Table of Contents
1. Introduction to Formwork
Formwork is a temporary or permanent mold into which fresh concrete is placed and shaped to achieve the desired structural element. It is one of the most significant cost components in concrete construction, typically accounting for 30-50% of the total structural frame cost. Proper formwork design balances strength, stiffness, economy, and speed of erection while ensuring safety during the construction process.
The formwork system must resist all vertical loads (self-weight of forms, reinforcement, and wet concrete) and lateral loads (concrete pressure, wind, and construction live loads) without excessive deflection or failure. The governing design standard in North America is ACI 347, Guide to Formwork for Concrete, while OSHA 29 CFR 1926.700-706 provides mandatory safety regulations for formwork construction and use.
The Civil Engineering Handbook covers formwork design principles in detail, and the Engineering Glossary provides definitions of formwork terminology.
2. Formwork Materials and Systems
Plywood (Douglas fir or Finnish birch) is the most common form facing material due to its availability, workability, and cost-effectiveness. Plywood sheathing thickness typically ranges from 12 mm to 21 mm, with HDO (high-density overlay) and MDO (medium-density overlay) surfaces providing improved finish quality and multiple reuses. Plywood is usually supported by lumber or engineered wood studs and walers at 300-600 mm spacing.
Steel formwork offers high strength, tight tolerances, and superior finish quality, making it ideal for visible architectural surfaces and repetitive elements in precast plants. Steel panels are modular (typically 600-1200 mm wide) and can achieve 100+ reuses with proper maintenance. The high initial cost is offset by durability and labor savings in large-scale applications.
Aluminum formwork is approximately one-third the weight of steel, offering faster handling and lower crane requirements. It is widely used in tunnel form systems and modular formwork for residential and commercial buildings. Typical aluminum forms achieve 150-300 reuses and produce high-quality concrete surfaces suitable for direct painting.
Plastic and fiberglass formwork systems are gaining adoption for specialized applications. Plastic modular systems are lightweight, corrosion-resistant, and suitable for column and slab forming. Glass-fiber-reinforced plastic (GFRP) formwork can create complex architectural shapes with high precision. Fabric formwork—using geotextile membranes—offers a novel approach for organic architectural forms with reduced weight and material use.
3. Lateral Concrete Pressure — ACI 347 Method
The lateral pressure exerted by fresh concrete on formwork is the critical design parameter for wall and column forms. Fresh concrete behaves as a fluid with internal friction and some cohesion, and its lateral pressure depends on the rate of placement, concrete temperature, slump, admixture use, vibration method, and form geometry.
ACI 347 provides the following formulas for calculating maximum lateral pressure:
| Case | Formula | Maximum | Units |
|---|---|---|---|
| Walls, rate R ≤ 2.1 m/h | pmax = CwCc[7.2 + 785R/(T + 17.8)] | ≤ 150 Cw kPa | kPa |
| Walls, rate R > 2.1 m/h | pmax = CwCc[7.2 + 1154/(T + 17.8) + 244R/(T + 17.8)] | ≤ 150 Cw kPa | kPa |
| Columns, rate R ≤ 4.3 m/h | pmax = CwCc[7.2 + 785R/(T + 17.8)] | ≤ 150 Cw kPa | kPa |
| Columns, rate R > 4.3 m/h | pmax = CwCc[7.2 + 1154/(T + 17.8) + 244R/(T + 17.8)] | ≤ 150 Cw kPa | kPa |
| Slump ≤ 100 mm | pmax = wh (full hydrostatic) | w = 24 kN/m³ | kPa |
| Slump > 100 mm | Use full hydrostatic + dynamic factor | p = wh × 1.2 | kPa |
Where: Cw = unit weight coefficient, Cc = chemistry coefficient (accounts for Type III cement or retarders), R = rate of placement (m/h), T = concrete temperature at placing (°C), w = unit weight of concrete (kN/m³), h = vertical distance from placing point (m).
