Ciria Report 108 Concrete Pressure On Formwork ^hot^ May 2026

CIRIA Report 108, Concrete Pressure on Formwork (1985), provides a widely adopted methodology for calculating lateral pressure exerted by fresh concrete on vertical formwork. Utilizing a semi-empirical formula based on factors like pour rate and concrete density, the report helps determine maximum design pressure while accounting for modern cement types and admixtures. The guide serves as a key reference for temporary works, defining hydrostatic limits and pressure envelopes for concrete construction. For more details, visit CIRIA.

An empirical model to predict fresh concrete lateral pressure

CIRIA Report 108: Concrete Pressure on Formwork - An In-Depth Review

Introduction

Formwork is a crucial component in the construction industry, particularly in the building of concrete structures. The temporary structure is used to hold and shape concrete until it sets and gains sufficient strength. One of the primary concerns with formwork is the pressure exerted by the concrete on the formwork, which can lead to deformation, instability, or even collapse. CIRIA (Construction Industry Research and Information Association) Report 108, titled "Concrete Pressure on Formwork," provides a comprehensive guide on the subject. This report aims to summarize and expand on the key findings and recommendations of CIRIA Report 108.

Background

The pressure exerted by concrete on formwork is influenced by several factors, including:

  1. Concrete properties: The type of cement, aggregate size and type, concrete temperature, and admixtures used can all affect the pressure on formwork.
  2. Formwork characteristics: The type of formwork, its stiffness, and the joint details can influence the pressure distribution.
  3. Placement conditions: The rate of concrete placement, the height of the pour, and the vibration method used can all impact the pressure on formwork.

Key Findings

CIRIA Report 108 provides a detailed analysis of the factors influencing concrete pressure on formwork. Some of the key findings include:

  1. Pressure distribution: The pressure distribution on formwork is not uniform and can vary with depth. The maximum pressure often occurs at the bottom of the pour.
  2. Influencing factors: The report highlights the significance of concrete temperature, placement rate, and formwork stiffness on the pressure exerted on formwork.
  3. Design methods: The report reviews various design methods for calculating concrete pressure on formwork, including the widely used "H" method.

Design Methods for Concrete Pressure on Formwork ciria report 108 concrete pressure on formwork

The "H" method, as presented in CIRIA Report 108, is a widely used approach for estimating concrete pressure on formwork. The method assumes a linear pressure distribution with depth, with the maximum pressure occurring at the bottom of the pour. The pressure (P) at any point is calculated using the following equation:

P = (ρ × g × h) × (1 + (α × V))

where:

  • ρ is the density of concrete
  • g is the acceleration due to gravity
  • h is the depth of the pour
  • α is a coefficient related to the type of cement and admixtures used
  • V is the rate of placement

Case Study

A case study was conducted on a recent high-rise building project in London. The project involved pouring a large concrete slab using a climbing formwork system. The CIRIA Report 108 method was used to estimate the concrete pressure on the formwork. The results showed that the maximum pressure occurred at the bottom of the pour, with a value of 65 kN/m². The formwork was designed to withstand this pressure, and the pour was completed successfully without any incidents.

Recommendations and Best Practices

Based on the findings of CIRIA Report 108, the following recommendations and best practices are proposed:

  1. Accurate estimation of concrete pressure: Engineers should use reliable design methods, such as the "H" method, to estimate concrete pressure on formwork.
  2. Formwork design and construction: Formwork should be designed and constructed to withstand the expected concrete pressure, with adequate stiffness and stability.
  3. Monitoring and control: The concrete placement process should be carefully monitored and controlled to minimize pressure on formwork.

Conclusion

CIRIA Report 108 provides a comprehensive guide on concrete pressure on formwork, highlighting the key factors influencing pressure and recommending best practices for design and construction. By understanding and applying the principles outlined in this report, engineers and contractors can ensure the safe and efficient construction of concrete structures. CIRIA Report 108, Concrete Pressure on Formwork (1985),

References

  • CIRIA Report 108: Concrete Pressure on Formwork (2017)
  • Construction Industry Research and Information Association (CIRIA)
  • British Standards Institution (BSI) - BS 8533-1:2017

Appendices

  • Appendix A: Calculation example using the "H" method
  • Appendix B: Case study - Concrete pressure on formwork for a high-rise building project

The appendices provide additional information and examples to support the report's findings and recommendations.

This report aims to provide an interesting and informative review of CIRIA Report 108. It is essential to consult the original report for detailed information and guidance on concrete pressure on formwork.

CIRIA Report 108 (1985) establishes the standard UK methodology for calculating lateral concrete pressure on vertical formwork, utilizing factors like placement rate, temperature, and mix composition to determine maximum pressure. Referenced in BS 5975:2019, the report addresses both hydrostatic and reduced pressure scenarios to optimize formwork design. Purchase the report at the CIRIA Official Bookshop. Concrete pressure on formwork - BS5975:2019 - Prontubeam

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

❌ Mistake 2: Ignoring pump startup surge

When pumping begins, the initial placement rate can be double the steady-state rate. CIRIA 108 recommends designing for the maximum instantaneous rate, not the average over the whole pour.

2. Additives and Retarders

Retarding admixtures extend the time before initial set. This effectively increases R/T ratio, raising pressure. The report suggests applying a temperature correction or using an "effective temperature" lower than the actual concrete temperature.

The Core Principle: Why Concrete Isn’t Water

To understand CIRIA 108, you must abandon the "liquid assumption." Concrete properties : The type of cement, aggregate

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 108 defines the Maximum Lateral Pressure (P_max) as a value between two extremes:

  1. The Upper Limit: Full hydrostatic pressure (rarely reached unless pouring extremely fast or using retarding admixtures).
  2. The Lower Limit: Zero pressure (fully hardened).

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.

3. Vibration

Internal vibration can locally increase pressure by 20–40% directly around the vibrator head. However, CIRIA 108 notes that proper vibration does not increase average formwork pressure if the placement rate accounts for vibration-induced fluidization. The key is to avoid over-vibration, which can segregate concrete and cause blowouts.

4. Form Friction and Surface Coating

Rough form liners (e.g., plywood) and stiff formwork can reduce measured pressure compared to smooth steel forms. CIRIA 108 provides reduction factors but recommends caution—friction effects are unreliable.

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.

Crucial Constraints (The "Caps")

The formula above does not apply infinitely. CIRIA 108 imposes two absolute limits:

  1. Hydrostatic Limit: P_max must never exceed D × H (where H = total height of the pour). In practice, this means if H < (1.2 × R × E), use hydrostatic pressure.
  2. Minimum Pressure: P_max must never be less than 25 kN/m² (to account for vibration effects near the bottom).

❌ Mistake 1: Using the wrong temperature

The temperature T must be the concrete temperature during placement, not ambient air temperature. For hot weather, adding ice or using cool aggregates significantly reduces pressure.

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