To understand the importance of Report 108, one must first understand the problem it solved. Before 1985, formwork designers often relied on simplistic assumptions.
The theoretical maximum pressure concrete can exert is full hydrostatic pressure. 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.
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.
The challenge for engineers was finding a reliable mathematical model that balanced safety (preventing formwork failure) with economy (not over-designing the formwork). This is where CIRIA Report 108 stepped in.
CIRIA 108 is not for SCC (Self-Compacting Concrete). For SCC, assume full fluid pressure (( \rho gh )) until the HPC (High Performance Concrete) has a measured yield stress > 300 Pa. ciria report 108 concrete pressure on formwork
Your turn: Do you still use the CIRIA 108 nomographs, or have you moved to pressure sensors and digital monitoring?
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Suggested Image for the Post: A graph showing "Lateral Pressure (kN/m²)" on the Y-axis vs "Height (m)" on the X-axis, with two lines:
Alt Text: Comparison graph of concrete pressure on formwork showing CIRIA 108 plateau vs hydrostatic pressure. To understand the importance of Report 108, one
Before the widespread adoption of limit state design in temporary works, there was significant ambiguity regarding how to calculate concrete pressure. Previous methods were often overly conservative or failed to account for the specific behavior of modern concrete mixes (particularly those with admixtures and pulverized fuel ash).
CIRIA R108 was a landmark publication because it moved away from purely prescriptive tables and provided a rational, semi-empirical method for calculating pressure based on the specific properties of the pour. Despite its age, it remains a cornerstone document referenced in the Temporary Works Forum (TWf) guidance and is often preferred over the Eurocode methods for its clarity.
The core of the report is the design pressure formula. For internal vibration (the standard method of compaction), the design pressure $P_max$ is given by:
$$P_max = C_1 \sqrtR + C_2 \sqrtH + C_3$$ Suggested Image for the Post: A graph showing
However, in practice, designers usually utilize the simplified charts derived from the report's regression analysis. The standard CIRIA equation is often presented as:
$$P = D \left[ C_1 \sqrtR + C_2 K \sqrtH \right]$$
Where:
Despite its clarity, engineers frequently misapply the report:
In the 1970s and 80s, engineers realized the "fluid head" method was wasting millions on over-engineered formwork. Concrete stiffens as it hydrates. CIRIA 108 introduced the concept of "pressure relief" based on setting time.