The pressure distribution is typically assumed triangular from the top of the fresh concrete pour, reaching a maximum value at a depth determined by the rate of placement and setting time. Below this depth, pressure remains constant at pmax (if the concrete below has begun to set and no longer transmits full hydrostatic pressure). The ACI 347 standard reference includes detailed pressure calculation examples. The Engineering Formula Library contains the ACI 347 formulas in calculation-ready format.
4. Shoring, Reshoring, and Backshoring
Shoring is the temporary support system that carries the weight of freshly placed concrete and formwork until the concrete gains sufficient strength to be self-supporting. The analysis of multistory shoring systems is critical because loads from upper-level construction are transferred downward through shores and slabs still gaining strength.
Shoring design must account for: dead load of fresh concrete (24 kN/m³), reinforcement (0.5-2 kN/m³), formwork self-weight (0.3-1.0 kN/m²), construction live loads (2.4-4.8 kN/m² per ACI 347), and wind loads. Shore spacing is determined by the load capacity of the shoring system and the slab strength at the time of loading. Common shoring systems include: steel pipe shores, aluminum adjustable shores, timber shores, and engineered scaffolding towers.
Reshoring refers to the process of replacing original shores after they have been removed, providing continued support to the slab while upper-level construction proceeds. Backshoring (or reshoring where no full removal occurs) provides local support beneath slabs that require additional capacity. Per ACI 347, the reshoring design should consider: the slab must have adequate strength to support its own weight before shore removal, reshoring should be positioned to reduce span lengths, and the number of reshores must account for the accumulated construction loads from multiple levels.
5. Stripping Time and Strength Requirements
Stripping time—the interval between concrete placement and formwork removal—is a critical scheduling and safety parameter. ACI 347 and ACI 318 specify minimum concrete strengths before formwork removal based on the type of structural element:
The maturity method (ASTM C1074) is widely used to determine in-place concrete strength for stripping decisions. Using temperature sensors embedded in the concrete, the maturity index M = Σ(T - T₀)Δt is computed and compared with pre-established strength-maturity relationships from laboratory testing. This approach allows early stripping when conditions are favorable, accelerating construction schedules.
The Learn: Concrete Technology module covers maturity testing and stripping time optimization in detail. The Concrete Mix Design Calculator can help estimate early strength gain for different mix designs.
6. Safety Requirements and Failure Prevention
Formwork failures are among the most catastrophic construction accidents, often resulting in multiple fatalities. OSHA 29 CFR 1926.700-706 sets mandatory safety standards for formwork, requiring that formwork be designed, constructed, and maintained to support all loads without failure. The competent person responsible for formwork must inspect all components before, during, and after concrete placement.
OSHA key requirements include: formwork must be designed by a qualified person; design drawings must specify loads, member sizes, and spacing; reshoring must be provided when original shores are removed; construction loads on formwork must not exceed design capacity; and formwork must be inspected by a competent person after each pour. ACI 347 additionally recommends minimum design factors: 1.5 for vertical loads (dead + live), 2.0 for lateral loads, and 2.5 for form ties and anchors.
Common causes of formwork failure include: inadequate lateral bracing (accounting for approximately 40% of failures), overloading during concrete placement (pumping concrete too rapidly in one area), premature stripping (removing forms before concrete reaches required strength), inadequate shoring foundation (soft or uneven ground), lack of diagonal bracing for wind loads, and poor connection detailing. Thermal effects from concrete hydration can also cause displacement and misalignment if not accounted for in design.
Inspection checklists per ACI 347 include: verification of member sizes and spacing per design drawings, tightness of connections and wedges, plumbness and alignment of forms, adequacy of lateral bracing, condition of form tie hardware, cleanliness and release agent application, foundation bearing capacity for shoring, and provisions for concrete placement and vibration access. The ACI 347 standard provides comprehensive inspection guidance.
