Ciria Report 108 Concrete Pressure On Formwork -

When constructing the massive diaphragm walls for Heathrow Terminal 5 (London), engineers faced pours up to 15 meters deep. Ordinary hydrostatic assumptions would have required 200 kN/m² formwork—impractical and expensive.

Applying CIRIA 108, they measured the setting time (E) of the site mix (a high-density concrete with PFA) at 3.5 hours and controlled the rate of rise (R) to 1.2 m/hour. The resulting P_max was just 120 kN/m².

By trusting the CIRIA 108 calculation, they saved over £2 million in formwork costs and completed the walls safely and on schedule.

The magic of CIRIA 108 lies in accurately capturing Rate (R) and Setting Time (E). Misjudging these is the primary cause of formwork failure.

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 ciria report 108 concrete pressure on formwork

Hydrostatic pressure (traditional): 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

Outcome: The formwork supplier designed ties at 1.2 m horizontal × 1.5 m vertical spacing, versus 0.6 × 0.8 m for hydrostatic. Material savings: 60% less tie hardware, lighter walers, and faster assembly. The pour completed without any deflection or leakage. This project alone saved over £15,000 in formwork materials.

CIRIA Report 108 is a classic example of industry research that successfully transferred into daily practice. While it requires careful application when dealing with modern, highly fluid concrete mixes (like SCC), it remains the "go-to" document for standard concrete pours.

It is written with the authority of research but the pragmatism of the construction site. For any engineer designing vertical formwork, R108 remains an indispensable tool, serving as a sanity check against more complex modern standards. When constructing the massive diaphragm walls for Heathrow

Rating: 4.5/5 (Deducted 0.5 only due to the need for supplementary guidance on modern admixtures).

CIRIA Report 108 is built on the understanding that fresh concrete is a thixotropic fluid—it behaves as a liquid when agitated but gains shear strength as it rests. The key variables affecting lateral pressure are:

Nearly 40 years after its publication, CIRIA Report 108 "Concrete Pressure on Formwork" remains the gold standard for rational formwork design. It shifted the industry from fearful over-design to intelligent, risk-aware engineering.

The formula P_max = 1.2 × D × R × E is elegant, powerful, and—when used correctly—profoundly safe. It recognizes that concrete is not an enemy to be contained, but a material to be understood. The magic of CIRIA 108 lies in accurately

Scenario: Pouring an 8-meter-high wall.

Calculation: P_max = 1.2 × 24 × 2 × 2 = 115.2 kN/m²

Hydrostatic check: D × H = 24 × 8 = 192 kN/m². Since 115.2 < 192, the CIRIA value controls. You can design formwork for 115 kN/m², not 192. That is a 40% reduction in required tie strength.