Formwork ACI 2014 / R2020

ACI 347-14 (R2020) — Guide to Formwork for Concrete

Guide for the design and construction of formwork for concrete structures, covering lateral pressure of fresh concrete, design loads, deflection limits, construction loads, shoring, and formwork removal times.

Scope

ACI 347-14 provides guidance for the design, construction, and safe use of formwork for cast-in-place concrete structures. The document covers formwork for walls, columns, beams, slabs, and other structural elements, including the determination of lateral pressure from fresh concrete, design loads, deflection criteria, shoring and reshoring procedures, and formwork removal times. It applies to both job-built formwork and manufactured forming systems.

The guide addresses formwork for all types of structural concrete including normal-weight, lightweight, and self-consolidating concrete (SCC). It does not cover slipforms, tunnel forms, permanent forms (stay-in-place), or formwork for precast concrete production, which have their own specialized guidelines.

Purpose

The primary purpose of ACI 347 is to promote safety, quality, and economy in formwork construction. Improperly designed or constructed formwork is a leading cause of construction accidents, and inadequate formwork can result in defective concrete surfaces, misaligned structures, or catastrophic collapse during concrete placement.

The guide provides engineers and contractors with standardized methods for determining lateral fresh concrete pressures, which is the most critical and frequently misunderstood design load on wall and column forms. By establishing consistent design criteria and construction practices, ACI 347 helps prevent formwork failures while allowing efficient reuse of forming systems across multiple projects.

[FIGURE — Lateral pressure distribution on wall forms: triangular pressure envelope from top (zero) to maximum at bottom, showing effect of rate of placement R and concrete temperature T on maximum pressure]

Engineering Applications

  • Wall formwork — lateral pressure from fresh concrete during placement (ACI 347 Cl. 2.2)
  • Column formwork — often governed by full hydrostatic pressure for rapid pours
  • Slab and beam formwork — vertical dead and live loads plus construction live loads
  • Shoring and reshoring systems for multistory buildings
  • Bridge falsework and temporary support structures
  • Self-consolidating concrete (SCC) formwork — higher lateral pressures due to fluid consistency

Design Philosophy

Formwork design follows limit states principles: strength (adequate capacity for all expected loads without failure) and serviceability (deflections within acceptable tolerances). The formwork must support its own weight, the weight of fresh concrete, reinforcement, construction live loads (workers, equipment, impact), wind loads, and lateral concrete pressure.

The lateral pressure of fresh concrete is the most complex load to determine. For walls, ACI 347 provides: p = CwCc[150 + 9000R/T] (psf), with maximum of 150h psf (hydrostatic) and minimum of 600 psf. For columns, the lateral pressure is p = wh (hydrostatic). The coefficients Cw (unit weight factor) and Cc (chemistry/retarder factor) adjust for specific concrete properties.

Note: The lateral pressure equation uses 150 psf as base unit weight and 9000 as an empirical constant. For metric, the equivalent is p = CwCc[7.2 + 785R/T] (kPa). The rate of placement R and temperature T are the two most influential variables in determining lateral pressure.

Important Requirements

  • Minimum Design Loads — Vertical: concrete weight + reinforcement (typically 24–26 kN/m³), construction live load (2.4 kPa min), and impact (0.75×vertical). Lateral: wind per ASCE 7 and concrete lateral pressure per Cl. 2.2.
  • Lateral Pressure Coefficients — Cw = 1.0 (normal weight, 2240–2400 kg/m³), Cc = 1.0 (normal setting, no retarding admixtures). Use Cc = 1.2 if retarding admixtures are used or if concrete contains more than 10% fly ash or 25% slag.
  • Deflection Limits — L/270 for exposed surfaces (rough formwork), L/360 for visible surfaces (architectural concrete), L/180 for concealed surfaces (soffits above ceilings).
  • Shoring/Reshoring — Sufficient shores to support all dead loads and construction loads until concrete has sufficient strength. Reshoring should redistribute loads to new floors without overstressing.
  • Formwork Removal Times — Based on concrete strength gain (curing temperature, cement type). Minimum 70% of f'c for slab forms, 50% for beam sides. Remove shores only when concrete can safely support its own weight and any superimposed loads.

Key Parameters

Parameter Value / Provision
Wall lateral pressure equation (English) p = CwCc[150 + 9000R/T] psf; min 600 psf, max 150h psf
Wall lateral pressure equation (SI) p = CwCc[7.2 + 785R/T] kPa; min 30 kPa, max 23h kPa
Column lateral pressure p = wh (full hydrostatic) for rates > 2 m/hr
Rate of placement R Vertical rise rate of concrete in form (m/hr)
Concrete temperature T Temperature of concrete in form (°C); T ≥ 10°C per Cl. 2.2
Deflection limit — exposed surface L/270 (rougher finish acceptable)
Deflection limit — visible surface L/360 (architectural concrete)
Deflection limit — concealed L/180 (soffits above ceilings)
Construction live load (min) 2.4 kPa (50 psf) per Cl. 2.1.4
Impact factor 0.75 × vertical load for power buggies, 0.2× for pumping
Formwork Removal Times (days) 10°C 15°C 21°C 27°C 32°C
Column/beam sides (50% strength)321.510.7
Slab soffits (70% strength)75432.5
Beam soffits (70% strength)107543
Shore removal (100% design load)1410765

Based on Type I cement, normal-weight concrete, moist curing. Adjust for Type III (high-early) by approximately 40% reduction or Type II by approximately 20% increase.