7. Worked Example: Design Formwork for a 4m High Wall
Design wall formwork per ACI 347 for a 4m high wall
Given: Wall height = 4.0 m. Rate of placement R = 1.5 m/h. Concrete temperature T = 25°C. Slump = 80 mm. Unit weight w = 24 kN/m³. Type I cement, no retarders (Cw = 1.0, Cc = 1.0). Use 18 mm plywood sheathing, 50×100 timber studs at 300 mm spacing, double walers at 600 mm vertical spacing. Form tie capacity = 45 kN (she-bolt type).
Step 1 — Lateral pressure. Since R = 1.5 m/h < 2.1 m/h: pmax = 1.0 × 1.0 × [7.2 + 785 × 1.5 / (25 + 17.8)] = 7.2 + 785 × 1.5 / 42.8 = 7.2 + 27.5 = 34.7 kPa. Check maximum: 150 × 1.0 = 150 kPa. Use pmax = 34.7 kPa. Equivalent fluid depth = p/w = 34.7/24 = 1.45 m. Pressure distribution: triangular from top to 1.45 m depth; constant 34.7 kPa below 1.45 m to the bottom of the wall.
Step 2 — Plywood sheathing check. Stud spacing = 300 mm. Consider a 1m wide strip. ws = 34.7 kN/m² × 1.0 m = 34.7 kN/m. Three-span continuous: Mmax = wL²/10 = 34.7 × 0.3² / 10 = 0.312 kN·m/m. Section modulus S = 1000 × 18²/6 = 54,000 mm³/m. Bending stress fb = 0.312 × 10⁶ / 54000 = 5.8 MPa. Allowable for 18 mm plywood (Class I) = 12.4 MPa. OK. Deflection: Δ = wL⁴/(145EI) = 34.7 × 300⁴ / (145 × 6500 × 1000 × 18³/12) = 0.81 mm. L/360 = 0.83 mm. OK.
Step 3 — Stud design (50×100 mm @ 300 mm c/c). Load per stud = 34.7 × 0.30 = 10.4 kN/m. Waler spacing = 600 mm. Mmax = 10.4 × 0.6²/10 = 0.374 kN·m. S = 50 × 100²/6 = 83,333 mm³. fb = 0.374 × 10⁶ / 83333 = 4.5 MPa vs. allowable 9.0 MPa (No. 2 Douglas fir). OK.
Step 4 — Waler design (double 50×100 mm @ 600 mm vertical). Tie spacing horizontally = 600 mm. Load per waler = 34.7 × 0.60 = 20.8 kN/m. Mmax = 20.8 × 0.6²/10 = 0.749 kN·m per waler. For double studs: S = 2 × 83333 = 166,666 mm³. fb = 0.749 × 10⁶ / 166666 = 4.5 MPa. OK.
Step 5 — Form ties. Tributary area per tie = 0.60 × 0.60 = 0.36 m². Max tie load = 34.7 × 0.36 = 12.5 kN. Allowable tie capacity = 45 kN. OK (factor = 3.6).
Summary: 18 mm HDO plywood, 50×100 mm studs @ 300 mm, double 50×100 mm walers @ 600 mm vertical, 45 kN she-bolt ties @ 600×600 mm pattern. All stresses and deflections within allowable limits per ACI 347. Verify with the Unit Weight Calculator for fresh concrete density assumptions.
Typical Wall Formwork Cross-Section
[SVG Diagram: Cross-section of a wall formwork assembly showing: plywood sheathing, vertical studs at 300 mm c/c, horizontal double walers at 600 mm c/c, form ties with cones and lifting hardware, lateral bracing (adjustable shores) at 45°, and a scaffold platform at the top of the wall. Pressure distribution diagram on the left side shows triangular to 1.45 m depth then constant 34.7 kPa to base.]
8. Frequently Asked Questions
What is the minimum design load for formwork per ACI 347?
ACI 347 requires a minimum vertical live load of 2.4 kN/m² for construction loads on formwork, plus the full weight of wet concrete (24 kN/m³), reinforcement (typically 0.5-2 kN/m³), and formwork self-weight. Lateral loads must include the greater of concrete pressure, wind (0.5 kN/m² minimum), or construction eccentricities.
How does concrete temperature affect lateral pressure?
