriendly in some sources?
ACI 347R has separate IP and SI versions. The IP version uses p in psf, R in ft/h, T in °F. We use the SI form: p in kPa, R in m/h, T in °C. The constants 7.2 and 785 are the SI conversion.
Related reading & tools
- ../calculators/formwork-pressure.html — Formwork Pressure calculator
- formwork-engineering-is14687.html — Formwork Engineering — IS 14687 (blog)
- formwork-types-and-liners.html — Formwork Types and Liners (blog)
Indicative tool — not a substitute for engineering judgement.
Results are computed from formulae and reference values cited to Indian Standards and other published codes as named on this page, using nominal default assumptions; clause and table references are given in good faith for study and preliminary checks. Always verify the current edition and amendments of any cited standard in the BIS catalogue before relying on it. Final mix proportions, structural checks and construction decisions must be confirmed by a qualified engineer against the project design brief, drawings and site conditions. ConcreteInfo accepts no liability for design, procurement or construction decisions taken on the basis of these tools. Code references & rights: Indian Standards are the property of the Bureau of Indian Standards (© BIS) — official PDFs at bis.gov.in. CIRIA references are copyrighted by the Construction Industry Research and Information Association. Shared in good faith as technical reference for the civil engineering fraternity — always verify against the official publication.