Introduction: The Hidden Force in Concrete Construction In the world of construction, few forces are as misunderstood—or as dangerous—as fresh concrete pressure on formwork. Every year, projects face blowouts, bulging forms, and even catastrophic failures because engineers and contractors underestimate the liquid-like pressure exerted by newly placed concrete. For decades, the industry struggled with inconsistent guidelines until a groundbreaking document changed everything: CIRIA Report 108, "Concrete Pressure on Formwork."
This article provides a comprehensive breakdown of CIRIA Report 108—its history, key formulas, practical applications, and why it remains indispensable for safety and efficiency in concrete placement. Before Report 108, formwork designers relied on empirical rules-of-thumb or overly conservative hydrostatic pressure models. The hydrostatic assumption—that fresh concrete behaves exactly like a liquid (pressure = density × height)—led to massively over-engineered (and expensive) formwork. Conversely, simplified rules like "pressure = 1.5 × height" often proved unsafe for high-slump, fast-pouring conditions.
This distribution has major implications for formwork design: tie rods and walers can be spaced more widely in the lower half of a tall wall, reducing congestion and cost. Over the years, engineers have applied CIRIA Report 108 to a wide range of conditions. The report explicitly addresses several modifiers: 1. Slump Influence Higher slump (e.g., self-consolidating concrete) increases fluidity and delays stiffening. CIRIA 108 recommends adjusting the coefficient C1 upward for slump >120 mm. For SCC (slump flow >600 mm), many designers conservatively revert to hydrostatic pressure. 2. Additives and Retarders Retarding admixtures extend the time before initial set. This effectively increases R/T ratio, raising pressure. The report suggests applying a temperature correction or using an "effective temperature" lower than the actual concrete temperature. 3. Vibration Internal vibration can locally increase pressure by 20–40% directly around the vibrator head. However, CIRIA 108 notes that proper vibration does not increase average formwork pressure if the placement rate accounts for vibration-induced fluidization. The key is to avoid over-vibration, which can segregate concrete and cause blowouts. 4. Form Friction and Surface Coating Rough form liners (e.g., plywood) and stiff formwork can reduce measured pressure compared to smooth steel forms. CIRIA 108 provides reduction factors but recommends caution—friction effects are unreliable. Comparison with Other Codes (ACI 347, EN 12812) To appreciate CIRIA Report 108, one must compare it to other major standards: ciria report 108 concrete pressure on formwork
Importantly, the report states that pressure never exceeds the (ρgh ≈ 24 × height in kN/m²), and it sets a minimum pressure for low rates or high temperatures.
24 kN/m³ × 2.8 m = 67.2 kN/m² CIRIA 108 pressure: P_max = 7.2 × (3/18) + 18 = 7.2 × 0.167 + 18 = 1.2 + 18 = 19.2 kN/m² Introduction: The Hidden Force in Concrete Construction In
CIRIA 108 is often considered more flexible for low- to moderate-rate pours, while EN 12812 is more prescriptive for safety-critical applications. Many formwork manufacturers now produce load tables that directly reference loading. Common Mistakes When Applying CIRIA Report 108 Despite its clarity, engineers frequently misapply the report: ❌ Mistake 1: Using the wrong temperature The temperature T must be the concrete temperature during placement , not ambient air temperature. For hot weather, adding ice or using cool aggregates significantly reduces pressure. ❌ Mistake 2: Ignoring pump startup surge When pumping begins, the initial placement rate can be double the steady-state rate. CIRIA 108 recommends designing for the maximum instantaneous rate, not the average over the whole pour. ❌ Mistake 3: Forgetting vertical form height limit The formula only applies up to the height where concrete begins to set. For very tall walls (e.g., 10 m), multiple lift pours are needed, as the lower concrete will have set before the top is placed. ❌ Mistake 4: Misapplying to slipforming or jump forms CIRIA 108 is intended for stationary forms. For slipforming, dynamic pressures are higher—consult separate guidance. Case Study: Real-World Application of CIRIA 108 Project: 12-story reinforced concrete core wall, London Formwork: Crane-lifted gang forms, 2.8 m tall panels Concrete: C35/45, slump 80 mm, rate of placement 3 m/h, temperature 18°C
Published by the Construction Industry Research and Information Association (CIRIA), Report 108 remains the gold standard for calculating lateral pressure exerted by fresh concrete on vertical and inclined formwork systems. Despite being originally released in the 1980s (with updates in subsequent years), its principles continue to inform modern design codes, including ACI 347 and EN 12812. Before Report 108, formwork designers relied on empirical
| | CIRIA 108 | ACI 347 (USA) | EN 12812 (Europe) | |-------------|----------------|--------------------|-------------------------| | Base formula | P_max = C1*(R/T) + C2 | P_max = C_w × C_c × (7.2 + 785R/(T+17.8)) | P_max = F + (R/(T+1)) × K | | Temperature | Explicit (°C) | Explicit (°C) | Explicit (°C) | | Slump influence | Coefficients for 0–50mm, 50–100mm, >100mm | C_c factor (0.5 to 1.2) | Built into K factor | | Rate limit | No strict cap, but pressure limited to hydrostatic | R ≤ 2.1 m/h for formula; above that, hydrostatic | R limited based on form class | | Minimum pressure | Yes (C2 term) | Yes (7.2 factor) | Yes (F term) |