Lower concrete temperatures slow the setting reaction, allowing the concrete to behave as a fluid longer, thus increasing lateral pressure. At 10°C, the calculated pressure is approximately 40% higher than at 30°C for the same rate of placement. Cold-weather concreting requires careful pressure calculation.
When can formwork be safely stripped?
Form stripping time depends on concrete strength gain, not elapsed time. Minimum strengths per ACI 347: beam/column sides at 5 MPa, wall forms at 1.5 MPa, slab soffits at 70% f'c. The maturity method is recommended for determining actual stripping time based on temperature history.
What is the difference between shoring and reshoring?
Shoring supports fresh concrete and formwork during placement and initial curing. Reshoring replaces original shores after they have been removed but before the structure is fully self-supporting for superimposed loads. Reshoring distributes construction loads from upper levels to slabs that have gained additional strength.
What causes most formwork failures?
The leading cause is inadequate lateral bracing (40% of failures), followed by overloading from rapid concrete placement or pumping, premature stripping, inadequate shoring foundations, and lack of diagonal bracing for wind loads. Most failures occur during or immediately after concrete placement.
How do superplasticizers affect formwork pressure?
Superplasticizers increase slump and can slow the loss of workability, increasing lateral pressure on formwork. For concrete with slump > 175 mm, ACI 347 recommends using full hydrostatic pressure or applying a chemistry coefficient Cc = 1.2-1.4 to account for retarded setting.
What is the maximum deflection allowed for formwork?
ACI 347 recommends deflection limits of L/360 for exposed concrete surfaces and L/270 for concealed surfaces. For architectural concrete requiring tight tolerances, L/540 or more restrictive limits may be specified. Deflection of formwork affects finished concrete appearance and member geometry.
How many times can formwork be reused?
Plywood forms: 5-15 uses with proper care and HDO surfaces. Steel forms: 100-500+ uses. Aluminum forms: 150-300 uses. Plastic modular forms: 100-200 uses. Reuse depends on handling care, cleaning procedures, release agent application, and the required quality of the finished concrete surface.
What are OSHA's competency requirements for formwork?
OSHA requires formwork to be designed by a qualified person (engineer or experienced designer) and inspected by a competent person (one who identifies hazards and has authority to correct them). The competent person must inspect formwork before, during, and after concrete placement.
What formwork is best for architectural concrete?
Steel forms produce the highest quality architectural finish with tight tolerances and uniform surface texture. HDO plywood with controlled pouring and vibration also produces acceptable architectural surfaces. GFRP and elastomeric form liners are used for specialized textured or patterned finishes.
Related Calculators
Concrete Volume Calculator
Estimate concrete volume per pour.
Unit Weight Calculator
Fresh concrete density and yield determination.
Concrete Mix Design Calculator
Estimate early strength gain for stripping decisions.
Retaining Wall Calculator
Wall stability and reinforcement design.
Live/Dead Load Calculator
Construction load combinations for formwork design.
Concrete Cost Calculator
Formwork and concrete cost estimation.
Related Articles
Concrete Mix Design Explained
Mix proportioning principles for quality concrete.
Concrete Curing Methods Guide
Curing methods, duration, and best practices.
Concrete Quality Control
Site quality control for concrete construction.
Structural Loads Explained
Dead, live, and construction loads in design.
References & Standards
- ACI 347-19. Guide to Formwork for Concrete. American Concrete Institute, 2019.
- OSHA 29 CFR 1926.700-706. Safety Standards for Formwork. U.S. Department of Labor.
- Hanna, A.S. Concrete Formwork Systems. McGraw-Hill, 1998.
- Peurifoy, R.L. and Oberlender, G.D. Formwork for Concrete Structures. 5th ed., McGraw-Hill, 2020.
- Ringwald, R.C. Formwork Design and Construction. Wiley, 2021.
- Civil Engineering Handbook — Construction Engineering chapter.
- Engineering Formula Library — Formwork pressure formulas.
- Engineering Glossary — Formwork and shoring definitions.