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.
To understand CIRIA 108, you must abandon the "liquid assumption."
Water exerts pressure equally in all directions. Concrete, however, is a granular material with thixotropy (it thickens when left undisturbed) and cohesiveness. Once the concrete begins to set, it forms an arching action against the formwork. ciria report 108 concrete pressure on formwork
CIRIA 108 defines the Maximum Lateral Pressure (P_max) as a value between two extremes:
The determining factor is internal friction, which the report links directly to the rate of rise (R) and the initial setting time (E) of the concrete.
Download a copy of CIRIA 108 (or the later CIRIA C660, a modern update) and keep it in your site office. It isn't just a technical report; it is a blueprint for fast, safe, and economical concrete construction.
Disclaimer: This article provides general guidance. Always consult a qualified structural engineer and reference the original CIRIA 108 publication for legal and contractual specifications.
CIRIA Report 108, published in 1985 by the Construction Industry Research and Information Association (CIRIA), is a seminal technical document providing guidance for calculating the lateral pressure exerted by fresh concrete on vertical formwork. It significantly improved upon earlier methods by accounting for modern concrete variables such as chemical admixtures, blended cements, and varying rates of placement. Quick Facts Full Title: Concrete Pressure on Formwork (Report 108). Authors: C.A. Clear and T.A. Harrison. To understand the importance of Report 108, one
Core Purpose: To provide a reliable, empirical model for predicting maximum lateral pressure ( Pmaxcap P sub m a x end-sub ) to ensure safe and cost-effective formwork design.
Status: Currently considered a standard reference in the UK and internationally, often cited alongside standards like BS 5975. The CIRIA 108 Calculation Model
The report proposes that concrete pressure follows a hydrostatic distribution (increasing with depth) until it reaches a maximum value ( Pmaxcap P sub m a x end-sub
), after which it remains constant to the bottom of the formwork. Key Factors Influencing Pressure
The model calculates pressure based on several critical parameters: The determining factor is internal friction , which
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
The report uses a simplified empirical equation to determine the maximum characteristic pressure ( cap P sub m a x end-sub
), assuming a hydrostatic pressure envelope that reaches a constant maximum value toward the bottom of the pour. ScienceDirect.com The design pressure is calculated as the smaller of: Full Hydrostatic Pressure: is density and is height). The CIRIA Formula: (simplified version). Key Variables in the Formula: (Rate of Rise): The vertical speed at which concrete is placed (m/h). cap C sub 1 (Size/Shape Coefficient):
Dependent on the formwork's dimensions and shape; it distinguishes between walls and columns. cap C sub 2 (Material Coefficient):
Accounts for the type of cement and use of admixtures (e.g., retarders). (Temperature Coefficient):
Adjusts for the temperature of the concrete at placement; lower temperatures typically result in higher pressures due to slower setting. Independent Design House Key Takeaways and Practical Use Concrete pressure on formwork - BS5975:2019 - Prontubeam
