CIRIA Report 108: Concrete Pressure on Formwork (1985) is a seminal industry standard used to calculate the lateral forces exerted by fresh concrete on vertical formwork. It replaced the older CIRIA Report 1 (1965) to better account for modern developments like chemical admixtures and blended cements. Core Calculation Methodology
However, fresh concrete is not water. It is a viscous material containing aggregates that interlock and cement that begins to hydrate and stiffen. As the concrete stiffens, it begins to support its own weight, reducing the lateral pressure exerted against the formwork walls. ciria report 108 concrete pressure on formwork
Placement rate: Faster placement (higher vertical speed of placing) increases lateral pressure because concrete has less time to dissipate flow and consolidate. CIRIA Report 108: Concrete Pressure on Formwork (1985)
To understand CIRIA 108, you must abandon the "liquid assumption." It is a viscous material containing aggregates that
The theoretical maximum pressure concrete can exert is . This assumes the fresh concrete behaves exactly like a liquid; as the concrete is poured, the pressure increases linearly with depth ($P = \rho g h$). While safe, this approach is incredibly conservative. Designing formwork to withstand liquid pressure for a 10-meter pour requires heavy, expensive, and cumbersome falsework.