Practical Engineering Notes

The lateral pressure exerted by fresh concrete depends critically on the rate of placement R (vertical rise in meters per hour) and the concrete temperature T. At low placement rates and high temperatures, the lower portion of concrete begins to stiffen and develop shear strength before the top is placed, reducing maximum pressure below hydrostatic. At high placement rates or low temperatures, the concrete remains fluid throughout the pour, and hydrostatic pressure governs.

For self-consolidating concrete (SCC), the lateral pressure is significantly higher due to its fluid consistency and slower stiffening rate. ACI 347 recommends using Cc = 1.5 for SCC without viscosity-modifying admixtures, and Cc = 1.2 with VMAs. Some formed elements using SCC may need to be designed for full hydrostatic pressure regardless of pour rate.

Field Tip: Monitor concrete temperature in the form, not the ambient temperature. Concrete temperature can be 5–15°C higher than ambient due to heat of hydration. Using ambient temperature in the lateral pressure equation underestimates the true pressure. Place a thermometer in the first concrete placed and record the temperature.

Typical Workflow

  1. Determine concrete properties: unit weight, setting characteristics, use of retarders/SCMs
  2. Determine pour conditions: rate of placement R, concrete temperature T, pour height h
  3. Calculate lateral pressure per ACI 347 Cl. 2.2
  4. Design form panel sheathing, studs, wales, and ties for calculated pressure
  5. Check deflection limits for each form component
  6. Design shoring/reshoring plan for multistory construction
  7. Establish form removal schedule based on concrete strength gain
  8. Inspect formwork before concrete placement; monitor during pour

Common Mistakes

  • Underestimating lateral pressure from SCC — SCC exerts significantly higher lateral pressures than conventional concrete. Using standard ACI 347 coefficients without the SCC adjustment factor is a common and dangerous mistake.
  • Using ambient temperature instead of concrete temperature — The concrete temperature in the forms can be much higher than ambient, leading to lower calculated pressure than actual. Always use measured concrete temperature.
  • Premature form removal — Removing slab or beam forms before concrete reaches adequate strength can cause excessive deflection, cracking, or collapse. Always verify strength through cylinder tests or maturity methods.
  • Inadequate shoring for multistory buildings — Fresh concrete on upper floors adds load to shores on lower floors that may not have reached design strength. ACI 347 requires a shoring/reshoring analysis for buildings over one story.
  • Ignoring vibration loads — Internal vibration adds dynamic loads to formwork. Design for an additional 50% of lateral pressure locally around vibrator insertion points in heavily reinforced sections.

Best Practices

  • Design formwork for the most critical combination of the three variables: highest rate of placement, lowest concrete temperature, and heaviest concrete unit weight expected during construction
  • Use modular forming systems (e.g., EFCO, PERI, Doka) where possible — they come with engineered design capacities and established safe working loads
  • Include a minimum construction live load of 2.4 kPa on all deck forms and consider concentrated loads from buggy paths and storage areas
  • Use form release agents consistently to prevent concrete adhesion and ensure clean surface finish — apply before reinforcement is placed
  • For multistory buildings, develop a detailed shoring/reshoring sequence showing load transfer at each floor level
  • Use the Bending Moment Calculator to analyze formwork structural components (studs, wales, ties) under lateral pressure

Limitations

ACI 347 is a guide, not a mandatory code. It provides recommended practices but does not have the legal status of a building code. Engineers must verify that local jurisdictions accept ACI 347 as the basis for formwork design. Some states and cities have specific formwork regulations that supplement or override ACI 347.

The lateral pressure equations are empirically derived for conventional concrete mixtures. For emerging materials (ultra-high-performance concrete, engineered cementitious composites, self-leveling toppings) or unusual placement methods (pumping from below, tremie placement), full-scale testing may be necessary to establish design pressures.

RC Beam Design Calculator

Calculate beam loads and design reinforcement.

Bending Moment Calculator

Analyze formwork structural components.

Shear Force Diagram Calculator

Analyze shear forces in formwork members.
Lateral wall pressure (metric): p = CwCc[7.2 + 785R/T] (kPa) Column hydrostatic pressure: p = w × h (kPa) Deflection limit: δallow = L/n (n = 180, 270, or 360) Shore load: P = w × A (tributary area per shore)

The Structural Engineering Formulas section includes beam deflection and load distribution formulas applicable to formwork design.

Refer to the Civil Engineering Handbook for guidance on formwork design, temporary structures, and construction engineering.

Formwork Design and Safety Guide

Safety and design principles for concrete formwork.

Top 25 Mistakes in Concrete Construction

Common pitfalls including formwork errors.

Deepen your understanding with Concrete Technology and Construction Management learning modules.

Review key terms in the Engineering Glossary: Formwork, Shoring, Reshoring, Lateral Pressure, Hydrostatic Pressure, Rate of Placement, Falsework, and Camber.

References

  • ACI 347-14 (R2020). Guide to Formwork for Concrete. American Concrete Institute, 2014 (Reapproved 2020).
  • ACI 318-19. Building Code Requirements for Structural Concrete. American Concrete Institute, 2019.
  • ASCE 37-14. Design Loads on Structures During Construction. ASCE, 2014.
  • Engineering Standards Reference — ACI 318, CivilFlow.
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