108 Concrete Pressure On Formwork — Ciria Report

Published in 1985, CIRIA Report 108 ( Concrete Pressure on Formwork

) provides a standardized method for calculating lateral pressure exerted by fresh concrete, accounting for variables like rate of rise and mixture design. It remains a primary reference in BS 5975:2019 for determining the maximum pressure ( cap P sub m a x end-sub

) on vertical formwork. Detailed information regarding the report can be found at Concrete pressure on formwork (R108) - CIRIA

Published in 1985, CIRIA Report 108 (R108) serves as a primary industry standard for calculating the lateral pressure of fresh concrete on formwork. The report provides an empirical formula to determine maximum pressure based on variables like concrete density, rate of rise, and temperature, which remains relevant for ensuring safe, cost-effective formwork design. For full details, visit CIRIA. Concrete pressure on formwork (R108) - CIRIA

CIRIA Report 108, "Concrete pressure on formwork," serves as a foundational guide for calculating the lateral pressure exerted by fresh concrete on temporary formwork structures, accounting for variables like rate of rise, temperature, and mix composition. It provides specific formulas to determine maximum pressure ( Pmaxcap P sub m a x end-sub ciria report 108 concrete pressure on formwork

) and design coefficients for walls and columns to prevent formwork failures. For more details, visit CIRIA. Construction and Building Materials

2. The CIRIA 108 Methodology

CIRIA Report 108 provided a semi-empirical method for calculating pressure. It moved away from the "worst-case scenario" of hydrostatic pressure and introduced a formula based on observed site behaviors.

The report identifies several variables that influence pressure, but the two most critical are:

1. Rate of Rise (R): How fast the concrete is placed in the forms (meters per hour).
2. Temperature (T): The temperature of the concrete, which affects the setting time.

Core Principles of Concrete Pressure (As Per CIRIA 108)

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: Published in 1985, CIRIA Report 108 ( Concrete

1. Rate of placement (R) – Vertical rise speed of concrete in the form (m/h). Faster rates mean less time for stiffening, hence higher pressure.
2. Temperature (T) – Affects cement hydration and setting. Cold concrete retains fluidity longer, increasing pressure.
3. Concrete mix design – Slump, cement type, water-cement ratio, and use of superplasticizers or retarders.
4. Formwork geometry – Height, width, and stiffness of the forms.
5. Vibration – Internal or external vibration temporarily liquefies concrete, increasing local pressure.

2. Key Content and Methodology

The report is structured to take the user from the fundamental properties of concrete to the final design pressure.

• Theoretical Basis: The report establishes that fresh concrete behaves as a fluid for a limited time. The pressure profile is not hydrostatic throughout; instead, it peaks at a certain depth and remains constant below that point (the "envelope" theory).
• The "CIRIA Method": The core of the report is the pressure calculation formula. It identifies the Critical Rise Rate and the Temperature of the concrete as the primary variables.
• Mix Proportions: A significant contribution of R108 is its categorization of concrete types. It distinguishes between:
  • Ordinary Portland Cement (OPC).
  • Concrete containing Pulverised Fuel Ash (PFA).
  • Concrete containing Ground Granulated Blastfurnace Slag (GGBS).
  • Retarded mixes. This differentiation is crucial because these admixtures alter the stiffening time of the concrete, directly affecting the pressure exerted on the forms.

3. Strengths of the Report

A. Clarity of Presentation The report utilizes clear charts and tables to derive coefficients ($C_1$, $C_2$) based on concrete temperature and cement type. For a site engineer or temporary works coordinator, the workflow is logical and easy to follow. The pressure diagrams (showing hydrostatic head vs. the design envelope) visually explain why the pressure is calculated the way it is.

B. The "Formwork Pressure" vs. "Hydrostatic" Distinction One of the most valuable sections explains the phenomenon of particle interference. As concrete stiffens, the aggregate particles begin to arch, transferring load to the sides of the formwork via friction, thereby reducing the pressure on the lower layers. R108 quantifies this reduction, leading to more economical formwork designs compared to assuming full liquid head.

C. Practical Guidance on Restraint The report provides excellent guidance on the "kick" or the outward force generated during the placement of the first lift of concrete, and the importance of adequate restraint systems (props and ties). It addresses safety factors and load combinations with a site-focused perspective. Rate of Rise (R): How fast the concrete

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.

1. Context and Relevance

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.

Formwork Height

Report 108 distinguishes between the height of the formwork and the pressure envelope. In tall columns, the pressure might reach a maximum peak and then drop off near the bottom because the concrete there has set. In short walls, the pressure might be hydrostatic because the pour finishes before the concrete has time to stiffen.